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Review Article
Hypes and hopes on the materials development
strategies to produce ammonia at mild conditions
Swati Singh
a,c
, Abdul Khayum Mohammed
b,c
,
Ali Abdulkareem AlHammadi
c,d
, Dinesh Shetty
b,c,**
,
Kyriaki Polychronopoulou
a,c,*
a
Mechanical Engineering Department, Khalifa University, P.O. Box 127788, Abu Dhabi, United Arab Emirates
b
Department of Chemistry,Khalifa University, P.O. Box 127788, Abu Dhabi, United Arab Emirates
c
Center for Catalysis and Separations (CeCaS), Khalifa University, P.O. Box 127788 Abu Dhabi, United Arab
Emirates
d
Chemical Engineering Department, Khalifa University, P.O. Box 127788, Abu Dhabi, United Arab Emirates
highlights
NH3 synthesis can follow associative, dissociative, and Mars Van Krevelen mechanism.
Defects engineering and structure manipulation are considered game changers.
Design of single-atom catalysts can tailor the rate of NH
3
synthesis reaction.
DFT tools guide the electrochemical and thermochemical catalysts design.
Both inorganic and organic materials are subjected into shape and structure tuning.
article info
Article history:
Received 7 March 2023
Received in revised form
2 May 2023
Accepted 18 May 2023
Available online xxx
Keywords:
Ammonia
Hydrogen
NRR
Ruthenium
abstract
The conventional Haber-Bosch (HB) process is the mainstream route to manufacture
ammonia (NH
3
); the latter being one of the most significant compounds and a carbon-free
energy carrier; HB operates at high pressure and temperature, resulting in significant en-
ergy consumption and CO
2
emissions. An alternative method that has lately received a lot
of attention is the electrocatalytic nitrogen reduction reaction (NRR), which produces NH
3
in ambient settings using renewable energy. The rate of NH
3
synthesis and faradaic effi-
ciency (FE) are both used as catalytic activity descriptors; their values decreased when the
competing hydrogen evolution reaction (HER) is picking up. The design of high-
performance NRR catalysts, operating under ambient settings, while suppressing HER, is
a major research field in the energy sector. This review discusses recent progress on
catalyst design, as well as challenges and opportunities on the catalytic NH
3
synthesis
pathways focusing on different classes of materials, such as Ru-based catalysts, inorganic
metal oxides, organic covalent-organic frameworks (COFs), and metal-organic frameworks
*Corresponding author. Mechanical Engineering Department, Khalifa University, P.O. Box 127788, Abu Dhabi, United Arab Emirates.
** Corresponding author. Department of Chemistry, Khalifa University, P.O. Box 127788, Abu Dhabi, United Arab Emirates.
E-mail addresses: dinesh.shetty@ku.ac.ae (D. Shetty), kyriaki.polychrono@ku.ac.ae (K. Polychronopoulou).
Available online at www.sciencedirect.com
ScienceDirect
journal homepage: www.elsevier.com/locate/he
international journal of hydrogen energy xxx (xxxx) xxx
https://doi.org/10.1016/j.ijhydene.2023.05.206
0360-3199/©2023 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Please cite this article as: Singh S et al., Hypes and hopes on the materials development strategies to produce ammonia at mild con-
ditions, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2023.05.206
Metal oxides
Catalysts
(MOFs). Experimental validation studies on the NRR, through isotopic studies, are pre-
sented as well as future directions for the most prominent catalytic substrates.
©2023 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Contents
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00
Mechanisms of NH
3
synthesis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00
Energetics of the process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00
Fundamentals and challenges in the NH
3
synthesis process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00
Thermodynamics and kinetics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00
The competition with HER . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00
BrønstedeEvansePolanyi relation, Sabatier principle, and scaling relation for N
2
activation . . . . . . . . . . . . . . . . . . . . . . 00
Contamination sources in NH
3
synthesis: detection and elimination . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00
Sources . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00
Strategies to control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00
The design of structure for catalysts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00
Defects engineering . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00
Structural manipulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00
Design of single atom catalysts (SACs) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00
X-mediated strategy (X:Li, Ta) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00
Interface tailoring . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00
Facet engineering . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00
Materials used for N
2
activation and conversion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00
Ru-based catalysts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00
Support effect case study . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00
Promoter effect case study . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00
Bimetallic effect case study . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00
Metal oxides based catalysts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00
Metal nitrides . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00
Other catalysts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00
Open framework materials (COFs/MOFs) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00
Operando techniques for the exploration of the NH
3
synthesis mechanism . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00
DFT studies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00
Future outlook and perspective . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00
Funding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00
Declaration of competing interest . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00
Introduction
Ammonia (NH
3
) is one of the widely used chemicals in
chemical industries, pharmaceuticals, and fertilizers in-
dustries [1e5]. In addition, as a replacement for hydrogen (H
2
),
NH
3
comprising 17.6 wt % H
2
is also an important H
2
carrier
that can be easily transported and stored in contrast to
gaseous H
2
. As an ideal fuel, H
2
reacts with oxygen (O
2
) to form
water (H
2
O) in a fuel cell for zero-emission vehicles, with no
harmful gases emission such as carbon dioxide (CO
2
) or car-
bon monoxide (CO) into the atmosphere. However, the storage
and transportation of H
2
is still a challenge. To store H
2
as a
liquid, high pressure or cryogenic temperatures are required
as H
2
boiling point is 253 C at atmospheric pressure.
Nevertheless, owing to the low vapor pressure of NH
3
, it can be
easily liquefied under mild conditions. Also, there is an
established system for bulk liquid NH
3
storage and trans-
portation. Thus, H
2
can be obtained on demand by electrolysis
or thermal decomposition of NH
3
[6,7]. Some solid oxide fuels
can use NH
3
directly as a fuel and have a performance com-
parable to H
2
fuels. Moreover, NH
3
is also considered as fuel
for gas turbines and internal combustion engines [8].
In nature, biological N
2
fixation happens with the help of
nitrogenase enzymes in bacteria, such as cyanobacteria and
rhizobacteriaunder ambient conditions [9,10]. The enzyme
cofactors containing FeeFe, FeeV, or FeeMo are accountable
for binding, activation, and reduction of N
2
by taking energy
from the adenosine triphosphate molecules. Inspired by na-
ture, different synthetic molecular catalysts have been re-
ported for N
2
reduction. Unfortunately, the extremely low
international journal of hydrogen energy xxx (xxxx) xxx2
Please cite this article as: Singh S et al., Hypes and hopes on the materials development strategies to produce ammonia at mild con-
ditions, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2023.05.206
yield rate of the natural process was unable to meet industrial
demands for NH
3
production. Additionally, nitrogenase en-
zymes and molecular catalysts'poor stability and recycling
problems limit their use in large-scale applications [11,12].
Currently, the industrial synthesis of NH
3
is via the Haber-
Bosch (HB) process using iron (Fe) catalyst at high pressure
(150e300 atm) and temperature (300e600 C) as shown in the
reaction scheme below [13].
1
2N2þ3
2H2/NH3;DHo
298K¼45:9kJmol1
;DGo
298K
¼16:4kJ mol1
;Keq ¼750
In this process, high pressure is used to shift the equilibrium
of the reactionabove. In a single reaction, ~15%of NH
3
is formed
and the unreacted H
2
and N
2
is recycled back to the reaction
chamber [10]. However, the industrial process for NH
3
synthesis
has an adverse impact on the atmosphere as it is energy
intensive and dependent on fossil fuels. For instance, H
2
pro-
duction is an endothermic process from fossil fuels comprising
coal, oil, and natural gas, while a cryogenic process is required
to obtain N
2
from the atmosphere. The percentage of natural
gas, coal, and oil used as feedstock for NH
3
production world-
wide is 22%, 4%, and 72%, respectively [14], contributing to over
420 million tons of CO
2
emissions annually, or more than 1% of
all emissions connected to energy [15]. H
2
production from
natural gas is a multistep method which includes desulfuriza-
tion, steamreforming, and the wateregas shift reaction [16,17].
Out of the total cost for NH
3
production, 75% is accounted for
cost of natural gas and H
2
production [18] and the remaining
25% is for gas compression and NH
3
synthesis and separation
[19]. Therefore, researchers have dedicated efforts to investi-
gating novel methods for lowering the energy consumption of
NH
3
synthesis and the associated CO
2
emissions. At the same
time, havingthe environmental regulations become stringestin
alignment with climate change control had the United Nations
FrameworkConvention on Climate Change (UNFCCC) set a two
degree scenario (2DS) as “economically feasible”and “cost
effective.”This scenario targets deep cuts in emissions, to the
extent of 70% by 2050, with a decarbonized or carbon-negative
economy by 2100. It is worth noting that to meet the COP 21
2DS targets, by 2050 for the NH
3
-related CO
2
emissions is 367
MtCO
2
/yr, whereas with the current HaberBosch process, the
CO
2
emissions are reaching the 900 MtCO
2
/yr.
Additionally, current efforts have been successful in
shifting away from the HB process towards a more sustainable
one, where the H
2
is renewable (e.g., water electrolysis) and
thus the process is called electrolysis-driven HB (i.e., eHB). In
the eHB, the objective is to match the pressure of NH
3
syn-
thesis with that of the electrolysis system for H
2
evolution
(~5 MPa, typically in the 1e3.2 MPa), to eliminate the need for
pressure ramping. Thus, the challenge is to design NH
3
syn-
thesis catalysts that can perform at mild pressures. Therefore,
there is a tremendous need to re-design the catalytic
manufacturing process currently used for NH
3
production, to
achieve the COP21 two-degree scenario (2DS) goals.
Several reviews have been published in the field of NH
3
[20,21], for instance, a review by Chen et al. [22] focuses on the
application of photocatalytic processes for renewable energy
production, while in another review, Wen et al. [23] focused on
the development of nanocatalysts for NRR. In contrast, the
review provided here discusses NH
3
synthesis and covers a
broad range of inorganic and organic catalytic systems for NH
3
production. There have been numerous catalysts developed to
date, such as Fe- [24], Ru- [25], Co- [26], and Ni- [27,28] based
catalysts, which are active both in synthesis and decomposi-
tion of NH
3
. As compared with Fe-based catalysts, the Ru
catalyst provides 20-fold greater activity in a wider range of N
2
and H
2
ratios. Additionally, renewable energy production of
green NH
3
promotes the NH
3
production under moderate
conditions, avoiding frequent ramping from intermittent en-
ergy production. The metal Ru is ideal for catalyzing NH
3
synthesis at low temperatures and pressures. Specifically, in
this review we include Ru-based catalysts and metal-oxide-
based catalysts in the inorganic catalyst section, along with
MOF and COF-based catalysts in the organic catalyst section,
for NH
3
production. The review aims to provide an overview of
the mechanism of NH
3
synthesis, the challenges associated
with the process, and strategies to overcome these challenges.
The focus of the review is to inspire more fundamental
mechanistic investigations in the field of NH
3
production and
to present future perspectives for research.
Mechanisms of NH
3
synthesis
The NRR is a proton-coupled electron transfer (PCET) process.
Proton coupled electron transfer (PCET) term was invented in
1981 to define a step in electrochemistry in which electrons
and protons transfer together in two separate steps or in one
step [29]. Clearly, it involves a complex electrochemical pro-
cess. The standard potential indicates that the reaction is
favorable for NH
3
production, but further investigation is
necessary to determine how its rate is determined. Due to
proton coupling, the NRR is pH-dependent, and its perfor-
mance is highly influenced by its electrolyte. Since a protic
medium is required for electrolyte reduction, undesirable H
2
production is difficult to avoid, which is the primary concern
for NRR. The solubility of N
2
in water is comparatively low, at
about 2 vol%, so mass transport must exert a substantial in-
fluence on the reaction [30].
The multistep reactions and the formation of several in-
termediate products in NRR process is depicted in below
equations:
N2þHþþe/N2H;E+
3:2V vs:reversible hydrogen electrode ðRHEÞ(1)
N2þ2Hþþ2e/N2H2ðgÞ;E+1:1V vs:RHE (2)
N2þ4Hþþ4e/N2H4ðgÞ;E+0:36V vs:RHE (3)
N2þ4H2Oþþ6e/N2H4;E+þ0:55V vs:RHE at pH¼14 (4)
N2þe/N
2ðaqÞ;E+3:37V vs:RHE at pH ¼14 (5)
As we can see from reactions (1) and (2), the redox poten-
tials are more negative, indicating that the reduction of H
þ
into H
2
is easier, while the generation of intermediate prod-
ucts (N
2
H and N
2
H
2
) is difficult in an aqueous solution and
therefore, H
2
is obtained as the major byproduct in NRR pro-
cess. These multiple electron-proton transfer reaction
international journal of hydrogen energy xxx (xxxx) xxx 3
Please cite this article as: Singh S et al., Hypes and hopes on the materials development strategies to produce ammonia at mild con-
ditions, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2023.05.206
processes makes it a complicated reaction mechanism and is
yet to be properly investigated [31].
NRR reaction pathway can be classified into dissociative or
associative mechanisms (Fig. 1). For the dissociative mecha-
nism (Fig. 1a), the triple bond breaks first on the catalyst sur-
face. NH
3
is then formed by adding protons and electrons to
the N atom. In early transition metals (such as Sc, Y, Ti, and
Zr), N atoms normally bind stronger on their surfaces than H
atoms, resulting in the dissociative mechanism at room
temperature. However, due to the strong triple N^N bond and
the high ionization energy of N
2
, NRR usually fails to go along
with the dissociative mechanism due to the high energy input.
In the associative mechanism, protons can enter the N
2
molecule before the NeN triple bond is broken, unaffected by
the breakage of the triple bond. It is therefore suitable for NRR
at mild conditions because it requires relatively little energy in
comparison to the dissociative mechanism. According to the
protonation order, the associative mechanism can be
classified into distal and alternating pathways. An alternating
pathway involves two N atoms bonding with protons one at a
time (Fig. 1b). As part of the distal reaction, protons are first
added to the N atom that is not bonded to the catalyst, until
NH
3
is formed (Fig. 1c). Afterward, the other N atom acquires
protons. Consequently, distal mechanisms do not generate
the byproduct N
2
H
4
; the latter derived from NH
2
eNH
2
desorption [32].
Energetics of the process
In conventional Haber Bosch plants, natural gas (50%), oil
(31%), and coal (19%) are used as feedstocks [34]. In a study by
Smith et al. [35], methane-fed processes served as the
benchmark to compare alternative technologies based on
their higher energy efficiency and lower greenhouse gas
emissions. Below is a simplified diagram of the methane-fed
HabereBosch process (Fig. 2a).
A sustainable future for the Haber Bosch process depends
on the use of renewable energy, which generally refers to the
electrification of the chemical industry [36], in comparison to
the conventional NH
3
process. Renewable energy sources can
replace methane as a feedstock and a fuel in this particular
case, thereby providing all energy requirements. Hydrogen is
generated by electrolysis of water, and NH
3
is produced using
a Haber-Bosch reactor similar to the conventional process
(Fig. 2b).
Haber Bosch process requires minimum energy of 18.6 GJ
t
NH31
based on the lower heating value of NH
3
(LHV). Fig. 3
shows the amount of chemical energy stored, and all energy
consumed above this value is considered a loss. It is estimated
that the theoretical minimum energy input for the methane
fed process is 22.2 GJ t
NH31
[37], with 17.7 GJ t
NH31
allocated to
the methane feedstock and 4.5 GJ t
NH31
allocated to methane
fuel to fire the steam methane reforming reactors SMR
reactor, which are used to produce H
2
(in order to use in NH
3
synthesis reaction). Since the required temperature is so high,
the latter heat cannot be recycled. Based on the LHV of a
Fig. 1 eThe dissociative (top), associative alternating
(middle), and associative distal (bottom) pathways for
potential NRR mechanisms. Reproduced from Ref. [33],
with permission from the Royal Society of Chemistry.
Fig. 2 eSchematic diagram of (a) a typical conventional methane-fed Haber Bosch process and (b) an electrically powered
alternative. Reproduced from Ref. [35], with permission from the Royal Society of Chemistry.
international journal of hydrogen energy xxx (xxxx) xxx4
Please cite this article as: Singh S et al., Hypes and hopes on the materials development strategies to produce ammonia at mild con-
ditions, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2023.05.206
stoichiometric amount of hydrogen, the required energy input
through an alternative route, such as electrolysis, is no less
than 21.3 GJ t
NH31
. Based on the direct electrochemical syn-
thesis of NH
3
from liquid water and nitrogen at 25 C and 1 bar,
the energy input is 19.9 GJ t
NH31
(1.17 V) [38]. Nevertheless, the
electrochemical synthesis of NH
3
presents low throughput
and low selectivity at present, which causes it to consume
much more energy than methane-based Haber Bosch syn-
thesis [35].
Fundamentals and challenges in the NH
3
synthesis process
Thermodynamics and kinetics
The large dissociation energy (941 kJ mol
1
) of the N
2
triple
bond (N^N) is not the only reason for the inertness of N
2
molecules. It is important to note that the energy required to
dissociate the triple bonds in carbon monoxide (1070 kJ mol
1
,
CO) or acetylene (967 kJ mol
1
,HC^CH) are larger than that of
N
2
, but they are significantly more reactive than N
2
. The inert
behavior of N
2
could be explained by the fact that almost half
of the total dissociation energy (410 kJ mol
1
) is required to
break the first sbond in N^N whereas only 222 kJ mol
1
is
sufficient for the first bond cleavage in HC^CH. With such a
significant difference, it is difficult to initiate the N
2
dissocia-
tion compared to HC^CH. Moreover, N
2
molecule does not
have a permanent dipole and therefore shows lower proton
affinity (493.8 kJ mol
1
), high ionization potential (15.84 eV),
negative electron affinity (1.90 eV), and a large energy gap
(10.82 eV) between lowest and highest unoccupied molecular
orbitals, which obstructs the electron transfer reactions. Thus,
it is difficult to activate N
2
molecules and it is unlikely to have
their direct protonation under ambient conditions and even
with strong acids. From the thermodynamic constraints
caused by the intermediates of the reaction, we can under-
stand the fundamental bottlenecks in the electrochemical
reduction of N
2
to NH
3
. Following are the equilibrium poten-
tials required for different NRR products using either revers-
ible hydrogen electrodes (RHE), standard hydrogen electrodes
(SHE), or normal hydrogen electrodes (NHE) as references
[33,39,40]:
N2þ6Hþþ6e#2NH3ðgÞ;Eᵒ¼þ0:55VvsNHE (6)
2Hþþ2e#H2E0¼0V vs SHE at pH ¼0 (7)
N2þ6H2Oþ6e#2NH3þ6OH
;
E0¼0:736 vs SHE at pH ¼14 (8)
2H2OðlÞþ2e#H2ðgÞþ2OH
;
E0¼0:828Vvs SHE at pH ¼14 (9)
N2þHþþe#N2H;E0¼3:2V vs RHE (10)
N2þ2Hþþ2e#N2H2ðgÞ;E0¼1:10V vs RHE (11)
N2þ4Hþþ4e#N2H4ðgÞ;E0¼0:36V vs RHE (12)
N2þ4H2Oþ6e#N2H4þ4OH(13)
E0¼1:16V vs SHE at pH ¼14
N2þe¼N
2ðaqÞ;E0¼4:16V vs NHE or E0
¼3:37V vs RHE at pH ¼14 (14)
In terms of equilibrium potentials, the electrochemical
reduction of N
2
to NH
3
is similar to that of the competing HER
(Reactions 6 and 7; Reactions 8 and 9). As a result, NRR in
aqueous electrolytes produces a significant amount of
hydrogen as a side product. However, NRR involves multiple
proton-electron transfer reactions and intermediates. It is
evident from the much negative redox potential of the N
2
H
intermediate that the initial addition of the hydrogen atom is
difficult (Reaction 10). Reaction 14 needs a sufficiently high pH
Fig. 3 eImprovement in the efficiency of NH
3
production over the last decades showing actual plant data compared to Best
Available Technique (BAT), the minimum energy requirement for a methane-fed plant, the minimum energy for electrolysis
(H
2
LHV), and current and future electrically driven processes. The amount of energy stored in NH
3
is the LHV and
everything above that is losses. Reproduced from Ref. [35], with permission from the Royal Society of Chemistry.
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to compete with Reaction 10. Moreover, the second H atom
can be harder to add than the third, resulting in higher redox
potentials with two-electron and four-electron reductions
than with six-electron reductions (Reactions 11, 12, and 13). As
illustrated by such negative potentials, N
2
hydrogenation is a
thermodynamically challenging process.
In addition, it is unclear whether NRR prefers sequential
proton-electron transfer (SPET) or conduction-proton-
electron transfer (CPET). In the case of solid metallic electro-
catalysts, the CPET pathway was usually considered; however,
SPET has been shown to be an important pathway in recent
years [41,42]. According to recent reports [42], the selectivity
between CPET and SPET is pH dependent; however, most
computational studies of NRR have not taken pH into account.
It is more likely that HER will occur in the CPET pathway due to
Hþand e- being transferred in a coupling similar to that of
hydrogen atoms. During the SPET pathway, Hþattacks the
activated nitrogen and adds an electron to the protonated
nitrogen intermediate, which is positively charged. According
to estimates, the SPET reaction has a lower activation barrier
than the CPET reaction for NRR; thus, the SPET pathway is
preferred [43]. However, more research must be conducted on
the competition or selection between SPET and CPET path-
ways in electro catalytic NRR mechanisms over heteroge-
neous catalysts.
The competition with HER
As we discussed above, the NRR contests with HER, which is
kinetically more preferred, and most of the experimental
work has noticed that there is a large overpotential and
significantly low activity towards NH
3
synthesis. Fig. 4 shows
the competing PCET steps in NRR and HER [44].
To underline the significance of selectivity, in contrast to
overpotential, some calculations were performed by Singh
et al. [45], showing that the main drawback is low selectivity,
even though catalysts with a less overpotential have a clear
benefit in terms of energy conversion efficiency. To under-
stand the cause of low selectivity in solid electrocatalysts, a
qualitative model was used to analyze the electrochemical
NRR, and it was observed that hydrogen evolution rate is first
order in the proton and electron concentrations, while its
zeroth order for NH
3
formation rate. This indicates that by
limiting the availability of either electron or proton at the
surface of the electrode, it is possible to improve NH
3
selec-
tivity. Accordingly, four strategies were provided to enhance
selectivity in electrochemical NH
3
synthesis. The first
approach was to limit the availability of protons at the elec-
trode surface by decreasing the proton donor's concentration
in the bulk solution (Fig. 5a). The second approach was to
protect the surface of the electrode with a hydrophobic,
aprotic layer that limits the rate of proton transfer rate while
allowing the N
2
molecules to pass (Fig. 5b). The third approach
makes use of the coulomb blocking effect, in which a thin
insulator was added at the interface of bulk solution and
metal electrode to limit the rate of electron transfer (Fig. 5c).
Briefly, a coulomb blockade (CB) is the decrease in electrical
conductance of a small electronic device that includes a low
capacitor tunnel junction at small bias voltages [46]. In a de-
vice with a CB, the conductance may not be constant at low
bias voltages, but disappears when the bias is below a certain
threshold, i.e. when current is not flowing. In the fourth
approach, the rate of electron transfer can be limited by
monitoring the flow of electrons photochemically (Fig. 5d).
However, limiting the rate of either electron or proton transfer
could most probably result in a reduced NH
3
formation rate
and thus the dilemma still exists [47].
BrønstedeEvansePolanyi relation, Sabatier principle, and
scaling relation for N
2
activation
The Brensted-Evans-Polanyi (BEP) relation states that the
activation energy and enthalpy change of the basic reaction
steps must be linearly related [48,49]. According to an exten-
sive set of density-functional theory (DFT) calculations [50], it
has been demonstrated that a number of diatomic molecules
(CO, N
2
,O
2
, and NO) have BEP-type relationships between the
activation energy for dissociation and the dissociative chem-
isorption energy. When these diatomic molecules are
employed as essential reactants in a series of simple catalytic
processes, the dissociative chemisorption energy is an
appropriate descriptor of catalytic activity, and the linear BEP
relation is combined with a microkinetic model to generate
volcano-shaped curves that show the catalyst activity as a
function of this descriptor [50,51].
Fig. 4 eThe competing PCET steps in NRR and HER. Reproduced from Ref. [44], with permission from the American Chemical
Society.
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The Sabatier principle is a qualitative idea that outlines the
qualities of an ideal catalyst for heterogeneous catalysis [52]. It
follows that the most effective catalysts should bind atoms
and molecules just rightdnot too tightly, not too loosely. A
strong binding affects catalyst activity, as it prevents desorp-
tion of the products, while a weak binding prevents surface
reactions. Plotting the reaction rate against a variable (e.g.,
bond strength) describes the chemical bond between the
catalyst surface and reactants, reaction intermediates, or
products producing a volcano-shaped plot [52,53].
An important characteristic for determining interaction
intensity between adsorbates and catalyst surfaces is the
adsorption energy of reaction intermediates. Scaling re-
lations describe linearly the adsorption energy of various
chemical intermediates on TM surfaces [54,55]. The adsorp-
tion energy of various chemical intermediates on TM sur-
faces is described linearly by scaling relations. It asserts that
the adsorption energies of H
2
-containing molecules AH
x
(DE
AHx
,A¼O, N, C, N, and S) are linearly associated with the
adsorption energy of the central atom A (DE
A
), as shown in
equations (15) and (16), on a variety of closely packed and
stepped TM surfaces [54].
DEAH ¼gðxÞDEAþε(15)
gðxÞ¼ðxmax xÞxmax 1 (16)
here,
x: a constant related to the surface geometry,
g(x): the slope of the scaling relations,
x: the actual number of H atoms that bond to A (x¼0, …,
x
max
), and
x
max
: is the maximum number of H atoms that can bond to
the central atom A (x
max
¼2 for O and S, 3 for N, and 4 for C).
Equation (16) signifies that for any central atom A, the slope
of the scaling relations only depends on the valency of the
adsorbate, i.e., on x
max
ex. In addition to estimating the re-
action energies of hydrogenation or dehydrogenation re-
actions for all TM based on the reaction energy for only one
metal, scaling relations allow us to estimate the adsorption
energies of various adsorbates on a TM surface. Furthermore,
scaling relations have also been applied to various other sur-
faces and adsorbates like TM compounds, graphitic materials,
and nanoparticles. Among other advanced surface science
techniques, the DFT method can also be applied to measure
adsorption energy with adequate accuracy. However, it is
difficult to deal in depth with all of the relevant adsorption
systems using theoretical or experimental methods. Scaling
relations are therefore frequently employed to improve
calculation efficiency, i.e., to lower calculation costs by
reducing the number of adsorption energy calculations
necessary to explain the energetic trends of a complicated
catalytic reaction [47].
Fig. 5 eApproaches to enhance NH
3
selectivity in electrochemical NH
3
synthesis. Reproduced from Ref. [47], with
permission from the American Chemical Society.
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Contamination sources in NH
3
synthesis: detection and
elimination
Sources
NH
3
formation and control (blank) tests have recently been
discussed among researchers in this field regarding the un-
derstanding of the N
2
source and improving the trust
worthiness of the process [56,57]. It is essential to identify and
exclude all possible contamination sources, as thoroughly as
possible, in order to confirm that the detected NH
3
originates
from dinitrogen and not from any other source. Recently,
several research teams have investigated various possible
contamination sources [57e59]. The contamination sources
can be categorized into two groups: out-system and intra-sys-
tem. Airborne NH
3
and NOx, as well as rubber gloves, are the
most common sources of out-system contamination. Due to
the requirement of conducting NRR tests in a closed system
[59,60], such out-system contamination is rationally excluded
with careful and rigorous precautions and it is anticipated
that they may not have a significant impact/interference on
the catalytic test. There is, however, an indeterminate level of
intra-system contamination, which is caused by nitrogen-
containing compounds in the feed gas, electrocatalysts, and
membranes, which cannot be detected independently; the
latter is expected to cause a significant impact on NH
3
measured yields in the lab scale and thus leading to unreliable
results. For example, the presence of NOx traces in the gas
supply especially in long-term experiments can lead to a
misleading observation of steady NH
3
production over
extended periods and give a falsified NRR stability of the
catalyst under investigation. The presence of NOx in labware
is also considered a common contamination. In spite of mul-
tiple rinsing steps, Ishibashiet al. [61], and Makelaet al. [62]
observed micromolar amounts of NO
3
and NO
2
contami-
nants on laboratory gloves and glassware. There is a related
problem when the electrode material or electrolyte contains
reducing agents, for example, N-species or nitrides. In the
absence of sufficient and rigorous control experiments, eval-
uating the NRR activity of electrocatalysts would be unreliable
and quite challenging.
Strategies to control
In order to remove background NH
3
or NH
3
from exogenous
sources (x-NH
3
), as well as NOx contaminants, gases used in
the experimental set up (e.g. Ar,
14
N
2
and
15
N
2
) must be
scrubbed, and the presence of such species in control samples
can be determined by ion chromatography, or equivalent.
Environmental pollutants like NO, NO
2
and N
2
O can be found
in both the air and water [61] and are readily reduced elec-
trochemically to NH
3
by forming anions. In terms of nitrate
and nitrite reduction, gold, silver, copper and platinum have
all been recognized as excellent electrocatalysts [63e66]. NOx
contamination in the gas supply can have a persistent effect
on the NH
3
yields; background subtraction cannot reverse the
effect. Therefore, its impact on reported NH
3
yields must be
quantified for relevant volumes of gas delivered. In the
context of NRR measurements, x-NH
3
concentrations in
electrolyte and trap solutions should be determined and
assessed for adequacy after passage of each gas used and for
relevant experiment periods. N
2
should produce x-NH
3
at or
below the level observed with argon. It is necessary to scrub
the gas further if it is not below this level. Moreover, it is
recommended that an argon control be conducted at every
potential measurement. In order to avoid NH
3
accumulation
on surfaces over time, these tests need to be performed
immediately before each NRR experiment for the same
amount of time. Electrochemical data plots should also show
these background currents, and the x-NH
3
amount measured
should be subtracted from the reported NRR NH
3
yields. For
long-running tests, this is particularly crucial, as the
measured yield may be affected to some extent by contami-
nation due to gas or other factors. When electrocatalysts
contain N
2
-containing active sites, or are prepared, at any
stage, using nitrogen-containing solvents or compounds, the
argon control may produce a variable amount of background
x-NH
3
. For further study of a material, it is recommended to
repeat argon control experiments over varying resting and
electrolysis periods.
To demonstrate that NH
3
was produced only from N
2
sources, results usually are quantitatively validated using
15
N
2
isotope reduction experiments. N
2
fixation communities have
developed a number of detection methods. A common tech-
nique is 1
H
NMR, which provides a characteristic triplet orig-
inating from
14
NH
4þ
that is clearly separated from the ones of
the
15
NH
4þ
species; the latter is detected as strong doublet
[67,68]. It is possible to perform quantitative measurements
either with
15
NH
4
Cl or with an internal standard. In order to
avoid background
14
N
2
and x-
14
NH
4þ
, either
15
NH
4þ
and
14
NH
4
need to be quantified, or
15
NH
4þ
needs to be demonstrated to
be within experimental error of total NH
4þ
determined for the
same
15
NRR experiment. For this experiment to serve as a
sufficient proof of NRR, the mol (
15
NH
4þ
) determined must be
in excess of mol (
14
NH
4þ
), and quantitatively consistent with
other measurements of NRR yield. Also, it is critical that the
electrocatalyst can produce NH
3
through NRR continuously
for over 5 h, with sustained activity. Experiments of this type
should be included in the mandatory toolbox, especially when
investigating nitrogen-containing materials such as metal
nitrides and N-doped carbons. In this regard, an integral
component of determining a catalyst's long-term perfor-
mance is demonstrating a turnover number of over one, with
respect to any species present that might potentially be
reacting. Furthermore, it is important that authors present
their data in such a way that this can be compared directly to
the results of other laboratories. The yield rate must always be
normalized to the geometric surface area in mol cm
2(geom)
s
1
. From fundamental point of view, normalizations based on
the catalyst mass or electrochemically active surface area are
of interest, mainly for mechanistic discussions. A hidden,
usually overlooked point here is the cost of a practical, large-
scale NRR cell which can be a dominant economic factor,
and thus calculations based on catalyst mass yield is
misleading.
To achieve accurate quantification of NH
3
produced, a
number of detection techniques have been adopted based on
the diverse range of experimental conditions used. Each
technique has its own limitations and conditions that must be
followed each time. In particular, Nessler or Berthelot re-
actions are the most commonly utilized techniques based on a
complexing agent which reacts with NH
3
produced [69e71].
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The appropriate controls must, however, be designed after
understanding the source and nature of potential in-
terferences. This method is interfered with metal ions, anions,
and organic solvents that interfere with chromophores for-
mation. Therefore, calibration data should be obtained using
the same solution components and concentrations as those
used for the NRR experiment. An effective way to ensure ac-
curacy is to spike (add a known amount of NH
3
to the solution
being analyzed). Because many solvents interfere with the
Berthelot reaction, it is crucial in non-aqueous NRR studies to
investigate and discuss the potential impact of the solvent on
the calibration data. Verifying headline results requires a
second validation method, regardless of the primary method
used. There are several alternative methods to NMR, including
ion chromatography with a routine limit of detection around
1mM[57].
The design of structure for catalysts
The major tools for effective N
2
reduction are defects engi-
neering, structure manipulation, design of single-atom cata-
lysts, interface tailoring, and facet engineering. In the below
sections we explain these individual tools in greater detail.
Defects engineering
A key strategy for enhancing the efficiency of NH
3
formation is
defect engineering (Fig. 6), which modifies the electronic
structures of catalysts to provide superior chemical and
physical behavior and overcomes the bottleneck of the NRR.
When combined with reaction intermediates, defect sites in
electrochemical catalysts typically have distinctive electronic
structures that increase the activity and selectivity of the
catalyst [72]. As a result, the defect engineering approach is
regarded as a novel and successful method of improving the
functionality of electrochemical catalysts, which may greatly
increase the electrocatalyst activity in NRR [73].
Liet al. [74] reported N
2
adsorption on TiO
2
(101) with an O
2
-
vacancy located between the five-coordinated (Ti
5c
,V
o2c
) and
four-coordinated (Ti
4c
,V
o2c
) Ti atoms near the vacancy site,
inclined towards the four-coordinated. Using the computed
electron density difference, it is also revealed that the four-
coordinated site is likely to be a primary component of the
subsequent NRR activity. In NRR, V
OS
lowers the activation
energy barrier by altering the electronic structure around
metal oxides and also functioning as reactive sites to adsorb
reactants [75]. Luo et al. found that Fe-doped SrMoO
4
produced
by a solvothermal technique with O
2
vacancies, reduced the
activation energy of N
2
reduction and promoted the adsorption
of N
2
molecules on the catalyst surface [76]. Schaub et al.
investigated the active sites for the dissociation of H
2
O mole-
cules on rutile TiO
2
(110). For each vacancy on the TiO
2
(110)
active surface, V
OS
dissociates water by transferring a proton to
adjacent O
2
atoms. Upon reaching the active site, the H
2
O
molecule begins to dissociate [77]. Furthermore, metal nitrides
with N
2
vacancies are also favorable for NRR, and N-doping
materials are widely documented. A study by Zhang et al. [78]
used N-doped V nanosheets arrays on Ti mesh to catalyze NRR,
resulting in an 8.40 10
11
mol s
1
cm
2
NH
3
yield rate. Nano-
array catalysts are also useful for exposing a variety of active
locations. For the HER [79e81], Sulfur-vacancies were pro-
duced on MoS
2
by electrochemical desulfurization. They sug-
gested that by controlling the vacancy concentration, S-
vacancies produced on the 2HeMoS
2
materials'basal planes as
active sites could elevate intrinsic activity. Uncoordinated Mo
atoms introduce interstitial states at sulfur-vacancy locations
that promote favorable H
2
binding for enhancing HER. Fei et al.
[82] reported a sulfur-vacancy-rich molybdenum disulfide
catalyst for N
2
reduction to NH
3
. Surface vacancy was moni-
tored by controlling the dopants (phosphorus) amount. High
NH
3
formation rate (60.27 mgh
1
mg
1cat
) of catalyst (P-M-1)
having large number of surface vacancy was observed along
with the enhanced FE (12.22%). Doping was beneficial in
creating surface vacancies as the active centers and for
modifying the electronic structure to improve the adsorption
and activation of N
2
molecules. Therefore, significantly
assisting the catalytic function. According to Matanovica et al.,
adding C vacancies can reduce the HER and the build up of H
atoms [83]. They eliminated half of the C atoms from molyb-
denum carbide (MoC) to produce a sub-stoichiometric MoC
0.5
composition in order to study the impact of the metal-to-C
ratio on the activity and selectivity of MoC. By adding C va-
cancies, the structure was transformed into a tetragonal
shape.
Structural manipulation
Considering structural manipulation, various promising cata-
lysts such as 1-D nanowire, 2-D nanosheets, and 3D porous/
hollow structure have been broadly used for NRR applications.
As compared to others, 3D hollow structures have some
unique benefits owing to the high electrical conductivity, large
surface to volume ratio, and good stability, thus increasing N
2
reduction and enhancing the conversion efficacy. Hollow
structures can also improve the reactant transfer and provide
Fig. 6 eAn overview of electrocatalytic NRR defect
engineering. Reproduced from Ref. [72], with permission
from the Elsevier.
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sufficient active sites on both surfaces. Specifically, cause of
the cage effect, the hollow cavity can assist in the entrapping of
the reaction intermediates. It helps to increase the content of
critical intermediate products used in rate-determining pro-
cesses for N
2
reduction [68]. In order to comprehend the driving
force behind lattice N diffusion and extraction, Shan et al. [84]
modified the geometric and electrical structures of well-
defined Mn
2
N and Mn
4
N lattices utilizing TM heteroatoms
(Fe, Cr, Co, Mo, Ni). It was noted that the early hydrogenation
product (NH) binding to lattice N follows a linear relationship
on a variety of close-packed surfaces. But the electrical and
geometrical structures have a greater impact on the binding of
NH
2
and NH
3
. Furthermore, the covalency obtained from the
Crystal Orbital Hamiltonian Population can be used to quan-
titatively quantify the chemical bonding. The total NH
3
for-
mation free energy (DG
NH3
) and lattice N diffusion barrier (E
a
)
are the corresponding thermodynamic and kinetic controlling
variables in the EleyRideal-Mars van Krevelan pathway. All
Mn
4
N modified with Fe, Co, Ni single-atom dopants showed
improved NH
3
production rate. Cao et al. [85] reported a top-
ochemical synthesis of a hexagonal nitride hydride, under
hydrogen at 673 K, by heating an orthorhombic nitride, along
with a rotational structural transformation. The H
2
intercala-
tion alters the CaeN rock-salt-like atomic packing in ortho-
rhombic nitride to a face-sharing octahedral chain in
hexagonal nitride hydride, mirroring a “hinged tessellation”
movement. Moreover, the hexagonal nitride hydride showed
stable NH
3
formation activity as a catalyst. Huang et al. [86]
used Ti
3þ
active sites to alter the amine-functionalized metal
organic frameworks (MOFs) and exhibited an NH
3
formation
rate of 12.3 mmol g
1
h
1
. Shang et al. [87] synthesized a
porphyrin-based MOFs with Fe as the active center and
observed a photocatalytic NH
3
formation rate of
7.5 mmol g
1
h
1
. The active sites in various crystal faces dis-
played the distinctive exposure degree due to the periodic
arrangement of metal clusters in MOFs. Additionally, certain
MOFs have metal clusters that are coordinatively unsaturated,
giving them well designability and modifiability in the control
of active sites [88].
Design of single atom catalysts (SACs)
A lot of effort has been dedicated in the field of heterogenous
catalysis by supported metal nanostructures, to enhance the
activity of supported metal catalyst through reducing the
metal particles size. In these catalysts, the metal particles are
usually homogenously distributed on a high surface area
support, in which only a small part of the metal components
are really involved in catalysis, with some of them serving as
the active centers in the catalytic process. Every metal particle
can have many active sites with various behaviors because
supported metal catalysts often contain a variety of metal
particles with varying sizes and asymmetrical morphologies.
This variability affects how well the metal's active sites are
utilized, which lowers the selectivity towards a certain prod-
uct. The size of the metal particles is thus a crucial factor in
influencing the specificity and reactivity. Other advantages of
metal size reduction include metal-support interactions, low-
coordination environments of metal centers, and quantum
size effects. Consequently, there is a distinct impact of size on
the reactivity of this nanoclusters of metals. Theoretical and
experimental investigation have showed that as compared to
nanometer sized metal, sub-nanometer sized metal clusters
can occasionally exhibit better catalytic activity or selectivity.
For instance, gold in the bulk form is chemically inert but when
they are reduced to nanometer or even sub nano meter size,
they show peculiar catalytic behavior. To enhance perfor-
mance and comprehend catalytic mechanisms, one must look
for catalysts with clearly defined single active centers [89].
Therefore, downsizing the metal nanostructures to well-
defined, atomically dispersed metal active centers, SACs, is
the most efficient technique to utilize every metal atom of
supporting metal catalysts. Based on the chemical interactions
among supports and mononuclear metal atom, there are
several types of SACs, comprising single metal atoms anchored
to metal surfaces, metal oxides, graphene, etc. Fig. 7aec illus-
trates these interactions. The bulk materials can be converted
into nanoparticles, subnanoclusters, and ultimately single
metal atoms, as shown in Fig. 7d. The metal species'unsatu-
rated coordination environment increases as a result of size
decrease. The size effects of metal nanocatalysts are explained
by an increase in the surface free energy of the metal compo-
nents and an increase in the activity of the metal sites for
chemical reactions with the support and adsorbates. The sur-
face free energy of metal species reaches a maximum in the
extreme case of SACs due to the extremely active valence
electrons, electron quantum confinement, and sparse quan-
tum level of metal atoms, which then promotes chemical in-
teractions with the support and distinctive chemical behaviors
of SACs. The position of metal single atoms and the attach-
ment locations on the support are interconnected. In case of
SACs, where metals are used as a support, the position of single
atoms are dependent on the chemical potential of the
component metals. For instance, gold (Au) has a higher
chemical potential compared to palladium (Pd) and thus Au
preferably stays on corner sites of Pd [90]. When we use
Fig. 7 eSchematic of various kinds of SACs: metal single
atoms attached to a) metal oxide, (b) metal surfaces, and c)
graphene, and d) Schematics illustrating the changes of
specific activity and surface free energy per metal atom
with metal particle size and the support effects on
stabilizing single atoms. Reproduced from Ref. [94], with
permission from the American Chemical Society.
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ditions, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2023.05.206
graphene as the support for SACs, the carbon vacancies aids as
the anchoring locations and the single atoms are placed on the
defects [91e93]. A direct geometric impact of these positions of
single atoms is that SACs have complete utilization of each
active metal atoms, which is the main reason to reduce the size
of metals to single atoms, particularly for noble metals.
The metal single atom also shows unique electronic prop-
erties as compared to the metal nano catalysts due to the
chemical interactions and special locations on support. Ac-
cording to their coordinated atmosphere, they have very vola-
tile electronic structures. The bonding between uncapped sites
on support and metal atoms causes transfer of charge among
support and metal atoms as there is difference in chemical
potentials. Consequently, the attached metal atoms generally
carry some charge. The presence of strong interaction among
support and metal atoms will have an unavoidable impact on
the support. In general, different kind of metal single atom has
different impact on the modification of support properties. For
instance, isolated La atoms on g-Al
2
O
3
surface can be used to
stabilize the support and inhibit catalyst degradation. This is
due to the mutual repulsions and strong binding of La atom,
which pins the surface and prevent sintering as well as sup-
ports transformation. In case of noble metals anchored on
reducible supports, such as Ir/CeO
2,
Pt/FeO
x
, etc., metal single
atoms aids in the reduction of supports. Ascribing to their su-
perior and unique properties, SACs provide a great potential as
a catalyst for various applications [47]. Zhen et al. [95] reported a
simple method to make pure siliceous zeolite-confined SAC(Ru
SAs/S-1) by monitoring the atmosphere to withdraw the amine
and template. As compared to traditional Ru based catalyst
(CseRu/MgO), Ru SAs/S-1 exhibited enhanced catalytic
behavior. Moreover, with the use of Ba as promoter, catalytic
activity was enhanced by 2 orders than for the catalyst with no
promoter. Further,it was noted that the N
2
triple bond breakage
in the Ru single-atom site follows after the reaction of adsorbed
H
2
with N
2
, resulting in a change of mechanism from the usual
Ru catalyst. Liu et al. [96] proposed single Ru-atom-embedded
boron monolayers SAC for NH
3
synthesis at mild conditions.
It was observed that the SAC showed exceptional NRR activity
following distal mechanism for the reaction pathway with a
DG
max
value of 0.42 eV, which is much lower than previously
reported for the flat Ru (0001) catalysts (1.08 eV). Thus, the
computational study indicated that an appropriate substrate
provides large space to optimize the TM for N
2
reduction at mild
conditions. Yu-Wu et al. [97] reported Fe SAC for active and
selective reduction of nitrate to NH
3
.NH
3
product selectivity
was improved as the SAC was able to inhibit the NeN coupling
step needed for N
2
owing to the absence of neighboring metal
sites. Geng et al. [98] fabricated Mo single atoms attached on
activated carbon for NRR. The as prepared catalyst had a high
NH
3
formation rate along with a FE of 57.54% with good dura-
bility and stability. The improved activity of the catalyst was
attributed to the formed MoeO
x
sites. The Mo precursor is
efficiently captured by the AC with surface-rich O
2
functional
groups, which also serve as coordination sites for Mo single
atom attachment by forming MoeO
x
bonds. Using a template-
free folic acid self-assembly method, Wang et al. [99] synthe-
sized SAC, which consists of Mn atomic sites on ultrathin car-
bon nanosheets. Due to the well-exposed atomic Mn active
sites connected to the 2D conductive carbon matrix, the
catalyst demonstrated remarkable NRR activity and selectivity.
Ruiyiet al. [100] synthesized RueHiseGQDeG electrocatalyst, a
hybrid of single atom Ruehistidine-functionalized graphene
quantum dot and graphene. The resultant catalyst had a well-
defined 3D structure and showed high catalytic behavior for
NRR. The theoretical and experimental studies revealed that
the Ru sites with HiseGQD are the main active centers that
permit N
2
adsorption, stabilization and destabilization of *NNH
and *H respectively. Peng et al. [101] prepared an electro-
catalyst, single-atomic Ru altered Mo
2
CT
X
MXene nanosheets
for NRR. It was noted that single atom Ru acts as a center for
electron back-donation in N
2
activation process, thus promot-
ing N
2
adsorption and catalyst activation behavior along with
lowering the thermodynamic energy barrier.
X-mediated strategy (X:Li, Ta)
In the early works of Fichtner et al., in 1931 [102] and Tsuneto's
group in 1993 and 1994 [103,104], lithium-mediated NRR was
envisioned starting with lithium electro-deposition. Dini-
trogen molecule is activated on lithium-containing deposits,
leading to NH
3
production [103,105]. Additionally, the use of
nonaqueous electrolytes restricts protons'access at the cata-
lytic active site, suppressing parasitic HER. As a result, active
lithium and an inert proton source can facilitate the activation
of N
2
and the suppression of HER, respectively. Lithium-
mediated NRR is a rapidly developing field [59,105e107] and
is becoming an important technological roadmap. While the
lithium-mediated NRR process has received widespread
attention by researchers [108], the mechanistic understanding
is still in its infancy, hampering performance improvement.
To improve understanding of lithium-mediated eNRR,
Suzanne et al. [106] developed a kinetic model. Based on the
results of the model, a voltage cycling procedure is then
developed that increases the energy and FE significantly.
Fig. 8 eA schematic of the electro-deposited Li surface
during nitridation at 220 C (up), Three-step Li-mediated
synthesis of NH
3
(down). Reproduced from Ref. [109], with
permission from the Wiley Online Library.
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Nikifaret et al. [105] developed a coupled kinetic-transport
model to optimize lithium-mediated NH
3
synthesis and
found that the highest FE was 18.5%, and the highest pro-
duction rate was 7.9 10
9
mol cm
2
s
1
. Kim et al. [109]
developed a three-stage NRR process using Li-ion conducting
glass ceramic materials, showing significantly improved FE.
Over time, both NH
3
and FE decreased attributing to the
decreased accessibility of N
2
molecules to the dwindling
amount of live Li, as shown in Fig. 8.
Using lithium-mediated cycling, McEnaney et al. [110]
developed a new electrochemical NH
3
synthesis approach for
producing NH
3
from N
2
and H
2
O. The electrochemical cycling
strategy is highly efficient and selective, allowing other metal
systems to improve upon the efficiency of NH
3
production,
opening up a new research field. Using N
2
and H
2
at ambient
temperature, Geng et al. [111] demonstrated that bare Ta
2þ
in
a fully catalytic cycle can mediate the NH
3
formation. A planar
Ta
2
N
2þ
intermediate, consisting of alternating Ta and N
atoms, was found to be a key intermediate. It is formed in an
exothermic reaction involving Ta
2þ
and N
2
(educt) or two
molecules of NH
3
(product). When Ta
2þ
reacts with N
2
, the
triple bond of dinitrogen is completely broken. It was found
that this unexpected reaction can be attributed to the inter-
play between vacant and doubly occupied d-orbitals involved
in the rate-determining step. A ditantalum center can accept
electrons as well as donate electrons during the cleavage of
triple bonds. In addition, Ta
2þ
is shown to be a multipurpose
tool using a less exothermic reaction to split the single bond of
H
2
.
Interface tailoring
Along the lines of surface engineering a facile approach is
proposed by Ren et al. [112] who fabricated Cu nanowires with
concave-convex surfaces (Cu@Cu
2þ1
O NWs) to be used for
nitrate electroreduction to NH
3
. Metal Cu components within
the nanowire structure allow efficient electronic transmission
capacity, while exterior concave-convex Cu
2þ1
O layers
contain abundant catalytically active sites. Further, Cu/
Cu
2þ1
O interactions and interface effects enable tuning of the
Cu d-band center and modulating intermediate's adsorption
energies. Chen et al. [113] used a flow electrochemical cell
operating in the gas phase to electro-catalyze iron supported
on carbon nanotubes (CNTs) and observed that the active sites
for NH
3
electrocatalysts synthesis may lie at the interface
between iron particles and CNT, where they are capable of
activating N
2
, which makes it more reactive towards hydro-
genation. Wang et al. [114] reported a lithium-iron ternary
hydride species at the surface/interface of the catalyst. As a
result of a redox reaction between dinitrogen and lithium
hydride (LiH) mediated by iron, the ternary hydride species
may act as centers for activating and hydrogenating dini-
trogen, resulting in Fee(NH
2
)eLi and LiNH
2
moieties distinct-
ing it from NH
3
formation mediated by conventional iron or Ru
catalysts. In Fig. 9,NH
3
synthesis activity increases mono-
tonically with a decrease in Fe content, with Fee693LiH having
an activity of approximately 7.4
˟
10
4
mmol
NH3 gcat-1h-1
. An in-
crease in activity with increasing Fe amount in Fe-xLiH may
reflect the important role of interfaces between LiH and Fe
particles in catalyzing NH
3
formation. In other words, if the
iron loading were lower, then Fe particles would be smaller,
and they would have a larger surface area for contacting
neighboring LiH particles.
Shen et al. [115] reported the development of an N
2
-
microextractor (NME), which can extract N
2
from water, and
feed it to catalysts on electrodes, in order to reduce the
amount of N
2
dissolved in aqueous electrolytes. In NME, the
polymer framework and extractant possess high solubility for
N
2
, forming ultra-thin interfacial systems around electrodes.
Aqueous NRR is enhanced by N
2
-rich layers formed on cata-
lyst surfaces due to the solubility gradient between water and
tetrahydrofuran. N
2
molecules are extracted from water by
the NME, and the accumulating N
2
molecules bind with the
catalyst surface. Furthermore, the polymer framework regu-
lates and controls water transfer during NRR and stabilizes the
interface between the solution and NME. Hence, as a result of
N
2
confinement effects, NRR activity and selectivity are
significantly enhanced. In an electrocatalytic NRR system, Tao
et al. [116] investigated the coupled reactions and transport
processes and observed that in addition to reaction-induced
N
2
concentration gradients, potential- and concentration-
dependent electric double layers (EDLs) also play an active
role in NRR selectivity and activity. The aggregated cations
near the cathode surface of the EDL inhibited N
2
diffusion,
thus reducing the rate of adsorption on active sites, limiting
the rate of NRR at deeply negative electrode potentials. Also,
the NRR performance is influenced by pore size if the NRR rate
is high, which results in N
2
depletion in the smallest internal
nanochannels.
Facet engineering
In recent years, it has been widely reported that crystal facet
engineering can enhance electrocatalytic performance and
material properties. In a study by Lim et al. [117], Pd nano-
particles with both (100) and (111) crystal facets showed more
NH
3
selectivity than catalysts with just (100) or (111), perhaps
because the edge sites were more likely to adsorb reaction
intermediates. Using Cu
2
O samples synthesized with multiple
crystal facets, Zhong et al. [118] studied the correlation be-
tween material properties and NO
3
RR performance. Oxygen
Fig. 9 eNH
3
synthesis performances of Fe-xLiH (x ¼1e693)
catalysts at 300C. Reproduced from Ref. [114], with
permission from the Wiley Online Library.
international journal of hydrogen energy xxx (xxxx) xxx12
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ditions, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2023.05.206
vacancies and hydroxyl groups were found to be more
concentrated in the (111) facet than in the (100). Cu
2
O (111) had
the highest NO
3
RR activity with significant quantities of O
2
vacancies and hydroxyl groups. Adsorption of reactants such
as *NO
3
,*NO
2
, and *NO
3
is enhanced by oxygen vacancies. On
the other hand, surface hydroxyl groups assist *H formation
and inhibit further HeH coupling. Consequently, the hydro-
genation reaction in NO
3
RR was promoted, and the HER side
reaction was suppressed. The Cu
2
O(111) surface achieves the
highest nitrate conversion, NH
3
selectivity, FE, and NH
3
for-
mation rate due to these two factors. Another attempt of facet
engineering is presented in which BiVO
4
was synthesized with
different ratios of facets (110) and (040). NH
3
generation ac-
tivity was linearly related to facets ratio, suggesting that the
ratio of exposed crystal faces greatly influences the activity
[119]. Facet engineering in the case of a noble metal is pre-
sented by Han et al. [120], who reported that a Pd nano-
crystalline with a well-desired facet was demonstrated to be
an effective NO
3
RR electrocatalyst for ambient NH
3
synthesis,
owing to its optimized NO
3
adsorption activity, poorer HER
activity, and smaller free energy change of the rate-limiting
step (*NH
3
to NH
3
). A detailed electrocatalytic study on Pt
3
Fe
nanocrystals with configurable morphologies having different
exposed facets revealed shape-dependent NRR electro-
catalysis [121]. As a result of a high index of facets on the
nanowires, the optimized Pt
3
Fe nanowires exhibits high ac-
tivity, excellent selectivity, and high NH
3
yield. Also, the Fe-3d
band acts as an efficient d-d coupling correlation center,
enhancing Pt-5d-electronic transfer activities toward the NRR.
Based on the results of Li et al. [122], oxygen vacancies (OVs) of
BiOCl can be used as catalysts to overcome the kinetic inertia
of N
2
to create molecular steps with lower energy, allowing
solar light to drive the cleavage of the NeN triple bond through
proton-assisted electron transfer. Further, OV structures on
different BiOCl facets influence both adsorption structure and
N
2
activation level, strongly determining N
2
fixation path-
ways. Using cyclodextrin, Wang et al. [123] synthesized CuNi
bimetal nanocrystals with well-defined facets. By providing a
special reaction environment, cyclodextrin was able to control
the shape of nanomaterials and disperse them. Exposed facets
and optimized d-band centers enabled the nitrate ion to
strongly adsorb and facilitate the electrochemical process of
forming NH
3
from nitrate. The findings of all these studies
confirm the possibility of exploring the effects of catalyst
properties on surface reaction kinetics using catalysts with
controllable crystal facets.
Materials used for N
2
activation and conversion
Ammonia synthesis is a crucial process in the field of chemical
engineering, and extensive research has been conducted to
explore different catalysts for this purpose. In this review, we
present an overview of key experimental studies on NH
3-
synthesis, focusing on the use of various catalysts. Specifically,
we discuss catalysts based on ruthenium (Ru), metal oxides,
metal nitrides, and other catalysts. The findings from these
studies are summarized in Table 1, providing valuable insights
into the performance and characteristics of these catalysts.
Ru-based catalysts
Ru-based catalysts are a potential candidate for the
HaberBosch process. DFT studies have shown that Ru is a
great NRR catalyst, owing to the adequate N
2
adsorption en-
ergy and an overpotential in dissociative and associative
Fig. 10 eVarious approaches for rational design of Ru-based catalysts. Reproduced from Ref. [173], with permission from
American Chemical Society.
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ditions, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2023.05.206
mechanism which is comparably lower than other noble
metals, such as Pd and Pt [170e172]. To achieve highly effi-
cient NH
3
synthesis and decomposition, Ru-based catalysts
must be constructed at the nanoscale in a controlled manner,
including particle size and structure control, support effects,
promoters, bi/multimetallic strategies and so forth (Fig. 10).
Size effect case study. Using the Ru single atom catalyst
(SAC) dispersed on N-doped carbon, an NH
3
formation rate of
120.9 mg
NH3
mg
1cat
h
1
and a FE of 29.6% was attained [124].
Zhou et al. [125] studied the effect of the size of Ru as a catalyst
for the synthesis of NH
3
(Fig. 11c). An array of Ru
x
/BaCeO
3
was
prepared having different sizes of Ru (x ¼1.1e3.0 nm) using
size-monitored Ru colloid. It was noticed that by decreasing
the size of Ru to 1.1 nm, the rate of NH
3
synthesis was
enhanced 5.7 times more than the one with Ru size of 3.0 nm;
this was associated with the generation of oxygen vacancies
and Ce
3þ
species in BaCeO
3
, which stimulate the N
2
dissoci-
ation by donating e
to Ru centers, while improves the H
2
spillover from Ru to BaCeO
3
to lessen H
2
poisoning. The effect
of various Ru size (1.4 ~ 5.0 nm) on NH
3
formation by colloidal
deposition of Ru has also been reported [141]. By reducing the
size of Ru from 5 to 1.4 nm, the NH
3
formation rate was
increased highlighting the dependence of Ru based catalyst on
the size. As compared to the catalyst having 5.8 nm Ru size,
the catalyst with 1.4 nm Ru size had 5.8 times more NH
3
for-
mation rate. It was observed that reduction in the Ru size
might reduce the amount of step and terrace site and escalate
the proportion of corner sites. Additionally, the catalyst with
lower Ru size having enough corner sites was beneficial for
reducing the work function and promoting the activation of
N
2
. Li et al. [174] have reported multiscale Ru catalysts for NH
3
synthesis ranging in size from single atoms to atomic clusters
to sub-nanometric clusters to nanoparticles (Fig. 11a). Ac-
cording to the studies, when Ru size decreases to sub-
nanometric levels, B5 and/or terrace sites decrease, which
leads to alterations in N
2
activation routes. Since Ru atomic
clusters exhibit strong intra-cluster interactions, N
2
molecules
can more easily activate over Ru atomic clusters than over Ru
nanoparticles due to strong interaction between Ru d-orbitals
and pand sorbitals of N
2
.
Support effect case study
Li et al. [129] prepared ceria supported Ru catalyst, in which
ceria was incorporated with titanium in various concentra-
tion. The addition of titanium in an optimum amount result in
the formation of CeTiO
2-x
, providing a better platform for the
homogenous dispersal of Ru species. The introduction of ti-
tanium reduces the amount of active oxygen species and
creates the lewis acidic sites affecting the electronic state of
Ce species. Ru catalyst based on various lanthanide oxides as
the support have been reported and compared with the
traditional Ru/MgO with respect to the NH
3
synthesis rate
[130]. As compared to Ru/MgO, Ru/lanthanide oxide catalysts
had lower H/Ru ratio and specific surface area. Yet, the NH
3
yield of all the considered lanthanide oxide-based Ru catalyst
were much higher than that of Ru/MgO at 400 C. Also, a
positive linear relation between turnover frequency and ba-
sicity of the catalyst was observed. Kinetic analysis disclosed
that the H
2
reaction order for the Ru/lanthanoid oxide cata-
lysts was close to zero, implying that H
2
poisoning was well
retarded in these catalysts. It was observed that rate of NH
3
synthesis was higher for the catalyst having light lanthanide
element (e.g., CeO
2
,La
2
O
3
, and Pr
2
O
3
) as compared to the one
having heavy lanthanide elements (e.g., Ho
2
O
3
,Yb
2
O
3
, and
Er
2
O
3
). Furthermore, carbon-based materials were used as
efficient Ru supports. In particular, Ma et al. [131] used carbon
nanotubes with surface defects as a support for the BaeRu/
CNTs-D catalyst; the latter catalyst had 5 times higher NH
3
synthesis rate compared to that of BaeRu/CNTs catalyst due
to the strong interaction among the surface defect of the CNTs
with Ru nanoparticles. Han et al. [132] used Ru/La
2
Ce
2
O
7
which showed higher rate of NH
3
synthesis than Ru/La
2
O
3
and
Ru/CeO
2
, along with improved stability during over-heating
test. The N
2
dissociation step in NH
3
synthesis is consider-
ably assisted by the strong metalesupport interaction
(Fig. 11b) among La
2
Ce
2
O
7
and Ru where charge transfer from
La
2
Ce
2
O
7
to Ru takes place favoring the N
2
dissociation step.
Another work described synthesis of NH
3
using electro-
chemical method in which nickel was used as anode and
Rueplatinum alloy catalyst was used as cathode at ambient
temperature and pressure. The high NH
3
generation rate was
Fig. 11 ea) Illustration of the formation of Ru SAC, Ru ACCs, Ru SNCs, and Ru NPs [174], b) the strong metalesupport
interaction (SMSI) between Ru and La
2
Ce
2
O
7
[132], and c) Effect of Ru size on NH
3
synthesis over BaCeO
3
eRu catalysts [125].
Reproduced from Refs. [125,132,174], with permission from the Elsevier.
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ditions, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2023.05.206
obtained owing to the synergistic effect of the RuPt alloy [134].
Borisov et al. [135] reported a Ru based catalyst (RueCs(Ba)/
Sibunit) with Cs and Ba as the promoter and carbon composite
Sibunit calcined at various temperatures were as the support
in which the molar ratio of Cs (Ba):Ru was 2.5. With an in-
crease in the calcination temperature, it was observed that the
carbon structure was organized with a significant decrease in
specific surface and increased specific activity. Zhang et al.
[143] prepared ZrO
2
having monoclinic phase and carbon
species from ZrCl
4
ensuring the preparation method of UiO-66
to develop an effective ZrO
2
-supported Ru catalyst. The elec-
tronic metal support interaction between ZrO
2
and Ru species
was lowered due to the presence of carbon species. Besides,
the introduction of carbon increases the amount of H
2
adsorbed and also assists their desorption, thus alleviating the
side effect of H
2
species on the N
2
adsorption-desorption and
NH
3
synthesis. Consequently, the Ru/ZrO
2
catalyst showed
four times higher NH
3
formation rate as compared to the
catalyst prepared using zirconium nitrate. For the production
of NH
3
, Karakaya et al. [136] formed a catalyst with nano-
phase Ru distributed on a (BaO)
2
(CaO) (Al
2
O
3
) (B2CA) support.
The stoichiometry of the catalyst support was strongly
correlated with measured results in the ((BaO)
x
(CaO)
y
(Al
2
O
3
)
z
)
ternary phase diagram space. To comprehend the perfor-
mance under a variety of operating scenaria, quantitative
microkinetic models were applied. The strong impact of sur-
face covering, which result in practically significant catalyst
poisoning behaviors, were demonstrated by the microkinetic
models. The optimal H
2
/N
2
feed mixture can deviate signifi-
cantly from the stoichiometric ratio (H
2
/N
2
¼3) due to
poisoning.
In another work, Cs promoted Ru/CeO
2
catalyst was used
for microwave assisted NH
3
synthesis [137]. A strong inter-
action between CeO
2
and Ru was observed, resulting in a well
distributed Ru particles and thus supporting the NH
3
synthe-
sis. The lower electronegativity of Cs promoter and higher
electron donating ability of CeO
2
resulted in higher electron
density on Ru decreasing the N^N dissociation barrier. Thus,
at a concentration of 2 wt% of Cs and 4 wt% of Ru, an optimum
performance was observed with a NH
3
generation rate of
1.18 mmol/h. g
cat
. Zhang et al. [145] reported carbonized Ru-
dispersed ZIF-8 electrocatalyst with a FE of 14.23% and a NH
3
yield rate of 16.68 mg
NH3
h
1
mg
cat.
1
. Yu et al. [144] reported Ru
single atoms spread on a graphitic carbon nitride matrix and
showed enhanced catalytic behavior and selectivity with
respect to the bulk Ru. Also, Ru SAs/g-C
3
N
4
had simpler re-
action thermodynamics as compared to bulk Ru. The
improved NRR behavior was initiated by the tuning of d-
electron energies, resulting in a shift of the d-band to Fermi
level. Ahmed et al. [138] showed surface alteration approach
to modify the electronic structure and increase the N
2
acces-
sibility on the surface of a catalyst, through metal-sulfur
bonds in the nitrogenase enzyme. To acquire the metal-
sulfur bonds, Ru was attached on graphene oxide and was
modified using various aliphatic thiols. The resultant catalyst
(Ru/rGO) had a high FE of 11% and enhanced NH
3
generation
rate of 50 mgh
1
mg
1
. It was also observed that the electronic
structure and surface alteration aids in suppressing the HER
along with achieving the intermediate adsorption and
desorption of N
2
. The catalyst also displayed excellent recy-
clability and stability for NH
3
production. Wei et al. [139] re-
ported a micro tubular Ru-CNT gas diffusion electrodes (GDEs)
and applied it for electrochemical synthesis of NH
3
in a H-type
cell under ambient conditions, providing a novel method to
fabricate catalyst-loaded, self-standing GDEs for NRR. Ni et al.
[140] showed that by using HNO
3
gas-phase oxidation treat-
ment, oxygen functional groups can be added into C-sup-
ported Ru catalyst. The results showed that the presence of
oxygen functional groups enables the H
2
desorption. More-
over, methane (CH
4
) resistance is improved along with an
alleviation in H
2
poisoning and acceleration of barium nitrate
decomposition.
In case of the Ru catalyst having less O
2
functional groups
(Fig. 12a), CH
4
is formed by direct reaction of adsorbed H
2
on
Ru particles with the C support. However, in case of the
catalyst prepared by using HNO
3
gas-phase oxidation treat-
ment, oxygen functional groups are mostly in the vicinity of
Ru particles (Fig. 12b). Thus, reducing the direct contact be-
tween C support and adsorbed H
2
on Ru particles by surface O-
groups. Consequently, adsorbed H
2
on Ru particles might
desorb as a H
2
molecule instead of forming CH
4
by reacting
with C. In brief, appropriate H
2
desorption would reduce CH
4
Fig. 12 eGraphic illustration of the impact of O
2
functional groups on C methanation over C-supported Ru catalyst.
Reproduced from Ref. [140], with permission from the Elsevier.
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ditions, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2023.05.206
formation, which is crucial for the stability of the catalyst.
Therefore, the improvement in NH
3
synthesis rate of the
catalyst prepared by HNO
3
gas-phase treatment was owed to
the amalgamation of the alteration in the existing form of Ba
promoter, exchange of H
2
species, the acceleration in the
desorption. According to Li et al. [175], modifying the cerium
nitrate solvents modifies the rate at which H
2
migrated from
Ru/CeO
2
. There are more different O
2
species, more Ce
3þ
concentration, more RueOeCe sites, and defects in Ru cata-
lysts based on ceria attained by ethanol-precipitated synthe-
sis, which results in a decrease in the quantity of exposed Ru
species and percentage of Ru metal. As a result, N
2
activation
as well as the migration and desorption of H
2
species are
blocked. The mobility and desorption of H
2
species as well as
N
2
activation are facilitated by a Ru catalyst supported on ceria
produced via water-precipitated synthesis, which also has a
greater ratio of Ru metal and a lot more Ru exposure, resulting
in higher catalytic activity of the Ru catalyst supported on the
water-precipitated synthesized ceria.
Promoter effect case study
Effect of using promoter on the NH
3
synthesis rate was re-
ported, where a Ru-based catalyst (Ru/Cs
þ
/CeO
2
) was prepared
using CeO
2
as the support incorporated with a large amount of
Cs
þ
as the promoter [126]. The catalyst containing 2 wt% of Ru
and Cs/Ce molar of 0.35 showed the optimum performance
with an NH
3
formation rate being almost 10 times more than
that of the catalyst without promoter (Ru (2 wt%)/CeO
2
)at
350 C and 0.1 MPa. FTIR studiues using CO adsorption [176],
showed that the exceptional performance was due to the
electron donating effect of Cs. Moreover, the order of adding
Cs
þ
and Ru was found to impact the catalytic behavior; the
enhancement was observed to be more when Cs
þ
was added
before loading of the Ru catalyst. Chen et al. [127] used stron-
tium niobates (Sr/Nb ¼2.0) as the support for Ru catalyst
having Cs and Ba as promoters in NH
3
synthesis process.
Incorporation of Cs or Ba as a promoter to the catalyst exhibi-
ted enhanced catalytic activity. The optimum molar ratio of
Ba/Ru and Cs/Ru was obtained at 4 and 8 respectively. The
exposed (002) and (151) and (002) facets of Sr
2
Nb
2
O
7
support
stimulated the epitaxial growth of Ru nano crystallites in
shortened pyramid shape with ample steps and B
5
sites; the
latter sites are very critical for NH
3
catalytic synthesis. In
another work, Chen et al. [128] prepared a Cs-promoted Ru
catalysts, supported on g-Al
2
O
3
with different molar ratio of Cs
and Ru. From all the catalysts, 1.5CseRu/g-Al
2
O
3
, with the Ru
size of approx. 2 nm and a molar ratio of 1.5 showed the highest
NH
3
formation rate. It was noted that by tuning the molar ratio
of Cs and Ru, the surface basicity/acidity can be altered and
new active centers can be created at Ru particles/CsOHeCs
0
Fig. 13 e(a) NH
3
synthesis rate and (b) TOF
surRu
under 1 MPa at different temperatures; (c) NH
3
synthesis rate with respect to
pressure at 350 C, and (d) thermal stability of Ru/xLaN/ZrH
2
catalysts. Reproduced from Ref. [177], with permission from the
Elsevier.
international journal of hydrogen energy xxx (xxxx) xxx16
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ditions, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2023.05.206
species interface. A super-growth carbon nanotube-
supported, Cs-promoted Ru, (CseRu/SGCNT) catalyst for NH
3
synthesis has been observed to have a NH
3
synthesis rate of
35 mmol g
1
h
1
at 5MPaG and 400 C[142]. The CseRu particles
restrained in the SGCNTs nanospace assists the H
2
and N
2
adsorption during the NH
3
synthesis, thus showing great
resistance to H
2
inhibition in H
2
-rich NH
3
formation condi-
tions, mainly under pressurization, and aided N
2
dissociation.
The study by Li et al. [177] investigated the NH
3
synthesis
process over a LaN-promoted Ru/ZrH
2
catalyst, using operando
spectroscopic and isotopic-label-directed observation.
Fig. 13aed shows the comparsion of the NH
3
synthesis activity
of the LaN-promoted Ru/ZrH
2
with respect to the Ru/ZrH
2
. The
study found that the reaction proceeded via an associative and
chemical looping pathway. Kinetic studies showed that
dissociative adsorption of N
2
was not the rate-determining
step, and isotopic exchange experiments confirmed that spe-
cies could undergo hydrogenation or nitridation to produce
NH
3
. Spectroscopic analysis demonstrated the presence of
HeN]NeH species, indicating an associative pathway rather
than a dissociative pathway. These findings were further
supported by in situ DRIFTS measurements.
Bimetallic effect case study
As of today, bimetallic catalysts integrated with Ru-based
catalysts are proving effective in reducing Ru usage and
enhancing catalytic performance. For the preparation of
bimetallic Ru-based catalysts, wet-chemistry methods like
polyol reduction and impregnation are commonly used. A
systematic study of bimetallic NH
3
synthesis was conducted
by Jacobsen et al. [178] and they found that an ideal strategy
for designing NH
3
synthesis catalysts is to combine metals
with high and low nitrogen interaction energies to produce
intermediate interaction strengths. A bimetallic active center
combining Ru with M (M ¼Ni, Co, Mo, and Fe) was investigated
by Yang et al. [179] for NH
3
synthesis. It was found that Ru-M
catalyst activities decreased in the order of
RueCo >RueNi >RueFe >RueMo. In a similar study,
Lucentini et al. [180] synthesized NieRu NPs anchored on CeO
2
by coimpregnation, and found that the close contact among
Ru and Ni and the strong metal-support interactions are
favorable for enhanced catalytic activity. A ZIF-8-assisted
strategy (Fig. 14a) was developed by Yang et al. [181] to pro-
duce RueCo clusters by encapsulating the precursor within
ultrafine pores to stabilize the alloy. As a result, the RueCo
Fig. 14 eScheme of the a) preparation process of the Ru clusters@NeC and Ru NPs@NeC, and b) integration of RueCo
nanoparticles in the pores of N-doped carbon hollow spheres. Reproduced from Ref. [181], with permission from the ACS
and from Ref. [182], with permission from Elsevier.
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ditions, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2023.05.206
clusters@NeC shows superior activity and stability in NH
3
synthesis. Using N-doped carbon support, Ni et al. [182]
confine RuCo nanoparticles in hollow pores (Fig. 14b). It is
demonstrated that the RuCo alloy NPs have high resistance to
particle sintering, excellent NH
3
synthesis activity, and long-
term stability. Incorporating Co lowers the particle size of
NPs and increases the electron density of Ru species. Ac-
cording to Chen et al. [183], RueFe/CNT catalysts can be pro-
duced through an integrated process that involves
impregnating Ru precursors onto CNTs followed by a liquid-
phase reduction process. By forming RueFe interfaces and
FeNx, surface nitrogen atoms are efficiently desorbed as the
most abundant reaction intermediate. As a result, bimetallic
Ru-based catalysts with rational composition and interface
control show high potential for catalyzing NH
3
synthesis.
Ru is situated at the top of the N
2
reduction volcano dia-
gram, which is beneficial for the activation of N
2
triple bond.
Nevertheless, the catalyst based on Ru tend to be covered by H
2
species to form H
2
gas which results in the reduction of FE for
NH
3
formation. To enhance the NRR activity and inhibit HER, a
good approach would be to introduce elements with weak H
2
adsorption to reduce H
2
binding over Ru. A potential material to
alloy with Ru was found to be Au which shows highly selective
hydrogenation of N
2
on Au and thus improve the catalytic
behavior. Moreover, the active sites of the catalyst can be
increased by designing porous structure to improve the cata-
lytic behavior. Generally, Ru is recognized as the most active
single-metal catalyst for NH
3
synthesis in which the catalyst
support has a crucial role. Various kinds of supports such as
hydrides, oxides, amides, and carbon has been found effective.
It has been observed from different reported studies that a
catalyst support can improve the catalytic behavior through
electronic or structural promotion of the active metal. How-
ever, since we still do not have better understanding of the
basic mechanism, it is difficult to explore or design the support/
catalyst interaction. Ru/Carbon catalysts shows a major limi-
tation of methanation where at increased temperature and in
H
2
-containing medium, carbon from the support goes through
methanation owing to the interaction with activated H
2
on Ru.
The methanation causes sintering of Ru particles, pore struc-
ture degradation of the support, and catalyst deactivation. The
possible mechanism of methanation at 400 and 900 Cwas
proposed in a reported study, where they also show ways to
suppress methanation by introduction of promoters and
graphitization of the support. The amount of edge carbon
atoms that interact with H
2
first is dramatically reduced as a
result of graphitization, which results in the ordering of
structure and an increase in carbon crystallite size. Promoter,
on the other hand, aids in blocking the surface areas at Ru
particle borders and stops H
2
from leaking onto the carbon
support. Sibunit, a carbon compound with strong mechanical
and chemical inertness, is a viable catalytic support for low-
temperature NH
3
production. Previously it has been observed
that resistance to methanation can be enhanced by the
graphitization of Sibunit. Moreover, graphitization aids an
effective charge redistribution between catalyst components
and improves catalytic activity. Moreover, through a study
[184], it has also been observed that Ru promoters like Barium
reduces the Ru activity for methanation while potassium en-
courages the methanation of carbon supports. Forni et al. [185]
employed gas chromatography to regulate CH
4
generation
during the production of NH
3
and observed that in addition to
the C treatment conditions, the characteristics and concen-
tration of promoters also had an impact on the methanation
onset temperature. The production of very stable catalysts was
made possible by the combination of a high C pretreatment
temperature and the existence of Cs, K, and Ba, which
considerably enhanced the initial temperature of CH
4
synthe-
sis. Over the past years, substantial investigations have
concentrated on the exploration of Ru catalysts for NH
3
gen-
erationin mild environment, and C-supported Ru catalysts
have been effectively used conventionally at pressure as low as
100 bar [186]. Although carbon methanation, which was pre-
viously believed to be the main reason for the deactivation of
the Ru/C catalyst, would be catalyzed by the presence of Ru
species [187,188]. According to Iost et al. [189], O
2
-containing
carbon species are significantly easier to gasify into CH
4
than
other C species. According to Lin et al. [190] during the heat
treatment of C-supported Ru catalysts, C oxidation to generate
CO was also seen in addition to C methanation, which had an
impact on the stability and activity of Ru/C catalysts. Ba is less
catalytically active than alkali compounds for the gasification
of carbon (CH
4
and CO) [191,192], therefore it can be used as a
promoter to create a Ru/C catalyst having good stability.
Though, the presence of O
2
surface groups not only made it
easier for Ru species to disperse [189], but it also altered the
characteristics of H
2
species that were adsorbed [193]. The su-
perior performance of C-supported Ru catalysts for NH
3
syn-
thesis may not be completely realized in this scenario because
the removal of O
2
surface groups may result in a conflict be-
tween catalytic stability and activity.
The crucial factors that affects the catalystic behavior of Ru
based catalyst.
1. The geometrical structure known as the B5-type sites is
one of the key factors on Ru that can influence NH
3
for-
mation [194]. On the surfaces of Ru, B5-type sites may exist,
and they are in charge of the robust physical adsorption of
N
2
[195].
2. The size and morphology of the Ru crystals have a major
influence on the fraction of B5-type sites, which can rise
with smaller particle sizes [196]. The support and promoter
have the ability to change the shape and size of Ru, and
these actions are regarded as structural modifications.
3. Ru's ability to synthesize NH
3
may also be significantly
increased by altering the electrical structure using basic
supports and alkali metal promoters [197]. The electron
donation density into the p*orbitals of N
2
and the N^N
cleavage are both aided by the transport of electrons from
promoters and supports to the Ru surface. According to
Ref. [198], these effects are considered electronic modifi-
cations. The geometrical and electrical properties of Ru are
impacted by the support properties. While the size and
dispersion of Ru particles are influenced by their in-
teractions with the support, the capacity to donate elec-
trons is essential for the development of extremely active,
electron-rich Ru [199].
Table 2 provides a summary of the catalytic performance of
NH
3
synthesis using typical Ru- and Fe-based catalysts. NH
3
international journal of hydrogen energy xxx (xxxx) xxx18
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ditions, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2023.05.206
production rates are generally higher at high temperatures
and pressures, but different catalysts might have different
optimal activity under different conditions. The performance
of Ru-based catalysts at low temperatures and pressures was
generally better than that of Fe catalysts. Some Ru-based
catalysts, however, show poor NH
3
production rates because
of their low Ru loading. Additionally, as can be seen from the
above graph, Ru-based catalysts need further improvement
compared to the KBR Advanced Ammonia Process (KAAP) Ru
catalyst, which is composed of alkaline earth metals and al-
kali metals, showing the importance of precise Ru-based
catalyst construction for efficient NH
3
production. In addi-
tion, to achieve a fundamental understanding of NH
3
syn-
thesis catalytic chemistry, advanced experimental
techniques, and theoretic calculations are required to identify
which pathway occurs in a specific catalyst.
Table 1 eAn overview of key experimental research on the production of NH
3
using different catalysts.
Catalyst Conditions NH
3
formation rate FE (%) Catalytic stability References
Ru SAs/NeCe120.9 mg
NH3
mg
1cat
h
1
29.6 12 h Geng et al. [124]
Ru
1.1
/BaCeO
3
1 MPa, 400 C 19.4 mmol g
1cat
h
1
e100 h Zhou et al. [125]
Ru (2 wt%)/Cs
þ
(0.35)/CeO
2
350 C 4.62 mmol h
1
g.cat
1
e100 h Osozawa et al. [126]
8Cs-2 wt%Ru/Sr
2
Nb
2
O
7
0.1 MPa, 673 K 4986 mmol g
1cat
h
1
e72 h Chen et al. [127]
4Cs-2 wt%Ru/Sr
2
Nb
2
O
7
0.1 MPa, 673 K 2317 mmol g
1cat
h
1
e72 h Chen et al. [127]
1.5CseRu/g-Al
2
O
3
1 MPa, 410 C 17 mmol NH
3
g
1
h
1
e50 h Chen et al. [128]
Ru/Ti
0.18
-Ce e18.9 mmol g
cat1
h
1
eeLi et al. [129]
Ru/Pr
2
O
3
1 MPa, 400 C 49 mmol g
1
h
1
eeMiyahara et al. [130]
BaeRu/CNTs-D 5 MPa, 450 C 110 mmol g
1
h
1
e14 h Ma et al. [131]
Ru/La
2
Ce
2
O
7
10 MPa, 450 C 10.34 vol% e12 h Han et al. [132]
PdRu NS-NF 25 C 20.46 mg h
1
cm
2
2.1 30 h Li et al.
PdRu TPs e37.23 mgh
1
mg
1cat
1.85 eWang et al. [133]
RuPt/C 50 C 1.08 10
8
g
NH3
s
1
cm
2
4 45 h Manjunatha et al. [134]
CseRu/Sib2200 6 atm, 400 C 0.591 mol NH
3
$g
Ru1
h
1
eeBorisov et al. [135]
Ru/B2CA 10 bar, 490 C 189 mmol g
1cat
h
1
eeKarakaya et al. [136]
CsRu/CeO
2
(2e4%) 0.1 MPa, 533 K 1.18 mmol/h.g
cat
eeWang et al. [137]
P-M-1 RT 60.27 mgh
1
mg
1cat
12.22 23 h Fei et al. [82]
Ru/rGO-C12 e50 mgh
1
mg
1
11 12 h Ahmed et al. [138]
RuCNT GDE ambient condition 2.1 10
9
mol/cm
2
s 13.5 48 h Wei et al. [139]
Ba-(Ru/AC)
O
1.0 MPa, 400 C 38.0 mmol g
cat1
h
1
eeNi et al. [140]
1.4 nm Ru NCs 1.0 MPa, 400 C 17.13 mmol
NH3
g
cat1
h
1
e100 h Peng et al. [141]
CseRu/SGCNT 5MPaG, 400 C 35 mmol g
1
h
1
e140 h Nishi et al. [142]
Ru/ZrO
2
1.0 MPa, 400 C 2.0 mmol g
cat1
h
1
e120 h Zhang et al. [143]
Ru SAs/g-C
3
N
4
e23.0 mgmg
cat1
h
1
8.3 12 h Yu et al. [144]
NC@Ru ambient condition 16.68 mg
NH3
h
1
mg
cat.1
14.23 2 h Zhang et al. [145]
MHCMs ambient condition 25.3 mgh
1
mgcat
1
6.78 24 h Zhang et al. [146]
3Cs-2wt%Ru/Ba
5
Ta
4
O
15
0.1 MPa, 400 C 263.7 mmol g
1Ru
h
1
e72 h Huang et al. [147]
BaeRu/gCeAl
2
O
3
1.0 MPa, 400 C5611mmol g
1
h
1
eeLin et al. [148]
Co/C12A7:e
e
0.1 MPa, 340 C912mmol g
1
h
1
e60 h Inoue et al. [149]
Ru/C12A7:e
e
0.1 MPa, 633 K 49 kJ mol
1
e6 h Hara et al. [150]
Ni/Al
2
O
3
1 atm, 35 C471mmol g
1
h
1
e6 h Wang et al. [151]
Ru/CeO
2
-r 1.0 MPa, 400 C 18 mmol g
1
h
1
e64 h Lin et al. [152]
SC-Ru/CeO
2
1.0 MPa, 400 C 6.9 mmolg
cat1
h
1
e100 h Fang et al. [153]
Ru/CeO
2
eC500H 1.0 MPa, 400 C 0.93 molgRu
1
h
1
e95 h Lin et al. [154]
Ru/CeO
2
eN
2
H
4
1.0 MPa, 400 C 5.52 mmolg
cat1
h
1
eeLi et al. [155]
RueBa/Al
2
O
3
-980 1.0 MPa, 400 C7217mmol g
1
h
1
e200 h Lin et al. [25]
t-Cr
2
O
3
(T) e2.72 mg. h
1
.cm
2
5.31 24 h Shi et al. [156]
Co@BaO/MgO-700red 1.0 MPa, 350 C 24.6 mmolg
cat1
h
1
e200 h Sato et al. [157]
Co@BaO/MgO-700red 3.0 MPa, 350 C 48.4 mmolg
cat1
h
1
e200 h Sato et al. [157]
Ru/La
0.5
Pr
0.5
O
1.75
_650red 1.0 MPa, 400 C 60.2 mmol g
1
h
1
50 h Ogura et al. [158]
Ru/MgOeEr
2
O
3
1 atm, 450 C 1325 ppm eeJavaidet al. [159]
Ru/MgOeCeO
2
2.5 MPa, 375 C 3000 ppm eeJavaidet al. [160]
Co/MgOeNd
2
O
3
6.3 MPa, 470 C 48 mmol
NH3
g
cat1
h
1
e72 h Rondudaet al. [161]
Ru/CeO
2
eBH 1.0 MPa, 400 C 5.45 mmol g
cat1
h
1
eeLi et al. [162]
0.31Ru/CeO
2
1.0 MPa, 400 C 0.78 molNH
3
gRu
1
h
1
eeLin et al. [163]
Ru/BaeCa(NH
2
)
2
0.1 MPa, 340 C 12.4 mmol g
1
h
1
e180 h Hattori et al. [164]
Activated Ru/Sm
2
O
3
1.0 MPa, 400 C 32 214 mmolg
cat1
h
1
e350 h Zhang et al. [143]
CoO QD/RGO ambient condition 21.5 mgh
1
mg
1
8.3 18 h Chu et al. [165]
Ru/LaCeeC 1.0 MPa, 400 C 20.3 mmol g
1
h
1
e120 h Li et al. [166]
Ba þCo/MgeLa 9.0 MPa, 400 C 0.63 g
NH3
g
cat.
1
h
1
e72 h Rondudaet al. [167]
OV-Ti
2
O
3
ambient condition 37.24 mgh
1
mg
cat.
1
19.29 eChen et al. [168]
TiO
2
/JE-CMTs ambient condition 20.03 mgh
1
mgcat.
1
10.76 36 h Chen et al. [169]
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ditions, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2023.05.206
Metal oxides based catalysts
Zhang et al. [146] prepared a selective, efficient, and stable
non-noble metal catalyst using multishelled hollow Cr
2
O
3
microspheres (MHCMs) for NRR. In 0.1 M Na
2
SO
4
solution and
at 0.9 V vs RHE, a high FE of 6.78% along with large NH
3
production rate of 25.3 mgh
1
mgcat
1
was observed. It was
noted that the multishelled hollow Cr
2
O
3
microspheres
follow both partially associative and distal associative path
forNRR.Inanotherwork[156], hollow Cr
2
O
3
nanocatalyst
was prepared using template-assisted method. The poly-
styrene template was used to control the thickness and
dimension of precursor and to feasibly remove the poly-
styrene template during calcination. The resultant hollow
Cr
2
O
3
nanocatalyst has a controllable cavity, small size and
very thin shell which helps in the N
2
reduction process. The
factors affecting the enhancement in the catalyst perfor-
mance of template assisted-Cr
2
O
3
were as follow. a) the
nanostructures with a smaller size distribution are antici-
pated to provide adequate active sites on both the outer and
inner surface, and accelerate the three-phase interactions
among the electrolyte, N
2
,andthetemplateassisted-Cr
2
O
3
catalyst, b) The hollow structure is advantageous to trap N
2
into the cavity and encourage the N
2
reduction enduring the
high-frequency collisions, and c) the particularly-thin shell
of t-Cr
2
O
3
catalyst can assist the reactant transfer across the
shell, and concurrently decrease the diffusion resistance on
the interface. Huang et al. [147] also prepared Cs-promoted
catalysts, in which Ba
5
Ta
4
O
15
nanosheets was prepared to
be used as the support. At 0.1 MPa and 400 C, the catalyst
3CseRu/Ba
5
Ta
4
O
15
with Cs/Ru molar ratio of 3 had shown
improved catalytic activity (263.7 mmol g
1Ru
h
1
)andsta-
bility (72 h) for NH
3
synthesis reaction. It was revealed that
the support could be reduced to form oxygen vacancies
which would improve the NH
3
synthesis rate of Ru based
catalyst. In addition, hydrogen poisoning was also inhibited
by increased hydrogen spillover from Ru to support of
Ba
5
Ta
4
O
15
.Linetal.[148] prepared barium-promoted Ru/
Al
2
O
3
catalyst, where the alumina was coated with carbon in
two different ways. First support (sCeAl
2
O
3
)waspreparedby
the sucrose pyrolysis and the second support (gCeAl
2
O
3)
was
attained by acetylene decomposition. Ba-promoted Ru/gC-
Al
2
O
3
catalyst showed improved catalytic behavior and sta-
bility for NH
3
synthesis. This was credited to the reduction in
Table 2 eComparison of catalytic activity over selected Ru-based catalysts for NH
3
synthesis.
Catalysts Ru (wt %) T (C) P (Mpa) NH
3
yield (mmol g
cat1
h
1
) References
Ru/C
12
A
7
:e
e
1.2 400 0.1 2757 [200]
Ru/BaO/CaH
2
10.0 340 0.1 10 500 [164]
Ru/BaCeO
3ex
N
y
H
z
4.5 400 0.9 30 000 [201]
LaNeRu/ZrH
2
2.0 350 1 12 800 [177]
Ru/BaTiO
2.5
H
0.5
0.9 400 5 28 200 [202]
CseRu/MgO 2.0 300 0.1 697 [203]
Ru/CaH
2
2.0 300 0.1 2549 [203]
Ru/Y
5
Si
3
7.8 400 0.1 1900 [204]
Ru/LaScSi 1.8 400 0.1 2800 [205]
Ru/La
0.5
Ce
0.5
O
1.75
5.0 350 1 31 300 [206]
Ru/BaCeO
3
1.25 400 0.1 24 000 [207]
Ru/GdHO 0.8 400 5 168 000 [208]
Ru/CaCN
2
5.0 300 1 3785 [209]
Ru/Sm
2
O
3
5.0 400 1 64 852 [210]
Ru/BaeCa(NH
2
)
2
10.0 300 0.9 23 300 [211]
Ru/TieCeeS 3.0 400 1 14 580 [212]
Ba/Ce/Ru ACCs 2.0 400 1 56 160 [174]
Ru/Ba/LaCeOx 5.0 350 1 52 300 [213]
Ru/3TiCN/ZrH 1.8 400 1 25 600 [214]
BaeRu/BN 4.5 400 10 186 600 [215]
Co1Ru SAA 1.05 100 1 4444 [216]
RuLa/HZ 2.53 400 1 14 730 [217]
Ru3Fe/CNTs w 1.67 400 1 6030 [183]
RuCo DSAC 1.34 400 1 0.016 [218]
RuCo/MgO 2 450 0.2 759 [179]
KeRueCo@NeC 1.23 400 1 494 [181]
CseRu/G1900/OR 17.6 430 10 ca. 245 535 [185]
KeRu/G1900/OR 17.6 430 10 ca. 103 571 [185]
BaeRu/G1900/OR 17.2 430 10 ca. 238 314 [185]
KeBaeCseRu/G1900 11.8 430 10 ca. 230 911 [185]
BaeCseRu/Cm 9.1 400 9 68 500 [219]
BaeRueK/AC 4.0 350 10 70 800 [220]
Mittasch's Fen 40.5 460 15 95 600 [194]
Mittasch's Fe 40.5 460 1 3600 [194]
Fe-cat. (KM1)i e400 10 37 800 [221]
m:similar compositions with KAAP catalysts.
n:commercial Fe catalyst having similar compositions as Mittasch's catalyst.
international journal of hydrogen energy xxx (xxxx) xxx20
Please cite this article as: Singh S et al., Hypes and hopes on the materials development strategies to produce ammonia at mild con-
ditions, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2023.05.206
N
2
desorption temperature, surface H
2
enrichment, and H
2
desorption via a favorable H
2
-formation path.
Sato et al. [157] discovered that by encasing cobalt (Co)
nanoparticles in barium oxide (BaO) during reduction of MgO
at 700 C, the rate of NH
3
production can be increased. The
highest NH
3
synthesis rate of 24.6 mmol h
1
g
cat
1
at 350 and
1.0 MPa was observed for Ba-doped Co/MgO catalyst. With
increase in pressure (3 MPa), NH
3
formation rate was
enhanced to 48.4 mmolg
cat
1
h
1
. SEM and EDX spectroscopy
showed that Co nanoparticles was encapsulated by BaO. The
mechanism explaining the structure formation was the
decomposition of BaCO
3
to BaO, reduction of oxidic to metallic
Co, and relocation of Co nanoparticles and BaO. Due to N
2
adsorption on the Co atoms at the catalyst surface and the
donation of an electron from Ba
2þ
of BaO via nearby Co atoms,
it was revealed that the N
2
triple bond was reduced to the
strength of a double bond. This weakening of N
2
triple bond
accelerated the cleavage of bond, the rate-determining step
for NH
3
generation. Inoue et al. [149] reported Co based cata-
lyst with enhanced NH
3
synthesis activity by using
12CaO$7Al
2
O
3
as the electride. The reaction for NH
3
formation
was initialized at 200 C and has a very low activation energy
of 50 kJ mol
1
. The activity of Co catalyst was directly
enhanced by the electride for N
2
dissociation reaction where
electron is injected from 12CaO$7Al
2
O
3
to the Co nano-
particles. This study shows that by donating electron from
electride with lower work function (2.4 eV) can enhance the
catalytic behavior of Co catalyst. Another electro-catalytic
material based on Co
3
O
4
and CNT had been reported to
generate NH
3
using an electro-catalytic NRR process [222].
Using CNT as a substrate for growing Co
3
O
4
can enhance
electronic conduction properties and improve N
2
adsorption.
Under ambient conditions, this catalyst produces a high NH
3
yield of 27 mgh
1
mg
cat.
1
along with excellent selectivity and
strong electrochemical durability. Hara et al. [150] prepared
catalyst based on Ru-loaded C12A7:e
e
electride derived from
12CaO$7Al
2
O
3
for NH
3
synthesis and observed that the
mechanism and properties of this catalyst is different from
other traditional catalysts (Fig. 13). The presence of Ru on the
electride avoids the inhibition of NH
3
synthesis by H
2
poisoning resulting in a very efficient catalyst. There isa direct
relation between the density of N and H adatom on the surface
of Ru, thus the amount of N adatoms increases immediately
with reduction in H adatoms. The reaction order for N
2
was
almost 0.5, representing highly effective N
2
cleavage, owing to
the N adatoms that populates the surface of Ru/C12A7:e
e
more densely in comparison to other catalysts surfaces. As a
result, it became apparent that the improvement in catalytic
behavior was caused by high electron mobility, which
improved N adatoms on the Ru-surface at the metaleinsulator
transition (MIT) point and caused the Fermi level to upshift,
which increased the power of electron donation. The sug-
gested mechanism of the reaction is demonstrated in Fig. 15.
Ronduda et al. [161] prepared cobalt-based catalyst where
mixed oxide system of MgO and Ln
2
O
3
(Ln ¼La, Nd, Eu) were
used as the support. The modification of the support config-
uration by choosing lanthanide elements having different
electronegativity modifies the physicochemical characteris-
tics of the catalyst like surface density of basic sites, accessi-
bility of active sites for H
2
, and active phase dispersion.
Among the three different lanthanide elements used, Nd
based catalyst (Co/MgeNd) showed the superior NH
3
synthe-
sis activity at 6.3 MPa and 470 C(Fig. 16), owing to the incre-
ment in the basicity of the support. The improved electron-
donating capacity of the cobalt surface resulted from the
increased surface basicity, which sped up the adsorption of N
2
and subsequent breaking of the N triple bond, which is the
rate-determining step in the synthesis of NH
3
.
In another work, Ronduda et al. [167] performed the kinetic
studies of NH
3
synthesis over a Ba-promoted Co catalyst
sustained on MgeLa mixed oxide and compared it with com-
mercial Fe catalyst. It was observed that the Co based catalyst
had better catalytic behavior as compared to the Fe catalyst.
Besides, the reaction order value for both catalysts with
respect to H
2
was similar, showing that H
2
does not inhibits
the NH
3
synthesis reaction. However, with respect to NH
3
, the
reaction order of the two catalysts were different. As
compared to the Fe catalyst, with respect to NH
3
, Co-based
catalyst had lesser negative value of the reaction order,
revealing a lower inhibition by NH
3
.
Fig. 15 eSuggested reaction mechanism of NH
3
formation over Ru/C12A7:e
e
. Reproduced from Ref. [150], with permission
from the American Chemical Society.
international journal of hydrogen energy xxx (xxxx) xxx 21
Please cite this article as: Singh S et al., Hypes and hopes on the materials development strategies to produce ammonia at mild con-
ditions, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2023.05.206
Wang et al. [151] used Fe, Ni, and Cu as the TMs supported
on Al
2
O
3
to form catalyst for plasma enhanced NH
3
synthesis
at ambient conditions with a water-electrode dielectric barrier
discharge (DBD) plasma reactor. The rate of NH
3
synthesis and
energy efficiency was significantly enhanced by using plasma
catalysis as compared to the plasma synthesis without cata-
lyst. The order of increment (Ni/Al
2
O
3
>Cu/Al
2
O
3
>Fe/Al
2-
O
3
>Al
2
O
3
) varied with different TMs where Ni/Al
2
O
3
exhibited
the highest performance with a NH
3
synthesis rate of
471 mmol g
1
h
1
. The synergistic influence of plasma-catalytic
synthesis of NH
3
was investigated using various character-
ization methods and was noted that the weak acidic sites of the
support can impact the behavior of M/Al
2
O
3
catalysts. As
compared to plasma only or Al
2
O
3
, Ni/Al
2
O
3
had a constant
plasma discharge which enhanced the reaction on the catalyst
surface and gas-phase radical reactions of NH, H and N in the
plasma. Moreover, the number of strong and medium acid
sites were altered during the reaction by altering the surface
acidity of the catalyst leading to improve the NH
3
synthesis
rate. Lin et al. [152] prepared Ru/CeO
2
catalysts using CeO
2
rods
(CeO
2
-r) and CeO
2
cubes (CeO
2
-c) as the support with a loading
of 10 wt% of Ru for NH
3
synthesis. The amount of oxygen va-
cancies was enhanced by converting Ce
4þ
to Ce
3þ
and forma-
tion of RueOeCe bonds. In case of CeO
2
nanorods, less
crystalline Ru species enriched with Ru
4þ
ions were present on
the surface while metallic Ru particles were present in case of
CeO
2
cubes. Ru/CeO
2
nanorods showed higher NH
3
synthesis
rate owing to the high amount of oxygen vacancies and less
crystalline Ru species which increases the adsorption of H
2
and
N
2
and leads to desorption of H
2
present on the surface in the
form of H
2
. However, for Ru/CeO
2
cubes the catalytic activity
was lower due to the large size of Ru species and less amount of
O
2
vacancies causing less favorable N
2
and H
2
adsorption. Fang
et al. [153] synthesized CeO
2
-supported Ru catalyst using a
sacrificial sucrose strategy without intreating Ru encapsula-
tion or strong metal support interaction (SMSI). The sucrose
layer inhibited the reversible O
2
spillover during the reduction
of H
2
leading to the enhanced metallic Ru proportion, exposed
Ru species, RueOeCe bonds, and Ce
3þ
concentration. As
compared to the catalyst without sucrose, this catalyst can
have a higher amount of H
2
desorbing by H
2
formation,
resulting in higher NH
3
synthesis rate. Therefore, sacrificial
sucrose strategy is an efficient way to prepare effective catalyst
for NH
3
synthesis. Lin et al. [154] reported Ru/CeO
2
catalyst and
observed that by CO activation we can increase the reduction
degree and exposure of Ru species along with oxygen vacancy,
active oxygen, and Ce
3þ
concentration. This leads to the
growth of electron-enriched Ru
d
species and Ru
d
eO
V
eCe
3þ
sites, resulting in increased NH
3
synthesis rate and reduced H
2
poisoning. Li et al. [162] used NaBH
4
treatment to induce
interaction between ceria and Ru for Ru/CeO
2
catalyst. Amount
of Ce
3þ
, proportion of metallic Ru, concentration of surface O
2
species, and amount of exposed Ru were enhanced by NaBH
4
treatment. Moreover, the N
2
dissociation is aided by the strong
metal-support interaction. As a result, as compared to the
catalyst with only H
2
reduction, Ru/CeO
2
catalyst with NaBH
4
treatment exhibited higher NH
3
synthesis rate. Lin et al. [163]
studied the effect of the surface density of Ru on the catalytic
behavior for a series of Ru/CeO
2
catalysts for NH
3
synthesis. For
the catalyst with Ru of less surface density less than (<0.68 Ru
nm
2
), it was observed that the Ru layers were closely linked
with CeO
2,
transferring electrons directly from the defect sites
of CeO
2
to Ru. Thus, the adsorption of H
2
on Ru sites was high
resulting in strong H
2
poisoning and enhanced NH
3
synthesis
rate. However, in case of Ru with higher surface density (>1.4
Ru nm
2
), aggregation of Ru species was observed resulting in
weak RueCeO
2
interactions and alleviated H
2
poisoning. In
another work based on Ru/CeO
2
by Li et al. [155], the catalyst
was obtained by N
2
H
4
treatment in normal conditions to
stimulate an interaction between ceria and Ru. It was observed
that the reduction of N
2
H
4
enhanced the interaction between
ceria and Ru, increased the amount of metallic Ru and fraction
of exposed Ru species resulting in improved NH
3
synthesis
activity. This strategy provides a new path to design catalyst by
altering the metal-support interaction. Lin et al. [25] described
that by calcining alumina at higher temperature, the phase
change occurs from g-to a-Al
2
O
3
(Fig. 17a) which results in
reduced surface are and fraction of tetrahedral Al
3þ
sites. As a
result, for RueBa/alumina catalysts, by raising the calcination
temperature, Ru's particle size distribution was widened,
metal to oxide ratio increased, surface hydroxyl groups were
reduced, and the temperature at which N
2
desorption occurs
Fig. 16 e(a) Temperature dependency of the Co/Mg-Ln catalysts'rates of NH
3
production. (b) The rates at which different
catalysts produce NH
3
. Reproduced from Ref. [161], with permission from the Elsevier.
international journal of hydrogen energy xxx (xxxx) xxx22
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ditions, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2023.05.206
was lowered. In addition, elevated calcination temperature of
alumina reduces the effect of H
2
poisoning and lowers the
activation energy required for NH
3
synthesis. The NH
3
syn-
thesis rate was three times more for the RueBa/Al
2
O
3
catalyst
with alumina calcined at 980 C than that where alumina was
calcined at 800 C(Fig. 17a).
Ogura et al. [158] prepared Ru/La
0.5
Pr
0.5
O
1.75
catalyst via
prereducing it at an elevated temperature. The NH
3
synthesis
rate of the catalyst was comparable to the many oxides sup-
ported Ru catalysts. From the kinetic analysis, it was revealed
that the H
2
poisoning was suppressed by the Ru/La
0.5
Pr
0.5
O
1.75
catalysts. The enhanced catalytic activity was due to the phase
change of the support (La
0.5
Pr
0.5
O
1.75
) during prereduction at
high temperature and high thermal stability of the support. By
increasing the electron donation from the support to the an-
tibonding porbital of the N^N bond of N
2
through Ru atoms
(Fig. 17b), the rate-determining step for the NH
3
production
was sped up. Javaid et al. [159] used mixed oxide of MgO and
lanthanide as the support to form an efficient Ru based cata-
lyst. The reported catalyst showed better catalytic perfor-
mance than other lanthanides for NH
3
synthesis. The mixed
metal oxides were prepared in different molar ratio using co-
precipitation technique and 1 wt% of Ru was deposited on
the support using impregnation method. As compared to the
catalyst where Ru was deposited on pure Er
2
O
3
support, Ru/
MgOeEr
2
O
3
catalyst with Mg/Er ratio of 25/1 had shown higher
catalytic activity with similar reaction conditions. It was due to
the similarity in the textural characteristics of the mixed metal
oxide supported Ru catalyst up to a Mg/Er ratio of 25/1. In
another work by Javaid et al. [223], Ru catalysts supported by
mixed oxide were studied through using MgO and CeO
2
. It was
noted that the morphological and textural properties of the
catalyst had a significant effect on the NH
3
synthesis activity.
By changing the molar ratio of Mg/Ce to 10/1, the catalyst
showed almost similar NH
3
synthesis, as compared to pure
CeO
2
supported catalyst. The possible reason for this
consistency in NH
3
synthesis is due to the nearly similar
texture of mixed metal oxide supported Ru catalysts and same
Ru dispersal tendency with favored accumulation on CeO
2
.
This method of employing mixed metal oxides would help in
reducing the cost of the catalyst as the expensive CeO
2
can be
diluted with inexpensive MgO. Hattori et al. [164] reported a
heterogenous catalyst made with a mixture of BaO and CaH
2
powder with Ru nanoparticles for NH
3
synthesis at low tem-
perature and with less activation energy. The enhanced cata-
lytic behavior was observed due to the formation of BaH
2
,a
stable and strong electron-donating material that facilitated
the reversible H
2
storage reaction and increased the electron
donation to Ru. Zhang et al. [143] reported Ru/Sm
2
O
3
catalyst
and showed that the NH
3
formation rate can be enhanced by
the activation of Ru through the formation of surface hydride.
The synergistic effect of Ru clusters and surface hydride spe-
cies enhances Ru clusters activity and increases NH
3
yield to
almost 90.1e100% of the thermodynamic equilibrium value.
Theoretical calculation and mechanistic studies showed that
surface hydride species also participates in NH
3
synthesis on
the Ru clusters. Chu et al. [165] reported CoO quantum dot
supported on reduced GO for NRR catalysts. It was observed
that the catalyst had favorable NRR activity with poor HER
activity. NH
3
formation rate of the catalyst was superior to
most reported NRR catalyst with a FE of 8.3% under mild con-
ditions. Moreover, the CoO QD/RGO displayed excellent sta-
bility and selectivity, indicating the possibility of Co-based
catalysts for NH
3
formation. Li et al. [166] used hydrothermal
and coprecipitation technique to introduce Ce inside La
2
O
3
and
observed that the reduction of Ru was enhanced along with
increased Ru
0
proportion and the amount of Ru exposure.
Furthermore, the integration of Ce species through coprecipi-
tation technique abates the water vapor adsorption, thus
preferentially desorbing the adsorption of H
2
species in H
2
pathway. Accordingly, the as prepared catalyst synthesized
through coprecipitation technique attains higher reactivity.
Fig. 17 ea) Schematic showing the phase transformation of Al
2
O
3
(left) and NH
3
synthesis rate of catalysts calcined at
different temperature [25], and b) Mechanism of N
2
activation over Ru/La
0.5
Pr
0.5
O
1.75
_650red [158]. Reproduced from Refs.
[25,158], with permission from the American Chemical Society.
international journal of hydrogen energy xxx (xxxx) xxx 23
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ditions, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2023.05.206
Chen et al. [168] reported an electrocatalyst for NRR containing
oxygen vacancies was developed from plasma-etched Ti
2
O
3
that exhibited excellent N
2
fixation performance with
maximum NH
3
yield and FE of up to 37.24 mgh
1
mg
cat
.1
and
19.29%, respectively. An electrocatalyst with a 3D cross-linked
hollow tubular structure derived from Juncus effusus and TiO
2
was proposed to develop NH
3
at ambient conditions [169]. It
exhibits superior electrochemical and structural stability in
addition to providing a large NH
3
yield of 20.03 mgh
1
mgcat.
1
and a high FE of 10.76%. Using ZnCo
2
O
4
nanosheet arrays as a
catalyst, ambient electrohydrogenation of nitrate to NH
3
has
been proven to be efficient, stable, and environmentally benign
[224]. In alkaline media, the catalyst had a FE of 98.33% and a
maximum NH
3
yield of 634.74 mmol h
1
cm
2
, which is higher
than that of its Co
3
O
4
counterpart. Furthermore, its stability for
cycling tests and electrolysis over a long period of time is
excellent. In another study by Liu et al. [225] showed that
NiCo
2
O
4
nanoarray catalyzes ambient NH
3
synthesis effi-
ciently, with a high FE of 99.0% and a large NH
3
yield of
973.2 mmol h
1
cm
2
. As revealed from DFT calculations,
NiCo
2
O
4
(311) had a half-metal feature, unlike Co
3
O
4
(311)
which had a semiconductor feature, which contributed to its
superior activity and promotes a thermodynamically feasible
nitrate-NH
3
conversion with a low energy barrier of 0.2 eV.
Metal nitrides
Owing to the advantage of using the Mars-van Krevelen
mechanism to produce NH
3
electrochemically under ambient
settings, TM nitride catalysts have been investigated. This
process states that a surface N atom is reduced to NH
3
in the
first step rather than adsorbing N
2
to the catalyst surface, and
the catalyst is then regenerated with gaseous N
2
. The most
promising catalysts among the several TM nitrides are found
to be VN, ZrN, NbN, and CrN utilizing theoretical studies [226].
Niobium oxynitrides are considered to be a good catalyst for
NRR as they are stable towards poisoning of surface vacancies,
catalytically active towards NH
3
formation, and compara-
tively stable towards decomposition upon polarization.
Hanifpour et al. [227] studied the catalytic behavior of niobium
oxynitrides (NbO
x
N
y
) for NRR at room temperature conditions
in an aqueous electrolyte. For that, NbO
x
N
y
thin films were
prepared having different stoichiometries of xþyto study the
impact of stoichiometric ratios on NRR behavior. As the stoi-
chiometric ratios increases from 1.22 to 1.52, substantial dif-
ferences and signs of catalytic NH
3
formation with the
instigation of an N
2
reliant charge transfer was observed.
Moreover, NbO
1.28
N
0.24
reveals high current efficiency and NH
3
formation rate in N
2
compared toAr. Humphreys et al. [228]
prepared samarium doped cerium oxynitrides (Ce
1-z
Sm
z
O
2-
x
N
y
)to enhance the amount of anion vacancies. The NH
3
synthesis of the cost effective Fe catalyst was promoted by
using these oxynitrides, showing increased activity compared
to previous documented non-Ru catalysts. It was concluded
that N
2
vacancy incorporation by doping facilitates the
mobility of N
2
vacancy. Thus, doped oxynitrides having high
amount of anion vacancies, mainly N
2
vacancies are excep-
tional co-catalysts/promoters for NH
3
formation. Bismuth is
another promising electrocatalyst for activating N
2
reduction
to NH
3
owing to the easy dissociation of nitrogen caused by
the strong interaction among N 2p orbitals and Bi 6p band.
Nevertheless, the electron transfer for N
2
reduction is limited
by the poor conductivity of bismuth. Also, the slow dissocia-
tion of H
2
O on the surface of bismuth causes inadequate
proton supply for the *N
2
protonation step, resulting in low
performance for NH
3
production. Recently, Li et al. [229] used
facile displacement reaction to prepare binder free and inte-
grated bismuth nanoparticles@nickel foam (BiNPs@NF) elec-
trode for ambient NH
3
synthesis. Owing to the high
conductivity of nickel, the poor conductivity of bismuth gets
improved. Additionally, because bismuth has a higher elec-
tronegativity than nickel (2.02 vs. 1.91), which causes elec-
trons to accumulate on its side, bismuth's intrinsic activity is
enhanced. Additionally, at high frequencies, severe water
dissociation on nickel surfaces will result in a large number of
protons, which will diffuse to a neighboring bismuth surface
and cause a spillover effect. A FE of 6.3% and a high NH
3
production rate of 9.3 10
11
mol s
1
cm
2
were observed
using an electrochemical H-type cell.
Other catalysts
Recently, metal-free catalyst was prepared for NRR and an
NH
3
yield of 72.3 mgh
1
mg
1
(0.3 V) was attained with a FE of
19.5% (0.2 V) [230]. This value was comparable to most of the
metal-based catalyst for NRR and nearly above all the reported
works based on metal-free catalyst. Graphitic carbon nitride
and boron nitride quantum dots was used to form a metal-free
heterostructure (BNQDs/C
3
N
4
). The drastic enhancement in
the NRR activity was owing to the stimulated N
2
adsorption
and activation at the BNQDs-C
3
N
4
interface, accelerating the
NRR procedure with a very low overpotential of 0.23 V. Wang
et al. [133] prepared PdRu tripods in an aqueous solution using
a one-pot synthesis method. The highest performance with
respect to electrocatalytic performance for NRR was observed
at 0.2 V, where the NH
3
yield rate was 37.23 mgh
1
mg
1cat
with a FE of 1.85%. The tripod structure offers sufficient active
regions exposed to react with the target species, assisting an
improved NRR performance. Unlike nanowires and nano-
particles, the tripod nano architectonics can efficiently control
the adverse accumulation of active sites, thus providing an
excellent stability for NRR. Moreover, bi-metallic configura-
tions can modify the coordination environments and elec-
tronic states of Ru and Pd, which help increase the adsorption
and activation of N
2
species thus stimulating the electro-
catalytic reduction of N
2
to NH
3
. An innovative method of
continuously regulating the FE was presented by Liu et al.
[231]. Based on a parallel plate capacitor, a general rule was
derived for how the membrane affects the electric field in-
tensity. In a three-electrode system, uniform FeAg nano-
clusters were synthesized using rapid heating and rapid
quenching strategies to achieve a FE of 10.81% vs. RHE. A
reduction in the number of pores on the membrane results in
a decrease in H
þ
ions transfer rate (V
H
), while a progressive
improvement in the FE is achieved up to 41.86%. It was found
that the physical model for FE and V
H
matched well with the
experimental data across a wide range of values. In addition to
having a wide range of applications, this physical model can
predict and explain the trend in FE affected by hydrophobicity
of catalysts, protonic solvent concentrations, etc. Another
international journal of hydrogen energy xxx (xxxx) xxx24
Please cite this article as: Singh S et al., Hypes and hopes on the materials development strategies to produce ammonia at mild con-
ditions, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2023.05.206
study proposes [232] the use of Cu
3
P nanoribbons as highly
efficient electrocatalysts for NH
3
formation under mild con-
ditions. The Cu
3
P nanoribbons achieved excellent FE of 37.8%
when measured in N
2
-saturated 0.1 M HCl. Furthermore, long-
term electrolysis tests over 45 h demonstrate excellent sta-
bility. Using a simple two-step synthetic method and theo-
retical predictions, Sun et al. [233] developed a new type of
nitrogenase-inspired catalyst (FeMoSe and FeMoTe) with
electrocatalytic N
2
fixation abilities. Compared to the refer-
ence FeMoS, the N
2
electrofixation activity is greatly improved
with joint electron pools. FeMoSe has also been shown to be
cyclically stable over a long period of time. Based on experi-
mental and DFT calculations, it is evident that Fe þSe elec-
trons, 2D nanosheets, and molecular dispersed Fe molecules
are crucial for N
2
fixation activity. HER signaling can be
inhibited by these specific structural characteristics, triple
bonds can be broken, and reaction intermediates can be
regulated so that N
2
can be electrofixed. CoeNieS/Ni foam, a
bimetallic catalyst, has been reported to achieve N
2
reduction
[234]. Co
9
S
8
and Ni
3
S
2
can be effectively combined without any
adhesive using the in-situ growth strategy. Since Ni and Co
bimetallic active centers regulate adsorption of various com-
plex intermediates in the N
2
activation process, the CoeNieS/
Ni foam has a high N
2
activity. An optimum FE of 6.23% with
good electrochemical stability and selectivity was achieved at
0.6 V vs. RHE with the designed electrocatalysts in 0.1 M
Na
2
SO
4
.
Open framework materials (COFs/MOFs)
The TM-based electrocatalysts are extensively researched for
nitrogen reduction activities due to their promising features to
produce NH
3
. However, TMs are generally known for their toxic
nature, limited availability, and expensive extraction and puri-
fication. Moreover, the continuous consumption of TMs at the
bulk-levelmay lead to the depletion of the material at the source
level. In this regard, the TM-based NH
3
production in futuristic
aspects may not be viable by economic and environmentally
friendly means. In addition, theTM-based catalysts face several
demerits for effective NH
3
production: the vacant d orbitals in
TMs promote metal-hydrogen interaction that can adversely
affect the efficient NRR. The dominating HER poisons the active
centers of TMs and results in a poor NH
3
production rate. The
selective nitrogen adsorption sites must be created on the cat-
alytic surface to avoid such an undesirable reaction. Taking all
these aspects, researchers developed metal-free or metal-
doped catalysts as better alternatives for pure TM compounds
in terms of economic viability, ample resources, and effective
nitrogen interactions to produce NH
3
. However, the organic and
inorganic materials composed from the main group elements
lack efficient electron donation and bonding facilities with ni-
trogen molecules. Hence, the strategic selection of elements to
construct the organic material is required for the effective
electrocatalytic NRR. For example, boron is a Lewis acid center
due to its electron deficiency nature. It can efficiently interact
with nitrogen molecules through acid-base interaction. The
grafting of boron in a catalytic-active surface shall enhance the
nitrogen interaction and the subsequent reduction processes
under suitable conditions. This concept has been successfully
demonstrated by Yu and co-workers in a two-dimensional (2D)
boron-doped graphene system [235]. The lower electronega-
tivity of boron (2.04) compared to carbon (2.55) makes it an
electron-deficient center in the 2D layers. As a result, the posi-
tively charged boron could interact with electronegative nitro-
gen molecules. Furthermore, the effective blocking of the
proton adsorption due to the Lewis acidnature of boron centers
could avoid the HER to a considerable extend. The boron-doped
graphene showed high efficiency in NH
3
production
(9.8 mgh
1
cm
2
) and FE of 10.8% at 0.5 V vs. RHE under
0.05 M H
2
SO
4
solution. The utility of boron centers for nitrogen
conversion to NH
3
was further extended to metal-free boron
carbides [4]. Boron carbide nanosheets were explored for their
selectiveNH
3
synthesisin both acid and neutral conditions. The
high electrochemical stability of boron carbide signified its
efficient feasibility in terms of more extended utilities. The NH
3
generation reaches up to 26.57 mgh
1
mg
1
with FE of 159.97% at
0.75 V. Researchers have also explored functionally diverse
materials like nitrogen-doped porous carbon for N
2
reduction
and NH
3
production activities. Liu et al. [236]preparedzeolites
imidazole framework derived N
2
-doped porous carbon for NH
3
production with high efficiency (1.40 mmol g
1
h
1
at 0.9 V vs.
RHE). The relatively high amount of NH
3
generation in this
electrocatalytic process was explained by the adsorption-
dissociation of N
2
molecules on several active sites like pyr-
idinic and pyrrolic functional moieties.
Effective N
2
activation is an essential step in the electro-
catalytic NRR process. The nitrogen molecules must be
adsorbed and chemically interacted with the catalysts, and its
triple bond should be weakened during the reduction activity.
As mentioned previously, unlike TM elements, metal-free
catalysts lack pback bonding feasibilities for the coordina-
tion of nitrogen molecules. In this aspect, novel strategies are
developed to mimic the pback bonding character to improve
the nitrogen interaction on the metal-free catalytic surface.
Defect engineering is one of the notable attempts to create an
electron back donation process in such systems. Herein, the
defect sites act as nitrogen coordination centers on the metal-
free catalytic surface. Recently, the defect generation effect in
a polymeric carbon nitride was demonstrated with the
enhanced nitrogen generation. In this defect engineering
method, Chade L. V. et al. [237] demonstrated the creation of
vacant spaces in 2D carbon nitride layers through thermal
treatment under argon atmosphere. This recalcination pro-
cess broke the long-range order 2D plane and created many
nitrogen vacancies. The nitrogen molecules could occupy
these created defective spaces through the electron back
donation process from the adjacent carbon atoms. Notably,
the electron back donation weakens the triple bond of nitro-
gen (1.26
A in sharp contrast with 1.0975
A in free N
2
), and the
molecules are activated for further reduction to NH
3
.Asa
result, the defect engineering strategy showed nitrogen
reduction performance with a good NH
3
production of
8.09 mg h
1
mg
1cat
. and with a FE of 11.59%.
Carbon-based organic porous materials are well known as
catalytic adsorptive platforms due to their physical and
chemical interactions with the adsorbate molecules or ions.
Amorphous carbon is one of the earliest candidates for het-
erogeneous catalytic activities due to its concurrent
international journal of hydrogen energy xxx (xxxx) xxx 25
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ditions, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2023.05.206
conductive and porous nature. For example, a thermally
treated nitrogen-free carbon cloth was reported for nitrogen
reduction activities with an NH
3
generation rate of
2.59 10
10
mol cm
2
s
1
, a FE of 6.92% [238]. However,
amorphous carbons lack precise functional and structural
tunability at the molecular level. Therefore, it results in the
potential utilities in efficient NH
3
production by strategic
molecular level functional amendments. Moreover, the
amorphous distribution of the pores in the porous carbon or
porous polymers may further retard the accessibility of ni-
trogen molecules for catalytic activities. Therefore, an ideal
NH
3
production catalyst should possess a large surface area
with ordered pores for the complete access of every possible
active site for the nitrogen occupation, activation, and sub-
sequent reduction.
In this regard, covalent organic framework (COFs) is
considered an emerging candidate for nitrogen reduction cat-
alytic (Fig. 18a) activities due to their excellent physicochem-
ical features [239e241]. In general, COFs are 2D or 3D crystalline
and porous polymeric materials composed of symmetric
organic building blocks of lightweight elements. The reticular
design approach of COF allowed the construction of desired
shape and size of the intrinsic pores within an ordered
arrangement of organic molecules. Furthermore, the strong
covalent bonds assemble the polymeric framework through
reversible dynamic covalent chemistry. Again, the covalent
pore walls can be functionalized with potentially catalytic-
active molecules/metal ions with known structural informa-
tion. Considering all these unique features, COFs can offer an
excellent catalytic activity for NH
3
production. However, only a
few attempts have been reported so far as summarized in
Table 3. Meanwhile, the theoretical studies suggested the po-
tential utility of COF for the effective production of NH
3
and
eliminating the HER in the catalytic system. Recently, it has
been suggested that incorporating molybdenum metals can
suppress the HER in an imine linked 2D-COF composed of 2, 3,
9, 10, 16, 17, 23, 24-octaamino-metallophthalocyanine and
pyrene-4, 5, 9, 10-tetraone [86]. In this work, Cong-Wang et al.
theoretically showed the electrocatalytic performance of 2D
imine-linked COF with various TMs (Ti, Cr, Sc, Mn, Fe, Ni, Nb,
Cu, Zn, Pt, Mo, Rh, Pd, Ag, V, Co, W, Ru, Ir, and Au). Notably, the
molybdenum incorporated COF model exhibits higher catalytic
activity than other TMs through the distal mechanism at a
lower overpotential (0.16 V) and with a reduced HER. The
positive charge on molybdenum sites electrostatically repels
the protons. Hence the possibilities of proton adsorption on the
catalytic surface were drastically reduced and subsequently
suppressed the hydrogen evolution activity.
The feasibility of metal-doped COF for nitrogen activation
has experimentally shown in an imine-linked Ni-doped
porphyrin-connected network [244]. In this work, a hybrid of
carbon nanotube and COF-366-Ni was investigated for nitro-
gen fixation activity. The 2D COF-366-Ni was synthesized from
C
4
symmetric Ni incorporated 5, 10, 15, 20-Tetrakis (4-
nitrophenyl)-21H, 23H-porphine, and C
2
symmetric 1, 4-
benzenedicarboxaldehyde. The hybrid electrode electro-
chemically reduced the nitrogen to NH
3
with an efficiency of
8.56 mgh
1
mg
1cat
. and a FE of 12.7%. The carbon nanotube
helps to improve electronic conductivity and Ni acts as an
active center for NH
3
production. The structure of COF facili-
tates a large display of active centers and effective mass
transfer of reactants and products.
Fig. 18 ea) Graphic representation of COF for nitrogen reduction to NH
3
, b)The boron-linked COF for nitrogen interaction and
activation, and c) 2D imine-linked COF as a proton filter membrane in electrocatalytic NH
3
production. Reproduced from
Refs. [242,243],with permission from the Nature.
international journal of hydrogen energy xxx (xxxx) xxx26
Please cite this article as: Singh S et al., Hypes and hopes on the materials development strategies to produce ammonia at mild con-
ditions, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2023.05.206
The boron-linked COFs are well known for their excep-
tional crystalline and porous features. The periodic grafted
boron atoms in the ordered intrinsic nanopores in COF ex-
hibits inherent Lewis's acid features. The abundant presence
of boron atoms in COF makes them suitable candidates in
electrocatalyticNH
3
production. The symmetric organization
of boron building blocks enhances nitrogen adsorption in two
ways: 1) the inherent ordered porosity facilitates lucid mass
transfer through catalytic surface matrix and 2) the Lewis acid
nature of boron easily captures the nitrogen molecules
through the acid-base interaction. Unlike the non-uniform
distribution of boron atoms in doped carbon materials, the
precise spatial occupation of boron atoms in COF improves
many catalytic properties. The recent report on the catalytic
activity of a boron-linked 2D-COF for NH
3
production validates
the above facts. The boron atom distributed molecular
framework was constructed from the self-condensation of 1,
4-benzenediboronic acid (BDBA) as a building block on sup-
porting monomer pre-treated nitrogen-doped carbon nano-
sheets (Fig. 18b) [242]. The hybrid material improved the
nitrogen reduction activity due to the synergic effect from COF
and nitrogen-doped carbon nanosheets. The boron-linked
COFs are not electrically conductive, which may affect the
efficiency of the NH
3
production due to the poor electronic
transfer. Herein, the COF act as a nitrogen reduction catalyst,
and N
2
-doped porous carbon nanosheets function for the
electrical conductivity. The electrochemically excited hybrid
material showed NH
3
production rate and FE as high as
12.53 mgh
1
mg
1
and 45.43% respectively at 0.2 V vs. RHE.
The efficient NH
3
production could be due to the excellent
interaction of nitrogen with the COF catalyst surface. An
efficient interaction of boron in COF with nitrogen molecules
was also observed. As a result, it mitigates the energy barrier
to break the strong N
2
triple bond and lucid transformation of
nitrogen to NH
3
with optimum applied potential. Notably, the
interaction boron with nitrogen distorts lattice planes of the
COF, which reduce the crystallinity of the material and turned
into an amorphous state.
In general, the electrocatalytic NRR is associated with a
HER, which drastically decreases the efficiency of desired NH
3
synthesis. Firstly, the solubility of nitrogen in an aqueous
solution is very poor compared with protons. Consequently,
the prominent accumulation of protons on the catalytic sur-
face compared with nitrogen leads to an unfavorable HER
during the desired nitrogen conversion reaction. Moreover,
the over the potential of the NRR is greater than the HER. The
possible strategies to avoid the HER and keep the NRR in favor
can be achieved by eliminating the proton adsorption at the
catalytic interfaces. Keeping these in perspective, a general
strategy has been demonstrated to reduce the proton accu-
mulation by introducing a filtration system in the catalyst
surface. Sisi Liu et al. [243] reported a selective proton filtering
2D-COF membrane to enhance the NRR of boron-doped
porous carbon. An exfoliated 2D-COF (ECOF) was con-
structed from C
3
symmetric 2, 4, 6-tris(4-aminoformyl)-s-
triazine and terephthaldehyde through Schiff base imine
condensation reaction (Fig. 18c). The 2D nanosheets of COF
were homogenously deposited on the catalytic surface of
boron-doped carbon paper. The exfoliated 2D-COF allowed
the selective mass transfer of nitrogen molecules and
effectively removed protons from catalytic interfaces. The
optimum van der Waals interaction between COF and nitro-
gen molecule allowed the transfer of it through the pores.
Meanwhile, the electrostatic interactions between COF and
proton block them at the membrane level. However, a few
amounts of protons were diffused through the membrane due
to the ordered pores in ECOF. These protons were consumed
for the reduction activities of N
2
into NH
3
. The high concen-
tration of nitrogen molecules at the catalytic interfaces im-
proves nitrogen reduction activity which yields FE and NH
3
production rate as high as 54.5 ±1.1% and
287.2 ±10 mgh
1
mg
1
respectively at 0.3 V vs. RHE. Zhang
et al. [245] reported four potential bi-atom catalysts to improve
the NRR activity, in which TM atoms like Fe, Mo, Co, Ru, and W
were embedded in the COF. Due to the synergistic interaction
between the 2D COF and metal atoms and the asymmetrical
charge reduction of metal dimers, the catalysts displayed high
theoretical FE in the range of 76e100% with a reduced limiting
potential range of 0.29 to 0.57 V. Jiang et al. [246] reported
conjugated 2D COF for superior NRR behavior incorporated
with ordered quasi-phthalocyanine N-coordinated TM (Co, Ti,
or Co) centers using a pyrolysis-free synthetic techniquefor
improved NRR behavior. The resultant TieCOF showed a FE of
34.62% and high NH
3
formation rate of 26.89 mgh
1
mg
1cat.
for
NRR. In comparison to pure COF, CoeCOF, and CueCOF,
studies showed that TieCOF can effectively adsorb and
excite N
2
molecules as well as prevent HER. Zhonget al. [247]
reported 2D conjugated-COF integrated with MeN
4
eC centers
as a new, potential catalyst for NRR to synthesize NH
3
. The 2D
conjugated-COFs were prepared using pyrene units and
metal-phthalocyanine (M ¼Co, Mn, Fe, Zn, Co, Cu, and Ni)
bonded via pyrazine bonds. It was observed that FeeN
4
eC
centers act as catalytic sitesand had strong interaction with N
2
as compared to the other M-N
4
-C centers, therefore faster
activation of N
2
.
Metal-organic frameworks (MOFs) are a subclass of porous
crystalline materials with periodic network structure. They
have drawn a variety of research interests in catalysis,
sensing, energy storage, and other fields [248]. MOFs are
frequently employed as the precursors to produce
nanocarbon-based materials [249]. MOFs have a space
confinement effect owing to their open framework and
controlled pore structures. They can be used to create nano-
materials with high levels of dispersion and mixing in situ.
MOFs are good precursors of oxides or sulfides because they
can have their morphology and size altered during prepara-
tion. However, there are currently just a few publications on
the use of MOFs in electrocatalysis for NRR, HER, ORR, and
other processes (Table 3). In a recent work, Liu et al. [244]
constructed a hetero-interface FeNi
2
S
4
/NiS electrocatalysts
with distinctive electronic structures using interface engi-
neering. Compared to the non-noble metal-based NRR elec-
trocatalysts in alkaline electrolyte, significantly high NH
3
production and FE of FNS/CC-2 are attained. The improved
electrocatalytic performance and selectivity of FeNi
2
S
4
/NiS
towards N
2
are attributable to modifications in the D-band
center and electronic structure of the electrocatalyst surface.
He et al. [250] created a MOF with disulfide trimers by
combining dynamic covalent and coordination chemistry.
This MOF was then used as the host matrix for encasing
international journal of hydrogen energy xxx (xxxx) xxx 27
Please cite this article as: Singh S et al., Hypes and hopes on the materials development strategies to produce ammonia at mild con-
ditions, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2023.05.206
extensively dispersed Au NPs to create the Au@MOFelec-
trocatalyst for NRR. The hydrophobic organosilicone layer
that has been applied to the surface of the Au@MOF can also
significantly increase the electrocatalytic activity for NRR in
ambient settings. Significantly, the optimized
hydrophictreated-Au@MOF electrocatalyst produces the most
NH
3
(49.5 mgh
1
mg
cat.
1
) and has the best FE (60.9% at 0.3 V
versus RHE). The superior NRR performances suggest that
MOFs with defined and abundant sulfide sites might work well
as carriers for active Au NPs, which have a considerable
advantage in attaining high-efficacy catalysts for NH
3-
synthesis. Additionally, the hydrophobic organosilicone is
capable of resolving the HER competition bottleneck during
NRR. In another study [251], Vo-rich Zn-doped Co
3
O
4-
nanopolyhedrons (ZneCo
3
O
4
) were prepared by a low-
temperature oxidation method using a variety of bimetallic
zeoliticimidazolate frameworks based on ZIF-67 and ZIF-8
with varying Co/Zn ratios as precursors (Fig. 19a). Under
ambient circumstances, ZneCo
3
O
4
exhibited a high FE of
11.9% for NRR and an NH
3
yield of 22.71 mgh
1
mg
cat.
1
(Fig. 19b).
It was thought that the exceptional catalytic capabilities
are due to the abundance of V
o
as Lewis acid sites and
electron-rich C
o
sites, which facilitate the adsorption and
dissociation of N
2
molecules. Unexpectedly, ZneCo
3
O
4
had a
significant level of electrochemical stability. To create effec-
tive NRR electrocatalysts, a series of 2D MOFs with various
metal atoms and organic linkersdM
3
C
12
X
12
(M ¼Cr, Mo, and
W; X ¼NH, O, S, and Sedwere proposed [252]. Our theoretical
simulations demonstrated the effectiveness of metal atoms in
M
3
C
12
X
12
in capturing and activating N
2
molecules. Owing to
thesuperior activity, selectivity, and low limiting potential
(0.59 V, 0.14 V, and 0.01 V, respectively), W
3
C
12
X
12
(X ¼O, S,
and Se) exhibited the greatest NRR performance of these
candidates. Through in situ growth, a Co-based metal-organic
framework (MOF), or zeoliticimidazolate framework-67 (ZIF-
67), described as ZIF-67@Ti
3
C
2
, was created [253]. The MOF's
high porosity, substantial active surface area, and Ti
3
C
2-
MXene's higher conductivity allowed for the effective elec-
trochemical synthesis of NH
3
using the composite.
Particularly, the produced ZIF-67@Ti
3
C
2
catalyst had a good FE
(20.2%) at 0.4 V and an excellent NH
3
yield
(6.52 mmol h
1
cm
2
). These results were much better than
those attained by Ti
3
C
2
and ZIF-67 alone (2.77 and
1.61 mmol h
1
cm
2
, respectively) (vs. the RHE). Cong et al.
developed and assessed a well-defined single-site MOFs, M-
TCPP; M ¼Fe, Co, or Zn as electrocatalysts for N
2
reduction
[254]. The produced Fe-TCPP demonstrated outstanding per-
formance, outperforming all previously reported molecular
Fig. 19 ea) Graphicalrepresentation of ZneCo
3
O
4
synthesis, b) NH
3
yield and FE of Co
3
O
4
and ZneCo
3
O
4
. Reproduced from
Ref. [251], with permission from the American Chemical Society.
Table 3 eSummary of various COF/MOF based catalysts for NH
3
synthesis using electrochemical method.
Catalyst NH
3
formation rate FE (%) Catalytic stability References
PCN-NV4 8.09 mgh
1
mg
1cat
11.59 -[237]
CC-450 2.59 10
10
mol cm
2
s
1
6.92 10 h [238]
COF-366-Ni/CNT 8.56 mgh
1
mg
1cat
12.7 e[258]
Eex-COF/NC 12.53 mgh
1
mg
1
45.43 e[242]
ECOF@BCP 287.2 mgh
1
mg
cat1
54.5 e[243]
TieCOF 26.89 mgh
1
mg
1cat
34.62 12 h [246]
2Dc-COFs with FeeN
4
eC center 33.6 mgh
1
mg
cat1
31.9 10 h [247]
FNS/CC-2 128 mgh
1
cm
2
28.6 12 h [244]
HT Au@MOF 49.5 mgh
1
mg
cat.1
60.9 6 h [250]
ZneCo
3
O
4
22.71 mgh
1
mg
cat.1
11.9 24 h [251]
ZIF-67@Ti
3
C
2
6.52 mmol h
1
cm
2
20.2 15 h [253]
FeeTCPP 44.77 mgh
1
mg
cat.1
16.23 e[254]
MIL-100 (Al) 10.6 mgh
1
cm
2
mg
cat.1
22.6 10 h [255]
FeS
2
/MoS
2
@RGO 41.1 mgh
1
mg
cat1
38.6 e[256]
HE-MOF 42.76 mgh
1
mg
1
14.75 100 [257]
international journal of hydrogen energy xxx (xxxx) xxx28
Please cite this article as: Singh S et al., Hypes and hopes on the materials development strategies to produce ammonia at mild con-
ditions, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2023.05.206
and MOF catalysts with a high NH
3
production of 44.77 mgh
1
mg
cat.
1
and a FE of 16.23%. The very efficient N
2
activation at the
Fe site, which profited from the overall reaction thermody-
namics advantage in the crucial reaction step of *NNH pro-
duction, was credited with the better performance. Fu et al.
[255] designed porous aluminum-based MOFs materials, MIL-
100 (Al), for the electrochemical nitrogen fixation. Because of
its distinctive structure, MIL-100 (Al) displays impressive NRR
characteristics (NH
3
yield: 10.6 mgh
1
cm
2
mg
cat.
1
and FE:
22.6%) at a lower overpotential. According to research, the
catalyst has excellent N
2
-selective captures because N
2
binds
to the unsaturated metal sites. More precisely, Al, a main
group metal, has a high and selective affinity to N
2
, as the Al 3p
band can interact strongly with N 2p orbitals. Feng et al. [256]
demonstrated a hosteguest-assisted strategy based on Fe-
based MOF (MIL-100) and molybdenum-based poly-
oxometalate (PMo
12
) for preparing nanostructured bimetallic
sulfides via one-pot hydrothermal synthesis route. FeS
2
/MoS
2
particles were distributed uniformly on RGO having high
conductivity, forming a well-defined nanoflower structure.
The combination of MoS
2
, FeS
2
, and RGO had a synergistic
effect that allowed the FeS
2
/MoS
2
@RGO to achieve electro-
catalytic activity and stability toward NRR in both acidic and
basic conditions. The electrochemical results indicated that a
RHE in an acidic potassium sulfate solution exhibits a high
NH
3
yield rate and FE. One study was to develop different
derivatives of high-entropy MOFs (HE-MOFs) based on tran-
sition metals (Co, Zn, Ni, V) [257]. N
2
electrofixation by HE-
MOFs and derivatives was shown to have favorable kinetics
in a wide range of pH electrolytes, specifically in acidic media
where nitrogen reduction occurs and in alkaline media where
oxygen evolution occurs. In order to buffer the pH mismatch,
bipolar membranes were used to construct an asymmetric
acidic/alkaline device prototype. Prototype showed remark-
able performance, with NH
3
yield rate of 42.76 mgh
1
mg
1
,
energy efficiency of 2.59% and FE of 14.75%.
Operando techniques for the exploration of the
NH
3
synthesis mechanism
Steady state, isotopic transient kinetic analysis (SSITKA) can
be performed by switching between two streams of either
14
N
2
or
15
N
2
which are mixed on-stream with H
2
and He. In a
similar manner N
2
/D
2
(D: Deuterium) mixtures can be used.
These experiments are used to evaluate the H or N pathway
towards NH
3
formation (mechanistic pathway). SSITKA
experiment is usually coupled with mass spectrometry and/or
infrared spectroscopy for in parallel monitoring of the gas
phase or surface intermediates formation, respectively.
For example, an understanding of how hydrogen atoms
originate in NH
3
is crucial to understand the electrochemical
promotion of NH
3
formation. NH
3
formation is followed by a
surface reaction with electrochemical promotion of catalysis
(EPOC) if the hydrogen atoms in NH
3
come from H
2
on the
cathode. However, if the hydrogen atoms are originated from
the anode, NH
3
will be formed through the charge transfer
reaction between the gas, the Fe catalyst, and the |BaCe
0.9-
Y
0.1
O
3
| (BCY) proton-conductor. A deuterium isotope (D)
analysis has been used to investigate the mechanism of
electrochemical promotion of NH
3
formation, where D is used
to follow the hydrogen mechanistic pathway. Using deute-
rium isotope analysis, Li et al. [259] examined the electro-
chemical promotion of NH
3
formation. The hydrogen/
deuterium content of NH
3
formed in the cathode was deter-
mined using FTIR. It was observed that the NH
3
products with
cathodic polarization correspond to the species of H
2
(or D
2
)in
the cathode, that is, NH
3
(or ND
3
) was mainly formed when H
2
(or D
2
) was introduced to the cathode. The isotopic analysis
showed that NH
3
is produced faster via EPOC than via charge-
transfer reactions, suggesting EPOC will lead to a significant
increase in the formation rate of NH
3
. Moreover, hydrogen
partial pressure and applied voltage are both factors influ-
encing NH
3
formation by EPOC. Li et al. [260] also conducted
the deuterium isotope analysis to characterize the electro-
chemical progression of NH
3
synthesis at 650C in the pres-
ence of H
2
eN
2
, 1%We10%Fe- |BCY|Pt, and 3%H
2
O-10%D
2
-87%
Ar. An increase in cathodic polarization increased the rate of
NH
3
formation by around 20 times. In addition, the NH
3
for-
mation rate and hydrogen partial pressure at the cathode
were positively correlated. Based on these results, it is likely
that accelerated N
2
dissociation was responsible for electro-
chemically promoting NH
3
formation via supplying Dþ.
Consequently, ND*and N*formed from N
2
D*, and then ND*
and N*reacted with H*to form NH
2
D and NH
3
.*Denotes
adsorbed species. In another study by Hunter et al. [261], a
potential NH
3
synthesis catalyst and catalyst for nitrogen
transfer reactions, Co
3
Mo
3
N, was investigated for its
14
N/
15
N
isotopic exchange pathways. Both heterolytic and homo-
molecular exchange processes were studied, and it was found
that lattice nitrogen species are exchangeable. There was no
pure homomolecular exchange observed when
15
N
2
and
14
N
2
molecules were scrambled over a surface, indicating a limited
N
2
dissociation ability of cobalt molybdenum nitride. A het-
erolytic exchange between
15
N
2
and nitrogen atoms of the
lattice was observed at 450 C during the temperature pro-
grammed nitrogen isotopic exchange experiment when the
Co
3
Mo
3
N was pretreated in 3:1H
2
:N
2
before 700C followed by
degassing at 400 C. The results of this study indicated that
pretreatment has a strong effect on nitrogen in the CoMoN
system. It was found that almost 25% of lattice N atoms were
exchanged after 40 min at 600C after N
2
pretreatment at
700C as compared to only 6% after similar Ar pretreatment at
600C. As far as the application of this material is concerned,
this observation is significant, considering that the potential
contribution of adsorbed N species can be discounted. In the
case of the Co
6
Mo
6
N phase, regeneration to Co
3
Mo
3
N under
15
N
2
at 600C occurs concurrently with
14
N
15
N formation,
further demonstrating that the reactivity of N
2
in the
CoeMoeN system is strongly dependent on pretreatment. In a
study of electrochemical NH
3
synthesis, Krempl et al. [262]
presented a highly sensitive and quantitative method for
analyzing dynamic reactions. The method was shown to be
generalizable and versatile in non-aqueous and aqueous
electrolytes. In the diglyme-based electrolyte, the intrinsic FE
towards NH
3
was 49% in a thin-layer cell with a well-defined
electrochemical environment coupled with rapid nitrogen
mass transport to the electrode, highlighting a potential ni-
trogen reduction under ambient pressure. In spite of the fact
that lithium electroplating has ceased, the electrochemistry-
international journal of hydrogen energy xxx (xxxx) xxx 29
Please cite this article as: Singh S et al., Hypes and hopes on the materials development strategies to produce ammonia at mild con-
ditions, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2023.05.206
mass spectrometry setup was able to detect ongoing NH
3
production directly. In this study, increased FE was hypothe-
sized to be caused by the electrode being cycled between
lithium electroplating and open circuit voltage.
DFT studies
Investigation of the DFT calculated reaction mechanisms of
different pathways in the electrochemical and thermochem-
ical NH
3
synthesis has surely enriched our understanding of
the elementary reaction stages. Liu et al. [263] used DFT to
fabricate 20 types of Ru/graphene catalyst models based on
altered N
2
atom doping, distribution of Ru atom, different
number of Ru atoms, and other aspects. The highest NRR ac-
tivity of the catalysts were determined by measuring the
highest occupied molecular orbital of every structure, the
energy difference between the lowest and highest electron
cloud distribution, and other related information. From
various catalyst, the best catalytic behavior was observed for
3Ru supported pyrrole N
2
doped Gr with double adsorption
process and the 2Ru supported graphite N
2
doped graphene
with single adsorption process. Fig. 20 displays the relative
energy change of the two superior catalysts catalyzing the
NRR. During the double adsorption step, the total relative
energy has a significant decrement, specifying that, as
compared to single adsorption the reaction rate was faster for
double adsorption process. However, hydrogenation is one by
one in single adsorption process, while the double adsorption
process combines two H*in the first step, indicating that
double adsorption process is more challenging. This study
offered a theoretic direction and designs for fabricating high-
performance NRR catalysts by creating models and improving
configurations.
Yuan et al. [129] used spin polarized DFT and designed a
thermodynamically stable photocatalyst with high selectivity
and activity for NRR via single TM atom supported by BeO
monolayer. Various TMs such as Cr, Ti, V, Sc, Mn, Fe, Ni, CO,
Zn, Cu, Zr, Y, Mo, Nb, Rh, Ru, Ag, Cd, and Pd were considered. It
was observed that the photocatalyst based on Ru/BeO had
comparatively lower limiting potential value (0.41 V) for NRR
and also had visible light adsorption along with suitable band
edge positions. Kang et al. [264] prepared various graphyne
materials doped with single N, P, S, O, and B and used DFT to
study their electrocatalytic processes for nitrogen reduction
reaction. It was observed that only sp-B doping effectively aids
the nitrogen reduction reaction procedure. Further, codoping
effect was investigated by doping second B, comprising sp-B
and sp-2B. Following a distal mechanism, B (sp
2
)-C (sp)-B
(sp) arrangement with graphyne had a very low limiting po-
tential of 0.12 V along with high dynamic stability, low HER
activity and good conductivity. It was also noted that the sp-C
directly connected to active sp-B could be an energy buffer
that makes a flat potential energy surface. Wang et al. [265]
reported the electrocatalytic performance of a Cu-based TM
dual atoms attached to N-doped phosphorene electro-
catalyst,Cu-TM/N
4
-BP, towards N
2
production using first-
principles computations. The N
2
adsorption property was
promoted by the synergistic interaction between CueRu dual
atoms along with stretching the N^N triple bond. The best
NRR behavior was observed for CueRu/N
4
-BP catalyst, which
also showed efficient reduction of N
2
to NH
3
with a low over
potential value of 0.30 V. Moreover, the energy required for
NH
3
desorption was reduced owing to the synergistic influ-
ence of dual atoms, thus balancing the interaction of NH
3
and
N
2
with catalyst and breaking the linear scaling relations and
ensuring the catalyst durability. 12 TMs attached on anatase
TiO
2
(0 0 1), which includes noble TMs (Nb), low-cost 3 d TMs
(Cr, Cu, Mn, V, Co, Fe, Ni), and noble TMs (Au, Ag, Pt, Pd) were
also studied [266]. Three different mechanisms involving
distal, enzymatic, and alternative were investigated and it was
observed that the most favorable mechanism is alternating
pathway with respect to energy (N
2
*þH*/NNH*as the
potential-determining step). According to the DFT calcula-
tions, Fe atom attached at topmost O
2C
on TiO
2
(0 0 1) servers
as the solo active site for N
2
activation and reduction and
showed potential as an active catalyst for photo electro-
chemical NRR. Yazdiet al. [267] reported DFT calculations to
understand the EleyeRideal/Marsevan Krevelen (associative)
and the LangmuireHinshelwood (dissociative) mechanism of
Co
3
Mo
3
N surfaces, in the presence of surface defects. The two
mechanisms are shown in Fig. 21. After contrasting the two
methods, it was discovered that there is a further mechanism
that uses diazine and hydrazine intermediates produced via
Eley-Rideal type chemistry and involves H
2
reacting directly
with surface-activated N
2
. Thus, it was determined that the
rate of NH
3
production at ambient settings can be boosted by
adding surface defects.
Suryantoet al. [268] described an extremely selective Ru-
decorated MoS
2
catalyst for NRR with controllable HER activ-
ity. From DFT study, it was demonstrated that the hydroge-
nated S-vacancy plays a crucial role as the H-provider in the
NRR mechanism as the *H formed can be transmitted directly
to nearby bound N
2
or combine with the N
2
reduction in-
termediates. This allows for the regeneration of the corre-
sponding S-vacancy for additional proton reduction steps. Lin
et al. [269] demonstrated a DFT study on investigating the
performance of various TMs with black phosphorus-based li-
gands for NRR and proposes a design principle for SACs. From
various catalysts explored, Ta@BP, Nb@BP, and W@BP had low
Fig. 20 eRelative energy of single and double adsorption
catalysts for N
2
reduction. Reproduced from Ref. [263], with
permission from the Elsevier.
international journal of hydrogen energy xxx (xxxx) xxx30
Please cite this article as: Singh S et al., Hypes and hopes on the materials development strategies to produce ammonia at mild con-
ditions, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2023.05.206
free energy change, high electrical conductivity, and high
stability for NRR. It was because the SACs with P-based ligands
adsorbed *N
2
more strongly than *H. Additionally, from vol-
cano plot it was demonstrated that moderate amount of
positive charge on metal center leads to a better ability to
insert electrons into *N
2
H, eventually resulting in superior
NRR activity. Similarly, Liu et al. [270] used DFT calculations to
investigate various TMs (Ti, Cr, Mn, Co, V, Ni, Fe, Zn, Cu, Mo,
Nb, Sc, Ru) attached to porphyrin sheets for NRR. From them,
2D Mo-porphyrin sheet showed the superior catalytic activity
for selectively capturing N
2
and NH
3
reduction. The distal re-
action path was preferred for N
2
reduction with high energy
barrier (0.58eV). Moreover, the molecular dynamics simula-
tion indicates that the catalyst had a high thermodynamic
stability making it a promising catalyst for NRR at mild con-
ditions. In another study, Yao et al. [271] reported DFT calcu-
lations for 2D transition metal-porphyrin (TM-PP) sheets
having high-loading intrinsic TMN
4
centers for NRR. It was
observed that as compared to 3dand 4d,5dTM-PP sheets had
better electron transfer capability and thus more efficient as a
catalyst. The most promising catalyst was W-PP sheet with
overpotential of 0.15 V and high mass loading of 38.33 wt%,
favorable for accelerating NRR with enhanced selectivity. Ji
et al. [272] used grand canonical DFT to examine the adsorp-
tion of NRR intermediates under constant electrode potential
(CEP) condition. They found Ru single atom bonded with 2C
atoms and 2N
2
atoms was recognized as the ideal NRR reac-
tion site. Ball et al. [273] reported a DFT study on Mo atom-
doped salphen-based COF based catalyst ((Mo-salphen COF))
for NRR. The catalyst showed thermodynamic and electro-
chemical stability along with enhanced electrocatlytic
behavior toward NRR. The Mo-salphen COF favored distal
mechanism having a lower limiting potential of 0.33 V vs
RHE. Moreover, Mo-salphen COF can subdue the competing
HER at 0 and NRR working potential with a theoretical FE of
41%. Based on recently-synthesized 2D binuclear Cu-salphen
COF. Ohashiet al. [274] used DFT to investigate COFs doped
with different 3D-metal atoms having diverse coordination
numbers to achieve an overall guideline for designing an
effective NRR catalysts. An optimum NRR catalyst shows an
adequate binding strength with intermediates. From various
metal-doped COFs investigated, the highest onset potential of
0.49 eV vs CHE was detected for the catalyst with Fe metal
center. Back et al. [172] theoretically examined the electro-
chemical NRR mechanism to generate NH
3
on the Ru catalyst.
The associative and dissociative pathways were evaluated
along with all possible NeN dissociation steps during the
reduction processes. On the basis of the calculated free energy
diagrams, it was evident that the kinetically facile dissociative
intermediate pathways require a thermodynamic limiting
potential similar to the associative pathways (0.68 V). As in
the Haber-Bosch process, the initial dissociative pathway for
NeN bonds faces a substantial kinetic barrier. It was observed
that electrochemical nitrogen reduction with high efficiency is
hampered by competitive hydrogen evolution. N
2
protonation
is thermodynamically less favorable at low overpotentials
than hydrogen adsorption, thus reducing the number of active
sites available to activate N
2
. In addition, free energies were
compared with different H-coverages on Ru to demonstrate
that the H-coverage can significantly increase the energy
barrier for protonation of N
2
, thus changing the potential
determining step and resulting in an increase in
Fig. 21 ea) L-H (Dissociative) mechanism, b) E-R/MvK (Associative) mechanism for NH
3
formation on Co
3
Mo
3
N. Reproduced
from Ref. [267], with permission from the American Chemical Society.
international journal of hydrogen energy xxx (xxxx) xxx 31
Please cite this article as: Singh S et al., Hypes and hopes on the materials development strategies to produce ammonia at mild con-
ditions, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2023.05.206
overpotentials. In the work of Liu et al. [275], single transition
metal atoms (Ag, Pd, Cu, Rh, Mo, Ti, Mn, Mo, Fe, Zn, Ru, Pt, and
Co) were systematically studied for N
2
fixation on a single
boron phosphide monolayer supported by a phosphorus
monovacancy with the presence of a P-point defect. Using DFT
calculations, the best catalytic performance was observed for
Mo/BP, and the reaction pathway favored sequential and
enzymatic mechanisms with an activation barrier as small as
0.68 eV. Based on DFT, Sun et al. [276] predicted that high-
entropy oxides (HEOs) promote NRR and OER, and then
develop a facile method for synthesizing HEOs with the
morphology of sea urchin-shaped hollow nanospheres. In
both NRR and OER, HEOs exhibit excellent electrocatalytic
activity (NH
3
yield rate 47.58 mgh
1
mg
1
and FE 10.74%). As a
result, a prototype of N
2
electrolysis driven by commercial
batteries was fabricated, which provides high NH
3
yield rate
and FE. According to further mechanism studies, HEOs'
excellent catalytic performance is attributed to the synergistic
effects between multiple metals and the effect of increasing
entropy on their electronic structures. Based on DFT and
microkinetic modeling, Banisalam et al. [277] examined the
NH
3
synthesis process of RueCo catalysts. Due to RueCo's
ability to induce spin symmetry breaking of Ru, the RueCo
surface promotes easier dissociation of N
2
than Ru's surface.
Furthermore, RuCo(0001) was analyzed with varying partial
pressures of H
2
and N
2
. Under Haber-Bosch pressure condi-
tions, the most stable phase of the RueCo surface consists of
N and H atoms. Furthermore, it was found that NH
3
could be
produced on these surfaces without causing severe surface
poisioning. Over a Ru catalyst, Logad
ottir et al. [278] presented
DFT results for the elementary steps in NH
3
synthesis.
Compared to flat terraces, steps have stronger bindings for
intermediates in the reaction. As a result, the reaction was
suggested to mainly occur at the step sites. There is a possi-
bility, however, that some of the elementary reaction steps
may take place on a terrace. A Ru (0001) surface is also shown
to be the rate-determining step in the reaction by the disso-
ciation of N
2
. According to the calculations, alkali metals
promoted reactants by stabilizing the transition state of N
2
dissociation and by destabilizing surface NH species. In
another study, Zhang et al. [279] conducted DFT calculations,
in order to understand stepwise addition reactions on NHx
(x ¼1e3) synthesis on Ru (0 0 0 1). There were significant re-
action barriers in stepwise addition reactions. Hydrogenation
of NH had the highest barrier, 1.28 eV, almost equal to the
dissociation of N
2
. It was found that an increase in surface
coverage reduced the reaction barrier and step sites were
active in stepwise addition reactions. The step sites reduced
the reaction barrier by about 0.5 eV for NH hydrogenation. It
was noted, however, that the step sites do not play as signif-
icant a role in lowering the transition states'energies in the
stepwise addition reactions as they do in the case of N
2
dissociation.
Future outlook and perspective
Various factors that affects the NRR activity include tempera-
ture, pressure, catalyst, electrolyte, cell configuration, ion-
exchange membrane, and reference/working electrode.
However, themost critical factor is the choice of electrocatalyst.
Based on earlier reported investigations[280e284] the following
characteristics are desired in an efficient catalyst: higher cata-
lytic activity for adsorbtion and activation of N
2
,adequate
electrical conductivity, rapid mass transfer (like 2D nanosheet,
nanoparticle, etc.), hydrophobicity for subduing HER (such as
MoS
2
,TiO
2
,Fe
2
O
3
, etc.), exposed catalytic site,and high stability.
Herein this review we summarized and discussed the chal-
lenges, recent progresses, approaches, and opportunities for
alternative NH
3
synthesis methods focusing on catalysts based
on ruthenium, inorganic metal oxides, organic COFs and MOFs.
An important challenge in mild-condition NH
3
synthesis is
the difficulty in using proper experimental methods, stan-
dards, and controls to quantify the products accurately owing
to the relatively low production rates and numerous inter-
fering factors. A range of colorimetric/spectrophotometric
methods is widely used to detect NH
3
(ammonium ion), such
as Nessler, phenate, and indophenol blue tests. However,
certain ions dissolved in solution strongly affect the total
amount of NH
3
. Combining ion chromatography, ion-selective
electrodes, fluorescence and
15
N isotope labeling with other
detection methods could enhance the results'reliability. The
origin of nitrogen atoms should be clarified by 15 N isotopic
measurements if nitrogen-containing catalysts, reagents,
electrolytes, etc. are used during the synthetic process. In
addition, the environment in which the measurements are
carried out must be free of NH
3
contamination. It will be
beneficial to explore the possibility of industrialization in the
future if online fast and accurate methods are developed to
facilitate the discovery of effective NRR catalysts. Further-
more, energy efficiency, process throughput, and catalyst
selectivity must be addressed. Experiments and theoretical
simulations leading to breakthroughs are highly demanded
and are foreseeable. Moreover, sophisticated techniques such
as the Steady State Isotopic Transient Kinetic Analysis
(SSITKA) have been used to understand the mechanism of
NH
3
formation; under SSITKA experiment either deuterium
isotope analyses, or
15
N/
14
N isotopic exchange is performed.
The SSITKA is coupled with mass spectroscopy (MS) or
infrared spectroscopy (DRIFTS) for the monitoring of the gas
phase at the reactor exit or the surface formed intermediates.
SSITKA is a powerful tool for the study of structure-sensitive
reactions, such as NH
3
synthesis since information about
the catalyst under working conditions can be extracted. This
means that no assumptions are needed about the metal active
sites for the calculation of TOF (turnover frequency).
One of the major issues of Ru-based catalyst is to control
the shape and size of Ru particle to optimize the amount of
active sites, well known active sites of Ru-based catalysts, i.e.,
B5 sites are thought to be the most productive sites in the
dissociation pathway for the synthesis of NH
3
. However, due
to the lack of simple and effective synthesis techniques, it has
proven difficult to date to design uniform Ru-based catalysts
having well-designed structures, high dispersion, and specific
metal-support interfaces in order to maximize catalytic per-
formance in NH
3
synthesis and decomposition. In addition to
traditional precipitation, impregnation, photo- and electro-
deposition, and chemical reductions, the development of
more precise synthesis methods could prove valuable. For
instance, the bottom-up synthesis of supported metal
international journal of hydrogen energy xxx (xxxx) xxx32
Please cite this article as: Singh S et al., Hypes and hopes on the materials development strategies to produce ammonia at mild con-
ditions, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2023.05.206
catalysts via chemical vapor deposition or atomic layer
deposition, which uses a gas phase method, allows one to
precisely control the size, make-up, and structure of metal
nanoparticles. Additionally, increasing research has demon-
strated the utility of single-atom catalysts in the production of
NH
3
, a process that is appealing but difficult to perform at a
high level due to the complexity of catalyst synthesis tech-
niques and the development of metal-support interactions. In
order to ensure the design of reliable catalysts with ideal
active sites for practical applications, it is urgently necessary
to identify the active sites (such as B5 sites) and establish their
thermal stability without surface reconstruction during the
reaction. Moreover, the use of second metal offers a signifi-
cant opportunity for the implementation of Ru-based cata-
lysts. Even though many efforts have been made to increase
the catalytic performance of Ru catalysts, these bimetallic
catalysts still perform much below what we had anticipated.
This could be improved with further research. Bimetallic
catalysts provide exceptional and significant prospects for
fine-tuning catalytic performance and building new catalysts
for NH
3
synthesis and decomposition with the goal of gener-
ating higher activity with a reduced cost of catalysts. Similar
to this, adding promoters such as structural and electronic
promoters significantly increase the activity of NH
3
produc-
tion and breakdown; nevertheless, both theoretical and
experimental research are required to fully understand how
these promoters affect Ru-nanoparticles. According to the
major breakthrough studies reported, instead of N
2
activation,
the NeH formation is the rate-determining step for Ru/
Ca12A7:e
[150]. Understanding of reversible hydride storage,
which prevented Ru poisoning by H
2
at high pressure in Ru/
Ca12A7:e
, attracted attention to substitute electrides with
superior hydride storage qualities, and sparked advanced
research on a variety of electrides and hydride materials for
NH
3
synthesis [204,285]. In addition, a thorough understand-
ing of the fundamentals of NH
3
production and breakdown
also requires a thorough research of the reaction processes
and reaction dynamics. Identifying the reaction mechanism
and routes is still very difficult. According to previous studies,
the catalysis of NH
3
with Ru catalysts follows a dissociative
pathway, where the B5 sites play a crucial role. Nonetheless,
an increasing number of studies have put forth novel hy-
potheses on the mechanism of the reaction, challenging the
conventional view that coordinated Ru atoms in cluster form
(B5 sites) exhibit significantly higher activity than isolated
sites. To achieve fundamental insights, more work must be
done to combine experimental and theoretical methodologies.
A new chemical cycling technique developed by
McEnaneyet al. [110] combines molten salt electrolysis, Li
nitridation, and Li
3
N hydrolysis to enable stepwise NH
3
pro-
duction with high initial FE at atmospheric pressure of 88.5%.
It is fundamentally based on the following principles: Metallic
Li's reducing power reacts with N
2
to produce Li
3
N, which can
then be reacted with protons or water to produce NH
3
and
LiOH. In order to facilitate Li collection, Kim et al. [109] used
water as an electron donor in a LISICON-based electrolytic
cell. This three-stage Li-mediated NH
3
synthesis is made
possible by Li nitridation and Li
3
N protonation, which allow
for high FE under milder conditions. Based on initial tech-
economic analysis of electricity costs and energy inputs, this
process offers promise for suitable markets, especially when
compared to conventional, centralized NH
3
synthesis pro-
cesses, this process is easily decentralized, uses renewable
resources, mitigates CO
2
emissions, and can easily be decen-
tralized. Also, unexpected, mechanistically unique NH
3
syn-
thesis has been described mediated by the cationic tantalum
dimer Ta
2þ
.
Furthermore, it is extremely desirable to design and create
novel, highly conductive MOFs and COFs to improve the
electrocatalytic performance. The addition of electron-
donating and collecting nodes helps enhance the electron
transfer procedure. Many MOFs are unstable in strongly polar
solvents, particularly water. Due to the presence of organic
ligands, substantial doubts have been raised about the sta-
bility of MOFs and COFs under catalytic conditions. The
backbone of MOFs and COFs collapses as a result of the
degradation of organic ligands, especially in extremely alka-
line solutions. Due to the aggregation of metal nanoparticles,
loss of active sites, and obstruction of effective mass transfer,
the structural collapse of conductive MOF-based materials
and COF-based materials significantly lowers the catalytic
performances. It is necessary to increase the stability of
conductive MOFs and COFs during the electrochemical reac-
tion process [286]. Moreover, a cell with a bifunctional elec-
trocatalyst is far more appealing than the current HER
catalysts. Despite the recent reports of several such bifunc-
tional conductive MOFs and COFs as electrocatalysts, their
research and development are still in the early stages.
Therefore, bifunctional electrocatalysts with high efficiency
and cheap cost are eagerly required [286]. The application of
MOF and COF can be broadened effectively by combining MOF
and COF-composite materials with other substances such as
Mxene, 2D TMdichalcogenides, graphene, perovskites, carbon
nanotube, carbon fiber, carbon nanohorn, carbon nitrides,
carbon black, black phosphorus, and layered double hydrox-
ides [287].
Funding
Authors acknowledge the financial support from Khalifa
University through the grant RC2-2018-024.
Declaration of competing interest
The authors declare that they have no known competing
financial interests or personal relationships that could have
appeared to influence the work reported in this paper.
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