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Cite this: DOI: 10.1039/d3mh00335c
Emerging transition metal and carbon
nanomaterial hybrids as electrocatalysts for water
splitting: a brief review
Ayaz Muzammil, †
ab
Rizwan Haider, †
b
Wenrui Wei,
b
Yi Wan,
b
Muhammad Ishaq,
b
Muhammad Zahid,*
a
Waleed Yaseen
c
and Xianxia Yuan *
b
Electrocatalytic water splitting has appeared to be a sustainable green technology for hydrogen and
oxygen production, and noble metal-based electrocatalysts, like Pt for hydrogen evolution reaction
(HER) and RuO
2
/IrO
2
foroxygenevolutionreaction(OER)havebeenprovedtobestate-of-the-artin
water electrolyzers. However, high cost and scarcity of noble metals hinder large-scale applications
of these electrocatalysts in practical commercial water electrolyzers. As an alternative, transition
metal based electrocatalysts have attracted great attention because of the exciting catalytic
performance, cost-effectiveness and abundant availability. However, their long-term stability in
water splitting devices is unsatisfactory because of agglomeration and dissolution in the harsh
operating environment. A possible solution to this issue is encapsulating transition metal (TM) based
materials in stable and highly conductive carbon nanomaterials (CNMs) to make a hybrid of TM/
CNMs, and the performance of TM/CNMs could be further enhanced by heteroatom (N-, B-, and
dual N,B-) doping to carbon network in CNMs to break the carbon electroneutrality due to the
different electronegativity, modulate the electronic structure to facilitate the adsorption of reaction
intermediates, and promotion of efficient electron transfer to enhance the number of catalytically
active sites for water splitting operation. In this review article, the recent progress of TM-based
materials hybridizing with CNMs, N-CNMs, B-CNMs, and N,B-CNMs as electrocatalysts towards HER,
OER as well as overall water splitting have been summarized, and the challenges and future
prospects are also discussed.
Wider impact
By dint of global warming and energy crises, hydrogen production from water splitting has attracted great attention and is anticipated as a zero-emission
alternative to fossil fuels. However, current water splitting devices on industrial scale need to employ scarce and expensive noble-metal catalysts for efficient
operation. In this context, widely available transition metals (TM) supported on nonmetal hetero atom-doped carbon nanomaterials (CNMs) with bifunctional
catalytic sites have been explored. Among the n-type dopants, nitrogen is considered the most efficient, while boron has been introduced as a proper p-type
dopant. The advantages of N-, B-, and N,B-doping in CNMs and the effects on TM/CNMs catalysts performance towards HER and OER as well as OWS, the
challenges and proposed solutions, and some future research directions have been discussed in this review. It is predicted that supporting transition metals on
dual N,B-CNMs could work as an efficient alternative to costly noble metal-based catalysts for water electrolyzer. Moreover, this review article could provide
steering to researchers working in the field of electrochemical water splitting.
1. Introduction
Global energy demand is increasing due to the rapid growth
in the world’s population and economy.
1
At present, most of
theenergydemandisfulfilled through fossil fuels such as
oil, coal, and natural gas, which are nonrenewable and
expected to deplete in 35 to 40 years.
2
And these resources
are not green as their utilization induces detrimental effects
on the environment owing to the discharge of greenhouse
a
Department of Chemistry, University of Agriculture Faisalabad, 44000, Pakistan.
E-mail: rmzahid@uaf.edu.pk
b
Department of Chemical Engineering, Shanghai Jiao Tong University,
Shanghai 200240, China. E-mail: yuanxx@sjtu.edu.cn
c
School of Chemistry and Chemical Engineering, Jiangsu University,
Zhenjiang 212013, China
†Authors contributed equally.
Received 5th March 2023,
Accepted 11th May 2023
DOI: 10.1039/d3mh00335c
rsc.li/materials-horizons
Materials
Horizons
REVIEW
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gases and other harmful substances. Therefore, massive
research has been stimulated into technologies for the
conversion and storage of sustainable green energy
resources.
3
Ayaz Muzammil
Ayaz Muzammil completed his Master of
Philosophy (MPhil) in Chemistry from
University of Agriculture Faisalabad,
Pakistan, in 2021. He is currently
workingasaPhDstudentatSchoolof
Chemistry and Chemical Engineering,
Shanghai Jiao Tong University, under
the supervision of Prof. Xianxia Yuan
with financial support from Shanghai
Government Scholarship (SGS). His
current research focuses on the synthesis
of platinum group metal free materials for
their applications in electrocatalysis with
main emphasis on overall water splitting. Rizwan Haider
Rizwan Haider completed his
Master of Philosophy (MPhil) in
Chemistry from University of
Agriculture Faisalabad, Pakistan, in
2014. He joined School of Chemistry
and Chemical Engineering, Shanghai
Jiao Tong University in 2018 for PhD
in Chemistry under the supervision of
Prof. Xianxia Yuan with financial
support from Chinese Scholarship
Council (CSC). His current research
focuses on the synthesis of platinum
group metal free catalysts for their
electrocatalytic applications in oxygen
reduction reaction and water splitting.
Wenrui Wei
Wenrui Wei received his BS degree
from Shandong University of
Technology in 2018, and MS degree
from Ocean University of China in
2021. He is currently a PhD student
under the supervision of Prof. Xianxia
Yuan at School of Chemistry and
Chemical Engineering, Shanghai
Jiao Tong University. His research
interest focuses on efficient materials
for electrochemical energy systems
including water electrolyzer and
rechargeable batteries. Yi Wan
Yi Wan received his Master degree
from Dalian Institute of Chemical
Physics, Chinese Academy of
Sciences, in 2021. Then, he joined
School of Chemistry and Chemical
Engineering, Shanghai Jiao Tong
University, in 2021 as a PhD
student under the supervision of
Prof. Xianxia Yuan. His current
research focuses on development of
efficient electrocatalysts for oxygen
reduction reaction (ORR) and
oxygen evolution reaction (OER).
Muhammad Zahid
Dr Muhammad Zahid completed his
Master degree in Chemistry from UET
Lahore, Pakistan, in 2004, and PhD
in Physical Chemistry from the
Institute of Physical and Theoretical
Chemistry, Graz University of
Technology, Austria, in 2011. Then,
he joined Department of Chemistry,
University of Agriculture Faisalabad,
Pakistan and currently serves as an
Associate Professor there. His
research interest includes the
development of various materials for
their application in wastewater
treatment, electrocatalysis and
energy conversion and storage.
Xianxia Yuan
Dr Xianxia Yuan is a full professor in
Shanghai Jiao Tong University
(SJTU), China. She received her PhD
in Material Physics and Chemistry
from Shanghai Institute of Micro-
system and Information Techno-
logy, Chinese Academy of Sciences,
in 2002. After that she joined the
School of Chemistry and Chemical
Engineering in SJTU as an assistant
professor (2002–2004) and was pro-
moted to associate professor (2004–
2016) and full professor (2016-
present). From 2008 to 2009, Dr
Yuan worked in the Pennsylvania State University, USA. Dr Yuan’s
research interest is now focused on advanced materials for
electrochemical energy storage and conversion systems.
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The energy density of molecular hydrogen (H
2
) is 2 to 3 times
higher than gasoline (39 kW h kg
1
),
4
and hydrogen energy has
robust potential to replace the fossil fuel to overcome renew-
able energy issue.
5
In recent decades, electrocatalytic water
splitting has received considerable interest as simple and
efficient route for hydrogen production, and the water electro-
lyzers (WEs) could be classified into polymer electrolyte
membrane (PEM), alkaline and solid oxide ones.
6
Since solid
oxide electrolyzer works at temperature high up to 600–100 1C,
the practical water electrolyzers are generally operated in PEM
(acidic) or alkaline systems,
7
wherein the oxygen evolution
reaction (OER) and hydrogen evolution reaction (HER) occur-
ring at the electrodes and the overall water splitting (OWS)
reaction could be given by eqn (1)–(6).
8–10
It is observed that a
theoretical potential of 1.23 V is needed to operate water
electrolysis under both conditions with an energy input of
237.1 kJ mol
1
.
11,12
In alkaline water electrolyzer:
OER at anode:4OH!O2þ2H2Oþ4e
E
OH=O2¼0:40 V=NHE
(1)
HER at cathode:2H2Oþ2e!H2þ2OH
E
H2O=H2¼0:83 V=NHE
(2)
Overall reaction:2H2O!O2þ2H2
DE¼0:40 ð0:83Þ¼1:23 V=NHE
(3)
In acidic water electrolyzer:
OER at anode:2H2O!O2þ4Hþþ4e
E
H2O=O2¼1:23 V=NHE
(4)
HER at cathode:2Hþþ2e!H2
E
Hþ=H2¼0:00 V=NHE
(5)
Overall reaction:2H2O!O2þ2H2
DE¼1:23 0:00 ¼1:23 V=NHE
(6)
The energy conversion efficiency of overall water splitting is
usually low due to the slow reaction rates of both HER and
OER, resulting in OWS potential of higher than 1.23 V in
practical electrolyzers.
13
Hence, some efficient electrocata-
lystsarenecessarytoreducethispotentialcloseto1.23Vand
enhance the water splitting efficiency. Till now, tremendous
work has been done to fabricate cost-effective electrocatalysts
with enhanced performance.
5
For instance, metal alloys,
borides, phosphides, carbides, selenides and nitrides have
been used effectively for HER electrocatalysis,
7
and many
metal-based oxides and hydroxides are being explored and
reported as remarkable non-precious electrocatalysts for
OER.
1
However, some challenges still need to be addressed:
(a) scarcity and high cost of noble metals and unsatisfactory
electrocatalytic performance of noble metal-free electrocata-
lysts limit the bulk production of hydrogen by water electro-
lysis;
14
(b) OER is a sluggish process and high overpotential
is required to carry out this process,
15
and the most efficient
iridium and ruthenium based OER catalysts are unstable in
acidic conditions; (c) non-noble metal based catalysts may
undergo structural or chemical changes in both acidic and
alkaline conditions, leading to poor water splitting effi-
ciency;
16
(d) HER and OER catalysts are usually evaluated
under different reaction conditions which may lead to their
incompatible or abnormal integration in the same electrolyte
media; (e) catalytic mechanism of transition metal based
catalysts in alkaline medium is far less explored as compared
to acidic conditions.
16
To find out an effective solution for all the above mentioned
issues, transition metal based nanoparticles have been in spot-
light because of their promising electrocatalytic performance.
17
However, their performance is still not substantial because of
the poor stability induced by the low electrical conductivity,
corrosion, dissolution and aggregation during water electrolyzer
operation.
18
To overcome these drawbacks, it is necessary to
couple them with highly conducting and stable nanomaterials
resulting in novel hybrid materials with remarkable electrocataly-
tic efficiency and prolonged stability.
Pristine carbon nanomaterials (CNMs) like carbon nano-
tubes, graphene and fullerenes do not show remarkable activi-
ties for electrochemical water splitting, because only the
vacancy induced defects are responsible for charge transfer
among local carbon atoms.
19–23
However, non-metal hetero-
atom like N and B doping in CNMs could modulate the electron
donor–acceptor properties of carbon, improve charge transfer,
provide high density of exposed active sites and facilitate
the adsorption of reactants on active sites, and circumvent
restaking of graphene nanosheets.
24–28
Hence, hetero atom
doped CNMs like N-CNMs
20,25,26
B-CNMs
29–31
and N,B-
CNMs
32,33
with high conductivity have shown considerable
activity and stability for HER
20,26
and OER
20,26,32,33
catalysis.
In addition to the intramolecular charge transfer induced by
heteroatom doping, the intermolecular charge transfer from
CNMs to transition metals or their compounds is also
reported to uplift the catalytic activity of TM/CNMs.
34,35
Actually, the charge is transferred from CNMs to metal in
TM/CNMs hybrids during water splitting, which increase the
electronic states near to the Fermi level of metals, resulting
in improved catalytic activity of TM/CNMs hybrids as com-
pared with pristine TM based or CNMs based catalysts.
36
Moreover, the intrinsically active transition metal atoms or
their compounds could be stabilized by confinement in the
mesoporous conductive carbon network in TM/CNMs
hybrids.
34
Overall, due to the excellent electrical and thermal
conductivity and prominent structural stability, carbon
nanomaterials are regarded as the most suitable candidates
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to support transition metal based catalysts for electrocataly-
tic water-splitting process.
37,38
Specifically, carbon nanofibers (CNFs)
39–41
and carbon
nanowires (CNWs)
42,43
show excellent electron transferability
when they are combined with electrocatalysts. Carbon nano-
tubes (CNTs) can encage catalysts to promote electrocatalytic
performance and stability.
44–46
Similarly, graphene oxide
(GO)
47
and reduced graphene oxide (rGO) render more active
sites, where adsorption of intermediates takes place resulting
in improved catalytic efficiency.
35,48–50
Moreover, CNMs doped
with heteroatoms like nitrogen,
39,46,48,51
phosphorus,
52,53
sulfur,
53–55
and boron
29,56,57
could reduce catalyst corrosion.
Generally, both pure transition metal-based catalysts and
CNMs undergo with the problem of poor stability for water
splitting especially during OER, because of the high oxidizing
potential required to operate the electrolyzer. Transition
metal chalcogenides, phosphides, nitrides and oxides are
oxidized,
58,59
while the intermediates of carbon support oxida-
tion like CQO and CO
32
have been reported as a driving force
for carbon degradation.
60
In order to overcome this challenge,
the major factors, including morphology, crystallite, particle
size, oxidation state of active metal, and nature of CNMs,
that influence the stability have been extensively modified in
TM/CNMs hybrids with the strategies of metal particles/
compounds encapsulation in carbon structure,
34
energy level
modulation in metals and carbon,
36
and presence of redox couples
in metal based species to suppress the side reactions.
61–63
Overall, transition metal based electrocatalysts supported on
carbon nanomaterials for electrocatalytic water splitting have
been under the spotlight in recent years, and their performance
could be further enhanced by cooperating with heteroatom-
doped carbon materials.
In the current state of development, carbon nanomaterials
supported transition metal based electrocatalysts for water
splitting can be classified into hybrids of transition metal
based materials with pure CNMs, N-doped, B-doped and dual
N,B-doped CNMs as shown in Fig. 1 with the abbreviations
of TM/CNMs, TM/N-CNMs, TM/B-CNMs and TM/N,B-CNMs,
respectively, where some examples for HER (H-2D/3D-MoS
2
-
rGO),
49
OER (boron doped graphene oxide encaged Ni
3
N
nanoparticles,
64
and OWS (N,B : Mo
2
C@BCN
65
and CoFe@N-
C
66
) are also demonstrated. In this article, recent advances
on these catalysts integrating transition metal-based
materials with pure CNMs as well as heteroatom doped CNMs
to attain enhanced efficiency and stability is reviewed,
the effects of boron, nitrogen and dual-N,B doping on the
structure of carbon nanomaterials and their performance
towards HER and OER are emphatically discussed, and the
present challenges and future research prospects are also
proposed.
Fig. 1 Classification of carbon nanomaterial (CNMs) supported transition metal based electrocatalysts for water splitting with some examples of H-2D/
3D-MoS
2
-rGO, reproduced from ref. 49. MDPI, (Open access) 2021; boron doped graphene oxide encaged Ni
3
N nanoparticles, reproduced
with permission from ref. 64. Copyright (American Chemical Society) 2020; N,B : Mo
2
C@BCN, reproduced with permission from ref. 65. Copyright
(American Chemical Society) 2018; and CoFe@N-C, reproduced with permission from ref. 66. Copyright (Royal Society of Chemistry) 2020.
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2 Transition metal-based materials
and CNMs hybrids
Pure transition metals like cobalt, nickel, copper, iron and
molybdenum, and their alloys, oxides, sulfides, nitrides and
phosphides are intensively reported and show considerable
water splitting activities due to the abundant unpaired d-orbital
electrons, which makes them active for the electrocatalysis.
35
However, they demonstrate poor durability and low activity for
HER and OER catalysis due to their thermodynamic instability,
structure complexities and physiochemical challenges.
67,68
By combining transition metals and/or their compounds with
highly conducting and stable carbon nanomaterials as displayed
in Fig. 2, the hybrids of transition metal based materials and
CNMs (TM/CNMs) have been developed as an efficient way to
achieve improved electrocatalytic performance as compared with
both pure transition metal based materials and CNMs.
46,47,50,69
In the TM/CNMs hybrids, the unique metal–carbon (M–C) inter-
face with more active sites and exposed surface area can be
achieved to promote the reaction of adsorbed intermediates and
accomplish efficient electron transfer. In addition, agglomeration
of transition metal nanoparticles could be prevented by encapsu-
lating them in carbon nanomaterials, leading to promoted cata-
lytic activity and prolonged stability.
42,46,70–72
Some representative
TM/CNMs hybrids selected on the basis of HER, OER and OWS
efficiency are presented in Table 1.
2.1 Hydrogen evolution reaction
Some of transition metal based electrocatalysts,
73
including
nickel based materials,
74–77
cobalt nitrides, phosphides and
chalcogenides,
78–81
molybdenum sulfides,
49,82–84
molybdenum
borides,
85–87
and transition metal alloy borides, supported on
CNMs,
88
have shown activities comparable with state-of-the-art
Pt-based materials for HER. Among them, nanocrystalline
nickel has the lowest Gibbs free energy for hydrogen adsorption
(DG
H*
), and is considered the most promising transition metal
for HER catalysis.
89
NiFeO
x90
and NiSe nanowires
91
exhibit
remarkable HER performance, but such Ni based materials
are prone to oxidation, inherent corrosion and formation of
nickel hydrides in highly acidic or alkaline environment lead-
ing to reduced performance.
55,92
In addition, the active sites in
nickel alloys are less exposed due to uneven and uncontrollable
thickness.
89
In order to improve the HER activity and stability, alloying
nickel with other metals
71,93–98
and supporting transition
metal-based materials on stable and highly conducting carbon
nanomaterials
99–102
has been developed and widely explored,
while carbon supported nickel shows superior HER perfor-
mance due to the carbon material induced efficient electron
transfer, high durability and more surface area.
103,104
For
example, Oluigbo et al.
105
encapsulated Ni nanoparticles into
carbon nanotubes with a facile pyrolysis method to synthesize
Ni-CNTs. It was observed that Ni nanoparticles were encapsu-
lated at the tips of CNTs (Fig. 3a), and the average inner
diameter, total diameter and wall thickness of CNTs were 7.3,
23.8, and 8.2 nm, respectively. During a comparative study of
Ni-CNTs pyrolyzed at 600, 650 and 700 1C, the one obtained at
650 1C (Ni-CNTs-650) exhibited best electrochemical perfor-
mance with lowest overpotential of 266 mV to attain a current
density of 10 mA cm
2
in 1.0 M KOH solution for HER (Fig. 3b)
and a smallest Tafel slope of 102 mV dec
1
(Fig. 3c) suggesting
fast reaction kinetics. When compared with other Ni based
HER catalysts without carbon support, this value of Tafel
slope for Ni-CNTs-650 was also lower than that of NiMn
Fig. 2 Representation of the effects of combining transition metal based electrocatalysts with carbon nanomaterials with some examples of Fe-
Ni
12
P
5
@rGO, reproduced with permission from ref. 50. Copyright (Elsevier) 2023; Rh-nanoparticles decorated on GO, reproduced with permission from
ref. 47. Copyright (American Chemical Society) 2020; and S,N-CNTs/CoS
2
@Co, reproduced with permission from ref. 46. Copyright (Elsevier) 2018.
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(130 mV dec
1
),
106
nanocrystalline Ni (173 mV dec
1
)
107
and
many others.
105
Moreover, the obtained Ni-CNTs-650 was
stable for 10 hours as the current density remained intact at
15 mA cm
2
throughout the chronoamperometry experiment at
a potential value of 1.42 V (vs. SCE) in alkaline media as
displayed in Fig. 3d and bears no loss in HER overpotential
after 10 h chronoamperometric test (inset of Fig. 3d). The
authors attributed the small overpotential, low Tafel slope,
and excellent stability of Ni-CNTs-650 to the synergistic effect
between Ni nanoparticles and CNTs which resulted in
enhanced charge transfer rate and less agglomeration of metal
nanoparticles.
In another study on nickel based compound,
104
Balaji et al.
reported that the HER performance of Ni
3
N/Ni can be signifi-
cantly uplifted in both acidic and alkaline electrolyte by
compositing it with carbon to make Ni
3
N/Ni@C. They com-
pared the materials annealed at 550, 650, 750 and 850 1C
under N
2
atmosphere, and the optimal one obtained at 750 1C
(Ni
3
N/Ni@C-750) demonstrated the best HER performance.
At a current density of 10 mA cm
2
, its overpotential was
reduced to 163 from 200 mV in 0.5 M H
2
SO
4
and 172 from
190 mV in 1.0 M KOH solution as compared with that of
carbon free Ni
3
N/Ni. It also showed 32 (90 to 58) and 31 (94 to
63) mV dec
1
lowering in Tafel slope values in acidic and
alkaline medium, respectively. Moreover, the Ni
3
N/Ni@C-750
showed no loss in current density after 10 h operation at
overpotentials of 163 and 172 mV in 0.5 M H
2
SO
4
and 1.0 M
KOH solutions, respectively, and no deviation in overpotential
was observed after 1000 CV cycles in both electrolytes, sug-
gesting good stability and durability in both acidic and alka-
line conditions. According to the authors, the improved HER
performance of Ni
3
N/Ni@C-750 could be attributed to higher
ECSA leading to uplifted double layer capacitance (C
dl
)from
10.1to21mFcm
2
in case of acidic and 6.1 to 11 mF cm
2
in alkaline medium, and the modulation in electronic struc-
ture leading to efficient charge transfer.
Bare MoS
2
nanoparticles with various morphologies as dis-
played in Fig. 3e (top) show electrochemical activity for HER.
82
Actually, the bulk MoS
2
has graphite like structure as displayed
in Fig. 3e (bottom), resulting in diverse surface sites, and high
electron and hole mobility along the basal planes. However,
thermodynamically favorable basal plane sites present in the
bulk of 2H-MoS
2
are catalytically inert (Fig. 3f), while the edge S
sites occupying only a small region are catalytically active
towards HER.
82,83,108–111
Hence, it could be inferred that rough
MoS
2
surface would have better HER activity because of high
density of available active sites. Moreover, the activity of this
material can be improved by increasing its electrical conduc-
tivity to facilitate electron transfer and forming large electrolyte
ion adsorption/desorption sites through increasing the
number of active edges via minimizing the size, shape, and
dimensions of MoS
2
to reduce free energy for hydrogen
adsorption.
84,108,112–114
In this regard, supporting MoS
2
based
materials on highly conducting and stable carbon nanomater-
ials supports
115–120
is an efficient way to improve the electrical
conductivity, and rGO is considered one of the most attractive
CNMs owing to its high conductivity, stability, and ability to act
as backbone network for the synthesis of 3D dispersed catalytic
materials.
121–124
The better HER performance uplift of rGO, as
compared with hollow carbon nanospheres (HCNs), single
walled carbon nanotubes (SWCNTs) and multi walled carbon
nanotubes (MWCNTs), was confirmed by supporting MoS
2
on
these carbon materials using simple hydrothermal method,
118
where the HER electrocatalytic activity of as obtained MoS
2
/C
materials in 0.5 M H
2
SO
4
was reported in the order of MoS
2
/
rGO 4MoS
2
/HNCs 4MoS
2
/SWCNTs 4MoS
2
/MWCNTs.
In another example, two-dimensional MoS
2
nanosheets
were supported on three-dimensional rGO substrate to obtain
Table 1 Performance comparison of some recent TM/CNMs hybrids for electrocatalytic water splitting
Electrocatalyst Electrocatalytic process Electrolyte Overpotential
(mV at 10 mA cm
2
)Tafel slope
(mV dec
1
) Stability Ref.
H-2D/3D-MoS
2
-rGO HER 0.5 M H
2
SO
4
286 77 40 h at 10 mA cm
2
49
Ni-CNTs-650 HER 1.0 M KOH 266 102 10 h at 0.351 V 105
Ni
3
N/Ni@C-750 HER 0.5 M H
2
SO
4
163 58 10 h at 0.163 V 104
1.0 M KOH 172 63 10 h at 0.172 V
Co-50Ni-B/CC HER 1.0 M KOH 80 88 36 h at 0.33 V 88
Ni
2
P/rGO OER 1.0 M KOH 221 106 12 h at 1.60 V 6
Fe
2
O
3
-CNT OER 1.0 M KOH 201 134 20 h at 1.47 V 145
NiRu
0.3
Se/rGO OER 0.1 M KOH 290 98 10 h at 1.53 V 149
Co
0.9
Fe
0.1
-CNF OER 0.1 M KOH 496 83 250 h at 20 mA cm
2
138
Fe-Ni
12
P
5
/rGO HER 1.0 M KOH 81 76 2000 CV cycles 50
OER 230 87 100 h at 1.5 V
OWS (1.57 V/10 mA cm
2
) — — 12 h at 1.7 V
NiVB/rGO HER 0.5 M H
2
SO
4
146 115 — 158
1.0 M KOH 151 88 12 h at 0.151 V
1.0 M PBS — — —
OER 0.05 M H
2
SO
4
353 77 —
1.0 M KOH 267 44 12 h at 1.497 V
1.0 M PBS 489 95 —
OWS (1.46 V/10 mA cm
2
) 1.0 M KOH — — —
mCo
0.5
Fe
0.5
P/rGO HER 1.0 M KOH 190 54 25 h at 10 mA cm
2
169
OER 250 42 18 h at 10 mA cm
2
OWS (1.66 V/10 mA cm
2
) — — 25 h at 1.66 V
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H-2D/3D-MoS
2
-rGO aerogels by hydrothermal method and
evaluated for their HER performance in 0.5 M H
2
SO
4
solution.
49
The results depicted a small overpotential of
286 mV for H-2D/3D-MoS
2
-rGO to attain a current density of
10 mA cm
2
, compared to that of more than 500 mV for pure
MoS
2
, and the Tafel slope of 77 mV dec
1
for H-2D/3D-MoS
2
-
rGO was much smaller than that of 125 mV dec
1
for bare MoS
2
nanosheets (Fig. 3g), indicating lower polarization and faster
HER kinetics of H-2D/3D-MoS
2
-rGO. The enhanced activity of
H-2D/3D-MoS
2
-rGO can be attributed to the increased number
of active sites and enhanced electrical conductivity due to the
integration of 3D rGO with 2D MoS
2
, which finally resulted in
efficient adsorption of reaction intermediates and fast electron
transfer kinetics for HER. Moreover, stability of the as-prepared
H-2D/3D-MoS
2
-rGO material was pretty good as there was
only slight increase in overpotential (7 mV) after 20 hours at
a current density of 10 mA cm
2
(Fig. 3h) and no further
potential loss was observed after 40 h operation in 0.5 M
H
2
SO
4
. In the authors’ opinion, the excellent stability of
H-2D/3D-MoS
2
-rGO was due to the chemical stability of rGO
in harsh conditions which protected the metal core from
agglomeration.
49
Metal borides were not studied much as water splitting
electrocatalysts until the synthesis of ultrathin Mo
3
B films as
displayed in Fig. 3i(i), which acted as an efficient HER electro-
catalyst in acidic medium.
87
The ultrathin Mo
3
B bears rhombo-
hedral geometry with hexagonal structure and space group of P6
3
/
mmc (R%
3m) (Fig. 3j). In this structure, boron and molybdenum
atoms are located at 2c and 2b sites, respectively (Fig. 3i(ii)), a unit
cell of Mo
3
B contains 14 atoms (Fig. 3i(iii)), and the clearly
exposed Mo and B surface sites could be observed in the side
view of Mo
3
B structure in bc projection (Fig. 3i(iv)). The calculated
free energies of hydrogen adsorption on Mo
3
Bsurfacestermi-
nated with Mo and Mo–B are 0.30 and 0.22 eV, respectively
(Fig. 3k), which are close to the Fermi level (0.56 eV) of Mo foil
and very close to the free energy of hydrogen adsorption on Pt
(0.13 eV). These results indicate that Mo
3
B structure with highly
exposed boron atoms should have excellent performance for HER.
Wang et al.
87
prepared Mo
3
B nanofilms with an average
thickness of 6.48 nm by chemical vapor deposition (CVD)
Fig. 3 (a) TEM image of Ni-CNTs-650. (b) HER polarization curves and (c) corresponding Tafel slopes of CNTs encapsulated Ni nanoparticles obtained at
different temperatures and Pt/C in 1.0 M KOH. (d) Chronoamperometric study (inset polarization curves before and after 10 h chronoamperometric
study) for the Ni-CNTs-650. Reproduced with permission from ref. 105. Copyright (Elsevier) 2019. (e) A cartoon of different types of bare MoS
2
nanoparticles showing HER activity (top) and top view of the Mo and S edges of a bulk MoS
2
crystal (bottom). (f) Two-dimensional representation of MoS
2
catalyst with electrochemically active surface area and projected geometric surface area. Reproduced with permission from ref. 82. Copyright (American
Chemical Society) 2014. (g) Tafel plots for H-2D/3D-MoS
2
-rGO, pure MoS
2
, and Pt/C in 0.5 M H
2
SO
4
solution. (h) Comparison of LSV curves for H-2D/
3D-MoS
2
-rGO before and after electrochemical stability tests. Reproduced from ref. 49. MDPI, (Open access) 2021. (i) (i) SEM images of the Mo foil
(below) and ultrathin Mo
3
B films on the Mo foil (top), (ii) top view of the ultrathin Mo
3
B film, (iii) structure of the basic unit cell of the thin film shown in the
ac projection and (iv) side view of the ultrathin Mo
3
B film. Unit cell vectors are shown in (ii–iv). (j) High-resolution TEM image of Mo
3
B film. (k) Free energy
evolution of H (vs. SHE) during HER catalyzed by various materials. Reproduced with permission from ref. 87. Copyright (Royal Society of Chemistry) 2017.
(l) H adsorption site on Pt {111}, Mo {110}, Mo-terminated MoB
2
and B-terminated MoB
2
{001} surfaces. Reproduced with permission from ref. 86.
Copyright (Royal Society of Chemistry) 2017.
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method using Mo foil as substrate under H
2
atmosphere and
different temperature of 700, 800, 900 and 1000 1C for 1 h. The
HER performance of as-prepared Mo
3
B materials and Mo foil
was comparatively evaluated in 0.5 M H
2
SO
4
solution and it was
observed that the Mo
3
B prepared at 900 1C bears the lowest
overpotential of 249 mV at a current density of 20 mA cm
2
,
compared to the values of 334, 305 and 391 mV for the Mo
3
B
prepared at 700, 800 and 1000 1C, respectively, while the
current density of Mo foil couldn’t reach 5 mA cm
2
at an
overpotential of 350 mV. The Tafel slope of 52 mV dec
1
for
Mo
3
B prepared at 900 1C was also evidently lower than that of
64, 72 and 94 mV dec
1
for Mo
3
B synthesized at 700, 800 and
1000 1C and 146 mV dec
1
for Mo foil, respectively. This
material also showed excellent durability with only 10 mV
increase in overpotential at current density of 20 mA cm
2
after 2500 CV cycles in voltage range of 0 to 0.6 V (vs. RHE)
and at scan rate of 50 mV s
1
. The excellent performance and
durability of Mo
3
B prepared at 900 1C was attributed to the
highly exposed Mo and B sites and crystalline metal like
structure providing high electrical conductivity, while the
Mo
3
B films prepared at 700, 800 and 1000 1C had amorphous
structure which limits the electron transport along the surface of
thin films and ultimately suppresses HER catalytic performance.
Jothi et al.,
86
comparatively studied the active sites present
in MoB
2
nanoparticles, including Mo-terminated MoB
2
(001)
and B-terminated MoB
2
(001), and Mo (110) along with Pt (111)
active sites in Pt/C, for hydrogen adsorption energy, where
three type of sites including top of boron or metal atom (T),
boron–boron or metal–metal bridge site (Bg) and hollow (Ho)
sites were considered as shown in Fig. 3l. It was revealed that
the Gibbs free energy (DG
H
) for Ho Pt (111) surface is 0.13 eV
at 25% hydrogen coverage, which increases with the increase in
hydrogen coverage and eventually becomes zero between 70 to
100% hydrogen coverage. On overall comparison of all other
active sites present on MoB
2
, it was concluded that B–B bridged
site present on the B-terminated MoB
2
has similar structure to
graphene and is the most active site, exhibiting zero DG
H
at
hydrogen coverage of 70–100% which is similar to that on Pt
(111) site. The authors prepared crystalline MoB
2
nanoparticles
with an average size of 50 nm with a redox assisted solid state
metathesis reaction at 650 1C, and compared the HER perfor-
mance in 0.5 M H
2
SO
4
solution with commercial 5% Pt/C,
molybdenum metal (Mo), amorphous boron (B) and carbon
sheets (CS). It was revealed that an overpotential of 154 mV was
required for MoB
2
to attain a current density of 10 mA cm
2
,
which was less than 25% of that for Mo metal (633 mV) but
higher than the value of 62 for 5% Pt/C, while CS and B only
showed negligible active for HER. Moreover, the overpotential
of as-prepared nanocrystalline MoB
2
is 150 mV lower than that
of bulk MoB
2
recorded at similar conditions.
85
Its Tafel slope
(49 mV dec
1
) was close to 5% Pt/C (37 mV dec
1
) and strongly
lower than that of 187 mV dec
1
for Mo metal, indicating swift
reaction kinetics of MoB
2
compared with pure metallic Mo.
This material showed excellent durability with only 7% loss in
current density at an overpotential of 200 mV after 1000 CV cycles.
With these examples, both Mo
3
BandMoB
2
display good HER
activity in acidic solution, but their performance is still lower than
state-of-the-art Pt based catalysts.
According to the best of our knowledge, no literature has
reported the synthesis and HER activity of Mo
3
B or MoB
2
material supported on CNMs, which might be a valuable future
research direction for high efficiency HER catalysts develop-
ment. But on the other hand, some other transition metal
borides hybridized with CNMs have been explored. Sheng
et al.
88
prepared highly amorphous and porous Ni-Co–B
supported on carbon cloth (CC) with varying Ni content of 17,
33, 50 and 67%, and revealed that the one with a 50% Ni
content (Co-50Ni-B/CC) demonstrates the best HER perfor-
mance in 1.0 M KOH solution with an overpotential of 80 mV
to achieve a current density of 10 mA cm
2
, which was only one
fifth of the unsupported Co-50Ni-B powders with an overpoten-
tial of about 400 mV and obviously lower than that of Co–B/CC
and Ni-B/CC prepared with similar procedure. Its Tafel slope
(88.2 mV dec
1
) was also evidently lower than that of Co-50Ni-B
powders (108.9 mV dec
1
), indicating the faster kinetics. The
excellent stability of Co-50Ni-B/CC was demonstrated by a long-
term chronoamperometric operation at an overpotential of
330 mV with negligible change in current density, along with
minute change (2 mV) in overpotential after 1000 CV cycles.
These results reflect the positive effects of CC support on HER
performance of transition metal borides and superior performance
of binary metal borides as compared with single metal ones.
Overall, different types of CNM supports could be employed
to uplift the HER activity and stability of transition metal-based
catalysts, because they can provide enhanced surface area
leading to more active sites, fast electron transfer kinetics
due to the inherent high electrical conductivity, and high
durability by shielding metal nanoparticles. Careful compari-
son finds that rGO as support can be introduced by using
simple one pot hydro or solvothermal process to give enhanced
performance, while metal particle encapsulation in CNTs gen-
erally requires high temperature pyrolysis
55,105,125
or complex
multistep synthesis routes.
126,127
Overall, rGO supported tran-
sition metal based electrocatalysts have significant potential to
be directly employed in wide commercial level applications of
water splitting devices.
2.2 Oxygen evolution reaction
In contrast to HER, OER is a kinetically sluggish process
involving four-electron transfer which requires high overpoten-
tial to attain the ideal current density.
128–130
In search of a
durable, efficient and easy to synthesize OER catalyst, NiFe-,
131–135
NiS-,
136
NiCo-,
137
CoFe-,
138–140
and CuCo-
141
based materials,
transition metal alloy borides
142–144
and iron oxides
145
have been
extensively evaluated. All these catalysts show significant OER
performance, and their electrochemical capability could be effec-
tively enhanced by supporting on CNMs.
Xu et al.
145
synthesized CNT supported Fe
2
O
3
(Fe
2
O
3
-CNT)
by in situ sprouting of Fe
2
O
3
nanoparticles on SWCNTs using
CVD method followed by annealing in open air at 280 1C, and
studied the effects of carbon support on OER performance in
1.0 M KOH solution. It was observed that in the presence of
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SWCNTs, the OER overpotential was significantly reduced to
201 mV from the reported value of 302 mV for bare Fe
2
O
3
.
146
However, Fe
2
O
3
-CNT undergoes slower reaction kinetics
according to the Tafel slope value of 134 mV dec
1
, which is
almost 2.5 times higher than pristine Fe
2
O
3
(51.8 mV dec
1
), in
which the (012) facet of Fe
2
O
3
was found to be OER active. The
authors presented that the sluggish reaction kinetics of Fe
2
O
3
-
CNT might be associated with the covering of some Fe
2
O
3
(012)
facets by carbon. However, it is worth noticing that during
the annealing process, SWCNTs were corroded due to their
oxidation by iron to make Fe
2
O
3
-CNT. This type of defective
carbon support enhanced the electron transfer from metal to
carbon with the creation of positive charge center after OER,
which is beneficial for OER as evident from the lower over-
potential (201 mV) of Fe
2
O
3
-CNT to achieve current density of
10 mA cm
2
, as compared with IrO
2
(B270 mV) and pristine
CNTs (B370 mV). It justifies that introduction of CNTs could
reduce the OER overpotential of Fe
2
O
3
to considerable extent.
Moreover, the stability of as-prepared Fe
2
O
3
-CNT was also
evaluated at a constant potential of 1.47 V (vs. RHE) in 1.0 M
KOH, and a stable current density of 20 mA cm
2
was observed
for 20 h. Although the oxidized intermediates of carbon support
like CQO and CO
32
have been reported as a driving force for
carbon degradation,
60
the addition of CNTs has exhibited little
effect on inherent stability of Fe
2
O
3
in 1.0 M KOH, which was
reported as only 14 mV increase in overpotential after 30 h
operation at 20 mA cm
2
.
146
This might be attributed to the
strong interaction between Fe
2
O
3
nanoparticles and CNTs,
since it was found from the post mortem analysis of catalysts
that the electronic coupling between Fe
2
O
3
nanoparticles and
corroded CNTs increases because of the self-optimization of
SWCNTs and electron movement from Fe
2
O
3
nanoparticles to
CNTs, resulting in long term stability and excellent performance
of as-prepared Fe
2
O
3
-CNT catalyst.
Supporting transition metal-based catalysts on rGO can also
uplift their OER activity and performance. Luo et al.
6
fabricated
rGO supported Ni
2
P nanoparticles by phosphating Ni(OH)
2
/
rGO hybrid synthesized by facile hydrothermal method, and
evaluated its OER activity in 1.0 M KOH solution. An over-
potential of 221 mV was demonstrated to attain a current
density of 10 mA cm
2
, which was obviously lower than that
of 269 mV for the counterpart of bare Ni
2
P nanoparticles, and
it is even lower than the state-of-the-art IrO
2
which has the
overpotential of 281 mV under similar conditions. For the
reaction kinetics, Ni
2
P/rGO hybrid showed a Tafel slope of
105.7 mV dec
1
, compared to the high value of 146.3 mV dec
1
for bare Ni
2
P nanoparticles. Moreover, the stability of Ni
2
P/rGO
hybrid was determined with chronoamperometry at a potential
of 1.60 V (vs. RHE), and a performance retention of 99.7% was
revealed after 12 hours. The excellent performance of Ni
2
P/rGO
hybrid was attributed to the synergistic effect of rGO support on
Ni
2
P nanoparticles resulting in increased electrical conductivity,
faster electron transfer, and prolonged stability to facilitate the
overall electrochemical process as compared to bare Ni
2
P.
Along with supporting transition metal-based catalysts on
CNMs, the introduction of hetero non-noble metal atom to
TM/CNMs hybrid catalysts is a good strategy for uplifting the
OER performance during water splitting. For instance, intro-
duction of selenium has been proved effective to significantly
uplift the OER performance of Ni-based catalyst in alkaline
media. Nickel selenide alone exhibits good OER perfor-
mance with an overpotential of 300 mV and a Tafel slope of
71 mV dec
1
to achieve a current density of 10 mA cm
2
in
1.0 M KOH solution.
147
This performance is far better than the
overpotential of bulk nickel foam (460 mV) with a high Tafel
slope of 189.6 mV dec
1 148
at same operating conditions.
With the introduction of CNMs as support and some ternary
heterometal atoms, the OER electrocatalytic performance of
NiSe could be further uplifted. Mehmood et al.
149
supported
NiSe on rGO along with introduction of Ru using hydrothermal
method, and studied the dependence of OER performance in
0.1 M KOH solution on varying ratios of Ru. It was revealed that
NiRu
0.3
Se/rGO exhibits better OER activity than NiRu
0.1
Se/rGO,
NiRu
0.2
Se/rGO and NiSe/rGO (Fig. 4a). It demonstrates an
overpotential of 290 mV to attain the current density of
10 mA cm
2
, while NiRu
0.1
Se/rGO, NiRu
0.2
Se/rGO and NiSe/rGO
render the overpotentials at 340, 360 and 460 mV, respectively,
at the same current density. Moreover, the Tafel slope of
98 mV dec
1
for NiRu
0.3
Se/rGO is greatly lower than those of
198, 246, and 257 mV dec
1
for NiRu
0.2
Se/rGO, NiRu
0.1
Se/rGO
and NiSe/rGO (Fig. 4b), respectively, and the EIS plot displayed
in Fig. 4c evidences the lowest charge transfer resistance and
fastest mass diffusion in the case of NiRu
0.3
Se/rGO. On a carful
comparison of these results, it is noted that the overpotentials
of NiRu
0.3
Se/rGO and NiSe/rGO in 0.1 M KOH are only 10 mV
lower and 160 mV higher, respectively, as compared with that of
NiSe (300 mV) in 1.0 M KOH solution, and the Tafel slopes of
98 and 257 mV dec
1
for NiRu
0.3
Se/rGO and NiSe/rGO,
respectively, in 0.1 M KOH solution are much higher than the
value of 71 mV dec
1
for NiSe catalyst in 1 M KOH solution.
This might be associated with the low concentration of
electrolyte in which oxygen is about 6 times more soluble as
compared with 1.0 M KOH, leading towards high resistance in
0.1 M KOH solution,
150
and it indicates that although structural
modification is essential and can uplift OER performance of
catalysts, the electrolyte concentration plays significant role as
well. At the overpotential of 300 mV, chronoamperometric
experiments revealed the good stability of NiRu
0.3
Se/rGO for
10 h, during which the slight decrease in current density was
attributed to the oxidation of metal selenides to hydroxides due
to anodic polarization under alkaline media.
151
The superior
OER performance of NiRu
0.3
Se/rGO was attributed to synergis-
tic effect between metal nanoparticles and rGO substrate which
facilitates OER activity by not only promoting the electron
transfer but also providing additional active sites as compared
to the pure nickel selenide.
The proposed OER mechanism in the presence of NiRu
0.3
Se/
rGO is depicted diagrammatically in Fig. 4d. During this
process, the reaction starts on the surface of black encircled
catalyst, in which M represents Ni and Ru particles (shown in
brown) embedded on Se clusters. In the initial steps, MOH and
MO intermediates are formed successively, followed by the
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formation of MOOH species, which is considered as the rate
determining step. Actually, the high oxidation state metal
cations (Ni
III/IV
/Ru
III/IV
) involved in the formation of M–O
species are active sites for OER.
152,153
Hence, it could be
postulated that the formation of M–O bond has important role
on the overall efficiency of OER catalysts, as it can affect the
formation of MOOH species as a result of nucleophilic attack
between MO and OH
ions. While in the above work on
NiRu
x
Se/rGO,
149
the presence of multicomponent (Se, Ru and
Ni) uplifts the formation of MOOH species leading to improved
OER performance.
The oxophilicity control of transition metal based catalysts
combined with highly graphitized carbon support has been
presented as another efficient method for the preparation of
highly durable OER catalysts.
138,154–156
For example, corrosion
resistant carbon nanofiber coated Co
0.9
Fe
0.1
alloy nanoparticles
(Co
0.9
Fe
0.1
-CNF) were synthesized by electrospinning followed
by carbonization at 1000 1C.
138
Along with the presence of
highly graphitized CNF obtained by metal iron content tuning
in Co
1x
Fe
x
-CNF, the existence of 10% Fe in CoFe alloy mod-
ulates the oxophilicity of Co metal to facilitate OH
adsorption
on Co
3+
ions, resulting in better stability and low OER over-
potential. The as-prepared Co
0.9
Fe
0.1
-CNF showed excellent
stability for 290 h with consecutive current densities from
20 (for 250 h) to 50 (for 40 h) mA cm
2
, and the current density
reached a high value of 794 mA cm
2
at 1.7 V, which is better
than most of the reported transition metal or Ir-based catalysts.
The lower Tafel slope value of 83.1 mV dec
1
for Co
0.9
Fe
0.1
-CNF,
as compared with 115 mV dec
1
for Co-CNF, indicates the facile
charge transfer in Co
0.9
Fe
0.1
-CNF due to the modulation of
charge transfer coefficient (a
a
) from 0.5 to 0.71.
From the above discussion, it might be figured out that OER
activity and durability of transition metal-based catalysts could
be uplifted by introduction of hetero metal atoms to tune the
oxophilicity of transition metals, and supporting them on
highly graphitized CNMs. These modifications could result in
electronic structural regulation, more exposure of active sites
and increase in number of active sites for OER. They can also
promote electron transfer kinetics and reduce corrosion/
oxidation of metal nanoparticles to uplift the activity as well
as durability of transition metal based OER catalysts. However,
there is still much room in fabricating cost-effective, stable, and
efficient OER electrocatalysts by the proper selection of metal
composites and highly stable and conducting carbon nano-
materials as supports.
2.3 Overall water splitting
In addition to their applications as single HER or OER electro-
catalyst, transition metal-based materials supported on CNMs
are also being studied as bifunctional catalysts for overall water
Fig. 4 (a) OER polarization curves, (b) Tafel plots and (c) EIS spectra of NiRu
x
Se/rGO nanocomposites in 0.1 M KOH. (d) OER mechanism of NiRu
0.3
Se/
rGO electrocatalyst. Reproduced with permission from ref. 149. Copyright (European Chemical Societies Publishing) 2021. (e) Synthesis of NiVB
nanoparticles supported on rGO substrate by chemical reduction method. (f) LSV curves and (g) Tafel slopes for OER, and (h) LSV curves and (i) Tafel
slopes for HER of NiVB with different Ni/V ratios, RuO
2
and NiVB/rGO heterostructure in 1.0 M KOH, and (j) the polarization curve for overall water
splitting of NiVB/rGO as catalyst on both electrodes. Reproduced with permission from ref. 158. Copyright (Elsevier) 2021. (k) Polarization curve of
mCo
0.5
Fe
0.5
P/rGO||mCo
0.5
Fe
0.5
P/rGO cell for overall water splitting in 1.0 M KOH (inset: i–tcurve at a voltage of 1.66 V). Reproduced with permission
from ref. 169. Copyright (Elsevier) 2022.
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splitting. Generally, alkaline water electrolyzer offers wide
range of electrolytes for non-noble metal catalysts, but the
HER activity in alkaline system is 2–3 times lower than in
PEM electrolyzer which needs acidic media to operate.
157
In
addition, PEM electrolyzer offers advantages of high proton
conductivity and low gas permeability resulting in swift hydro-
gen production and high energy efficiency, but the OER process
in PEM electrolyzer requires noble metal (Pt) or noble metal
oxide (IrO
2
, RuO
2
) catalysts for efficient performance resulting
in high cost.
16
Therefore, it is a challenge to fabricate an
economical non-noble metal based electrocatalyst which can
efficiently catalyze overall water splitting in both alkaline and
acidic conditions. To achieve this target, many researchers have
focused to develop different types of transition metal-based
materials and CNMs hybrids (TM/CNMs), including transition
metals and their alloys, transition metal borides (TMBs) and
phosphides (TMPs) supported on CNMs.
Among these catalysts, TMBs have emerged as an excellent
candidate to electrolyze water in both acidic and alkaline
medium. However, according to the best of authors’ knowl-
edge, only few TMBs have been reported as OWS catalysts in
acidic or universal pH, while most of them are demonstrated
active in alkaline medium or have not been evaluated in acidic
medium. The TMBs active in universal pH range as OWS
catalysts include only NiVB
158
and NiMoB,
159
While that active
in alkaline medium include Co
1
-Fe
1
-B-P,
160
Co–B@CoO/Ti,
161
and Ni-B-O@Ni
3
B.
162
The efficient catalytic activity of TMBs
comes from the back transfer of electrons from boron to metals
which results in increased electron density at the catalytic sites
promoting water-splitting process.
163–165
Based on these dis-
coveries, much attention has been focused to uplift the OWS
performance of TMBs by integrating them with carbon nano-
materials and other transition metal heteroatoms.
Arif et. al.
158
combined NiVB with reduced graphene oxide
using chemical reduction approach and synthesized NiVB/rGO
with different Ni/V ratios of 2 : 1, 4 : 1, 6 : 1 and 8 : 1. In the
as-prepared NiVB/rGO materials, rGO provides a platform for
the uniform in situ growth of NiVB particles and distribution of
active sites on ultrathin nanosheets as displayed in Fig. 4e. It is
clear from Fig. 4f that the catalyst with an optimal Ni/V ratio of
4 : 1, entitled as NiVB/rGO, depicted the lowest OER overpoten-
tial of 267 mV to attain a current density of 10 mA cm
2
in
1.0 M KOH, while the NiVB materials having Ni/V ratio of 2 : 1,
4 : 1, 6 : 1, and 8 : 1 without rGO and RuO
2
exhibited evidently
higher overpotentials of 335, 302, 313, 318 and 275 mV,
respectively. It should be noted that the overpotential of NiVB
with a Ni/V ratio of 4 : 1 was shifted 35 mV towards negative side
by introduction of rGO, indicating the obvious effect of CNMs
in alkaline medium. The obtained NiVB/rGO not only showed
better performance than pristine NiVB, but also possess 8 mV
more negative overpotential than state-of-the-art RuO
2
electro-
catalyst. Moreover, the superior OER activity of NiVB/rGO
was also justified with its small Tafel slope of 44 mV dec
1
,
which was much lower than that of RuO
2
(72 mV dec
1
)
suggesting faster OER reaction kinetics in alkaline solution as
displayed in Fig. 4g.
Similarly, NiVB/rGO was also evaluated for its OER perfor-
mance under acidic (0.05 M H
2
SO
4
) and neutral (1.0 M PBS
buffer) pH bearing overpotentials of 353 mV and 489 mV,
respectively. Although these overpotential values are higher
than that in alkaline conditions, they are still lower than NiVB
without rGO in both acidic and neutral pH. Also, an obvious
difference of 15 (77 vs. 92) and 38 (95 vs. 133) mV dec
1
was
observed in acidic and neutral pH, respectively, for NiVB/rGO
and pristine NiVB (4 : 1) catalysts. With these results, it is clear
that OER activity of NiVB could be significantly improved with
the introduction of rGO and the as-obtained NiVB/rGO exhibits
remarkable OER activity in all pH range.
Concurrently, the NiVB with a Ni/V ratio of 4 : 1 also demon-
strated better HER performance than the other NiVB materials
having Ni/V ratio of 2 : 1, 6 : 1, and 8 : 1 in 1.0 M KOH, and
its performance could be further improved by combing with
rGO (Fig. 4h). In 1.0 M KOH solution, the overpotential of
as-obtained NiVB/rGO was 45 (151 vs. 196) mV lower than its
pristine counterpart of NiVB with a Ni/V of 4 : 1, and its Tafel
slope of 88 mV dec
1
was lower than all of the NiVB materials
with various Ni/V ratios and 63 mV dec
1
lower than that of
Pt/C (151.0 mV dec
1
) as depicted in Fig. 4i. It was also active
towards HER in 0.5 and 0.1 M KOH with overpotentials of
268 and 315 mV and Tafel slopes of 113 and 128 mV dec
1
.
In acidic media of 0.5 and 0.05M H
2
SO
4
solution, the HER
overpotentials of 146 and 278 mV were recorded, respectively,
to achieve a current density of 10 mA cm
2
, compared to the
values of 230 and 365 mV for pristine NiVB (4 :1), and the Tafel
slopes of 115 and 117 mV dec
1
were 22 and 27 mV dec
1
lower,
respectively. So, it is similar to the case of OER that NiVB/rGO is
active towards HER process in universal pH range.
Chronoamperometric experiment was employed for 12 h
continuous operation in 1.0 M KOH solution for both OER
and HER using NiVB/rGO catalyst, which showed negligible
change in current density in both cases. The overall water
splitting performance of NiVB/rGO was evaluated in 1.0 M
KOH solution with a device fabricated using NiVB/rGO as
catalyst on both anode and cathode, and the current densities
of 10 and 100 mA cm
2
were achieved at 1.46 and 1.76 V,
respectively (Fig. 4j). According to the authors, the overall water
splitting performance of NiVB/rGO was better than many other
bifunctional transition metal based catalysts, and also superior
to the Pt/C||RuO
2
device which requires 1.53
166,167
or 1.594 V
168
to achieve 10 mA cm
2
.
Typical examples of TMPs/CNMs for OWS include meso-
porous CoFeP nanoparticles anchored on reduced graphene
oxide (mCo
0.5
Fe
0.5
P/rGO),
169
NiFeP supported on amorphous
carbon matrix (NiFeP@C),
167
and nickel phosphide (Ni-P) sup-
ported on carbon fiber paper (CP) (CP@Ni-P).
170
In detail,
Huang et al.
169
fabricated mCo
0.5
Fe
0.5
P/rGO with nano casting
method and compared its overall water splitting performance
with Co
0.5
Fe
0.5
P/rGO without mesopores and mCo
0.5
Fe
0.5
P
nanoparticles without rGO substrate in 1.0 M KOH solution.
To achieve a current density of 10 mA cm
2
, an OER over-
potential of 250 mV for mCo
0.5
Fe
0.5
P/rGO was recorded, which
was much lower than the values of 350 and 297 mV for
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Co
0.5
Fe
0.5
P/rGO and mCo
0.5
Fe
0.5
P, respectively. The faster OER
reaction kinetics of mCo
0.5
Fe
0.5
P/rGO was justified with the
lower Tafel slope of 42.4 mV dec
1
as compared with that of
64.57 and 56.83 mV dec
1
for Co
0.5
Fe
0.5
P/rGO and mCo
0.5
Fe
0.5
P
nanoparticles, respectively. Moreover, the chronopotentiometry
operation for 18 h at a current density of 10 mA cm
2
displayed
no significant change in OER potential, confirming the excel-
lent stability of mCo
0.5
Fe
0.5
P/rGO catalyst.
For HER process, a lower overpotential of 190 mV was
observed to attain a current density of 10 mA cm
2
, as com-
pared with the values of 258 and 250 mV for Co
0.5
Fe
0.5
P/rGO
hybrid and bare mCo
0.5
Fe
0.5
P nanoparticles, respectively. The
better HER performance of mCo
0.5
Fe
0.5
P/rGO was also justified
with its lower Tafel slope of 53.8 mV dec
1
, as compared
with that of 75.3 and 69.4 mV dec
1
for Co
0.5
Fe
0.5
P/rGO and
mCo
0.5
Fe
0.5
P, respectively. Moreover, chronopotentiometry
experiment at 10 mA cm
2
showed no change in HER potential
after 25 hours continuous operation, suggesting excellent long-
term stability of mCo
0.5
Fe
0.5
P/rGO hybrids.
The overall water splitting performance of a two electrode
mCo
0.5
Fe
0.5
P/rGO||mCo
0.5
Fe
0.5
P/rGO device was evaluated in
1.0 M KOH and a potential of 1.66 V was required to reach
10 mA cm
2
(Fig. 4k). Moreover, during continuous operation
at 1.66 V for 25 h, only slight decrease in current density
was observed at the beginning and a stable current density of
10 mA cm
2
was then retained (inset in Fig. 4k), indicating
good stability of the device as well as the mCo
0.5
Fe
0.5
P/rGO
catalyst. The superior overall water splitting activity and stabi-
lity of mCo
0.5
Fe
0.5
P/rGO might be attributed to the introduction
of rGO which is responsible for increased electrical conductivity
and chemical stability, and the availability of more active
sites due to mesoporous structure which is responsible for
the efficient adsorption of reaction intermediates leading to an
enhanced electrocatalytic process.
On the whole, TM/CNMs hybrids are far less explored for
their OER and HER performance/mechanism in both alkaline
and acidic environment, and TMBs/CNMs and TMPs/CNMs are
two emerging classes of TM based OWS catalysts. However,
both of them lack intensive experimental work especially in
acidic electrolyte, and TMBs/CNMs are much more efficient in
universal pH because of the presence of Pt like active sites and
boron which can accept electrons more easily than P in metal
phosphides to produce positive charge centers beneficial for
sluggish OER reaction. rGO has been approved effective to
enhance the activity, reaction kinetics and stability of TMBs
and TMPs towards both OER and HER because of the intrinsic
high electrical conductivity and porous structure, but CNTs
as an important and representative member of CNMs is far
less explored as carbon support probably due to the difficult
synthesis.
3. Transition metal-based materials
and nitrogen doped-CNM hybrids
CNMs like rGO and CNTs as support can elevate the activity and
durability of transition metal based HER, OER and overall
water splitting catalysts up to a considerable level, but the
performance is still unsatisfactory, and it could be further
enhanced by non-metal hetero atom doping like nitrogen and
boron. N-doped CNMs differ significantly in their physical
and chemical properties from undoped CNMs, and are being
evaluated extensively as electrocatalyst support owing to the
remarkable activity, long-term stability, cost-effectiveness, and
high conductivity.
171
Specifically, the effects of N-doping on TM/CNMs hybrids
are summarized in Fig. 5. The presence of nitrogen could
reduce the number of oxygen groups present at graphene
surface resulting in reduced corrosion,
172
and induce activation
region in graphene because of the difference in electronegativity
between nitrogen and carbon.
173
In addition, nitrogen doping
creates structural defects on graphene surface which could
Fig. 5 Effects of N-doping on TM/CNMs hybrids as water splitting catalysts.
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promote the formation of active sites leading to enhanced electro-
catalytic activity.
174
The morphological and structural integrity of
TM/N-CNMs hybrids could be retained because the detachment,
agglomeration, corrosion and structural collapse of catalyst nano-
particles are prevented in the presence of nitrogen-doped gra-
phene supports,
175
which facilitates mass and electron transfer
leading to enhanced interactions between electrocatalysts and
alkaline electrolyte, and increases electron density on the surface
of CNMs resulting in the better adsorption of reaction inter-
mediates during the reactions.
176
Moreover, nitrogen doping
decreases the binding energy between intermediate species and
electrocatalyst surface enabling more efficient electrocatalytic
performance.
177
Some representative TM/N-CNMs hybrids
selected on the basis of HER, OER and OWS efficiency are
presented in Table 2.
3.1 Hydrogen evolution reaction
With the merits of natural abundance, cost-effectiveness, and
remarkable catalytic potential induced by the unique electronic
structure, 3d transition metals and their alloys are considered
to be promising candidates for replacing noble metal-based
HER electrocatalysts.
178,179
However, their application in HER
is limited because of the strong binding interactions with
reaction intermediates leading to poor desorption of adsorbed
H
+
-ions to produce H
2
.
180
Moreover, their low stability under
harsh conditions is another challenge which needs to be
addressed.
181
Actually, nature of transition metals plays important role on
the performance of TM/N-CNMs catalysts. Morozan et al.
182
compared Co, Fe, Ni, Cr, Mn, W, Mo, Cu and Zn–N–C catalysts
for their HER activity and it was revealed that Co–N–C, Ni–N–C
and Fe–N–C are the most active in alkaline (pH = 13)
environment, while Co–N–C and Cr–N–C were found to have
the highest HER activity in acidic (pH = 1) medium. After that, a
huge number of researches are focused on the development of
Co based N-CNMs hybrids because of their high HER activity in
universal pH. Some examples of Co/N-CNMs hybrids include
Co@N-CNT,
183
Co–N–C,
184
Co-g-C
3
N
4
/rGO,
185
Co@NC/NG,
186
Co@NC/Ti,
187
and CoP/NCNWs.
42
Besides, Ni with various
morphologies has been proved excellent catalyst in alkaline
medium,
188–190
but only few works have been published for
Ni/N-CNMs hybrids including Ni
x
Cu
y
@NG-NC,
191
Ni–C–N,
192
NiNC,
193
and Ni/WC@NC.
194
Moreover, Fe–N–C materials for
HER catalysis are mainly focused on Fe single atom supported
on NC (FeNC).
193,195–197
From all these materials, encapsulation of
transition metals (especially Co) by carbon nanomaterials has
been proved most effective for the development of an efficient
HER catalyst, because hydrogen adsorption energy on electro-
catalyst surface could be significantly decreased by such an
association of transition metal atoms and N-CNMs resulting in
enhanced HER activity.
180
Moreover, manipulation of metal
core is also an important factor for enhancing the catalytic
efficiency of TM/N-CNMs hybrids.
191,198
Metal organic framework (MOF) derived CoP nanoparticles
enclosed in nitrogen doped carbon nanowires (CoP/NCNWs)
were synthesized using two step pyrolysis followed by phos-
phorization.
42
This material showed good HER activity in both
alkaline and acidic media due to structural integrity and
intrinsic catalytic activity of both CoP and NCNWs. To attain
a current density of 10 mA cm
2
, an overpotentials of 95 mV
was required in acidic media of 0.5 M H
2
SO
4
, compared to
that of 125 and 131 mV for CoP nanoparticles and CoP/C,
respectively. Moreover, CoP/NCNWs bears a smaller Tafel slope
at 50 mV dec
1
, indicating faster reaction kinetics as compared
Table 2 Performance comparison of some recent TM/N-CNMs hybrids for electrocatalytic water splitting
Electrocatalyst Electrocatalytic process Electrolyte Overpotential
(mV at 10 mA cm
2
)Tafel slope
(mV dec
1
) Stability Ref.
Co@N-CNT HER 1.0 M KOH 44 94 30 h at 10 mA cm
2
183
Co@NC/NG HER 0.5 M H
2
SO
4
49 79 1000 CV cycles 186
Co@NC/Ti HER 0.5 M H
2
SO
4
56 78 8 h at 0.150 V 187
CoP/NCNWs HER 0.5 M H
2
SO
4
95 50 20 h at 0.098 V 42
1.0 M KOH 154 59 20 h at 0.172 V
Ni
3
Cu
1
@NG-NC-700 HER 0.5 M H
2
SO
4
95 77 40 h at 0.095 V 191
1.0 M KOH 122 84 80 h at 10, 20, 30
and 40 mA cm
2
Ni NPs@N-CNTs OER 0.1 M KOH 460 106 400 CV cycles 211
Co
2
P/Co–N–C OER 0.1 M KOH 420 115 18 h at 1.65 V 205
N-C QDs/Ni
2
P OER 0.1 M KOH 350 50 30 000 s at 10 mA g
1
212
NCNTs@NF HER 1.0 M KOH 171 (20 mA cm
2
) 119 5.5 h at 0.28 V 20
OER 250 (20 mA cm
2
) — 5.5 h at 1.58 V
OWS (1.75 V/10 mA cm
2
) — — 40 h at 1.99 V
CoP/PNC HER 0.5 M H
2
SO
4
99 46 24 h at 0.99 V 37
1.0 M KOH 165 70 20 h at 0.165 V
OER 0.5 M H
2
SO
4
300 77 20 h at 1.53 V
OWS (1.68 V/10 mA cm
2
) 1.0 M KOH — — 24 h at 1.68 V
FCP@NG HER 1.0 M KOH 187 76 20 h at 0.195 V 221
OER 269 58 20 h at 1.495 V
OWS (1.63 V/10 mA cm
2
) — — 60 h at 15 mA cm
2
NCNT-NP@NF HER 1.0 M KOH 96 84 100 h at 0.2 V 148
OER 240 31 450 000 s at 1.47 V
OWS (1.61 V/10 mA cm
2
) — — 150 h at 10 mA cm
2
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with the values of 73 and 69 mV dec
1
for CoP nanoparticles
and CoP/C, respectively, and it retains 86% of the initial current
density after a long period of operation for 20 h at a fixed
overpotential of 98 mV, which is much higher than that of both
CoP nanoparticles and CoP/C experiencing significant loss in
current density after only 10 h operation at overpotential of
about 130 mV, respectively.
In addition to acidic media, the HER activity of CoP/NCNWs
was also evaluated in 1.0 M KOH solution and a small over-
potential of 154 mV was recorded to attain a current density of
10 mA cm
2
, as compared to CoP nanoparticles and CoP/C
bearing 172 and 174 mV overpotential, respectively. Similarly,
the Tafel slope of 59, 75, and 73 mV dec
1
was demonstrated by
CoP/NCNWs, CoP/C, and CoP nanoparticles, respectively. After
20 h chronoamperometric operation at an overpotential of
158 mV in 1.0 M KOH, no loss in current density was observed
for CoP/NCNWs.
Hence, it could be inferred that CoP/NCNWs is more stable
as HER catalyst in alkaline media as compared to harsh acidic
environment where it lost about 14% of the current density
after 20 hours, though the HER overpotential in acidic environ-
ment is smaller. The authors attributed the remarkable HER
performance of CoP/NCNWs to the intrinsic catalytic activity of
CoP nanoparticles, optimized pyrolytic strategy which rendered
an integrated heterogeneous structure to the hybrid helping in
prevention of agglomeration of nanoparticles, and enhanced
charge transfer.
Although the lowest HER overpotentials of only 44 and
47 mV were recorded for Co@N-CNT
183
and Co-g-C
3
N
4
/rGO
single atom catalyst (Co-CNG),
185
respectively, in 1.0 M
KOH solution with corresponding Tafel slopes of 96 and
44 mV dec
1
, the use of Ni foam as a substrate led to poor
stability of the catalysts which makes them controversial.
199
Hence, a catalyst with more reliable HER activity in universal
pH range is more desirable. To achieve this target, Liu et al.
191
fabricated nickel-copper alloy encaged in few-layered nitrogen-
doped graphene on N-doped graphitic carbon framework (Ni
x
Cu
y
@
NG-NC) using one-pot hydrothermal method followed by calci-
nation at high temperature in inert N
2
atmosphere as repre-
sented in Fig. 6a. In an alkaline electrolyte of 1.0 M KOH, it was
found that the catalyst calcinated at 700 1C exhibits the best
performance among the materials obtained in the temperature
range from 600 to 800 1C, and the overpotential at 10 mA cm
2
,
Tafel slope and mass activity of the catalysts obtained at 700 1C
with different Ni/Cu ratios have been comparatively evaluated
(Fig. 6b). It is suggested that the Ni
3
Cu
1
@NG-NC (Ni
3
Cu
1
@
NG-NC-700) exhibits the best HER electrocatalytic efficiency
with lowest overpotential of 122 mV and Tafel slope of
84.2 mV dec
1
. This performance was superior to the reported
NiCu alloy,
200
Ni
3
Cu
1
nanoparticles and porous nitrogen-doped
carbon,
200
indicating the importance of hybridizing both porous
nitrogen-doped carbon and metallic alloy. After 80 h chrono-
potentiometry operation in 1.0 M KOH solution at current density
of 10, 20, 30 and 40 mA cm
2
(Fig.6c),Ni
3
Cu
1
@NG-NC-700 only
showed slight change in HER potential, implying the excellent
stability in alkaline condition.Inanacidicsolutionof0.5M
H
2
SO
4
,Ni
3
Cu
1
@NG-NC-700 showed a HER overpotential of 95 mV
at 10 mA cm
2
, a Tafel slope of 77.1 mV dec
1
, and an appreciable
stability for 40 h with negligible potential change.
According to the authors, the electrocatalytic active sites for
HER are mainly located on graphene surface, and the remark-
able HER activity of Ni
3
Cu
1
@NG-NC-700 is accredited to the
incorporation of copper which modulates the morphology of
Ni
3
Cu
1
core and optimizes the electronic property and thick-
ness of graphene shell. In this catalyst, the electronic property
of graphene shell can be significantly altered by electrons
transferred from Ni
3
Cu
1
core, which further tunes the binding
energy of the reaction intermediates on graphene surface. Since
electron transfer is affected by the work function of metallic
core and structure property of graphene layer, the catalytic
activity can be adjusted by tuning the chemical composition
of metallic core and thickness of carbon layers. Therefore, the
flower-like structure of Ni
3
Cu
1
@NG-NC-700 provides increased
number of active sites, enhanced surface area, and efficient
mass transport, and the synergistic effects between bimetallic
core and optimized thickness of graphene shell greatly boosted
the intrinsic catalytic activity of Ni
3
Cu
1
@NG-NC-700. Moreover,
the introduction of N-dopants into graphene induces polarization
in the carbon network, and they interact with protons more
efficiently during HER process as compared to undoped compo-
sites to improve the HER performance.
42
It is clear from the above discussion that the introduction
of hetero metal atom like Cu in Ni
3
Cu
1
@NG-NC, presence of
transition metal atom with innate HER activity like Ni and Co,
and presence of graphitic moieties in N-doped CNMs support could
significantly affect the HER performance of transition metal-based
catalysts. But the reported work is very limited and intensive
research is urgently required to meet the increasing demand of
hydrogen energy, which works as a renewable fuel in many
applications including transportation to stationary grid stations.
3.2 Oxygen evolution reaction
Many approaches have been devised to overcome the slow
kinetics and high overpotential of OER process during water
splitting. Enhancing the number of catalytically active sites
by decreasing particle size of the catalyst, facilitating mass and
electron transport by inducing porosity in the structure, and
achieving high stability and conductivity by the integration
of carbon nanomaterials are the best strategies in this
regime.
201–203
Some of the typical TM/N-CNMs catalysts for
OER include MN
4
C
4
where M stands for Ni, Co or Fe,
204
Co
2
P/Co–N–C,
205
Co-NC/CoFe,
206
Co/N-CNTs,
207
Ni and N
doped nano porous graphene,
208
NiFe
3
alloy supported on
nitrogen-doped graphene hollow spheres (NGHS) entangled
with N-doped carbon nanotubes (NCNTs) (NiFe
3
@NGHS-
NCNTs),
209
CNH-D-NiMOF-400,
210
Ni nanoparticles supported
in N-CNTs (Ni NPs@N-CNTs)
211
and nitrogen doped quantum
dots supported on Ni
2
P nanoparticles (N-C QDs/Ni
2
P).
212
It should be noted that most of the TM/N-CNMs hybrids for
OER catalysis are cobalt and nickel based materials, among
which nickel based materials show better performance in most
of the cases.
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For example, Ni nanoparticles were encapsulated by Han
et al.
211
with nitrogen doped carbon nanotubes (N-CNTs) to
fabricate the catalyst of Ni NPs@N-CNTs through calcining
Ni-MOF at 600 1CinN
2
atmosphere (Fig. 6d). In 0.1 M KOH,
the as synthesized Ni NPs@N-CNTs bears an OER overpotential
of 460 mV to acquire a current density of 10 mA cm
2
, which is
160 mV lower than state-of-the-art RuO
2
which renders an
overpotential of 620 mV under similar conditions, while the
materials obtained at 700 (Ni-MOF-700) and 800 (Ni-MOF-800)
1C cannot achieve a current density of 5 mA cm
2
at an
overpotential of 620 mV (Fig. 6e). The Tafel slops for Ni
NPs@N-CNTs and RuO
2
were recorded as 106 and 90 mV dec
1
,
respectively, indicating comparable reaction kinetics. More-
over, the mass activity of Ni NPs@N-CNTs was the highest with
a value of 55.70 A g
1
at overpotential of 600 mV (Fig. 6f),
justifying the better OER performance as compared to N-CNTs
free Ni-MOF-700 (8.78 A g
1
), Ni-MOF-800 (3.21 A g
1
) and
state-of-the-art RuO
2
(19.69 A g
1
). Ni NPs@N-CNTs achieved a
turn over frequency (TOF) of 0.01 s
1
with an overpotential of
539 mV, while the other three catalysts were unable to achieve
this TOF value even with an overpotential of 600 mV. These
results indicate that introduction of N-CNTs significantly
elevates the activity of Ni based OER catalysts by facilitating
charge transfer and uplifting active site density.
As for the stability, Ni NPs@N-CNTs retained 92% current
density after 400 CV cycles in N
2
saturated 0.1 M KOH solution
in the potential range from 1.0 to 1.85 V (vs. RHE), while only
62.5% retention was achieved by state-of-the-art RuO
2
electro-
catalyst, and Ni NPs@N-CNTs needed only 10 mV increase in
overpotential to regain the current density of 10 mA cm
2
. This
result signposted the superior stability of Ni NPs@N-CNTs as
compared to RuO
2
for OER process in alkaline environment.
The outstanding OER performance of Ni NPs@N-CNTs catalyst
might come from three reasons: (1) large surface area and
Fig. 6 (a) A schematic illustration for the synthesis of Ni
x
Cu
y
@NG-NC catalysts. (b) Comparison of the overpotential at 10 mA cm
2
(Z
10
), Tafel slope, and
mass activity of Ni
x
Cu
y
@NG-NC calcinated at 700 1C in 1.0 M KOH solution. (c) Chronopotentiometry curves of Ni
3
Cu
1
@ NG-NC recorded at 10, 20, 30,
and 40 mA cm
2
for a total duration of 80 h. Reproduced with permission from ref. 191. Copyright (John Wiley and Sons) 2019. (d) A schematic for the
formation of Ni NPs@N-CNTs. (e) LSV curves and (f) mass activity of Ni NPs@N-CNTs, Ni-MOF-700, Ni-MOF-800 and RuO
2
for OER in N
2
-saturated
0.1 M KOH. Reproduced with permission from ref. 211. Copyright (Elsevier) 2017. (g) LSV curves of 20 wt% N-C QDs, Ni
2
P nanoparticles, N-C QDs/Ni
2
P
and 20 wt% RuO
2
/C in 0.1 M KOH. (h) The 1st and the 200th LSV curves of N–C QDs/Ni
2
P and 20 wt% RuO
2
/C at 1600 rpm. The inset:
Chronopotentiometry curve of N-C QDs/Ni
2
P composite at 10 mA g
1
(without rotation). (i) The representation of N-C QDs/Ni
2
P structure and OER
mechanism occurring on its surface. Reproduced with permission from ref. 212. Copyright (Elsevier) 2017.
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porosity provided by the metal organic framework; (2) enhanced
electrical conductivity provided by N-CNTs; (3) introduction of
nitrogen in the carbon network promotes adsorption of reaction
intermediates, resulting in decrease of binding energy between
adsorbed reaction intermediates and catalyst surface, which leads
to efficient release of reaction products.
Among Co based TMs/N-CNMs hybrids as OER catalysts,
Co
2
P/Co–N–C
205
bears an OER overpotential of 420 mV to
achieve 10 mA cm
2
in 0.1 M KOH solution, which is 40 and
70 mV lower, respectively, than the reference catalysts of RuO
2
and Co–N–C, and this overpotential is also lower than the
reported values of 460 mV for Co-NC/CoFe
206
and 500 mV for
Co/N-CNTs.
207
The Tafel slope of Co
2
P/Co–N–C (115 mV dec
1
)
was also lower than that of RuO
2
(119 mV dec
1
) and Co–N–C
(130 mV dec
1
). Moreover, the long-term stability of Co
2
P/
Co–N–C was confirmed from chronoamperometry experiment
at 1.65 (vs. RHE) in 0.1 M KOH. After 18 h operation, it
displayed a small current density loss of only 12%, which was
evidently lower than that of Co–N–C (17%) and RuO
2
(42%).
These results suggest that along with N-CNMs, phosphating
treatment of transition metals also has significant effects on
the OER performance of TM/CNM hybrids.
Liao et al.
212
deposited nitrogen doped carbon quantum dots
(N-C QDs) as satellites on Ni
2
P nanoparticles and evaluated the
electrocatalytic performance towards OER in 0.1 M KOH. Among
the obtained composites with 5–20% weight percentage of N-C
QDs, the one with 10 wt% N-C QDs, named as N-C QDs/Ni
2
P,
bears the lowest overpotential of 350 mV to achieve a current
density of 10 mA cm
2
as compared with that of 20 wt% RuO
2
/C
(430 mV) and Ni
2
P nanoparticles (440 mV), while 20 wt% N-C
QDs/C was unable to achieve 10 mA cm
2
even at a high over-
potentialof520mV(Fig.6g).TheTafelslopeofN-CQDs/Ni
2
P
(49.7 mV dec
1
) was about half of the N-C QDs free Ni
2
P
nanoparticles (88.9 mV dec
1
) and much lower than that
of 20 wt% RuO
2
/C (127.5 mV dec
1
) and 20 wt% N-C QDs/C
(90.1 mV dec
1
), indicating fast charge transfer in the case of
N-C QDs/Ni
2
P. The as-prepared N-C QDs/Ni
2
P also bears very
good OER stability in 0.1 M KOH, as negligible change
was observed in the onset potential and current density after
200 CV cycles in the voltage range of 1.2–1.8 V (vs. RHE), while
significant loss of current density was demonstrated for 20 wt%
RuO
2
/C after 200 CV cycles (Fig. 6h), and the chronopotentio-
metry i–tcurve measured at 10 mA g
1
without electrode
rotation shows no change in potential even after 30000 s as
displayed in inset of Fig. 6h. The authors attributed the uplift in
activity and stability of N-C QDs/Ni
2
P to the synergistic effects
between Ni
2
P nanoparticles, surface covering N-C QDs and
NiO
x
/Ni(OH)
x
particles formed after OER as depicted in Fig. 6i.
After a careful observation, it might be suggested that
N-CNMs are extremely effective to enhance the activity and
stability of transition metal based OER catalysts, and phosphating
treatment could further enhance the performance to achieve
efficient TM/N-CNMs hybrids for OER. However, it is worth
noticing that in the OER catalysis regime, N-CNMs have only
been hybridized with Ni, Co and Fe based transition metal
electrocatalysts, and a lot of room is open to support N-CNMs
with other transition metals and their compounds to develop
efficient TM/N-CNMs OER catalysts.
3.3 Overall water splitting
The main recently reported TM/N-CNMs catalysts for overall
water splitting include nickel foam supported nitrogen doped
carbon nanotube encapsulated nickel nanoparticles (NCNT-
NP@NF)
148
and Co nanoparticles (Co/NCNTs-NF),
213
nickel
quantum dots anchored in nitrogen doped carbon (Ni@C-N),
214
N-doped carbon nanosnakes (NCNSs) encapsulated Fe-doped
CoSe nanoparticles (FeCoSe@NCNSs),
215
CoFeN embedded into
1D N-doped carbon nanotubes modified 3D cruciform carbon
matrix (CoFeN-NCNTs//CCM),
216
core–shell nickel–iron oxide
on porous N-doped carbon nanosheet (CS-NFO@PNC),
217
Ni
enhanced Mo
2
C nanoparticles supported on N-doped graphi-
tized carbon (Ni-Mo
2
C/NC),
218
nanoscale hetero Mo
2
C–CoO
encapsulated in N-doped carbon nanofibers (Mo
2
C–CoO@
N-CNFs),
219
heterogeneous structure Co
2
P/CoP hollow nano-
spheres supported on cobalt-embedded nitrogen doped carbon
nanotubes (Co
2
P/CoP@Co@NCNT) hybrid,
220
nitrogen doped
carbon shell coated CoP nanocrystals encaged in porous carbon
substrate (CoP/PNC)
37
and iron–cobalt bimetallic phosphide
nanoflakes supported on nitrogen doped graphene (FCP@NG).
221
Among these catalysts, cobalt based N-CNMs hybrids are consi-
dered the best OWS catalysts, and CoP is believed to have the best
electrocatalytic efficiency
222
to combine N-CNMs which has
enhanced active sites for adsorption of H
+
-ion during HER
due to the N-induced polarization into carbon network.
223,224
Peng et al.
37
fabricated CoP nanocrystals coated with nitrogen
doped carbon shell encaged in porous carbon substrate (CoP/
PNC) with sol–gel and pyrolysis-oxidation-phosphorization
method (Fig. 7a), and evaluated the HER activity in both 1.0 M
KOH and 0.5 M H
2
SO
4
solutions along with CoP/NC and Co/PNC
for comparison. In 0.5 M H
2
SO
4
, a small overpotential of 99 mV
was required for CoP/PNC to attain a current density of
10 mA cm
2
, while 124 and 185 mV were exhibited by CoP/NC
and Co/PNC, respectively, and this value was also greatly lower
than that of bare CoP nanospheres
225
which bear an overpoten-
tial of 226 mV. Similarly, the HER overpotential of CoP/PNC
in 1.0 M KOH solution (165 mV) was obviously lower than that
CoP/NC (204 mV) and Co/PNC (228 mV).
37
However, these values
of overpotential for CoP/PNC are higher than that of less than
40 mV and about 50 mV for Pt/C in 0.5 M H
2
SO
4
and 1.0 M KOH
solution, respectively. In addition, the Tafel slope of CoP/PNC in
both 0.5 M H
2
SO
4
and 1.0 M KOH was obviously lower than that
of CoP/NC and Co/PNC, but higher than Pt/C, and its Tafel slope
of 46 mV dec
1
under acidic condition is considerably lower than
that of 76 mV dec
1
for bare CoP nanospheres.
225
These results
justify the positive effects of PNC on HER performance of CoP
nanoparticles. Moreover, minute change in current density
was observed after 1000 CV cycles in both acidic and alkaline
conditions, implying excellent stability of CoP/PNC.
On the other hand, OER activity of CoP/PNC was also
evaluated in 1.0 M KOH solution,
37
its overpotential of
300 mV to attain a current density of 10 mA cm
2
was lower
than that of 343, 405, 315 and 410 mV for CoP/NC, Co/PNC,
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state-of-the-art RuO
2
, and bare CoP nanoparticles,
226
respectively,
and its Tafel slope of 77 mV dec
1
was also evidently lower.
Moreover, the durability of CoP/PNC was monitored to show
negligible decrease in current density after 1000 CV cycles in
1.0 M KOH solution for OER.
The OWS performance of CoP/PNC was evaluated in 1.0 M
KOH using a symmetrical two-electrode cell (Fig. 7b) which
requires a cell voltage of 1.68 V to achieve 10 mA cm
2
, and
a current retention of 78.7% could be sustained after 24 h
continuous operation at 1.68 V. In addition, it is worth men-
tioning that the H
2
and O
2
volume released during overall water
splitting at anode and cathode, respectively, was equal to the
theoretically calculated volume as depicted in Fig. 7c. These
results indicate that nitrogen doped porous carbon substrate
could significantly enhance the catalytic performance of CoP
nanoparticles for water splitting, and CoP/PNC nanohybrid
could potentially be used for full water splitting applications.
The superior OWS efficiency of CoP/PNC could be assigned
to synergistic effect between CoP nanocrystals and N-doped
carbon nano shells. Herein, carbon nano shell is crucial in
stabilizing the metal nanoparticles in harsh conditions and is
also helpful in preventing the agglomeration of CoP nanocrystals
leading to catalytic sites active for a long period of time, and the
polarization induced by nitrogen doping plays a key role in
adsorption of reaction intermediates and stabilizes the binding
energy between catalyst surface and adsorbed species to release
the formed products.
Yang et al.
221
synthesized iron–cobalt bimetallic phosphide
nanoflakes supported on nitrogen doped graphene using facile
spray drying method followed by annealing and phosphoriza-
tion as displayed in Fig. 7d, and compared the effects of Fe : Co
ratio on catalytic performance towards HER, OER and overall
water splitting in 1.0 M KOH solution. For HER process, the
smallest overpotential of 187 mV was achieved by the material
with a 1 : 1 ratio, abbreviated as FCP@NG, to reach a current
density of 10 mA cm
2
(Fig. 7e). Its Tafel slope of 76 mV dec
1
was lower than that of 80 and 78 mV dec
1
for FP@NG and
CP@NG, respectively, and equal to that of both F
2
C
1
P@NG and
F
1
C
2
P@NG, and it showed no significant difference in HER
activity after 1000 CV cycles in the potential range from 0.1 to
0.4 V (vs. RHE) and 20 hours of chronoamperometric opera-
tion at 195 mV overpotential. For OER process, FCP@NG
showed a smallest overpotential of 269 mV to achieve 10 mA cm
2
and a lowest Tafel slope of 58 mV dec
1
among all the studied
catalysts (Fig. 7f). And it also demonstrated excellent long-term
stability for OER with negligible change in current density after
Fig. 7 (a) A synthetic scheme of CoP/PNC nanoparticles, (b) overall water splitting performance of CoP/PNC8CoP/PNC system in 1.0 M KOH using a
two-electrode device. (c) Practical H
2
or O
2
volumes over time versus theoretical gas volumes. Reproduced with permission from ref. 37. Copyright
(Elsevier) 2019. (d) A schematic illustration of the synthesis of FCP@NG. (e) HER and (f) OER overpotential (Z
10
) and Tafel slope in 1.0 M KOH of various
catalysts. (g) Overall water splitting performance of FCP@NG, FCP and bare Ni foam (NF) in 1.0 M KOH. Reproduced with permission from ref. 221.
Copyright (Royal Society of Chemistry) 2019.
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1000 CV cycles and 20 hours chronoamperometry experiment at
an overpotential of 265 mV.
In a two-electrode symmetrical cell fabricated with FCP@NG
at both electrodes, a cell voltage of 1.63 V was required to
achieve a current density of 10 mA cm
2
for overall water
splitting (Fig. 7g), which is very close to the voltage of commer-
cial Pt||RuO
2
cell bearing 1.53 V at 10 mA cm
2
,
166
and this cell
can work stably at 15 mA cm
2
for 60 h. Moreover, it is worth
noting that the overall overpotential of 400 mV for FCP@NG
cell was less than the sum of half-cell overpotentials (456 mV).
The authors attributed this difference to the use of nickel foam
current collector, which has much larger surface area and
better current collection ability as compared to glassy carbon
electrode, and high loading (2 vs. 0.27 mg cm
2
) of catalyst for
full cell fabrication. Moreover, the superior activity of FCP@NG
can be accredited to the combined effects of both transition
metals along with synthetic strategy which provides a chemically
stable and highly conducting nitrogen doped graphene support.
This type of structural configuration not only enhanced the
stability of catalyst but also offers larger area of activation for
the adsorption of HER and OER reaction intermediates.
Cheng et al.
148
prepared a bifunctional electrocatalyst of
NCNT-NP@NF, which was self-supporting, multi interfacial
nickel foam (NF) supported nitrogen doped carbon nanotube
(NCNT) encapsulated nickel nanoparticles (NPs). In 1.0 M
KOH solution, the as-prepared catalyst showed excellent OER
performance with a low overpotential of 240 mV to achieve
10 mA cm
2
, which is much lower than that of 400 mV for the
commercial IrO
2
catalyst at the same operating conditions.
It also showed an extremely lower OER Tafel slope of
30.7 mV dec
1
, compared to the value of 68.9 mV dec
1
for
commercial IrO
2
, and an excellent durability with negligible
loss in current density even after 50 000 s operation at an
overpotential of 240 mV. Concurrently, NCNT-NP@NF also
showed a low HER overpotential of 96.1 mV to approach
10 mA cm
2
, and a Tafel slope of 84.4 mV dec
1
, indicating
Heyrovsky mechanism during HER catalysis.
The overall water splitting performance of NCNT-NP@NF
was recorded in 1.0 M KOH solution using a two-electrode
symmetrical cell fabricated with it on both anode and cathode
as electrocatalyst. This cell started splitting water at a voltage of
1.54 V and achieved 10 mA cm
2
at 1.61 V, which is very close to
the cell voltage of RuO
2
/NF8Pt/C (1.54 V) system at similar
conditions, and it could work stably at 10 mA cm
2
for 150 h.
The excellent performance and durability of NCNT-NP@NF
were attributed to the graphite coated nickel foam induced
uplift in electrical conductivity leading to fast charge transfer.
In addition, 3D conducting network formed due to vertically
aligned NCNTs anchored carbon surface also facilitates the
charge transfer during HER and OER catalysis. DFT calcula-
tions also supported these statements by clarifying that
enhanced performance was due to the synergistic effects of
homogeneous nickel nanoparticles distribution and distinct
confinement effects of NCNT, which results in multi-interfacial
electron transfer to uplift the overall performance and activity
of catalyst.
In summary, several TM/N-CNMs catalysts have demon-
strated promising activity and considerable stability for overall
water splitting in alkaline environment, but the cell potential to
achieve current density of 10 mA cm
2
is still lagging from the
benchmark of state-of-the-art RuO
2
8Pt/C cell. Hence, extensive
work is still required to match the performance of noble metal-
based HER and OER electrocatalysts to meet the global energy
demands, and careful manipulation of transition metal-based
composites with carbon nanomaterials could be vital strategy
in achieving this milestone in near future.
4. Transition metal-based materials
and boron doped-CNMs hybrids
Except for nitrogen, CNMs can also be doped with other
heteroatoms like boron to achieve desirable performance for
electrochemical water splitting. The formation energy of boron-
doped graphene (BG) is approximately 5.6 eV per atom, which is
muchlowerthanthatofnitrogen-dopedgraphene(8eVperatom),
resulting in energetically more favorable doping of boron as
compared with nitrogen in CNMs.
227
The specific effects of B-doping on TM/CNMs hybrids are
summarized in Fig. 8. When sp
2
hybridized boron atoms are
doped to graphene, the planar structure of graphene is retained
with a stable in-plane doping mode as shown in Fig. 9a.
64
In this structure, the length of C–C bond (1.42 Å) is shorter than
that of B–C bond (1.50 Å), due to which there exists a minute
lattice mismatch of boron doped graphene which is beneficial
for maintaining the structural integrity, and there are three
types of boron sites including one central graphitic (B–3C), two
edged (B–2C–O) and (B–C–2O) boron species, along with some
B
2
O
3
precursor residues (Fig. 9b).
228
Moreover, boron has one
electron less than carbon, so it could induce polarity among
carbon atoms to provide more active sites and promote electro-
catalysis process. Boron doping to graphene or other CNMs
Fig. 8 Effects of B-doping on TM/CNMs hybrids as water splitting
catalysts.
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could induce p-type semiconducting behavior which lowers the
Fermi level towards the Dirac point,
229–231
and the bandgaps
ranging from 0 to 0.54 eV can be achieved by changing the level
of boron doping to graphene.
227
Doping of boron renders more
holes to the valence band of graphene showing an increase in
its conductivity. As a result, there is a significant increase in the
carrier concentration of boron-doped graphene (0.5 carriers
per dopant) and similarly in other CNMs. Some representative
TM/B-CNMs hybrids selected on the basis of HER, OER and
OWS efficiency are presented in Table 3.
4.1 Hydrogen evolution reaction
Till now, there are only few reports on TM/B-CNMs hybrids as
HER electrocatalysts and no remarkable results have been
obtained. Hence, this field offers vast opportunities to develop
novel and efficient HER catalyst materials. A representative
work was done by Yusuf et al.,
232
they fabricated Ni nano-
particles enclosed in boron doped carbon (Ni@B-C) via one pot
chemical reduction method followed by pyrolysis at different
temperatures, and evaluated the HER performance in alkaline
medium. The results revealed that the material obtained at
500 1C (Ni@B-C 500) exhibits the best HER activity by showing
an overpotential of 176 mV to attain the current density of
10 mA cm
2
in 1.0 M KOH solution, along with that of 188, 304,
and 556 mV for Ni@B-C 400, Ni@B-C 600, and Ni@C 500,
respectively, and its Tafel slope of 78 mV dec
1
was evidently
lower than that of Ni@B-C 400 (87 mV dec
1
), Ni@B-C 600
(128 mV dec
1
) and Ni@C 500 (231 mV dec
1
). The difference
of 380 mV in HER overpotential and 153 mV dec
1
in Tafel
slope of Ni@B-C 500 and Ni@C 500 demonstrates how the
doping of boron can effectively uplift the HER performance of
TM/CNMs materials. Moreover, unremarkable change in current
density was recorded during continuous 100 h chronoampero-
metric operation at an overpotential of 176 mV, demonstrating
the good stability of Ni@B-C 500 in 1.0 M KOH solution (Fig. 10a).
The superior performance of Ni@B-C 500 could be attri-
buted to the facts that the rate of electron transfer increases
significantly due to the synergistic effects between Ni and
boron, carbon shell is responsible for prolonged stability of
catalyst in alkaline medium, and the crystalline structure of
Ni@B-C 500 provides enhanced number of catalytically active
sites to accelerate the HER process as depicted in green part
of Fig. 10a. This work draws a new outlook for the synthesis of
TN/B-CNMs electrocatalysts for HER process.
Another example of TM/B-CNMs catalyst toward HER is
NiCo@BC
233
prepared by a facile reduction method followed
Fig. 9 (a) A diagrammatic representation of in-plane boron doping to graphene. Reproduced with permission from ref. 64. Copyright (American
Chemical Society) 2020. (b) K-edge NEXAFS spectra for B-doped graphene materials (left) and the corresponding atomic model (right). Reproduced with
permission from ref. 228. Copyright (Springer Nature) 2016.
Table 3 Performance comparison of some recent TM/B-CNMs hybrids for electrocatalytic water splitting
Electrocatalyst Electrocatalytic process Electrolyte Overpotential (mV at 10 mA cm
2
) Tafel slope (mV dec
1
) Stability Ref.
Ni@B-C HER 1.0 M KOH 176 78 100 h at 0.176 V 232
NiCo@BC 500 1C HER 1.0 M KOH 209 60 200 h at 0.209 V 233
BGO/Ni
3
N OER 1.0 M KOH 280 78 48 h at 20 mA cm
2
64
Ni
1
Co
3
-BDC OER 1.0 M KOH 309 62 20 h at 1.54 V 247
GH-BGQD2 HER 0.1 M KOH 130 95 10 h at 0.41 V 30
OER 370 70 10 h at 1.55 V
OWS (1.61 V/10 mA cm
2
) — — 70 h at 1.61 V
Ni
3
Fe@BC-500 1C HER 1.0 M KOH 330 127 20 h at 0.396 V 57
OER 280 77 20 h at 1.554 V
FeCoNiB@B-VG HER 0.5 M H
2
SO
4
148 87 10 h at 10 mA cm
2
255
1.0 M KOH 31 30 10 h at 10 mA cm
2
0.5 M PBS 413 — —
OER 0.5 M H
2
SO
4
635 — 10 h at 10 mA cm
2
1.0 M KOH 387 51 10 h at 10 mA cm
2
0.5 M PBS 905 — —
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by pyrolysis in nitrogen atmosphere. In 1.0 M KOH solution,
the NiCo@BC material prepared at 500 1C, which is named as
NiCo@BC 500 1C, displayed the best HER performance among
those obtained in the temperature range of 400–600 1C, and its
overpotential of 209 mV at 10 mA cm
2
and Tafel slope of
60 mV dec
1
were also lower than that of 464 mV/199 mV dec
1
for the B-free NiCo@C 500 1C prepared with the same procedure,
suggesting the significance of boron doping on TM/CNMs per-
formance. Moreover, NiCo@BC 500 1C showed ultra-high stabi-
lity by retaining current density of 10 mA cm
2
and negligible
increase in overpotential after 200 h continuous operation at
0.209 V (vs. RHE). The surprising stability and satisfying activity
of NiCo@BC 500 1C was attributed to the paired and synergistic
inter-orbital electronic co-existence and unique structure induced
high conductivity and exposed active sites. Besides, the HER
performance of NiCo@BC 500 1C with overpotential/Tafel slope
of 209 mV/60 mV dec
1
was superior to many other reported HER
catalysts like CoS
2
/CC (426 mV/207 mV dec
1
),
234
Ni
2/3
Fe
1/3
-rGO
(560 mV/210 mV dec
1
),
235
Mn doped Ni foam (360 mV/not
given)
236
and 3D MoS
2
-G-Ni (4600 mV/98 mV dec
1
),
237
to
achieve a current density of 10 mA cm
2
in 1.0 M KOH solution.
Comparison of the above examples suggests that incorpora-
tion of binary transition metal in TM/B-CNMs could signifi-
cantly improve the HER durability in alkaline environment.
However, only few researches on TM/B-CNMs as HER electro-
catalysts have been reported in literatures, and much more
experimental work is required to understand the HER catalysis
mechanism and improve the performance.
4.2 Oxygen evolution reaction
Among diverse 3d transition metal-based electrocatalysts for
OER, transition-metal nitrides have exhibited promising elec-
trocatalytic performance as they possess intrinsic metal like
properties, and the presence of nitrogen in metallic core could
induce faster mass transport and high electron density close to
Fermi level owing to the modulation of electronic structure and
contraction of d-orbitals in transition metals.
238,239
However,
poor stability under harsh conditions and conversion of metal
Fig. 10 (a) HER mechanism and chronoamperometry results for the stability of Ni@B-C 500 in 1.0 M KOH. Reproduced with permission from ref. 232.
Copyright (Elsevier) 2020. (b) A schematic representation of the synthesis of BGO/Ni
3
N. (c) LSV curve and (d) Tafel slope of different electrocatalysts for
their OER performance in 1.0 M KOH, XPS spectra of (e) Ni 2p and (f) N 1s in BGO/Ni
3
N (red line) and Ni
3
N (black line). Reproduced with permission from
ref. 64. Copyright (American Chemical Society) 2020. (g) A schematic illustration of the formation of Ni
1
Co
3
@BC hybrid, the gray, red, green and blue
represent C, Co, Ni and C atoms, respectively. (h) LSV curves of various materials in for their OER performance 1.0 M KOH solution. Reproduced with
permission from ref. 247. Copyright (Elsevier) 2020.
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nitrides to oxyhydroxides under anodic oxidation limit their
large scale applications in water splitting.
240,241
This drawback
is expected to be overcome by the integration of boron doped
carbon nanomaterials which not only act as high conducting
support but also enhance OER efficiency.
242–244
Li et al.
64
made full use of p-type and n-type doping to
synthesize Ni
3
N nanoparticles encaged in boron-doped graphene
oxide (BGO/Ni
3
N) by the procedure described in Fig. 10b. The
positive effects of this p-n junction were observed on OER
efficiency in alkaline medium, and the as-prepared BGO/Ni
3
N
rendered a small overpotential of 280 mV to attain 10 mA cm
2
in 1.0 M KOH solution, while GO/Ni
3
N without boron doping
showed an overpotential of 390 mV under similar conditions
(Fig. 10c). This overpotential of BGO/Ni
3
N surpassed state-of-the-
art RuO
2
as OER electrocatalyst, which shows an overpotential
of 295 mV to attain the same current density under similar
conditions. In addition, BGO/Ni
3
N exhibited a smaller Tafel slope
of 77.9 mV dec
1
as compared to that of 170.8 mV dec
1
for
GO/Ni
3
N, justifying its faster reaction kinetics than boron free
GO/Ni
3
N (Fig. 10d). This value is comparable to the benchmark
RuO
2
that exhibits a Tafel slope of 72.5 mV dec
1
. Moreover,
excellent stability of BGO/Ni
3
N for 24 h at current density of
10 mA cm
2
,48hat20mAcm
2
and 10 h at 100 mA cm
2
in
1.0 M KOH was demonstrated with no change in potential.
The uplifted performance of BGO/Ni
3
N might be assigned to
several facts: (1) facile adsorption of hydroxyl ions induced by
the decreased electron density and partial positive charge on
the carbon network due to lower electronegativity of boron as
compared with carbon;
245
(2) BGO facilitates the formation of
Ni
3
N nanoparticles on itself and also prevents agglomeration of
Ni
3
N particles leading to uniformly distributed high density
metallic catalytic sites for OER; (3) electron transfer between
Ni
3
N nanoparticles and BGO interface was accelerated because
of the closely packed structure of BGO/Ni
3
N and strong
coupling effect between Ni
3
NandBGOsheets(Fig.10eandf);
246
(4) electrical conductivity of graphene oxide was increased from
1.17 to 1.72 10
3
Sm
1
by doping of boron to carbon network.
Shuai et al.
247
enclosed NiCo nanoparticles in boron-doped
carbon by heating Ni
1
Co
3
-BDC nanosheets
248
and H
3
BO
3
at
high temperature in Ar atmosphere (Fig. 10g) and obtained the
material of Ni
1
Co
3
@BC. In 1.0 M KOH solution, Ni
1
Co
3
@BC
bears a smallest overpotential of 309 mV to obtain the current
density of 10 mA cm
2
, compared to the other materials
prepared with the same procedure by adjusting the concen-
tration of precursors and pyrolysis temperature (Fig. 10h).
Specifically, its overpotential was smaller by 54 mV than that
of 363 mV for boron free Ni
1
Co
3
@C and even smaller than that
of 322 mV for state-of-the-art RuO
2
. It also exhibited a lowest
Tafel slope of 62 mV dec
1
, which was lower by 19 and
10 mV dec
1
, respectively, than Ni
1
Co
3
@C and RuO
2
, indicating
the faster OER kinetics. Moreover, the excellent stability of
Ni
1
Co
3
@BC was verified by quite similar LSV curves before and
after 1000 CV cycles, along with only 0.5% and 5.6% loss in
current density after 12 and 20 h chronoamperometric operation,
respectively at a constant overpotential of 0.309 V.
247
The superior
performance of Ni
1
Co
3
@BC was assigned to the fact that boron
doping induced more catalytically active sites for reactive oxygen
intermediates adsorption.
249–251
Overall, synthesis technology, nature of active transition
metals, and nature of CNMs are the major factors affecting
the OER activity of TM/B-CNMs catalysts, but similar to the case
of HER active TM/B-CNMs materials, very few reports have been
published for OER active TM/B-CNMs so far and there exist
a huge gap to discover reliable and efficient OER catalysts
applicable in practical water electrolyzers.
4.3 Overall water splitting
In recent years, NiFe alloys supported on CNMs have gained
significant interest because of their excellent electrocatalytic
ability, and a lot of efforts have been conducted to promote
their performance with boron doping to CNMs, since boron
doped transition metals such as Ni, Mo, and Co have shown
remarkable HER efficiency in universal pH range.
165,252,253
Adegbemiga et al.
57
encaged Ni–Fe alloy by boron doped
carbon using a facile chemical reduction method followed by
annealing at various temperatures, and a series of catalysts
entitled Ni
3
Fe@BC-400 1C, Ni
3
Fe@BC-500 1C and Ni
3
Fe@BC-
600 1C were synthesized and evaluated for the water splitting
performance in 1.0 M KOH solution. Among them, Ni
3
Fe@BC-
500 1C exhibited the best OER performance by showing an
overpotential of 280 mV to achieve 10 mA cm
2
(Fig. 11a), and
this value is greatly lower than that of 405 mV required by
unsupported NiFe alloy
254
delivering the same current density
in 30 wt% KOH solution. A smallest Tafel slope of 77 mV dec
1
was obtained for Ni
3
Fe@BC-500 1C, compared to that of 106,
82 and 87.8 mV dec
1
for Ni
3
Fe@BC-400 1C and Ni
3
Fe@BC-
600 1C and NiFe alloy,
254
respectively, indicating facile reaction
kinetics. As far as the stability of Ni
3
Fe@BC-500 1C is con-
cerned, chronoamperometry experiment was performed at
1.554 V (vs. RHE) and only a slight change in current density
was depicted after 20 h operation.
Similarly, Ni
3
Fe@BC-500 1C exhibited a smallest overpoten-
tial of 330 mV as HER catalyst to attain the current density of
10 mA cm
2
in 1.0 M KOH solution (Fig. 11b), compared to the
value of 545 mV for NiFe alloy to attain same current density in
30 wt% KOH solution.
254
It also bears a small Tafel slope value
of 127 mV dec
1
, which is obviously lower than that of 134
and 140 mV dec
1
, respectively, for Ni
3
Fe@BC-400 1C and
Ni
3
Fe@BC-600 1C followed by 233.4 mV dec
1
of NiFe alloy in
30 wt% KOH solution.
254
After 20 h of chronoamperometry
experiment at an overpotential of 396 mV, only slight decline in
current density was observed, indicating the excellent stability.
It is clear from these results that Ni
3
Fe@BC-500 1Ccouldactas
an efficient electrocatalyst for overall water-splitting process. This
efficiency could be attributed to the combined effects of Ni and Fe
which are responsible for enhancing the electron density at
reactive sites leading to improved activity of electrocatalytic pro-
cess, the integration of carbon network which protects the metal
core from harsh conditions leading to prolonged stability, and the
doping of boron to carbon network causing electronic redistri-
bution to induce polarization which is beneficial in capturing the
reaction intermediates to facilitate HER and OER process.
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Jiang et al.
255
supported a series of transition metal borides
on boron doped vertically aligned graphene (B-VG) with elec-
troless plating technique, and the electrochemical performance
of as-prepared FeCoNiB@B-VG, FeCoNiB@VG, CoNiB@B-VG,
FeNiB@B-VG, FeCoB@B-VG, NiB@B-VG, FeB@B-VG and
CoB@B-VG towards water splitting was comparatively investi-
gated. It was revealed that the ternary FeCoNiB@B-VG has the
highest activity towards both HER and OER, and it’s HER
overpotential and Tafel slope were even less than that of
Pt/C@VG. In this material, vertically aligned graphene sheets
interconnect with each other by twisted and curled fibers of GO
produced during cooled quenching process (Fig. 11c), FeCoNiB
nanoparticles are uniformly distributed on surface of B-VG
(Fig. 11d), and the structure could be drawn as Fig. 11e where
a schematic description of its mechanism for water splitting to
produce hydrogen and oxygen is also presented.
In 1.0 M KOH, 0.5 M H
2
SO
4
, and 0.5 M phosphate buffer
solution (PBS), FeCoNiB@B-VG bears HER overpotentials of
31 mV, 148 mV, and 413 mV, respectively, to achieve 10 mA cm
2
(Fig. 11f). It is exciting that the overpotential of 31 mV for
FeCoNiB@B-VG in 1.0 M KOH is 122 mV lower than that of the
material prepared with boron free VG (FeCoNiB@VG), highlight-
ing the obvious positive effect of boron doping in CNMs based
supports. Moreover, the HER Tafel slope of FeCoNiB@B-VG is
30 mV dec
1
in 1.0 M KOH, and it is even smaller than that of
44 mV dec
1
for benchmark Pt/C@VG. Similarly, OER perfor-
mance of FeCoNiB@B-VG was also evaluated in a wide pH range,
and it requires an overpotential of 387, 635 and 905 mV in
1.0 M KOH, 0.5 M H
2
SO
4
, and 0.5 M PBS, respectively, to attain
10 mA cm
2
(Fig. 11g), and the overpotential of 387 mV for
FeCoNiB@B-VG in 1.0 M KOH is 81 mV lower than that of 468 for
FeCoNiB@VG, depicting again the advantage of boron doping in
CNMs supports.
Moreover, the small Tafel slope of 51 mV dec
1
for FeCo-
NiB@B-VG in 1.0 M KOH is quite close to that of 45 mV dec
1
for IrO
2
/VG. The stability of FeCoNiB@B-VG for HER (Fig. 11h)
and OER (Fig. 11i) processes was evaluated in both alkaline and
acidic media at a current density of 10 mA cm
2
, and only slight
change was observed in potential after continuous operation
for 35 000 s, confirming the excellent stability in a wide range of
Fig. 11 (a) OER and (b) HER polarization curves of Ni
3
Fe@BC-400 1C, Ni
3
Fe@BC-500 1C, Ni
3
Fe@BC-600 1C and IrO
2
/C in 1.0 M KOH solution.
Reproduced with permission from ref. 57. Copyright (Elsevier) 2020. (c) SEM and (d) TEM images of FeCoNiB@B-VG and (e) drawing of FeCoNiB@B-VG
structure. (f) HER and (g) OER polarization curves of FeCoNiB@B-VG in 0.5 M H
2
SO
4
, 1.0 M KOH and 0.5 M PBS. (h) HER and (i) OER stability of FeCoNiB@
in 0.5 M H
2
SO
4
and 1.0 M KOH solutions (j) HR-TEM image and SAED pattern of FeCoNiB@B-VG. Reproduced with permission from ref. 255. Copyright
(Elsevier) 2021.
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pH environment. The plausible source for the superior perfor-
mance of FeCoNiB@B-VG might be the unique amorphous
structure as shown in the HR-TEM image of Fig. 11j with
no clear lattice fringes, which was further justified with the
SAED pattern given in the inset of Fig. 11j showing broad-
ened diffraction rings. The other factors involved behind the
uplifted water splitting performance of FeCoNiB@B-VG include
synergistic effects between substituted boron and FeCoNiB
alloy and the integration of FeCoNiB with B-VG leading to
increased conductivity, enhanced charge transfer, increased
number of active sites and promoted long-term stability in
harsh environment.
In a word, the electrocatalytic efficiency of TM/CNMs
hybrids could be enhanced significantly by boron doping to
CNMs due to the advantages discussed above with Fig. 8.
However, there are not so many reports available on TM/
B-CNMs catalysts towards water splitting at present, and there
is a huge scope to explore the mechanism and develop more
efficient materials.
5. Transition metal-based materials
and N,B co-doped CNMs hybrids
For TM/CNMs electrocatalysts towards water splitting, doping
of electron efficient nitrogen and electron deficient boron in
CNMs can break the electroneutrality of carbon along with the
production of novel active sites,
256
so the HER, OER and OWS
catalytic activities of N and B co-doped TM/CNMs, entitled as
TM/N,B-CNMs, could be significantly enhanced as compared
with either TM/N-CNMs or TM/B-CNMs.
257–262
This enhance-
ment in performance could be attributed to the high density of
catalytically active sites and simultaneous presence of both
electron accepting (B–C) and electron donating (N–C) sites.
263
In this section, recent progress of TM/N,B-CNMs based cata-
lysts for water splitting has been summarized and an overall
performance comparison of some representative TM/N,B-CNMs
hybrids selected on the basis of HER, OER and OWS efficiency
is presented in Table 4.
5.1 Hydrogen evolution reaction
As discussed in the above sections, cobalt-based phosphides
exhibit promising HER performance, which could be enhanced
by using porous one dimensional cobalt based MOF as the
precursors to fabricate CoP nanoparticles enclosed in nitrogen
doped carbon nanowires (CoP/NCNWs).
42
However, Li et al.
presented that weak interaction exist sometimes in this type of
material between doped carbon nanomaterials and metal
nanoparticles along with a little lack of graphitization,
264
which
is harmful to overall electrocatalytic performance.
To resolve this issue, Tabassum et al.
265
encaged MOF-
derived cobalt phosphide nanoparticles with nitrogen and
boron co-doped carbon nanotubes and fabricated the material
of CoP@BCN by combining pyrolysis and phosphidation as
displayed in Fig. 12a. In this material, the polarization of
carbon network was modulated by integration of nitrogen
and boron in carbon nanotubes,
266
and the electron density
between carbon and heteroatoms is richly enhanced because
the electronegativity of carbon is more than boron but less than
nitrogen, which is beneficial for increasing the electrocatalyti-
cally active surface area and improving the performance.
267–269
In this work, two different concentrations of phosphoric acid
(0.015 and 0.03 g) were used during material synthesis
to prepare the catalysts of CoP@BCN-1 and CoP@BCN-2,
respectively, in which CoP particles are uniformly distributed,
crystalline CoP nanoparticles could be verified from the bright
spot and amorphous CNTs structure is confirmed from the
concentric circles in the SAED image (Fig. 12b).
In 0.5 M H
2
SO
4
, CoP@BCN-1 achieved a HER current density
of 10 mA cm
2
at a small overpotential of 87 mV, while
the overpotentials of CoP@BCN-2, Co@BCN, and CoP@CP
prepared with the same procedure were high up to 219, 257,
and 379 mV, respectively. It also demonstrated a smallest
Tafel slope of 46 mV dec
1
, compared to that of 68, 142 and
Table 4 Performance comparison of some recent TM/N, B-CNMs hybrids for electrocatalytic water splitting
Electrocatalyst Electrocatalytic process Electrolyte Overpotential
(mV at 10 mA cm
2
)Tafel slope
(mV dec
1
) Stability Ref.
CoP@BCN HER 0.5 M H
2
SO
4
87 46 3.6 h at 0.105 V 265
1.0 M KOH 215 52 8.5 h at 0.15 V
1.0 M PBS 122 59 8.5 h at 0.15 V
FeMn@BNPCFs-900 HER 1.0 M KOH 247 105 8 h at 0.3 V 273
G-BNG OER 0.1 M KOH 580 143 14 h at 1.8 V 32
BN-codoped carbon OER 1.0 M KOH 270 100 9 h at 1.65 V 33
BN/CA-NiCoFe-600 OER 1.0 M KOH 321 42 8 h at 10 mA cm
2
260
CoMoS
3.13
@BN-CNTs HER 0.5 M H
2
SO
4
168 82 10 h at 0.17 V 303
OER 1.0 M KOH 400 68 10 h at 0.4 V
Co
2
B/Co/N–B–C/B
4
C HER 0.1 M KOH 220 105 1000 CV cycles 304
OER 360 111 1000 CV cycles
OWS (1.62 V/10 mA cm
2
) 1.0 M KOH — — 10 h at 1.62 V
Co/NBC-900 HER 1.0 M KOH 117 146 5000 CV cycles 69
OER 302 70 10 h at 10 mA cm
2
OWS (1.68 V/10 mA cm
2
)——6hat10mAcm
2
Ni@BNPCFs-900 HER 1.0 M KOH 164 61 8 h at 0.2 V 168
OER 287 19 10 h at 1.6 V
OWS (1.584 V/10 mA cm
2
) — — 40 h at 1.585 V
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162 mV dec
1
for CoP@BCN-2, Co@BCN, and CoP@CP, respec-
tively, and this value is close to 32 mV dec
1
for state-of-the-art
Pt/C catalyst. Moreover, the LSV curves before and after 2000 CV
cycles in 0.1–0.24 V (vs. RHE) matched well with each other and
only slight change in current density could be observed after
3.6 h continuous operation at an overpotential of 105 mV.
Similarly, CoP@BCN-1 also showed the best HER performance
in both 1.0 M KOH, and 1.0 M PBS, with overpotentials of
215 and 122 mV, respectively, at current density of 10 mA cm
2
,
Tafel slopes of 52 and 59 mV dec
1
, and reliable stability after
2000 CV cycles and 8.5 h continuous operation at 150 mV
overpotential. These results declared that CoP@BCN-1 is an
efficient HER electrocatalyst in full pH range.
The superior activity and durability of CoP@BCN-1 was
attributed to synergistic effects between metal core and BCN,
along with encapsulation of CoP particles inside the CNTs: (1) P
atoms play significant role by maintaining charge separation
through accepting electrons from Co and transferring them to
BCN network;
270
(2) P in CoP@BCN-1 also acts as an adsorp-
tion site for hydrogen ions leading towards enhanced HER
activity;
271
(3) MOF-derived CoP@BCN-1 exhibits large surface
area with enhanced porosity which provide a large number of
accessible active sites for adsorption of hydrogen; (4) hetero-
atom-doped carbon nanomaterials not only show HER activity
by themselves but also are involved in improving the electro-
catalytic performance of CoP nanoparticles.
272
Liu et al.
273
synthesized FeMn@BNPCFs consisting of MnO,
Fe
3
C and N,B-codoped porous carbon nanofibers by using the
procedure displayed in Fig. 12c at various calcining tempera-
tures of 600, 700, 800, 900 and 1000 1C. In 1.0 M KOH solution,
the material prepared at 900 1C, FeMn@BNPCFs-900, bears a
lowest onset potential of 49 mV and overpotential of 247 mV to
achieve a current density of 10 mA cm
2
(Fig. 12d), and it also
achieves a lowest Tafel slope of 104.89 mV dec
1
(Fig. 12e).
These results indicate the higher HER activity and faster HER
catalysis kinetics of FeMn@BNPCFs-900 than all the other
studied catalysts. Moreover, the as prepared FeMn@BNPCFs-
900 catalyst also showed better stability than commercial 20%
Pt/C which displayed only 81.26% current density retention
after 8 h continuous chronoamperometric operation at 300 mV
Fig. 12 (a) A schematic depiction of CoP@BCN synthesis. (b) TEM image of CoP@BCN-1, inset (bottom) is SAED and low magnification of respective
tubes (right top). Reproduced with permission from ref. 265. Copyright (John Wiley and Sons) 2017. (c) A schematic for the synthesis of FeMn@BNPCFs-T
Networks with ample 3D hierarchical pores, abundant Edges/defects, and high N and B doping amounts. (d) Histograms of HER onset potential (E
onset
)
and the overpotential to achieve 10 mA cm
2
(E
10
) and (e) Tafel slopes of different FeMn@BNPCFs-Tcatalysts in 1.0 M KOH solution. Reproduced with
permission from ref. 273. Copyright (American Chemical Society) 2022.
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overpotential in 1.0 M KOH solution, while the current density
of FeMn@BNPCFs-900 was retained at 90.87% at similar
conditions.
The authors attributed the superior activity and durability of
FeMn@BNPCFs-900 to synergistic effects of macro and meso-
porous 3D structure, high density of carbon edges and defects,
facile charge transport and abundant active sites including
pyridinic N–M species, Fe
3
C@BNC structures, nanocrystalline
MnO particles, and B and N atoms doped in carbon nanofibers.
This strategy was also employed by the same group to develop
catalysts of Co@BNPCFs-800
261
and Ni-BNPCFs-900
168
which
showed very good overall water splitting performance and their
details will be discussed in Section 5.3.
5.2 Oxygen evolution reaction
In order to develop efficient catalysts with outstanding perfor-
mance, durability and wide scale availability altogether to
replace state-of-the-art noble metal based catalysts, N,B-codoped
CNMs materials, such as transparent G-BNG stacked nanofilms,
32
NB-codoped carbon,
33,274
nanoscale NiCoFe embedded in N,B-
codoped carbon aerogel (BN/CA-NiCoFe-600),
260
and boron doped
Co–N–C active sites confined in carbon sheets (Co-N,B-CSs),
259
have been proved helpful.
Abdullah et al.
275
investigated the detailed structure and
heteroatom interactions in N,B-codoped bilayer graphene
(BLG) with DFT calculations, and it was revealed that two types
of interactions, named as attractive and repulsive, exists
between N and B atoms. Attractive interaction induces AA-
stacking of BLG and bestows small bandgap, unstable struc-
ture, and poor mechanical properties resulting in bad thermal
and optical properties. While during repulsive interaction
between N and B atoms, AA-stacking is converted to AB-
stacking leading towards excellent mechanical, thermal and
optical properties owing to the reduced interlayer distance and
large band gap. These properties are more relevant for opto-
electronic applications, but it can also affect the conductivity of
charge transport channels present in different N and B codoped
graphene-based catalysts.
For example, a single layer of pure graphene stacked on
BN-codoped graphene (G-BNG)
32
showed enhanced OER per-
formance due to the efficient charge transfer which could be
attributed to the heteroatom doped p–p* modualtions and
corresponding electrostatic strain induced by the electro-
negativity difference between N,B and C atom as displayed in
Fig. 13a. In an electrolyte of 1.0 M KOH, the OER onset
potential and Tafel slope of the as-prepared G-BNG were
recorded as 370 mV and 143.22 mV dec
1
, respectively. These
values were much better than many other reported bilayer
graphene materials including graphene on graphene (G–G)
(470 mV/301.95 mV dec
1
), graphene on N-doped graphene
(G–NG) (440 mV/321.87 mV dec
1
), graphene on B-doped
graphene (G–BG) (430 mV/191.97 mV dec
1
), and single layer
Fig. 13 (a) Stacked layer of graphene on BNG showing p–p* modulations and efficient charge transfer in G-BNG structure. Reproduced with permission
from ref. 32. Copyright (American Chemical Society) 2020. (b) Schematically illustrating the solution plasma process system to synthesize BN-codoped
carbon. (c) OER polarization curves of 5 wt% RuC, N-doped C, 1 mM B-N, 5 mM B-N and 10 mM B-N codoped carbon catalysts in 1.0 M KOH and (inset)
Tafel slopes. (d) The structure and OER mechanism on the surface of BN-codoped carbon prepared by solution plasma process. Reproduced with
permission from ref. 33. Copyright (Elsevier) 2020. (e) Overpotentials of BN/CA-NiCoFe-600, B-CA-NiCoFe-600, N-CA-NiCoFe-600, IrO
2
and RuO
2
at
10 and 20 mA cm
2
(f) Tafel slopes and overpotentials at 10 mA cm
2
for some reported non-noble metal-based electrocatalysts for OER in basic
medium. Reproduced with permission from ref. 260. Copyright (Elsevier) 2022.
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N,B-codoped graphene (BNG) (540 mV/572.49 mV dec
1
). More-
over, the G-BNG catalyst also showed excellent OER stability
with a current density retention of 81.6% in 1.0 M KOH after
50,000 s operation at 1.8 V (vs. RHE).
Li et al.
33
prepared N,B-codoped carbon with different con-
centrations of boric acid by using a plasma synthesis process as
displayed in Fig. 13b. Comparative investigation revealed that
the material prepared with 10 mM boric acid has the best OER
performance in 1.0 M KOH solution, as compared to those with
1 mM and 5 mM boric acid, N-doped carbon and even the
benchmark 5 wt% Ru/C electrocatalyst. As shown in Fig. 13c, it
showed the smallest onset potential of 230 mV which is closely
similar to that of 5wt% Ru/C (240 mV), the lowest overpotential
of 270 mV to achieve 10 mA cm
2
which is slightly lower
than that of 5 wt% RuC (275 mV), and smallest Tafel slope of
100 mV dec
1
which is much lower even than that of 5 wt%
Ru/C (143 mV dec
1
). After continuous chronoamperometric
operation at 1.65 V (vs. RHE) for 9 hours, the 10 mM N,B-
codoped carbon exhibited 17% decline in current density, while
the value for 5wt% Ru/C was 29% decrease. The authors
proposed that the superior efficiency of 10 mM BN-codoped
carbon could be attributed to the formation of B-C and B-N
active sites in addition to N-C interactions (Fig. 13d).
In these two examples, no metal was incorporated in the
studied catalysts, and the positive effects of N,B-codoping on
OER electrocatalytic performance of carbon-based catalysts
have been evidently demonstrated with considerable uplift in
OER activity and durability.
Among the few reported TM/N,B-CNMs OER catalysts, tran-
sition metal alloy containing nanoscale Ni, Co and Fe sup-
ported on N,B-codoped carbon aerogel prepared by high
temperature pyrolysis at 600 1CinN
2
atmosphere (BN/CA-
NiCoFe-600) showed excellent performance.
260
In 1.0 M KOH
solution, it showed a lowest overpotential of 321 mV at a
current density of 10 mA cm
2
, as compared with that of 473,
423, 394 and 339 mV, respectively, for N-CA-NiCoFe-600, B-CA-
NiCoFe-600, IrO
2
and RuO
2
, and it also displayed the lowest
overpotential to achieve 20 mA cm
2
(Fig. 13e), while the metal
free BN/CA prepared with the same procedure showed no OER
activity. In addition, its Tafel slope of 42 mV dec
1
was
significantly lower than that of N-CA-NiCoFe-600 (84 mV dec
1
),
B-CA-NiCoFe-600 (71 mV dec
1
), IrO
2
(102 mV dec
1
) and RuO
2
(93 mV dec
1
). These results demonstrated the outstanding
effects of N,B-codoping on OER performance improvement of
TM/CNMs catalysts, and the overpotential/activity and Tafel
slope/reaction kinetics achieved here by BN/CA-NiCoFe-600 are
obviously lower than many other N or B doped carbon contain-
ing TM/CNMs hybrids as shown in Fig. 13f. Moreover, 30 000 s
continuous chronoamperometric operation showing no loss in
current density and negligible change in onset/overpotential
after 1000 CV cycle in non-faradic region justify the high
stability of BN/CA-NiCoFe-600 towards OER catalysis in alkaline
environment.
According to the above discussion, the OER overpotentials of
270 mV for 10 mM BN-codoped carbon and 321 mV for BN/CA-
NiCoFe-600 in 1.0 M KOH solution imply that transition metal
free N,B-codoped carbon can even be used as efficient OER
catalyst with better performance than the metal incorporated
ones. However, solution plasma method was employed for
10 mM BN-codoped carbon preparation, which is less common
as compared with the widely used high temperature pyrolysis
or hydrothermal method. Moreover, only very few works on
TM/N,B-CNMs hybrids have been reported as OER catalysts to
the best of our knowledge, and extensive research work is still
required to understand the effects of N,B-codoping on the
properties and performance of CNMs as well as TM/CNMs to
develop efficient OER catalysts for practical applications.
5.3 Overall water splitting
In recent years, transition metal dichalcogenides (TMDs),
MoS
2
-based in particular, have been explored as remarkable
candidates for HER and OER activity due to their intrinsic
catalytic efficiencies.
117,276–286
It has been reported that HER
activity of MoS
2
could be enhanced by introduction of 3d
transition metals like iron, cobalt, and nickel,
238,287–296
Mo-based ternary sulfides especially cobalt-based Mo-sulfides
also have good OER activity,
294–296
and N,B-codoped CNMs
could be used as efficient support materials to enhance the
performance towards overall water-splitting in addition to
undoped CNMs as well as N-doped CNMs.
168,261,297–302
Zhang et al. supported CoMoS
3.13
on highly conducting N,B-
codoped carbon nanotube by a simple hydrothermal process
followed by high temperature pyrolysis, in which CoMoS
3.13
nanosheets were homogenously anchored on N,B-CNTs as
displayed in high resolution SEM image given in Fig. 14a.
303
In 0.5 M H
2
SO
4
solution, the as-prepared CoMoS
3.13
@BN-CNTs
revealed an HER onset potential of 63 mV, which was less than
half of that for pristine CoMoS
3.13
(150 mV), its overpotential
(168 mV) required to achieve a current density of 10 mA cm
2
was about 100 mV lower than that of CoMoS
3.13
(Fig. 14b),
and its lower Tafel slope of 82 mV dec
1
compared to that of
94 mV dec
1
for CoMoS
3.13
justifies faster reaction kinetics.
After 1000 CV cycles in 0.5 M H
2
SO
4
solution, no change in
HER overpotential and current density was observed, and a
constant current density of 10 mA cm
2
was retained after
10 hours of continuous chronoamperometry operation at over-
potential of 0.170 V, confirming the remarkable stability of
CoMoS
3.13
@BN-CNTs.
Similarly, this material bears smaller OER overpotential
(400 mV) to attain 10 mA cm
2
(Fig. 14c) and lower Tafel slope
(68 mV dec
1
) than CoMoS
3.13
in 1.0 M KOH solution. Its
excellent stability was demonstrated by a minute decrease in
current density and minor increase in overpotential after 1000
CV cycles, and also the negligible change in current density
after 10 hours of continuous chronoamperometric operation at
a fixed overpotential of 0.4 V.
In addition to MoS
2
based materials, Co based catalysts
supported on N,B-codoped CNMs have also been reported as
promising candidates towards overall water splitting. For instance,
Liu et al.
304
immobilized Co nanoparticles and Co
2
BonN,B-
codoped carbon to prepare Co
2
B/Co/N–B–C/B
4
Cbyin situ pyrolysis
of nano-B
4
C supported Co(OH)
2
with melamine (Fig. 14d). In this
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material, the newly formed Co nanoparticles and Co
2
B were
anchored on fresh surface of N–B–C and undecomposed B
4
C,
and the proposed structure is shown in Fig. 14e. In 0.1 M KOH
solution, the as synthesized Co
2
B/Co/N–B–C/B
4
C showed an
OER overpotential of 360 mV to achieve a current density of
10 mA cm
2
, which is identical to that of state-of-the-art RuO
2
,
much lower than that of nano-B
4
C and N-doped nano-B
4
C
(greater than 670 mV), and 80 mV lower than that of Co
2
B/
Co/B-C/B
4
C (Fig. 14f). Its faster OER kinetics was justified from
the lowest Tafel slope (111 mV dec
1
) as compared with that of
RuO
2
(118 mV dec
1
), Co
2
B/Co/B–C/B
4
C (129 mV dec
1
), nano-
B
4
C (492 mV dec
1
) and N-doped nan-B
4
C (495 mV dec
1
). This
catalyst also showed very good HER performance in 0.1 M KOH
solution with an overpotential of 220 mV, which was slightly
lower than that of Co
2
B/Co/B–C/B
4
C (235 mV), and much lower
than that of nano-B
4
C (421 mV) and N-doped nan-B
4
C (344 mV)
as displayed in Fig. 14g. It’s Tafel slope (105 mV dec
1
) was also
slightly lower than Co
2
B/Co/B–C/B
4
C(108mVdec
1
), and greatly
lower than nano-B
4
C (136 mV dec
1
) and N-doped nano-B
4
C
(149 mV dec
1
). These results indicate that N,B-codoping could
improve the electrochemical performance of transition metal-
based catalysts more significantly than boron doping alone.
Moreover, this catalyst also showed excellent durability for both
OER an HER with negligible loss in current density and over-
potential after 1000 CV cycles in 0.1 M KOH solution.
In a symmetrical OWS device of Co
2
B/Co/N–B–C/B
4
C8Co
2
B/
Co/N–B–C/B
4
C, a voltage of 1.622 V was required to achieve a
current density of 10 mA cm
2
with1.0MKOHsolutionas
electrolyte, and this cell also demonstrated excellent stability
during 10 h continuous overall water splitting. The superior
OWS performance of Co
2
B/Co/N–B–C/B
4
Ccouldbeattri-
buted to the following aspects: (1) the boron induced lattice
strain in Co structure leading to reduction of energy
barrier for the formation of OOH intermediates; (2) B doping
activates adjacent carbon atoms resulting in facile charge
transport and enhanced catalytically active sites leading
towards facilitated OH
chemisorption and uplifted OER
performance; (3) high density of Co-N species (Fig. 14e left)
in the catalyst take part actively in charge transfer to uplift
OER catalysis.
305,306
Fig. 14 (a) High magnification SEM image of BN-CNT@CoMoS
3.13
and the (b) HER polarization curves in 0.5 M H
2
SO
4
and (c) OER polarization curves in
1.0 M KOH. Reproduced with permission from ref. 303. Copyright (Elsevier) 2018. (d) A schematic illustration for the synthesis of Co
2
B/Co/N–B–C/B
4
C
and (e) the possible types of Co/N-doped and B-doped graphitic carbon structure derived from XPS data. The iR corrected (f) OER and (g) HER
polarization curves of Co
2
B/Co/N–B–C/B
4
C in 0.1 M KOH. Reproduced with permission from ref. 304. Copyright (American Chemical Society) 2018.
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Liu et al.
69
supported cobalt nanoparticles on N,B-codoped
carbon via direct carbonization of cobalt-based boron imida-
zole framework, and comparative investigation revealed that
the hybrid obtained at 900 1C has the best performance among
the materials prepared in the temperature range from 600 to
900 1C. In 1.0 M KOH solution, the optimal Co/NBC-900
exhibited a small OER overpotential of 302 mV to attain
10 mA cm
2
, which is comparable to state-of-the-art Ir/C
catalyst showing an overpotential of 298 mV under similar
conditions. Its Tafel slop of 70 mV dec
1
was also very close
to that of Ir/C, and its long-term stability was confirmed by
chronopotentiometry operation for 10 hours at 10 mA cm
2
showing only a slight change in potential. For HER process, the
as-prepared Co/NBC-900 revealed a small overpotential of
117 mV which was lower than many reported Co/NBC hybrids
in literatures and a small Tafel slope of 146 mV dec
1
implying
fast HER kinetics, and its good durability was verified by a
minute increase in overpotential from 117 mV to 142 mV
to retain a constant current density of 10 mA cm
2
after
5000 CV cycles.
In a two-electrode symmetrical cell with Co/NBC-900 on
both electrodes, a cell potential of 1.68 V was required to
achieve a current density of 10 mA cm
2
for overall water
splitting using 1.0 M KOH solution as electrolyte, and only
slightly increased voltage was observed after 6 hours of cell
operation at 10 mA cm
2
, demonstrating superior stability
of the Co/NBC-9008Co/NBC-900 OWS system. The authors
proposed that the good performance of Co/NBC-900 was bene-
fitted from the synergistic effects of partial oxidation of metallic
cobalt, conductive N,B-codoped graphitic carbon and carbon
nanotube, and the coupled interactions among these compo-
nents. Moreover, the carbonization temperature greatly influ-
ences pore size in the synthesized material, and the optimal
size obtained at 900 1C could lead to enhanced adsorption
of reaction intermediates during HER and OER processes
resulting in excellent performance.
Methodology of electrospinning followed by high tempera-
ture pyrolysis (Fig. 12c) was also employed to synthesize
Ni@BNPCFs
168
and Co@BNPCFs
261
catalysts for overall water
splitting. Among all the Ni@BNPCFs materials pyrolyzed in
temperature from 600 to 1000 1C,
168
the one obtained at 900 1C,
named as Ni@BNPCFs-900, showed the best HER, OER and
OWS performance in 1.0 M KOH solution. Specifically, it bears
an HER overpotential of 164.2 mV to achieve 10 mA cm
2
,
a Tafel slope of 61.35 mV dec
1
, and an excellent stability of
8 h chronoamperometric operation at an overpotential of 200 V
showing a current density retention of 97.28%, which was
evidently better than that of 88.67% for commercial 20 wt%
Pt/C at similar conditions. As for OER process, it required an
overpotential of 287 mV to achieve 10 mA cm
2
, which was
obviously lower than that of 327 mV for state-of-the-art RuO
2
,
displayed a small Tafel slope of 19.31 mV dec
1
which was
lower than one-third of that for RuO
2
(64.03 mV dec
1
), and a
good durability of 8h chronoamperometric operation at 1.6 V
(vs. RHE) retaining 91.78% current density which was signifi-
cantly higher than the value of 82.15% for RuO
2
. These results
suggest the superior HER and OER performance of Ni@
BNPCFs-900 to the benchmark noble metal based electro-
catalysts.
In a two-electrode device of Ni@BNPCFs-900||Ni@BNPCFs-
900 cell, the overall water splitting process delivered a current
density of 10 mA cm
2
at a cell voltage of 1.584 V, which was
10 mV lower than the cell voltage of 1.594 V for a precious
metal-based Pt/C||RuO
2
system, and it retained 96.1% of the
initial current even after 40 h of continuous operation at
1.585 V. Similarly, the OWS device using the optimal
Co@BNPCFs-800 on both electrodes demonstrated a cell
potential of 1.596 V to achieve 10 mA cm
2
in 1 M KOH
solution
261
and an outstanding stability of 30 hours at a cell
voltage of 1.66 V. Moreover, the OWS performance of the
devices based on both Ni@BNPCFs-900 and Co@BNPCFs-800
is better than that with Co
2
B/Co/N–B–C/B
4
C (1.622 V)
304
and
Co/NBC-900 (1.68 V) as catalysts to deliver 10 mA cm
2
.
69
In a word, N,B-codoped CNMs as catalyst support can
efficiently uplift the OWS performance and stability of TM/
CNMs hybrids, and the synthesis conditions, especially pyro-
lysis temperature, and extent and type of metal atoms present
in the material are the major factors affecting the performance
of water electrolyzers. It should also be highlighted that most of
the researches are conducted in alkaline electrolytes and less
focus has been devoted to acidic medium.
6. Conclusion and future prospects
Overall water splitting is an emerging technology for producing
green H
2
and pure O
2
gases, and different types of water electro-
lyzers based on innovative technologies are being increasingly
introduced in the market. In the current state of technology
development, the electrocatalysts used in water electrolyzers for
facilitating the performance are mainly noble metal-based
materials, which are rare, costly and also have stability issues
especially in the most efficient PEM electrolyzers. Therefore,
development of novel, low cost, highly active and durable
catalysts to be used in OWS is need of the day. In this regard,
widely available transition metal-based materials are being
evaluated as HER and OER catalysts, and some of them have
demonstrated promising performance. However, the poor
stability especially in acidic media and slow reaction kinetics,
make them unsuitable for commercial scale applications in
OWS system.
Supporting transition metal-based catalysts on carbon nano-
materials is a good strategy to enhance the activity and reaction
kinetics as well as stability due to the large surface area, high
porosity, high conductivity, increased active sites and synergis-
tic effects between metal cores and carbon support. However,
the comprehensive performance of TM/CNMs hybrids is still
not up to the mark for replacing noble metal-based catalysts. In
this case, non-metal heteroatom like N and B doping to the
carbon network in CNMs was developed to further increase the
catalytic performance benefiting from the different electro-
negativity as compared with C, which can easily modulate the
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electronic structure causing polarization on adjacent carbon
atoms to facilitate the adsorption of reaction intermediates.
In this article, recent progress of transition metal-based
catalysts towards HER, OER and OWS have been reviewed in
four groups of TM/CNMs, TM/N-CNMs, TM/B-CNMs and
TM/N,B-CNMs hybrids, and N,B-CNMs have been proved to
be the most efficient support for uplifting the water splitting
activity of transition metal-based catalysts because of the
simultaneous presence of electron donating and electron
accepting sites. However, the stability, which is a major para-
meter and challenge for the commercialization of TM/CNMs
hybrids as water splitting catalysts, is generally evaluated for
several hours only, as summarized in Tables 1 to 4. For
example, the maximum overall water splitting stability for the
catalyst family of TM/CNMs, TM/N-CNMS, TM/N,B-CNMs and
TM/B-CNMs, to the best of our knowledge, is reported as 25 h
operation with 10 mA cm
2
at 1.66 V for mCo
0.5
Fe
0.5
P/rGO,
169
150 h at 10 mA cm
2
with cell potential of 1.61 V for NCNT-
NP@NF,
148
and 40 h with 10 mA cm
2
at 1.585 V for
Ni@BNPCFs-900
168
in 1.0 M KOH solution, and 70 h with
10 mA cm
2
at 1.61 V for GH-BGQD2 in 0.1 M KOH
solution.
30
In addition, to the best of authors information,
none of the reported TM/CNMs hybrid catalysts has been
evaluated at an industrial-level current density of 2–3 A cm
2
for practical applications. These stability data are significantly
lower than the US department of energy (DOE) targets of
40 000 h operation at 2 A cm
2
with cell potential of 1.9 V for
year 2022 and 80 000 h operation at 3 A cm
2
with cell potential
of 1.8 V for year 2026 with maximum voltage loss of 10% in
both cases.
307
Therefore, it could be concluded that TM/CNMs hybrids
with N and/or B doping have shown considerable catalytic
activities for OER, HER and OWS, but their stabilities still need
to be uplifted by screening some appropriate strategies like
optimization of metal contents, reducing overpotentials by
tuning d-bands of metals, and encapsulation/confinement of
metals or their compounds in shells/pores of carbon materials.
Moreover, it is strongly suggested that all the water splitting
catalysts should be evaluated for longer hours of time to get a
clear aspect of their stability for commercial applications.
It would be worth mentioning that in most of TM/CNMs
hybrids for water splitting, the transition metal used are gen-
erally Ni, Fe and/or Co, while many other transition metals,
such as Cu, Mn, Zn, Sb and Sn and their compounds, that
displayed considerable performance in many other electro-
catalytic applications
308–310
have not been explored in-depth
towards HER, OER and OWS. Hence, many transition metals
other than Ni, Fe and Co, and their compounds in combination
with CNMs should be extensively evaluated/screened during
future research for their application in water splitting.
Another concern is that very less researches have been
reported for TM/B-CNMs and TM/N,B-CNMs hybrids as com-
pared with the extensively explored TM/N-CNMs materials, and
little effort has been devoted to understand the actual mecha-
nism and role of metal atom cations and dual non-metal hetero
atoms present in carbon matrix especially in the case of
TM/N,B-CNMs hybrids, hindering the development of efficient
catalysts as alternate for state-of-the-art noble metal catalysts
and the practical large scale commercialization of water
electrolyzers.
Therefore, there is a huge room in the field of water splitting
to develop more efficient electrocatalysts to replace the expen-
sive and scarce noble metal-based catalysts, and this goal can
be achieved by proper selection of metal core, optimization of
synthesis conditions especially the precursors and reaction
temperature, and selection of suitable CNMs material, based
on theoretical calculations assisted experimental research and
in situ observation helped in-depth mechanism understanding.
List of abbreviations
H
2
Molecular hydrogen
WEs Water electrolyzers
HER Hydrogen evolution reaction
OER Oxygen evolution reaction
OWS Overall water splitting
PEM Polymer electrolyte membrane
NHE Normal hydrogen electrode
RHE Reversible hydrogen electrode
CNMs Carbon nanomaterials
CNFs Carbon nanofibers
CNWs Carbon nanowires
CNTs Carbon nanotubes
rGO Reduced graphene oxide
GO Graphene oxide
TM Transition metals
N-CNMs Nitrogen-doped carbon nanomaterials
B-CNMs Boron-doped carbon nanomaterials
N,B-CNMs Nitrogen and boron-codoped carbon
nanomaterials
wt% Weight percent
G Gibb’s free energy
SCE Saturated calomel electrode
TMPs Transition metal phosphides
BGO Boron-doped graphene oxide
ECSA Electrochemically active surface area
3D Three dimensional
HCNs Hollow carbon nanospheres
SWCNTs Single-walled carbon nanotubes
MWCNTs Multi-walled carbon nanotubes
2D Two dimensional
CVD Chemical vapor deposition
TMBs Transition metal borides
CP Carbon paper
CC Carbon cloth
PBS Phosphate buffer solution
TMP Transition metal phosphides
SACs Single atom catalysts
C
dl
Double layer capacitance
CV Cyclic voltammetry
LSV Linear sweep voltammetry
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QDs Quantum dots
MOF Metal–organic frameworks
NPs Nanoparticles
TEM Transmission electron microscope
SEM Scanning electron microscope
DOE Department of energy
Conflicts of interest
There is no conflict of interest for this article.
Acknowledgements
This work was supported by the Oceanic Interdisciplinary
Program of Shanghai Jiao Tong University (SL2022ZD105).
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