Recent Progress on Boron-Doped Diamond Electrodes for Electrochemical CO 2 Reduction: A Mini-review

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Recent Progress on Boron-Doped Diamond Electrodes for
Electrochemical CO2Reduction: A Mini-review
Ayesha Tariq, Muhammad Zain Akram, Muhammad Daniyal Ghouri, Sabir Hussain, Sandeep Kanade,
Bharat B. Kale,*and Manu Gautam*
Cite This: https://doi.org/10.1021/acs.energyfuels.4c00410
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ABSTRACT: In recent years, boron-doped diamond (BDD) electrodes
have attained great significance and emerged as outstanding potential
candidates for electrochemical carbon dioxide (CO2) conversion to
valuable products. The features like chemical stability, abundant
economical raw material, and long cyclic stability of BDD electrodes
made them highly competitive as compared to the conventional metal-
based electrodes. However, the direction of research approach is not
focused and not adequate for improvement in the design, yield, and
selectivity. Most of the countries have targeted the achievement of “net
zero”, i.e., utmost removal of CO2from the atmosphere that has been
emitted by human activities within the next decade. In this context, we
have reviewed electrochemical CO2reduction using a diamond electrode.
In this mini-review, we used the curated literature available in the CAS
content collection to present a systematic analysis of the various approaches applied by scientists on recent developments on BDD
electrodes for electrochemical reduction of CO2. More significantly, we wisely addressed the challenges and future perspectives to
improve the yield and selectivity of CO2reduction products as a direction to researchers in this field. Multiple strategies have been
discussed allied to tackle the high overpotential and low carbon monoxide yield issues. The review highlights the current status and
developments with focus on understanding the reaction mechanisms, impact of dopant concentration on performance, improved
electrolyte designs, surface characteristics, choice of electrolytes, and challenges such as low yield and unsatisfactory selectivity of
BDD based CO2electroreduction. Our analysis highlights the latest trends alongside the associated challenges with BDD based CO2
electroreduction and future direction for researchers.
1. INTRODUCTION
Increasing atmospheric CO2concentration emitted from
anthropogenic sources has forced researchers to work on CO2
conversion into green chemicals.
1,2
In recent years, electro-
chemical reduction of CO2(eCO2RR) has become a promising
approach to reduce harmful CO2to valuable single as well as
multicarbon products.
3−8
eCO2RR has shown its ecacy as a
potential alternative route for hydrogenation and re-formation
in terms of providing a more ecient and convenient
methodology and product formation pathways. Utilization of
renewably empowered technologies in coupling with eCO2RR
has further raised the attention toward this method in mitigating
environmentally harmful impacts using renewable energy.
9−11
Various electrocatalysts have exploited conversion of CO2into
various single and multicarbon products such as CO, HCOOH,
CH3OH, C2H2, etc.
12−18
The formation of these products
depends on the stabilization of the intermediate during
mechanistic pathways.
19−23
Various methodologies have been
employed in terms of electrolyzer design to enhance the overall
performance of eCO2RR in both aqueous and nonaqueous
electrochemistry.
24−29
The eciency and the product formation
in eCO2RR are mainly dependent on the activity and
characteristics of the electrocatalyst used in the electrochemical
system. For examples, Ag, Pb, and Bi are selective to HCOOH/
formate and Cu, Zn, Sn are selective toward CO and other
products.
30−36
Sn and Pb electrodes have been reported more
frequently owing to their high faradaic eciencies for formic
acid production and cost-eectiveness.
13
However, their
instability during long-term electrolysis and environmental
load are the issues that have prompted researchers to look for
more stable and environmentally friendly alternates.
37
First reported in 2014, boron-doped diamond electrodes
emerged as sought-after candidates for high yield electro-
Received: January 24, 2024
Revised: May 12, 2024
Accepted: May 15, 2024
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chemical reduction of CO2to formaldehyde.
38
Since then, BDD
electrodes have gained tremendous consideration in eCO2RR
owing to their outperforming electrochemical character-
istics.
39,40
A wide potential window, low background current,
decent long-term stability, and outstanding physiochemical
properties make this material highly ecient for eCO2RR.
41
Introduction of metal oxides into this material have also been
able to show the ecacy of this material in terms of further
reduction in overpotential.
42,43
Multiple factors are responsible
for influencing eCO2RR with the boron-doped electrodes. Out
of those, one is the nature of the electrolyte. It has been reported
that KCl when used as electrolyte favors the formation of
HCOOH, whereas KClO4is used directly in the formation of
CO as a main product.
44
The performance of boron-doped
electrode in eCO2RR is also aected by the electrolyzer design
and electrolyte conditions used.
45−47
The durability of BDD
through all of the above-mentioned influential factors has made
it a desirable electrocatalyst for eCO2RR. Multiple studies have
been reported for BDD application in eCO2RR. Nevertheless,
more and more work is being done on this material, unveiling
new properties that have extended the scope of reporting the
cumulative review of these studies.
This mini-review highlights the importance of boron-doped
diamond electrodes toward eCO2RR with all recent studies
representing the significance of these electrocatalysts in terms of
faradaic eciency and stability. This further explains the
mechanism followed on this electrocatalyst for eCO2RR
followed by numerous studies reported on a modified boron-
doped diamond electrocatalyst, Figure 1. Moreover, the
challenges associated with this system have been explained
which further govern the future perspectives.
2. FUNDAMENTALS OF BDD FOR ELECTROCHEMICAL
CO2REDUCTION
Pure carbon-based materials with pure sp2and sp3species do not
possess much activity for CO2RR; however, the doped carbon
materials such as BDD have shown excellent CO2RR perform-
ance.
40,48
The BDD-based electrodes are suitable for electro-
chemical CO2reduction, as they oer high conductivity, a wide
potential window in aqueous solution, and a negligible
background current. Since the boron content determines the
electrical conductivity of diamond, it can greatly influence the
electrochemical properties. Other factors that contribute to
electrochemical properties of BDD electrodes are sp2carbon
concentration and surface functionalities.
39
The presence of sp2
carbon impurities also influences the electrochemical perform-
ance of BDD electrodes and aects the selectivity of BDD in
CO2RR. Similarly, boron doping concentration is a driving
factor for BDD electrodes performance and selectivity. The
boron doping level dictates the electrical conductivity of the
electrodes. Higher doping levels increase the carrier concen-
trations and in turn increase the electrical conductivity.
49,50
The selectivity in the eCO2RR of BDD can be impacted by the
high concentration of sp2impurities, especially when the boron
doping levels are low. The sp2carbon provides adsorption sites
that promote higher intermediate CO2adsorption, leading to
the generation of carbon monoxide (CO) as the primary
product on the BDD surface.
44,51
Electrochemical CO2RR
comprises multiple charge (electron/proton) transfer processes
leading to the reduction of CO2into dierent gaseous and liquid
products such as hydrocarbons (CH4and C2H4), formic acid
(HCOOH), carbon monoxide (CO) and alcohols (CH3OH
and C2H5OH). The reaction mechanism and selectivity of
products are dependent on the electrocatalysts and the
electrochemical reaction conditions such as applied potential
and type of electrolyte. To trigger the CO2reduction
electrochemically, the foremost step is to generate the CO2
•−
radical intermediate, which can be dicult in the absence of a
catalyst. By the means of an electrocatalyst, a negative redox
potential can be generated when chemical bonding is developed
between CO2and the electrocatalyst, leading to the stabilization
of CO2
•− radical. As discussed earlier, the sp2carbon impurity
concentration directly impacts the product selectivity due to its
adsorption sites. Higher sp2carbon content promotes higher
intermediate CO2adsorption, leading to the formation of CO as
the primary product. On the other hand, HCOOH is produced
on the electrocatalyst surface in the absence of sp2carbon
impurity in the BDD electrode.
3. INFLUENCE OF BORON AND SP2CONTENT ON THE
BDD-BASED ELECTROCHEMICAL CO2REDUCTION
As it is known, the BDD electrode surface is mainly selective to
two eCO2RR products, i.e., HCOOH and CO. It is been widely
established that the boron doping level can have a strong impact
Figure 1. Schematic representation of electrochemical reduction of CO2on a boron-based diamond electrode.
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on the electrochemical properties of BDD electrodes.
Schwarzova-Peckova et al.
52
have reported that how variation
of the boron content can influence the morphology as well as the
electrochemical properties of BDD electrodes. Cyclic voltam-
metric performances having outer and inner sphere redox
species performed to dierentiate among the semiconductive
films (500 and 1000 ppm) and films with metallic conductivity
(2000−8000 ppm). Reversible behavior with peak separation,
i.e., ΔEpof 59.8 ±0.9 mV (n= 5) and Ipa/Ipc ratio 1.00 at 100
mV s−1scan rate was observed with higher boron content value.
The electrochemical reduction of CO2follows the two-steps
reaction mechanism which involves the formation of anion
radical (CO2
•−) as intermediate followed by the formation of
HCOOH and CO with their corresponding steps. The
production of CO and HCOOH depends on the ability of the
BDD surface to bind the CO2
•− radical. The electrode surface
that poorly adsorbs CO2
•− mainly produces HCOOH. In this
case, the electron transfer from the electrode to CO2occurs via
an electrical double layer and the unstable CO2
•− immediately
reacts to produce HCOOH. On the other hand, the electrode
surfaces that can bind CO2
•− adequately, mainly forms CO.
In general, it is considered that increasing the boron doping in
diamond enhances the adsorptions of intermediate species,
which, in turn, promotes the production eCO2RR products of
BDD electrodes. Xu et al.
50
performed a quantitative analysis to
establish a relation between boron content in diamond
electrodes and the resulting CO2reduction products. They
developed BDD electrodes with 0.01, 0.1, 0.5, 1, and 2% boron
content and observed that, for the lowest boron content, i.e.,
0.01%, the faradaic eciency of hydrogen evolution was more as
compared to HCOOH formation. The highest faradaic
eciency for HCOOH production was achieved with a 0.1%
boron-doped diamond electrode.
50
In general, the faradaic
eciency for hydrogen evolution increased with increasing
boron content, which was attributed to the narrower potential
window of BDD achieved with higher boron content. Mean-
while, the faradaic eciency for CO production increased
slightly as the boron content increased in the electrodes and the
faradaic eciency for producing HCOOH decreased between
the range of 0.1 and 2%.
50
Their study established that the
intermediate adsorption species can be controlled by altering the
boron doping level in diamond electrodes, Figure 2.
50
Besides boron content, the concentration of sp2carbon and
surface termination are the factors that influence the electro-
chemical properties of BDD electrodes for CO2electro-
reduction. The eect of various sp2/sp3ratios on a 0.1% BDD
electrode on the electrochemical reduction of CO2was
systematically studied by Xu and Einaga.
51
They found that,
with increasing sp2content, the faradaic eciency for the H2
production increased under constant current density, whereas
the production of HCOOH decreased. This study unveils that
the presence of sp2carbon influences the rate of CO2reduction
under constant current, which is a competition between HER
and eCO2RR. Without the involvement of sp2carbon impurity
on BDD electrodes, HCOOH emerged as a main product while
H2and CO were the side products. Considering these outcomes,
CO2RR outperforms HER on a pure sp3surface BDD with a
high selectivity of HCOOH. Meanwhile, BDD electrodes
Figure 2. (a) Schematics showing the electrochemical reduction of CO2to CO and HCOOH on a boron-doped diamond electrode and (b) FEs of
variation with increased boron content representing the lower HCOOH and higher CO FEs at higher boron content. Reprinted with permission from
ref 50. Copyright 2018 Elsevier.
Figure 3. Dierent cell design for eCO2RR (a) H-cell configuration where both the compartments are separated by ion exchange membrane or frit, and
(b) flow cell mode where anolyte and CO2saturated catholyte flow to the electrocatalyst with a constant flow.
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containing sp2species produce a smaller amount of HCOOH
and a relatively higher amount of CO which is attributed to a
higher adsorption capability of CO2
•− on the sp2surface that
aects the selectivity of CO2RR products.
51
4. UTILIZATION OF DIFFERENT CELL DESIGNS IN
ECO2RR ON BDD-BASED ELECTRODES
The eciency of electrochemical reduction of CO2is largely
dependent on the cell design.
45,46
The most basic electrolyzer
setup exists in the form of H-configuration, Figure 3a, and is
often called an H-cell having two compartments either separated
by ion exchange membranes or a frit.
24,53
This setup has been
widely explored, because of its easy operation and suitable
product quantification. This also provides the independence of
influence of any anodic product generated during electrolysis
compared with the single compartment electrolyzer cell. Besides
these advantages, some major challenges restrict the utilization
of this setup in practical applications. Mass transport limitations
due to limited solubility of CO2is a major challenge which
restricts the current density of this electrolyzer cell setup. In this
regard, flow cells have gained tremendous consideration to
enhance the overall performance during electrolysis because of
sucient mass transport of electroactive species, lower cell
resistance due to decreased electrode in-between distance, and
flexibility in electrolysis during liquid and gas phase electrolyzer,
Figure 3b.
40,47
Natsui et al.
37
have utilized the flow cell for electrochemical
reduction of CO2to HCOOH with using boron-doped diamond
(BDD) electrodes and reported superior electrochemical
properties in terms of 94.7% HCOOH faradaic eciency and
stability up to 24 h. The production of electrochemical product
also depends on how much time electroactive species spend near
the electrode−electrolyte surface. Irkham et al.
46
have reported
eCO2RR on BDD electrode by controlling and optimizing the
mass transport of CO2to the electrode using a synchronized
dual-phase double-action cylindrical pump. They have designed
a continuous liquid-fed intermittent flow electrolyzer by giving
sucient time to CO2to reduce to HCOOH by controlling the
flow. 9.6 times higher faradaic eciency have been reported by
this intermittent flow cell system than that of a continuous flow
cell, and the HCOOH amount generated after 4 h of electrolysis
was 8 times that produced by the continuous flow cell system.
46
The formation of the product also depends on the variation of
some parameters such as potential and boron concentration in
the flow cell configuration. Tomisaki et al.
44
have shown the
dependency of eCO2RR products upon the boron concen-
tration, the nature of the electrolyte, and applied potential. CO
tended to increase with the utilization of KClO4as electrolyte,
whereas HCOOH tended to increase when the electrolyte was
KCl, Figure 4.
44
Also, the selectivity of CO was enhanced when
boron concentration in BDD electrode increased to 1% from
0.1%.
44
The flow rates of electrolytes also play a crucial role in
enhancing the faradaic eciency. Tomisaki et al.
44
also reported
the increase in faradaic eciencies of CO and HCOOH with
increase in flow rates of electrolyte. This is further expanded by
Otake et al.,
54
where they have demonstrated the electro-
chemical generation of HCOOH in a halogen-free electrolyte.
Accelerated electron transfer rate on BDD electrode has been
observed in the K2SO4electrolyte solution.
54
These studies
show the crucial dependency of eCO2RR on electrolyte and flow
conditions using the BDD electrode. The influence of
convection on eCO2RR using BDD electrode is further studied
by Nihongi et al.
47
They have shown that variation in frequency
can aect the overall FEs of the products. This is because of
promoted mass transfer of CO2to the electrode surface with the
increase in the frequency.
47
All of these studies unveil the
importance of flow electrolyzer cell in eCO2RR using a BDD
electrode.
5. MODIFIED BDD ELECTRODES FOR ECO2RR
Metal modification of BDD electrodes has proven to be a
sought-after way of customizing the electrode’s electrochemical
performance for various applications. The idea behind this
concept is to use the metal of choice to make BDD electrodes
perform better for targeted electrochemical processes and
sensing applications. Previous research indicates that by
modifying the BDD surface with copper particles, it is possible
to produce organic compounds with more than one carbon. A
stable deposition of Cu on a BDD surface led to the production
of ethanol, acetaldehyde, and acetone as a result of CO2
reduction at −1.0 V vs Ag/AgCl.
55
Meanwhile, Au modified
BDD electrodes have shown tendency of producing CO as the
main CO2electroreduction product, improving the process
selectivity.
56
Recently, BDD modification by bimetallic (Cu−
Au) particles was reported wherein mixed Cu−Au medication
was conducted on the BDD surface by Saprudin et al.
57
They
were able to electrochemically reduce CO2to HCOOH and
acetic acid. They found that mixed metal modification oers
better stability of the metal particles on the BDD surface as
compared to solo modification by either metal.
57
Some research
groups have dedicatedly conducted studies on the BDD surface
modification impact on electrochemical CO2reduction. Jiwanti
et al.,
58
at first, employed a wet chemical seeding approach to
modify the surface of the BDD electrode with copper−nickel in
order to stabilize the metal particles as the common electro-
deposition methods would lead to instability and detachment of
metal particles. They took NaBH4as a reducing agent in 0.1 M
NaOH solution and drop casted it on the surface of the BDD
electrode followed by the metal precursor, resulting in
significant adsorption on the electrode surface. The electrode
that had been altered by wet seeding was then subjected to
electrochemical deposition with the goal of increasing the
number of deposited metal particles. Thereafter, to ensure the
attachment the metal particles on the BDD surface, thermal
annealing was carried out for 5 min at 700 °C. The thermal
annealing, however, passivates the modified electrode, neces-
Figure 4. Schematics showing the dependency of FEs of CO2RR
products on the nature of electrolytes. Reprinted with permission from
ref 44. Copyright 2019 American Chemical Society.
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sitating another polishing step in the electrochemical process. As
an electrochemical polishing method, multiple cycles of cyclic
voltammetry were used in this work.
58
Figure 5a shows an SEM
image of the CuNi-BDD electrode. The Cu and Ni particle
contents, according to EDS, were 0.11% and 0.14%, respectively
(C-99.5% and O-0.59%). Further, elemental analysis of the
modified BDD electrode was performed by XPS spectrum,
Figure 5b. Cu 2p3/2 is represented by the peak at 933.9 eV, and
Ni 2p3/2 is represented by the peak at 856 eV, proving that there
were metal particles on the modified BDD electrode surface.
Additionally, Raman spectroscopy was performed to examine
the stability of the BDD electrode after all of the deposition
treatments, Figure 5c,
58
although the previous research
suggested that the sp3carbon bonding of BDD was aected by
the high temperature thermal annealing and multiple deposition
processes.
59
CuNi modified BDD electrode retained its
distinctive characteristics, i.e., the B−B bond peak at 1220
cm−1and the sp3carbon bonding represented by the 1332 cm−1
peak. No evidence of sp2carbon bonding was seen at around
1500 cm−1. Therefore, it was established that the fabrication
approach utilized in this study to deposit Cu and Ni did not
harm the intrinsic properties of BDD electrode. While
unmodified BDD led to the formation of HCOOH as the
main CO2reduction product, the Cu−Ni modified BDD
generated CH3OH and CH4along with HCOOH because of
CO2electroreduction.
58
Palladium-doped BDD (Pd-BDD) electrodes were also
synthesized by Jiwanti et al. via a chemical vapor deposition
technique with microwave plasma assistance.
60
Pd-BDD
electrodes were developed at a potential of −0.15 V with
various deposition times, i.e., 30, 100, 300, 500, and 1000 s. CO2
were electrolyzed in two cells that were divided by a Nafion
membrane. The Pd-BDD with a deposition time of 100 s is
represented by SEM analysis in Figure 5d. They found that
unlike Pd-BDD electrodes with 500 and 1000 s deposition times
where the agglomeration of particles leads to larger sized
particles (∼160 nm), the Pd particles got deposited evenly over
the active portions of the Pd-BDD (100 s deposition) surfaces
with an average of ∼50 nm particle size. The areal density of the
particles on PdBDD30 is 2.75 μg cm−2which increases with
prolonged deposition times, Figure 5e. The faradaic eciency of
eCO2RR productions for all the Pd-BDD electrodes at a
potential of −1.5 V for 1 h is shown in Figure 5f. The
dependency of the faradaic eciency on the electrode potential
Figure 5. (a) CuNi-BDD electrode SEM micrograph, (b) XPS analysis, and (c) Raman spectrum. Reprinted with permission from ref 58. Copyright
2019 Directorate of Research and Community Engagement, Universitas. (d) SEM image of the Pd-modified BDD electrode prepared at −0.15 V with a
deposition time of 100 s; (e) Pd loading on BDD vs deposition time; and (f) faradaic eciencies related to the generation of products from the
electrochemical reduction of CO2at a potential of −1.5 V versus Ag/AgCl for 1 h. Reprinted with permission from ref 60. Copyright 2019 Royal
Society of Chemistry. (g) MXene-BDD SEM picture at 1.0 mg mL−1MXene concentrations. (h) HCOOH concentration derived from the
electrochemical reduction of CO2at various concentrations of MXene at a potential of −1.7 V (vs Ag/AgCl). Reprinted with permission from ref 61.
Copyright 2019 Multidisciplinary Digital Publishing Institute.
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Table 1. Overview for All Reported Conditions Such As Dopant Concentration, Choice of Electrolytes, Electrolyzer Conditions, and CO2RR Product Eciency
Working electrode Geometric
area, cm2
Boron
doping
concn, % Work highlight Potential or j
applied Main
product FE, %
a
Electrolytes used ref
BDD 9.62 0.1 Electrolyte flow rate control −2 mA cm−2HCOOH 35.4 (20 mL/min) and
94.7 (200 mL/min) Catholyte, 0.5 M KCl;
anolyte, 1 M KOH 37
CO 2.7 (500 mL/min) and 0.4
(20 mL/min)
BDD 0.790 0.1 Electrochemical reduction of CO2using a BDD cathode
in a two-electrode system 3.5 V HCOOH 96 Catholyte, 0.1 M KCl;
anolyte, 0.1 M Na3PO4
45
BDD 16.4 0.1 Intermittent flow electrolyzer vs continuous flow cell −2.5 V vs the
Ag rod HCOOH 96 Catholyte, 0.5 M KCl;
anolyte, 0.5 M K2SO4
46
BDD 9.62 0.1 Influence of convection in a flow cell system −2 mA cm−2HCOOH 96 (f, 7.14 s−1) and
62.7 (f, 0.26 s−1)Catholyte, 0.1 M H2SO4;
anolyte, 0.5 M KOH 47
BDD with dierent
boron doping
content
9.62 0.01 Eect of boron doping concentration −2 mA cm−2HCOOH ∼35 Catholyte, 0.5 M KCl;
anolyte, 1 M KOH 50
0.1 75
BDD with dierent sp2
carbon content 9.62 0.1 Influence of sp2carbon content −2 mAcm−2HCOOH 84 (no sp2), 36 (mid-sp2), and
16 (high-sp2)Catholyte, 0.5 M KCl;
anolyte, 1 M KOH 51
−2.9 V (vs Ag/
AgCl) CO 7 (high-sp2) and 1 (mid-sp2)
Cu-BDD, Au-BDD,
and CuAu-BDD Not
determined 1 Cu, Au, and CuAu modified BDD electrodes −1.0 V
(vs Ag/AgCl) HCOOH 30.32 (Cu-BDD),
35.57 (Au-BDD), and
40.31 (CuAu-BDD)
0.5 M KCl 57
Pd-BDD 0.754 1 Pd modified BDD −1.6 V
(vs Ag/AgCl) CO 53.30 Catholyte, 0.1 M NaCl;
anolyte, 0.1 M Na2SO4
60
MXene-BDD 0.754 1 MXene-BDD 2.0 −1.3 V
(vs Ag/AgCl) HCOOH 97 Catholyte, 0.5 M KCl;
anolyte, 0.5 M KOH electrolyte 61
Surface modified BDD 3.300 1 Surface modification to improve the electractive area;
rough surface BDD vs flat and porous surface BDD
−1.9 V
(vs Ag/AgCl) CO 52.7 (f-BDD) and 48 (r-BDD) Catholyte, 0.1 M KClO4;
anolyte, 1 M KOH 64
a
mL/min is electrolyte flow rate. no sp2, mid-sp2and high-sp2refers to no, medium and high sp2carbon content, respectively. fis the frequency of pressure change. Cu-BDD is copper-doped BDD, Au-
BDD is gold-doped BDD and CuAu-BDD is copper and gold-doped BDD. f-BDD is flat surface BDD and r-BDD is rough BDD.
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from −1.4 to −1.9 V was further evaluated for Pd-BDD
electrode with various Pd deposition times, and it turned out
that the maximum production rate for CO is 53.3% with
HCOOH 9.3% and H239.9% was achieved for Pd-BDD with
300 s Pd deposition time at the potential of −1.6 V, which was 5-
fold higher than that obtained with a BDD electrode with no
metal deposition indicating that the surface decoration of BDD
with Pd helped in reducing the overpotential for CO2reduction.
Jiwanti et al.
61
also synthesized MXene, using LiF-HCl
treatment and found that the presence of the MXene in BDD
helped in reducing the electroreduction overpotential of CO2.
Figure 5g illustrates the SEM micrograph of MXene-BDD with
MXene concentrations of 0.5 mg mL−1. The quantification of
CO2electroreduction products at a potential of ∼1.7 V (vs Ag/
AgCl) was performed using each of the bare BDD and MXene-
BDD electrodes at dierent concentrations, Figure 5h. The
amount of HCOOH products generated by the MXene-BDD
electrode grew in proportion to the variety of MXenes coated on
the surface of the electrode. With an HCOOH concentration of
11.41 ppm, the MXene-BDD 2.0 electrode generated the highest
HCOOH concentration. The SEM and EDS analyses revealed
that the mass percentage of MXene particles deposited did
increase with increase in MXene concentration on the electrode
surface.
61
It was determined that MXene, which had a high
surface area and high electrical conductivity, promoted the
electrochemical reduction of CO2to produce HCOOH. Due to
the increased electronegativity on the BDD surface caused by
the increased MXene concentration, CO2was able to approach
the electrode surface and decrease even more readily.
61
Table 1
represents the overview of all reported conditions such as
dopant concentration, choice of electrolytes, electrolyzer
conditions and CO2RR product eciency.
In essence, the electrochemical reduction of CO2on BDD
electrodes shows promising performances for addressing
environmental concerns, producing valuable chemicals, and
advancing the development of renewable energy technologies. It
represents a multifaceted approach with implications for carbon
capture, utilization, and storage, as well as the integration of
electrochemical processes with sustainable energy sources.
6. CHALLENGES
The generation of multicarbon products in higher eciencies
from electrochemical reduction of CO2on BDD electrodes has
remained a challenge. In eCO2RR, Cu-based electrocatalysts
have shown tremendous potential to produce multicarbon
products but due to the sluggish kinetics of C−C coupling raises
the challenge of stabilization of these products. BDD electrodes
have not been well-utilized for the electrochemical reduction of
CO2to multicarbon products. Poor selectivity and low yield are
still the main challenges that must be addressed. Although, the
active research has been conducted on BDD electrodes for
eCO2RR for almost a decade now but still the yield reported in
the literature is not as per expectation. The highest recorded
yield rate ∼190 μmol h−1cm−2for CO2to HCOOH conversion
was reported by a design modification of cell that allowed the
control of electrolyte flow.
45
Surface engineering techniques
hold promise for eCO2RR activity enhancement and based on
that, eorts have been accomplished in the past to improve the
surface area and porosity of carbon-based electrodes.
62,63
In
attempt to overcome the low-yield issue, a surface modulation
strategy was recently introduced wherein and a flat 1% BDD film
(f-BDD) was compared to a porous 1% BDD film (p-BDD) and
a rough-surfaced 1% BDD film (r-BDD) for electrochemical
properties and CO2reduction activity.
64
Despite possessing the highest real surface area, p-BDD
exhibited the lowest e-CO2RR activity. On the other hand, r-
BDD outperformed f-BDD for CO production by providing
approximately 1.7-fold increases in current density and yield.
The ecient eCO2RR activity in r-BDD was attributed to its
larger electroactive area with a comparable kinetic performance
in comparison to f-BDD. A wider potential window has been
seen for the flat and rough surfaced BDD with a visible oxidation
peak around +1.5 V in cyclic voltammetry measurements which
was attributed to be a characteristic of sp2-bonded carbon in the
grain boundaries of polycrystalline BDD films. Meanwhile, the
narrower potential window and higher capacitive current was
seen for porous BDD films. An assignable cause for the low
activity for eCO2RR to CO for p-BDD was implicated to be the
diculty of allowing CO2molecules to diuse inside the pores.
Although the porous surface of BDD has the largest real surface
area, the unreachable and inactive area inside the holes made it
dicult for CO2molecules to diuse inside leading to poor
eCO2RR activity.
The eect of the morphology modulation on the electro-
chemical active area was investigated, which revealed that the r-
BDD demonstrated the largest electroactive area followed by p-
BDD and f-BDD. These findings promote the significance of
surface morphology modulation to achieve a large electroactive
exposed surface area, which can be helpful for the interactions
between the reactants and the electrode surface leading to high
CO2electrochemical reduction yields. This highlights the
ecient utilization of modified electrode surface by influencing
the surface morphology in the direction of enhancing the
electrochemical properties. More significantly, the honeycomb
like morphology and other hierarchical 2−3D surface structures
needs to be examined.
In terms of cell design, the flow cell electrolyzers have shown
their significance over two compartment H-cell configuration in
terms of current densities and eciencies by avoiding the mass
transport limitations due to the limited solubility of CO2in
aqueous electrolyte. Utilization of gas diusion electrodes
(GDEs) has gained much attention to provide the CO2
availability near the electrocatalyst through backside of its
porous structure. To further reduce the performance barrier and
enhance the eciency, zero gap or MEA-based electrolyzers
have shown their ecacy. However, CO2, carbonate crossover,
and pH variations during the electrolysis have triggered
researchers to explore other well-functioning electrolyzers.
Recently, bipolar membrane based and three compartment
based electrolyzers have emerged as potential candidates for
enhancing the underutilized performance for formation of
HCOOHs and other eCO2RR products.
65,66
These config-
urations have not been explored with the BDD electrode.
Moreover, the utilization of nonaqueous electrolytes for CO2
reduction has obtained much consideration in recent years due
to higher solubility of CO2in these electrolytes compared with
aqueous electrolytes. Utilization of nonaqueous solvents with
BDD electrodes in flowing conditions with multiple config-
urations is needed to further show the eectiveness of this
electrocatalyst.
7. CONCLUSION AND FUTURE PERSPECTIVES
We have discussed how the research in past decade has helped in
performance enhancement of performance BDD electrodes for
electrochemical CO2reduction. Strategies like dopant concen-
Energy & Fuels pubs.acs.org/EF Review
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Energy Fuels XXXX, XXX, XXX−XXX
G

tration control, surface modification with active metals,
enhanced surface area and conductivity, modification of cell
design, and working on the choice of electrodes have greatly
assisted in excellent performance of BDD electrodes. Electro-
chemical properties of BDD are remarkable and the
fundamental aspects discussed in this review provide guidelines
for the design of BDD electrodes suitable for eCO2RR
application and development. The future research should be
focused on improved synthesis methods for better electrode
quality, modification of electrodes, and cell designs to tackle the
major challenges like diculty of CO2
•− intermediate
generation at low overpotentials, selectivity, and low yield.
The development of electrocatalyst with remarkable stability in
particular solvent and higher activity toward CO2RR products
by simultaneously representing the higher overpotential window
for unwanted hydrogen evolution reaction are some key
challenges, and BDD electrodes have shown promising behavior
in overcoming these setbacks. The underutilized applications of
these electrodes can be further enhanced by the addition of
metal catalyst (metal codoping) into BDD electrocatalyst.
Multiple factors such as catalyst thickness, surface coverage,
morphology, porosity, cocatalyst composition and distribution
defect densities etc. direct the overall activity of electrocatalyst
for CO2RR. The introduction of metal-based catalysts into BDD
and maintaining the optimum values of these CO2RR eciency
deciding factors are highly crucial and can be controlled by
modifications in doping techniques such as electrodeposition.
Detailed investigations of the stability of the intermediate or its
binding energy on the catalyst surface and the favorable
formation of the CO2RR product based on the metal
characteristics and composition should be explored further.
Theoretical investigations are highly required to understand this
binding energy and the synergistic eect of composites. The AI/
ML technique might be helpful for cocatalyst doping and
composition.
BDD electrode consists of a wide potential range in both
aqueous and nonaqueous electrolytes which makes it a suitable
candidate for electrochemical reduction of CO2in nonaqueous
media which recently gained much consideration due to the
suppression of parasitic hydrogen evolution reaction. The
utilization of BDD electrodes and their composites in flow cell
configurations would be crucial to obtaining a higher current
density with reasonable faradaic eciencies of the CO2RR
products by simultaneously eliminating the mass transport
limitations. Moreover, the eect of cations and pH are also
crucial parameters in the CO2RR because of their tendency to
influence the absorption of the intermediate. The behavior of
BDD electrode toward CO2RR in the presence of cations and
pH needs be investigated further. Ionic liquids (ILs) have
recently contributed to capturing CO2and its further conversion
to valuable fuels. With the benefits of having a wider potential
range and HER suppression, ILs tend to be suitable electrolytes
for the CO2RR. BDD electrode can be further explored in these
electrolytes for CO2conversion applications that are thermally
quite stable and safe. In-situ characterization techniques such as
surface-enhanced Raman spectroscopy (SERS) can be used to
investigate the mechanistic pathways of the CO2RR inter-
mediate using BDD electrodes in these ILs.
The BDD electrode market is expected to grow significantly in
the upcoming years. BDD is mainly grown via high-temperature
and energy-intensive chemical vapor deposition (CVD) and
sub-CVD techniques. The operating parameters such as
temperature, deposition time, and pressure have impacts on
the final product’s characteristics. The cost associated with the
manufacturing techniques is the main barrier in large-scale
diamond production. With technological advancements, the
cost of manufacturing and modifying BDD and BDD-based
electrodes can be regulated. However, a proper analysis to
explore the cost model for the commercialization of these
technological advances is indispensable and is highly recom-
mended. This would be essential in establishing an unexplored
cost-reduction strategies for streamlining the manufacturing
technologies and operating conditions for commercial level
boron-doped diamond-based electrochemical CO2reduction.
■AUTHOR INFORMATION
Corresponding Authors
Manu Gautam −Conn Center for Renewable Energy Research,
University of Louisville, Louisville, Kentucky 40292, United
States; orcid.org/0000-0002-2061-4646;
Email: manu.gautam@louisville.edu
Bharat B. Kale −MIT World Peace University (MITWPU),
Kothrud Campus, Pune, India &Centre for Materials for
Electronics Technology (C-MET), Government of India, Pune
411008, India; orcid.org/0000-0002-3211-717X;
Email: bbkale1@gmail.com
Authors
Ayesha Tariq −CAS Key Laboratory of Nanosystem and
Hierarchical Fabrication National Center for Nanoscience and
Technology, Beijing 100190, China; University of Chinese
Academy of Sciences, Beijing 100049, China
Muhammad Zain Akram −Deposition Technology
Innovations, Jeersonville, Indiana 47130, United States
Muhammad Daniyal Ghouri −University of Chinese Academy
of Sciences, Beijing 100049, China; CAS Key Laboratory for
Biomedical Eects of Nanomaterials and Nano Safety and
CAS Center for Excellence in Nanoscience, National Center for
Nanoscience and Technology, Beijing 100190, China;
orcid.org/0009-0009-4019-1757
Sabir Hussain −Tyndall National Institute, University College
Cork, Lee Maltings Complex, Cork T12R5CP, Ireland
Sandeep Kanade −Department of Chemistry, Indian Institute
of Science Education &Research, Pune, Maharashtra 411008,
India
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.energyfuels.4c00410
Notes
The authors declare no competing financial interest.
Biographies
Ayesha Tariq obtained her bachelor’s degree in Nanoscience and
Nanotechnology. Later she joined CAS Key Laboratory of Nanosystem
and Hierarchical Fabrication, National Center for Nanoscience and
Technology of China, and is currently pursuing her Ph.D. Her research
interests mainly focus on advanced nanomaterials for renewable energy.
Currently, she is working on modern electrochemical CO2reduction
techniques and next-generation lithium-ion batteries.
Muhammad Zain Akram completed his Ph.D. in Material Science and
Engineering from the National Center for Nanoscience and
Technology, Chinese Academy of Sciences, in collaboration with the
Conn Center of Renewable Energy Research, University of Louisville,
USA. His research interests include CVD synthesis of single and
polycrystal diamond, PVD thin films, and, one-dimensional metal oxide
nanomaterials for renewable energy applications. Muhammad currently
Energy & Fuels pubs.acs.org/EF Review
https://doi.org/10.1021/acs.energyfuels.4c00410
Energy Fuels XXXX, XXX, XXX−XXX
H

works as a Senior Process/Research Engineer at Deposition
Technology Innovations, Jeersonville, Indiana, USA.
Muhammad Daniyal Ghouri is a final year Ph.D. candidate at the CAS
Key Laboratory for Biomedical Eects of Nanomaterials and Nano-
safety, National Center for Nanoscience and Technology, Beijing,
China. His research interest is focused on nanomaterials for renewable
energy applications, advanced therapeutics, and drug delivery. His
current research is based on understanding the physiological interaction
of nanomaterials and the role of protein corona on cell−cell junction.
Sabir Hussain is currently a postdoctoral researcher at Tyndall National
Institute/UCC, Cork, Ireland. From 2021 to 2023, he served as a
postdoctoral researcher at the National Center for Nanoscience and
Technology (NCNST). He received his Ph.D. degree from the
University of Chinese Academy of Sciences (UCAS), Beijing, China in
2018−2021. His research interest is on 2D-TMCDs/multiferroics
materials including synthesis via exfoliation/CVD approach, crystal
identifications, surface physics properties, device applications, and
multifrequency AFM-based techniques.
Sandeep Kanade is a currently a technical assistant in the Indian
Institute of Science Education and Research, Pune, India. He received
his Ph.D. degree from Savitribai Phule Pune University in 2022. His
research primarily revolves around the development and utilization of
cost-eective transition metal catalysts for energy applications.
Bharat B. Kale is a distinguished chemist with a Ph.D. degree in
chemistry, earned in 1992 from India. He serves as an emeritus scientist
at the Centre for Materials for Electronics Technology, Pune, India,
where he was also the former director general and scientist. Currently,
he holds the position of emeritus professor and director of materials
science (College of Engineering) at MIT World Peace University
(MIT-WPU), Pune, India. His extensive research portfolio spans
nanotechnology, photocatalysis, batteries, and photonic glasses.
Manu Gautam is a postdoctoral research associate at the University of
Louisville, Louisville, KY, USA. He received his Ph.D. degree from the
Department of Chemistry, Indian Institute of Science Education and
Research, Pune, India, in 2021. His area of research is electrochemical
CO2reduction, direct flue gas electroreduction, CO2capturing,
batteries, fuel cells, and electrolyzers.
■ACKNOWLEDGMENTS
We thank our organizations. There is no financial support for
this work.
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