Review
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Electrocatalysts by Electrodeposition: Recent Advances,
Synthesis Methods, and Applications in Energy Conversion
Manoj B. Kale, Rahul Anil Borse, Aya Gomaa Abdelkader Mohamed, and Yaobing Wang*
are already satisfying the need for energy
demand.[2] But the limiting energy supply
of these technologies needs the support
of other energy conversion and energy
storage technologies to balance the continuous energy demands.[3] Therefore,
in the past two decades the various economical and highly efficient methodologies for energy conversion and energy
storage have been proposed, such as traditional batteries, metal-air/CO2 batteries,
and electrocatalytic energy conversion.[4]
Out of these, the electrocatalytic energy
conversion includes the conversion of
chemical energy into electrical energy
and contrariwise has become wide attention for researchers. To understand the
importance of value-added chemicals
and fuels such as hydrogen, formic acid,
carbon monoxide, ethanol, or methanol,
the two electrolysis strategies, including hydrogen evolution
reaction (HER) and carbon dioxide reduction reaction (CDRR),
have been reported elsewhere, where the HER and CDRR take
place at the negative electrode (i.e., cathode), and the oxygen is
generated at the positive electrode (i.e., anode), called oxygen
evolution reaction (OER). These kinetically inert and thermodynamically complicated reactions require highly efficient electrocatalysts to increase the energy conversion efficiency, minimize
the energy losses, and deplete the energy barriers. Therefore,
the desired electrocatalysts must meet all the conditions such
as high catalytic activity, durability, economical, and environmentally friendly.
In recent years, with the help of advanced nanomaterials
synthesis and characterization techniques there are varieties of
electrocatalyst proposed such as carbon nanotubes, 2D nanomaterials such as metal, nonmetal, alloy, and metal–organic
frameworks[5] which are cost-effective, available in abundant
amount, show high stability and high efficiency in energy conversion reactions. However, the high performance of energy
conversion is mainly dependent on the highly active structure and intrinsic properties of electrocatalysts. Therefore, to
mitigate the high conversion efficacy of the electrocatalyst, the
synthesis method can play a significant role in controlling the
electrocatalyst’s structure and properties. In the past few decades, various techniques used for the synthesis of electrocatalysts are impregnation, colloidal, microemulsion, sputtering,
etc.[4e] These electrocatalyst synthesis methods are multistep
processes, require higher temperatures, gives uncontrolled
morphology structures, and need capping agents or surfaceactive agents. In addition, the conventional chemical reduction
The conventional environment polluting energy sources and the continuous
growing energy demand compelled researchers to find alternative energy
sources. Therefore, in recent years, extensive research has been carried out in
synthesizing catalysts for energy conversion applications. This review focuses
on the application of various electrodeposition methods in the synthesis of
energy-related electrocatalyst and briefly discusses different electrocatalyst
characterization techniques. Further, the influence of various parameters on
the electrocatalyst activity and stability is highlighted. Electrocatalyst application in clean energy conversion reactions, such as the hydrogen evolution reaction, oxygen evolution reaction, oxygen reduction reaction, carbon
dioxide reduction reaction, nitrogen reduction reaction along with the metalair/CO2 battery, are reviewed. Finally, the comparative experimental data are
provided as a reference to synthesize the next-generation electrodeposited
electrocatalyst in clean energy conversions and beyond.
1. Introduction
The sociological development and technological advancements in the last century have a significant impact on the environment. The use of traditional fossil fuel combustion, the
greenhouse effect, has adversely affected the climate resulting
in global warming, increased sea level, pollution, etc.[1] The
excessive consumption of fossil fuel by mankind and further
increasing demand may result in the exhaustion of natural
fossil fuel resources. The wind and solar energy technologies
Dr. M. B. Kale, R. A. Borse, A. Gomaa Abdelkader Mohamed,
Prof. Y. Wang
CAS Key Laboratory of Design and Assembly of Functional
Nanostructures
and Fujian Provincial Key Laboratory of Nanomaterials
State Key Laboratory of Structural Chemistry
Key Laboratory of Optoelectronic Materials Chemistry and Physics
Fujian Institute of Research on the Structure of Matter
Chinese Academy of Sciences
Fuzhou, Fujian 350002, China
E-mail: wangyb@fjirsm.ac.cn
R. A. Borse, A. Gomaa Abdelkader Mohamed
University of Chinese Academy of Sciences
Beijing 100049, P. R. China
Prof. Y. Wang
Dalian National Laboratory for Clean Energy
Dalian 116023, P. R. China
The ORCID identification number(s) for the author(s) of this article
can be found under https://doi.org/10.1002/adfm.202101313.
DOI: 10.1002/adfm.202101313
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Figure 2. Representative electroplating cell.
2. Fundamentals of the Electrodeposition
2.1. Electrodeposition
Figure 1. Overview of the main topics in this review.
methods generate impurities in the deposits, which can merge
in the catalyst and step down the catalytic activity. Owing
to all these reasons, there is a need to carry out a detailed
study of appropriate synthesis method used to synthesize
various electrocatalysts in energy conversion reactions. With
this context, the electrodeposition technique proven to be a
promising alternative method with advantages such as 1) the
amount and the size of the deposits can be easily controlled
by varying the plating parameters (deposition time, voltage,
and electrolyte concentration, etc.), 2) fast and facile route for
controlled synthesis of uniformly dispersed electrocatalysts
without needing the additional capping agents or reductants,
3) simple and highly scalable at ambient condition, and 4)
allowing controlled, patterned, and faceted crystal growth of
nanostructures.
In this review, we have presented the pioneering work to
recent progress in the application of the electrodeposition
method for the synthesis of electrocatalysts for energy conversion reactions. Figure 1 represents the main topics covered in
this review. Herein, we have discussed various electrodeposition methods, including co-electrodeposition, potentiostatic,
galvanostatic, modified microwave electrodeposition, and
hydrothermal electrodeposition used in electrocatalyst preparation, along with their advantages and challenges in electrochemical energy conversion applications. Besides, to understand the performance of synthesized electrocatalyst, the
various affecting factors such as electrolytes, concentration,
deposition time, pH, pressure, and so on are also discussed.
Further, to understand the real active site and stability of electrocatalysts, the specific concentration is provided on different
characterization techniques. Finally, we also represent the comparative study on the energy-conversion application concerning
the electrodeposition methods along with the challenges and
future perspective.
Adv. Funct. Mater. 2021, 31, 2101313
The electrodeposition method is used in electroplating. Generally, two metal electrodes are immersed in the specific electrolyte
solution, and then the external electric field is applied to deposit
the required metal on the working electrode (cathode). The
electrodeposition method is used in synthesizing various thick
and thin films metal’s, supercapacitor materials, nanomagnetic
materials, etc.[6] The thickness of the metal films is controlled
by varying the electrode potential and current density. In the
case of synthesizing the semiconducting materials, there is an
arrangement of three electrodes, such as a working electrode,
a reference electrode, and a counter electrode immersed in the
electrolyte containing ions. Typically, with the milliwatt range dc
power supply given to the cathode and anode electrodes, where
the positive ions moved toward the cathode and discharged and
chemically react to synthesize the desired material.
2.2. Principles and Basic Mechanism of Electrodeposition
Figure 2 shows the representative electroplating cell. The direct
current (DC) power source is the externally applied unidirectional current source across the electrode system. The flow of
current is the flow of electrons in the circuit. In the electrolyte
solution, the electrical current flow in the form of ions. The
negative ions (anions) attract toward the anode, whereas the
positive ions (cations) attract toward the cathode on applying
the appropriate external power source. The electrolyte solution
contains a particular type of material that influences the deposition process.[7]
The electrodeposition setup usually contains the following
components:
A. Working electrode (WE)
The selection of working electrode materials depends on
the material’s excellent electron transfer properties subject to
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the substrate while illustrating enhanced activation energy to
transfer electrons in the performing reaction. Most commonly,
smooth platinum, lead dioxide, and graphite are used as an
anode material. The very important reactions performed in
water are oxygen evolution at the anode and hydrogen evolution
at the cathode.
B. Counter electrode (CE)
The counter electrode is used to complete the circuit of the
electrochemical cell. Mercury, lead, cadmium, graphite, and
platinum are frequently used cathode materials, which gives
high overpotentials in hydrogen evolution in water solution.[8]
The CE does not involve in the chemical reaction and the current travel between CE and WE, the CE surface area should be
more than that of WE and thus will not hinder the rate of an
electrochemical reaction process.
C. Reference electrode
The electrode potential of the reference electrode is known,
and it is used as a reference to control and measure the potential
of electrochemical arrangement. The high stability of the reference electrode potential is usually reached by using a redox
system with constant (buffered or saturated) concentrations of
each redox reaction participant. Also, the current through the
reference electrode is maintained around zero by keeping the
circuit current closed using CE and with the high input impedance (around >100 GΩ). Overall, one of the most challenging
issues is to find the most suitable RE, depending on the experimental conditions. According to Caton,[9] an “ideal RE” should
possess the following properties: i) have a stable potential; ii)
meet the demands of a charge transfer imposed by the measuring instrument without changing its potential (be nonpolarizable); iii) return to its fixed reference potential after accidental
polarization; iv) obey the Nernst equation for some species in a
solution; and v) if it is an electrode of the second kind, the solid
compound must be only sparingly soluble in the electrolyte.
Examples of electrodes that have these properties for use in
aqueous solutions are the saturated calomel electrodes (SCE).[9]
The SCE seems to be the scientists’ “favorite RE” because of
its simplicity, quietness, and robustness. It works at moderate
temperatures up to 90 °C, is filled with KCl, NaCl, and LiCl
solutions, and equilibrates quickly when refilled.
3. Electrodeposition: Synthesis Methods,
Techniques, and Structural Characterization
pyrolysis, impregnation, vapor deposition, and ball milling.[10]
These methods are mainly followed depending upon the surface,
structural, and electrochemical properties of catalysts. Although
these electrocatalyst synthesis methods have some advantages,
such as catalysts with high surface area and less agglomeration, they still have their own limitations, such as the need of
surface-active agents or capping agents to produce controlled
morphology structures, toxic, multistep processes, and need
higher temperature conditions. Further, the contaminants generated by the traditional reduction methods mix with the electrocatalyst and affect the catalytic activity. In the past few years, the
researcher reported and proven the electrodeposition technique
as a superior alternative method to overcome these issues.
This section will explain the detailed study of different electrodeposition techniques used to synthesize electrocatalyst for
clean energy conversions, and the superior electrocatalyst’s performance, catalytic activity, stability, factors affecting, and structural characterization have also been discussed.
3.1.2. Conventional Electrodeposition Method
Electrodeposition is a traditionally followed procedure to
enhance the surface properties, designing, and decorative purposes of numerous materials. The electrodeposition is used as
an efficient procedure to synthesize the different types of nanomaterials. The electrodeposition method is a significant process
for synthesizing nanomaterials as electrocatalysts for different
energy conversion reactions in recent years. In this method,
the small variations in the electrochemical deposition conditions can control the morphology and sizes of electrocatalysts
deposited on the conducting surfaces. The electrodeposition
method’s biggest advantage is that there is no involvement of
capping agent or surface-active agents, which further simplifies the procedure for their real application. Sarfraz et al. prepared the bimetallic electrode through the electrodeposition of
Sn species on the surface of oxide-derived copper (OD-Cu).[11]
The Cu surface, when decorated with an optimal amount of Sn,
resulted in a Faradaic efficiency (FE) for CO greater than 90%
and a current density of −1.0 mA cm−2 at −0.6 V versus RHE,
compared to the CO FE of 63% and −2.1 mA cm−2 for OD-Cu.
Excess Sn on the surface caused H2 evolution with a decreased
current density. This study represented a strategy to provide
low-cost non-noble metals that can be used as a highly selective
electrocatalyst for the efficient aqueous reduction of CO2.
3.1. Synthesis
3.1.3. Galvanostatic Electrodeposition Method
3.1.1. Common Methods
The electrodeposition method of preparing dispersed metal
nanoparticles for electrocatalysts is favorable as it does require
surfactants or activating agents. Rather, the regulated addition
of electrons can be used. Thus, the electrodeposition technique is cost-effective and gives pure nanomaterials deposits.
In the galvanostatic electrodeposition, the current is constant
to achieve the uniform size of nanoparticle deposits. Generally, the galvanostatic deposition consists of three electrodes,
the glass carbon electrode as a working electrode, the platinum
coil electrode as an auxiliary/counter electrode, and Ag/AgCl
It is always difficult to adapt the well-regulated synthesis procedure to prepare electrocatalysts with enhanced electrocatalytic
activity. The synthesis route followed to synthesize the catalysts plays a major role in changing the structure and properties of the catalyst, which eventually affects the electrocatalytic
activity. There are numerous methods proposed to synthesize
the electrocatalysts, including microemulsion, electrochemical,
low-temperature chemical precipitation, colloidal, sol-gel, spray
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Figure 3. Preparation and O2 electroreduction of MnO2@RGO catalyst. Reproduced with permission.[12] Copyright 2020, SpringerNature.
electrode as a reference electrode. Figure 3 shows the fabrication process of a composite of spherical MnO2 coated with
reduced graphene oxide by a facile and green step galvanostatic
electrodeposition for oxygen reduction reaction (ORR).[12] In a
typical procedure, first, the MnO2 was deposited on the electrode surface by the anodic galvanostatic method as follows
Mn 2+ + 2H2 O → MnOOH + e − + 3H+ → MnO2 + 4H+ + 2e −
(1)
Consequently, a 3D composite of RGO confined MnO2 was
obtained through cathodic galvanostatic reduction of GO to
RGO. This unique structure can address the structural damage
of graphene sheets caused by the crowd pressure of MnOx on
the graphene’s surface, thus improving the composite conductivity. This structure can also prevent the agglomeration of
MnOx particles. Besides, owing to the larger specific surface
area of graphene on the outer layer and its stronger electrondonating ability compared to MnO2, the as-prepared composite
can adsorb and activate O2 more easily.
Paoletti et al. used the galvanostatic polarization technique
to deposit the Pt nanoparticles electrocatalysts on different
carbon substrates (carbon black and carbon nanotubes).[13] The
electrocatalysts were prepared by single and multiple pulse
galvanostatic polarization in 1 m sulfuric acid + 5 × 10−3 m
hexachloro-platinic acid solution. They have used a standard
three-electrode cell. A Pt foil as a counter electrode, an Ag/
AgCl saturated electrode (SC) was used as a reference electrode, and the working electrode was prepared by spraying the
homogeneous alcoholic suspension of carbon black or carbon
nanotubes mixed with Nafion solution over the cleaned glassy
carbon (GC) disc inserted in Teflon holder. Pt nanoparticles
were electrodeposited on the substrates by galvanostatic polarization from 5 × 10−3 m H2PtCl6 + 1 m H2SO4 aqueous solution.
For the deposition of Pt, two strategies were used, such as the
galvanostatic electrodeposition at constant (GED) and pulsed
current (PED). In Figure 4a inset, a typical GED pulse profile is
shown: tp is the polarization time, and ip is the applied current
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density. Electrodeposition charge density (QED) is calculated as
the product between ip and tp. Further, PED current profile is
reported in Figure 4b inset, characterized by three parameters:
the current on-time ton, the current off-time toff, and the applied
current density ip. They have obtained the Pt deposits with a
fine nanostructured surface and uniform electrocatalyst distribution on both CB and CNTs substrates using galvanostatic
electrodeposition. Electrodeposition parameters allow to significantly control the Pt particles morphology, leading to spherical,
dendritic, and lamellar shapes. Further, the methanol oxidation
reaction (MOR) was influenced by the deposit morphology.
Toghraei et al. fabricated self-supported Ni-Mo-P coating on
3D Ni foam substrate (Ni-Mo-P/NF) by single-step galvanostatic
electrodeposition method and studied the HER activity of this
electrode in 1 m KOH electrolyte.[14] They have implemented
the three-electrode electrodeposition configuration with a SCE
as the reference electrode, a platinum as a counter electrode,
and Ni foam as the working electrodes (Figure 5). The Ni foam
acted as a substrate, and the electrodeposition was carried out
under various concentrations of dissolved MoO4–2 and at different applied current densities. The optimized Ni-Mo-P/NF
electrode required a low HER overpotential of −63 mV to
achieve a cathodic current density of −10 mA cm–2 in 1 m KOH
electrolyte. The magnificent electrocatalytic performance of the
electrode was only because of the synergistic effect of Ni, Mo,
and P and playing an essential role in altering the electronic
structure of the catalyst.
3.1.4. Potentiostatic Electrodeposition Method
Potentiostatic electrodeposition is a method where the constant
potential is applied for a certain time in the electrodeposition
procedure. Bocchetta et al. prepared the cobalt/polypyrrole
ORR electrocatalysts by a potentiostatic anodic/cathodic pulseplating procedure.[15] The three-electrode was used for electrochemical syntheses with a glassy carbon rod, graphite disk, and
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Figure 4. a,b) Chronopotentiometric curve in H2SO4 1 m + H2PtCl6 5 × 10−3 m of Pt deposited on carbon substrates by galvanostatic polarization at
single and multiple pulses, respectively. A–D represent the main steps of the nucleation process. Inset: a) typical single pulse galvanostatic electrodeposition (GED) where tp is the polarization time and ip is the applied current density; b) typical multiple pulses galvanostatic electrodeposition (PED)
where ton is the current on time, toff is the current off time, and ip is the applied current density. Reproduced with permission.[13] Copyright 2008, Elsevier.
gold rod as working electrodes, a Pt wire spiral as a counter
electrode, and an aqueous silver/silver chloride (Ag/AgCl/3 m:
0.209 V/normal hydrogen electrode (NHE)) as a reference electrode. They have synthesized the Co/PPy catalysts on graphite
disks or glassy carbon electrode (GCE) by using a step pulsed
potentiostatic co-electrodeposition in deaerated acetonitrile
solutions containing 0.1 m pyrrole, 0.05 m CoCl2 1% v/v H2O,
and 0.1 m TBAP supporting electrolyte. The step pulsed electrodeposition procedure includes the repetition of an optimized
cycle composed by an initial step at 0 V for 1 s to relax the compositional double layer, a subsequent anodic pulse at 1.2 V for
0.5 s to electrodeposit PPy, a cathodic step at −1.8 V for 0.5 s
to incorporate into PPy reduced Co species, and a final anodic
step at 1.2 V for 0.2 s to deposit another layer of PPy. Figure 6
shows the a) the potential program, b) the c.d. response to the
whole electrosynthesis pulse train, and c) the c.d. response to
a representative potential pulse period (the second one). In
the electrodeposition process, the initial step 1 does not lead to
faradaic reactions, but it is required to relax the compositional
double layer. After this relaxation step, a layer of PPy is electrodeposited during the first anodic pulse 2. During the sub-
sequent cathodic pulse 3, reduced Co species are incorporated
into PPy. In the final anodic step of each cycle 4, another layer
of PPy is deposited. The pulsed co-electrodeposition process
facilitated the formation of a composite containing Co in both
metallic and oxidized forms. The as-electrodeposited Co/PPy
material showed good electrocatalytic activity toward ORR.
Elrouby et al. used the potentiostatic electrodeposition
method to prepare a NiCo alloy flower-like structure loaded on
C-steel.[16] They have used the three-electrode system containing
Pt sheet as a counter electrode, Ag/AgCl as a reference electrode, and Cu sheet/carbon sheet as a working electrode. The
electrodes were dipped in the electrodeposition bath containing
the mixture of 0.02 m CoCl2·6H2O, 0.05 m NiCl2·6H2O, and
0.1 m KCl. Figure 7 shows the different potentials applied for
the electrodeposition of Ni-Co alloys on carbon steel at room
temperature and at a pH of 6.5. In Figure 7, there is a sudden up
or down in the current time depending on the applied potential
owing to the presence of a double layer between the negatively
charged electrode and cations of electrolyte. Also, the applied
potential of −1.2 V shows an increase in the negative current
owing to the increase in the electrodeposition rate at a higher
Figure 5. Schematics illustrating a) the fabrication method of Ni-Mo-P/NF and b) Ni-Mo-P coating formation on the Ni foam surface. Reproduced
with permission.[14] Copyright 2020, Elsevier.
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Figure 6. Pulse-plating potential program and c.d. response for the electrosynthesis of Co/PPy catalyst precursors. a) The potential program; b) the
c.d. response to the whole electrosynthesis pulse train of 60 periods; c) the c.d. response to a representative potential pulse period (second period).
Reproduced with permission.[15] Copyright 2014, Elsevier.
potential. The higher electrodeposition rate increases the nucleation and hence reduces the progress of the electroreduction
reaction. The electrodeposited Ni-CO alloy had the flower-like
structure only at the −1.0 V conditions, which had very high
stability in highly corrosive media and the alloy structures had
good HER capabilities in HCl solution.
Liu et al. applied a pulsed electrodeposition method to deposit
the platinum electrocatalyst on the indium tin oxide substrate.[17]
Figure 8 shows the schematic of pulse potentiostatic electrodeposition. They investigated the effect of low potential pulse duration
on the morphology of electrodeposition and structure of Pt particles. It was observed that with the decrease in pulse duration
(from 1 to 0.01 s), the shape of Pt particles on the ITO substrate
changed from flower nanosheet to prickly and smooth sphericallike, as can be seen in Figure 8b–f. The flower and nanosheet
morphologies had the highest mass-specific activity for the methanol oxidation, followed by the prickly surface and smooth structure of Pt particles. Such an improvement in the electrocatalytic
activity was attributed to the large electrochemically active surface
area of Pt nanosheets and the high electrocatalytic activity per
unit electrocatalytic activity surface area related to its morphology.
Figure 7. Potentiostatic deposition curves for the Ni–Co alloy on steel
substrates, at different deposition potentials: −800, −900, −1000, −1100,
and −1200 mV, at pH = 6.5, and at room temperature. Reproduced with
permission.[16] Copyright 2020, Elsevier.
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Shan et al. modified the ITO using the electrospinning
method with the chitosan (CS) nanofibers.[18] Then, the potentiostatic electrodeposition route was used to directly synthesize
the Prussian blue (PB) nanoparticles on the CS nanofibers in
an acidic solution containing single ferricyanide. They observed
the increase in the PB deposition amount on CS nanofibers
with an increase in electrodeposition potential value. The modified electrode exhibited an electrocatalytic activity toward the
reduction of H2O2. The CS nanofibers/PB-modified ITO electrodes when subjected to the electrochemical experiments in
the presence of H2O2, the complete electrode structure was still
intact, but the redox current fell to 90%. Further, the increase
in electrochemical experiments to two weeks, the redox current
has remained only about 70%. Gao et al. prepared the Ni-Se-Mo
electrocatalysts deposited on nickel foam (NF) substrate by a
facile one-step electrodeposition method.[19] They implemented
a conventional three-electrode system with the commercial NF
as a working electrode, the graphite flake as a counter electrode,
and SCE as a reference electrode. The NF substrate was exposed
to a deposition electrolyte containing 100 g L–1 NiSO4 6H2O,
20 g L–1 SeO2, 100 g L–1 LiCl, and 20 g L–1 Na2MoO4 ·2H2O.
The electrodeposition was carried out at −0.6 V against SCE for
the deposition time of 600 s. The prepared Ni-Se-Mo film had
shown high electrocatalytic activity and stability toward HER,
with a low overpotential of 101 mV to under a current density of
10 mA cm–2 in 1.0 m KOH medium. The enhanced HER performance of the Ni-Se-Mo film generated by the Mo-doped Ni-Se
film’s synergistic effects leads to the fast electron transfer.
Zeng et al. reported the transformation of the copper substrate into CuxS using the potentiostatic electrodeposition
method.[20] Compared to the regular electrodeposition method,
where the electrochemical synthesis of copper composites primarily takes place at the working electrode; in this work, they
have explored the practicability of simultaneously fabricating
copper composites at both the working and the counter electrodes. A saturated Ag/AgCl electrode was used as the reference electrode. The utilization of sulfide-mediated underpotential oxidations of copper led to the simultaneous formation of
CuxO nanoparticles at the deployed copper counter electrode,
doubling the efficiency of this modification method. They
have observed that the sizes of the synthesized nanoparticles could be controlled by varying the relative surface areas
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Figure 8. a) Schematic diagram of the potentiostatic pulsed electrodeposition. b–f) EM images of the Pt/ITO electrodes fabricated at the different
lower potential pulse duration of 1, 0.5, 0.1, 0.05, and 0.01 s, respectively. Reproduced with permission.[17] Copyright 2013, Elsevier.
of the working and counter electrodes. Further, the prepared
CuxS/CuxO modified copper electrodes showed better electrocatalytic activity for HER and better stability. Wang et al.
proposed the micelle-assisted potentiostatic electrodeposition method for direct fabrication of porous Au film on Ni
foam (pAu/NF) by using diblock copolymer poly(1-vinylpyrrolidone-co-styrene) (PVP-co-PS) micelles as a soft template.[21]
The potentiostatic electrodeposition was performed at a potential of 0.5 V for 2000 s on three-electrode systems where the Ni
foam, a Pt wire, and an Ag/AgCl/saturated KCl electrode were
used as the working electrode, the counter electrode, and reference electrode, respectively. The micelles of PVP-co-PS helped
to form the nanopores. The nanopores had an average size of
about 59.5 nm.[21] The interconnected porous structure of pAu/
NF had an excellent nitrogen reduction reaction (NRR) activity.
in a concentrated KOH solution to etch the SiO2 component,
leading to the formation of po-Co3O4 films. In a typical experimental setup, a three-electrode compartment was used for the
electrodeposition. A stainless steel (S) sheet, a KCl saturated
Ag/AgCl electrode, and a platinum plate with an exposed area of
4.0 cm2 were used as the working, reference, and counter electrodes, respectively. The cathodic electrodeposition was under
−1.0 V versus Ag/AgCl for 300 s under stirring at 25 ± 1 °C. The
obtained films were blue in color. Further, in the second step to
remove the SiO2 template from the Co3O4-SiO2 composite film
to get a po-Co3O4 film, they have performed the 50 consecutive
CV cycles. Figure 10 shows the scanning electron microscopy
(SEM) images of the obtained Co3O4(a), Co3O4-SiO2(b), and poCo3O4(c) films. Figure 10d,e shows the energy-dispersive X-ray
(EDX) spectra derived from Figure 10b,c, respectively. The poCo3O4 film performed considerable electrocatalytic property
toward OER. The presence of an E-silica template increased the
3.1.5. Co-Electrodeposition Method
The co-electrodeposition is a direct one-step method to prepare
nanocomposites and large-area films at room temperature.
The thickness and the composition can be easily controlled by
varying the deposition time, applied potential, and precursor
compositions. Further, the co-electrodeposition method helps
to prepare porous nanomaterials with high specific surface
area, which have a short diffusion and path length to reactants.
These porous nanoarchitectures help to prepare electrocatalyst
with very high electrocatalytic activity. Figure 9 shows the generalized procedure of the two-electrode co-electrodeposition
of copper (Cu) nanocomposites with a high content of carbon
nanotubes.[22] Wu et al. prepared the nanoporous Co3O4
(po-Co3O4) films on stainless steel substrate by a two-step
method.[23] First, a composite Co3O4- SiO2 film was fabricated
via electrochemical codeposition under a cathodic potential
from the mixed sol-gel precursor. Then, the obtained composite film was applied under consecutive cycling voltammetry
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Figure 9. The generalized co-electrodeposition procedure of CNTs-Cu
nanocomposite. Reproduced with permission.[22] Copyright 2016, Elsevier.
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Figure 10. a–c) SEM images of the Co3O4, Co3O4-SiO2, and po-Co3O4 films, respectively. The scale bars correspond to 500 nm. d,e) EDX spectra derived
from (b) and (c), respectively. Reproduced with permission.[23] Copyright 2014, Elsevier.
surface roughness factor, which was beneficial to the transfer of
electrons and electrolyte ions during OER.
Ahn et al. synthesized the Pt–Ru electrocatalyst on carbon
paper by one-step co-electrodeposition method using a standard
three-electrode system.[4e] They controlled the particle size
and density by varying deposition potential and time. 1 cm2
of carbon paper (TGPH-090; Toray) was exposed to the solution as a working electrode, and other conducting parts were
sealed with a home-made Teflon holder. Pt wire and a saturated
calomel electrode were used as the counter and reference electrodes, respectively. The potentiostat was used to control all
the electrochemical processes, and all of the potentials were
converted to a NHE. The co-electrodeposition of Pt–Ru was performed by using various solutions consisting of H2PtCl6·6H2O
and RuCl3·xH2O. The ratio of two metal sources was varied
while the total concentration was fixed at 20 × 10−3 m (PtxRuy;
x + y = 20), e.g., Pt4Ru16 means that the electrolyte is composed
of 4 × 10−3 m Pt and 16 × 10−3 m Ru. The pH of the solution
was controlled at 2.0 using HCl or KOH. Pt–Ru particles
deposited on the carbon paper were observed by field emission SEM (FESEM). Figure 11 shows FESEM images of Pt–
Ru particles obtained by varying the deposition potential and
Figure 11. a) FESEM images of Pt–Ru particles deposited on carbon paper under various deposition potentials and time with Pt10Ru10 electrolyte; (i)
−0.76 V for 2 s, (ii) −0.96 V for 2 s, (iii) −1.16 V for 2 s, (iv) −0.76 V for 5 s, (v) −0.96 V for 5 s, and (vi) −1.16 V for 5 s. b) FESEM images of (i) bare carbon
paper and Pt–Ru particles deposited at −1.16 V for 5 s with various electrolytes: (ii) Pt4Ru16, (iii) Pt6Ru14, (iv) Pt8Ru12, (v) Pt10Ru10, (vi) Pt12Ru8, (vii)
Pt14Ru6, (viii) Pt16Ru4, and (ix) Pt10. Reproduced with permission.[4e] Copyright 2012, Elsevier.
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Figure 12. Tenth cycle in the electrodeposition process of (A) G-Pd/CC, G-Ag/CC, (B) G-PdAg/CC, and all 10 cycles in the electrodeposition process of
(C) G-PdAg/CC by cycling potential from –1.5–1 V at 25 mV s–1. Reproduced with permission.[24] Copyright 2018, Elsevier.
time with Pt10Ru10 electrolyte. With the increase in the negative
potential, the particle density increased while the particle size
decreased at the constant deposition time. From the FESEM
results in Figure 11, it can be said that the particle size and density indicate that the coverage of Pt–Ru catalyst particles were
proportional to the Ru concentration of the electrolytes.
Furthermore, without the Ru, the particle density (Figure 11b ix)
was remarkably lower than that obtained with Pt10Ru10 electrolyte
(Figure 11b v). All Pt–Ru electrocatalysts showed catalytic activity
for methanol oxidation and tolerance against carbon monoxide
poisoning. The catalyst deposited with Pt10Ru10 electrolyte showed
superior performance compared to the other catalysts.
Ghiabi et al. reported the preparation of graphene-supported
nano-palladium-silver on carbon cloth (G-PdAg/CC) electrode for electrocatalytic oxidation of methanol in an alkaline
medium.[24] The electrocatalyst’s performance was compared
with graphene-supported nano-palladium and silver on CC
(G-Pd/CC and G-Ag/CC, respectively) electrodes. The electrodeposition was carried out on a conventional three-electrode
cell consisting of the modified CC electrode (G-Pd/CC, Pd/
CC, PdAg(m:n)/CC or G-PdAg(m:n)/CC) as working electrode,
platinum rod as the counter electrode, and Ag|AgCl|3 m KCl
electrode as the reference electrode. CC acted as a substrate
for electrodeposition of G-PdAg, G-Pd, and G-Ag, and one
side of CC was covered with Teflon paper and using a copper
wire for establishing electrical contact. G-PdAg composite was
electrodeposited on CC from ammonia buffer solution (0.5 m,
pH 9.18) containing 0.6 mg mL−1 GO, 1 × 10−3 m palladium
chloride, 1 × 10−3 m silver nitrate, and 50 × 10−3 m potassium
nitrate (as supporting electrolyte) by applying ten successive
cyclic voltammograms (CVs). The CVs were performed in a
potential range of −1.5 to 1 V with a scan rate of 25 mV s–1.
For the comparison purpose, the G-Pd/CC and G-Ag/CC electrodes were also prepared by electrodeposition under the same
conditions in the absence of silver and palladium salt in the
electrolyte, respectively. In Figure 12A, the CV curves illustrate
the growth of G-Pd and G-Ag on CC. In both cases, the reduction peaks current observed in lower potentials (lower than
−1.0 V) indicate the graphene nanosheets’ irreversible codeposition with the respective metal. By comparing the last two
voltammograms in Figure 12A, and as seen in Figure 12B, the
composite containing graphene nanosheets with PdAg catalyst
was codeposited on the CC surface under the same electrodeposition process. Finally, as seen in Figure 12C, the increase
in the anodic and cathodic currents with continuous scans
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implies simultaneous electrodeposition of metals and graphene
nanosheets and the layered structure formation.
Lamaison et al. reported the application of the co-electrodeposition method to prepare highly active Ag-alloyed Zn dendritic
electrodes with enhanced CO2-to-CO selectivity.[25] They have
controlled the electrodeposition parameters to investigate the Ag
content, porosity, thickness, and surface area of the electrodes.
With the addition of a small amount of Ag+ to the Zn2+ solution,
the maximum surface area of 3133 cmphys2 cmgeo–2 was achieved.
The Ag and Zn electrocatalyst composite produced the CO with
Faradaic efficiency of 91% without the loss in selectivity.
3.1.6. Hydrothermal Electrodeposition Method
The porous nanomaterials with a high surface area are considered to be highly efficient catalytic performance materials.[26]
Constructing nanomaterials with a large specific surface area is a
key challenge for OER catalysts. For example, Liu et al. fabricated
mesoporous NiFe2O4 nanorods as an efficient oxygen evolution
catalyst, which shows an overpotential of 342 mV at 10 mA cm–2
and a Tafel slope of 44 mV dec–1 .[27] In comparison, Zhang et al.
synthesized NiFe LDH hollow microspheres with a small onset
overpotential of 239 mV at 10 mA cm–2, and a low Tafel slope
of 53 mV dec–1.[28] As reported in previous studies, most NiFebased OER catalysts are powders. They are coated onto the surface of conductive substrates with the help of binders.[27–29] The
use of binders reduces the conductivity of electrodes giving rise
to degraded OER performance. Another common phenomenon,
which is a disadvantage, is that the glued OER catalysts tend to peel
off and aggregate during long-time electrochemical operations.[30]
Electrodeposited metal alloy coatings onto the surface of substrates
have also been wildly investigated as OER catalysts. However, the
coatings usually show poor electrocatalytic performance because
only the surface layers are in contact with electrolyte. Therefore,
exploring a new strategy to electrodeposit metal alloys with abundant active sites for OER catalysts is of great interest.
Yao et al. reported and investigated the hydrothermal electrodeposition method, for the first time, to fabricate mesoporous
Ni0.8Fe0.2 film self-supported on SLS as an OER catalyst.[26a]
Compared with traditional electrodeposition, hydrothermally
driven electrodeposition possesses the following advantages: 1)
higher temperature (110 °C) of hydrothermal electrodeposition
method makes gas (H2) generated on the cathode to overflow faster than that of traditional electrodeposition method
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Figure 13. a) Schematic diagram and b) digital photos of modified hydrothermal equipment for electrodeposition. Reproduced with permission.[31]
Copyright 2019, ACS.
(50–60 °C). Also, high temperature facilitates plating solution
to gasify and leads to a high-pressure environment inside the
confined space of Teflon autoclave. Both the gas generated on
the cathode and higher pressure impact the coating surface,
and hence, the film possesses more active sites to boost OER
properties; 2) by calculating the size of NiFe particles, we found
that the crystallization of the coating is better than that of traditional electrodeposition method so that the electrode presents
excellent stability under OER test. The mesoporous Ni0.8Fe0.2
film synthesized here showed a significantly enhanced electrochemical active area of 25.3 mF cm–2, a low overpotential of
206 mV at 10 mA cm−2, and a small Tafel slope of 64 mV dec−1.
The newly developed process also led to excellent stability for a
long time of up to 2 × 105 s (more than 55 h) at 10 mA cm−2,
which is better than that achieved by the traditional electrodeposition method. The mesoporous Ni0.8Fe0.2 film displayed a low
overpotential, small Tafel slope, and excellent stability owing to
its porous structure resulting from the novel synthesis process.
Yao et al. designed a hydrothermal electrodeposition method
of Ni/Zn alloy and in situ electrochemical dealloying followed
by sulfuration to fabricate 3D mesoporous nickel sulfide
nanosheets assembled tightly on Ni foam for oxygen evolution reaction.[31] For this purpose, they have followed the typical procedure shown in Figure 13. The NiZn alloy film was
electrodeposited onto the surface of 2 × 3 cm2 Ni foam under
the hydrothermal condition at 110 °C with a current density
of 15 mA cm−2 for 3 min in modified equipment in a Teflon
reactor. Zn in the alloy was used as a self-template to construct
a mesoporous structure and enhance the number of active sites.
Then the polarity of the electrode was reversed. NiZn film/
NF, as an anode, was electrochemically dealloyed for 4 h with a
constant potential of 1.1 V under the hydrothermal condition to
obtain mesoporous NiO NS/NF. The as-prepared mesoporous
Ni NS/NF was loaded into a ceramic boat, which was in the
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center of a tube furnace. A quartz boat loaded mesoporous
Ni3S2 nanosheets/Ni foam exhibited a highly mesoporous structure with a specific surface area of 60.1 m2 g−1 and showed a
low overpotential of 223 mV at a current density of 10 mA cm−2
with a small Tafel slope of 60.5 mV dec−1; this overpotential was
superior to that of IrO2/Ni foam. As-prepared Ni3S2 nanosheets/
Ni foam achieved a high turnover frequency value of 0.61 mol
O2 s−1 at an overpotential of 500 mV. Impressively, the asobtained catalyst possessed excellent conductivity and outstanding stability for over 240 h. The superior catalytic property
could be ascribed to the rational synthetic process, morphologycontrolled mesoporous structure, and highly exposed active
sites. The articles reported an efficient route to fabricate porous
nanosheets as a stable and efficient electrocatalyst for water
oxidation. Finally, the as-prepared mesoporous Ni NS/NF was
loaded into a ceramic boat, placed at the center of the furnace.
In another procedure, Yao et al. designed the new growth
mechanism of NiCoFe-PS nanorod/NF.[32] NiCoFeZn alloy film
was first directly synthesized onto NF by novel hydrothermal
electrodeposition (Figure 14a), and as far as we know, this is the
first report of electrodeposition of a quaternary alloy. After selective dealloying by controlling the potential, NiCoFe nanorod/
NF was reacted with P and S to form NiCoFe-PS nanorod/NF.
The decomposition of NaH2PO2 formed P. The 1 h sulfidization
and phosphorization processes resulted in crystallized catalysts.
Figure 14b is a schematic of the designed electrodeposition
device, while Figure 14c shows the corresponding digital photographs of a home-built hydrothermal electrodeposition device.
Compared with the traditional electrodeposition at standard
pressure and temperature, this method’s alloy film exhibits a
much stronger bonding force with the substrate and better cry
stallization.[26a,31] The NiCoFe-PS nanorods self-supported on
NF as highly efficient, stable, and low-cost bifunctional electrocatalyst for electrochemical overall water splitting. NiCoFe-PS
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Figure 14. a) Schematic illustration for the growth of NiCoFe-PS nanorod/NF; b) diagram of hydrothermal electrodeposition device; and c) digital
images of hydrothermal electrodeposition device. Reproduced with permission.[32] Copyright 2019, Wiley-VCH.
nanorod/NF can reach 10 mA cm−2 at the overpotential of
195 mV for OER, which is one of the best among the non-noble
metals-based OER catalysts reported to date, and 97.8 mV for
HER. Besides, the as-obtained catalyst shows small Tafel slopes
of 40.3 mV dec−1 for OER and 51.8 mV dec−1 for HER. The twoelectrode cell using NiCoFe-PS nanorod/NF as both cathode
and anode in the same electrolyte presents low potentials of
1.52 and 1.76 V to achieve 10 and 50 mA cm−2, respectively, with
excellent durability for 200 h (≈1% fluctuation), which are superior to those of precious metal-based catalysts.
3.1.7. Microwave Electrodeposition Method
Microwave has been used in reactions where localized heating
and inverted temperature gradient are required.[33] Sur et al.
reported the effects of microwave radiation on the electrodeposition of copper metal on platinum, gold, and carbon
electrode surfaces are studied by cyclic voltammetry and scanning electron microscopy.[34] Electrochemical experiments
under microwave conditions were carried out in a conventional
three-electrode electrochemical cell placed into a 2.45 GHz
microwave cavity. They have implemented the three-electrode
electrochemical flow cell, which was placed through a port into
the microwave cavity, as seen in Figure 15. The working electrode is in the microwave cavity, and electrochemical experiments are performed as a function of microwave intensity
(controlled by setting the magnetron anode current). The electrochemical cell consists of a three-electrode arrangement with
a large Pt gauze area as a downstream counter electrode, a SCE
as an upstream reference electrode, and a microwave working
electrode. The platinum, gold, or carbon fiber of diameter 50,
Figure 15. A) Electrochemical system for microwave enhanced voltammetry: a) port for electrochemical cell, b) flow cell connected to Viton inlet/
outlet tubing, c) vacuum degassing system with Goretex tubing, d) reference electrode, e) working electrode, and f) Pt gauze counter electrode.
B) Schematic representation of the thermal and convective effects induced by focused microwaves. Reproduced with permission.[34] Copyright 2004, RSC.
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25, or 33 mm were used as microwave working electrodes,
respectively. The microelectrodes were fabricated by sealing a
coiled Pt wire[35] together with gold microwire or glassy carbon
microfiber (33 mm diameter). For microwave activation experiments, they have implemented a multimode microwave oven
(Panasonic NN-3456, 2.45 GHz) with a modified smooth power
supply, a water-energy sink, and a port for the electrochemical
cell. They have observed with the application of microwave
electrodeposition at the microelectrodes of 25–50 mm diameter, there can be a current enhancement of up to three orders
of magnitude. There is an increase in the copper deposition
process, and the type of copper growth was affected by microwave radiation.
Cabello et al. followed the same experimental procedure and
setup to as reported in ref. [34] to deposit Fe-Co alloy at a stainless steel electrode under fast mass transport high-temperature
deposition conditions.[33] They have reported that the microwave Fe-Co alloy was produced with a Fe:Co ratio close to unity
(mas transport controlled deposition). The new alloy material’s
unusual properties include an excellent electrocatalytic performance for the HER, showing low onset overpotential (145 mV),
high exchange current density (20 mA cm–2), and Tafel parameters close to those for Pt on stainless steel.
3.2. Characterization Methods
There are many characterization instruments to carry out the
sophisticated study of electrocatalysts. The phase study and
composition analysis are studied using X-ray diffraction (XRD)
and electron diffraction spectroscopies (EDS). The Brunauer–
Emmett–Teller (BET) is used to study the surface area, while
electrochemical hydrogen adsorption/desorption is used to
investigate the electrochemically active surface areas. The morphological studies are carried out on SEM and transmission
electron microscopy (TEM). The structure and crystallographic
investigation are performed on X-ray photoelectron spectroscopy (XPS), energy-dispersive spectroscopy (EDS), and induced
photoelectron spectroscopy (UVPS). The differential scanning
calorimetry (DSC) and thermogravimetric analysis (TGA) are
used to study electrocatalysts’ thermal analysis. There are other
instruments like Fourier transform infrared, UV–vis, etc., to
study the electrocatalysts’ properties.
3.2.1. Structural Characterization
Here, we have given a few examples of some of the characterization instruments used to characterize electrodeposited electrocatalysts. Figure 16 shows the XRD pattern of the
electrodeposited mesoporous nickel-iron nanosheet on the
nickel foam (NiFe/NF) of as-prepared and calcinated NiFe/
NF samples.[30a] The presence of three diffraction peaks of NF
at 44.5°, 51.8°, and 76.4° depicts the successful deposition of
amorphous NiFe on NF.
The SEM analysis is used to study the morphology and distribution of the electrocatalysts. Figure 17a represents the SEM
images of the NiFe/NF electrode.[30a] The NiFe catalysts seem
to be uniformly distributed on NF’s 3D structure, facilitating a
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Figure 16. XRD patterns of as-prepared and annealed NiFe/NF samples.
Reproduced with permission.[30a] Copyright 2015, Nature Research.
large amount of NiFe loading and more active sites. Figure 17b
shows the magnified SEM image of the square marked area of
NiFe/NF in Figure 17a, showing highly shredded NiFe nanostructure. Figure 17c–e represents the SEM images of bimetallic CuZn catalysts with various concentrations of Zn, which
were fabricated by electrodeposition.[36] The CuZn shows the
rough morphology and shows the strong adhesion between Cu
and Zn, which would entertain the fast electron flow during the
reaction. Figure 17f illustrates the EDX spectra of CuZn-1, confirming the elemental composition of the CuZn catalyst.
Figure 18 shows the morphological and compositional characterization study of thiocyanate modified porous silver nanofoams (AgNF) using the H2 bubble templated electrodeposition
method for CO2 reduction to CO.[37] Figure 18a,b shows the
TEM images of AgNF at different magnification. The inset of
Figure 18a shows the diffraction pattern of Ag ligaments with
different diffraction rings, which are assigned to several Ag
facets. Figure 18b illustrates the Ag lattice fringes with d-spacing
of 0.23 nm and are from Ag (1 1 1). The Fourier-transformed
image depicts a highly symmetric pattern owing to an extended
crystalline network. In Figure 18c, the high angle annular darkfield mode scanning TEM (HAADF-STEM) showed a thin layer
(≈1 nm) with poor crystallinity at the Ag ligament surface with
reduced contrast owing to surface modification. The corresponding elemental mappings by EDX confirm the existence of
S, C, N, and O on the surface Ag ligaments.
The XPS is essential to verify the composite elements of the
catalyst. The XPS spectra of various components or similar
components with various valences can have different peaks.
Figure 19 shows the XPS spectra of the Ni/Co/Fe phosphosulfide nanorods on nickel foam NiCOFe-PS nanorods/NF,
which were fabricated by hydrothermal electrodeposition and
in situ electrochemical dealloying followed by a process for P/S
cotreatment.[32] In Figure 19b, the two Ni 2p3/2 peaks for Ni 2p
are observed at 855.5 eV (Ni2+) and 852.5 eV (Ni0). Also, peaks
situated at 869.9 and 873.2 eV corresponded to Ni0 and Ni2+
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Figure 17. a) SEM image of the NiFe/NF electrode. b) High-resolution SEM image of the area squared in (a). Reproduced with permission.[30a] Copyright
2015, Nature Research. SEM images of c) CuZn-1, d) CuZn-0.5, and e) CuZn-0.1; f) EDX spectra of CuZn-1. Reproduced with permission.[36] Copyright
2019, ACS. Scale bars 200 µm, 200 nm, and 5 µm in (a), (b), and (c–e), respectively.
species, respectively. In Figure 19c, for Co 2p, the binding energies of Co 2p3/2 peaks are present at 775.7 eV (Co0), 780.3 eV
(Co2+), and 781.7 eV (Co3+), while the peaks of Co 2p1/2 can be
seen at 797.6 eV (Co2+) and 795.9 eV (Co0). In Figure 19d, the
FeP and FeO peaks for Fe 2p were observed at 707.2 and
715.7 eV, respectively. In Figure 19e, the Fe-P (2p3/2), Fe-P
(2p1/2), and Fe-P-O 4e (P 2p) peaks were observed at 127.9, 129.8,
and 134.8 eV, respectively. In Figure 19f, the S 2p spectrum
shows binding energies of 161.9 eV (S 2p3/2) and 163.0 eV (S
2p1/2) and is attributed to metal-S species. The binding energy
located at 168.2 eV suggests the characteristic of metalSO.
4. Activity and Stability
The electrocatalytic activity of the synthesized electrocatalysts
has been reported along with the geometric activity, intrinsic
activity, and mas activity.[38] The geometric activity, intrinsic
activity, and mass activity can be measured by normalizing the
documented current with a geometric area of the electrode,
the electrochemical active surface area (ECSA), and catalyst
mass, respectively.[39] The geometric activity is used to report
the catalytic activity of electrocatalysts. But the geometric
activity is often used to measure solar devices’ efficacy and so
Figure 18. a,b) Bright-field TEM images at different magnifications. The inset of (a) shows the corresponding diffraction pattern, and the inset of (b)
is the Fourier transformed image. e) HAADF-STEM image and its corresponding EDX elemental mapping results. Ag and S signals are overlaid for
comparison. Reproduced with permission.[37] Copyright 2019, ACS.
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Figure 19. The film of NiCoFe-PS/NF: a) XRD pattern, b) Ni 2p spectrum, c) Co 2p spectrum, d) Fe 2p spectrum, e) P 2p spectrum, and f) S 2p spectrum. Reproduced with permission.[32] Copyright 2019, Wiley-VCH.
may not report a true area and intrinsic activity of the electrocatalysts.[38] The ECSA can give the more dependable intrinsic
activity calculations of the electrocatalysts related to the amount
of electrochemically active sites, which is the real activity value
catalyzed by each active site, i.e., turnover frequency.[38,39] Wang
et al. reported the amorphous CoMoPO electrodeposited
on nickel foam with enhancing activity as an electrocatalyst
toward water splitting.[40] They have reported the high density
of active sites with large ECSA because of the amorphous structure owing to the incorporation of Mo and P in CoMoPO
film. The electrochemical activation triggered the formation of
highly active CoOOH nanostructures and the enhancement in
the ECSA, OER kinetics, and charge transport. As a result, the
activated CoMoPO demonstrated excellent OER catalytic
activity with a small overpotential of 168.5 mV at 10 mA cm–2.
Further, only a 97 mV overpotential needed to drive 10 mA cm–2
toward HER for CoMoPO.
Wang et al. reported the one-step electrodeposition of Mn
on CoP via direct one-step electrodeposition to prepare MnCoP
nanosheets.[41] The MnCoP showed an overpotential of 65 mV
for HER and 261 mV for OER in 1 m KOH solution with a current density of 10 mA cm–2 to give an excellent catalytic performance (Figure 20a,f). The MnCoP/CC had the highest Cdl
of 293 mF dec−1 for HER (Figure 20c) and 162.25 mF cm–2
for OER (Figure 20h). This study demonstrates that the Mn
doping modifies the surface electronic structure of CoP and
Figure 20. Polarization curves for a) HER and f) OER; Tafel slopes for b) HER and g) OER; capacitive current as a function of scan rate for the electrodes for c) HER and h) OER; the current density which starts at 10 mA cm–2 and ends at 500 mA cm–2 with an increase of 50 mA cm–2 every 500 s
for d) HER and i) OER; chronopotentiometry curves of MnCoP/CC at a constant current density of 10 mA cm–2 without iR compensation for e) HER
and j) OER. Reproduced with permission.[41] Copyright 2020, Elsevier.
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improves the ECSA, which revealed more accessible active sites
and resulted in enhanced intrinsic activity for HER and OER.
Figure 20d,i depicted the multistep chronopotentiometric curve
for MnCoP, demonstrating that the MnCoP/CC had better
mass transport property and mechanical robustness.
Further, the MnCoP/CC as both anode and cathode in electrolyzer cell for overall water splitting demonstrated a very low
voltage of 1.62 V at 10 mA cm–2 and improved long-term durability within 24 h, as shown in Figure 20e,j. Haq et al. fabricated
the Gd-CoB electrocatalysts (Gd-CoB@Au) on the surface of
Au sputtered glass slide by electrodeposition of Gd and Co precursor.[42] They have reported the freestanding Gd-CoB@Au had
enhanced electrochemical performance, high intrinsic activity,
and lower reaction resistance with long-term stability. The electrocatalyst’s high intrinsic activity was confirmed using small
charge transfer resistance, lower Tafel slope value, and high turnover frequency. The enhancement in the activity and selectivity of
electrocatalyst could be because of the adsorption of the intermediates on the active surface sites, which further depends on the
compatibility of the metal orbital energy level/electronic structure of the incoming intermediates, the population of electron
density, and surface engineering of catalyst.[42] The smooth Au
film and the controlled electrodeposition of Gd form the novel
uniform nanosheets, which improves the complete catalytic
potential, reduces the catalyst poisoning, and increases the stability even at a high anodic potential. An excellent electrocatalyst
should possess the high catalytic activity and increase stability.
4.1. Factors Affecting Electrodeposition
The electrodeposition process is dependent on several factors,
including the nature of deposition metal particles, the electrolyte concentration, pH, temperature, the current density, and
electrode structure.[43] However, there are mainly three major
factors affecting the electrodeposition: current density, the electrode movement, and the concentration and type of particles.[43]
The current density is a vital parameter when considering
the particle content in the deposit. Chen et al. reported the
increase in the CNTs content with the increase in current density to a certain value and then decreased with further increase
in the current density.[44] As seen in Figure 21, the content was
maximum at 3.0 and at 2.4 A dm−2 for long and for short CNTs,
respectively. The increase in the current density can increase
the deposition rate of the CNTs owing to the electrostatic attraction between the CNTs and cathode. The nature of the deposit
and the current density can affect the smoothness, deposition
rate, and deposit structure.
The deposit surface structure is dependent on the concentration of particles in the bath. Some researchers reported the content of deposits increased with the increase in the particles bath
concentration up to a certain limit and then remain constant even
for higher concentration with rough deposit surfaces.[4e,45] Further,
particle deposit structure and content depend on the density, size,
and shape of metal particles.[45a] The agitation plays an essential
role in the suspension and transfer of deposit particles. Whitehead et al. reported for the electrodeposition of 226Ra, the agitation
increased the deposition rate by a factor of 5.[46] The agitation facilitates the increase in the operating current density, which allows a
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Figure 21. CNT’s content in the deposits as a function of current density.
Reproduced with permission.[44] Copyright 2002, Elsevier.
higher operating current density. This affects the structure of the
deposit particles with an increase in the deposit concentration.[47]
The proper use of an agitation along with the other controlling
parameters can enhance the plating performance as it gives enough
support for the mixing of deposits for the plating solution.[47,48]
The pH of the electrolyte affects the rate of deposition.[43]
Jesmani et al. studied the effects of pH on the phase formation, morphology, particle size, and electrocatalytic properties
of NiMo/Ni alloy coating for MOR.[49] It was observed that
with the increase in pH value, the crystalline structure size
decreased, and the amorphous structure increased. Also, they
achieved high catalytic activity at the neutral pH. The metal particles with low hydrogen overvoltage are highly affected owing
to the pH variation. In electrodeposition, the change in the pH
can change the cathode current efficiency and deposit structure.[48] The increase in the bath temperature can increase the
crystal size, while lower bath temperature can give smaller crystals.[48] Also, the rate of deposition increases with the increase
in the bath temperature.[43] The higher temperature can facilitate salts’ higher solubility, enhance the conductivity, and rise
in the current densities. Also, the high temperature reduces
the adsorption of hydrogen in the deposits, thus there is less
chance of cracking of deposit surface.
5. Applications of Electrodeposition
in Electrocatalyst
In the past few decades, the synthesis of various electrocatalyst via different methods such as hydrothermal, solvothermal,
chemical vapor deposition, and electrodeposition has been
extensively studied. These promising strategies show wide
attention toward water splitting (bifunctional electrocatalyst):
for H2 and O2 formation, HER, OER, ORR, CDRR, NRR, and
metal-air/CO2 batteries. In this section, we will discuss the
electrodeposition techniques and their clean energy conversion
applications and utilization.
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5.1. Water Splitting
In the overall water-splitting system, the two-half reactions, i.e.,
HER at cathode and OER at the anode, take place to released H2
and O2 gases, respectively. It is well understood that Pt-based
and Ru/Ir oxides electrocatalysts are widely reported for
HER and OER. However, owing to the high price and low durability during the electrolysis process, these electrocatalysts scale
down their real applications. Therefore, to overcome these problems, various non-noble metal electrocatalysts such as metal
oxides, sulfide, and selenides for O2 evolution and H2 evolution
nitrides, phosphides, and metal chalcogenides are reported. In
this context, Wang and co-workers successfully synthesized a
“raisins-on-bread” heterostructure NiSP film on NF, an
excellent bifunctional electrocatalyst via a facile and controllable
two-step electrodeposition method for overall water splitting.[50]
As synthesized, NiSP achieved a high current density of
10 mA cm−2 at a very low overpotential of 120 mV for HER and
219 mV for OER in 1 m KOH solution. Moreover, NiSP electrocatalyst reached maximum durability of 160 h@10 mA cm−2
or 120 h@50 mA cm−2 in alkaline media. The high performance of NiSP electrocatalyst was mainly attributed to
the synergetic effect of heterointerfaces with the highly active
surface area and the long-term durability owing to the direct
growth of heterostructure nanosheet on NF. In addition, Shi
and co-workers developed an amorphous CoMoPO electrocatalyst on NF for overall water splitting.[40] CoMoPO
electrocatalyst achieved large ECSA and high conductivity;
therefore, CoMoPO showed superior performance toward
O2 and H2 evolution at very low overpotential 168.5 and 97.6 mV
for 10 mA cm−2 current density with 80 h stability, respectively,
in 1 m KOH aqueous solution.
Recently, an advanced novel hydrothermal electrodeposition strategy has been developed by Komarneni and co-workers
for the synthesis of low-cost and highly efficient bifunctional
mesoporous Ni/Co/Fe phosphosulfide nanorods on NF
(NiCoFe-PS/NF) electrocatalyst for overall water splitting.[32]
Owing to the nanorod-like morphology with large surface
area and excellent conductivity, the NiCoFe-PS/NF achieved
10 mA cm−2 current density at very low overpotential 195 mV and
a Tafel slope of 40.3 mV dec−1 and 97.8 mV with 51.8 mV dec−1
for OER and HER, respectively. As synthesized bifunctional
NiCoFe-PS/NF electrocatalyst achieved current density
10 and 50 mA cm−2 at low potentials of 1.52 and 1.76 V toward
overall water splitting with superior 200 h durability. The excellent electrocatalytic performance was attributed to a novel fabrication strategy, i.e., hydrothermal electrodeposition, which
could enhance the catalyst roughness, and high temperature
enhanced the catalyst crystallization. Besides, P and S elements doping into catalytic materials modified the electronic
structures and improved the conductivity; thus, bifunctional
NiCoFe-PS/NF electrocatalyst improved the overall catalytic
property. The authors claimed that this type of work provides
a new direction toward synthesizing multiple metal-P/S composite bifunctional electrocatalyst in the future for overall water
splitting. Furthermore, in the recent year metal−organic frameworks (MOFs) have proven to be promising catalyst in electrocatalysis. Therefore, Ni and co-workers successfully designed
and synthesized a hierarchical Co,Fe-MOF-74/Co/CC electrode
Adv. Funct. Mater. 2021, 31, 2101313
via a simple electrochemical deposition technology and further
subsequent solvothermal treatment.[51] As-synthesized catalyst
shows the superior electrochemical activity toward full water
splitting, where a low overpotential of 226 mV@20 mA cm−2
for the OER and 94 mV@−10 mA cm−2 for the HER are
observed. Their study revealed that the superior catalytic activity
attributed controlled catalyst design, superhydrophilic surface
of the catalyst, and more important, Co and Fe synergetic effect.
5.1.1. Hydrogen Evolution Reaction
To solve the increasing environmental issues and global
warming caused by fossil fuel combustion, the HER has been
considered a promising approach, which produced hydrogen
(H2) as a clean, green, and high-density future energy carrier.[52]
It is well understood that platinum (Pt) has been widely used
and exhibits the highest efficiency in the HER;[53] however, for
large-scale applications, H2 production has been limited owing
to the high price and scarcity, and low durability of Pt metal.
Therefore, in the past few years, the different types of nonprecious metals including a) single metals: Ni, Co, FeP, MoS2,
W, and Cu, b) binary metals: CoNi,[54] CoMoSx,[55] NiCoS,[56]
FeMoP,[57] and so on, and c) ternary metals: NiCoW/C[58] have
been reported by cost-effective and simple electrodeposition
method. For instance, Du et al. synthesized nickel-containing
hydrophilic graphene composite on Ni foam Ni–HGX/NF electrocatalyst by co-electrodeposition method HER-to-H2 conversion.[59] The Ni–HG0.5/NF composite achieved high current
densities of 10 and 100 mA cm−2 at very low overpotentials of
30 and 140 mV, respectively, with a low Tafel slope 81 mV dec−1
in alkaline solutions for H2 evolution. Ni–HG0.5/NF composite’s catalytic activity was attributed to charge transfer between
Ni and hydrophilic graphene and the strong intensity of Ni
(111) of Ni-HG0.5/NF catalyst. Liu and co-workers synthesized
well-defined hierarchical radial NixP nanospheres on Ni foam
by a facile one-step co-electrodeposition method for HER.[60]
NixP electrocatalyst achieved a current density of 10 mA cm–2,
low Tafel slope of 55 mV dec–1 in 1 m KOH at only 63 mV
overpotential.
Recently, Wang and co-workers developed the NiCoS,
NiS, and CoS electrocatalyst on NF via a one-step electrodeposition method for H2 production.[56] Their study showed
that after electrodeposition, the NF cathode has an excellent
HER activity owing to the increased surface-active sites and
reduced overpotential. Furthermore, various research groups
have been reported as cost-effective copper-mediated electrocatalysts for electrocatalytic HER.[20,52b] In 2018, Min and coworkers reported a highly active 3D porous Cu foam@MoSx
electrocatalyst synthesized by a controlled electrodeposition
method for H2 evolution.[52b] Cu foam@MoSx electrocatalyst
exhibited high activity and durability for H2 production in 0.5 m
H2SO4. The current densities reached 10 and 100 mA cm−2 at
very low overpotential 200 and 250 mV, respectively, and the
Tafel slope of ≈44 mV dec−1.
Wang and co-workers synthesized nanocomposite having a
pine leaf shape or a spherical morphology CuXS/CuXO-coated
electrocatalyst by electrodeposition method for the efficient
HER.[20] As-synthesized CuXS/CuXO exhibited an excellent HER
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performance toward hydrogen production, where the onset
potential CuXS was found at −90 mV, and −100 mV versus RHE
for CuxO-coated electrodes in 0.5 m H2SO4 solution and Tafel
slope ≈100 mV dec−1. Their study showed that high catalytic
performance is attributed to the in situ modification along with
interconnection between the CuxS/CuxO coating and copper
substrate facilitating charge transfer.
5.1.2. Oxygen Evolution Reaction
In the water-splitting reaction, OER occurs at the anode, an
energy-exhaustive four-electron process, leading to a kinetically
slow step and significant efficiency loss. For the high production of O2 evolution, an efficient electrocatalyst plays a crucial
role, reducing the overpotential and enhanced energy efficiency.
Therefore, noble metal oxides, including IrO2 and RuO2 electrocatalyst, have been reported as a benchmark for OER. But
owing to their high cost and scarcity, the practical application
for large-scale synthesis has become challenging. To overcome
these problems recently, various types of active and stable nonprecious earth-abundant elements, including Co, Ni, Fe, and
their alloy (NiFe, CoFe, etc.), have been reported as electrocatalysts for OER. It is well understood that to achieve highly efficient OER, the following two essential observations have been
considered: 1) covalency around the transition metal center
could directly be proportional to catalytic activity, and 2) transition metal doping enhanced catalytic activity by reducing the
overpotential. In this regard, in 2016, Gewirth and co-workers
synthesized additive controlled fractal-like behavior Ni and NiFe
film containing an inhibitor/additive 3,5-diamino-1,2,4-triazole
(DAT) electrocatalyst (NiFe-DAT) by single-step galvanostatic
electrodeposition method for OER.[61] Their study showed that
NiFe-DAT electrocatalyst has an excellent performance toward
O2 evolution in alkaline solutions with high current density
(100 mA cm−2), high mas activity (≈1200 A g−1 of catalyst) at
low overpotential (≈300 mV) for ≈72 h stability. This high activity’s origin is fractal-like behavior, i.e., film roughness, caused
by the DAT additive inhibition of electrodeposition. Their
hypothesis electrodeposition method revealed a simple strategy
that provides high electrocatalytic activity with any conductive
materials.
Furthermore, the co-electrodeposition method has been
used for the synthesis of the most attractive, low cost, and
environmentally friendly (MnO2) metal ions (Fe, V, Co, and
Ni)-doped ultrathin nanosheets (≈5 nm) electrocatalyst on
carbon fiber paper (CFP) for OER.[62] As-synthesized doped
MnO2/CFP showed high catalytic performance with current density 20 mA cm−2 at 390 mV overpotential and ≈28 h
stability in 1 m KOH than pure MnO2/CFP composite electrode. Because metal ions (Fe, V, Co, and Ni) doped into MnO2
improved the conductivity of MnO2. Nath and co-workers have
developed earth-abundant quaternary mixed transition (Ni, Fe,
and Co) metal-doped selenides (Se) incorporating electrocatalyst
(FeCoNi)3Se4 via combinatorial electrodeposition method
for OER.[63] According to their comparative study, the quaternary (Ni0.25Fe0.68Co0.07)3Se4 exhibited the best performance at a
very low overpotential of 230 mV (vs RHE) at 10 mA cm–2 with
8 h stability than binary and the ternary metal selenides.
Adv. Funct. Mater. 2021, 31, 2101313
In 2019, a two-step electrodeposition method was used to
synthesize binder-free nano-bilayer films electrocatalyst NiCoPNiCoSe2 on CC for OER.[64] NiCoP-NiCoSe2 film showed a higher
performance of O2 evolution with 243 mV for 10 mA cm–2 with
long stability (80 h) than that of NiCoP (275 mV) and NiCoSe2
(302 mV) films. Their study revealed that Ni/Co codoping, facile
synthesis electrodeposition method, and the rational integration of NiCoP and NiCoSe2 played a crucial role in high OER
performance. Similarly, Zhang et al. synthesized highly effective binder-free and quaternary electrocatalyst (NiFeWMo
alloy on Ni mesh) by facial electrodeposition method for OER.[65]
Because of alloying and binder-free nanostructures’ synergistic
effect, the NiFeWMo achieved excellent catalytic OER performances 10 mA cm–2 at a low overpotential of 152 mV.
In the past decade, various electrocatalysts have been
reported for OER; however, most electrocatalysts do not fulfill
the large-scale application. To overcome this issue recently,
Haik and co-workers developed and successfully synthesized
a facile, inexpensive, and scalable, a new hybrid gadolinium
(Gd) dopant-based cobalt boride nanosheet on gold film
(Gd-CoB@Au) electrocatalyst via electrodeposition strategy for
OER.[42] As synthesized, Gd-CoB@Au exhibited excellent electrochemical performance with FE (>98%), high current density,
and turnover frequency (TOF) (1600 s−1) and at very low overpotential for OER. The authors claimed that this type of work
could provide a new direction for the fabrication of transition
metal-derived electrocatalyst toward O2 evolution for largescale application. Furthermore, noble metal-free Ce-doped Ni
foam-supported Ni3S2 electrocatalyst has been synthesized by
a one-step electrodeposition method for alkaline OER.[66] Their
study revealed that compared to pure Ni3S2, the Ce-doped Ni3S2
electrocatalyst has high performance with a current density of
50 mA cm−2 at overpotential 257 mV for OER.
5.2. Oxygen Reduction Reaction
The ORR is one of the most important cathodic electrochemical reactions, which takes place in energy conversion devices
such as fuel cells and energy storage devices such as metal-air
batteries. It is known that the Pt-based catalyst is considered
as the highest degree active ORR electrocatalyst that has a
more significant positive onset and half-wave potential but are
affected by scarcity, high cost, low corrosion resistance, and low
stability in real application. The ORR is a kinetically slow and
highly irreversible electrochemical reaction; therefore, the synthesis of the cost-effective, highly desirable electrocatalysts is an
urgent need. Sookhakian et al. synthesized a nonprecious metal
Rh@rGO-modified GCE by in situ electrochemical deposition
for ORR.[67] Their study revealed that the ORR’s high activity
and stability were because of the well-homogenous decoration
of the Rh nanoparticles and a 2D structure of rGO.
Similarly, a cheap and efficient non-noble metal electrocatalyst MnO2/rGO composite with a novel yarn-rod shape was
developed by a simple, economic, and environmentally friendly
electrodeposition method.[68] Electrochemical studies showed
that the ORR’s MnO2/rGO composite electrocatalysts in an
alkaline medium proceed predominantly by the four-electrontransfer pathway. The experimental results indicated that the
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MnO2 nanoparticles with rod-like morphology scatter over
the yarn-shaped rGO sheet through the electrodeposition procedure. Therefore MnO2/rGO showed better ORR stability,
higher electron transfer numbers, and stronger methanol-tolerant ability than the commercial Pt/C catalyst.
Furthermore, Tantavichet and co-workers developed the different carbon-supported CC and GC PtCo bimetallic electrocatalyst by the pulse-reverse electrodeposition method for
ORR.[69] Their study showed that the ORR of PtCo on CC
is significantly higher than on GC owing to its highly porous
structure. In addition, the authors also claimed that pulse electrodeposition (PED) pulse current (PC) and pulse reverse current (PR) has advantages over DC electrodeposition in terms of
the flexibility of controlling the applied current.
and 50 h stability. Despite this progress, to achieve the high
selectivity and production of formate from CO2 at low overpotential is still relatively challenging. In this context, recently,
Jiang and Wang’s group’s fabricated a nontoxic Bi metal electrocatalyst on Cu foil via simple constant potential electrodeposition method for formate production.[73] Their comparative study
showed that, at low overpotential of 0.65 V a Bi catalyst with a
hexagonal sheet structure achieved ≈100% FE format production from CO2.The structural and physical study revealed that
the Bi hexagonal sheet structure was superior to cuboid and
dendritic structures. Because the high-curvature hexagonal
sheet structure contains a lot of sharp edges and corner sites
these could enhance ECSA of the electrode and facilitate fast
electron transfer and further can suppress the HER.
5.3. CO2 Reduction Reaction
5.3.2. CO Formation
Electrochemical CO2 reduction (ECDRR) to value-added
chemicals and fuels ranging from formate, carbon monoxide,
methane, ethylene, methanol, ethanol, and so on provided an
avenue reduce the globally increasing amount of CO2 emissions.[70] For this conversion, various electrocatalysts such as
pristine metal, nanomaterials, bimetallic, single-atom catalyst,
porous organic framework (MOF and COF), and metal-free
have been reported. The synthesis of these electrocatalysts has
been achieved using hydrothermal, solvothermal, spin-coating,
and electrodeposition methods. Among them, the electrodeposition method has been extensively studied and extended for
ECDRR toward the production of value-added fuels because of
cost-effective, user-friendly, and usefulness for the synthesis of
highly air and moisture-sensitive metal.
In addition to bulk Zn metal, nanostructure Zn electrocatalyst
has also been reported, enhancing the electrocatalyst activity
and selectivity than a bulk catalyst for ECDRR. However, in
air, moisture, and prolonged contact with aqueous electrolytes,
Zn can be easily oxidized and may significantly affect catalytic
activity. To overcome this problem, Jiao and co-workers developed an electrodeposition strategy to synthesize nanostructured
Zn dendrite electrocatalyst, which could minimize the surface
oxide layer formation and create a highly active catalyst a dendritic structure.[74] The synthesized Zn dendrite electrocatalyst
provided the selective CO formation by electroreduction of
CO2 with FE ≈ 80% at −1.1 V versus RHE in 0.5 m NaHCO3,
which was threefold higher than bulk Zn metal. Their study
also claimed that at this negative potential, the electrocatalyst is
structurally more stable under working conditions by ruling out
the oxidation of Zn. This study revealed that the electrodeposition approach is to overcome the Zn electrode’s sensitivity and
an enhanced CO selectivity and current density (≈13 mA cm–2)
owing to a very high surface area. However, this result could
not meet the industrial requirements; therefore, Luo et al.
recently reported a low-cost porous Zn (P-Zn) electrocatalyst
using a facile electrodeposition method to enhanced ECDRR
toward selective CO formation.[75] The porous Zn catalyst
showed remarkable CO formation with FE ≈95% and achieved
current density ≈27 mA cm–2 at −0.95 V in the H-cell reactor.
Besides, a novel strategy to transform the porous Zn electrode
into a gas diffusion electrode (GDE) revealed CO FE ≈84% and
higher current density ≈200 mA cm–2 at −0.67 V versus RHE
in the flow-cell reactor, which showed the best performance
to date over non-noble ECDRR catalysts. The dramatically
improved catalytic activity and selectivity for CO2-to-CO are
primarily attributed to the highly porous structure of P−Zn,
which increases the number of active sites and strengthens the
local pH effect. This study will guide a new direction for the
future development of other low-cost and active electrocatalysts
by applying the facile electrodeposition strategy, fulfilling the
industrial requirement for ECDRR applications.
Recently, H2 bubble templated electrodeposition method
in a thiocyanate (SCN) containing aqueous electrolyte has
been reported to synthesize a hierarchically porous Ag nanofoam (AgNF). This AgNF exhibited excellent performance for
5.3.1. Formate Formation
Rabiee et al. synthesized an asymmetric porous Cu hollow fiber
gas diffusion electrode (HFGDE) with controlled Sn surface
electrodeposition.[71] The HFGDE derived from the optimal Sn
electrodeposition condition exhibited a formate FE of 78% and
a current density of 88 mA cm–2 at −1.2 V versus RHE, which
was twofold the pristine Cu HFGDE. This high performance
is attributed to the Sn catalyst’s-controlled electrodeposition
on the porous Cu hollow fiber to fabricate composite HFGDEs
substantial changes in formate formation selectivity. However,
excess Sn deposition resulted in a significant HER. Besides, the
enormous effect of minute amounts of surface adatoms on the
electrocatalytic activity and selectivity of the ECDRR has been
studied. Where bimetallic adatom electrodes were prepared
using underpotential electrodeposition (UPD), this study demonstrated that bimetallic catalysts consisting of only minute
sub-monolayer amounts of Pb adatoms deposited on Cu surfaces (Pd/Cu) exhibit and maintain unusually high selectivities
for formate (HCOO−) over a broad range of overpotentials.[72]
Furthermore, Cui and co-workers studied the density functional
theory (DFT) to investigate the effect of alloying Cu and Sn on
the activity and selectivity toward formate.[70a] Theoretical trends
suggested that the designed CuSn3 catalysts by co-electrodeposition exhibited formate with FE of 95% at −0.5 V versus RHE
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converting CO2 to CO with maximum FE 97% at the potential range −0.5 to −1.2 V versus RHE and current density of
33 mA cm−2. DFT calculations suggested that SCN ligands’
roles promote COOH* intermediates by inducing charge
redistribution on the Ag surfaces. The authors also hypothesize that this synthesis method of AgNF can be extended to
other catalyst substrates. Furthermore, Lamaison et al. successfully synthesized an industrial-based, highly active porous
silver-zinc alloy (AgZn) electrocatalyst via co-electrodeposition
strategy to convert CO2 into CO, as high as CO 90% FE with
40 h durability.[25] The electrode optimization study achieved a
new record-high CO partial current density of −286 mA cm–2
at 9.5 bar CO2 pressure. Compared to atmospheric conditions
(pressure: 1 bar pressure), the high-CO2 pressure condition
(≈9.5 bar) showed record-high industrial-scale −286 mA cm–2
partial current densities, this is because of the mass transport
of CO2 limitation that could be overcome at high pressure.
Besides metal-based electrocatalyst for ECDRR, the metalfree electrocatalyst has also been reported elsewhere.[76]
Rosenthal group developed a cost-effective bismuth−carbon
monoxide evolving electrocatalyst (Bi-CMEC) formed under
either aqueous or nonaqueous conditions using versatile electrodeposition method.[76] The Bi-CMEC promoted the reduction
of CO2 to CO with high FE 90%, with an efficiency of 80%, and
large current densities ≈25−30 mA cm–2.
5.3.3. Hydrocarbon and Alcohol Formation
In addition to monometallic electrocatalysts, in the past few
decades, the cost-effective electrodeposition strategy has also
been extensively studied for ECDRR. Therefore, Gewirth
and co-workers have successfully synthesized a nanoporous
copper-silver alloy (CuAg) at very low Ag contents ≈6% electrocatalyst by additive-controlled electrodeposition method for
ECDRR.[77] The CuAg-wire electrocatalyst exhibited the highest
FE C2H4 ≈60% and C2H5OH ≈25% at −0.7 V versus RHE and
a high current density −300 mA cm–2 compared to CuAgpoy and Cu-wire in an alkaline 1 m KOH flow electrolyzer for
ECDRR. Their study showed that the presence of Ag could help
in the formation of Cu2O on the Cu surface because Ag has
high redox potential than Cu and can accept electrons from Cu,
resulting in a slight positive charge on Cu in the CuAg sample.
To be specific, Ag plays a crucial role as a promoter for Cu2O
formation in the CuAg electrocatalyst, leading to the production of C2H4 from CO. Also, the appropriate loading of Ag
could enhance the CO production so that neighboring Cu may
have to participate in CC coupling.
Furthermore, oxide-derived (OD) Cu bimetallic AgCu metal
foams with a ratio of 15:85 have been synthesized by an additiveassisted electrodeposition process using the dynamic hydrogen
bubble template approach annealing strategy for alcohol formation in ECDRR.[78] The bimetallic Ag15Cu85 foam electrocatalyst
showed high electrocatalytic selectivity toward alcohol formation with FE EtOH = 33.7% and FE n-PrOH = 6.9% at −1.0 and
−0.9 V versus RHE, and 100 h stability. Both operando X-ray
diffraction and operando Raman spectroscopy confirmed the
electrocatalyst activation process, which involved the reduction
of the oxidic Cu precursors prior to the ECDRR onset.
Adv. Funct. Mater. 2021, 31, 2101313
Similarly, Zhu et al. successfully synthesized a high-aspectratio CuxAuy nanowire arrays (NWAs) by a potentiostatic pulseelectrodeposition electrocatalyst for the conversion of CO2 to
ethanol production.[79] This electrocatalyst showed high performance toward ethanol formation with EF 48% at the very low
potential of −0.5 to −0.7 V versus RHE. The Cu:Au ratio in the
CuxAuy NWAs could be easily controlled by the electrodeposition potential, which affects the surface electronic structures
of the fabricated CuxAuy NWAs. This excellent EtOH selectivity could be attributed to the synergistic effect of morphology
and electronic structure that enhances CO’s adsorption on the
CuxAuy nanowire surface and facilitates the subsequent reduction of the ∗CO EtOH via the CO dimerization pathway. These
all-mentioned Cu-based bimetallic electrocatalysts provided a
new direction for designing Cu-based electrocatalysts with high
selectivity for C2 products. In addition to alcohol products, the
formation of methane product has also been extensively studied
and extended owing to economic benefits and represents one
promising solution for energy and environmental sustainability.[80] Therefore, Qiu et al. developed a Cu electrocatalyst
by PED method. The PED exhibited high selectivity toward
CH4 production with 85% at −2.4 V versus RHE and partial
current density 38 mA cm–2. This high electrocatalytic activity
and selectivity were because of the electrocatalyst roughness
factor and extending surface area, which was achieved by PED
strategy. This study demonstrated that the surface morphology
of Cu electrodes could significantly influence the catalytic
activity and product selectivity for CO2 reduction, making pulse
electrodeposition an excellent technique for electrode fabrication of electrochemical reduction of CO2.
5.4. Nitrogen Reduction Reaction
Various literature and reviews have reported the electrocatalysts for the NRR toward highly desirable chemicals such as
ammonia (NH3) production.[21,81] The electrochemical reduction
of N2 to NH3 (N2 + 6H+ + 6e− → 2NH3 at room temperature)
has an alternative facile and sustainable strategy that could
replace the well-known industrial Haber–Bosch process.[21,81d,82]
Among the various synthesis method (as shown in Section 3),
Wang and co-workers successfully synthesized a porous Au
film on Ni foam (pAu/NF) via micelle-assisted electrodeposition method for electrochemical reduction of N2 to NH3.[21]
Benefiting from its interconnected porous architectonics and
active Au composition, the pAu/NF showed excellent NRR performance with a high NH3 production rate of 9.42 µg h−1 cm−2
and a high FE of 13.36% at −0.2 V versus RHE at 0.1 m Na2SO4
electrolyte solution. The authors proposed that this micelleassisted electrodeposition strategy could provide a future
direction for designing a future NRR electrocatalyst toward
various electrocatalysis fields. Furthermore, owing to the high
solubility of N2 in ionic liquid (IL) and favorable hydrophobic
nature, MacFarlane and co-workers synthesized the Fe-based
electrocatalyst mixture of lLs and a trace amount of water by
electrodeposition technique as the working electrode for NH3
production from NRR. Among [C4mpyr][eFAP] and [P6,6,6,14]
[eFAP lLs the [P6,6,6,14][eFAP showed high conversion efficiency
of 60% for N2 reduction to ammonia (60%) higher than that
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in [C4mp][eFAP] (30%). The DFT calculations indicate that the
particular ionic structure of [P6,6,6,14][eFAP] allows N2 accumulation at the cathode to increase NRR performance, and the additional delocalization of charge onto the three C2F5 groups can
modulate the charge on the P-bound fluorine atoms to enhance
the interaction with N2 in the [C4mpyr][eFAP] leading to the
high N2 solubility.
5.5. Metal-Air/CO2 Batteries
In addition to the above-discussed energy storage systems,
the metal-air/CO2 rechargeable batteries were also extensively
studied by many researchers as an energy storage system. It
is well understood that the metal-air battery, i.e., Li-O2 battery
(LOBs), showed high energy density (3500 Wh kg−1), almost
3–10 times higher than that of Li-ion battery (LIBs).[83] However,
metal-O2 batteries have not been explored in a large-scale application because of unsatisfactory cycling performance, high
overpotential, and low capacity.[84] It was caused by a sluggish
ORR 2Li + O2 → Li2O2 and OER 2Li + O2 ← Li2O2. Therefore,
to overcome these problems, the synthesis and design of effective cathode electrocatalysts play an important role. In recent
years, in addition to LOBs, Zn-O2 battery (ZOBs), Na-O2 battery,
and Al-O2 battery have been reported in many literatures.
Moreover, transition metal oxide (MnO2, Mn2O3, and
Mn3O4) electrocatalysts showed high ORR performance owing
to their multiple valence states. In this context, Li et al. developed and successfully synthesized a low-cost, highly abundant, and binder-free MnOx (Mn3O4 nanoparticles and Mn2O3
nanosheet) electrocatalyst on carbon cloth MnOx@C for LOBs
by convenient and environmentally friendly electrodeposition
method along with heat treatment (200–400 °C).[84] MnOx@C350 °C showed a limiting specific capacity of 1000 mAh g−1
with operate stably for 80 cycles at 340 mA g−1 and achieved
8000 mA h g−1 first discharge specific capacity at 200 mA g−1.
The high electrocatalytic performance was attributed to the control synthesis of nanosheets with nanoparticles and the high
valence of Mn, which could favor ORR. Despite the high-performance ORR of MnO2 electrocatalyst, the OER activity MnO2
has been still challenging. Therefore, Wang and co-workers
first time reported the cost-effective Janus MnO2-NiFe/Ni electrocatalyst by a combination of MnO2 and highly OER active
catalytic layered double hydroxides (NiFe LDHs) via a facile
two-step electrodeposition strategy.[85] The ZOBs assembled by
the Janus MnO2-NiFe air electrode exhibits a peak power density of 93.95 mW cm−2, the high energy efficiency of 52.43% at
the current density of 50 mA cm−2, and superior rechargeable
durability even under a large current density of 50 mA cm−2,
which was higher than bare MnO2. This type of strategy could
be applicable for future high-performance Jasus electrocatalyst
for metal−air batteries.
Furthermore, Song et al. reported a low-cost bifunctional
cobalt-based electrocatalyst Co(OH)2 and Co3O4 for OER, ORR,
and ZOBs.[86] First, Co(OH)2 electrocatalyst was synthesized
by electrodeposition method, and by post thermal treatment,
Co3O4 was obtained as a high geometrical area of mesoporous
nanosheet than Co(OH)2. The Co3O4 nanosheets on carbon
cloth Co3O4NS/CC assembled primary or rechargeable cells
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ZOBs showed a large peak power density of 106.6 mW cm−2,
low charge, and discharge overpotentials (0.67 V), high discharge rate capability (1.18 V at 20 mA cm−2), and long-term
cycling stability (400 cycles).
In addition to metal-air batteries, metal-CO2 batteries such
as Li-CO2 have also been reported for CO2 capture and utilization.[87] In this context, Archer and co-workers reported a novel
primary Li-CO2 battery, where CO2 is consumed at the cathode
producing lithium carbonate (Li2CO3) as a primary discharge
product and CO as a by-product (4Li + 3CO2 → 2Li2CO3 +
CO).[87] Inspired by the coupling of Li-battery and CO2 electroreduction, Wang and co-workers successfully synthesized a 3D
porous fractal Zn (PF-Zn) as cathode material by redox-couple
electrodeposition method and fabricated with Li-CO2 battery for
CO gas generation as a primary product.[88] As prepared, the
PF-Zn cathode with the Li-CO2 battery system showed a maximum FE of 67% CO generation as the major product, and the
proposed reaction was 2Li+ + 2CO2 + 2e− → CO + Li2CO3. This
study revealed the future direction toward generating valueadded carbon-based fuels and chemicals by the configuration
of the metal-CO2 battery system and highly selective and active
cathodic materials.
6. Conclusion, Challenges, and Future
Perspectives
In this review, we have discussed the advances of the electrodeposition technique as a promising alternative method for
the synthesis of highly active and stable electrocatalysts for
clean energy conversion. Considering an importance of synthesis methods in preparing highly efficient electrocatalysts,
the various electrodeposition methods such as the conventional electrodeposition method, the galvanostatic electrodeposition method, the potentiostatic electrodeposition method,
and the advanced modified hydrothermal-electrodeposition
and microwave-electrodeposition method have discussed in
detail. In the galvanostatic and potentiostatic electrodeposition method, the constant current and constant potential are
applied during electrodeposition, respectively. The co-electrodeposition method helps to prepare highly porous nanomaterials with high specific surface area and can be used to prepare
electrocatalysts with high intrinsic electrocatalytic activity.
The modified hydrothermal method and modified microwave
electrodeposition method are useful for preparing highly
crystalline deposited electrocatalysts and highly mesoporous
coatings, respectively, that are very effective in achieving an
enhanced electrocatalytic activity.
The electrodeposition methods have various advantages such
as: 1) electrodeposition is fast and low cost to prepare metal
deposits, 2) facile route for controlled synthesis of uniformly
dispersed electrocatalysts, and 3) it does not need additional
capping agents or reductants, which makes it easy to scale up
and fulfill the industrial requirements. In the electrodeposition technique, the experimental parameters’ influence, such as
current density, electrolyte concentration, pH, or temperature,
and chemical composition, play a significant role in obtaining
the desired composite coatings. The deposit particle content
may increase or decrease with the variation in current density
2101313 (20 of 24)
© 2021 Wiley-VCH GmbH
www.advancedsciencenews.com
www.afm-journal.de
Table 1. Electrochemical performance of synthesized electrocatalysts using electrodeposition techniques.
Potential (mV vs RHE) J [mA cm–2] Tafel slope
or another parameter
[mV dec–1]
Reaction
FE [%]
Ref.
HER = 71
OER = 82
HER/OER
–
[50]
10
20
HER = 89
OER = 85
HER/OER
100
98
[51]
250
100
44
HER
–
[52b]
0.69 (V vs SHE)
1.91
192
HER
–
[54]
Electrocatalyst
Synthesis method
(electrodeposition)
Morphology
Electrolyte
NiS/Ni2P/Ni(OH)2
Two-step
electrodepositions
Nanosheet
1 m KOH
HER = 120
OER = 219
10
Co,Fe-MOF-74/
Co/CC
Electrodepositions/
solvothermal
Nanosheet
1 m KOH
HER = 94
OER = 226
–
Thin sheet-like
0.5 m H2SO4
MoSx/Cu
Co-Ni/AAO
Alternating current
electrodeposition
Nanowire array 0.025 m H2SO4
CoMoSx/CC
Co-electrodeposition
–
Acidic
180
100
70
HER
–
[55]
Ni-Co-S/NF
Co-S/NF
–
Nanosheet
1 m HCl
0.6 V
0.8 V
5.3
10.6
–
HER
–
[56]
Ni-Co-(WC)x/Ni
–
3D porous
0.5 m H2SO4
0.4 V
I0 = 12.08
10
0.258 V dec–1
HER
–
[58]
Ni-HG/NF
Co-electrodeposition
–
1 m NaOH
30
I0 = 5.012
10
81
HER
–
[59]
NixP/NF
One-step
co-electrodeposition
Nanosphere
1 m KOH
6
10
55
HER
–
[60]
NiFe/DAT
Single-step galvanostatic
electrodeposition
Nanocluster
1 m NaOH
300
Mas activity = 1200 A g−1
100
–
OER
–
[61]
(Fe, V, Co, and Ni)doped MnO2/CFP
Anodic electrodeposition
Nanosheet
1 m KOH
390
10
104.4
OER
–
[62]
Ni0.25Fe0.68Co0.07)3Se4
Combinatorial
electrodeposition
Rhombus-like
nanostructure
0.3 m KOH
230
ECSA = 1.725
10
41.7
OER
–
[63]
NiCoP-NiCoSe2
Two-step
electrodeposition
Nanofilm
1 m KOH
243
10
52
OER
–
[64]
Ni-Fe-W-Mo
Electrodeposition
Nanoparticle
Alkaline
152
10
–
OER
–
[65]
Ce-doped Ni3S2/Ni
foam
One-step
electrodeposition
Nanofilm
1 m KOH
257
50
81
OER
–
[66]
Rh@rGO/GCE
Electrodeposition
Nanoparticle
0.1 m KOH
Onsite potential
−0.11 V (Ag/AgCl)
n = 3.7
−2.14
–
ORR
–
[67]
MnO2/rGO
–
Yarn-rod shape
0.1 m KOH
n = 3.62
0.43
–
ORR
–
[68]
Pt-Co on CC
Pulse-reverse
electrodeposition
–
0.5 m H2SO4
n = 4.3
ECSA = 6.5
20
–
ORR
–
[69]
–
–
0.5 m KHCO3
−1.2 V
88
–
CDRR to formate
78
[71]
Constant potential
electrodeposition
Hexagonal
shape
0.1 m KHCO3
−0.6 V
–
–
CDRR to formate
≈100
[73]
Cu-HFGDE/Sn
Bi/Cu foil
Zn dendrite
Electrodeposition
−0.9 V
4
–
CDRR to CO
80
[74]
P-Zn
Electrodeposition
Porous
0.1 m KHCO3
−0.64 V
200
–
CDRR to CO
84
[75]
Ag-Zn
Co-electrodeposition
Porous
0.1 MCsHCO3
−0.9 V
BET-3133
−286
@9.5 bar
CDRR to CO
≈90
[25]
Cu-Ag-DAT
Electrodeposition
–
1 m KOH
−0.7 V
−300
–
AgCu foam
Additive-assisted
electrodeposition
Foam
0.5 m KHCO3
CO = −0.3 V
C2H4 = −1.1 V
EtOH = −1.0 V
−0.24
−11.32
−8.67
–
CDRR to C1
and C2
CO ≈ 82
C2H4 ≈ 37
EtOH ≈ 34
[78]
0.1 m KHCO3
−0.7 V
1
–
CDRR to ethanol
45
[79]
Nanoparticles 0.5 m NaHCO3
[77]
CDRR to C2H4
C2H4 = 60
and C2H5OH C2H5OH = 25
CuxAuy NWA
Potentiostatic
Nanowire array
pulse-electrodeposition
Cu-P-ED
Pulse electrodeposition
–
0.1 m NaHCO3
−2.8 V
38
–
CDRR to
methanol
85
[80]
pAu/NF
Micelle-assisted
electrodeposition
Porous film
0.1 m Na2SO4
−0.2 V
ECSA = 72 cm2
0.25
–
NRR
13.36
[21]
Note: I0 = exchange current density (mA cm–2), n = electron transfer number, BET = specific surface area (cm2phys cm–2geo).
Adv. Funct. Mater. 2021, 31, 2101313
2101313 (21 of 24)
© 2021 Wiley-VCH GmbH
www.advancedsciencenews.com
www.afm-journal.de
and electrolyte. The variation in the pH can significantly vary
the cathode current efficiency and deposit structure; also, the
deposit particles crystal structures changes with the change in
bath temperatures. The Table 1 depicts the detailed insights on
electrochemical performance parameters of synthesized electrocatalysts using electrodeposition techniques. Though the
electrodeposition technique shows multiple advantages toward
clean energy conversion, the major challenges of electrodeposition are: 1) re-electrodeposition and 2) generate uneven distribution and smaller sizes of nanoparticles. But these issues
can be solved by varying one or all the parameters such as the
deposition potential time and composition of the electrolytes,
through which the particles size and particles densities can
be managed.[89] Finally, there is a future demand to prepare
the industrial-scale electrocatalysts through cell design membrane electrode assembly (MEA),[90] which could improve the
local CO2 concentration and intrinsic activity and stability of
electrocatalyst.
Acknowledgements
M.B.K. and R.A.B. contributed equally to this work. This work was
supported by the National Natural Science Foundation of China, China
(Nos. 21872147 and 21805277), the Natural Science Foundation of Fujian
Province, China (Nos. 2018J05030 and 2019J05152), the Key Research
Program of Frontier Sciences, CAS, China (No. ZDBS-LY-SLH028), DNL
Cooperation Fund, CAS, China, (DNL201924), and the Strategic Priority
Research Program, CAS, China, (No. XDB20000000). R.A.B. acknowledges
the partial scholarship sponsored by the University of Chinese Academy of
Sciences, Chinese Council, and Fujian Institute of Research on the Structure
of Matter, Chinese Academy of Sciences, for international students.
Conflict of Interest
The authors declare no conflict of interest.
Author Contributions
M.B.K. contributed to writing—original draft, writing—review, editing,
and Investigation. R.A.B. contributed to writing—original draft,
review, editing, and Investigation. A.G.A.M. contributed to review. W.Y.
contributed to conceptualization, funding acquisition, writing—review,
editing, and supervision.
Keywords
CDRR, electrocatalysts, electrodeposition, energy conversion, water
splitting
Received: February 6, 2021
Revised: February 25, 2021
Published online: April 1, 2021
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Manoj B. Kale is a postdoctoral researcher at Fujian Institute of Research on the Structure of
Matter, Chinese Academy of Sciences (P. R. China). He received his Ph.D. from Fujian Institute
of Research on the Structure of Matter, Chinese Academy of Sciences (P. R. China) in 2020.
His research interest is synthesis and application of novel electrocatalysts materials for energy
conversion.
Rahul Anil Borse received his B.S. degree (2014) and M.S. degree (2016) in chemistry from Pune
University, India. In 2018, he joined Prof. Yaobing Wang’s group at Fujian Institute of Research
on the Structure of Matter, Chinese Academy of Sciences (P. R. China) as a Ph.D. candidate. His
current research interests are focusing on design and synthesis of advanced materials including
carbon-based metal free electrocatalysts and MOF, for CO2 reduction and their implications to
electrochemical energy storage and conversion.
Yaobing Wang is a professor in Fujian Institute of Research on the Structure of Matter, Chinese
Academy of Sciences. He received his Ph.D. degree from the Institute of Chemistry, Chinese
Academy of Sciences. His current research is focused on novel material’s synthesis and application in energy conversion and storage.
Adv. Funct. Mater. 2021, 31, 2101313
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