Energy Conversion and Management 261 (2022) 115648
Contents lists available at ScienceDirect
Energy Conversion and Management
journal homepage: www.elsevier.com/locate/enconman
Review
Photovoltaic/photo-electrocatalysis integration for green hydrogen:
A review
Piyali Chatterjee a, 1, 2, Mounika Sai Krishna Ambati b, 1, Amit K. Chakraborty a, c, 3, *,
Sabyasachi Chakrabortty d, Sajal Biring e, f, Seeram Ramakrishna g, Terence Kin Shun Wong h,
Avishek Kumar i, Raghavendra Lawaniya i, Goutam Kumar Dalapati b, e, g, i
a
Carbon Nanotechnology Lab, Department of Physics, National Institute of Technology, Durgapur 713209, WB, India
Department of Physics, SRM University, Andhra Pradesh 522502, India
Centre of Excellence in Advanced Materials, National Institute of Technology, Durgapur 713209, WB, India
d
Department of Chemistry, SRM University, Andhra Pradesh 522502, India
e
Organic Electronics Research Center, Ming-Chi University of Technology, 84 Gungjuan Rd., New Taipei City 24301, Taiwan
f
Department of Electronic Engineering, Ming-Chi University of Technology, New Taipei City 24301, Taiwan
g
Center for Nanofibers and Nanotechnology, National University of Singapore, Singapore 117576, Singapore
h
School of Electrical and Electronic Engineering, Nanyang Technological University, Block S2, Nanyang Avenue, Singapore 639798, Singapore
i
Sunkonnect, 1 Cleantech Loop, Singapore 637141, Singapore
b
c
A R T I C L E I N F O
A B S T R A C T
Keywords:
Photovoltaic
Photo-Electrocatalysis
Photoelectrochemical
Solar to Hydrogen
Tandem Solar Cells
Metal-oxides
The Sun is an inexhaustible source of renewable energy, although under-utilized due to its intermittent nature.
Hydrogen fuel is another clean, storable, and renewable energy as it can be readily produced by electrolysis of
water, a naturally abundant resource. However, the necessary voltage for water electrolysis (>1.23 V) is high for
the process to be cost effective, and therefore requires photoelectrocatalytic (PEC) cells for lowering the voltage.
Powering the PEC cells with solar driven photovoltaic (PV) devices offers an all-clean efficient technology purely
relying on renewable sources and therefore warrants large research attention. This review aims to provide an up
to date account of the PV-PEC integrated technology for green hydrogen. We begin with the fundamentals of PV
and water splitting technologies (electrolysis, photocatalysis, electrocatalysis (EC), photoelectrocatalysis (PEC)),
as well as why and how the unassisted solar water splitting technology gradually progressed from PV with
external electrolysers (PV-EC) to integration of PV with EC (IPV-EC) and PEC (PV-PEC). We then discuss the
major challenges in PV-PEC integration and outline the major breakthroughs in design and materials develop­
ment for high Solar to Hydrogen (STH) efficiency and long device lifetime. The importance of material selection
and metal-oxide semiconductor nanostructures for PV-PEC integration are also discussed with a special focus on
Cu-oxide as an emerging material. An outlook toward commercialization including the major guiding factors and
related technologies (for e.g., PV-Thermal integration) that can maximize solar energy utilization to reduce
payback time has been discussed.
1. Introduction
Global energy demand is on the rise. Among the alternative energy
solutions (solar, wind, hydro, etc.,) found to derive electrical energy
with low carbon emission, solar energy is more reliable and ample.
[1–12] But still sunlight is intermittent[13–20] (being only diurnal) and
unpredictable due to weather disruptions. So, one of the limitations
during practical application of electricity derived from renewable en­
ergies arise from the time gap between power production and its con­
sumption. At present, our energy demands are met to a great extent by
fossil fuels such as coal, natural gas, oil, etc.[21–25] which are not only
* Corresponding author.
E-mail addresses: amit.chakraborty@phy.nitdgp.ac.in (A.K. Chakraborty), sabyasachi.c@srmap.edu.in (S. Chakrabortty), biring@mail.mcut.edu.tw (S. Biring),
goutam.dalapati@sunkonnect.co (G.K. Dalapati).
1
These authors made equal contribution.
2
Orcid id 0000-0003-4259-0267.
3
Orcid id- 0000-0001-5741-0009.
https://doi.org/10.1016/j.enconman.2022.115648
Received 24 January 2022; Received in revised form 5 April 2022; Accepted 16 April 2022
Available online 29 April 2022
0196-8904/© 2022 Elsevier Ltd. All rights reserved.
P. Chatterjee et al.
Energy Conversion and Management 261 (2022) 115648
Nomenclature
PEC
Photoelectrocatalysis /photoelectrochemical
PEC cell Photoelectrocatalytic cell with at least one photoelectrode
PEC tandem cell PEC cell with two photoelectrodes (cathode and
anode)
PEM
Polymer electrolyte membrane
PMMA Polymethyl methacrylate
PSC
Perovskite solar cells
PV
Photovoltaic
PV-EC
Photovoltaic cell with external electrolyser
PV-PEC Photovoltaic cell powered photoelectrocatalytic device
PVT
Device with integration of photovoltaic and thermal
technology
P3HT
PCBM -poly(3-hexylthiophene-2,5-diyl): 6,6- phenyl C61
butyric acid methyl ester
RHE
Reversible hydrogen electrode
STH
Solar to hydrogen
AZO
CPV
DSSC
EC
EIS
HER
IPCE
IPV-EC
Aluminum doped zinc oxide
Concentrated photovoltaic
Dye sensitised solar cells
Electrocatalysis
Electrochemical Impedance Spectroscopy
Hydrogen-evolution reaction
Incident photon to current conversion efficiency
Electrocatalytic device with integrated photovoltaic cell
(immersed in electrolyte)
LDH
Layered double hydroxide
MEA
Membrane electrode assembly
MWCNT Multiwalled carbon nanotube
NREL
National Renewable Energy Laboratory (USA)
OER
Oxygen-evolution reaction
PCE
Power conversion efficiency
exhaustible resources but also their burning produces greenhouse gases
(like CO2)[26–35] including toxic chemicals such as SOx and NOx,
leading to air pollution and global warming.[36,37] Thus, it is clear that
for production of clean energy without causing environmental pollution,
one should look for alternatives to fossil fuels. Renewable energy har­
vesting technologies, especially photovoltaics (PV) based systems are
thus fast-growing, although they require combining with additional
energy storage and management systems for uninterrupted power sup­
ply.[38–41].
Among storable and portable fuels, lightweight hydrogen has very
high gravimetric energy density ~ 120 kJ/g[58] (more than gasoline)
and its combustion in fuel cells [55–57] to derive electrical energy forms
the clean by-product, water (H2O). Nevertheless, it requires high pres­
sure, low temperature, large volume, or advanced techniques to store it
properly.[59] Then it can be employed to run electric vehicles,[60]
work as drop-in liquid fuels (by CO2 hydrogenation)[61]and even as
feedstock for production of valuable chemicals for e.g., ammonia
[62,63]. Fig. 1 shows the importance of hydrogen production due to its
utility and global consumption. Presently hydrogen is derived primarily
from thermochemical processing of hydrocarbon (most popularly,
methane), steam reforming and electrolysis or electrocatalysis (EC) of
water, all generally powered by fossil fuels and thereby depleting 6% of
global natural gas and 2% of global coal and resulting in ~ 830 million
tonnes of CO2 emission per year[64]. Lately, nuclear reactors are also
being utilised to power electrolysis.
Photoelectrocatalysis (PEC) of the abundant natural resource, water
is a clean and sustainable way to produce hydrogen as this effectively
utilizes solar energy to provide the required thermodynamic potential of
1.23 V [48–51] needed for splitting of water.[52–54] This way, solar
energy can be stored as chemical energy in hydrogen and is thus an
alternative energy harvesting and storage technology. [65].
Among photoelectrochemical (solar) water splitting devices,[67,68]
PEC cells contain at least one light absorbing electrode (generally either
a single semiconductor or two semiconductors in heterojunction) at its
simplest. ‘PEC tandem cells’[69–73], on the other hand, have dual light
absorbers (i.e., a n-type photoanode and p-type photocathode)[74] in
wired or wireless mode and use the photovoltage generated by the two
photoelectrodes themselves. This is one of the attractive paths to achieve
unassisted water splitting at low cost since no PV is required.[72] But
combining PV [36,42,43] with EC[44] or PEC[45–47] can be a very
interesting technology for self-driven solar water splitting with high STH
efficiency and cost effectiveness. In such PV-EC devices, water splitting
can occur in the dark using the voltage generated by PV [75,76] and in
the PV-PEC integrated devices[77], at least one catalytic electrode is
light active, thereby reducing the voltage output needed from PV.[78].
The complication and high cost in scaling up individual units of PV-EC
for large-scale hydrogen production has motivated research on inte­
grated PV-EC (IPV-EC) and especially PV-PEC which harvests solar en­
ergy at multiple stages of the integrated device for efficient water
splitting. [79–85] Khaselev et al.,[43] demonstrated the first monolithic
device that combined PV with PEC in 1998 and many others followed
them but such devices are expensive to scale up due to the use of group
III-V compounds in PV.
During the last decade, there have been many innovations in the
combination and optimisation of materials (generally semiconductors)
in nano-architecture model to enable the harvesting of intense visible
light in solar spectrum (popularly by anion/cation doping) in stable
photocathode/photoanode[86] capable of highly efficient sustainable
production of hydrogen/oxygen.[78,87] Also, their long-term perfor­
mance is enhanced with the help of thin surface protection layer by
different innovative techniques to reduce photo-corrosion.[88] Many
useful properties of a semiconductor can also be tuned just by changing
the particle size and morphology.[89] For example, low cost and
chemically stable iron oxide (Fe2O3/ Fe3O4)[90] nanotubes with band
gap ~ 2.1 eV [91] that can absorb light up to 600 nm, is a nanoelectrode
of choice.[92–95].
Presently, unassisted high-STH technologies are costly and upscaling
remains impractical.[96,97] But theoretically, covering only 1% of the
land area on earth by PEC of 10% efficiency will suffice the predicted
energy consumption even in the year 2050.[98] Recent developments
suggest semiconductors based on metal nitrides, oxides, chalcogenides,
Si, organics and III-V compounds as good material choice for PEC. In
particular, the emerging trends of solution processed PV and photo­
catalytic semiconductors of earth-abundant metal oxides have great
potential for a sustainable future.[99,100].
For our readers to follow the evolution of the technologies that have
led to today’s PV-integrated solar water splitting technology for
hydrogen production, we now briefly discuss the fundamentals of the
two main constituents, viz, solar energy harvesting and solar water
splitting technologies along with major classifications. Since solar pho­
tovoltaics is a well-established topic, we give a very brief outline only to
inform the readers about the evolution of various PV technologies over
the years with description of the main technologies. Since the review’s
focus is to integrate PV with various water splitting methods for
hydrogen generation, we then discuss in detail the different solar water
splitting technologies including their operating principles, evolution
chronology and relative advantages/limitations. In the literature, there
are a few reports that reviewed PV or EC or PEC as standalone tech­
niques [42,69,84,101] whereas integration of PV-EC or PV-PEC has been
reviewed only as part of broad reviews[102] thus indicating the need for
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Energy Conversion and Management 261 (2022) 115648
Fig. 1. Schematics showing some of the main applications of Hydrogen. Data represented in pi-chart is from ref. [66].
a timely update of the integrated systems. In this article, we review the
state of the art of PV-PEC integration technologies, challenges and op­
portunities through appropriate material selection and design using lowcost metal oxides such as Cu oxides for practical scalable green hydrogen
production.
electrochemical techniques[112] for fuel conversion, like electrolysis of
water[113] and thus utilizing the concept of photovoltaic energy for the
production of hydrogen received scientific attention. PV cells basically
absorb photons (sunlight, generally) to generate electron hole pairs
which are then readily separated to drive an external circuit instead of
allowing their recombination. Power conversion efficiency (PCE) of PV
cells is measured as given below in equation (1).
1.1. Basic photovoltaic (PV) technologies
In 1839, Becquerel discovered the photovoltaic effect[103] which
later found application in attractive technologies that are now easily
available in market as silicon (Si)[104] or organic solar cells.[105–107]
The main idea is to convert sunlight into electrical power[108] and
comprehensive reviews[56,109,110] have been published on the stateof-art of PV technology in the literature.
Historically, silicon photovoltaics emerged as an energy option in
everyday life from the first modern energy crisis in 1973.[111] Gener­
ation of solar electricity enabled cheap access to abundance of
PCE =
output electrical power
X100%
incident light power
(1)
Single crystalline devices based on Si, InP, GaAs and GaInP show
highest PCE with good stability and photovoltaic parameters. Alterna­
tively, heterojunction-based PV devices with suitable band alignment
with the addition of dipole layer at junction interface are extensively
used now for optimum results due to their decreased recombination
losses at interface and increased band bending.[114] In addition, multijunction PV has allowed broadening of light absorption range with wide
Fig. 2. Classification of the PV technologies along with schematic representation of 6 selected device configurations (colour coded).
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Energy Conversion and Management 261 (2022) 115648
tunability and the charge transfer to PEC components is more efficient.
[115] The most commercialized technologies are mainly based on Si, in
spite of the cost of mining and purification steps to get crystalline or
multi crystalline Si wafers.[110] As shown in the classification in Fig. 2,
technological development of PV has come a long way.[116] Various
newer device configurations (schematic representations of selected ones
in Fig. 2) were revealed as potential candidate for PV applications. The
solution-processed thin film[117] based devices like organic solar cells,
dye-sensitised solar cells/DSSC,[118] or Perovskite solar cells (PSC)
[119] could be alternatives to costly wafer-based PV for scalable high
efficiency (PCE) devices. Chronological development of champion NREL
certified PV module efficiencies as available from their website (revised
in 2022) [120] suggests that crystalline Si based PV reached PCE of
24.4% whereas, the best efficiency of the less costly amorphous Si based
devices is ~ 12.3%. The organic PVs which are cheaper and easier to
process also offer comparable efficiency of 11.7% although with much
lower stability. Chalcogenide based PV has ~ 19% PCE (not yet for the
cheaper CZTS) and certified PSCs recorded 17.9%, both lower than the
theoretical maximum obtainable from such devices. But PSCs hold the
promise of greater cost effectiveness if stability is enhanced. GaAs PV
with and without concentrator has achieved as high as 38.9% and 31.2%
respectively, but is way too expensive for widespread
commercialisation.
1.2. Basic water-splitting technologies:
Water splitting by electrolysis[121] is an age-old H2 production
method that requires two conducting electrodes immersed in an elec­
trolyte between which an external bias of at least 1.23 V is applied.
During operation, the electronic current passes through the electrodes
and ionic current passes through the electrolyte. The oxidation occurs at
the anode, forming O2 and the reduction occurs at the cathode, forming
H2. A separator is often used between the two electrodes to segregate the
two gases for easy collection and also to avoid explosion. However, since
this method requires an input voltage of 1.23 V, several efforts are in
place to reduce the input voltage and/or to produce this energy using a
renewable source. Primarily these can be classified into following three
classes:
particulate
photocatalysis,
electrocatalysis
and
photoelectrocatalysis.
Particulate photocatalysis is one of the simplest methods for unbi­
ased solar water splitting which employs photo-active semiconductors
(photocatalyst) suspended in water (or immobilized suitably) along with
sacrificial reagents (to reduce wastage by radiative recombination)
(Fig. 3(a)). The semiconductor must be band aligned, implying the
valence band should be below the oxidation potential of water and the
conduction band must be above the reduction potential of water. Under
light irradiation (only low band gap can allow optimum sunlight
Fig. 3. (a) Water splitting by suspended particulate photocatalyst; (b)&(c) energy diagrams based on (b) one-step photo-excitation, (c) two-step photo-excitation (so
called “Z-scheme”).[126] Adapted from The Royal Society of Chemistry[126] (d) Spectral irradiance at AM 1.5 G and maximum values of thermodynamic
photocurrent density from single system to produce H2 from PEC water splitting.[122] Adapted from Wiley[122].
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Energy Conversion and Management 261 (2022) 115648
semiconductor has to straddle the reduction and oxidation potentials of
water (0 V and + 1.23 V vs. NHE i.e., normal hydrogen electrode at pH 0,
respectively) with band gap>1.23 eV (as detailed in Fig. 3(b)).[123] In
Fig. 3(c), two semiconductors connected by a reversible redox shuttle is
shown, where reduction of water to H2 and oxidation of water to O2
occur on each photocatalyst. Such two-step photocatalysis is called ‘‘Z-
absorption), electron and hole pairs are generated although only a small
fraction get consumed in preferred redox reactions. Fig. 3(a) shows
the photocatalytic water splitting process and Fig. 3(b) shows the band
diagram for a single photocatalyst. Photogenerated electron-hole pairs
take part in reduction and oxidation reactions if charge transfer to the
electrolyte is thermodynamically favourable. The band edges of the
Fig. 4. Solar water splitting technologies discussed in this review: (a) PV-EC at zero level of integration (left) and fully integrated PV-EC device (IPV-EC) (right), (b).
PEC cell with single photoelectrode (anode), (c) PEC tandem cell, (d) PV-PEC at zero level of integration (left) and fully integrated PV-PEC device (right). Note, that
the performance required from PV unit is less in PV-PEC than IPV-EC, as pictorially demonstrated.
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Energy Conversion and Management 261 (2022) 115648
scheme’’[123–125] and can be found even in heterogeneous photo­
catalysts without the presence of reversible redox shuttles, where
interparticle electron transfer by a physical contact between H2 and O2
evolution photocatalysts play the decisive role.[126] But particulate
photocatalysis is plagued by very low rate of production along with
mixed H2 & O2 output, which is prone to backward reaction.
Alternatively, electrocatalysts[127] can counter both the problems
of low production rate and mixed gas output but needs external bias to
initiate and accelerate the water splitting reaction at their interface with
electrolyte instead of being able to use light. Electrocatalysis is a cata­
lytic process which involves oxidation and reduction reactions via direct
electron transfer, where lowering the overpotential of that particular
reaction is required.[129] Fig. 3(d) shows the plots of maximum
attainable photocurrent densities and STH efficiencies according to the
bandgap energy under standard AM 1.5 G illumination.[122].
PV-EC (Fig. 4(a)) addresses the drawbacks of both particulate pho­
tocatalysis and electrocatalysis. PV-EC consists of coupling the output of
a PV module to an electrolyser. So, water splitting can now occur in a
two-step process: (i) photon-to-electric conversion using PV and (ii)
conversion of electric-to-chemical energy by electrolysers whose elec­
trodes are made of materials with electrocatalytic property.[128] The
H2 and O2 are produced separately by using proton exchange mem­
branes between electrodes. But again, the PV module is often too
expensive in this case since it is the only component utilising light en­
ergy in the system and must render the entire bias requirement for water
electrolysis.
Photoelectrocatalysis (PEC) offers a more straightforward path than
PV-EC for a light-driven chemical transformation due to direct interface
[130] of photoactive semiconductor electrode with electrolyte as shown
in Fig. 4(b). Solar-driven PEC cells[131–133] involve absorption of
incident light at the surface of the electrode(s) where H2 and/or O2 is
generated/liberated. Thus, these devices are necessary for imple­
mentation of multiple absorbers which can supply ample photovoltage
to get efficient solar hydrogen conversion at very low or no external bias.
[74] Hence we are going to discuss this technology in greater detail
including common ideas related to both EC and PEC.
PEC system consists of anode and cathode, where either one or both
(Fig. 4(c)) may be made of materials acting as light absorbers (to reduce
the external bias needed to initiate catalysis) with reaction rateaccelerating active electro-catalytic sites (separate sites for hydrogenevolution reaction (HER) and oxygen-evolution reaction (OER)), medi­
ated by electrolyte.[74] The HER and OER sites must be separated to
control the unwanted but more easily progressed backward reaction as
water splitting reaction is an endothermic reaction and requires a min­
imum of 237 kJ mol− 1 Gibbs free energy.[68,134,135] The fundamental
reactions for water splitting in acidic and alkaline medium for HER and
OER are given below -.
In acidic conditions:
electrolyte solution, the electron transfers from semiconductor to elec­
trolyte and their Fermi level equilibrium with its redox potential will
take place.[138] Fig. 5 (a)-(c) shows the energy diagram for water
splitting based on the photoelectrode used: photocathode and/or pho­
toanode.[126] An n-type semiconductor electrode acts as photoanode,
and the photogenerated holes accumulated on the surface of semi­
conductor are utilised in the oxidation reactions. Simultaneously, the
photoexcited electrons pass to counter electrode through external cir­
cuit, which are consumed in the reduction reactions (Fig. 5 (a)). The
valence band maximum should be more positive than water oxidation
potential so as to allow O2 generation on the photoanode. Likewise, a ptype semiconductor electrode acts as photocathode for H2 evolution, for
which the conduction band minimum is more negative than water
reduction potential (Fig. 5 (b)). In Fig. 5 (b), photoanode and photo­
cathode are connected in tandem, instead of having one photoelectrode
and a counter electrode. In such “PEC tandem’’ cells, operating potential
and photocurrent are measured from the intersection point of the linear
voltametric curves obtained from respective photoelectrodes in 0-1.23V
range.[126] Fig. 5 (d) shows the 3 major steps of water splitting in a
simple PEC cell with n-type semiconductor photoanode externally wired
to metal counter-electrode in alkaline conditions. The external bias
applied is necessary for electron transfer when the conduction band is
positive to water reduction potential.
Photocurrent is a measure of the rate of charge carrier being gener­
ated and thus proportional to the amount of H2 or O2 that can be
generated. Therefore, this is a popular way to characterise photo­
electrodes at research level. The overall STH (Solar-to-Hydrogen) effi­
ciency of a two electrode PEC cell can be calculated using Eq. (6) given
below.
STH =
+
(3)
-
2H2O ↔ O2 + 4H +4e
In alkaline conditions:
4H2O + 4e-↔2H2 + 4OH-
4OH ↔O2 + 4H2O + 4e
−
(6)
where, faradaic efficiency accounts for the loss of photogenerated
electron-holes and is defined as the efficiency of charge carrier transfer
for utilisation in an electrochemical reaction.
The intermediate technologies bridging PV-EC and PEC are shown in
Fig. 6 which illustrate the smooth shift of technology suggesting
simplification for easy integration. The first transition no.1 from
monolithic PEC-cell as the starting point is (a) and (b), which differen­
tiates the monolithic architecture into two separate electrodes con­
nected by wire, thus conceptually associate the PEC-cell to PV-EC. The
next transition no.2 into (c) only illustrates the addition of protective
layer on photo absorber with the catalyst deposited on its surface to
enhance the stability. Transition no.3 explores the perpendicular
placement of catalyst with respect to photo absorber, in configuration
(c) using a solid-state p-n junction or as (d) with a vertical catalyst,
rendering difference in charge transport distance. Configuration (d)
requires the very straightforward transition no.4 to configuration (e)
which involves a protective polymer coating. The transition no.5 allows
the photo absorber to be separated out of electrolyte by just effectively
placing the catalyst farther, as in configuration (f). This enables the final
transition no.6 into a standalone PV-module externally wired to an
electrolyzer as demonstrated in configuration (g). Thus, we can realise a
PEC cell by advancing the PV-EC technology itself but with more ma­
terial optimization w.r.t the redox potentials of the water.
(2)
4H++4e-↔2H2
photocurrent density × 1.229V × faradic efficiency
illumination power density
(4)
(5)
1.3. Integration of PV with water splitting technologies (EC and PEC)
Especially OER is the rate determining step and hence significant
amount of research is done especially on the photoanode for water
oxidation.
There are three major physical and chemical processes involved in
the working principle of a water splitting PEC cell. The main steps are:
(I) light absorption; (II) charge carrier separation and transportation;
(III) surface redox reactions[78,136] at the interface of electrode and
electrolyte.[137].
When illuminated semiconductor electrode is placed in an
Although both PV[140] and PEC[141] devices are used for solar
energy absorption and conversion, the difference lies in the fact that PEC
device[126,142] contains an obvious liquid phase where the electrode/
water-based electrolyte interface is necessary for the ions to migrate and
take part in electrochemical reactions. On the other hand, commercial
PV cells,[143,144] are purely solid-state device where electrons or holes
are the charge carriers solely. But both PV[145] and PEC[146,147]
devices are designed for photo-generation of electron-hole pairs
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Energy Conversion and Management 261 (2022) 115648
Fig. 5. (a)-(c) energy diagrams of PEC using (a) photoanode, (b) photocathode, (c) photocathode and photoanode (PEC tandem configuration)[126] Adapted from
The Royal Society of Chemistry[126] (d) Schematic representation of an externally biased PEC cell with n-type semiconductor photoanode externally wired to metal
counter-electrode in alkaline electrolyte.[78] Adapted from The Royal Society of Chemistry[78].
followed by their separation (before recombination can occur) to ulti­
mately pass through their intended route, i.e the direct load/ any storage
mechanism or water splitting.[148].
But space saving integration maximizes the employment of sunlight
in generating both the electricity and the hydrogen.[149] Hence the two
distinctly different devices have been integrated into PV-PEC and IPVEC by various groups. The PV-EC of 1970′ s evolved into IPV-EC in
1990′ s.
In our nomenclature, IPV-EC basically refers to the devices where PV
is in contact with electrolyte via electrocatalytic electrode. PV-EC differs
from IPV-EC only because the PV component is separate and not
immersed in electrolyte. Mainly there can be 3 approaches to IPV-EC:
fully integrated, partially integrated, and non-integrated i.e., back to
PV-EC (configurations are analogous to PV-PEC as shown in Fig. 7(a-c))
except for the fact that none of the water splitting electrodes are photoactive in IPV-EC).[43,150] When PV is connected to external electro­
lyser (PV-EC), losses occur during lateral collection and electricity
transmission. But the advent of IPV-EC could check this specific problem
greatly. PV-EC is by far a back dated technology and Table 1 can give an
idea about how the budding technology of integrated PV-PEC can
generate H2 more cost effectively when scaled up compared to IPV-EC
for the same area of integration, if some obstacles are properly
addressed.
Unassisted water splitting with maximum utilisation of solar energy
can be realized with both PEC tandem cells and PV-PEC. Tandem cells
are a frequently used configuration for an unassisted overall watersplitting due to its advantages, where each component is connected to
form highly efficient integration. The PEC tandem devices (Fig. 7 (d-f))
are based on different absorbers for photocathode and photoanodes.
[151] In fact, PEC tandem cells with minimal changes in system results
in the reduction of upscaling cost as there is no need for multijunction
PV cells or PV modules with two or three cells connected in series to
provide necessary bias voltage.[42,150].
Various PV-PEC and PEC tandem cell design combinations at
different levels of integration (made of either layered or separate
component electrodes, with PV unit in case of PV-PEC externally wired
or attached to photoelectrode and immersed in electrolyte, and photo­
electrodes illuminated in parallel or tandem mode) have been presented
in Fig. 7(a)-(f). Referring to the schematics of possible configurations, it
is worth mentioning that light absorption and device integration is more
efficient when the photoelectrode first absorbs the higher frequencies
allowing the transmitted lower frequencies to be utilized by band gap
matched PV or photocathode in the back. The separate PV-PEC is easy to
assemble, and the PV is not prone to electrolytic corrosion, but its scaling
up proves to be most difficult to achieve with retained efficiency. On the
other hand, the layered cell involves complex fabrication in lieu of
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Energy Conversion and Management 261 (2022) 115648
high performance commercial InGaP/GaAs/GaInNAsSb, a triplejunction solar cell where sunlight is concentrated to match the
maximum power point of PV with the operating capacity of their poly­
mer electrolyte membrane (PEM) electrolysers, thereby achieving
average 30% STH efficiency for 48 hr.[81] In 2017, by using GaInP/
GaAs/Ge cell and low-cost Ni electrode hybrid system, STH as high as
22.4% was achieved.[84,85] Also, there has been progress in finding
membrane-free, safer and more economic ways to generate H2 in sepa­
rate cell from that of O2 and thus possibly in a remote location, enabling
easier H2 distribution as demonstrated by Landman et al. (2017).
[82,83].
Due to low-energy density of sunlight and low STH efficiency as of
now, it is essential to harvest sunlight over a large area by scaling up the
units.[161] Although, triple junction solar cells and PV-PEC tandem
designs are giving the highest efficiencies of hydrogen production, the
fabrication of such systems are complex and expensive.[101].
Compared to III-V multi-junction PV units and triple-junction
amorphous silicon for direct water splitting, usage of inexpensive
earth-abundant stable metal-oxide photoelectrodes in PV-PEC designs
can decrease the cost and device complexity. Metal oxides (such as, CuO,
WO3, TiO2 and Fe2O3) are ideal candidates for photocatalysis due to its
suitable optical bandgap, facile synthesis, tuneable electronic property,
and improved stability with electrolyte.[162] But even with the emer­
gence of scalable thin film solar cells and metal oxide photocatalysts, Si
is still now one of the perfect materials for cost-effective commercial
production of solar hydrogen mostly because of developed technology
for optimised light absorption and long-term stability along with earth
abundance.[163–165].
To be economically competitive with simple PV-EC strategies for
production of solar fuels (i.e., H2) using fossil fuels, a practical PV-PEC
or IPV-EC device must optimize the installation cost and longevity too
along with operational cost and performance. Even if the materials are
optimised for low losses and degradation, other challenges are equally
important. PEC enlargement is compromised as large area is correlated
with mass transport and solution resistance in liquid-phase since the ions
have to traverse long distances. Moreover, in PV-EC, IPV-EC and PEC
based cells, sacrificial reagents need to be replenished often along with
maintaining high flow of electrolyte to reduce bubble accumulation and
keep the concentration overpotentials low. The major problem with PV
unit is instability in aqueous solutions, so a protective layer to the device
for the PV-PEC system should be considered.
It is ideal if any system converting the solar energy should find a
balance between maximizing device performance (energy conversion
efficiency and device longevity) and minimizing system complexity
(directly related to device cost) to harness relatively low energy dense
solar irradiation. The economically feasible system of production of
solar hydrogen should compete with the price of H2 generated from
conventional sources.[166].
Fig. 6. Illustration of a gradual transition in six steps, starting from (a) a
monolithic PEC-device to (b) separation of monolith into the two free standing
electrodes connected by wire. In next step, protective surface layer is added
with (c) catalyst deposited over it or (d) catalyst deposited on a perpendicularly
placed conductor. (e) Stable transparent polymer is coated over photoelectrode
but penetrated by a catalyst loaded conductor perpendicularly. (f) photo­
electrode is removed from electrolyte and this transforms the system into PVEC. (g) PV-EC connected to power grid.[139] Adapted from The Royal Soci­
ety of Chemistry[139].
having the highest scope for integration. The wired configuration is a
tolerable compromise between the two and it is mostly used. Fig. 7(g)
gives a guideline on how to choose the front and rear absorber to
maximise the utilisation of solar spectrum and increase the highest STH
theoretically possible to obtain.
But the best reported 1 sun efficiencies for PEC tandem cell[152] and
PV-PEC tandem cell[153] are 13.1% and 19.3% respectively, which
begged our focussed attention to PV-PEC and also IPV-EC integration in
order for solar hydrogen to compete with existing conventional methods
of H2 production. IPV-EC device developments have been thoroughly
discussed along with PV-PEC devices in our paper because just by
choosing photo-active electrocatalytic electrode materials, one can
switch from one to another.
Without taking cost-effectiveness into account, PV-PEC/ IPV-EC
systems found off-grid applications in hydrogen fuel generation for
navigation, military, aerospace, etc.[84] Unlike separate PV-EC, the
integrated IPV-EC and PV-PEC system is still in the R&D stage only.
[154–159] Therefore analytic calculations to determine economic ad­
vantages[160] during commercialisation[99] is neither easy nor always
dependable.
3. Major historical development: Technology to materials
The solar water splitting technology is not so readily feasible for
large and secure application[167–170] but integrated devices for solar
water-splitting can incorporate key design features (w.r.t say, membrane
technology, device corrosion, etc) from the well-optimized commercial
electrolysers. In that vein, we must give due attention herein to the
chronological development of innovations in water electrolysis dating
from first report of H2 production by William Nicholson and Anthony
Carlisle in the 1800′ s to PV-EC/IPV-EC of the present times.[171,172]
Acidic PEM electrolysers arrived in 1960′ s and was marked by faster
response to power input than the alkaline electrolysers of 1900′ s
(operating in KOH electrolyte, generally) along with higher rate of H2
generation.[173–175] PEM has been incorporated in IPV-EC and even
PV-PEC using vapour phase reactants.[176,177] Lately cost of PEM has
been reduced by creating alkaline anion exchange type polymer mem­
branes (AEM) to get performance similar to acid-based systems with the
2. Major challenges to large scale integration of PV-PEC and IPVEC devices
The generation of solar fuels, in particular H2, from renewable
resource like water and being able to use only sunlight as input energy
for the photon-to-chemical energy conversion is an attractive goal for
decades.[130] But a viable route should be considered for converting the
solar energy on a scale which should be equivalent with global energy
demand. Commercial PV have a power conversion efficiency (PCE) of
11.5–17.5% and the overall STH efficiency of optimized PV-EC systems
is around 12 %.[79] In 2016, Jia et. al., developed a system combining
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Energy Conversion and Management 261 (2022) 115648
Fig. 7. (a)-(f) Schematic diagram of cell configurations for self-biased PEC water splitting. PV-PEC tandem cells: (a) partially integrated (wired); (b) fully integrated
(layered); (c) non-integrated (separated). PEC tandem cells: (d) wired (mode T); (e) layered (mode T); (f) wired (mode P). T = tandem irradiation and P = parallel
irradiation. Adapted from ACS Publications[105] (g) Contour plot showing the maximum predicted STH for AM 1.5G incident radiation (1000 W m− 2) depending
upon the semiconductor band gap energies, Eg,i, i = 1, 2 (front absorber Eg1 > back absorber Eg2) [130] Adapted from ACS Publications[130].
possible use of non-precious Ni based catalysts.[121] Solid Oxide Elec­
trolyser Cells have also come up. Commercial electrolysers have reached
80% efficiency these days.[178] The PV unit too evolved from wafers of
1960′ s to thin films[179,180] of 1990′ s as discussed in section 1.1.
The timeline of PEC cells begin with Fujishima and Honda’s break­
through PEC cell for ‘electrochemical photolysis of water’ using TiO2
photoanode in 1972.[181–183] Later, many other photoactive semi­
conductors were used such as Fe2O3,[184,185] BiVO4,[186,187] WO3,
[188–191] ZnO,[192–194] Cu2O,[195–199] and SrTiO3,[200] due to
their narrow/tuneable band-gap, low cost, simple morphology
controlled synthesis options, outstanding electrical properties, thermal
and chemical stabilities.[201].
Hydrogen production by PEC cell was demonstrated at laboratory
scale by using a variety of materials and reactor schemes to produce
sufficient solar-to-hydrogen (STH) conversion efficiency but their
durability remained a problem. Although PEC water splitting is often
termed as ‘‘artificial photo-synthesis’’;[202] in reality, PEC cells are yet
to outperform natural photosynthesis to become a practical commer­
cialized technology for hydrogen production. While STH conversion
efficiencies up to 18% were demonstrated in the laboratory using
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CuZnSnS4 (CZTS) [212–214] have been regarded as effective photo­
cathode materials, on account of excellent properties such as suitable
band structure, high absorption coefficient and tuneable bandgap (1.0
~ 2.4 eV). CZTS [117,215,216] overcame the scarcity of bulk raw ma­
terials (In and Ga) previously popular as photoreceptive components in
these devices. CZTS photocathode with In2S3/CdS coating attained a
photocurrent of 9 mA/cm2 at 0 V vs (reversible hydrogen electrode)
RHE,[214] and TiO2/CdS coatings resulted in 13 mA/cm2 photocurrent
at − 0.2 V vs RHE.[213].
As for efficient photoanodes, Xu et al (2018) presented a study on
WO3/BiVO4 heterojunction photoanodes with 1D-WO3 nanofiber cores
and BiVO4 film shells. It showed maximum photocurrent density of 2.8
mA cm− 2 at 1.23 V vs. RHE under the AM 1.5 simulated solar light
illumination, almost 20 times of that from pure 1D-WO3 counterparts
(0.15 mA cm− 2). The onset potential was reduced and maximum IPCE
was enhanced to ~ 4 times compared to 1D-WO3 (see Fig. 9).[49].
Unbiased PEC of both monolithic and separate component archi­
tecture with single/multiple photo absorbers have been thoroughly
researched but the current state of PEC development indicates that some
bias is needed for efficient performance of the cell and thus combination
with PV technologies is a way forward as dealt in greater detail in the
following sections and demonstrating heavy dependence on choice of
materials.
Amongst the semiconductors shown in Fig. 10, which are more useful
as photocathodes. Fig. 10 charts the band gap and band edge positions of
many popular semiconductor photocatalysts or cocatalysts including
oxides, nitrides, and chalcogenides which are used for hydrogen evo­
lution. Oxides are in general preferred and more widely used for their
easier synthesis routes and atmospheric stability and band gap is tuned
by doping and other techniques to meet requirements. The recombina­
tion losses and required bias can be greatly reduced if the band edges are
favourable w.r.t water splitting potentials. We can observe that all ma­
terials do not have both valence band below the water oxidation po­
tential although the conduction band is above the water reduction.
Hence even if overall water splitting is not possible without combination
with a complementary semiconductor in Z-scheme, all of these materials
are of interest as photocathode for PEC.
Fig. 11 shows the disparity between the number of reported studies
on PV integrated water splitting devices and those related to only PEC.
We hope this review paper may help more groups to come forward and
take up the challenge to optimise the combinations of PV and water
splitting units and scale up the devices thoroughly for potential com­
mercialisation, shoulder to shoulder with other H2 generation methods.
Table 1
comparison of PEC and PV-PEC device with PV-EC and IPV-EC with respect to
pros and cons of component integration in device.[133].
Device
PEC and PV-PEC device
PV-EC and IPV-EC device
Light
absorption
Pros: For PV-PEC, more solar
utilisation can occur as
photovoltage develops at both
PV and PEC stages.
Cons: hindered absorption of
water and scattering of light
due to bubbles formed on
photoelectrode.
Pros: device efficiency improve
due to low current densities
since the surface area for
catalysis and light gathering is
common.
Cons: recombination losses due
to longer path lengths needed
for ion-migration in general.
Pros: water-splitting elements
do not depend on light
absorption.
Cons: PV unit has to generate
the entire photovoltage
required for electrolysis.
Catalysis
Charge
transport
Device design
and
integration
Pros: easy (cheaper) to
integrate or scale-up due to
above mentioned reasons.
Cons: Limited material
selection scope due to same
material for both absorption
and catalysis and is constrained
by both band gap and
alignment with redox
potentials.
Net energy
balance
Positive impact more feasible
due to better integration
facilities.
Pros: Reduction in
electrocatalyst loading is
possible by concentrating
electric current.
Pros: low electrolyte
resistance.
Cons: losses through
transparent conductor,
connecting wires and collector
grid.
Pros: choice of material for
different components is not
inter-dependent and
unburdened with mandatory
optical properties. More
configurations are possible.
Cons: For PV-EC, need for
separate units hamper easy
scaling up.
For PV-EC, requirement of
distinct units add up to the
overall cost of manufacturing.
crystalline semiconductors as thin films or otherwise (with cost effec­
tiveness atleast in small scale), it must be targeted to > 30% with good
device stability to compete with fossil fuel derived H2 and even other
sustainable H2 technologies with zero carbon emissions or carbon
recycling (like renewable energy namely wind, hydro or solar powered
water electrolysis, biowaste derived liquid fuel reforming, solar or nu­
clear waste powered thermochemical water splitting).
However the challenge to realise fundamental science by engineer­
ing such architectures is difficult to achieve experimentally.[203,204]
Among different lab scale PEC devices, [205] compact “Cappuccino”
PEC cell[142] (closely situated electrodes minimize the ohmic losses) by
Swiss lab EPFL (2007) is most popular. Light reflectors and concentra­
tors are often useful for PEC devices, and it is advisable to increase the
ratio of electrode area to electrolyte volume to improve ionic transport.
The front and back light absorbers in Cappuccino cell (Fig. 8(a)) face
each other and only the front absorber is generally transparent. But this
configuration hampers anion/cation exchange without the use of a
special membrane to minimize the pH gradient. Another way to address
this issue has been demonstrated by Pihosh et al. (Fig. 8(b)). A tilted
photoanode gathers the light with a PV module (or a photocathode)
vertically placed in the cell which can absorb reflected light from the
mirror-backed photoanode. Here choice of front light absorber is not
stringent since its transparency is not required. [266] Another design
called ‘Porto cell’ (LEBAPE, Porto) reported in 2014 (Fig. 8(c)), is a 10
× 10 cm2 cell[151] suitable for continuous operation and easy gas
collection. Teflon diaphragm (porous and highly reflecting) has been
used to be able to comply with different aqueous electrolyte solutions.
The CoolPEC design [17] update (Fig. 8(d)) in 2018 is better for tandem
mode continuous operation.
Coming back to materials selection, in recent times, zinc-blende
related chalcogenides, such as CuInxGa1-xSe2 (CIGSe),[206–208]
CuGaSe2 (CGSe),[209,210] Cu2BaSnS4-xSex (CBTSSe)[211] and
4. III – V based tandem solar cells for PV-PEC/IPV-EC integration
In 1998, Khaselev and Turner constructed the first monolithic PVPEC device for hydrogen generation from water by using GaInP2/GaAs
tandem cell and obtained an impressive efficiency of 12.6% STH by
connecting p/n GaAs bottom cell to GaInP2 top cell through tunnel diode
interconnect.[43] Here the upper GaInP2 photocathode is somewhat
protected from photocorrosion because it is p-type.
In 2018, Kistler et al. presented an IPV-EC cell consisting of a
membrane-electrode-assembly that used a limited mass transport
regime (i.e., a solid electrolyte and water vapor feed) to increase the
durability of their device>4 times compared to subjection of PV to liquid
electrolyte. Their IPV-EC device as shown in Fig. 12(a)&(b) assisted by
commercial triple-junction III-V PV with InGaP/GaAs/Ge sub-cells
attained a 7.5% efficiency of STH whereas the PV-EC mode (with
externally wired PEM electrolyser) demonstrated 12% because of no
light shading. Stable and continuous operation over 100 hr have been
tested successfully. The setup is suitable for PV-PEC mode too if anode/
cathode material is chosen to be photoactive.[218].
Later in 2019, the same group invented a fully integrated IPV-EC
device using III-V triple-junction PV cell (of 22.5% efficiency)
embedded in Nafion proton exchange membrane. The catalyst loading
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Energy Conversion and Management 261 (2022) 115648
Fig. 8. Short overview on various popular PEC cells in use (a) Schematic representation of the Cappuccino cell by EPFL (b) Schematic representation of the PEC cell
with tilted photoanode configuration as used by Pihosh et al (c) Porto cell, a 10 × 10 cm2 cell shown during operation (left) and the innovative feeding system (right).
[205] Adapted from Elsevier[205] (d) CoolPEC (Vilanova et al.) Adapted from Elsevier[17].
(catalyst is integrated by the compression of metal sputter-coated car­
bon-electrodes on front and back of PV contacts) in wireless monolithic
architecture of MEA type helped to reduce the ion transport path lengths
to the extent that the PEC is operable in neutral-pH water with up to
12.6% efficiency, deteriorating to 7% in 4 days as illustrated in Fig. 12
(c)-(e).[219].
5. Organic/Inorganic hybrid solar cells for PV-PEC integration
PV-EC[81,220,221] and integrated PV-PEC or IPV-EC cells
[218,222,223] show high STH energy conversion efficiency. Many
significantly high-performance designs have come up using low-cost
thin film photovoltaics which may be even further improved by
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Fig. 9. (a) schematic of the FTO/1D-WO3/BiVO4 heterojunction photoanode, (b-e) PEC performance - (b) current–potential characteristics, (c) transient photo­
current densities vs. time, (d) EIS (Nyquist plots) and (e) IPCE spectra.[49] Adapted from Elsevier[49].
replacing water splitting components with photoactive metal oxide al­
ternatives. Using a bimetallic NiFe oxide electrocatalyst and PSC, Luo
et al. reported achievement of 12.3% of STH efficiency in 2014.[224]
12.7% of STH was also reported by using PSC and electrocatalytic cell
where anolyte and catholyte are separated by a bipolar membrane.
[223].
In 2015, Luo et al., designed a new multilayer CuInxGa1− xSe2
photocathode, which exhibits excellent performance and by pairing
with semi-transparent perovskite (CH3NH3PbBr3)-based PSC, the STH
efficiency is 6%. It is reportedly the highest value for PV-PEC devices
under 1 sun illumination employing a single-junction solar cell for bias.
With optimization of perovskite top absorber, the efficiency exceeds
20%.[225].
In 2016, Turan et al. designed a scalable device which is adaptable to
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Fig. 10. Positions of selected semiconductors along band-edge and band-gaps with respect to vacuum level and Normal Hydrogen Electrode (NHE) which indicates
the conduction-band edges (horizontal red lines), valence-band edges (horizontal green lines), water redox reaction potentials (two dashed lines).[217] Adapted from
Wiley[217].
6. Si heterojunction solar cell for PV-PEC integration – ‘Cool
PEC’
In 2017, Vilanova et al designed a new compact and optimised PVPEC tandem cell with 50 cm2 photo-active area named as Cool PEC
cell as shown earlier in Fig. 8(d) and it remained stable over 42 days
delivering a photocurrent density of 0.45 mA cm− 2 at 1.6 V supplied by
two Si heterojunction solar cells. The Si heterojunction PV is below the
PEC but enjoys an open light path and the photoelectrode (hematite on
FTO glass) also acts as a window for the cell.[17] The device has a
electrolyte flow path and shows good heat dissipation and efficiently
separated and collectable H2-O2 generation.[17].
Another stable device reported in 2019 by Fan et al. consisting of Si
based components deserves mention. They demonstrated a PV-PEC
tandem cells having 2 ordinary series connected Si-PV along with dual
Si-photoelectrodes (p+pn+-Si photoanode and n+np+-Si photocathode)
modified additionally with protective Ni layer and bifunctional Ni–Mo
catalyst. The self-biased PV-PEC tandem cells results in efficiency of
9.8% STH with stability over 100 h under parallel AM 1.5G 1 sunillumination in alkaline conditions.[228] Fig. 14 charts the compara­
tive performance of state-of-the-art Si based photoelectrodes.
Fig. 11. Number of Scopus indexed articles published between 2015 and 2021
which mention keywords related to PEC and PV-PEC /IPV-EC integration in the
article title.
multiple PV thin-film technologies and the configuration allowed inde­
pendent optimisation of PV and electrochemical components. They in­
tegrated a wireless device and obtained stable unassisted operation for
40 hr. The optimised IPV-EC was scaled up to a 64 cm2 device by easily
repeating the base units 13 times to give a STH efficiency of 3.4%. The
electrochemistry happened in nickel foam electrodes and it was envis­
aged that a wide scope for improvement is possible through appropriate
material choice.[226].
In 2021, Alfano et al., demonstrated a system comprising different
hybrid photocathodes with a PSC and a Ru-based oxygen evolution
catalyst which results in > 2% of STH efficiency, as represented in
Fig. 13.[227] The hybrid photocathodes consist of a P3HT:PCBM bulk
heterojunction where the electron-hole pair is photogenerated, with CuI
and TiO2 as hole and electron selective transporter respectively. Optical
transparency of photocathode is key to tandem performance, hence
thickness/coverage of metallic Pt for example is especially monitored.
They showed that photocathode-PSC tandems with optimised band gap
of top and bottom absorbers may help to achieve the milestone 10% STH
efficiency by using methylammonium lead iodide PSC and semi­
conducting polymers. This may even go up to 20% by using optimised
photocathodes.
7. Metal-oxide based semiconducting nanostructures for PV-PEC
application
It is essential to identify the right materials especially for the parts
which are prone to corrosion: the photoanode/photocathode, for
obtaining stability and cost effectiveness from PV-PEC devices. In that
respect, metal oxide semiconducting nanostructures[229,230] are
attractive for renewable energy conversion technologies because of their
in general stability combined with unique properties such as tunable
band gap,[231–234] excellent optical properties,[234–236] good elec­
trical conductivity,[237] swift reactivity for electronic transitions,
[238,239] high dielectric constant,[240–242] electrochromicity and
many other electrical properties.[243–245] Some of the prototypical
metal oxide-based PV-PEC are addressed in Table 2. There is also pos­
sibility of exploring various nano-architectures of metal oxides, for
example, 0D, 1D, 2D and especially 3D structures[246] (Fig. 15) for
example- nanoflowers,[247] vertical array of nanopillars,[248] nano­
cubes,[249] nanotubes,[250] nanoplates,[251] nanowires,[252] nano­
sheets,[253] nanoparticle decorated nanowires,[254] nanoprisms,[255]
hyperbranched core shell structures,[256] with chemical/physical
properties which are different from their bulk counterpart (due to
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Energy Conversion and Management 261 (2022) 115648
Fig. 12. (a)&(b) Vapor (liquid) phase
PEC test in two different configurations:
(a) photocathode-PV sitting in cathode
compartment; (b) photoanode-PV sitting
in anode compartment.[218] Adapted
from The Electrochemical Society[218]
(c)-(e) (c) A triple-junction photovoltaic
cell embedded into Nafion membrane
(PIM) as core of the device. (d) Cross
section of active components in mono­
lithic configuration (without wires). (e)
Diagram of monolithically-integrated
PEC device.[219] Adapted from The
Electrochemical Society[219].
quantum confinement) [257,258] namely, light absorption,[259]
charge exchange/collection[260] and surface reactivity.[261] Metal
oxides are widely applicable in electrochemical water-splitting, solar
cells,[262] photoelectrochemical cells and photocatalysis.[263] There
are many reports of metal oxides with 3D networks with enhanced
specific surface area, modulated interface, tunable bandgap, good
electron transport. In 2002, Shukla et al., developed a porous poly­
crystalline rutile TiO2 thin film by anodic oxidation of Ti. They reported
an open circuit voltage of 780 mV and short circuit current density 9.27
mAcm− 2 and the hydrogen evolution rate was about 37.4 and 24.6
Lh− 1m− 2 respectively for PV-PEC and IPV-EC.[264].
In 2011, Lee et al., developed one of the largest PV-PEC devices (130
cm2) based on tungsten oxide (WO3) photoanodes and TiO2 based DSSC.
The photoanodes are prepared by screen printing WO3 films with 130
cm2 active area on conducting FTO substrate with and without the
embedded inter-connected Ag grid lines and tested under 1 Sun
illumination in H2SO4 (0.5 M) electrolyte. The rate of hydrogen gener­
ation for photoanode (130.56 cm2) was 3 mL/min.[265].
Pihosh et al., developed a WO3/BiVO4 + CoPi core–shell nano­
structured photoanode in 2015, and obtained a photocurrent of 6.72
mA cm− 2 at 1.23 V vs RHE under 1 sun illumination corresponding to ~
90% of theoretical possibility and also demonstrated a self-biased
concept of photoanode with expensive double-junction GaAs/InGaAsP
PV cell in tandem to get photocurrent of 6.56 mA cm− 2 (8.1% STH ef­
ficiency).[266] The remarkable PEC cell configuration has been dis­
cussed earlier in section 3. The state of the art review on PV-PEC and
PEC tandem cells by Chen et al. in 2020 highlights metal oxide based
tandem cells giving ~ 8% of STH efficiency as viable for practical use.
[69].
Kornblum et al (2017) demonstrated 100% Faradaic efficiency and
IPCE>50% from an epitaxially grown thin SrTiO3 layer as photocathode
on III-V GaAs based PV. This performance resulted from the suitable
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Energy Conversion and Management 261 (2022) 115648
Fig. 13. (a & b) - Representation of PV-PEC (a) 3D sketch of arrangement of device components under illumination (anode is in dark) with their electrical
connection; (b) schematic depicting the materials, interconnection of device components (Ru anode and hybrid photocathode in electrolyte, powered by PSC),
conduction band minima and valence band maxima of each material with electron-hole separation and transfer pathway under UV–visible light passing through from
left to right; (c) Separately recorded Current-voltage profiles of three device components where the point intersection shows the expected operating parameters, with
dotted line following the STH efficiency; (d) tandem device under operation, dotted line is STH efficiency under zero applied bias.[227] Adapted from Cell Press,
Elsevier[227].
Fig. 14. (a) Graph comparing the maximum Applied Bias Photon-to-current conversion Efficiency (ABPE) of various state-of-the-art Si photocathodes with earthabundant catalysts. (b) Graph comparing the maximum ABPE of various state-of-the-art Si based photoanodes.[228] Adapted from Royal Society of Chemistry[228].
band alignment of the used material and good interfacial contact.[278].
In 2018, Cheng et al. tailored a monolithic PV-PEC device with
photovoltaic dual junction tandem heterojunctions as shown in Fig. 16
(a)-(b) using corrosion resistant, anti-reflective and band aligned crys­
talline TiO2 interfacial layer which supports high-activity Rh catalyst
nanoparticles well-distributed to greatly minimize dependent light ab­
sorption. It gave outstanding STH efficiencies, around 19.3% in acidic
and 18.5% in neutral electrolyte under simulated AM 1.5G irradiation,
demonstrating 85% of the theoretical limit of efficiency obtainable from
photoelectrochemical water splitting using the band gap combination of
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Table 2
Various components and performance parameters of some of the metal oxide-based PV-PEC prototypes.
Photoanode material
Photocathode material
PV type
Electrolyte
STH
(%)
Stability
Reference
WO3
WO3/W, Mo:BiVO4/FeOOH/
NiOOH
IrO2
Pt
Pt
DSSC
DSSC
1.0 M HClO4
0.5 M Na2SO4
3.1
5.7
9h
2h
[267]
[268]
PSC
2h
[269]
3.4
6.3
6.5
7.9
4.6
8.1
120 s
6h
10 h
100 h
10 h
60 min
[270]
[271]
[272]
[273]
[274]
[266]
W:BiVO4/ CoPi
Mo:BiVO4/ Ti:Fe2O3
Pt
Pt
PSC
PSC
PSC
PSC
PSC
InGaAsP/
GaAs
Si
Si
0.5 M Na2SO4 + 0.1 M KPi
(pH 5.0)
1.0 M NaOH (pH 13.6)
0.5 M KPi (pH 7)
1.0 M KBi (pH 9.5)
1.0 M KBi
1.0 M KBi (pH 9.1)
KPi (pH 7)
2.5
Fe2O3/SnOx/CoPi
Mo:BiVO4/FeOH/NiOOH
BiVO4/FeOOH/NiOOH
BaSnO3/FeOOH/NiOOH
BiVO4/Co (OH)2Pt
WO3/BiVO4 /CoPi
Cu2O/AZO/
TiO2/RuO2
Pt
Pt
Pt
Pt
Pt
Pt
0.1 M KPi (pH 7)
1.0 M KCi (pH 9.2)
5.2
7.7
N/A
8h
[275]
[276]
Mo:BiVO4/CoP
CoP/Ni
Si
0.5 M KPi (pH 7)
5.3
2h
[277]
Fig. 15. Different 0 D, 1 D, 2 D structures as building blocks for designing 3D assemblies by facile techniques in the case of metal oxides due to their
inherent stability.
the particular system.[153].
In 2019, Ahmet et al. demonstrated a new prototype for large-area
PV-PEC device with a 50 cm2 stand-alone made up of cobalt
phosphate-coated W doped BiVO4 (CoPi/W:BiVO4) photoanodes. These
are integrated into tandem Si heterojunction PV-PEC devices with single
and dual photoanode configuration producing 1.9% and 2.1% STH ef­
ficiency respectively. But the optimised lab scale PV-PEC device of 0.24
cm2 showed STH efficiency up to 5.5%.[279].
Later, in 2020, A. Villanova et al developed the second largest PVPEC device (200 cm2), ever reported till-date having four Si hetero­
junction PV backed ‘CoolPEC’ cells of 50 cm2 with modular array (using
Ti doped Fe2O3 photoanode) as shown in Fig. 16(c)-(d) for continuous
operation under concentrated sunlight. The photoelectrodes were
fabricated by spray pyrolysis which is a well-known reproducible tech­
nique to get compact films ensuring full coverage of substrate and
thereby long term stability of electrodes. The module could generate a
stable current density of 2.0 mAcm− 2 at 1.45 V. The hydrogen produc­
tion rate is 5.6 × 10 -5g h -1cm2. This module of PV-PEC device paved the
way for large-scale PEC H2 production.[280].
8. Cu-oxide as an emerging photocathode material for PV-PEC
Modern challenge is to identify the marketable innovations for solar
energy harvesting and storage and the biggest hurdle in emerging as a
competitive technology is to reduce the cost of scalability. The
achievement of high STH efficiency and device lifetime at reasonable
cost ultimately depend on perfecting the materials.[281] In this section
we focus on copper oxide based hybrid photoelectrodes because of their
earth abundance with easily tunable properties and popularity for
having multifunctional use in sensing, catalysis and energy conversion
and storage.[282] Articles on PEC cells repeatedly report studies on ntype photoanodes like Fe2O3,[283] BiVO4,[284] WO3,[285] ZnO,[286]
TiO2[287] vs Pt cathode (see Table 2). However, p-type photocathodes
for H2 generation are rarely reported and hence more research efforts
are needed.[288,289] Among different types of photocathodes, earth
abundant and low cost cuprous oxide (Cu2O) and cupric oxide (CuO) are
excellent p-type semiconductor photocathode which can increase the
photoconversion efficiency. CuO having an indirect band gap (1.3–1.5
eV)[231] is more usable than Cu2O (~2 eV).[198,290–292] They are
electrochemically stable[293] under illumination and in contact with
aqueous electrolyte[294] with superior light absorption in visible light
spectrum.[295] Moreover, CuO conduction band is favourably located
16
P. Chatterjee et al.
Energy Conversion and Management 261 (2022) 115648
Fig. 16. (a) schematic diagram of de­
vice and (b) Enlarged view after design
modifications with interfacial films and
electrocatalysts.[153] Adapted from
American Chemical Society[153] (c)
Graphical representation of 200 cm2 PVPEC device (top), (d) experimental pro­
cedure used to prepare α-Fe2O3 photoe­
lectrodes for module (bottom) SP: Spray
pyrolysis, PE: Photoelectrode. 50
cm2 bare α-Fe2O3 PEs were prepared by
SP and 2.4 × 2.3 cm2 Ti-doped
α-Fe2O3 PEs (used in four multi-PE win­
dows) were prepared by SP/hydrother­
mal method.[280] Adapted from
Elsevier[280].
photocurrent of 14.7 mA cm− 2 with STH conversion efficiencies upto
18% under one sun illumination[108] has been predicted making it an
attractive earth abundant metal oxide photocatalyst for solar-driven
water splitting into H2 generation.[195,291,308,309] Figure-17(a)
shows energy band diagram of Cu-based metal oxides alongside other
semiconductors. Cu-based metal oxide (such as, Cu2O (2.0 eV), CuFeO2
(1.55 eV) and CuBi2O4 (1.5–1.8 eV))[282] photocathodes are interesting
candidates due to their unique properties for water splitting. Stability of
Cu based photocathodes can also be tuned by pH of buffer solution to
avoid the favourable pathway for self-reduction under illumination
while in electrolyte. Especially, STH efficiency of Cu2O was increased
from ~ 0.78% in 2011 to ~ 5.51% in 2019[310,311] as one can see in
Fig. 17(b) and the detailed information of device structures and pa­
rameters are shown in Table 3.
Generally metal oxides show wide variation in the bandgap and band
edge positions, which furnish more opportunities to construct effective
tandem cells [69] and likewise the band edge position of the Cu-based
metal oxides alongside other semiconductor candidates (e.g. chalco­
genides) are easily tunable for hydrogen production through solar water
splitting.[282].
Binary oxides of copper (CuO and Cu2O) have suitable band gaps that
can utilize the sunlight effectively. Whereas ternary oxides of copper are
having more possibilities when compared to Cu-based binary oxides,
where one can tune the band structure and also the optoelectronic
properties of the materials. Among different Cu-based ternary oxides
CuBi2O4 and CuFeO2 are promising materials due to their favourable
band edges for unbiased solar water splitting. Photocathode material
CuBi2O4 is effective in utilizing visible light and producing a high
photovoltage[327] whereas, CuFeO2 is made of earth-abundant ele­
ments and have positive onset potential.[328–330].
In 2014, Tilley et al., used electrodeposited Cu2O photocathode with
TiO2 as protective overlayer (to stabilize the Cu2O in water) and RuO2 as
co-catalyst to demonstrate a greatly enhanced stability versus Pt nano­
particles (Fig. 18(a)) with 94% current even after 8 hr of chopped-light
chronoamperometry. STH conversion efficiency is > 6% for the PEC
tandem cell if DSSC type PV device provides the bias. The measured
faradaic efficiency of H2 production is ~ 100% as clearly shown in
Fig. 18(b).[312].
more negative to the water reduction potential, with the conduction
band minima around − 0.8 to − 1.0 eV vs. RHE.[296] In 1982, Koffyberg
and Benko[231] characterized p-type CuO for photo-electrochemistry
with respect to its band alignment and on aiming for further improve­
ment,[297] CuO thin films showed better PEC performance with 2–3
times increased transmittance and higher electrical conductivity. Later,
Lim et al.,[298] confirmed that CuO thin films have improved perfor­
mance, and modified it to demonstrate better light absorption over the
whole solar spectrum in spite of indirect band gap. According to theo­
retical study, CuO generates around 35 mAcm− 2 of photocurrent
[299,300] and maximum power conversion efficiency of solar cell is ~
31%. Even though, CuO is a promising photocathode, its low carrier
mobility, high bulk resistance and poor photo corrosion stability be­
comes critical during application as photocathode in PEC water split­
ting.[301] Developing a good quality of CuO thin film is extremely
important for electronic and PV application. Fabrication of CuO based
photocathode should be simple as well as cost-effective for large area
application. In-situ deposition/doping with metal/metal oxide can
enhance the stability which is effective for H2 evolution. Furthermore,
crystallinity, morphology, surface active site, optical and electrical
properties can be tuned through synthesis process. For example, Cots
et al.[302] worked on CuO photocathode to improve faradaic efficiency
of H2 evolution from ~45% to 100% by incorporating ternary copper
iron oxide to CuO. Thus, interface engineering, band alignment, and also
carrier transport strongly affects the PEC performance of Cu-based
heterojunction photocathodes.
Nevertheless, cuprous oxide (Cu2O) also served as a promising
candidate as their conduction band is suitably positioned to split water
to generate hydrogen[130] and CuO was widely used as protection layer
for Cu2O cathodes, thus facilitating the Cu2O/CuO heterojunction ar­
chitecture.[303,304] The Hall mobility can be as high as 100
cm2 V− 1 s− 1 and the minority carrier diffusion length ranges from 1 to
10 μm in case of Cu2O assuming Hall mobility > 50 cm2 V− 1 s− 1. Very
efficient stabilization of Cu2O with TiO2,[305] ZnO/rGO (reduced
Graphene Oxide),[306] by atomic layer deposition or RF sputtering was
also reported. Cu2O has band gap which is accessible to visible light and
still wide enough to act as top cell in tandem configuration and be in­
tegrated in PV-PEC cell to split water.[269,307] A theoretical
17
P. Chatterjee et al.
Energy Conversion and Management 261 (2022) 115648
Fig. 17. (a) Energy diagram of some Cu-based metal oxides alongside other semiconductors. The dotted lines stand for redox potentials of water splitting, (b)
Photocurrent vs. onset potential for Cu based photocathodes. Maximum STH conversion efficiency and tandem devices is represented by coloured disk. Radius of disk
is proportional to STH conversion efficiency values.[282] Adapted and modified from Royal society of Chemistry[282].
In 2014, Azevedo at el. designed a simple low-cost solution to
improve the aqueous stability of Cu2O photocathodes enormously and
independent of the co-catalyst used as shown in Fig. 18(c)-(d). It
required only steam treatment of the multilayers in a autoclave between
100 and 150 0C for 1–3 hr. Cu2O/AZO/TiO2 photocathodes with RuOx
and Pt as co-catalysts shown photocurrent over 5 mA cm2 with 90%
stability for>50hr of chopped-light (biased at 0 VRHE in pH 5 electro­
lyte).[331].
In 2016, Qi et al. used layered double hydroxides (LDHs)[332] as cocatalysts for PEC systems as shown in Fig. 19. Cu2O/NiFe-LDH elec­
trodes increased the photocurrent intensity seven-fold under a low
applied voltage like − 0.2 V vs Ag/AgCl with good photostability. There
is no photocurrent loss even after 40 hr of operation and 8 hr of H2
generation showed 78% faradaic efficiency. This proved Cu2O/NiFeLDH as an alternative photocathode material for H2 generation.[333]
Also, Ni3FeN nanoparticles from NiFe LDH exhibits an excellent cata­
lytic performance and a high stability in overall electrochemical water
splitting, and hence another promising co-catalyst.[334].
In 2017, Niu et al. reported a thermally oxidised Cu2O/Ga2O3/TiO2/
RuOx photocathode, which gave rise to a photocurrent of 6 mA cm− 2 at
0 V versus RHE and 3.5 mA cm− 2 at 0.5 V versus RHE where onset
potential is 0.9 V versus RHE due to proper band alignment with Ga2O3
for maximizing photovoltage. The complementary quantum efficiency
curves of thermally oxidised and electrodeposited Cu2O gave the idea to
make a dual photocathode to enlarge the light absorption range, and it
generated photocurrent of 7 mA cm− 2 at 0 V versus RHE, with an un­
changed onset potential of 0.9 V versus RHE.[318].
Pan et al. (2018) developed a coaxial radial heterojunction of pCu2O/n-Ga2O3 photocathode to achieve light harvesting across visible
region up to 600 nm with better charge transport and more light
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Energy Conversion and Management 261 (2022) 115648
Table 3
Selected Cu-based metal oxide photocathodes along with its parameters in PEC cells.
Device
structure
Photocurrent (mA/
cm2); applied bias
Onset potential
(V vs. RHE)
Stability (J/J0);
time; applied bias (V
vs. RHE)
Maximum STH or
IPCE (%)
Faradaic
efficiency
Electrolyte; light sourceAM
1.5G
(100 mW/cm2)
Ref
FTO/Au/Cu2O/ AZO/TiO2/
Pt
FTO/Au/Cu2O/ AZO/TiO2/
Pt
− 7.6; 0 V vs. RHE
0.35
~33%; ~20 min; 0
~0.78
~100%
[310]
− 6.0; 0 V vs. RHE
0.55
–
~1.5
~100%
FTO/Au/Cu2O/ AZO/TiO2/
RuOx
− 5.0; 0 V vs. RHE
0.5
~100%; 4 h; 0
~1.1
~100%
FTO/Au/Cu2O/ AZO/TiO2/
RuOx
FTO/Cu/Cu2O nanowires/
AZ O/TiO2/RuOx
FTO/CuO:NiO /Cu2O/AZO/
Ti O2/RuOx
FTO/Au/Cu2O/ SnO2/RuOx
5.5; 0 V vs. RHE
0.5
–
~100%
− 8.0; 0 V vs. RHE
0.48
75%; 55 h; 0
2.5 for Cu2O PSC
tandem device
0.88
~100%
− 5.2; 0 V vs. RHE
~0.55
~100%; 5 h; 0
~1.1
~100%
− 4; 0 V vs. RHE
~0.35
90%; 57 h; 0
~0.4
~100%
FTO/Au/Cu2O/ ZnS/TiO2/Pt
− 2; 0 V vs. RHE
0.72
–
~0.6
~100%
Cu/Cu 2O/ Ga2 O3/TiO2/Pt
− 3; 0 V vs. RHE
− 1.02
~60%; 2 h; 0
0.78
–
Dual Cu2O/Ga2 O3 / TiO 2/
RuOx photocathode
~7; 0 V vs. RHE
0.9
~100%; 2 h; 0
1.9
~100%
FTO/Au/Cu2O nanowires/
Ga2O3/TiO2/RuOx
FTO/Au/CuSCN/Cu2O/
Ga2O3 / TiO2/RuO x
p-n Si microwires/Cu2O/
Ga2O3/TiO2 /RuOx
tandem structure
FTO/Au/CuO/ CdS/TiO 2/Pt
~10; 0 V vs. RHE
~1
~100%; 100 h; 0.5
~100%
− 6.4; 0 V vs. RHE
~1
~94%; 60 h; 0.5
− 10; 0 V vs. RHE
1.35
100%; 200 h; 1
~3 for Cu2O BiVO4
tandem device
4.55 for Cu2O PSC
tandem device
5.51
–
1 M Na2SO4-0.1 M potassium
phosphate (pH = 4.9)
0.5 M Na2SO4- 0.1 M
potassium phosphate (pH =
5)
0.5 M Na2SO4- 0.1 M
potassium phosphate (pH =
5)
0.5 M Na2SO4– 0.1 M
KH2PO4 solution (pH = 5)
0.5 M Na2SO4– 0.1 M
KH2PO4 solution (pH = 5)
0.5 M Na2SO4– 0.1 M
KH2PO4 solution (pH = 5)
0.5 M Na2SO4- 0.1 M
KH2PO4 solution (pH = 5)
KH2PO4 buffer solution (pH
= 7)
0.5 M Na2SO4 – 0.1 M
KH2PO4 solution (pH = 4.3)
0.5 M Na2SO4 − 0.1 M
KH2PO4 − 2 M KOH (pH =
5.1)
0.5 M Na2SO4, 0.1 M sodium
phosphate (pH = 5)
0.5 M Na2SO4, 0.1 M sodium
phosphate (pH = 5)
0.5 M Na2SO4 and 0.1 M
phosphate solution (pH = 5)
− 1.68; 0 V vs. RHE
0.45
100%; 30 min; 0
~0.24
~100%
− 5.3; 0 V vs. RHE
0.53
87%; 5 h; 0
~0.8
98%
− 1; 0 V vs. RHE
~0.6
~60%; 3 h; 0
~0.13
~91%
FTO/CuBi2O4/ CdS/TiO2/
RuOx behind BiVO4
− 0.2; 0.4 V vs. RHE
~0.73
~80%; 1 h; 0
IPCE: ~6% @ 450
nm, 0.6 V vs. RHE
~100%
FTO/ITO CuFeO2/ C60/CoFe
LDH
− 4.86; 0 V vs. RHE
0.65
~60%; 1 h; 0.25
85–100%
FTO/CuFeO2/ CdS/ TiO2/ Pt
− 0.4; 0 V vs. RHE
0.40
FTO/O intercalated CuFeO2/
RGO/ NiFe LDH
− 2.4; 0.4 V vs. RHE
~0.8
~100%; 50 min;
− 0.2
~100%; 20 min; 0.4
IPCE: 17.5% @
600 nm, 0 V vs.
RHE
–
FTO/CuO/AZ O/TiO 2/AuPd
FTO/CuBi2O4/ CdS/TiO2/Pt
gathering surface in the presence of nanowires to get an external
quantum yield of 80% for H2 generation with onset photocurrent of + 1
V vs RHE and photocurrent density is ~ 10 mA cm− 2 at 0 V vs RHE.
Additionally, TiO2 as protective coating allowed the stability to exceed
100hr and NiMo as hydrogen evolution catalyst also helped the stability
of Cu2O in weak alkaline electrolyte. Finally by incorporating a state-ofthe-art 1% Mo doped BiVO4 photoanode as we can see in Fig. 20, the
unbiased all-oxide PEC tandem cell made of earth abundant elements
achieved ~ 3% STH conversion efficiency.[305].
In 2019, Septina et al[321] prepared CuO thin films via oxidation of
electroplated Cu but photocorrosion in 1 M phosphate buffer solution
(pH 7) reduced faradaic efficiency for H2 evolution to ~0.01%. By
depositing n-type CdS buffer layer under a protective TiO2 layer, onset
potential of ca. 0.45 V vs RHE and photocurrent of 1.68 mA cm− 2 at 0 V
RHE was achieved. As shown in Fig. 21, CuO/CdS enhances photo­
voltage and TiO2 layer on sulfide surface gives high stability of
hydrogen-producing photocurrents with Faradaic efficiency ~100%.
[321].
In 2019, Yoon et al. tailored a PV-PEC cell on conductive oxide
substrate by integrating p-n Cu2O thin films and n-ZnO nanorods for
IPCE: 7.5% @ 600
nm, 0.4 V vs. RHE
~100%
~100%
94%
[312]
[312]
[269]
[313]
[314]
[315]
[316]
[317]
[318]
[305]
[319]
[320]
1 M phosphate buffer (pH =
7)
0.1 M Na2SO4 (pH = 5.84)
[321]
Ar-purged 0.3 M K2SO4 and
0.2 M phosphate buffer (pH
= 6.65)
Ar-purged 0.3 M K2SO4 and
0.2 M phosphate buffer (pH
= 6.8)
Ar-purged 1 M NaOH (pH =
13.5)
[323]
Ar-purged 0.5 M Na2SO4
(pH = 6.1)
Ar-purged 1 M NaOH
[322]
[324]
[325]
[326]
[291]
water splitting. The built in electric field helped to enhance performance
and photocurrent onset appeared at − 0.330 V for ZnO/Au/p-n Cu2O
(which is more cathodic than ZnO/Au/n-Cu2O by 0.167 V) and its
photocurrent value at 0.2 V vs SCE is 0.206 mA/cm2 (237% increase
upon ZnO/Au/n-Cu2O) as shown in Fig. 22.[132].
In 2019, Kunturu et al. demonstrated stable water splitting for 75 hr
by PV-PEC tandem device made of micropillar array Si/Cu2O hetero­
structure with Pt catalyst and pulsed laser deposited homogeneous ZnO/
TiO2 layer as hole transporter cum anti-corrosion passivating overlayers.
They obtained 0.85 V vs RHE onset potential and a photocurrent of 7.5
mA cm− 2 at 0 V vs RHE.[335] It is also important to replace precious,
opaque and poorly electron blocking gold (Au) as back contact for Cu2O
photocathodes. In 2017, Son et al showed that Au coated back contact
can be replaced by transparent low-cost thin films of NiO/CuO.[314]
Later, as shown in Fig. 23, Pan et al. (2020) devised PSC based PV-PEC
where they replaced Au with solution-processed CuSCN to improve
electron hole separation in Cu2O photocathode. Hole transport between
Cu2O and CuSCN was aided by band-tail states and the PSC PV-PEC
achieved a solar-to-hydrogen (STH) efficiency of 4.55%.[336].
Chemical and physical properties of thin films depend on
19
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Energy Conversion and Management 261 (2022) 115648
Fig. 18. (a) Schematic of the photocathode (b) Photocurrent densities and Faradaic efficiency of photocathode biased at 0 V vs RHE in standard (pH 5) electrolyte.
[312] Adapted from Wiley[312] (c) Coloured FESEM image of photocathode (d) Stability variation with steam treatment, under continuous illumination mea­
surement, quantified as time taken for initial photocurrent to drop to 90% about its initial value.[331] Adapted from Royal Society of Chemistry[331].
Fig. 19. Schematic diagram of photogenerated electron transfer which occurs in PEC system by (a) bare Cu2O and (b) modified Cu2O/NiFe-LDH electrodes. Red dots
denote electrons.[333] Adapted from Nature[333].
20
P. Chatterjee et al.
Energy Conversion and Management 261 (2022) 115648
Fig. 20. (a) The tandem unbiased PEC device consisting of Cu2O as photocathode and Mo-doped BiVO4 as photoanode (b) Current-voltage curves obtained under
simulated 1.5 AM G illumination for Cu2O photocathode and BiVO4 photoanode individually and Cu2O photocathode behind BiVO4 photoanode in 0.2 M potassium
borate (pH 9.0). The crossing point is ~ 2.4 mA cm− 2 with STH efficiency is ~ 3% (c) wavelength-dependent IPCE spectra (d) stability test of unbiased system in 0.2
M potassium borate (pH 9.0), inset: corresponding quantification of gas evolved with time from photocathode and photoanode.[305] Adapted from Nature[305].
Fig. 21. (a) Schematic of photocathode with the role of each layers (CuO/CdS/TiO2/Pt) (b) Current density vs potential curves measured with and without Ptcatalyst in a 1 M phosphate buffer solution (pH 7) under chopped 1 sun illumination.[321] Adapted from American Chemical Society[321].
21
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Energy Conversion and Management 261 (2022) 115648
Fig. 22. (a) Band diagram of ZnO/Au/n-Cu2O and (b) ZnO/Au/p-n Cu2O showing efficient electron-hole separation (c) photocurrent density vs potential under
illumination (pink and green) and dashed line indicates dark current (d) Transient photo response of ZnO/Au/p-n Cu2O &ZnO/Au/n-Cu2O.[132] Adapted from
Elsevier[132].
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Energy Conversion and Management 261 (2022) 115648
Fig. 23. (a) Schematic representation of CuSCN incorporated Cu2O photocathode in layered configuration. (b) Current density vs potential curves of Cu2O pho­
tocathodes (c) Enlarged valence band spectra with shadowed area depicting the band-tail states. (d) Energy band diagram showing the hole transport between Cu2O
and CuSCN assisted by band-tail. (e) graphic of PV-PEC tandem configuration for stand-alone solar water splitting based on CuSCN-incorporated Cu2O photocathode
with PSC and IrOx anode. (f) Unbiased chronoamperometry of assembled PV-PEC under simulated one-sun illumination in pH 5 buffered electrolyte.[336] Adapted
from Nature[336].
crystallinity, thickness and morphology which can significantly change
with thermal treatment[337] thereby modifying optical and electrical
properties of the thin film.[338] Deposition rate and pH needs to be
precisely controlled during thin film growth. Sultana et al. reported pCuO thin film with 60 – 178 nm thickness grown via chemical bath
deposition process on Si. Film with thickness of 110 nm showed best
performance in terms of refractive index, crystal quality, band-gap and
dielectric constant. An interesting study developed a film of CuO
nanoleaves with light trapping engineering and light absorption in
visible-NIR region which demonstrated lower bandgap than nanosheets
film. The nanoleaves based electrode generated 1.5 mA/cm2 photocur­
rent whereas that from nanosheets is 1.1 mA/cm2 at potential 0 V v/s
RHE with photocurrent conversion efficiency of 1.8% and 1.4%
respectively.[339] Some of the parameters of selected stable photo­
cathodes which exhibit high photocurrent density along with their
synthesis processes are listed in Table 4.
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Energy Conversion and Management 261 (2022) 115648
Table 4
Some stable CuO based photocathodes fabricated by scalable techniques.
Sl.No
Photocathode
Synthesis Process
Photocurrent Density
− 2
1
2
Nanostructured CuO with Cu foil
Tree branch-shaped CuO
Chemical bath deposition
Hybrid microwave annealing
3
CuO nanoparticles
Electrodeposition with annealing
4
5
Microwave deposition
Electrodeposition
6
7
8
9
10
C-doped CuO/g-C3N4
FTO/CuO/
NiOx
Cu2O/CuO bilayered
CuO nanoparticles
Cu2O/CuO composite
Semi-transparent CuO film
CuO/CuBi2O4
Electrodeposition
Flame spray pyrolysis
Electrodeposition
Reactive-sputtering
Electrodeposition
11
FTO/CuO
In-situ deposition through rapid microwave
12
CuO thin film
spray pyrolysis
− 1.3 mA cm at 0 V versus RHE
− 4.4 mA cm− 2
at 0 V versus RHE
− 0.55 mA cm− 2
at 0.5 V versus RHE
− 2.85 mA cm− 2 at 0 V vs. RHE
− 1.02 mA cm− 2
at 0 V vs. RHE
3.15 mA cm− 2 at 0.40 V vs. RHE
1.20 mA cm− 2
− 1.54 mA cm− 2 at 0 V vs. RHE
6.4 mA cm− 2
− 0.9 mA cm− 2 at 0.1 V vs
RHE
− 1.15 mA cm− 2 at 0
V vs. RHE
24 mA cm− 2 at 0.25 V vs. RHE
PEC Efficiency
Ref
0.276%
–
[366]
[367]
–
[368]
3.13 µmol H2 h-1cm
–
–
1.48%
–
–
~0.19 %
–
21.5%
− 2
[369]
[370]
[371]
[372]
[289]
[373]
[374]
[375]
[376]
Fig. 24. (a) I-V plots of PEC measurement of different CuO based photocathodes growth at varying sputtering power, (b) I-V plots of PEC measurement for different
film thickness;[342] Adapted from American Chemical Society. [342] (c) PEC current–voltage measurements of CuO photocathode annealed at different temper­
atures; (d) PEC current–voltage measurements for sputter grown CuO photocathode with different thickness.[343] Adapted from Royal Society of Chemistry[343].
24
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Energy Conversion and Management 261 (2022) 115648
Fig. 25. (a) Schematic figure, (b) TEM
image of combined CuO film, (c) Ab­
sorption spectra of CuO:N (~40 nm),
Cu2O:N (~25 nm), and Cu2O:N/CuO:N/
CuO:Pd/CuO thin films; (d) Schematic
figure, (e) TEM image of combined CuO
based solar cell with Ti incorporated for
passivation and n-silicon being the sub­
strate (f) Comparison of current–voltage
curves for combined CuO based solar
cells with and without Ti passivation
layer. Ti passivation improves the Voc
and fill factor of CuO solar cell.[107]
Adapted from Elsevier[107].
Fig. 26. (a) H2 gas evolution of CuO photocathodes by varying O/Cu composition, (b) H2 evolution from the best performing O-rich CuO photocathode with and
without Au − Pd coating.[349] Adapted from American Chemical Society[349].
Magnetron sputtering is one of the most favourable method for good
reproducibility of film properties, long-term stability, simplicity of
deposition process, and ease to scale up from laboratory size for a largescale industrial application.[104,340,341] It was observed that the CuO
thin film photocathode deposited by varying sputtering power and rapid
thermal annealing treatment exhibited a highly crystalline film with
enhanced stability and photocurrent for PEC water splitting. MasudyPanah et al.[342] developed a sputter grown CuO film (150 nm) on
FTO substrate which exhibits a photocurrent about ~0.92 mAcm− 2 (0 V
vs RHE) and increased upto 2.5 mAcm− 2, and photocurrent conversion
efficiency about 3.1%, with thickness, as represented in Fig. 24 (a)-(b).
[342] By sputtering CuO at high power, crystallinity improved signifi­
cantly to enhance the charge transport property and photocurrent
generation capabilities. Masudy-Panah et al. designed a stable and
efficient CuO photocathode by tuning crystallinity, optical absorption,
and surface morphology. Film annealed at 550˚C showed highest PEC
performance for the CuO photocathodes with thickness of 550 nm, with
photocurrent of 1.68 mA cm2 and better stability against photocorrosion, as shown in Fig. 24 (c)-(d).[343].
It is worth noting that CuO is also an excellent candidate for
photovoltaic devices. Masudy-Panah et al.[107] demonstrated CuO
based solar cells with very high photocurrent (30 mA/cm2) and PCE of
~ 8.5%. In this work, CuO thin film was grown via sputtering by
incorporating Pd nanoparticles CuO (CuO:Pd) and nitrogen to enhance
the optical absorption and charge transport properties, as shown in
Fig. 25.
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Fig. 27. (a) TEM image of p-(CuO/CuO:Al)/n-ZnO:Al/TiO2 photocathode (b) Photocurrent stability of p-(CuO/CuO:Al)/n-ZnO:Al/TiO2/Au–Pd photocathodes. (c)
Comparison of photocorrosion stability of p-(CuO/CuO:Al), p-(CuO/CuO:Al)/n-ZnO:Al, and p-(CuO/CuO:Al)/n ZnO:Al/TiO2/Au–Pd photocathodes, (d) IPCE spectra
of photocathodes (e) Linear voltammetry showing record-high Jsc of ~ 5.4 mA cm− 2 at 0 V vs RHE, and (f) Hydrogen evolution of photocathodes.[322] Adapted from
Royal Society of Chemistry[322].
PEC water splitting measurements show that the stability of CuO
electrode is significantly influenced by the amount of Cu or O elements
in film. O-rich CuO electrode shows improved photocurrent and stability
against photocorrosion. Furthermore, incorporation of Au − Pd nano­
structures may enable the harvesting of a wider portion of the solar
spectrum.[344,345] Light trapping and consequently enhanced photo­
generation of electron-hole pairs due to interaction of plasmonic nano­
structures with semiconductor results in improvement of energy
conversion efficiency.[346–348] Stable CuO photocathode with Au-Pd
integrated demonstrated photocurrent as high as ~4 mA/cm2 at 0 V
vs RHE. The hydrogen evolution increases for O-rich CuO photocathode
(Fig. 26(a)-(b)).[349].
Not just Cu2O, photocorrosion is an issue for CuO also for long
duration contact with electrolyte and thus protective layers become
necessary.[350–359] Masudy-Panah et al.[322] fabricated an aluminum
(Al) - incorporated p-CuO/n-ZnO photocathode with TiO2 as a protec­
tive layer for PEC water splitting to generate H2 as shown in Fig. 27. TiO2
stabilizes the photocathode and improves the PEC activity. The highest
photocurrent density (~5.4 mA cm− 2) and a photocorrosion stability
(~87%) was observed after 5 hr.
CuO can be easily processed to make thin films and since it has high
optical absorption coefficient, the photocathodes can be highly efficient
[289,310,342,343,360–363]. But, very low cost techniques such as, thin
film sol–gel deposition can result in low charge transfer rate, high bulk
resistance and high recombination rate of the photogenerated carriers.
[364] Fortunately, charge transfer property can significantly improve by
incorporating a carbon nanostructure (such as., graphene) into the metal
oxide thin films. As shown in Fig. 28, Dalapati et al. developed a stable
and efficient photocathode by introducing graphene into CuO film (CuO:
G) via sol–gel process. Functionalized graphene reduces the conversion
of Cu2+ to Cu+ phase during photoelectrochemical reaction due to
effective charge transfer which leads to more stable photocathode. In­
tegrated CuO:G with TiO2 protecting layer and Au–Pd nanostructured
co-catalysts resulted in efficient and stable photocathode for solar H2
generation.[365].
Materials selection, synthesis process, devices integration and STH
performance are critical for the development of integrated PV/PEC
infrastructure. Towards this, copper oxides are excellent candidates, as
they are suitable for both PV and PEC devices. The present article pro­
vides information about the materials selection and design principle for
the PV/PEC integration. Fig. 29 summarises copper oxides-based PV
devices performance over the years and proposed PV/PEC integrated
structure to generate green hydrogen.
9. Towards commercial PV-PEC through cost effective CPV and
PVT technology:
Use of concentrators (for e.g., lens or curved/flat mirrors made of
recyclable low-cost materials like steel, aluminium, and plastic) with the
PV panels (concentrated PV or CPV) to increase the light absorption and
thereby the efficiency (up to 40%) of the PV is a promising approach to
commercialization of integrated PV-EC or PV-PEC. Generally, mass
producible PMMA based Fresnel lens or parabolic mirrors, coupled to
sunlight tracking devices are used as concentrators. It is also worth
noting that the quantum dot-based concentrators can work with diffuse
light over a broad and tuneable range of wavelength.[377] Fujii et al.
[378] demonstrated the concept of CPV-EC (Fig. 30) for water splitting
which could achieve a comparatively high STH efficiency of over 12%.
[379,380] Proper cooling mechanism and polycrystalline Si solar cell
can also be used.
Concentrators may result in device overheating to even ~ 400 ◦ C at
times and thus only useful with proper radiative and convective cooling
arrangements [381]. Some of the popular cooling methods are natural or
forced air/water/nanofluids circulation with/without the use of phase
change materials (PCM).[382] Heat pipes work to remove the heat from
site with condensation elsewhere. Although costly, nanofluids (based on
ZnO, SiC SiO2, MWCNT, TiO2, Al2O3 nanoparticles), have high thermal
conductivity, help increase Brownian motion and thermophoresis to
26
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Energy Conversion and Management 261 (2022) 115648
Fig. 28. (a) Absorbance spectra of CuO and CuO:G films, (b) I-V plot of CuO and CuO:G photocathodes, (c) Transient photo-response and stability of CuO:G
compared to CuO photocathode, (d) & (e) Hydrogen evolution from CuO and (CuO:G) photocathodes with TiO2 passivation layer and TiO2-Au-Pd nanostructure
under illumination.[365] Adapted from Wiley[365].
Fig. 29. Copper oxides-based PV device efficiency till-to-date and its proposed integration in PV/PEC architecture for green hydrogen production.
handle high heat flux.[383,384] PCMs (like paraffin wax, etc) are
wonderful passive cooling agents as they have large latent heat capacity
enabling their melting in the daytime by heat absorption and solidifi­
cation in the evening by release of heat.[385] Although water has ach­
ieved upto 65% efficiency, it entails leakage problems and thus
scalability is simpler for air (although with 40% efficiency). Neverthe­
less, the type of cooling mechanism must be suitable for the level of
device integration and the choice of coolant is dependent on climate
conditions like humidity and temperature. Passive cooling methods are
more preferable for commercialisation as they do not need additional
power to operate and requires less maintenance. Notably, special ge­
ometries for heat sink, microchannels, porous/high surface area heat
sink structures are on the way to replace conventional fins for more
effectiveness in heat dissipation.[386]
Due to the mismatch between solar spectrum and the band gap of the
semiconductor absorber, a significant portion of the solar energy is
27
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Energy Conversion and Management 261 (2022) 115648
Fig. 30. (a) Schematic of CPV-EC using Polymer Electrolyte electrochemical Cell (b) detailed layered structure of CPV for tandem light absorption [378].
Fig. 31. (a)Schematic of synergistic CPV-Photothermal reactor operating in a cascade pathway (b) Experimental apparatus (c) Photo-Thermal H2 generation rate
from an UV–Vis/IR Spectrum; (d) Photo-Thermal reaction under varying solar concentration and catalyst loading; (f) Efficiency of the co-generation system [390].
converted to heat which could be utilized effectively for a number of
applications such as space heating, crop drying and dehydrating, desa­
lination, water heating and even thermoelectric generators.[387,388] A
proper solar energy partitioning in photovoltaic/thermal (PVT) tech­
nology can achieve minimum wastage and thus increase overall effi­
ciency. Spectral splitting is done for this purpose by employing filters (e.
g., dichroic mirrors) to split the incident sunlight such that the ultra
violet and infra-red region is reflected towards the thermoelectric unit
and the visible portion is utilised by the photovoltaic unit thus ensuring
the optimized use of solar energy.[52,389].
Tang et al. [390] demonstrated a PV connected Photo-Thermal H2
generation where concentrated sunlight with spectral selectivity and
absorptivity is used as shown in Fig. 31(a-c). A solar collector with a
parabolic trough concentrates full spectrum of sunlight into 15 suns. A
near-homogeneous volumetric liquid absorber with UV–vis/IR absorp­
tion and Vis-NIR transmission (700–1100 nm) was developed by
dispersing Au-TiO2 in 10% vol methanol. To reduce optical losses, a
quartz reactor with a transmittivity of over 95% was used to hold the
liquid absorber. When sunlight is concentrated to 3–15 suns, H2
productivity increases linearly [392,393]. Fig. 31(d) depicts the effect of
catalyst mass concentration on photothermal H2 generation. The solar
concentration is high in the first condition (red colour), but the low
catalyst loading has low absorption and loses its UV–vis band in trans­
mission. The second condition (blue colour) has high catalyst loading
combined with lower solar concentration, resulting in a very slow H2
generation rate. The third condition (yellow colour) shows a substantial
interaction between high sunlight and high catalyst concentrations.
Fig. 31(e) depicts the overall system efficiency in which one can notice
that in comparison to separate PV and photo-thermo catalytic systems,
the hybrid system is more cost effective with a higher overall efficiency.
[390]
The influence of parameters like solar incident flux (500 W/m2 to
2000 W/m2), temperature contours, STH efficiency and H2 volume
production rate on the system was explored by Qureshy et al., (2018)
[391] using a PEC reactor as shown in Fig. 32. According to the results,
the rate of H2 volume production and STH efficiency increase when
applied solar incident flux is increased. Calculated STH efficiencies was
12.65% for 78.3 L/m2 h H2. Another usable technology, thermoelectric
28
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Energy Conversion and Management 261 (2022) 115648
Fig. 32. (a) STH conversion efficiency and (b) H2 production rate at different incident solar flux; (c-f) Temperature contours for (c) 500 W/m2, (d) 1000 W/m2, (e)
1500 W/m2, (f) 2000 W/m2.[391].
generators work following the principles of Seebeck and Peltier effects
and depend on the temperature difference between the top and the
bottom of the PV.[394] It is also worth noting that these are lowmaintenance devices since there are no moving parts. Thus, commer­
cial use of PV-PEC may benefit from PVT installations for co-generation
of thermal and electric power with reduced payback time. Prototype
CPV-EC [395] and PV-PEC devices [17] have been recently reported in
which the authors demonstrated the successful utilization of thermal
energy either by heating something or by some optimised cooling routes
through innovative management. But there is need for more demon­
stration of the viability of such CPV and integrated PVT-PEC in case of
different device configurations and solution processed PV materials with
known thermal stability issues.
production of hydrogen fuel from water. Generally, PV-PEC design is
more efficient and cheaper as solar energy is utilized at more than one
steps unlike PV-EC designs. STH efficiency and cost effectiveness have
been considerably improved over the last decade by appropriate selec­
tion of materials, their modifications/surface functionalization and
optimisation of parameters for each of the device components. Consid­
ering the importance of affordable sustainable energy, it is critical to
develop integrated PV-PEC devices for green hydrogen through water
splitting. Replacing non photo-active electrocatalytic materials in
already upscaled IPV-EC device architectures by low band gap earth
abundant transition metal oxide alternatives (like, copper oxide, sulfide
based materials) could be an effective strategy for development of
commercial PV-PEC technology and therefore demands for widespread
exploration.
In case of dual photo-absorbers in tandem, front (wider, preferably
< 2.2 eV) and rear (narrower, preferably < 1.7 eV) absorber band gap
combination should be selected carefully for proper sunlight utilization
so that the STH is increased > 10%. Such desired materials can be
10. Summary and outlook
In this article, we have reviewed the current status of integrated
photovoltaic and photoelectrocatalytic technologies for the clean
29
Energy Conversion and Management 261 (2022) 115648
P. Chatterjee et al.
practically realized by doping of highly tuneable metal oxides along
with passivation for stability.
To reduce loss due to recombination in scalable devices, well
controlled defect-free synthesis techniques (such as electrodeposition,
spray pyrolysis or solvothermal method along with spin coating for
multiple layering) deserve attention for designing of better nano­
structures and band aligned interfaces that increase minority carrier
lifetime and facilitate charge transfer. Corrosion prone electro­
de–electrolyte combinations and energy intensive deposition methods
or those requiring inert atmosphere need to be strictly avoided.
Precious metal co-catalysts could be replaced by using first-row
transition metal-based (e.g. Ni, Co) alternatives. Earth abundant and
stable materials (like many of the transition metal oxides) can be cost
effective when devices of large area are built up from the lab miniatures.
Hence, a balance between STH, lifetime and initial/replacement costs
should drive the practical implementation.
Integrated PV-PEC reports are yet to be scaled up considerably for a
meaningful study on practicability. Device complexity often becomes a
hindrance in integration. But the fact remains that analysis of profit­
ability becomes easier from those studies which gave an actual
demonstration of their upscaled prototypes operating under ambient
conditions. Large installation by repetition of individual water splitting
units and using established high efficiency multijunction thin film
photovoltaics that have band alignment with PEC electrode is an opti­
mizable and presently realisable configuration that can keep in-built
losses in check.
Thus, PV-PEC devices as a combination of a high-STH metal oxidebased dual absorber consisting of a PEC unit with low onset potential
and a solution-processed stable PV can exhibit remarkable performance
at low upscaling cost. Inexpensively acquired barren locations with low
chance of extreme weather conditions are preferable. Installation angles
may have to be selected as per round the year data of wind speed, dust
accumulation tendency, etc in order to reduce maintenance costs.
Coupling of PV/thermal technology for space-saving energy-saving
multi-utility and thereby low payback time is recommended.
However, PV-PEC or PVT-PEC integrated devices may only help to
achieve the sustainable development goal of clean energy but it may not
be affordable unless it receives additional impetus from policy makers.
H2 produced by conventional air polluting methods will continue to be
cheaper until say, a carbon emission tax is implemented to usher in the
more responsible choice. Active research should go on to make solar H2
production economic enough to shift the dependence on fossil fuels for
the sake of the environment.
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Declaration of Competing Interest
The authors declare that they have no known competing financial
interests or personal relationships that could have appeared to influence
the work reported in this paper.
Acknowledgements
MSKA would like to thank SRM University, Andhra Pradesh for
providing the fellowship to carry out the work. PC thanks NIT Durgapur
for her fellowship. SB would like to thank the Ministry of Science and
Technology, Taiwan under grant no. MOST 110-2221-E-131 -019 for
financial support.
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