Emerging Copper-Based Semiconducting Materials for Photocathodic Applications in Solar Driven Water Splitting

Abstract

Hydrogen production through solar-driven water splitting is a promising approach and an alternative to the conventional steam reforming of natural gas and coal gasification. The growing energy demand and environmental degradation through carbon-emitting fossil fuels urge a transition in the usage of non-renewable to renewable sources of energy. The photocathodes in a photoelectrochemical (PEC) water-splitting cell are essential for the direct evolution of hydrogen. Among the known photocathodes, Cu-based p-type semiconducting materials are the most promising photo-absorber materials owing to their low-cost, low toxicity, natural abundance, suitable bandgaps, and favorable band edges for reduction. Moreover, the chemical stability and the rate of recombination significantly limit the longevity, the PEC performance, and practical applicability of Cu-based photocathodes. To overcome these problems, it is critical to have a thorough understanding of the constraints, improvement strategies, and an assessment of current developments in order to construct and design highly stable and efficient photocathodes. Here, in this review we have summarized the development of Cu-based metal oxide and sulfide photocathodes with the significant operational challenges and strategies that have successfully been employed to enhance the PEC performance. Furthermore, the emphasis is placed on recent reports and future perspectives regarding emerging challenges.

1. Introduction

The global energy demand is rising daily, while the supply comes from the same non-renewable energy sources [1]. Carbon emissions and environmental damage are liabilities of the energy derived from fossil fuels. However, the diminishing fossil fuel sources do not provide a steady fuel supply to meet the growing population and demand. Alternative energy sources should be considered to mitigate the impact of carbon emissions and meet the energy demand. H₂ is considered a fuel for future generations with a high energy density and a lower molecular weight. At the moment, the majority of H₂ is being produced using non-renewable sources of energy processes, such as steam methane reforming, coal gasification, and coal pyrolysis. In comparison, renewable sources of H₂ production include biomass gasification & pyrolysis, thermochemical, solar-driven plasmolysis, and photo-electrolysis [2]. Presently, renewable energy sources are being scrutinized for developing reliable energy sources with no carbon emissions and a sustainable environment [3,4]. Solar energy-based technologies are attracting a vast number of applications in the form of renewable sources of energy. Solar-driven water splitting, or photoelectrochemical (PEC) water splitting, is one of the most attractive techniques used to decompose water into hydrogen (H₂) and oxygen (O₂) [5,6]. The energy falling on the earth’s surface from solar radiation is a driving force for H₂ production. PEC cells use photoactive materials coated over a transparent conductive oxide surface, mainly known as a photoanode (where oxidation of water occurs on the surface resulting in the oxygen evolution reaction OER) and photocathode (where water reduction occurs on the surface, resulting in hydrogen evolution reaction (HER)) [7,8]. The photoactive material coated films over the photoanode are generally n-type semiconductor materials and p-type semiconductor materials on the photocathode.

PEC water splitting is also known as “artificial photosynthesis.” The concept of this phenomenon is nature-inspired plant leaves that convert solar light into carbohydrates. The sunlight absorbed by plant leaves is utilized by a photosystem I (PS I) and photosystem II (PS II) to generate and transfer electrons through a series of transfer processes across several redox systems, to reduce CO₂ to hydrocarbons [9]. Similarly, semiconductors, absorbing the minimum threshold wavelength of light equivalent to their bandgaps, generate photoexcited electron/hole (e⁻/h⁺) pairs, and the photoexcited electrons are diffused to the conduction band. Holes remain in the valence band of semiconductors, and electrons travel to the surface of the photocathode. The HER and OER occur on the surface of the photoelectrodes, and the active surface sites govern the kinetics of the reaction. The active surface sites contribute toward the adsorption of H⁺ and OH− molecules on the surface of the photoelectrodes [10,11,12].

The seminal report on PEC water splitting, in 1972, used a thin layer of TiO₂ (E_g ~ 3.2 eV) as a photoanode for a PEC half-cell. Since then, photoanodes have been studied extensively with several material modifications and fabrication techniques. The OER takes place on the photoanode while the H₂ evolution occurs on the counter electrode [13]. The n-type semiconductors, such asTiO₂ (3.2–3.4 eV) [14], BiVO₄ (2.2–2.4 eV) [15], WO₃ (2.6–3 eV) [16], g-C₃N₄ (2.5–2.8 eV) [17], CdS (2.2–2.4 eV) [18], SrTiO₃ (3.2–3.4 eV), and Fe₂O₃ (2–2.2 eV) have been widely applied for the photoanode application in the PEC cell. Among all of these materials, TiO₂ comes out as the best material in terms of stability, while BiVO4 is the best material in abundance and light absorbing capacity in the visible region (Figure 1) [19].

Moreover, photocathodes have received very little attention when compared to photoanodes. Meanwhile, the direct evolution is more crucial than indirect evolution of H_2, which occurs in the photoanodic half-cells. Developing a p-type semiconductor material with a suitable bandgap, band alignment, and stability toward the oxidizing environment is paramount. Along with these physical and optical characteristics, the semiconductor must be tested for applications using non-precious and earth-abundant metals that are neither poisonous nor harmful with a sustainable approach(Figure 2) [20,21,22,23,24].

Copper-based oxide/chalcogenide semiconductor materials are promising materials owing to their tunable band gaps, band alignment with respect to water reduction potential (Figure 3), absorption coefficients, and various synthesis procedures. In this review, we have discussed recent advancements in the synthesis and fabrication of earth-abundant copper-based oxides/chalcogenides as a functional photocathode material for solar-driven water splitting, in light of their prospective use in PEC solar-to-hydrogen systems. In addition, several modifications that took place over a period and critically assess the ongoing debates in this area, have been discussed. Finally, this evaluation provides a chance to compare Cu-based semiconductor materials to give increasing emphasis to visible light-driven solar water electrolysis.

2. PEC Water Splitting

2.1. Understanding PEC Water Splitting

PEC water splitting uses a thin semiconductor film to decompose water into hydrogen and oxygen when irradiated with adequate light energy. Photogenerated charge carriers aid the process at the semiconductor-electrolyte interface. The thermodynamics of PEC water splitting requires 285.8 kJ mol⁻¹ energy, which is the same amount of energy released when hydrogen is transformed into water. The Gibbs free energy (237.2 kJ mol⁻¹, the maximum amount of energy that can be taken from the process) and the heat generated by the reaction (48.6 kJ mol⁻¹) contribute to the energy. A fuel (H₂) and an oxidant (O₂) participate as reactants in the combustion process, which is redox in nature and results in an exothermic reaction that produces water vapor. When taking into account the reverse reaction (Equation (1)), a system containing H₂O(l) may be given energy equal to Gibb’s free energy (237.2 kJ mol⁻¹), which can thermodynamically transform H₂O into H₂(g) and O₂(g).

$${\mathrm{H}}_2\mathrm{O}+237.2{ \mathrm{kJ} \mathrm{mol}}^{-1}+48.6{ \mathrm{kJ} \mathrm{mol}}^{-1}\to { \mathrm{H}}_2\left(\mathrm{g}\right)+\frac{1}{2}{\mathrm{O}}_2\left(\mathrm{g}\right)$$

(1)

The theoretical thermodynamic barrier for an overall water splitting process is proven to be 1.23 V by the fact that Gibb’s free energy (237.2 kJ mol⁻¹) equates to 1.23 eV per electron. The extra heat produced by the reaction must also be considered to drive the reaction at a reasonable rate. The overpotential, or 1.48 V, is the result of converting the total energy (285.8 kJ mol⁻¹) to the potential. Traditionally, the energy needed to split water into H₂(g) and O₂(g) is known as free energy, which is equal to 237.2 kJ mol⁻¹ or 1.23 V [6]. A semiconductor may theoretically generate a maximum photovoltage of 400 mV, less than the bandgap under ideal circumstances. To dissociate the water, the photoelectrode’s photovoltage must be V_ph > 1.23 V. For PEC applications, semiconductors with a bandgap > 1.6 eV are the best options [25].

2.2. Working Principle of Water Splitting

The processes involved in the PEC water splitting process’ essential operation are light absorption (step 1), charge carrier generation (step 2), charge carrier separation (step 3), and charge carrier transport to the SCLJ (step 4), where the surface reaction takes place (Figure 2). The suitable bandgap semiconductors, as previously said, absorb the light. The electron-hole pairs are excited by light absorption and subsequently separated. The electron then moves in the direction of the photocathode, leaving a hole behind. The conduction band (CB) and valence band (VB) band positions must be in the appropriate alignment for the reduction or oxidation of the water molecules in order for a semiconductor to meet the fundamental requirement. The reduction occurs on the photocathode and oxidation at the photoanode surface (Figure 2). The half-cell reactions taking place on the photoanode and photocathode are: The water-splitting reaction in the acidic electrolyte is shown in Equations (2) and (3)

$$\mathrm{Anode}: 2{\mathrm{H}}_2\mathrm{O}+4{\mathrm{h}}^{+}\leftrightharpoons {\mathrm{O}}_2+4{\mathrm{H}}^{+} \Delta {\mathrm{E}}_{\mathrm{OX}}^0=1.23{ \mathrm{V}}_{\mathrm{RHE}}$$

(2)

$$\mathrm{Cathode}: 4{\mathrm{H}}^{+}+4{\mathrm{e}}^{-}\leftrightharpoons 2{\mathrm{H}}_2\hspace{1em}\hspace{1em}\Delta {\mathrm{E}}_{\mathrm{Red}}^0=0{ \mathrm{V}}_{\mathrm{RHE}}$$

(3)

The water-splitting reaction in the basic electrolyte is presented in Equations (4) and (5)

$$\mathrm{Anode}: 2{\mathrm{OH}}^{-}+4{\mathrm{h}}^{+} \leftrightharpoons {\mathrm{O}}_2+2{\mathrm{H}}_2\mathrm{O} \Delta {\mathrm{E}}_{\mathrm{OX}}^0=-0.404{\mathrm{V}}_{\mathrm{RHE}}$$

(4)

$$\mathrm{Cathode}: 4{\mathrm{H}}^{+}+4{\mathrm{e}}^{-}\leftrightharpoons 2{\mathrm{H}}_2\hspace{1em}\hspace{1em}\Delta {\mathrm{E}}_{\mathrm{Red}}^0=-0.826{ \mathrm{V}}_{\mathrm{RHE}}$$

(5)

3. Photocathode in a PEC Cell

Photocathodes are responsible for the reduction in a PEC cell serving the purpose of the direct evolution of hydrogen in a 2e⁻ transfer process over its surface. The minority charge carriers drive the reaction on the surface of the photoelectrode. To date, a number of materials have been investigated as efficient photocathodes for hydrogen generation such as p-Si (E_g = 1.1 eV) with a theoretical maximum photocurrent density −44 mA/cm² [26], InP (E_g = 1.3 eV) with a theoretical maximum photocurrent of −35 mA/cm² [27], p-GaP (E_g = 2.2 eV) [28] Sb₂Se₃ (E_g = 1.2 eV) [29], BiSbS₃, a p-type material with narrow bandgap (E_g = 2.2 eV)[30]. Cu-based photocathode materials have a suitable conduction band maximum for the reduction of H⁺ ions and bandgap (Figure 3). The non-toxicity, earth abundance, low-cost, high absorption coefficient, and ease of synthesis make them a promising and sustainable component for photoabsorber materials. The sluggish reaction kinetics of the HER and the recombinations require the engineering of the interfaces with the different layers which give several advantages.

4. Photocathode Materials

4.1. Cu-Based Metal Oxides for the Photocathode

Copper-based metal oxides, such as Cu₂O and CuO are among the studied metal oxides, owing to their non-toxicity, availability, nature, and bandgaps. Due to their unique optoelectronic properties, they have been extensively used in energy conversion, storage, and sensing devices. However, the chemical instability of these materials restricts their PEC performance [31]. Cu₂O has a narrow bandgap (E_g = 2.0 eV) with a theoretical photocurrent density of −14 mA/cm² and a 18% photoconversion efficiency (PCE) owing to its light absorption in the AM 1.5 spectrum [32]. Cu₂O, as a photocatalyst, showed a stable H₂/O₂ evolution for 1900 h in visible light (λ > 460 nm) [33]. Although Cu₂O presents an immense opportunity for the hydrogen evolution, the photodegradation and recombination rate limits its photoactive material usage. Several strategies, including surface passivation, interfacial engineering, and cocatalyst decoration, have been investigated to overcome photodegradation and suppress the recombination to fabricate efficient photocathodes [34].

Cu₂O nanowires synthesized via sputtering, anodization, and annealing sequentially showed higher J_ph (Figure 4a) and IPCE (Figure 4b) than planar Cu₂O. Cu₂O/AZO/TiO₂/RuO_x delivered J_ph = −10 mA/cm² at 0 V_RHE and photostability beyond 55 h [35]. It is important to study the impact of pH on the photocathode and develop robust configurations for basic, neutral, and acidic mediums. The PEC activity of Cu₂O/AZO/TiO₂/RuO_x was also examined at various pH levels (i.e., pH = 1, 5, and 13.6), and the deposition of the protective layer of TiO₂ played an important role in the photostability of the photocathode [36]. The deposition of NiCO-LDH over pSi/Au/Cu₂O increased the photocurrent to 330% compared to pSi/Au/Cu₂O, due to the formation of the type-II heterojunction (Figure 4c,d), enhancing the photocarrier separation, accelerating the surface catalytic reduction reaction, and improving the stability of the pSi/Au/Cu₂O photocathode [37].

A Cu₂O/CuO heterojunction photocathode modified with a Cu₂S-Pt composite exhibited an enhanced hydrogen evolution. The optimum Cu₂S and Pt used as cocatalysts deposited using SILAR and sputtering sequentially, facilitated the charge transport and suppressed recombination. Cu₂O/CuO/Cu₂S-9/Pt delivered J = −5.7 mA/cm² at 0 V_RHE (2.5 times of Cu₂O/CuO), while a higher onset potential (E_op) for Cu₂O/CuO/Cu₂S−9/Pt was ~0.64 V_RHE observed, compared to Cu₂O/CuO (E_op = 0.54 V_RHE). The excessive Cu₂S deposition resulted in a parasitic light absorption and the creation of the recombination centers, resulting in J_ph fading [38].

The back contact material is essential in suppressing the recombination and charge transfer. Au is the best contact material for Cu₂O due to its large work function aligned with the valence band of Cu₂O. Recently, a CuO/NiO-based composite was investigated in place of Au using sputtering and aerial oxidation for the back contact with Cu₂O. A CuO/NiO thin film significantly improved and enhanced the transparency and hole collection (electron blocking) at the back contact of the Cu₂O photocathodes [39].

Surface plasmon resonance (SPR), a technique that allows for the prolonged light absorption throughout the whole UV-visible spectrum of the sun, has recently been applied in the PEC water electrolysis process. SPR is a naturally occurring property of metal nanoparticles, and the collective oscillation frequency is susceptible to the size and form of the metals [40]. Photocathodes with plasmonic sandwiched metal NPs have shown great potential in boosting the PEC performance. A p-Cu₂O/AuAg/n-Cu₂O photocathode fabricated with the additional benefit of the plasmonic activity of AuAg bimetallic NPs. AuAg NPs served as a plasmonic sensitizer that increased the charge carrier concentration and, as an electron relay to boost the charge transfer between the homojunction (Figure 5a). As a result, p-Cu₂O/AuAg/n-Cu₂O achieved a higher E_op (~0.8 V_RHE) and J_ph when compared to the homojunction p-Cu₂O/n-Cu₂O (E_op = 0.7 V_RHE) and p-Cu₂O (E_op = 0.43 V_RHE) (Figure 5b). X-ray absorption spectroscopy (XAS) revealed the hot electron injection from plasmonic AuAg alloy to the photocathode [41].

CuO is a p-type semiconductor with a narrow bandgap (E_g = 1.3–1.7 eV) suitable for light absorption and H₂ evolution. However, the photo corrosion of CuO in the presence of light is a significant drawback for the application. The majority of J_ph for the CuO photocathode is due to the photo corrosion of CuO to Cu with a low faradaic efficiency of the HER (0.01%) [43]. The deposition of CdS (buffer layer) and TiO₂ (as a protective layer), and Pt as a cocatalyst sequentially resulted in an enhanced PEC performance and faradaic efficiency (~100%) [43]. Various methods have been developed to synthesize CuO having different morphologies and bandgaps. The synthesis of CuO layers using sputtering and using variable power and thickness, exhibited a different photoactivity for hydrogen evolution. The variable power of sputtering (i.e., 30, 100, 200, and 300 W) in the synthesis resulted in a different morphology of the fabricated films, while the thickness of the layer of CuO played a role in light absorption. PXRD patterns showed the poor crystallinity of CuO due to low kinetic energy for diffusion and formation of Cu-O bond, while at higher the sputtering power, good crystalline films were obtained [44]. The deposition of a protective and stable metal oxide layer has proven to improve the faradaic efficiency of the H₂ evolution. The CuO/CuFe₂O₄ heterostructure was fabricated using the ion impregnation method. Although the faradaic efficiency increased from 45% for the bare CuO nanowires to 100% for CuO/CuFe₂O₄ with an improved stability, Jph decreased to one-third of the initial J_ph [45]. Generating Defects in the crystal structure (e.g., copper vacancies (V_Cu) in CuO) is an effective way to improve the charge separation and transfer. A study showed that V_Cu in CuO can be tuned by changing the O₂ partial pressure in the annealing process. The creation of vacancies in CuBi₂O₄ and CuFe₂O₄ resulted in an enhanced carrier concentration [46].

The incorporation of a foreign metal ion in the crystal structure can mitigate the issue of a photoinduced degradation. Several mixed metal oxides have been synthesized, such as CuBi₂O₄ [47], CuFe₂O₄ [48], CuAl₂O₄ [49], CuNbO₃ [50], and CuCrO₂ [51]_, and tested for the PEC photocathode application. CuBi₂O₄ has a bandgap of 1.5–1.8 eV and is a p-type mixed-metal oxide semiconductor that can be produced at low cost and with little environmental impact. By contributing their conduction band (CB) from the secondary metal rather than Cu 3d, ternary oxides have an advantage over the binary copper oxides in that they improve the photostability by preventing the conversion of Cu²⁺ to Cu⁰. CuBi₂O₄ is regarded as a good photocathode material because of its advantageous narrow bandgap, band positions, low cost, visible light absorption, non-toxicity, and high flat band potential (E_fb) values (>1 V vs. RHE), which are advantageous for unaided solar water splitting with a small bias given by the tandem photoanode. The theoretical photocurrent density reported for CuBi₂O₄ under AM 1.5 G (19.5–24.5 mA/cm²) is very far from achieved [52]. The practical photocurrent densities of the CuBi₂O₄ photocathodes are lower due to their poor charge transport, lower absorption coefficient, and higher charge recombination rates. Doping Ag⁺ in CuBi₂O₄ replacing Bi³⁺ increased the hole concentration, suppressing the anodic photo corrosion [53]. The sandwiched metal NPs between the heterojunction is a new approach for different configurations of the photocathode, which have shown potential in boosting the PEC performance. The PEC performance of the N,Cu-Codoped Carbon Nanosheets/Au/CuBi₂O₄ photocathode was examined. Au served as a plasmonic sensitizer and electron relay to transfer the charge from CuBi₂O₄ to the N,Cu-Codoped Carbon nanosheets [54]. Recently, a CuBi₂O₄-based photocathode having Au NPs sandwiched between CuBi₂O₄ and Sb₂S₃ (i.e., CuBi₂O₄/Au/Sb₂S₃), showed an enhanced photocurrent density and PCE with respect to the bare CuBi₂O₄ and CuBi₂O₄/Sb₂S₃. Enhancement was attributed to the dual role of Au NPs: (1) plasmonic sensitizer and (2) electron relay between CuBi₂O₄ and Sb₂S₃ (Figure 5c). The CuBi₂O₄/Au/Sb₂S₃ exhibited J_ph = −3.2 mA/cm² which was >200% then the bare CuBi₂O₄ photocathode (Figure 5d) [42].

Ferrites are considered one of the best candidates, owing to their merits, including earth abundance, non-toxicity, and stability in aqueous solutions. They have narrow band gaps and suitable band positions to drive the redox reaction over their surface [55]. Copper ferrite (CuFe₂O₄) is a mixed metal oxide and p-type semiconductor material for the PEC photocathode with a narrow bandgap (E_g = 1.5–1.9 eV). Theoretically, it can yield a high J_ph (~27 mA/cm²) and a STH efficiency (~ 33%). Although CuFe₂O₄ is an excellent p-type material, the crystallization temperature is high (~800–1000 °C), concerning the glass transition temperature of FTO (~564 °C). Rapid flame annealing (> 980 °C) of CuFe₂O₄ crystallized the film with a porous and high surface area structure which increased the light absorption (Figure 6a) and decreased the annealing time from 9 h to 16 min, when compared to conventional annealing. Changes in the bandgap and band position of CBM and VBM was observed (Figure 6b). Lesser oxygen vacancies were observed in the case of the flame-annealed CuFe₂O₄ films, which exhibited three times higher J_ph than the conventional heated CuFe₂O₄ (Figure 6c), and an enhanced IPCE was observed for the rapid flame-annealed CuFe₂O₄ films (Figure 6d) [48]. Various synthetic procedures have been reported. One is by forcibly impregnating the capped Cu²⁺ ion in a hematite (Fe₂O₃) crystal structure which resulted in drastic changes in the formation of highly porous flakes of the CuFe₂O₄ morphology and the crystal structure without the formation of Cu oxides. The temperature-dependent control over the degree of the spinel inversion (δ = 0.77) showed the enhanced photoelectric properties of CuFe₂O₄ [56]. Fabrication of the heterojunction with CuFe₂O₄ has been extensively examined for the enhanced charge separation. CuFe₂O₄/Amorphous MnO₂ (AMO) was examined for the H₂ evolution in the neutral electrolyte, and 502.8 μmol H₂ was evolved in 90 min for a 1:4 ratio of CuFe₂O₄/AMO, which is higher than CuFe₂O₄/TiO₂ (130 μmol/h) and CuFe₂O₄/g-C₃N₄ (76 μmol/h) [57]. Photonic crystals (PC), consisting of CuFeO₂ decorated microspheres served as self-light harvesting architectures, allowing a high transmittance (~76%) and an amplified light absorption. The synthesis proceeded over the silica microspheres and the polymer-assisted synthesis. The novel design exhibited −0.2 mA/cm² at 0.6 V_RHE [58]. Various studies with Cu-based metal oxide photocathodes have been listed in

Table 1. Photoelectrochemical performances of the Cu-based metal oxide photocathodes.

Device Structure	Photocurrent (mA/cm2); Applied Bias	Onset Potential (VRHE)	Stability (J/J0); Time; Applied Bias (VRHE)	Maximum STH or IPCE (%)	Faradaic Efficiency	Electrolyte; Light Source	Ref.
Cu2O/TiO2	−0.7; −1 VAg/Agcl	~ 0 VAg/Agcl	-	-	-	0.1 M Sodium Acetate, Xe Lamp (700 mW/cm2)	[59]
Cu2O/Carbon	−3.95; 0 VRHE	~ 0.6	~80%; 20 min; 0	0.56	-	1 M Na2SO4, AM 1.5G	[60]
FTO/Au/Cu2O/AZO/TiO2/Pt	−4.5; 0 VRHE	0.4	100%;1 h; 0	0.66	-	1 M Na2SO4/0.1 M potassium phosphate (pH = 4.9); AM 1.5G	[61]
FTO/Au/Cu2O/AZO/TiO2/Pt	−6.0; 0 VRHE	0.55	-	~1.5	~100%	0.5 M Na2SO4/0.1 M potassium phosphate (pH = 5); AM 1.5G	[62]
FTO/Au/Cu2O/AZO/TiO2/RuOx	−5.0; 0 VRHE	0.5	~100%; 4 h; 0	~1.1	-	0.5 M Na2SO4/0.1 M potassium phosphate (pH = 5); AM 1.5G	[62]
FTO/Au/Cu2O/AZO/TiO2/RuOx	−5.2; 0 VRHE	0.55	~100%; 25 h; 0	-	-	1 M Na2SO4/0.1 M potassium phosphate (pH = 4.9); AM 1.5G	[63]
FTO/Au/Cu2O/AZO/TiO2/MoSx	−4.8; 0 VRHE	0.45	~100%; 10 h; 0	-	100%	0.5 M Na2SO4/0.2 M potassium phosphate (pH = 4); AM 1.5G	[64]
FTO/Au/Cu2O/AZO/TiO2/Ni-Mo	−6.3; 0 VRHE	0.53	~25%; 10 h; 0	-	~100%	1 M KOH (pH = 13.6); AM 1.5G	[65]
FTO/Al/Cu2O/NiS	−5.16; 0 VRHE	0.6	-	1.12	-	0.1 M Na2SO4; AM 1.5G	[66]
ITO/Cu/Cu2O/TiO2	−1.5; 0 VRHE	0.55	-	0.28	98%	1 M Na2SO4; AM 1.5G	[67]
FTO/FeOOH/Cu2O/Pt	−1.5; 0 VRHE	0.6	66%, 1 h, 0		20%	0.1 M Na2SO4; AM 1.5G	[68]
FTO/Au/CuSCN/Cu2O/Ga2O3/TiO2/RuOx	−6.4; 0 VRHE	1.0	~100%; 10 h	4.2	100%	0.5 M Na2SO4/0.1 M NaH2PO4 (pH = 5); AM 1.5G	[69]
FTO/H:Ti3C2TX/Cu2O	−5.41; 0 VRHE	0.4	-	0.55	-	1 M Na2SO4; AM 1.5G	[70]
Ti/Cu2O/ZnO	−7.23; 0 VRHE	0.83	-	1.77	-	0.5 M Na2SO4; AM 1.5G	[71]
Cu2O/Ga2O3/TiO2/RuOx	−4.0; 0 VRHE	0.8	-	60% @ 450 nm at 0 VRHE	-	0.5 M Na2SO4/0.1 M Sodium phosphate (pH = 5); AM 1.5G	[72]
FTO/CuBi2O4/MoS2	−0.182; 0.6 VRHE	0.9	100%; 200 s; 0	-	-	0.1 M NaOH (pH = 12.5)	[73]
FTO/CuBi2O4	−0.3; 0.6 VRHE	~0.8	20%; 15 min; 0.6	~14% @ 550 nm, 0.6 VRHE	-	Ar-purged 0.3 M K2SO4/0.2 M phosphate buffer (pH = 6.65); AM 1.5G	[74]
FTO/CuBi2O4/Pt	−0.5; 0.4 VRHE	~1	~10%; 3 min; 0.6	~10% @ 400 nm, 0.6 V vs. RHE	-	Ar-purged 0.3 M K2SO4/0.2 M phosphate buffer (pH = 6.65); AM 1.5G	[75]
FTO/CuO/CuBi2O4/Pt	−0.72; 0 VRHE	-	100%; 600s; 0	-	-	0.3 M K2SO4/0.1 M Phosphate buffer pH = 6.8; AM 1.5G	[76]
SrTiO3/SrRuO3/NiO/CuBi2O4	−0.4 at 0 VRHE	1.22	~100%; 3 h; 0	~11% @ 345 nm, 0.2 VRHE~	-	0.1kPi Buffer solution (pH = 8.55); AM 1.5G	[77]
FTO/NiO/CuBi2O4	−0.5; 0.4 VRHE	~1.0	~50%; 3 h; 0.4			Ar-purged 0.1 M potassium phosphate (KPi) buffered solution (pH = 8.55); AM 1.5G	[78]
FTO/CBO/ZnSe/P25	−0.43; 0.3 VRHE	~1.0	~50%; 5000s; 0.3	-	-	0.3 M K2SO4/0.2 M phosphate buffer (pH = 6.65); 300 W Xe lamp	[79]
Ov/CBO/Zn-CBO	−0.6; 0.3 VRHE	~1.0	~50%; 300s; 0.3	-	-	0.3 M K2SO4/0.2 M phosphate buffer (pH = 6.65); 300 W Xe lamp	[80]
FTO/Au/CBO/Pt	−1.24; 0.1 VRHE	~1.0	~50%; 3000 s; 0		84.49%	0.1 M Na2SO4 (pH = 6.8); 300 W Xe lamp	[81]
FTO/CuO/CuBi2O4	−0.9 at 0.1 VRHE	~1.0	75%; 2500 s; 0.1	0.19	-	0.5 M Na2SO4 solution (pH = 7); 250 W Xe lamp	[82]
FTO/CuBi2O4/CdS/TiO2/Pt	−1; 0 VRHE	~0.6	~60%; 3 h; 0	~0.13	∼91%	Ar-purged 0.3 M K2SO4/0.2 M phosphate buffer (pH = 6.65); AM 1.5G	[83]
FTO/CuFeO2/AZO/TiO2/Pt	−1.25; 0.4 VRHE	~0.9	100%; 600 s; 0.4 VRHE	-	-	Ar purged 0.5 M Na2SO4; AM 1.5G	[84]
FTO/CuFeO2/NiFe-LDH/rGO	−2.4; 0.4 VRHE	~0.65	100%; 1200 s; 0.4 VRHE	-	94%	1 M NaOH; AM 1.5G	[85]
FTO/CuAlO2/CuFeO2	−2.6; 0.4 VRHE	~0.75	-	-	-	1 M NaOH purged with O2	[86]

.

4.2. Cu-Based Sulfides for Photocathode

Copper-based sulfides are promising semiconducting materials for several applications, including photovoltaics, photo-electrocatalysis, energy storage, energy conversion, sensing, CO₂ reduction, and organic degradation. Cu₂S and CuS are the most sought out of the studied materials in the recent past. Recently, copper-based ternary sulfides such as CuInS₂, CuSbS₂, CuGaS₂, and CuFeS_2, and quaternary sulfides such as CuInGaS₂, Cu₂BaSnS4, and Cu₂ZnSnS₄ have gained much attention in solar cells and PEC water splitting. Their high absorption coefficient, tunable bandgaps, optical properties, suitable band positions for a redox reaction, and tunable crystal structure make them the most prominent candidates for their application in solar harvesting. These copper-based sulfides lack photostability and crystal defects which create recombination centers. The most prominent problem is the leaching of sulfides in harsh conditions, which is the major drawback in their application in photocathodes. The scarce availability and cost aspect of In and Ga in CuInGaS₂ is a major drawback for its large-scale production. Researchers have developed techniques, such as the heterojunction formation, doping, metal ion substitution, creating vacancies, decoration of plasmonic metal NPs, a passivation layer, and cocatalysts to enhance the PEC performance of photoelectrodes. Engineering the surface of the photoactive material with novel robust architectures and the deposition of interlayers, such as an electron transporting layer (ETL), hole transporting layer (HTL), cocatalysts, and passivation layers which inhibit the direct contact of electrolytes with the photoactive semiconductor, is required. The deposited layers must have a good optical transparency and little to no parasitic absorption of light.

Cu₂S is a p-type material with a narrow bandgap (E_g = 1.6–2 eV) with a remarkable absorption coefficient. Cu₂S can be synthesized through various processes such as annealing, hydrothermal, hot injection method, electrodeposition, sulfurization, etc. Cu₂S nanowires arrays (NWAs) synthesized via a self-growth mechanism over the Cu foil followed by the decoration of carbon quantum dots (CQDs) remarkably enhanced the PEC performance four times that of the pristine Cu₂S NWAs [87]. Cu_xS (0 < x ≤ 1) NPs with a copper deficiency show an inherent localized plasmonic resonance [88]. A novel solution processed ion exchange reaction to fabricate the Cu₂S films from the chemical bath deposited CdS films showed remarkable J_ph and PEC performances. The deposition of CdS formed a facile heterojunction with Cu₂S, TiO₂ (100 nm) served as a protective/passivation layer (Figure 7a), and RuO_x as cocatalyst. Cu₂S/CdS/TiO₂/RuO_x delivered −7.0 mA/cm² at −0.3 V_RHE with E_op = 0.48 V_RHE (Figure 7b) with 90% photostability retention after 200 min [89]. MoS₂ has gained immense attention for the HER cocatalyst in recently focused research on cocatalysts. The heterojunction of Cu₂S/MoS₂/Pt showed an enhanced photothermal HER performance near the NIR irradiation.

Ternary sulfides have an appropriate energy band structure, a wide photo-absorption range, and fast charge carrier dynamics due to the less positive valence occupied by the S 3p orbital (when compared to O 2p) and small effective mass carriers, making them promising alternatives in photovoltaic, photocatalytic, and PEC devices. For instance, altering the rate of cation exchange or the proportion of the sulfur precursor during the synthesis process, may be used to create ternary Cu-In-S materials with various crystal morphologies, compositions, and switchable n-type and p-type semiconducting characteristics [91]. CuInS₂ (E_g ~ 1.5–1.8 eV) is a photoactive material in solar cells and PEC photocathodes. Along with the merits, CuInS₂ suffers from a high rate of recombination. Therefore, the CuInS₂-based photocathodes have been modified with other materials to improve PCE. Fabrication of the heterojunction generates an inbuilt electric field that helps in the charge separation and the enhanced light absorption.

The deposition of a hole transporting layer below the CuInS₂ layer helps in the charge separation and inhibits the recombination in the photoexcited charge carriers. FeOOH/CuInS₂/Pt was fabricated with FeOOH as a hole transporting layer, selectively (Figure 7c). FeOOH/CuInS₂/Pt delivered J_ph = −6 mA/cm² at −0.4 V_RHE (Figure 7d) [90]. In another study, a NiO/CuInS₂/NiS photocathode was fabricated where NiO served as hole transporting layer while NiS served as a cocatalyst and passivation layer [92]. The investigation of a facile heterojunction formation using CdS and In₂S₃ with CuInS₂ was studied by fabricating CuInS₂/CdS/Pt and CuInS₂/In₂S₃/Pt. The CuInS₂/In₂S₃/Pt photocathode delivered a higher STH efficiency (~ 2.9%), while CuInS₂/CdS/Pt delivered (~1.8%). XPS revealed the notch-type positive conduction band offset with In₂S₃/CuInS_2, while CdS/CuInS₂ formed an unfavorable negative cliff type negative offset [93]. Recently, the charge transfer and photoelectrochemical activity of the CuInS₂-based photoelectrodes have been improved by an atomic gradient passivation layer (Ta, Mo) _x (O, S)_y [94]. A metal ion substitution is another approach that improves the optical properties and modulates the crystal structure, changing the material’s bandgap, open circuit potential (OCP), and photostability in a PEC cell [95,96]. The Ga substitution with In in a CuInS₂ crystal structure has shown an enhanced PEC performance. Different Ga/(Ga + In) ratios (0, 0.10, 0.25, and 0.40) were synthesized with respect to CuInS₂. E_g increased with the amount of Ga (1.52, 1.58, 1.67, 1.80 eV), respectively. CBM and VBM were observed to change, which aided the enhanced light absorption and the OCP. Pt-CdS/CIGS(25) corresponding to the Ga/(Ga + In) ratio (0.25) exhibited 6.78 mA/cm² at 0 V_RHE in 0.1 M Na₂SO₄ (pH 9) and a higher E_op = 0.89 V_RHE, as compared to other ratios [97]. The Ag substitution in CuGaS₂ fabricating Cu_1-xAg_xGaS₂ (x = 0–1.0) and the insertion of Ag in the crystal structure was observed with a change in VBM [98].

Recently, CuSbS₂ has gained attention in the PEC photocathodic approach. It has also been used as HTL in solar cells for the hole extraction [99,100]. Synthetic procedures include thermal evaporation [101,102], electrodeposition [103], sputtering [104], CBD [101], pyrolysis [105], etc. In a recent study, CuSbS₂ exhibited a temperature-dependent bandgap (E_g = 1.57–1.58 eV). A CuSbS₂/Sb₂Se₃/TiO_x/Pt photocathode was fabricated which exhibited a remarkable J_ph = −18 mA/cm² at 0 V_RHE in 1 M H₂SO₄. The CuSbS₂ film acted as a hole transporting layer and a photoactive material, suppressing the recombination rate. In another study, the facile heterojunction of CuSbS₂ with CdS was synthesized and examined for the PEC photocathode. CuSbS₂/CdS/Pt delivered J_ph = −4.1 mA/cm² at 0 V_RHE and E_op = 0.45 V_RHE [106].

Quaternary copper-based sulfides are the newly emerging most interesting p-type materials (e.g., Cu₂ZnSnS₄) with the promising potential for solar harvesting applications. Synthesis procedures involved in the fabrication of films are hydrothermal, molecular ink, electrodeposition, and vacuum-based synthesis. The rapid progress in materials to achieve the target efficiency of these sulfides are hindered by the narrow phase stability of the quaternary phase and the existence of secondary phases, such as ZnS, Cu₂S, SnS, SnS₂, and Cu₂SnS_3, and to defects leading to poor performance and repeatability [107]. In early reports, Mo/Cu₂ZnSnS₄/CdS/AZO/TiO₂/Pt was fabricated via electrodeposition (sequential and simultaneous). Moreover, the examined photocathode showed over −1 mA/cm² at 0 V_RHE in the simultaneous electrodeposition [108]. The buffer layer formed over Cu₂ZnSnS₄ was scrutinized with CdS and In₂S_3, and the Cu₂ZnSnS₄/CdS/In₂S₃/Pt fabricated showed a higher J_ph and PCE, as compared to Cu₂ZnSnS₄/CdS/Pt. The modification with an In₂S₃/CdS double layer followed by the deposition of Pt exhibited STH (%) ~ 1.63% [109]. A HfO₂ layer was particularly effective at surface passivating the CdS/Cu₂ZnSnS₄ photocathode, which increased the photoelectrochemical stability. With a 6 nm thick HfO₂ layer added, the CdS/Cu₂ZnSnS₄ photocathode demonstrated a long-term photocurrent stability of over 10 h while maintaining a high half-cell solar-to-hydrogen efficiency (HC-STH) of 2.7% at 0.36 V_RHE. A full PEC cell was fabricated using a BiVO₄-based photoanode [110]. The molecular ink-derived Cu₂ZnSnS₄ (CZTS) films were synthesized via spin coating. The study showed the sequence of the precursor addition in the 2-methoxyethanol (2-ME) and the films synthesized with the configuration CZTS/CdS/ALD-TiO₂/Pt. CdS served as a buffer layer, TiO₂ as a passivation layer, and Pt coated as a cocatalyst for the HER. CZTS/CdS/ALD-TiO₂/Pt delivered −21.5 mA/cm² at −0.2 V_RHE with a 40–45% faradaic efficiency, showing side reactions as the surface of CZTS is not fully covered [111].

The metal ion substitution has been studied and proved to improve the optical properties of Cu₂ZnSnS₄. The substitution of Cu with Ag and Zn with X (X = In⁺³, Cd⁺², Sb⁺³, Bi⁺³) have been studied extensively with the application in the PEC cells and solar cells. The photocurrent produced by a Cu₂Cd_0.4Zn_0.6SnS₄ (CCZTS) photoabsorber coated with CdS/TiMo/Pt is claimed to be −17 mA/cm² at 0 V_RHE, which is at least three times larger than the photocurrent produced by a pure Cu₂ZnSnS₄. The XPS studies revealed a 0.13 eV spike-like offset when integrated with CdS, which enhanced the charge separation and transfer [112]. A low Ag substitution in Cu₂ZnSnS₄ revealed an enhanced J_ph and E_op. Cu⁺ was partially substituted with Ag⁺ ion in the (Ag_xCu_1-x)₂ZnSnS₄ (x-ACZTS) (x = 0.04, 0.08, 0.10) crystal structure synthesized via a molecular ink precursor spin-coated over the transparent conductive oxide. In comparison to the CZTS/CdS/Pt photocathode, which has a photocurrent of −13 mA/cm² and an onset potential of 0.65 V_RHE, the ACZTS/CdS/Pt photocathode produces a maximum photocurrent of −17.7 mA/cm² at 0 V_RHE with 4% Ag (x = 0.04) and a maximum onset potential of 0.85 V_RHE with 8% Ag (x = 0.08) [113].

Another new class of materials when Zn is replaced with Ba in CZTS is an earth-abundant emerging material for PV applications [114]. Cu₂BaSnS₄ is a p-type material (E_g ~ 1.5–2.0 eV) with a readily available and cost-effective material that can be synthesized on a large scale. The maximum theoretical maximum J_ph obtained is −14 mA/cm², according to the Schokley–Queisser limit (Q–S limit) [115]. At a high temperature, CBTS decomposes to Cu₄BaS₃ and Cu₂Ba₃Sn₂S₈ (E_g = 2.19 eV) in the presence of a low partial pressure of sulfur [116]. Large grain Cu₂BaSnS₄ (CBTS) films were synthesized using a green synthetic approach using polyethyleneimine and EDA. The Mo/CBTS (260 nm) photocathode exhibited J_ph = −4 mA/cm² at 0 V_RHE in 0.1 M Na₂SO₄ (pH = 7) with no degradation of J_ph up to 2 h. With a remarkable charge carrier concentration of 1.8 × 10²¹ cm⁻³, the charge carrier mobility in the large grains was 1.29 cm²/V·s [117]. In another study, CBTS/CdS/ZnO/TiO₂ was fabricated using co-sputtering Cu, SnS, and BaS on the target FTO (Figure 8a,b). The CdS shows a smaller lattice mismatch with CBTS, while ZnO and TiO₂ CBM are aligned so that the facile electron transfer can occur for the surface reduction. CBTS/CdS/ZnO/TiO₂ showed a higher J_ph (−7.8 mA/cm² at −0.1 V_RHE) and IPCE (%) in the neutral electrolyte, as compared to the bare CBTS (−4.8 mA/cm² at −0.2 V_RHE) (Figure 8c,d) [118]. The substitution of S with Se forming CBTSSe is another effective way to improve the crystal structure and electronic properties of the CBTS. Cu₂BaSnS_4-xSe_x (x = 3) (CBTSSe) was synthesized by co-sputtering followed by sulfurization. The CBTSSe (x = 3)/CdS/TiO₂/Pt photocathode was fabricated using a chemical bath deposition for CdS followed by ALD for TiO₂ and the electrodeposited Pt. The resulting photocathode showed −12 mA/cm² at 0 V_RHE with STH (%) 1.09% with a remarkable stability up to 10 h [119]. CBTSSe (x = 3) was powder synthesized via ball milling, and a photocathode with the configuration Mo/CBTSSe (x = 3)/CdS/TiO₂/Pt exhibited a photocurrent of −4.69 mA/cm² at 0 V_RHE with STH (%) 0.49%. The synthetic procedures also affect the STH (%) of the photocathode with the solution based CBTSSe (0.89%), vacuum-based (1.09%) and ball milling (0.49%) [120]. Various studies with Cu-based metal sulfides photocathodes have been listed in

Table 2. Photoelectrochemical performances of the Cu-based metal sulfide photocathodes.

Device Structure	Photocurrent (mA/cm2); Applied Bias	Onset Potential (VRHE)	Stability (J/J0); Time; Applied Bias (VRHE)	Maximum STH or IPCE (%)	Faradaic Efficiency	Electrolyte; Light Source	Ref.
FTO/Au/Cu2S/CdS/TiO2/RuOx	−2.5; −0.3 VRHE	0.42	76%; 12 h; 0	-	-	1 M kPi buffer solution (pH = 7); AM 1.5G	[121]
FTO/Cu2S/Cu2O/Cu foam Au/Cu2S/CdS/TiO2/RuOx	−5.05; 0 VRHE	0.35	80%; 1 h ; 0	40% @ 450 nm at 0 VRHE	-	1 M Na2SO4/0.1 M KH2PO4 at (pH 4.9); AM 1.5G	[122]
FTO/Cu2O/Cu2S	−4.1; −0.6 VAg/AgCl	−0.29 VAg/AgCl	-	0.38	-	0.5 M Na2SO4; AM 1.5G	[123]
FTO/Cu2O/Cu2S-Ni	−1.70; 0 VRHE	0.5	45%; 500 s; 0	-	-	0.5 M Na2SO4; 300 W Xe lamp with AM 1.5G filter	[124]
FTO/CuInS2/CdS@MBAs	−0.487; −0.15 VRHE	-	~100%; 400 s; 0	10% @ 400 nm at 0 VRHE	-	1 M KCl Solution (pH = 5.97); AM 1.5G	[125]
FTO/CuInS2/Sb2S3/Pt	−2.48; −0.6 VRHE	0.6	~88%; 180 s; −0.6	21.41% @ 550 nm at −0.6 VRHE	-	0.1 M Na2SO4 (pH = 7.1); AM 1.5G	[126]
FTO/CuInS2/CdS	−0.71; −0.2 VRHE	0.25	~100%; 1500 s; 0	9% @ 425 nm at 0 VRHE	-	1 M KCl Solution (pH = 5.97); 500 W Xe lamp with AM 1.5G filter	[127]
Mo/CuInS2/In2S3/Pt	−5.6; 0 VRHE	0.7	~100%;80 min; 0.1	0.7	100%	0.1 M Na2SO4 (pH = 10); AM 1.5G	[128]
FTO/CuInS2/CdS/AZO/TiO2/Pt	−3.5; −0.3 VRHE	0.6	80%; 2 h; 0	~20% @ 500 nm at 0 VRHE	-	0.5 M Na2SO4/0.1 M KH2PO4 (pH = 5.0); 300 W Xe lamp with AM 1.5G filter	[129]
FTO/CIS NR/CdS/ZnS	−2.0; 0.3 VRHE	1.06	~100%; 3000 h; 0.3	-	-	0.5 M Na2SO4 (adjusted to pH 10 by adding NaOH).; AM 1.5G	[130]
FTO/CuInS2/SnS2–1.6/C60	−4.51; −0.45 VRHE	-	-	8% @ 450 nm at −0.45 VRHE	-	0.5 M Na2SO4; AM 1.5 G	[131]
FTO/Au-CuInS2	−15.2; 0 VRHE	0.3 VSCE	~100%; 400 s; −0.5 VSCE	4.29	-	0.5 M Na2SO4; AM 1.5G	[132]
FTO/CuSbS2/Sb2Se3/TiO2/Pt	−18.0; 0 VRHE	0.2	-	-	-	1 M H2SO4 (pH = 0); AM 1.5G	[133]
FTO/CZTS/CdS/TiO2-Pt	−9.0; 0 VRHE	0.6	-	1.2	-	0.1 M Na2SO4 (pH = 9.5); 300 Xe lamp with AM 1.5G filter	[134]
FTO/CZTS/CdS/ZnO/Pt	−8.0; 0 VRHE	0.63	~100%; 2 h; 0	2.1	-	0.2 M Na2HPO4/NaH2PO4 (pH 6.5); 300 W Xe lamp with AM 1.5G filter	[135]
FTO/CZTS/HfO2/CdS/HfO2/Pt	−28.0; 0 VRHE	0.7	~100%; 24 h; 0	2.4	-	0.2 M Na2HPO4/NaH2PO4 (pH 6.5); AM 1.5G filter	[136]
FTO/ACZTS/CdS/Pt	−3.78; 0 VRHE	0.33	~100%; 1 h; 0	0.32	95%	0.2 M Na2HPO4 (pH = 10); AM 1.5G	[137]
FTO/ACZTS/CdS/In2S3/Pt	−15.0; 0 VRHE	0.7	~50%; 3 h; 0	2.4	98%	0.2 M K2HPO4/KH2PO4 (pH = 6.85); AM 1.5G	[138]
FTO/CGZTS/CdS/In2S3/Pt	−11.1;	0.6	90%; 7000 s; 0	1.7	-	0.2 M K2HPO4/KH2PO4 (pH = 6.85); AM 1.5G	[139]

.

5. Conclusions

PEC water splitting, a promising method to generate hydrogen using renewable solar energy, still suffers from issues such as photostability, longevity, efficiency, recombination, and side reactions. The development of robust and energy-efficient photoabsorber films is of prime importance, along with the overall efficiency of PEC cells. Cu-based metal oxides/sulfides are cost-effective, earth-abundant, and prime candidates as p-type semiconductors in photocathodes. In this review, the latest Cu-based photocathodes for PEC water splitting have been outlined, along with the different photocathode materials modifications and the operational challenges associated with the visible-light-driven water splitting. In theory, the reduction potentials of water must be overlapped by the band gap energy levels for the water splitting to occur independently, and as a result, the photoinduced charge carriers must have the proper overpotential for the HER. Notably, the semiconductor must have an excellent absorption coefficient, and the Cu-based semiconductors constitute all of the currently used materials’ inexpensive and scalable photocathode semiconductors. Although they have not yet reached the theoretical maximum photocurrent, they are unstable because of the material degradation. It is critical to identify the source of the instability. The chemical stability at the surface of the photoelectrodes should be examined in great detail, and the surface of the photocathodes should be provided with photostable layers that can withstand harsh conditions. Using stable interlayers should avoid the direct contact of the layers with electrolytes that are unstable under light illumination conditions. The interlayers with a low parasitic light absorption that can act as a selective charge transporting layers must be investigated. To balance the light absorption and the parasitic light absorption, the thickness optimization of these interlayers, a passivation layer, and a photoabsorber layer with an excellent transparency will be an intriguing challenge.

Numerous primary and complex synthetic techniques for the Cu-based binary metal oxides with morphological control are known. Even though these oxides have a remarkable PEC conversion efficiency, their photostability is relatively poor. Meanwhile, ternary metal oxides are more resistant to photo corrosion than binary oxides, although their photocurrents obtained are by far pretty minimal, compared to the theoretical maximum photocurrent. Furthermore, these ternary oxides suffer from strong recombinations, leading to poor a PEC performance. The likelihood of the crystal structural alterations is more likely in the ternary metal oxides than in the binary metal oxides, which might be helpful in numerous ways, including increasing the photostabilities and reducing the recombinations. While the Cu-based metal sulfides are good light absorbers, sulfur leaching in the presence of light during the water splitting is a concern for photoactive material deterioration. However, ternary and quaternary metal sulfides have demonstrated a higher activity and the PEC efficiency than oxides. They fail the longevity tests for their application. The multi-step PEC water splitting process begins with a charge carrier generation, separation, and migration, which are crucial aspects regulating the fate of a specific PEC efficiency. The recombinations in the semiconductor layer restrict the charge production by the photoexcitation of the semiconductors. The Cu-based metal oxide/sulfides may be manufactured using novel manufacturing procedures to achieve longer diffusion lengths and lower recombination rates. Due to the striking improvement in the crystal structure, the electrical characteristics, and the photocurrent density in the PEC cells, the cation substitution in the Cu-based sulfides, such as CIGS, CZTS, CBTS, etc., has attracted enormous attention. Currently, the ions used in the replacements are either hazardous or costly. Earth-abundant, non-toxic cations must be investigated for the cation substitution in the crystal structure replacement. A novel synthesis and layer loading strategies over the photoactive layer must be investigated to provide the effective interaction between the layers for the efficient charge transfer and to minimize the charge transfer resistance.

The main concerns of photostability and the PEC efficiency can be overcome by engineering the interfaces in photoelectrode. Using electron transporting layers (ETLs) and hole transporting layers (HTLs), along with stable buffer layers to form efficient heterojunctions can be fruitful in developing a robust photoelectrode. The decoration of an optically transparent cocatalyst that can serve as a passivation layer and boost the kinetics of the surface reactions would be of great potential. The cocatalyst layer deposition can be advantageous in minimizing the side reactions and enhancing the faradaic efficiency. The study of the interaction between different layers, including the semiconductor layer and the cocatalyst with different rational configurations, such as core-shell structures and 3D porous structures with a minimum mismatch in the crystal lattice, energy levels, and electronic structure. In order to allow for an adequate light absorption and charge separation, semiconductor photocathodes must control their shape and semiconducting nature.

The development of highly active photocathode materials will continue, and it will not be long until a confident band gap design becomes a norm.