Recent Progress and Perspectives on Photocathode Materials for CO₂ Catalytic Reduction

Abstract

The continuous consumption of fossil energy and excessive emissions of carbon dioxide (CO₂) have caused a serious energy crisis and led to the greenhouse effect. Using natural resources to convert CO₂ into fuel or high-value chemicals is considered to be an effective solution. Photoelectrochemical (PEC) catalysis utilizes abundant solar energy resources, combined with the advantages of photocatalysis (PC) and electrocatalysis (EC), to achieve efficient CO₂ conversion. In this review, the basic principles and evaluation criteria, of PEC catalytic reduction to CO₂ (PEC CO₂RR), are introduced. Next, the recent research progress on typical kinds of photocathode materials for CO₂ reduction are reviewed, and the structure–function relationships between material composition/structure and activity/selectivity are discussed. Finally, the possible catalytic mechanisms and the challenges of using PEC to reduce CO₂ are proposed.

1. Introduction

With the increasing demand for energy in modern society, the crisis of fossil fuels (such as oil, natural gas, etc.) and the greenhouse effect have ensued [1,2,3,4,5]. Therefore, the need for the development of clean and renewable energy is urgent. Solar energy, as a kind of renewable, clean energy, has limitless potential [6]. Using solar energy converts carbon dioxide (CO₂) into fuel or high-value chemicals, such as carbon monoxide (CO), formic acid (HCOOH), methanol (CH₃OH), ethanol (CH₃CH₂OH), acetic acid (CH₃COOH), and so on, and is considered an effective means of solving the energy crisis and mitigating the greenhouse effect [7,8,9,10,11].

Photoelectrochemical (PEC) characteristics of semiconductor nanoparticles convert and store solar energy in the form of chemical bonds, making them an effective catalyst for the carbon dioxide reduction reaction (CO₂RR) [12,13]. Compared with photocatalysis (PC) [14,15] or electrocatalysis (EC) [16,17], PEC has obvious advantages in CO₂RR. For instance, PEC CO₂RR can not only promote the separation of photo-generated charges by an external voltage, efficiently overcoming the energy barrier and achieving higher solar energy conversion efficiency [18,19], but can also decrease the overpotential under the action of solar energy to save energy. In other words, the synergistic effects of light and the applied bias potential could jointly reduce the CO₂ activation energy and facilitate the reduction of CO₂ [20].

Generally, PEC CO₂RR includes three processes: (i) the generation of electron-hole pairs on the semiconductor under photo-excitation, and the adsorption of CO₂ on the surface of the catalyst; (ii) the separation and transfer of electron-hole pairs on the semiconductor; (iii) the surface catalytic reaction, including H₂O oxidation by holes and CO₂ reduction by electrons [21,22]. Some p-type semiconductor materials have been applied for PEC CO₂RR, such as CuO₂ [23,24,25], GaP [26,27,28], TiO₂ [29,30], etc. An ideal p-type semiconductor should meet certain conditions for its conduction band (CB) and valence band (VB) positions, such as the energy level of the CB being more negative than that of CO₂ reduction, and the bandwidth being within the range of light absorption. Although PEC has made significant progress in CO₂RR, there are still significant challenges ahead, mainly including further improving the current density and Faraday efficiency (FE), overcoming the high redox potential to activate CO₂ and break the C=O bond [31,32], and inhibiting the hydrogen evolution reaction (HER) competing with CO₂RR [33].

2. Basic Configuration of PEC Devices

PEC devices for CO₂ reduction are generally divided into two types: single chamber electrolytic cell and H-type electrolytic cell, which are closed containers made of Teflon and quartz glass, respectively.

As shown in Figure 1 [34], in the single-chamber cell, the working electrode, the counter electrode, and the reference electrode are in the same reaction compartment. When CO₂ is reduced, some of the reduction products (such as HCOOH, CH₃OH, etc.) can be transferred to the counter electrode and oxidized, resulting in a significant decline in the yield, which is one of the most fatal drawbacks of the single reactor.

In the H-type electrolytic cell, the working electrode and the reference electrode are in one compartment, the counter electrode is in another compartment, and the two compartments are separated by a proton membrane. Compared with the single chamber cell, the oxidation reaction occurs in the anode compartment, while the reduction reaction occurs in the cathode compartment, which can effectively prevent the reduction products from being oxidized and can significantly improve the yield of reduction products. However, the transfer of hydrogen protons in the H-type electrolytic cell will be weakened. The three-electrode system of H-type electrolytic cells usually includes the following four types: (1) photocathode (PC, generally p-type semiconductor) and dark anode (A, Figure 2a); (2) photoanode (PA, generally n-type semiconductor) and dark cathode (C, Figure 2b); (3) PA and PC (Figure 2c); (4) photovoltaic cells and electrodes containing electrochemical catalysts (Figure 2d). Each of the above four electrolytic cells also contains a reference electrode (RE), and the electrolyte can be pure water or a salt solution.

3. General Parameters to Evaluate the PEC CO₂ Reduction

The PEC CO₂RR efficiency on different catalysts can be evaluated by the following specific standards:

3.1. Faraday Efficiency (FE)

FE is one of the important standards to evaluate CO₂ reduction performance, which can be expressed as the ratio of the actual product yield to the theoretical product yield. FE is a more intuitive way to illustrate the selectivity of a catalyst, and its calculation formula is shown in Equation (1) [35,36,37]:

$$F{E}_P=\frac{I_P}{I_t}=\frac{n_P \left(\mathrm{mol}\right)\cdot N\cdot F \left(\mathrm{C}/\mathrm{mol}\right)}{Q \left(\mathrm{C}\right)}$$

(1)

In Equation (1): FE_p is the Faraday efficiency of p (p represents the reduction product of CO₂, such as HCOOH); I_p is the current density of p; I_t is the total current density. n_p is the amount of the substance; N is the number of electrons that are created for every mole of the corresponding substance. F is Faraday constant (F = 96,485 C/mol). Q is the total amount of charge (Q = I_t).

The higher the FE value, the better the selectivity of the catalyst for a specific product. Its value is influenced by multiple factors, such as temperature, electrolyte concentration, applied voltage, solution acidity, and even electrode material purity.

3.2. Solar to Fuel Efficiency (STF)

STF is the conversion efficiency of the PEC reduction of CO₂ system under light irradiation when the action potential between the working electrode and the counter electrode is zero [38,39]. There are two different calculation methods for STF, such as Equations (2) and (3) [35,38,40]:

$$STF=\frac{C_{fuel} \left(\mathrm{mmol}/\mathrm{s}\right)\cdot △G^0 \left(\mathrm{kJ}/\mathrm{mol}\right)}{P_{solar }\left(\mathrm{mW}/{\mathrm{cm}}^2\right)\cdot Area \left({\mathrm{cm}}^2\right)}$$

(2)

$$STF=\frac{J_{sc} \left(\mathrm{mA}/{\mathrm{cm}}^2\right)\cdot △E^0\left(\mathrm{V}\right)\cdot FE}{P_{solar} \left(\mathrm{mW}/{\mathrm{cm}}^2\right)}$$

(3)

In Equation (2): C_fuel is the amount of fuel produced per unit of time; ΔG⁰ is Gibbs free energy that converts CO₂ into fuel; P_solar is the light power density of incident light and Area is the effective area of the working electrode exposed by the incident light.

In Equation (3): J_sc is photocurrent density under short circuit conditions; ΔE⁰ is the thermodynamic potential of CO₂ converted to fuel; FE is the Faraday efficiency; P_solar is the light power density of incident light.

3.3. Current Density

Current density is the ratio of the tested current to the geometric area of the working electrode, as calculated in Equation (4), which is related to the yield of the product.

$$J_{cd}=\frac{I \left(\mathrm{A}\right)\cdot 1000}{Area \left({\mathrm{cm}}^2\right)}$$

(4)

In Equation (4): J_cd is current density (mA/cm²); I (A) represents the total current of the i-t test, and Area is the geometric area of the working electrode.

Based on the total current density and the FE of the corresponding product (such as HCOOH), the partial current density of the corresponding product can be calculated, as shown in Equation (5).

$$J_{pcd}=J_{cd}\cdot F{E}_P$$

(5)

In Equation (5): J_pcd is the partial current density of the corresponding product; FE_p is the Faraday efficiency corresponding to the product.

3.4. Linear Sweep Voltammetry Curve (LSV)

The onset potential and overpotential of the reaction can be obtained from the LSV curve, which is also an important performance for catalysts.

3.5. Incident Photon to Current Efficiency (IPCE)

IPCE is the conversion efficiency of the incident photon into photocurrent under irradiation of incident light with a certain wavelength, which is also called external quantum efficiency [41,42]. IPCE can be calculated by the ratio of the total energy of the converted electrons to the total energy of the incident photons, as shown in Equation (6).

$$IPCE=\frac{1240\cdot J_{sc} \left(\frac{\mathrm{mA}}{{\mathrm{cm}}^2}\right)}{\lambda \left(\mathrm{nm}\right)\cdot P \left(\frac{\mathrm{mW}}{{\mathrm{cm}}^2}\right)}$$

(6)

In Equation (6): J_sc is photocurrent density under short circuit conditions; λ is the wavelength of incident light; P is the light intensity of incident light at a certain wavelength.

3.6. Absorbed Photon-Current Conversion Efficiency (APCE)

APCE is the conversion efficiency of absorbed photons into the photocurrent under light irradiation, which can evaluate the recombination efficiency of photo-generated carriers on the semiconductors as shown in Equation (7) [38,43].

$$APCE=\frac{IPCE\left(\lambda \right)}{A\left(\lambda \right)}$$

(7)

In Equation (7): A(λ) is the absorbance.

3.7. Turnover Number (TON) and Turnover Frequency (TOF)

TON and TOF are used to characterize the catalytic activity and stability of the catalyst [44]. TON is the mole number of the conversion substrate divided by the mole number of the catalytic activity center, which can also be expressed as the conversion rate of the active site within a certain period. TOF is the number of conversions of a single active site per unit time. In the photoelectrochemical reduction of CO₂, TON and TOF are defined as follows (Equations (8) and (9)) [19,24]:

$$TON=\frac{n_{pro}}{n_{cat}}$$

(8)

$$TOF=\frac{n_{pro}}{n_{cat}\cdot t}$$

(9)

In Equations (8) and (9), n_pro is the mole number of the product; n_cat is the mole number of the catalyst; t is reaction time.

3.8. Tafel Slope

Tafel slope reflects the relationship between overpotential and current density. The smaller the Tafel slope, the lower the overpotential under the same kinetic current density, indicating better catalytic performance. Moreover, the Tafel slope is also often used to explain reaction mechanisms [45].

3.9. Reaction Stability

Stability testing is necessary because most semiconductors are unstable due to photo-corrosion. The amperometric i-t curve is generally used to describe the stability of the catalyst. In the test, when the current density decreases, the stability attenuates.

4. Photocathode Materials for CO₂RR

Generally, CO₂RR involves many proton and electron reactions which can produce various reduction products, including CO, CH₄, HCOOH, CH₃OH, CH₃COOH, CH₃CH₂OH, etc. To date, metal oxides, metal sulfides, phosphates, etc. have been reported as photoelectrochemical catalysts for PEC CO₂RR [46,47]. Moreover, a variety of strategies have been proposed to further improve the activity, stability, and selectivity of the catalyst in PEC CO₂RR, such as the modification of the co-catalyst, and the formation of the protective layer [48]. In this section, we mainly focus on the recent progress of representative photocathode materials to convert CO₂ into different C products, and their performance enhancement strategies.

4.1. Types of Catalysts Commonly Used in PEC CO₂RR

The development core of PEC technology lies in the development of highly active photoelectric-catalysts, and there are many types of catalysts reported so far for CO₂ reduction. Among them, metal/oxides (Pt, Au, Fe₂O₃, CuFeO₂, Cu₂O, TiO₂, etc.), metal sulfides (CuS, etc.), metal phosphates (GaP, InP, etc.), MOF-based materials (ZIF9-Co₃O₄, etc.), and non-metallic carbon materials (g-C₃N₄, etc.) have been widely studied. To overcome the overpotential for CO₂ reduction and drive the reaction with sufficient electrical potential, the conduction band edge of the semiconductor should be much more negative than the potential for CO₂ reduction. Due to the sufficient negative CB positions, II-VI group materials in the periodic table have received extensive research in the early stages, mainly focusing on the properties of quantum dot materials such as CdTe, ZnTe, CdSe, and CdSeTe. These catalysts typically also have appropriate band gap width and band guide position, which are beneficial for absorbing visible light and reducing CO₂. In IV group materials, Si has good absorption of ultraviolet light, visible light, and even infrared light. Although the bandgap position of Si satisfies the multi-electron reduction potential of CO₂RR, it is easily oxidized and the recombination probability of photogenerated carriers is high, making it often used as a substrate. Apart from Si-based catalysts, GaP, InP, Cu₂O, and CuFeO₂ are also usually adopted as photocathode substrates.

Based on different semiconductors and reaction systems, the products of PEC CO₂RR also gradually shift from C₁ products (CH₄, CH₃OH, CO, HCOOH, etc.) to C₂ products (C₂H₄, CH₃CH₂OH, CH₃COOH, etc.) and multicarbon (C₂₊) products (acetone, polyols, etc.).

Table 1. Summary of the catalysts or active sites for PEC CO₂ reduction to different products.

Products	Cocatalysts or Active Sites
H2	Pt, Ni, Si, etc.
CO	Au, Ag, Pt-TiO2, CuAu3, AgP2, Cu(btc)3, ZnO, ZnTe, etc.
HCOOH	Sn, Bi, In, CuS, RuCt, FDH enzyme, CuFeO2, Co3O4, etc.
CH3OH	TiO2, TiO2-Cu+, O-doped Cds, etc.
CH4	GaN/Cu, GaN/CuFe, Cu-Ag, etc.
CH3COOH	CuFeO2, etc.
C2H4 (g)	Cu, Ag/Cu2O in AcCN, etc.
CH3CH2OH	Carbon/Cu2O composite, N, O-doped Cds, g-C3N4, etc.

summarizes the representative catalysts or active sites for PEC CO₂ reduction according to the types of CO₂-reduced products [49]. Similar to the situation in electrocatalytic CO₂ reduction, Au, Ag-based and Bi, Sn, In-based materials are excellent catalysts for the photocatalytic reduction of CO₂ to CO and HCOOH in aqueous electrolytes, respectively. The highest FEs for PEC CO and HCOOH production, using these metal-based catalysts, are close to 100%. For example, Fe-based catalysts have been studied extensively for CO₂RR. The element dopants, together with the local coordination environment of the Fe center, play remarkable roles in affecting the electronic structures and catalytic performances [50]. The reported Fe-based catalysts mainly produce CO with high Fes up to 90% [51,52,53]. However, there are few reports about the selective formation of multicarbon (C₂₊) products over Fe-based catalysts, due to their low FE and low current density. As a comparison, though Cu-based electrocatalysts exhibit unique advantages in electrocatalytic CO₂ reduction, as they can reduce CO₂ to various carbon products (CH₃OH, CH₄, or C₂₊ chemicals) through multiple electron transfer reactions, it is still a huge challenge to improve the selectivity of a single product, due to the complex and diverse properties. Biological catalysts, including molecular catalysts and enzymes, are also applied in PEC CO₂ reduction, where the corresponding photocathodes have achieved a record FE of 100% for CO or HCOOH production [54]. In recent years, graphitic carbon nitride (g-C₃N₄), as a non-metallic semiconductor material, has shown broad application prospects in CO₂ reduction research, due to its excellent catalytic performance, which comes from its sufficiently negative CB. Research has shown that pyridine N enriched g-C₃N₄, as an active site, can contribute to C-C coupling in PEC CO₂RR to produce CH₃CH₂OH. In addition, vacancy-rich TiO₂ [55], oxygen-doped CdS, CuFeO₂, and carbon/Cu₂O composites are also explored as active sites for PEC CO₂ reduction to produce CH₃OH, CH₃COOH, or CH₃CH₂OH, but most of them showed low selectivity for their primary products during the PEC CO₂ reduction process. To sum up, it remains a significant challenge for electrocatalytic CO₂RR to produce C₂₊ products efficiently over non-copper catalysts, and there is a lack of photocathodes using metal-based or biological co-catalysts for PEC CO₂ reduction to produce CH₃OH, CH₄, or C₂₊ chemicals with high FEs (>90%).

4.2. Strategies for Enhancing the Performance of Photocathodes in PEC CO₂RR

In recent years, numerous achievements have been made in the research of photocatalytic CO₂ reduction in photocathode materials. Despite significant progress, there are still many challenges in the efficient reduction of the CO₂ reaction process. This review is based on the aforementioned PEC CO₂RR process and briefly elaborates on the basic strategies for enhancing the efficiency of PEC CO₂RR from three perspectives: light absorption, carrier separation, and surface reaction kinetics.

4.2.1. Improving Light Absorption

The light absorption capacity of semiconductors is closely related to their band structure, therefore, optimizing the band structure of semiconductors is a major strategy for enhancing the absorption performance of photocathodes. In addition, loading light absorbing material onto catalysts is an effective method to enhance their light absorption performance.

Self-doping or doping with other elements can alter the band structure of catalytic materials, thereby improving their light absorption performance. At the same time, the conductivity of the photocathodes is greatly improved, and the current density increases. With regard to PEC systems, there are only a few studies on the doping of photoelectrodes, including Mg-doped CuFeO₂ [56], B-doped C₃N₄ [57], N-doped ZnTe [58], and S, N-co-doped nanoporous carbon [59] photocathodes for PEC reduction of CO₂.

Plasma metal nanostructures can also improve the efficiency of light energy collection and energy conversion, and have been widely used in semiconductors [60]. Under the action of an incident light wave electric field, the outer free electrons of metal nanoparticles are polarized and move, generating a new electric field that applies a linear internal restoring force within the original system. This type of electronic dipole oscillation, limited to the interior of metal nanoparticles, is known as localized surface plasmon resonance (LSPR). Common precious metals with LSPR effects include Au, Ag, Pt, etc., while non-precious metals mainly include Cu. In catalytic reactions, the abovementioned metal cocatalysts exhibit excellent performance in PEC CO₂RR.

Incorporating dye molecules and quantum dots into the catalyst can enhance its light absorption ability. Xu et al. [30] utilized multifunctional TiO₂ thin films as photocathodes to convert CO₂ into CO, and lower CH₃CH₂OH through PEC. The light absorption ability of the photocathodes is improved by combining the dye molecule, Eosin Y Disodium, with TiO₂, and Pd nanoparticles and amine ligands are introduced to capture protons and CO₂, respectively.

4.2.2. Enhancing the Separation Efficiency of Photogenerated Carriers

In the process of PEC CO₂ reduction, a large amount of recombination of photogenerated carriers occurs on the catalyst body and surface, greatly reducing the catalytic reaction efficiency. Therefore, enhancing the separation efficiency of photogenerated carriers is an important means of improving the photoelectrocatalytic efficiency.

Constructing homo/heterojunctions is considered a highly effective strategy for improving the separation of photogenerated electrons and holes, as well as enhancing the selectivity for specific products in PEC CO₂ reduction [61]. The heterojunction photocathode is usually composed of two or more semiconductors with alternating band structures, such as ZnO/ZnTe [62], CdTe/ZnTe [63], CuO/Cu₂O [64], and CuFeO₂/CuO [65], etc.

4.2.3. Accelerating Surface Reaction Kinetics

Interface reaction is a crucial step in the PEC process. As mentioned above, after the generation and separation of photogenerated carriers, they ultimately need to be transferred to the surface of the catalyst to participate in the redox reaction. For the photoelectric reduction of the CO₂ system, when the photogenerated carriers reach the catalyst surface, electrons participate in the CO₂RR, and holes participate in the oxidation reaction of H₂O. Research has shown that strengthening the interface reaction process is crucial for improving the efficiency of PEC CO₂RR.

Controlling the surface morphology of catalysts is an effective way to accelerate interfacial reactions. By adjusting the surface morphology of the catalyst, it has a high specific surface area and exposes a specific crystal plane, thus promoting the absorption of CO₂ at its surface active site. In addition, precise control of the morphology of the co catalyst can be an effective method to ensure illumination on the photocathode.

Modifying semiconductor surfaces, using different techniques, is another effective strategy for accelerating interface reactions. The sluggish reaction kinetics of CO₂ reduction, as a result of the high activation energy or overpotential, largely impedes the reaction process. By using techniques such as chemical etching, electrochemical treatment, or chemical vapor deposition (CVD), catalyst deposition (noble/non-noble metals, MOFs [66], molecular complexes [67], polymers, carbon materials [68]), and passivation with thin protective materials on the surface of photocathodes, the surface of photocathodes can be functionalized, which help to accelerate interface reaction kinetics (accelerate multi-electron reactions, reduce reaction overpotential, and promote the transfer of photogenerated charge carriers to the catalyst surface). In some cases, surface catalysts can also improve the stability of the photoelectrodes by rapidly consuming the surface charge carriers. Furthermore, specific functional catalysts can increase the selectivity for the desired products by manipulating the adsorption strengths of certain reaction intermediates.

4.3. Representative Photocathode Materials and Their Performance Enhancement Strategies

4.3.1. Zinc (Zn)-Based Photocathodes

Zinc-based photocathodes, such as ZnTe and ZnO, used in PEC CO₂RR, mainly produce CO. ZnTe is a p-type semiconductor with a direct bandgap of 2.3 eV, and the conduction band position of ZnTe is relatively negative, which is conducive to transport interface electrons [69,70]. Jang et al. [71] first reported a new Zn/ZnO/ZnTe photocathode for PEC CO₂RR (Figure 3a). An obvious photoelectric reaction can be observed on the Zn/ZnO/ZnTe photocathode, and under the Hg lamp (with >420 nm cutoff filter), the photocurrent density of the Zn/ZnO/ZnTe photocathode approaches 20 mA·cm⁻² at −0.7 V vs. the reversible hydrogen electrode (RHE) (Figure 3b). Compared with Zn/ZnO, Zn/ZnO/ZnTe showed significantly improved visible light absorption performance, which is consistent with the result that the light absorption edge can be extended to approximately 570 nm (Figure 3c) [62]. However, due to the competitive reaction of HER, the FE of CO at −0.7 V vs. RHE is as low as 22.9%, and the stability of the Zn/ZnO/ZnTe photocathode also needs to be enhanced. To further improve its performance, Jang et al. coupled Au nanoparticles with ZnTe/ZnO/Zn (ZOZT) to obtain a ZOZT-Au photocathode, as shown in Figure 3d. ZOZT-Au not only further enhanced photocurrent density, but also improved the FE of CO to 58% at −0.7 V vs. RHE, in comparison with ZOZT (Figure 3e–f).

This work offers an effective strategy by using a highly conductive metal substrate and heterojunction structure, which can improve the current density and facilitate the separation of photogenerated carriers. In particular, the deposition of Au nanoparticles could further improve the selectivity for CO and inhibit the competitive H₂ evolution, as well as promote the electron transfer by forming a Schottky junction with ZnTe at the interface. In addition, Au enhances the stability of the electrode. Coupling metal electrocatalysts with semiconductors or forming heterojunctions between semiconductors can effectively increase current density and promote carrier separation, which is worth learning in the design of optoelectronic materials.

4.3.2. Cobalt (Co)-Based Photocathodes

The most common catalyst in Co-based photocathodes is Co₃O₄, which is also a semiconductor material with a band gap of 2.07 eV and a conduction band of −1.05 eV [72,73], and the appropriate band gap width and band guide position are beneficial for absorbing visible light and reducing CO₂. Zhao et al. carried out a systematic study on Co₃O₄ for PEC CO₂RR. Firstly, the hierarchical porous Co₃O₄ (HA-Co₃O₄) was designed for PEC CO₂RR to HCOOH [74], which showed the top micro-flowers and the bottom rhombus structure (Figure 4a–c). This typical hierarchical structure added the active site of photocatalysis and electrochemistry, making the photocurrent density on the HA-Co₃O₄ electrode much higher than that on the Co₃O₄ rhombus nanorod electrode (NR-Co₃O₄) and the Co₃O₄ nanoparticle-coated electrode (NP-Co₃O₄) (Figure 4d), and the HCOO⁻ yield reached 384.8 ± 7.4 μmol in 8 h, which is higher than the yield of pure EC, PC and EC + PC (Figure 4e). Secondly, the metal Cu nanoparticles were modified on the Co₃O₄ nanotube arrays to form a metal/metal oxide composite material (Cu-Co₃O₄) for PEC CO₂RR (Figure 4f) [75]. The Co₃O₄ nanotube arrays obtained by in situ growth in cobalt foil, through anodizing technology, can not only reduce the resistance between the substrate and the nanotube arrays, but also provide more active sites to promote light absorption. (Figure 4g). In addition, Cu nanoparticles can also promote the adsorption of reducing intermediates, thereby improving the selectivity of formic acid, resulting in a yield of 6.75 mmol·L⁻¹·cm⁻² of HCOOH in PEC CO₂RR, with selectivity approaching 100% within 8 h (Figure 4h). The successful application of Cu-Co₃O₄ in PEC CO₂RR once again proves that the coupling between metal and semiconductor is a very desirable design in PEC CO₂RR.

The composite Ru(bpy)₂dppz-Co₃O₄/carbon aerogel (Ru(bpy)₂dppz-Co₃O₄/CA) (Figure 5a), prepared by solvothermal reaction and droplet casting method, also showed superior PEC CO₂RR performance [72]. The photocurrent response (Figure 5b) and the yield of HCOO⁻ (Figure 5c) on the sample Ru(bpy)₂dppz-Co₃O₄/CA is significantly higher than that of Co₃O₄ and Co₃O₄/CA, such as the FE of HCOO⁻ is 86% at −0.6V vs. normal hydrogen electrode (NHE). Such a good activity is mainly ascribed to the following aspects: (1) carbon aerogel has rich micropores and a large specific surface area, which can enhance the adsorption of CO₂; (2) Ru(bpy)₂dppz can be used as an electron transfer medium and CO₂ activator, and the synergistic effect between bpy and dppz can also effectively achieve the rapid transfer of electrons. It is believed that modifying the metal/oxide catalysts with organic molecules or linkers, to construct a microenvironment for CO₂ activation and reduction, will be a new strategy to regulate the photocathode activity or product selectivity.

Additionally, the Co₃O₄ nanowires (Co₃O₄ NWs), combined with Co-MOF (ZIF9), are conducive to the adsorption, and the capture of CO₂ has also been applied in PEC CO₂RR (Figure 5d) [76]. Under light illumination, the yield of formic acid is 578.7 mol·L⁻¹·cm⁻² and the FE of HCOOH is 70.6% in 8 h at −0.9V vs. saturated calomel electrode (SCE) (Figure 5e). DFT theoretical calculation results showed that when CO₂ was absorbed into the micropores of ZIF9, the structure of linear CO₂ would change and the bond angle would reduce, due to bending, which could promote the activation of CO₂ (Figure 5f–g). It has been concluded that the structure of Co₃O₄ NWs can enhance light capture, while ZIF9 can improve the adsorption performance of CO₂. Moreover, the p-p type heterojunction between Co₃O₄ NWs and ZIF9 is highly beneficial in promoting the separation of photogenerated electron-hole pairs, showing great potential in CO₂ reduction. In brief, the research progresses on Co-based photocathodes has paved the way for the development of non-precious metals to achieve PEC reduction of CO₂.

4.3.3. Silicon (Si)-Based Photocathodes

Si, as a potential p-type semiconductor, has a narrow energy bandgap and is widely used in PEC. Moreover, Si is abundant on Earth and has great industrial value [77]. Bradley et al. first reported p-Si photocathode materials in 1982 [78]. In PEC CO₂RR, p-Si is usually used for substrates and compounded with metal or other semiconductor catalysts. This is because, although Si has strong light capture ability, its CO₂ reduction activity is weak and it has high selectivity for hydrogen. It has been reported that Si photoelectrodes modified with nanoporous Au thin film mesh, formed by electrochemical reduction of the anodized gold thin film, can be used for PEC CO₂RR (Figure 6a) [77]. Compared with pure Si photoelectrodes, when the potential is greater than 1.1 V vs. RHE, the photocurrent density of Si photoelectrodes, with nanoporous Au thin film mesh, is reduced, but the initial potential shifts to a positive potential (Figure 6b). Additionally, the FE of CO on Si photoelectrode, with reduced anodic Au (w/RA Au), was 91% at −0.03 V vs. RHE, which is higher than that of pure Si and Si photoelectrode with untreated Au (Figure 6c). The excellent PEC CO₂RR of the w/RA Au was ascribed to the Au co-catalyst and p-n heterojunctions formed on Si, which jointly facilitated the separation of the photogenerated carrier.

In addition, Choi et al. [79] reported that Ag-assisted etchants were mixed with HF and H₂O₂ to form Si nanowires by etching the Si surface and combined with co-catalyst Sn to convert CO₂ into HCOOH through PEC CO₂RR (Figure 6d). Under AM 1.5G light (100 mW·cm⁻²), p-Si wire arrays have higher photocurrent density, more positive onset potential, and better stability. As the etching time increases, the photocurrent first increases and then remains unchanged (Figure 6e). The length of nanowires and the coupling of Sn have an effect on the selectivity of PEC CO₂RR (Figure 6f). In addition, the FE for HCOOH in the H-type electrolytic cell reached 88.4%, which is higher than that in the single-chamber electrolytic cell. Although p-Si wire arrays can promote photon absorption and carrier separation, due to their high geometric optical path and low reflectivity, the preparation process generally requires highly corrosive hydrofluoric acid or complex etching technologies, making it unsuitable for large-scale operation.

A p-Si/metal oxide (nitrides)/metal composite was also used for PEC CO₂RR. Sheng et al. [80] photo-deposited ZnO and Cu on the GaN/n⁺-p Si substrate to convert CO₂ to syngas using PEC, in which the FE of CO was as high as 70% at −0.2 V vs. RHE in CO₂-saturated 0.5 M KHCO₃ solution (pH = 8) (Figure 7a–b). The photocurrent density and FE of CO have no obvious attenuations at −0.2 V vs. RHE under illumination for 10 h (Figure 7c), indicating their high stability, mainly ascribed to the photoabsorption of p-n Si heterojunction, effective electron extraction by GaN, and the fast surface kinetics of the Cu-ZnO co-catalyst. LSV results show that Cu-ZnO/GaN/n⁺-p Si has the highest photocurrent density among all presented samples (Figure 7d), which could be attributed to the ZnO as a cocatalyst to absorb and activate CO₂ molecules, leading to the bend of the C=O bond and the decrease in CO₂ chemical stability (Figure 7e). In the Cu-ZnO/GaN/n⁺-p Si photocathode, Si serves as the substrate for light capture, and GaN promotes the separation of photogenerated carriers. Combined with the Cu-ZnO cocatalyst, PEC reduction of CO₂ can be effectively achieved, which demonstrates the combination of photocatalysts and electrocatalysts, fully achieving the synergistic effects of photo and electricity in material design.

4.3.4. Iron (Fe)-Based Photocathodes

Fe-based photocathode materials mainly include CuFeO₂ and Fe₂O₃. Among them, the bandwidth of CuFeO₂ is 1.5~1.6 eV, which is beneficial to photon absorption. Furthermore, CuFeO₂ has the structure of delafossite, and it has been reported that the conductivity of the delafossite structure can be improved by adding trivalent or bivalent metals [56,81]. For example, Gu et al. [56] synthesized Mg-doped CuFeO₂ by traditional solid-state methods, which showed a high photocurrent density. However, the FE of HCOO⁻ is as low as 10% on the photocathode Mg-doped CuFeO₂ at −0.9 V vs. SCE, due to the serious hydrogen evolution competitive reaction. Interestingly, CuFeO₂ and CuO mixed p-type catalysts were prepared to improve the PEC CO₂RR activity, and the selectivity of HCOO⁻ is close to 90% under simulated light irradiation [65]. More importantly, the activity can last for a week. Moreover, the onset potential of CuFeO₂/CuO is +0.9 V vs. RHE, which is a very positive potential in PEC CO₂RR.

The bandwidth of transition metal oxide Fe₂O₃ is 2.20 eV, therefore it has good visible light absorption performance [82]. Compared with the pure TiO₂ nanotubes, the TiO₂ nanotubes modified with Fe₂O₃ (Fe₂O₃/TiO₂ NTs) not only expanded the optical absorption region (Figure 8a) but also achieved good PEC CO₂RR for HCOOH [83]. At an incident wavelength of 500 nm and a bias voltage of −1.0 V vs. RHE, the selectivity and yield of HCOOH can reach 99.89% (Figure 8b) and 74,896.13 nmol·h⁻¹·cm⁻² (Figure 8c), respectively. As shown in Figure 8d, Fe₂O₃/TiO₂ NTs have an embedded heterojunction, and electrons in the VB of TiO₂ gradually transfer to the CB of Fe₂O₃ under incident light irradiation_. Meanwhile, the Fe₂O₃ can absorb CO₂ to form CO₂^·− (ads) and further obtain [Fe^IIICOOH]²⁺ under the effect of electrons and H₂O. Finally, the HCOOH was formed during the transformation process of O-Fe^III and O-Fe^II. Since Fe-based photocathodes are beneficial for future large-scale applications, the development of efficient non-noble-metal-based co-catalysts should receive much more attention.

4.3.5. Copper (Cu)-Based Photocathodes

Among the non-noble metals, many recent studies on CO₂ reduction have primarily focused on designing Cu-based materials due to their high abundance, low toxicity, unique catalytic activity, and good stability. There are two main forms of copper oxides, CuO and Cu₂O. Cu-based photocathodes, especially Cu₂O, are the most common photoelectrochemical catalysts, due to their good light absorption performance [84,85]. The bandwidth of Cu₂O is 1.9–2.2 eV, and the CB position is conducive to the reduction of CO₂ [86]. Additionally, the maximum photocurrent density of Cu₂O can reach 14.7 mA·cm⁻² under the illumination of AM 1.5 G light [87]. However, the biggest disadvantage of Cu₂O is its susceptibility to light corrosion. Therefore, heterojunction and core–shell structure, etc. are often used to effectively avoid photo-corrosion. Additionally, CuO has also a narrow bandwidth of 1.3–1.6 eV, leading to a high absorption coefficient in the solar spectrum [88]. Moreover, Cu-based photocathodes have complex and diverse properties, thus it is challenging to improve the selectivity of a single product.

The CuO-Cu₂O nanorod arrays, as a photocathode, have been prepared on a Cu substrate by a thermal oxidized and electrodeposition process for the reduction of CO₂ (Figure 9a) [64]. By changing the time of Cu₂O electrodeposition, the FE of CH₃OH can reach 94–96% on the sample CuO-Cu₂O nanorod. Under continuous illumination, the photoexcited electrons can transfer from Cu₂O to CuO, or directly to CO₂ adsorbed on the surface of the catalyst, improving the photoelectrochemical performance (Figure 9b). Later, Li et al. prepared Fe₂O₃ nanotubes modified with double-layer Cu₂O spheres (Cu₂O/Fe₂O₃ NTs) by electrodeposition method for PEC CO₂RR (Figure 9c) [89]. Cu₂O/Fe₂O₃ NTs-30 showed a three-layer structure: the bottom layer was Fe₂O₃ nanotubes, the middle layer was Cu₂O spheres of 200–500 nm, and the top layer was Cu₂O spheres with a larger diameter (Figure 9d,e). This novel three-layer structure successfully converted CO₂ to CH₃OH in 6 h and the FE of CH₃OH was as high as 93% at −1.3 V vs. SCE, which was higher than Cu₂O/Fe and Fe₂O₃ NTs (Figure 9f). The efficient conversion of CO₂ into CH₃OH is mainly because the novel three-layer structure has a conduction band position suitable for CO₂ reduction and the synergy between PC and EC can promote electron transfer (Figure 9g). Further focus on the effect of Fe:Cu ratio on the product distribution over Cu₂O/Fe₂O₃ will be more helpful in exploring its reaction kinetics and product selectivity.

Recently, Kang et al. [90] prepared a hierarchically structured photocathode material Cu₂O/TiO₂ by controlling the synthesis temperature, atmosphere and pressure (Figure 10a). The single-phase Cu₂O could be obtained by analyzing the chemical potential and accurately calculating the Gibbs free energy of CuO, Cu₂O, and Cu (Figure 10b,c). In PEC CO₂RR, TiO₂ can protect Cu₂O from corrosion, but the photogenerated electrons, on the CB of Cu₂O, directly participate in CO₂RR through the TiO₂ layer (Figure 10d,e). Interestingly, when the thickness of the TiO₂ layer is higher than 5 nm, TiO₂ and Cu₂O mainly form a p-n heterojunction and the photogenerated electrons on Cu₂O can transfer to TiO₂. Therefore, whether TiO₂ participates in the transfer of electrons mainly depends on its thickness. Through the ingenious design of fewer than 5 nm thick TiO₂ and pure phase cuprous oxide composite material, the stability and photoactivity have been greatly improved compared with pure Cu₂O or only deposition TiO₂ on the pure Cu₂O but not thermodynamically programmed calcination (Figure 10f). The selectivity of CH₃OH exceeds 90% on the Cu₂O/TiO₂ with 5 nm TiO₂ layers (Figure 10g), effectively demonstrating that the products based on Cu-based photocathodes achieve high selectivity. Nevertheless, this synthesis method is relatively complex, which is not conducive to large-scale promotion.

4.3.6. g-C₃N₄-Based Photoelectrodes

g-C₃N₄ is a polymer semiconductor with a bandwidth of 2.7 eV, which has high chemical stability under visible light irradiation, high reduction capacity, and sensitivity to visible light [91,92]. Recently, Nobuhiro Sagara et al. [57] prepared B-doped g-C₃N₄ (BCN_x) for PEC CO₂RR to CH₃CH₂OH. Although g-C₃N₄ is an n-type semiconductor, B-doped g-C₃N₄ is a proven p-type semiconductor. Compared with the pure g-C₃N₄, the potentials of the CB and VB of the sample BCN_x are significantly increased, but the potential of the CB is still sufficient to reduce the CO₂ (Figure 11a). It has been found that the photocurrent response of the sample BCN_x could be improved after doping B (Figure 11b). Moreover, the photocurrent response on the different co-catalyst (Au, Ag, Rh)-loaded BCN_3.0 could be further improved (Figure 11c). Rh or Au-loaded BCN_3.0 show a higher number of products than BCN_3.0 (Figure 11d), but the FEs of CH₃CH₂OH on Rh-loaded BCN_3.0 (36%) and Au-loaded BCN_3.0 (47%) are lower than Ag-loaded, B-doped BCN_3.0 (73%) and BCN_3.0 (78%).

The composite g-C₃N₄ and ZnTe, as a photocathode, can form a type II heterojunction and convert CO₂ to CH₃CH₂OH [93]. Through the analysis of the UV-vis absorption spectra, the composite g-C₃N₄/ZnTe presents similar visible light adsorption performance and bandwidth to pure ZnTe (Figure 12a). According to PL spectra, it has been found that the peak intensity of the sample g-C₃N₄/ZnTe is significantly lower than ZnTe, indicating that the g-C₃N₄/ZnTe has a lower electron-hole recombination rate (Figure 12b). The yield of CH₃CH₂OH on the heterojunction g-C₃N₄/ZnTe is 17.1 μmol cm⁻²·h⁻¹ at −1.1 V vs. Ag/AgCl, and the competitive hydrogen evolution is efficiently suppressed (Figure 12c), which is ascribed to the ZnTe with high CO₂ adsorption capacity, and g-C₃N₄ rich in pyridine N contributes to C-C coupling. Moreover, a pipeline mechanism has been proposed to explain the PEC CO₂RR process (Figure 12d), consisting of three steps: the adsorption and activation of CO₂, the formation of CO intermediate, and the adsorption of CO intermediate on pyridine N. DFT theoretical calculations indicated that pyridine N sites were very important for the adsorption of intermediates and the formation of ethanol. The results of the studies show that polymer semiconductors, and metal loads used for the construction of heterojunctions, are also effective for PEC CO₂RR. It is expected that advanced film preparation methods will further improve the film quality of g-C₃N₄/ZnTe photocathodes to achieve effective charge separation, which will enhance the activity of PEC CH₃CH₂OH production.

4.3.7. Titanium (Ti)-Based Photoelectrodes

The main structures of TiO₂ are rutile and anatase, with band gaps of 3.2 eV and 3.0 eV, respectively [94]. TiO_2, as the most popular semiconductor, is very stable under ultraviolet light and can be used as a photoanode or a photocathode. However, TiO₂ shows a weak adsorption performance in the visible range. Cardoso et al. [95] reported that Ti/TiO₂ nanotubes (TiO₂NT) modified ZIF-8 as a photocathode for the PEC CO₂RR (Figure 13a). It has been found that pure ZIF-8 absorbs very little visible light, while the Ti/TiO₂NT-ZIF-8 shows an enhanced visible light absorption performance and reduced bandwidth, compared with Ti/TiO₂NT, implying an interaction between nanotubes of Ti/TiO₂NT and ZIF-8 (Figure 13b,c). In the PEC CO₂RR, the selectivity to CH₃CH₂OH of the sample Ti/TiO₂NT-ZIF-8 increased significantly, and its conversion rate increased by nearly 430 times in 1 h at −0.7 V vs. Ag/AgCl (Figure 13d). Compared with PC CO₂RR, the conversion of CO₂ significantly improved in the PEC CO₂RR, owing to the synergy of light and electricity (Figure 13e). Moreover, ascorbic acid (AA) as a sacrificial agent was also studied in the PEC CO₂RR [96]. Sacrificial agents, as donors of electrons, can promote the conversion of CO₂ and increase CH₃CH₂OH conversion, and the FE of CH₃CH₂OH reached 86% on the sample Ti/TiO₂NT and ZIF-8 in the presence of AA (Figure 13f). The reduced N sites of imidazolate groups in ZIF-8 can enhance the adsorption of CO₂ and form carbamate to promote CO₂ conversion. Meanwhile, imidazole compounds can act as electron acceptors to enhance light absorption, and thus improve photocurrent response. Such a combination of titania with MOF materials shows great potential in PEC CO₂RR. Additionally, the results show that adding a sacrificial agent to the reaction is also an effective way to improve the selectivity of the product.

The products and performance of different representative catalysts mentioned above were summarized in

Table 2. Summary of products and PEC CO₂ reduction performance of different representative photocathodes.

Category	Product	Catalyst	Current Density	FE (%)	Ref.
Zi-based	CO	ZnTe/ZnO (ZOZT)	−7.89 mA·cm−2 (at −0.7 V vs. RHE)	7.2	[62]
	CO	ZOZT-Au	−15.97 mA·cm−2 (at −0.7 V vs. RHE)	58.0	[62]
Co-based	HCOOH	Cu-Co3O4 NTs	−0.122 mA·cm−2 (at −0.9 V vs. NHE)	-	[75]
	HCOOH	Ru(bpy)2dppz-Co3O4/CA	8.1 mA·cm−2 (at −0.6 V vs. NHE)	86	[72]
	HCOOH	ZIF9-Co3O4	1 mA·cm−2 (at −0.9 V vs. SCE)	70.5	[76]
Si-based	CO	Si-w/RA Au	−2.94 mA·cm−2 (at −0.6 V vs. RHE)	96	[77]
	HCOOH	p-Si/Sn	~−1 mA·cm−2 (at −0.6 V vs. RHE)	88.4	[79]
	CO	Cu-ZnO/GaN/n+-p Si	~−2 mA·cm−2 (at +0.07 V vs. RHE)	70	[80]
Fe-based	HCOOH	CuFeO2/Mg	−1 mA·cm−2 (at −0.9 V vs. SCE)	10	[56]
	HCOOH	CuFeO2/CuO	−0.2 mA·cm−2 (at +0.9 V vs. RHE)	90	[65]
Cu-based	CH3OH	CuO-Cu2O nanorod	0.25 mA·cm−2 (at −0.2 V vs. SHE)	94–96	[64]
	CH3OH	Cu2O/Fe2O3 NTs	~15 μA·cm−2 (at −0.6 V vs. SHE)	93	[89]
g-C3N4-based	CH3CH2OH	BCN3.0	~5 μA·cm−2 (at −0.4 V vs. Ag/AgCl)	78	[57]
	CH3CH2OH	BCN3.0/Ag	~10 μA·cm−2 (at −0.4 V vs. Ag/AgCl)	73	[57]
Ti-based	CH3CH2OH	Ti/TiO2 NT-ZIF-8	~−1.5 mA·cm−2 (at −0.7 V vs. Ag/AgCl)	86	[96]

.

5. Possible PEC CO₂RR Mechanisms

CO₂ has stable chemical properties and low reactivity; thus, high energy is required to realize the conversion of CO₂^.. Moreover, CO₂RR is also a complex process involving the coupling of many protons and the electron transfer processes. Using different catalysts and controlling the reaction conditions in the PEC CO₂RR, different products could be generated. Generally, the conversion of CO₂ to C₁ and C₂ products is easier than other complex hydrocarbon products. Up to now, though the mechanism of CO₂RR is not particularly clear, many possible reaction pathways have been proposed [21].

Table 3. The CO₂ reduction potential in aqueous solutions for the production of different hydrocarbon fuels [97].

Possible Half-Reaction Electrochemical CO2 Reduction	Electrode Potentials (V vs. SHE) at pH 7
CO2 (g) + e− → *COO−	−1.90
CO2 (g) + 2H+ + 2e− → HCOOH	−0.61
CO2 (g) + 2H+ + 2e− → CO (g) + H2O (l)	−0.53
CO2 (g) + 4H+ + 2e− → HCHO (l) + H2O (l)	−0.48
CO2 (g) + 6H+ + 6e− → CH3OH (l) + H2O (l)	−0.38
CO2 (g) + 8H+ + 8e− → CH4 (g) + 2H2O (l)	−0.24
CO2 (g) + 12H+ + 12e− → C2H4 (g) + 4H2O (l)	0.06
2CO2 (g) + 12H+ + 12e− → CH3CH2OH (l) + 3H2O (l)	0.08
2CO2(g) + 14H+ + 14e− → CH3CH3 (g) + 4H2O (l)	−0.27

provides the possible half-reactions of electrochemical CO₂ reduction and the electrochemical potential of thermodynamically stable CO₂ molecules into C₁, C₂ and C₃ hydrocarbon molecules. Wherein, the CO₂ molecule combines with an electron to form anion radical intermediate CO₂^·−, which is generally regarded as the rate-limiting step, since this step needs to overcome a large redox potential, which is much higher than that required by other processes.

At present, researchers have explored CO₂ reduction pathways using elemental catalysts, especially metals, and proposed possible mechanisms of CO₂ reduction in the aqueous solution, as shown in Figure 14 [97]. In the activation process, the straight CO₂ molecule can bend as it gains electrons. Additionally, different catalysts have different adsorption strengths for the intermediate and can stabilize different intermediates, thus producing various products, which is essential for the efficient conversion of CO₂. The main products of CO₂RR on the transition metals are CO and HCOOH, and the binding energy of the first intermediate and the catalyst influences the final product. When the intermediate CO₂^·− is weakly adsorbed on the catalyst surface, HCOOH or HCOO⁻ is the main product, and the metal catalysts applied in this reaction are mainly Sn, In, Hg, Pb, etc.

Furthermore, some researchers also believe that the intermediate CO₂^·−, that produces HCOOH or HCOO⁻, may also bind to the catalyst by one or two oxygen atoms, but the source of hydrogen for this mechanism is unclear. It is generally believed that hydrogen can include at least two sources: (1) a metallic hydrogen bond; and (2) hydrogen protons in solution (Figure 15a–c) [98]. With regard to the metal catalysts Au, Ag, and Zn, etc., they can be closely combined with CO₂^·− and *COOH, but it is difficult to combine with the generated *CO intermediate, thus the reduction product is mainly CO. Based on previous research, it is known that Cu is a special metal, which can produce more complex carbon products. Firstly, the *CO intermediate is generated, then the formation of subsequent production products can be roughly divided into three pathways: (1) the combination of electrons and hydrogen protons to form the CO and H₂O; (2) the conversion of intermediate *CO to *CHO or *COH and the formation of CH₃OH or CH₄; (3) dimerizing itself to obtain different products, such as CH₃CH₂OH, HCHO, etc. by getting different numbers of electrons and protons (Figure 14 and Figure 15b).

In addition, another mechanism is to avoid the formation of CO₂^·− intermediate by proton-coupled electron transfer (PCET) [99]. The activation barrier caused by high recombination energy and unstable intermediates is avoided by the transfer of a proton with an electron, but the proton concentration, pH, and temperature and other synthesis conditions have a greater impact on the PECT process [100,101].

In summary, there are still many unclear factors and processes in the PEC CO₂RR, and the research on its catalytic mechanism is still in its infancy. Therefore, we need to further explore the related mechanisms of CO₂RR, based on the experiment dates, advanced characteristic techniques, and DFT calculations.

6. Conclusions, Remarks, and Outlook

In the face of the global energy crisis and increasingly serious environmental issues, the reduction of CO₂ and the utilization of natural resources (light) have attracted significant attention. The current research, including the design of photocathodes to achieve effective CO₂ reduction by doping metal and non-metal; construction of heterojunctions such as Z-type heterojunctions, type II heterojunctions and even p-p type heterojunctions; as well as modification of co-catalysts, etc., has made great progress. Nevertheless, there are still great challenges in the practical applications; for example, the problems of low photocurrent density, low FE of reduction products, poor yield of products, and serious photo-corrosion of photocathode materials, etc.

The design of photocathode materials is crucial to the PEC CO₂RR process. How to improve the light absorption of the designed photocathode material, as well as how to promote the separation of charge carriers and improve the selectivity of the product, etc., will be the emphases of the research. To this end, the following issues should also be taken into consideration:

Firstly, it is crucial to develop a new p-type semiconductor photocathode material with good stability, ultra-visible light adsorption, and CO₂ conversion performance. Although some p-type semiconductor materials have been developed as photocathodes to achieve the application of PEC CO₂RR, most of them have serious issues such as photo-corrosion and the complexity of reduction products. Therefore, the developed new p-type photocathode material should have photo corrosion resistance and good selectivity for CO₂ reduction products. However, the development of new photocathode materials is relatively difficult and requires a long-term process, which requires deeper theoretical foundations and the assistance of DFT calculations.

Secondly, a highly efficient catalyst can be designed by reasonably optimizing the structure and composition, which is currently one of the most universally applicable measures to achieve highly efficient CO₂ reduction. For example, the combination of photocatalysts with good light absorption and electrocatalysts with super selectivity in CO₂RR not only preserves the advantages of each component but also regulates the selectivity of the product. More importantly, by loading co-catalysts, visible light can be effectively utilized and photogenerated electrons can be effectively separated. In addition, metals have stronger conductivity, and metal co-catalysts can thus increase current density, making them closer to industrial applications. At the same time, in the design of photocathode materials, combining semiconductor materials with appropriate conduction band positions is also an effective mean of promoting CO₂ reduction.

Thirdly, improving or optimizing the catalyst preparation process is also an effective way to improve its performance, such as surface coating, surface modification, or a combination of surface technology. Different preparation processes can expose different crystal planes or generate different surface states and configurations, which are conducive to the adsorption of CO₂ and the desorption of intermediates and products, improving light utilization efficiency, and enhancing the activity and stability of catalytic reactions. Moreover, it is also crucial to control and regulate the contact interface of photocathode materials. Additionally, the contact interface generally serves as a barrier for electron transfer, so precise control of the contact interface can effectively promote the separation of photo-generated charge carriers. For example, more active sites or defects can be exposed at the interface contact with electrolytes, to promote CO₂ activation.

Last but not least, it is necessary to further strengthen the exploration of CO₂ reduction mechanisms. A better understanding of the mechanism helps to design the structure of the target catalyst and achieve effective CO₂ conversion, especially through theoretical calculations, to strengthen the study of the internal relationship between kinetics and thermodynamics.

Overall, PEC CO₂RR is a promising approach for addressing fossil fuel depletion and mitigating greenhouse effects. Based on the existing technologies, combined with the development of new materials, reasonable design of catalyst structure and components, and optimization or improvement of preparation methods, it is expected to achieve more efficient CO₂ conversion.