Peng Chen, Yuxin Zhng, Ying Zhou, Fn Dong,＊

a School of New Energy and Materials, Southwest Petroleum University, Chengdu, 610500, China

b Research Center for Environmental and Energy Catalysis, Institute of Fundamental and Frontier Sciences, University of Electronic Science and Technology of China,Chengdu, 611731, China

c College of Materials Science and Engineering, Chongqing University, Chongqing, 400044, China

Keywords:Photoeletrocatalytic CO2 reduction Basic principles Influencing factors Reaction mechanisms

ABSTRACT With the rising global population,increasing energy demand and rapid climate change,great concerns have been raised for environment and energy security in future. Solar-driven CO2 reduction provides a promising way to deal with energy crisis and global warming, which has been widely concerned. Photoelectrocatalysis technology can effectively utilize solar energy and avoid to use high-temperature and high-voltage reduction environment by integrating the vantages of both photocatalysis and electrocatalysis, which exhibits a broad application prospect of CO2 reduction with a high efficiency and excellent selectivity.In this review,basic principles of CO2 reduction by photocatalysis, electrocatalysis and photoeletrocatalysis are briefly reviewed, also comparing the technical characteristics of the above technologies.Different photoelectrocatalytic systems for CO2 reduction are described and compared. The several key influencing factors of photoelectrocatalytic performance of CO2 reduction are discussed, including interaction between reaction molecules and catalysts, reaction conditions and influence of photoelectrode. Then, the advances on reaction mechanisms and strategies of performance enhancement by optimizing photoexcitation, charge separation efficiency and surface reaction were reviewed. Besides, the challenges and prospects of photoelectrocatalytic CO2 reduction will be also discussed.

1. Introduction

With the increase of global population, energy demand and climate change, great concerns have been raised for future energy security. Energy consumption mainly comes from the combustion of fossil fuels such as coal,oil,and natural gas,which also releases excessive carbon dioxide(CO2) into atmosphere and thus exacerbates greenhouse effect to cause global warming [1-3]. Converting atmospheric CO2into low-carbon fuels or small-molecule organic compounds not only benefits CO2emission reduction,but also can be used as a carrier of energy storage,which exhibits great significance to alleviate energy shortage and global environmental pollution[4-7].

At present,the artificial methods for CO2conversion mainly include high-temperature catalytic hydrogenation, electrocatalytic reduction,photocatalytic conversion and photoelectrocatalytic methods[8-11].For the CO2reduction by high-temperature catalytic hydrogenation, the process must input high energy and hydrogen sources that is usually provided by the combustion of fossil fuels. Therefore, such conversion itself will cause the energy consumption and the generation of more CO2[12]. Due to the sufficient electrons in the electrocatalytic reduction,which will simultaneously realize the multi electron reduction process,resulting in a variety of products. Additionally, the different reduction medium (gas phase or liquid phase) and reaction temperature (high or low temperature)will further lead to the increase of product types.If the external voltage is too high,it will also lead to the competitive reaction of hydrogen evolution, resulting in the reduction of Faraday efficiency in the process of electrocatalytic reduction. In addition, the type of electrode or catalyst and the magnitude of external voltage lead to the fact that the electrocatalytic reduction does not have absolute selectivity for a product (usually a mixture of several substances), which is a grand challenge for electrocatalytic reduction of CO2[13,14].Contrast to above technologies, photocatalytic CO2reduction can effectively utilize the solar energy and avoid the use of high-temperature and high-voltage reduction environment, which has a broad application prospect. However,photocatalysts generally exist limited utilization efficiency of solar energy and low separation efficiency of photogenerated carrier, which leads to the low efficiency of photocatalytic CO2reduction. Besides,abundant sacrificial agents are often added in the liquid-solid photocatalytic system of CO2reduction, leading to the increase of economic cost.Additionally,the current catalytic efficiency is relatively low in the gas-solid photocatalytic system of CO2reduction.Therefore,the practical application of current photocatalytic CO2reduction technology is restricted [11,15,16]. Considering those problems, the researchers put forward the technology of photoelectrocatalytic CO2reduction by integrating the characteristics of photocatalysis and electrocatalysis. Photoeletrocatalytic system can promote the carrier separation by adjusting overpotential and also realize the quick transfer of multi electrons and protons, which results in improving catalytic reaction efficiency to rapidly obtain the products of CO2reduction on the cathode.And product selectivity can be tuned by optimizing the main reaction steps of photoelectrocatalytic CO2reduction process on cathode. Moreover, photoelectrocatalysis can overcome the energy barrier owing to insufficient redox potential by using external voltage,realizing higher solar conversion efficiency compared with photocatalysis [11,17-19]. These excellent characteristics make the technology of photoelectrocatalytic CO2reduction show great application potential.

Scheme 1. The whole framework of this review.

In this review (Scheme 1), basic principles of CO2reduction by photocatalysis, electrocatalysis and photoelectrocatalysis will be briefly reviewed. The several key influencing factors of photoelectrocatalytic performance are discussed,including interaction between reaction molecules and catalysts(initial activation and C-C bond formation),reaction conditions(such as light source,electrolyte effect and overpotential,etc.)and influence of photoelectrode (thermodynamics and kinetics). The advances on the design of photoeletrocatalytic materials, reaction mechanisms, influence factors and strategies of performance enhancement will be comprehensively reviewed. The main strategies of enhancing product activity and selectivity on photoelectrode for CO2reduction are proposed,including photoexcitation(e.g.,doping element,introducing surface plasmon and quantum dots),charge separation efficiency(e.g.,loading co-catalysts,constructing heterojunction and defect structure),surface reaction(e.g., adsorption and activation of reactants,adsorption/desorption of intermediates), and their synergistic effects.

2. Basic principles of catalytic CO2 reduction

2.1. Photocatalytic CO2 reduction

The most common CO2conversion method is photosynthesis in natural world, which means that green plants or photosynthetic bacteria convert CO2in the air into oxygen and/or energy materials necessary for growth under light conditions. Photocatalytic (PC) reduction of CO2is similar to plant photosynthesis in Fig. 1a [20,21], which is that the electrons and holes on surface of photocatalyst respectively reduce CO2and oxidized water to produce oxygen under light irradiation.When the incident light energy is larger than the band gap(Eg),the electrons would be excited to conduction band (CB) while holes will be generated in valence band (VB), as shown in Fig. 1a. On the one hand, the photogenerated electrons and holes would recombine(volume recombination and surface recombination). On the other hand, the photogenerated electrons react with CO2adsorbed on the surface to generate CO,CH4and CH3OH,etc.,while photogenerated holes oxidize water to H+and O2[15,16,22,23].

Over the years,researchers have developed and designed many types of photocatalysts, including metal oxides [24-29], metal chalcogenides[30,31], metal nitrides [32], metal phosphides [33], layered double hydroxides (LDHs) [34] and non-metal semiconductors [35-37], etc.Although there are various photocatalytic materials, their practical application efficiency of CO2reduction is still not ideal. Therefore, researchers developed plentiful strategies of catalyst modification to optimize photocatalytic performance,including morphology and size control[38,39],crystal face control[40],doping[41,42],noble metal deposition[43,44],semiconductor recombination[45],dye sensitization[46],and defect construction [47,48], etc. To some extent, these modification strategies can improve efficiency of light utilization and carrier separation, thus enhancing the reactivity and product selectivity. However, a series of problems still exist in the practical application. For examples,many sacrificial agents usually need to be added in liquid-solid reaction system of photocatalytic CO2reduction that leads to increase the cost,while the catalytic efficiency is relatively low in the gas-solid reaction system, which further restricts the application of this technology.Therefore, it is necessary to develop new photocatalysts or new CO2reduction systems. Additionally, the studies about surface reaction process,absorption mechanism of solar energy,separation and migration of electrons/holes, are all conducive to elucidation of the surface microstructure and energy band structure of the catalyst,so as to improve the catalytic efficiency of CO2reduction[7,49-51].

2.2. Electrocatalytic CO2 reduction

The process of electrocatalytic (EC) CO2reduction is that using external electric field as the main energy source to induce redox reaction on electrodes (Fig. 1b). The H2O is oxidized to produce oxygen and protons on anode. And the protons migrate to participate in CO2reduction on cathode through proton exchange membrane. The reaction process of electrocatalytic CO2reduction is controllable by adjusting voltage and reaction condition. Due to the sufficient electron source in the electrocatalytic reduction, it will simultaneously realize the multi electron reduction process, resulting in a variety of products [13,14]. It should be noted that if the external voltage is too high,it will also lead to the competitive reaction of hydrogen evolution, which will lead to the reduction of Faraday efficiency of electrocatalytic CO2reduction.Moreover,the type of electrode or catalyst and the magnitude of external voltage would lead to the fact that the electrocatalytic reduction does not have absolute selectivity for a product,which usually includes a mixture of several substances.Therefore,one of the main challenges is to improve product selectivity of electrocatalytic CO2reduction by optimizing the electrode or catalyst. At present, the technology of electrocatalytic CO2reduction is still not mature enough due to the obstacles such as low energy efficiency,poor reaction selectivity and total conversion rate[13,14,52-54]. Birdja et al. points out that the systematic understanding of the intricate interaction among initial activation of CO2, formation of C-C bond, surface structure, electrolyte effect and mass transfer is still insufficient [13]. In addition to focusing on the design of catalysts, researchers should also recognize the importance of electrode surface structure, reaction and process conditions. Therefore, combining with theory calculation and in-situ electrochemistry technology is necessary to develop more rigorous experiments and standardized procedures,to have a deeper understanding of the electrocatalysis on the catalyst surface,and to develop multi-scale calculation and modeling methods[13,49,55-57].

Fig. 1. Basic principles of catalytic CO2 reduction. (a) Analogy diagram of natural photosynthesis and schematic of photocatalytic CO2 reduction [20]. Reproduced with permission:Copyright 2017,American Chemical Society.(b)Schematic diagrams for electrocatalytic CO2 reduction system.Photoelectrocatalytic systems:(c)photocathode + dark anode, (d) dark cathode + photoanode, (e) photocathode + photoanode [58], Reproduced with permission: Copyright 2016, Royal Society of Chemistry. (f) solar cell + electrodes.

2.3. Main systems and principle of photoelectrocatalytic CO2 reduction

Due to the limitation of photocatalytic and electrocatalytic CO2reduction in practical application, the researchers combined the two technologies to develop photoelectrocatalytic (PEC) reduction of CO2.The photoelectrocatalytic CO2reduction refers to the process that the semiconductor photoelectrode generates electrons by photoexcitation,and then the electrons migrate to the electrode surface under the guidance of external voltage to carry out the catalytic reduction of CO2[59-62]. The action of external electric field is helpful to promote the directional transfer of photogenerated electrons and holes, and thus enhancing the separation efficiency of photogenerated carriers to greatly improve the redox ability. Simultaneously, when the band position of photocatalyst is not conducive to CO2reduction and H2O oxidation,the redox potentials can be adjusted by applying appropriate bias voltage in the photoreduction system [17]. Compared to photocatalytic CO2reduction process,photoelectrocatalysis can realize the rapid transfer of electrons to reduce CO2on the photocathode at a lower overpotential,greatly enhancing reduction efficiency. Compared with single electrocatalysis, photoeletrocatalysis can use sunlight as the energy source to excite the generation of carriers, then participates in the oxidation-reduction reaction to finally realize the CO2reduction with low energy consumption. The whole process can reduce the input of external energy and realize the utilization of low-carbon clean and sustainable energy. The excellent photoelectrocatalytic CO2reduction system can not only use photocatalysis activity to excite and generate carriers under light conditions, effectively reducing the energy input of external electrons and energy consumption,but also use electrocatalytic activity to improve the electron-hole separation and transmission,greatly enhancing reduction efficiency.

Fig. 2. Continuous-flow PEC CO2 reduction system [81]. Reproduced with permission: Copyright 2020, American Chemical Society.

Most experiments of photoelectrocatalytic CO2reduction are mostly conducted in a three-electrode system including working electrode,counter electrode and reference electrode respectively.According to the different semiconductor materials of photoelectrode, the CO2reduction system can be divided into following categories(Fig.1c-e):I.dark anode and p-type photocathode,II.dark cathode and n-type photoanode,III.ptype photocathode and n-type photoanode [58,63-66]. The p-type semiconductor photocathode not only works as a center of generating electrons and holes but also need to act as the catalyst for CO2activation to carry out reduction reaction. Generally, the p-type semiconductor photocathodes have negative CBs which are favorable for CO2reduction.But VB potentials of most p-type semiconductor are insufficient to oxidize water, which needs a higher bias potential generally to excite the reaction[58,67].Additionally,H2is usually generated with CO and HCOOH on p-type semiconductor photocathodes,affecting the selectivity of CO2reduction. The frequently-used p-type materials as photocathode including p-GaP, p-InP, p-Cu2O, p-CuO and p-CdTe, but which are generally expensive,toxic and/or unstable in aqueous solutions[68-71].Instead, the most n-type semiconductors (e.g., TiO2, WO3, ZnO, Fe2O3,and BiVO4) that made of earth-abundant elements are generally low-toxic or nontoxic and/or also highly stable in aqueous solutions[72-74].Which makes the system(dark cathode and n-type photoanode)would be an attractive alternative. Furthermore, the combination of appropriate p-type and n-type semiconductors with matched band structure respectively as photocathode and photoanode to form Z-scheme heterojunction can realize efficient CO2reduction without external voltage[17,58].However,it is worth noting that not all of this kind PEC CO2reduction systems can avoid bias voltage. For instance, the system composed of an n-type TiO2photoanode and a p-type Si photocathode was still needed a bias voltage to promote CO2reduction [75]. Additionally,a novel tandem device is developed by integrating a solar cell for supplying external voltage to promote CO2reduction reaction (Fig. 1f),also showing the excellent efficiency and promising potential [76-78].When the external bias voltage generated by the solar cell is high enough,the system can realize the unassisted CO2reduction and avoid the strict conditions that needs to form Z-scheme heterojunction. Recently, some researchers considered the challenges of CO2conversion in traditional aqueous solution such as serious mass transfer limitation stemmed from low solubility and slow diffusion of raw material CO2in water [79,80],developing a continuous-flow system for PEC CO2reduction by direct introduction of gas CO2onto the surface of catalysts[81-84],as shown in Fig.2.This new system shows an obvious advantage is that no limitations in CO2concentration exist due to solubility and diffusion through the double layer and competitive chemisorption of water is largely reduced[81,82].

3. Evaluation parameters for photoelectrocatalytic CO2 reduction

Fig. 3. Schematic diagram of evaluation parameters for photoelectrocatalytic CO2 reduction. Where t represents the reaction time, nproduct and ncatalyst respectively refer to molar numbers of products and photoelectrocatalysts.The α is the needed electron numbers for products. For examples, α is 2 when generated CO and α is 8 when generated CH4. And nphoton and n'photon are respectively the number of absorbed and incident photons. F and Q is the Faraday's constant and the total passed charge, respectively.

The evaluation parameters for photocatalysis and electrocatalysis can combine to assess photoeletrocatalytic CO2reduction,as shown in Fig.3[61]. Which mainly include product conversion rate, catalytic current density, turnover number (TON), turnover frequency (TOF), quantum yield (QY) and Faradaic efficiency (FE). The product conversion rate is generally calculated the yield per hour of per unit mass or per unit area on photoelectrode. The catalytic current density is also an important index to evaluate the reaction efficiency, which needs to be compared with the reference electrode(RE).TON and TOF can reflect the activity of catalytic active centers.The utilization and conversion efficiency of solar energy can be evaluated by calculating the QY.Additionally,the product selectivity can be assessed by calculating FE.

4. Influence factors of photoelectrocatalytic CO2 reduction

The influence factors of photoelectrocatalytic CO2reduction should be considered comprehensively from two aspects of photocatalysis and electrocatalysis [13,85-87]. Fig. 4 summarizes the main influencing factors of photoelectrocatalytic reduction of carbon dioxide, including influence of photoelectrode (thermodynamics and kinetics), interaction between reaction molecules and catalysts (initial activation and C-C bond formation), reaction conditions (such as light source, electrolyte effect and overpotential,etc.).

4.1. Thermodynamics and kinetics on photoelectrode

Photoelectrocatalytic CO2reduction belongs to complex multistep reactions, intrinsic properties of semiconductor photoelectrode can influence the reduction efficiency and product selectivity[87].Firstly,the critical start step is to produce electrons and holes in semiconductor photoelectrode by photoexcitation. The energy of light absorption determines whether the semiconductor can be excited and how many electron-hole pairs can be generated, which affects reaction rate and product selectivity of CO2reduction from thermodynamic.Significantly,the different reduction potentials will determine the specific products in CO2reduction. Photogenerated electrons need sufficient reduction ability to trigger a specific reduction reaction in thermodynamics, thus affecting the product selectivity of CO2reduction. The reduction ability of photogenerated electron is correlated with the CB position of semiconductor photoelectrode. Fig. 4a outlines the correlation between standard redox potential of various products and the positions of CB and VB in some typical semiconductors. The effective electron-hole separation can increase the electron density on the surface of the photoelectrode, which can dynamically increase reaction rate and obtain higher reduction state products. In addition, the interaction between reactants and surface catalytic active sites can directly affect multi-step reaction of CO2reduction, thus determining the product selectivity.Exploring the relationship between the dynamic changes of surface active sites and the reaction efficiency is the key to achieve accurate control of CO2reduction [88]. Moreover, the adsorption/desorption of reactants/intermediates can affect the exposure of active sites and the reaction rate on the surface photoelectrode, which will also determine the product selectivity from kinetics[87,89-92].

4.2. Interaction between molecules and catalysts

4.2.1. Initial activation of CO2molecules

The activation of CO2molecule is first step in photoelectrocatalytic CO2reduction.In general,the activation and reduction of CO2is difficult owing to the negative reduction potential of forming the CO2·-radical intermediate by first electron transferring and because CO2is a very stable molecule[93].A less negative reduction potentials can be induced via forming chemical bonds between CO2and suitable photoelectrocatalysts, thus stabilizing CO2·-radicals or intermediates. Therefore,selecting the right catalyst can directly reduce CO2to two-electron products(CO or HCOOH)at low overpotential[94-96].Birdja et al.[13]considered four reduction processes related to CO2activation (equation(1)-(4)):

Considering the simultaneous transfer of protons and electrons, the initial activation of CO2can be deduced according to equations(1) and(2), and four possible bonding modes were shown in Fig. 4b, which are called concerted proton-electron transfer (CPET) reactions [97].Whether＊COOH or＊OCHO is the first intermediate of generating CO or HCOOH still remains controversial.By calculating the binding energy,it is found that post-transition metals (e.g., Pb and Sn) are more likely to combine CO2through oxygen and have selectivity for formic acid,while transition metal electrodes are more likely to combine CO2through carbon [97]. When the initial activation of CO2involves only one-electron transfer,the binding step of CO2can be deduced via equation (3). If the formation of ＊CO2-is rate determining step, the product selectivity is strongly influenced by the pH value [98].Additionally,equation (4) shows the formation of anionic hydride on the surface of catalyst. The Rh, Sn and In protoporphyrins can reduce CO2to formic acid with a high selectivity,and the main reason is that the formation of the key intermediate(anionic hydride)to attack C atom of CO2according to density functional theory(DFT)results[99,100].The steps involved in equations(3) and(4) called sequential proton-electron transfer(SPET),Fig. 4b exhibits a summary of the discussed activation mechanisms for CO2reduction [13]. Compared with the formed intermediates in CPET,the intermediates generated via CO2activation in SPET are generally charged(or strongly polarizable), which is susceptible to pH and cation effects [13]. Therefore, the activation of CO2is important to affect the reaction efficiency and product selectivity.

Fig. 4. Influence factors of photoelectrocatalytic CO2 reduction. (a), Thermodynamics and kinetics on photoelectrode [20]. Reproduced with permission: Copyright 2017,American Chemical Society.(b),Initial activation of CO2.(b)The C-C bond formation to generate C2H4 and C3+products.(d)The external conditions,including light source, PH, Ion effect and so on.

4.2.2. C-C bond formation

During the CO2reduction reaction(CO2RR),one or more C-C bonds were usually formed on copper-based electrodes during CO2reduction,which can realize the selectivity improvement of C2products by tuned the structure of copper [101,102]. In addition, NiGa, PdAu, and NiP catalysts were also reported to generate C2or other more carbon products while the efficiency is different from that of copper[103-106].The most typical multi-carbon products in liquid phase reduction of CO2are ethylene and ethanol, which have high energy density and economic value[107].The production of ethylene include two pathways:one is the high-overpotential pathway mainly occurred on Cu (111) with the generation of intermediate such as methane, while another is the low-overpotential pathway mainly occurred on Cu (100) without producing C1products[108].The pH dependence of these two pathways is also different, the former pathway is PH dependent while the latter pathway is not,which makes CO2and CO have(local)high pH sensitivity on copper-based catalyst. Several experimental and theoretical studies have indicated the key intermediate is a CO dimer in this pathway to form C2product [109-113]. The DFT calculation shows that CO dimerization is not easy to occur at a more negative potential,and the coupling reaction of ＊CO and ＊CHO has a lower activation energy [113]. Combined with the results of experiments and DFT calculations, the rate of C-C coupling is determined by the proton-electron transfer involved in the step, and explain its PH sensitivity [111]. In alkaline LiOH electrolyte, the presence of hydrogen CO dimer intermediate (OCCOH)can be observed in Fourier infrared spectroscopy, which further certified the formation of C-C coupling is sensitive for structure and PH [114]. In addition to ethylene, other C2products including acetaldehyde and ethanol can also be observed in CO2reduction. The hydrogenation of intermediates promotes acetaldehyde and ethanol formation, and then the hydrogenolysis of intermediates induces the formation of ethylene[111]. The formation of advanced (C3+) hydrocarbons with the FE of 20%has also been observed on appropriated copper[115].Additionally,the propanol and propanal are main C3products observed,also including a small amount of hydroxyacetone, allyl alcohol and acetone.The basic formed pathways of these C3products have not been studied clearly yet.There is literature has been reported that C-C coupling between CO and C2H4precursors would form n-propanol [116]. Fig. 4c shown the formation of C-C bond related to C2H4and C3+products on the surface of catalyst e format. Based on the above discussion, the selectivity of products can be optimized by tuning photoelectrocatalyst structure to form C-C bonds during CO2reduction.

4.3. Reaction conditions

4.3.1. Light source

For the system of photoelectrocatalytic CO2reduction,the energy of light plays an important role for semiconductor to generate excited electrons in the system[64].First,different reaction systems(mainly gas reaction system and liquid reaction system)have different requirements for light sources. Research shows that the ultraviolet ray drastic decay from gas phase into liquid phase [117,118]. Therefore, the UV light source should be fully considered the light source intensity, distance of light source to electrode and other factors in the liquid-phase reaction system.Additionally,different photoelectrodes need different excitation energies due to the differences of band gaps. When selecting a light source, the emitted light must contain the wavelength portion of the excitable semiconductor. At present, the most ideal light source is the sun. Its spectrum consists of about visible light(wavelength ≈400-760 nm), infrared light (wavelength ＞760 nm) and ultraviolet light (wavelength ＜400 nm). Among them, UV can stimulate most semiconductor catalysts to produce photogenerated electrons.The synthetic light source mainly includes metal halide lamp, mercury lamp, ultraviolet lamp,xenon lamp and LED [119-121]. The wavelength of the synthetic light source can be adjusted according to the different experimental parameters, but the main drawback is the need for additional power to drive,thus increasing energy consumption.So far,most of the basic research in laboratory is to use artificial light source for conventional research,and finally to carry out experiments with the sun as the light source for industrial application. In the future, the solar light source is the ultimate goal for photoelectrocatalytic CO2reduction from laboratory to large-scale use.Therefore,the reaction system integrated solar cell might be a good development direction.

4.3.2. Reaction temperature and pressure

The influence of temperature on the efficiency of photoelectrocatalytic CO2reduction mainly focused on these aspects including dissolution, diffusion and/or migration of CO2molecules. In the liquid phase reaction system, there are two following situations. On the one hand, the HCO3-or CO32-ions would be generated by CO2dissolved in water,and movement speed of them also accelerates with the increase of temperature, which can promote the reaction [61,122]. And there is molecular state of CO2in the liquid phase system saturated with CO2,which can directly obtain the photogenerated electrons and be converted when the temperature is appropriate. But on the other hand, the temperature causes the gas solubility to decrease and the high temperature causes the system energy consumption to increase greatly [61]. In photoelectrocatalytic CO2reduction, the main purpose is to achieve energy conservation and environmental protection,so the reaction temperature should be carefully considered. Additionally, the pressure of CO2also affects the solubility.The CO2concentration in solution can be increased under high-pressure environment, promoting higher photoelectrocatalytic current density and CO2reduction rate.In addition,some reports showed that the high CO2pressure could also enhance the CO2reduction rather than proton reduction, improving the stability of photocathode and product selectivity[61,123,124].

4.3.3. Electrolyte effect

The electrolyte effect(pH,ion effect,the CO2concentration and mass transfer process, etc.) will affect the reduction efficiency and product selectivity [125]. The pH, electrolyte composition and buffer capacity can influence on CO2/HCO3-,H2CO3/HCO3-and HCO3-/CO32-equilibria in water,which makes carbonaceous species have different concentrations in solution [126-128]. Compared with other electrolytes, bicarbonate electrolyte can enhance CO2reduction activity because of the formation of bicarbonate-CO2complex,which is as the main carbon source during the CO2reduction period,inducing CO2concentration to increase in near the electrode.The existence of bicarbonate was proved by isotope labeled infrared spectroscopy[129].During CO2reduction,the alkaline pH close to the electrode was higher than that of base solution because of the formation of hydroxide. This locally alkaline pH is the result of mass transfer and changes in local pH are proportional to current density,depending on factors such as the nature of the electrolyte and buffering capacity,as well as the shape of the electrode[130-132].

The properties of cations and anions in electrolytes importantly affect the reduction efficiency and product selectivity in CO2reduction.Generally, larger cations in electrolytes can bring about a higher C2/C1ratio and a higher CO2reduction rates on copper electrodes [13,133].The cation hydrolysis effect can increase local concentration of dissolved CO2because the buffering was enhanced near the electrode for larger cations [133]. Researches show that the onset potential for ethylene depends on the nature of cation while that for methane was unrelated to cation size[134].In terms of halide adsorption on catalyst surface,anion effects are generally explained as changing the electronic structure,and subsequently enhancing reduction efficiency and product selectivity.And the halide size and concentration also affect the efficiency of CO2reduction [135]. The relationship between reaction conditions and the individual effects mentioned above is very important, but which is still poorly understood.

Fig. 5. Main optimizing process for enhancing product selectivity of photoelectrocatalytic CO2 reduction on photoelectrode.

4.3.4. External bias voltage

In the process of photoelectrocatalytic CO2reduction, the external bias voltage can effectively separate photogenerated electron-hole pairs,thus improving the photoelectrocatalytic efficiency on photoelectrode.Hasan et al. [136] studied the CO2reduction performance during the voltage range was -1.5-0.0 V, finding that the products were mainly formic acid (HCOOH) and methanol (CH3OH) under -0.61 V bias.Generally speaking, there is an optimization condition between the external bias voltage and the photoelectrocatalytic CO2reduction conversion rate. Under the optimal bias voltage condition, separation efficiency of photogenerated electron-hole pairs reaches the maximum. If the external bias voltage continues to increase,the photoelectrocatalytic CO2reduction efficiency will no longer increase.Additionally,excessive bias voltage will not only increase the reduction products and reduce the selectivity of the products,but also increase the energy loss of the whole photoelectrocatalytic CO2reduction system. Therefore, the effect of external bias voltage on the reduction performance should be investigated in the actual photoelectrocatalytic CO2reduction system so as to find the best bias voltage.

5. Enhancement strategy of photoelectrocatalytic performance

Recently,much progress has been developed in photoelectrocatalytic CO2reduction. Castro et al. [59] reviewed the main materials of photoelectrodes, photocatalytic reactors of different configurations and their characteristics. Which provides a deep understanding on the photoelectrocatalytic CO2reduction from the aspects including photoelectrode structures and photoelectrocatalytic reactor. Simultaneously, the application of photoactive materials and key variables of configuration in reaction performance are also discussed. Gong et al. [60] reviewed the research progress of cocatalyst modified semiconductor based photoelectrode from the perspective of material modification, discussed the role of cocatalyst in reaction,and summarized the relationship between the performance of the photoelectrode and its structure.Xiong et al.[61]started from the fine structure of the photocathode, summarized strategies including improving the light utilization rate, providing catalytic active sites and controlling reaction paths,and summarized various novel photoelectrocatalytic CO2reduction reaction devices.

Crucially,the important aim in photoelectrocatalytic CO2reduction is to obtain high yield of target product. However, the undesired byproducts were generally produced by consume valuable photogenerated electrons on the surface, which makes the output of target products obviously lower. The poor product selectivity usually leads to the mixing of multiple products,which leads to another challenge that is to separate valuable target products from mixed products. Therefore,enhancing the product selectivity in the reaction process of photoelectrocatalytic CO2reduction is one of the keys to realize its application[87,90,91]. The main process of photoelectrocatalytic CO2reduction is that the semiconductor photoelectrode generates electrons by photoexcitation,and electrons transfer to electrode surface under the guidance of external voltage to carry out the catalytic reaction of CO2reduction.The structure and reaction characteristics on photoelectrode are the key factors affecting the efficiency and selectivity of photoelectrocatalytic CO2reduction. Therefore, product selectivity of photoelectrocatalytic CO2reduction can be adjusted by optimizing the multiple process in terms of photoexcitation, charge separation efficiency and surface reaction on photoelectrode, as shown in Fig. 5. The main strategies for enhancing the reaction efficiency and product selectivity of photoelectrocatalytic CO2reduction on photoelectrode include photoexcitation (e.g., doping element, introducing surface plasmon and quantum dots), charge separation efficiency (e.g., loading co-catalysts, constructing heterojunction and defect structure), surface reaction (e.g., adsorption and activation of reactants,adsorption/desorption of intermediates),and their synergistic effects. In the following parts, we will discuss the methods, structural properties and mechanisms involved in above process.

5.1. Photoexcitation

Fig.6. (a)Reduction mechanism of CO2 on the Co-doped MoS2 NPs and(b)Electrochemical impedance spectroscopy of samples[139],Reproduced with permission:Copyright 2015, Elsevier. (c) The band potential diagram of B-doped g-C3N4 (BCN1.5) and (d) photocurrent of CO2 reduction on BCNx [140]. Reproduced with permission: Copyright 2016, Elsevier.

The energy of light absorption determines whether the semiconductor can be excited and how many electron-hole pairs can be generated, which affects reaction rate and product selectivity of CO2reduction from thermodynamic [87,137]. In semiconductor photoelectrode, the band gap can generally determine the excitation energy needed, thus wavelength range of the incident light to be used. Additionally, the CB position (i.e., reduction potential) of photoelectrode is greatly related to the reduction abilities of photogenerated electrons.Adjusting the band structure of photoelectrode can optimize reduction potentials of electrons, which can effectively regulate the product selectivity from thermodynamics.Doping is an effective method to adjust band structure of photoelectrode,which can control the thermodynamic CO2reaction and thus greatly affect the product selectivity. Gu et al.[138] reported a p-type Mg-doped CuFeO2electrode, which realized photoelectrochemically reduce CO2to formate at an underpotential(400 mV). The introduction of Mg enhanced the visible light absorption and current density. When the applied electrode potential is -0.4 V vs SCE,irradiating with wavelength of ～800 nm can produce photocurrent,and a value of 14% for photon-to-current efficiency can be obtained at 340 nm. Peng et al. [139] prepared Co-doped MoS2NPs and used for photoelectrocatalytic CO2reduction.The Co element upshifted CB position,which makes the band gap of MoS2is narrowed and resistance is also reduced. Therefore, the photoelectrocatalytic performance of reducing CO2to methanol is excellent(Fig.6a and b).Sagara et al.[140]prepared a p-type boron-doped g-C3N4(BCNx)electrode for photoelectrochemical reduction of CO2under visible light irradiation. The BCNxnot only obviously changed the energy band structure(Fig.6c),but also showed a remarkable p-type conductivity. Additionally, the photocurrent was greatly enhanced in BCN3.0(Fig.6d),and the main product is C2H5OH.At present, the self-doping of photoelectrode was rarely reported for photoelectrocatalytic CO2reduction.But we believe it can be developed for optimizing photoelectrodes to improve the reduction efficiency of CO2in near future.

In addition, if plasmonic photocatalyst is used as photoelectrode,which shows different light absorption and excitation modes compared with traditional semiconductor-based photoelectrode, which can obtain strong light absorption via localized surface plasmon resonances. The adsorbed reactants can be activated by energetic (hot) electrons, promoting the reaction of CO2reduction. The energy and density of the photons greatly affect the distribution of photoexcited hot electrons,leading to different reaction rates and pathways [141-145]. DuChene et al. [141] fabricated gold/p-type gallium nitride(Au/p-GaN)Schottky junctions and studied the capture and conversion plasmon-induced hot-hole in photoelectrochemical reactions. The Au/p-GaN can keep a sustained photovoltage by plasmon excitation(Fig.7a),which makes the selectivity of CO generation greatly improve in aqueous electrolytes by plasmon-driven photoelectrochemical CO2reduction(Fig.7a).Kim et al.[142]demonstrated the enhanced photoelectrochemical performance of CO2reduction via surface plasmon on silver nanostructured electrodes.Under light irradiation, the photogenerated plasmonic hot electrons of nanostructured silver electrodes would transfer to the lowest unoccupied molecular orbital (MO) acceptor energy levels of adsorbed CO2or their reductive intermediates, which can enhance the yield, efficiency and selectivity of photoelectrochemical processes.In addition to Au, Ag and other precious metals, Cu also has plasma effect. Shen et al. [144]decorated metallic Cu nanoparticles in Co3O4nanotube arrays(Fig.7b),which showed a high selectivity of formate in photoelectrocatalytic CO2reduction. Therefore, it is feasible to control the product selectivity by adjusting the photoexcitation of plasmonic photoelectrode.

Moreover,the introduction of dye molecules and quantum dots(QDs)into the photoelectrode can also effectively enhance the light absorption.Xu et al.[146]reported that the introduction of Eosin Y disodium salt can improve light absorption. Simultaneously, the introduced Pd nanoparticles and amine ligands can capture protons and CO2, respectively.The major liquid product was methanol, and the highest selectivity reached approximately 100%. The schematic diagram of catalyst action is shown in Fig. 8a. Isaacs et al. [147] reported that it was possible to modify ITO electrodes with assemblies (polycations/QDs), with polycations PDDA and PMAEMA.Polycations can limit the effects of light on the QDs photoelectrocatalytic properties because different monomer structures of Polycations will generate different arrangements in the assemblies. The modified electrodes with (PMAEMA/QDs)6assemblies exhibited better selectivity of HCOH during photoelectrocatalytic CO2reduction(Fig.8b).

5.2. Charge separation efficiency

Fig. 7. The plasmonic photoelectrodes improved activity and selectivity of photoelectrocatalytic CO2 reduction. (a) Au/p-GaN photocathodes with plasma effect[141], Reproduced with permission: Copyright 2018, American Chemical Society. (b) Cu-Co3O4 NTs photocathode with surface plasmon [144]. Reproduced with permission: Copyright 2015, American Chemical Society.

The separation efficiency of photogenerated electron-hole pairs greatly determines the density of photogenerated electrons on photoelectrode surface[148-151].The electron density would strongly affect the multi-step reactions of CO2reduction,leading to different conversion pathways to obtain different products. Therefore, enhancing the electron-hole separation efficiency in photoelectrode should be a critical strategy to regulate product selectivity[152-154]. The external voltage can promote the separation and transfer of photogenerated carriers,and obtain higher density of photogenerated electrons on photoelectrodes for CO2reduction, which is a major feature and advantage of photoelectrocatalytic CO2reduction. The higher surface electron density can obviously change the product selectivity [136,147]. Besides, utilizing surface modification or interface control to optimizing photoelectrodes,such as loading metal co-catalysts, constructing heterojunction and defect engineering, can be an effective way to enhance the efficiency of photogenerated electron-hole separation and obtain high surface electron density.

5.2.1. Loading metal co-catalysts

A large overpotential generally needs to overcome caused by kinetic sluggishness of multi-electron reactions, which make the efficiency of CO2reduction be limited. Loading co-catalysts on semiconductor is believed an effective method to enhance efficiency of photocatalytic CO2reduction [70,155-159]. The overpotential and energy barrier can be reduced via the effects of co-catalysts, promoting CO2activation. Additionally,the co-catalysts can also promote the transfer of photogenerated charge carriers to reactants, accelerating surface reaction from kinetics.Moreover, selecting appropriate co-catalysts can restrain side or back reaction to design reduction pathway, improving product selectivity.Therefore,co-catalysts can be used to improve the reaction efficiency and product selectivity of photoelectrocatalytic CO2reduction. The mechanisms and fundamentals of loading co-catalysts on semiconductor photoelectrodes will be discussed in this section.

Fig. 8. (a)Schematic reduction mechanism of CO2 on multi-functionalized TiO2 photocathodes [146], Reproduced with permission: Copyright 2017, Elsevier. (b)Schematic diagram of electrode assembly and photoelectrocatalytic CO2 reduction of quantum dot-modified electrodes[147].Reproduced with permission:Copyright 2015, American Chemical Society.

The metal co-catalysts generally have two positive roles:1)capturing photogenerated electrons to promote carrier separation;and 2)acting as catalytic active sites to reduce overpotential and carry out the reaction[156,160,161]. The Schottky junction at the interface would be formed by directly loading metal nanoparticles on semiconductor, which will affect the charge transfer.If the work function of p-type semiconductor is larger than that of metal,the electrons on metal surface would transfer to semiconductor after contact.But in photoelectrocatalytic system,which is different. For p-type semiconductor photocathode, the separated photoinduced holes would transfer to the counter electrode to trigger oxidation reaction through the external bias under light irradiation.And the electrons would accumulate on metal of photocathode surface to promote CO2reduction.Although many metals as co-catalysts exhibited excellent performance, not all metals are suitable candidates for photoelectrocatalytic CO2reduction. And the biggest concern is that some metal co-catalysts would lead to serious competitive reactions such as H2evolution [93,122]. Fig. 9 shows that the typically metal electrodes are categorized by their primary products in solution: formic acid, carbon monoxide, or hydrogen. Copper is widely used in electrocatalytic CO2reduction due to the ability to produce a variety of hydrocarbons[122].Therefore, an excellent metal co-catalyst should simultaneously realize the improvement of Faradaic efficiency and suppression of competitive reactions in photoelectrocatalytic CO2reduction.

In recent years, the effect of loading metal co-catalysts on photoelectrodes was investigated for photoelectrocatalytic CO2reduction[162-164].For example,p-Si photocathode was modified by Cu,Ag and Au nanoparticles for photoelectrocatalytic CO2reduction. Fig. 10a and b showed the mechanism diagram of photoelectrocatalytic CO2reduction.The main products were CO and HCOOH for Ag and Au,while the Cu cocatalyst could induce CH4and C2H4to produce[163].Similarly,Pb,Ag,Au,Pd,Cu and Ni as co-catalysts on p-InP photocathode were also studied in photoelectrocatalytic CO2reduction.Both CO and HCOOH can be produced by Pb, Ag, Au and Cu co-catalysts while the only product was CO via Pd modification according to the experiments.The selectivity of products was related to the different enthalpy and Gibbs energy of dissociative adsorption of CO(g)on metal surface[162].Jang et al.[165]designed a new Au-coupled ZnTe/ZnO-nanowire photocathode for selective CO2reduction to CO.In general,the H2evolution was verified to be the dominant competitive reaction on ZnTe/ZnO,which is attributed to its negative CB position.When Au nanoparticles were as co-catalyst on ZnTe/ZnO surface, the activity and selectivity were enhanced(Fig. 10c-e). Additionally, Au, Ag, Cu, etc. metals have plasmon effect and can be loaded on the photoelectrode to achieve enhancement of light excitation and charge separation to control the product selectivity,which have been discussed in above section of photoexcitation[141,142,144].Therefore, selecting suitable metals as co-catalysts loaded on photoelectrode is feasible to improve efficiency and selectivity of photoelectrocatalytic CO2reduction.

Fig. 9. Periodic table depicting the primary reduction products in CO2-saturated aqueous electrolytes on metal and carbon electrodes [122]. Reproduced with permission: Copyright 2015, American Chemical Society.

Fig. 10. (a) Schematic of metal particles modifying p-Si electrodes and (b) Energy band diagrams of particulate-metal/p-Si [163], Reproduced with permission:Copyright 1998, American Chemical Society. (c) Schematic of the Au coupled ZOZT composite, (d) Photocurrent response and (e) FEs of CO and H2 from the photoelectrocatalytic CO2 reduction with ZnTe/ZnO (A) and Au-coupled ZnTe/ZnO (B) photocathodes [165]. Reproduced with permission: Copyright 2015, Royal Society of Chemistry.

5.2.2. Constructing heterojunction

Fig. 11. Schematic diagrams for photoelectrode: (a) photocathode, (b) photoanode.

The construction of heterojunction is an effective strategy to improve the separation efficiency of photogenerated electron-hole pairs in photocatalytic process. The heterojunctions include various types, such as conventional type-II heterojunction, metal-semiconductor (m-s) junction, p-n junction, direct Z-scheme heterojunction, and surface heterojunction, which are feasible to separate photogenerated electrons and holes [166-168]. By constructing heterojunction, the recombination of photogenerated carriers can be greatly reduced during migration process,promoting more electrons to reach the surface of photocatalyst and thus obtaining higher surface electron density. The multi electron reactions would be more easily to occur under higher surface electron density,which can facilize the generation of higher reduced state products.Therefore, product selectivity is related with the migration process of carriers through heterojunction.

Although constructing heterostructure to effectively improve charge separation efficiency is widely studied in photocatalysis,it is developing and need to strengthen the research in photoelectrocatalysis [169]. In terms of photoelectrocatalytic system, the type-II heterostructure was most widely studied and Fig. 11 shown the heterostructure of photoelectrode. The photogenerated electrons migrate from the semiconductor with a higher CB to that with a lower CB Under light irradiation, while holes would be accumulated at the semiconductor with higher VB. Driven by external voltage, photogenerated electrons are accumulated at the CB of photocathode to carry out CO2reduction,while holes are migrated to the counter electrode to participate in H2O oxidation. For photoanode, the photogenerated electrons are moved to the counter electrode and conduct CO2reduction while holes are left to carry out H2O oxidation. Through designing heterostructure and adjusting external electric field,the charge will be highly separated and thus enhancing the efficiency of photoelectrocatalytic CO2reduction.

Fig. 13. (a) Density of states for co-doping SnO2, (b) Transient photocurrent responses, (c) Mechanisms of photoelectrocatalytic CO2 reduction on the (Cu, N)-SnOx[185],Reproduced with permission:Copyright 2020,Elsevier.(d)Schematic diagram of the CO2(CO)reduction catalytic cycle on the defective anatase surface:(e)at the S/C interface, (f) at the E/C interface [180]. Reproduced with permission: Copyright 2020, American Chemical Society.

Ghadimkhani et al. [170] prepared CuO-Cu2O nanorod arrays as p-type semiconductor photocathode to carry out photoelectrocatalytic CO2reduction. Fig. 12a and b showed the prepared approach and microstructure of CuO-Cu2O nanorod arrays. Fig. 12c exhibited the mechanism of charge separation and migration at the interface by strong electric field built-in electric field. At an underpotential, CuO-Cu2O photocathode exhibited an excellent photoelectrocatalytic CO2reduction to methanol. Additionally, Takanabe et al. [171] also reported a Cu2O/CuO heterojunction photocathode on Cu foil, which made CO2reduce to CO with 60% selectivity under constant current conditions(-1.67 mA·cm-2) at -0.6 V versus RHE. Moreover, Won et al. [172]pointed out that Cu2O/Cu would be produced on the surface by reduced from CuO (CuO/Cu2O photocathodes), which would destroy the electron-hole pathways.Other Cu-based heterojunction photoelectrodes also showed excellent performance for CO2reduction. Kang et al. [173]prepared a CuFeO2/CuO catalyst for photoelectrochemical CO2reduction. The CuFeO2/CuO photocathode exhibited an enhanced photocurrent (Fig. 12d) and improved photoelectrocatalytic performance of CO2reduction (Fig. 12e). The selectivity of formate was over 90% under simulated solar light. Fig. 12f showed the mechanism of photoelectrocatalytic CO2reduction on CuFeO2/CuO photocathode. Additionally,Xu et al.[174]fabricated Ti3C2/g-C3N4(TCCN)heterojunctions for photoelectrocatalytic CO2reduction. The heterojunction not only enhanced light absorption but also promoted the charge separation to increase the photocurrent(Fig.12g).On the basis of the heterojunction,the modification by nanometal particles would further accelerate charge transfer to improve surface electron density.And mechanism for charge separation and CO2reduction is proposed in Fig.12h.

5.2.3. Defect engineering

For photocatalysis and electrocatalysis, defect engineering has been developed to increase the carrier separation efficiency, promote adsorption activation of reactants, enhance reaction efficiency and product selectivity [47,175-181]. Higher electron density can be obtained at defect positions,and importantly affect the product selectivity of multi-electron reaction involved in CO2reduction. However, some other studies present that defects could also adversely perform as the electron-hole recombination centers,leading to decrease photocatalytic activities[182,183].Notably,these contradictory effects of defects might due to the complex defect structures closely related to photocatalytic activity and selectivity.However, there is still a lack of advanced in-situ characterization techniques to characterize the complex relationship between structure and properties.Zhang et al.[184]pointed out that the defects cannot only serve as active sites for molecular chemisorption,but also spatially supply channels for energy and electron transfer. They emphatically outline how the parameters of defects(e.g.,concentration,location,geometric and electronic structures)can serve as the knobs for maneuvering molecular adsorption and activation as well as altering subsequent reaction pathway.Moreover,Wang et al.[177]summarized the design methods of surface defects and functional interface from atomic-level in electrocatalysts to promote efficiency and selectivity.Xue et al. [178] summarized the construction of the diverse defect types for carbon materials in electrocatalytic CO2reduction, and revealed the relationship between structure and activity of CO2reduction.

According to above analysis, we believe that adjusting the defect structure of the catalyst should be applicable for photoelectrocatalytic CO2reduction. But the current photoelectrocatalytic research is in a relatively initial stage, and the understanding of defect promoting the photoelectrocatalytic performance remains to be developed. Yang et al.[185]reported a Cu and N co-doped SnO2-based catalyst with defect for photoelectrocatalytic CO2reduction. The DFT revealed that the defect levels can narrow band gap for accelerating charge transfer, thus improving the photoelectrocatalytic performances in CO2reduction to formate (Fig. 13a-c). Additionally, Yuan et al. [180] uses DFT to study photoelectrocatalytic CO2reduction on the defective TiO2surface(Fig. 13d), at both solvent/catalyst and electrolyte/catalyst interfaces.Their study revealed an essential role of oxygen vacancy in the reduction process.The defect can promote the adsorption and activation by charge transfer between defective site and chemical adsorbed molecules on the surface (Fig. 13e and f). They also explained why the main product is methanol in electrolyte and why the reaction and/or diffusion process can control the overpotential.

Fig.14. (a)ZIF-8 formation on Ti/TiO2NT,(b)Formation mechanism of alcohol on Ti/TiO2NT-ZIF-8 electrodes[196],Reproduced with permission:Copyright 2018,Elsevier. (c) The adsorption mode of CO2 and Gibbs energy by DFT calculation (d) Mechanism of PEC CO2 reduction on ZIF9-Co3O4 NWs [197], Reproduced with permission:Copyright 2017,Elsevier.(e)Conversion pathways of CO2 on this photocathode,(f)Variation of the yield rate of formate(circles)and hydrogen(squares)on Ru(bpy)2dppz-Co3O4/CA with the applied potentials [198]. Reproduced with permission: Copyright 2016, Royal Society of Chemistry.

5.3. Surface reaction

Surface reaction is an important step in the process of photoelectrocatalysis. After generating electron/hole pairs, they need to be separated and transferred to the catalyst surface to carry out redox reaction. The adsorption of CO2on the active sites on catalyst is very important,which is the basis of starting catalytic CO2reduction reaction.Therefore, enhancing CO2adsorption can effectively improve the efficiency of photoelectrocatalytic CO2reduction. Additionally, CO2is a stable nonpolar linear molecule,which is difficult to be directly reduced.Enhancing the activation of CO2on photoelectrocatalyst can reduce the reaction barrier and improve reduction efficiency of CO2. Moreover,activation of CO2also can affect the reaction pathway to determine product selectivity of photoelectrocatalytic CO2reduction. In photoelectrocatalytic CO2reduction, various intermediates or byproducts can be obtained. Notably, the adsorption and desorption of intermediate or byproducts can also affect the selectivity of target product. The section will discuss the effects of surface reaction on photoeletrocatalytic CO2reduction and also summarized how to control the surface reaction of catalyst to enhance the photoelectrocatalytic performance.

5.3.1. Adsorption and activation of reactants

Fig. 15. (a) The high-index (730) facets of PtCu co-catalysts on C3N4 [203], Reproduced with permission: Copyright 2017, Royal Society of Chemistry. (b) Correlations between CH4 selectivity and surface site proportion as functions of the size of Pt NPs [39], Reproduced with permission: Copyright 2018, The Author(s). (c)Schematic illustration about the product selection for CO2 reduction depends on the surface chemistry of Ta3N5 and LaTiO2N photocatalyst [204], Reproduced with permission: Copyright 2018, Royal Society of Chemistry. (d) Mo sites especially Mo-terminated edges on MoS2 nanosheets benefit the stabilization of intermediate products (CHxOy) [205]. Reproduced with permission: Copyright 2017, Royal Society of Chemistry.

Generally,the main reactants include CO2and H2O in photocatalytic CO2reduction. CO2is a stable nonpolar linear molecule and the bond energy C--O is 799 kJ mol-1,which determine the activation and direct reduction of CO2is difficult[186].Enhancing adsorption and activation of CO2on photocathode can improve CO2utilization efficiency and reduce reaction barrier, which greatly affects product selectivity [187].Adjusting the coordination modes with CO2by changing the surface atomic construction of photocathode can significantly enhance adsorption and activation of CO2[87]. In addition, H2O not only consumes photogenerated holes to undergo oxidation half-reaction but also provides proton source for CO2hydrogenation process[64,188].Moreover,H2O can affect the adsorption structures of CO2on catalyst surface due to the co-adsorption of H2O and CO2[189]. Therefore, enhancing adsorption activation of CO2and balancing the competitive adsorption of H2O and CO2are the key to determine the reaction efficiency and product selectivity of CO2reduction[87,189-195].And for photoelectrocatalytic CO2reduction system with two electrode chambers mentioned in Fig.1,the H2O oxidation and CO2reduction are carried out in anode chamber and cathode chamber, respectively. Therefore, the competitive adsorption of H2O and CO2would be obviously inhibited.The key of regulating the reaction efficiency and product selectivity of photoelectrocatalytic CO2reduction is to enhance the adsorption and activation of CO2. Cardoso et al. [196] reported a Ti/TiO2nanotubes by zeolite imidazole framework-8(ZIF-8)for photoelectrocatalytic CO2reduction to alcohols(Fig. 14a). The experiment results exhibited the increased photocurrent of ZIF-8 at Ti/TiO2electrodes. And the concentration of CO2was dramatically increased in saturated solution with CO2, which indicated the efficient preconcentration of CO2on the electrode. When CO2adsorbed on ZIF-8,the nitrogenated sites of the imidazolate groups in the ZIF-8 complex would interact with CO2(Fig. 14b) to form stable carbamates,which promoted the adsorption and activation of CO2.Under light irradiation, photoelectrocatalytic efficiency of CO2reduction to form alcohols was greatly improved.Under UV-vis irradiation and an external potential, the formation mechanism of alcohol on Ti/TiO2NT-ZIF-8 electrodes was proposed in Fig.14b.

Additionally, Shen et al. [197] constructed a biomimetic photoelectrocatalytic interface. The cobalt-containing zeolite imidazolate framework (ZIF9) acts as CO2fixation and activation substrate, and Co3O4nanowires (NWs) performs as the photoelectrocatalyst. For ZIF9 modified Co3O4NWs,CO2could be concentrated on ZIF9 and activated via binding Co atom to the O atom of CO2(Fig. 14c). By photoelectrochemical CO2conversion,formate was produced with conversion rate (72.3 μmol·L-1·cm-2·h-1) and selectivity (～100%) at a low overpotential of 290 mV. The proposed mechanism of photoelectrocatalytic CO2reduction on ZIF9-Co3O4NWs was shown in Fig. 14d. Moreover,Huang et al. [198] established a photoelectrocatalytic interface with semiconductor/metal-complex hybrid, which can enhance adsorption and activation of CO2by using a carbon aerogel as the CO2fixation substrate.Compared the CO2surface concentration on Co3O4/FTO,that in this hybrid interface has a 380-fold increase.The photoeletrocatalytic mechanism of CO2reduction was exhibited in Fig. 14e, CO2was firstly activated to form CO2·-and then turned into HCOO-by protonation,conversing to the formate in the end.The yield of formate reached at 110 μmol·cm-2h-1(Fig.14f)with a selectivity of 99.95%.

5.3.2. Active sites and adsorption/desorption of intermediates

The intrinsic properties of active sites would determine their practicability for different target catalytic reactions,which makes the product selectivity is closely related to the number and type of active sites[199].Therefore, designing and regulating active sites can effectively adjust product selectivity of CO2reduction [51,87,200-202]. The low coordinated atoms on surface of catalyst can generally perform as good active sites to activate CO2and promote reaction. And the high-index facets may form more low coordinated atoms. For example, Lang et al. [203]found the high-index (730) facets of PtCu co-catalysts showed better improvement effect on generating efficiency and selectivity of CH4thanlow-index (100) facets. Which might be attributed to the formation of more low-coordinated metal active sites in high-index (730) facets(Fig. 15a). Moreover, the surface structure of catalyst will be greatly affected by size and might induce the formation of different active sites on surface,which can significantly determine product selectivity of CO2reduction.For instance,Dong et al.[39] studied the size effect of Pt NP co-catalysts for CO2reduction.The selectivity of CH4shows a consistent trend with the terrace site fraction but shows the opposite relationship with the low-coordinated site fraction over the variously sized Pt NPs(Fig.15b).Therefore,they conjectured that the active sites were terrace sites to generate CH4on Pt NPs while the low-coordinated sites might be active sites to hydrogen evolution. The structure of semiconductor photoelectrode is essentially similar to that of photocatalyst or electrocatalyst,so the construction and design of active sites on photoelectrode to control the efficiency and selectivity of CO2reduction can actually find inspiration from the cases of photocatalysis or electrocatalysis. At present, the research progress on the construction of active sites in photoelectrocatalytic CO2reduction is relatively slow and insufficient,and the typical cases of active site design in photocatalyst or electrocatalyst can guide and develop the design of photoelectrodes with unique active sites.According to the above analysis, it might be feasible that the efficiency and selectivity of photoelectrocatalytic CO2reduction could be effectively improved by constructing suitable active sites on photoelectrode.This is also an important research direction of photoelectrocatalytic CO2reduction.

Table 1 Some photo-electrocatalytic systems reported for the CO2 reduction.

Table 1 (continued)

(continued on next page)

Table 1 (continued)

Table 1 (continued)

Additionally, many kinds of intermediates or byproducts can be obtained during the process of CO2reduction. The adsorption and desorption of intermediate or byproducts would influence on the target product selectivity.If the interaction between intermediate and catalyst is strong,it may be difficult for the intermediate to desorb from the catalyst surface, which makes further hydrogenation reduction reaction more possible. On the contrary, the weak interaction would lead to easier desorption and release from the surface of catalyst for byproducts,which might greatly affect the active site exposure and product selectivity[87,204-208].For example,the adsorption and desorption properties of CO,as an important intermediate,would significantly determine the product selectivity in photocatalytic CO2reduction[204,206,207].Lu et al.[204]reported that the weak adsorption of CO on the surface of Ta2N5leaded to the rapid desorption of CO, which became the main product on Ta3N5.However,CO can strongly adsorb on the surface of LaTiO2N,which made CH4become the final product. Besides CO, the adsorption properties of the methoxy (·OCH3and ·CH3) radicals and CHxOyintermediates also greatly influence on product selectivity of photocatalytic CO2reduction[205,208].For example,Tu et al.[205]reported that CO2can selectively reduce to form CH3OH on MoS2-modified TiO2(Fig. 15d). During photocatalytic CO2reduction,the Mo sites can stabilize CHxOyintermediates by electrostatic attraction, which is beneficial to selectively reduce CO2to form CH3OH. For photoelectrocatalytic CO2reduction on semiconductor photoelectrode, the conversion process of CO2is similar to that on semiconductor photocatalyst, which means that the adsorption and desorption of intermediate also can determine product selectivity.According to above discussions, there is a strong correlation between active sites and adsorption/desorption of intermediates.Therefore,when designing and constructing the active sites, it is necessary to comprehensively consider the effects on adsorption/desorption of the intermediates, enhancing product selectivity in photoelectrocatalytic CO2reduction.

6. Some reported PEC CO2 reduction system application

Up to now, various photoelectrocatalysts have been reported to be used in typical systems of photoelectrocatalytic CO2reduction (Figs. 1 and 2)and Table 1 summarized the detailed experimental conditions and obtained efficiencies as follows. Generally, photoeletrocatalytic CO2reduction is carried out in a three-electrode system including work electrode, counter electrode and reference electrode. Additionally, light source,electrolyte and applied bias voltage are usually used as reference for basic experimental conditions.According to the reported results,the most evaluation parameters of efficiency include Faraday efficiency(FE),product conversion rate or current density.And a few results calculated the turnover number(TON)or quantum yield(QY).In fact,there is still no standard test specification,which make it difficult to reliably compare the results of different laboratories. We recommend the evaluation parameters discussed in the third part of this paper, which integrated the efficiency evaluation of electrocatalysis and photocatalysis. Moreover,the CO2reduction reaction on photoelectrode is very complex involving the adsorption-conversion-desorption of intermediates. Therefore, the reaction stability of the photoelectrode is very important for the practical application of photoelectrocatalytic CO2reduction and a stability test is needed in the experiments. Although various kinds of photoelectrocatalytic materials have been developed for CO2reduction, they are basically in the laboratory research stage and far from practical application.

7. Summary and prospect

With development of society and increase of world population, the demand for energy is soaring year by year.Among various applications,solar-driven CO2reduction to generate low-carbon fuels or smallmolecule organic compounds not only benefits CO2emission reduction,but also can be used as a carrier of energy storage to some extent,which is significant to alleviate energy shortage and global environmental crisis.Among diversiform methods for CO2conversion, the photoelectrocatalysis can endow more remarkable performance by integrating photocatalysis with electrocatalysis and developing the merits of both approaches. In recent years, the photoelectrocatalytic CO2reduction developed rapidly, which have been systematically summarized and discussed in this review.

To better understand the principle of photoelectrocatalysis,technical principles of photocatalysis and electrocatalysis were firstly summarized and expounded, respectively, and their advantages and disadvantages were compared. Then the main system and principle of photoelectrocatalysis are summarized. According to the different materials of photoelectrode, the photoelectrocatalytic CO2reduction system can be divided into following categories:I.dark anode and p-type photocathode,II. dark cathode and n-type photoanode,III. p-type photocathode and ntype photoanode.Recently,some novel system have been also developed to carry out photoeletrocatalytic CO2reduction. Ⅳ, a novel tandem device is developed by integrating a solar cell for supplying external voltage to promote CO2reduction reaction,also showing the excellent efficiency and promising potential.Ⅴ,a continuous-flow PEC CO2reduction system was also developed,which directly introduce gas CO2onto the surface of cathode to carry out reduction reaction, showing excellent selectivity.Then, the influence factors of photoelectrocatalytic CO2reduction were also discussed and analyzed, including influence of photoelectrode(thermodynamics and kinetics), interaction between reaction molecules and catalysts (initial activation and C-C bond formation), reaction conditions (such as light source, electrolyte effect and overpotential, etc.).Notably, enhancing the product selectivity in the reaction process of photoelectrocatalytic CO2reduction is one of the keys to obtain high yield of target product and realize practical application.Considering that the structure and reaction characteristics on photoelectrode are the key factors affecting the efficiency and selectivity of photoelectrocatalytic CO2reduction, we discussed the main strategies for enhancing the reaction efficiency and product selectivity of photoelectrocatalytic CO2reduction on photoelectrode include photoexcitation (e.g., doping element, introducing surface plasmon and quantum dots), charge separation efficiency (e.g., loading co-catalysts, constructing heterojunction and defect structure),surface reaction(e.g.,adsorption and activation of reactants,adsorption/desorption of intermediates),and their synergistic effects.

Although these strategies have been made in enhancing reaction efficiency and product selectivity of CO2reduction,many explorations and approaches are still in the initial stage,which leads to a fact that realizing large-scale practical applications still need a long way to go.Some typical challenges and issues are listed below:(1)the conversion efficiency and product selectivity of CO2are still insufficient for practical application;(2) the comprehensive and reliable detection system and standard is lacking;(3)developing more advanced in-situ characterizations to reveal reaction mechanism and conversion pathway is necessary;(4)combining with DFT calculation and experiment is powerful to guide the design of photoelectrocatalyst.Although challenges and hidden dangers still exist in this field, we believe optimistically that the technology of photoeletrocatalytic CO2reduction can realize the practical application. With the further development and combination of experiment and computation to deeply explore conversion mechanism, novel and efficient photoelectrocatalytic CO2reduction systems will be designed to accommodate large-scale applications in the near future.

Declaration of competing interest

None.

Acknowledgement

This work was supported by the National Natural Science Foundation of China (Grant Nos. 21822601, 21777011, and 52002054), the Fundamental Research Funds for the Central Universities(ZYGX2019Z021),the 111 Project(B20030),and the Plan for“National Youth Talents” of the Organization Department of the Central Committee.