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General Review on the components and parameters of photoelectrochemical system for CO2 reduction with in-situ analysis Amol Uttam Pawar, Chang Woo Kim, Minh-Tri Nguyen-Le, and Young Soo Kang ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b06303 • Publication Date (Web): 15 Mar 2019 Downloaded from http://pubs.acs.org on March 16, 2019

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ACS Sustainable Chemistry & Engineering

General Review on the components and parameters of photoelectrochemical system for CO2 reduction with in-situ analysis Amol U. Pawar †, Chang Woo Kim ‡, Minh-Tri Nguyen-Le † and Young Soo Kang †* † Korea

Center for Artificial Photosynthesis and Department of Chemistry, Sogang University,

35 Baebeom-ro, Mapo-gu, Seoul 121-742, Republic of Korea ‡

Department of Graphic Arts Information Engineering, College of Engineering, Pukyong

National University, Busan 48547, Republic of Korea. *Correspondence and requests for materials should be addressed to Prof. Dr. Young Soo Kang ([email protected]) Content list  Abstract  Keywords  Introduction  Principle of PEC CO2 reduction  CO2 reduction mechanism  Different type of PEC systems:o Photoanode based PEC system o Photocathode based PEC system o Photoanode-Photocathode based PEC system  Electrolyte solution  Figure of merits:o Solar to fuel (STF) efficiency o Current density o Faradaic efficiency o Stability

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o Production rate o Turnover number (TON) and turnover frequency (TOF) o Reaction temperature and pressure  PEC cell design  In-situ analysis  Conclusion  Acknowledgements  References


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Abstract One of the promising renewable energy sources is converting CO2 into useful chemical fuels; it will help to overcome both energy supplying and global warming issues. Among various methods for CO2 reduction, photoelectrochemical (PEC) CO2 conversion method is most promising and easy method so far because it can be useful to control chemical reaction thermodynamics and kinetics of CO2 reduction reaction for product selectivity and fast reaction kinetics by mimicking natural photosynthesis without high in-put energy cost. However, it has some limitation factors that creating difficulties for their widespread utilization. The detection of CO2 products and intermediates in a small scale is another challenging part in this field. The small amount of the CO2 product can be further oxidized or evaporated which could be difficult to be detected. Therefore, an in-situ technique is good approach to study proper reduction mechanism during CO2 reduction process. This article starts with basic concept of solar to fuel (STF) conversion by CO2 reduction and covers various aspects related to photoelectrochemical CO2 reduction. The main theme of article is completely depending on important parameters such as photoanode materials, photocathode materials, electrolyte solutions for photo-electrochemical CO2 reduction with most important in-situ measurement techniques for product analysis. Keywords Photoelectrochemical CO2 reduction, photoanode, photocathode, electrolyte, in-situ analysis



Introduction Fossil fuel is very useful in day today’s life as an energy source, but it also has severe disadvantages such as air pollution and global warming. Specifically, the climate change is one of the most important environmental issues of the earth in this century and increasing concentration of carbon dioxide (CO2) in atmosphere due to human activity with the use of fossil fuel is a key factor behind it.1 By addressing this issue it is necessary to decrease concentration of CO2 in the atmosphere as well as man made artificial emission of CO2. Hence, various strategies are introduced by various groups and scientists; such as CO2 capture and storage technology and CO2 conversion technology with different ways.2 CO2 reduction into fuels or chemicals has been studied with solar thermo-catalytic,3 biological technologies,4 photoanodic,5-7 electrochemical (EC),8-10 photochemical (PC)11-16 and photoelectrochemical (PEC)16-18 technologies. In the above listed methods, thermo-catalytic CO2 reduction techniques need a high operation cost because of high in-put energy cost. Biological method by using enzyme or bacteria has a very slow reaction kinetics because it occurs through the host-guest reaction system. PC and PEC methods are solardriven CO2 reduction process with comparatively low cost, easy to handle for final targeted product such as methanol by thermodynamic and kinetics control. More advantages and disadvantages of above-mentioned systems are presented in Table 1. Especially, liquid fuels have attracted considerable attention in these days because liquid fuels are easy to be stored and transported compared to gas fuels like methane and hydrogen gases.18,19 A PC process can be done by homogeneous or heterogeneous system. In a typical homogeneous system made up of a photocatalyst and reactants present in same phase. Usually, it is consisted of photosensitizer, molecular catalyst of organo-metallic complex and sacrificial agent.16 When light is incident on the photocatalysts, electron-hole will be generated. The produced electrons will be


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transferred to organo-metal complex of the homogeneous photocatalysts for CO2 reduction reaction and hole will transfer towards sacrificial agent (in some cases without sacrificial agent, holes can be utilized for water oxidation). This homogeneous organo-metallic photocatalysts are facing some critical problems with chemical stability and durability issues. It also has a low efficiency, moreover, for metal complex it requires very costly rare earth elements. In a heterogeneous photocatalytic system, photocatalysts and reactants are in different phase. In this case most common photocatalyst materials are used as metal oxides and semiconductor materials. But those semiconductor materials should have appropriate band energies and bad gap that can absorb visible light with highest radiation density of solar light. To have product selectivity of CO2 reduction, its conduction band energy level should be more negative than CO2 reduction level and valence band energy level should be more positive than water oxidation level. These conditions minimize the range for the selection of semiconductor photocatalyst materials for CO2 reduction.18 Another most important approach for solar light driven CO2 reduction is PEC CO2 reduction, it shows great potential in broad range of solar spectrum in case of water splitting.20,21 It may be due to external bias potential, which can help to minimize electron-hole recombination rate and the maximum generated electrons take part into CO2 reduction process. However, in CO2 reduction case, PEC systems are in the initial phase. PEC system consists of working and counter electrodes, among them one or both being photoelectrodes such as photoanode/cathode system or anode/photocathode system or photoanode/photocathode system. In laboratory scale, it contains reference electrode to observe the exact reduction potential value of half reaction of working electrode in a cell. In every case, the role of photoelectrode materials as well as choice of electrolyte are very important. Different combination of materials shows different product with different yield.



Basically, n-type semiconductor materials are used for photoanode and p-type semiconductors are used as photocathode. When the light is incident on photoanode, it generates electron-hole pairs and those electrons will be transferred towards cathode for reduction reaction while the holes will be utilized for oxidation reaction on the surface of photoanode. Conversely, in photocathode case, it generates electrons-holes after light irradiation and photogenerated electrons will be utilized for reduction reaction on the surface of photocathode, at the same time, the oxidation reaction occurs on the surface of the photoanode by photoproduced holes. In the case of light irradiation on both photoanode and photocathode, the photo-generated electrons in photoanode will be transferred to photocathode by external bias potential and added up to the electrons photoproduced in the photocathode. At the same time, the produced holes in the photoanode will be used for the oxidation in the electrolyte of the photoanode compartment. The energy levels of valence band and conduction band and the additional energy supplied by external bias potential are most important for product selectivity. In the CO2 reduction reaction, electrolyte plays an important role for solubility of reactant CO2 and proton diffusion from anode compartment to cathode compartment, which is very critical points for the efficiency of CO2 reduction into liquid fuel products. There are several PEC CO2 reduction products reported so far such as methanol, ethanol, formic acid, formaldehyde, methane, CO etc. Although the efficiency of PEC system is developing, several parameters need to be optimized for higher efficiency of CO2 reduction. Scheme 1 shows the proposed schematic representation of thermodynamic energy level diagram of water splitting and CO2 reduction reaction via PEC system with and without catalyst. In water oxidation reaction it shows green color arrow from H2O energy level to O2 energy level with proton production by water oxidation, it is known as activation energy curve for water splitting reaction. A highly stable material with high performance (photocurrent) can be used as photoanode. TiO2


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or modified TiO2, BiVO4 etc. are some of the candidate materials for photoanode. On the other hand, CO2 reduction reactions with red and blue lines are the activation energy curves with and without catalysis, respectively. If we use proper activator (a material which can activate CO2 for easy electron transfer process), it can reduce activation energy of CO2 reduction reaction via electron transfer pathway and it forms activated CO2 complex as [CO2*] in the reactant state and decreased thermodynamic energy level of transition state of electron transfer as [e- + CO2*]≠ complex, this complex requires less activation energy of electron transfer to form transition state of rate determining step for the further products of CO2 reduction reaction such as formaldehyde, methanol or methane. Here, it is assumed that the energy difference between Gcat and Gcat is amount of the reduced activation energy due to reactant of activated [CO2*] complex and the lowered transition state energy level of [e- + CO2*]≠ complex. (Note that* and ‘≠’ indicate the activated state and transition state of CO2 by adsorption on the cathode surface). There are few good materials related to H2 production from water splitting, in that case same H2 production materials can be used as CO2 reduction with some CO2 activator materials such as polyvinylpyridine (PVP) or polypyrrole (PPy). Hence, they will minimize water splitting and will enhance CO2 reduction. Hence, in this article, many examples of materials for photoanode, photocathode and electrolytes are discussed with very recent literatures. Principle of PEC CO2 reduction: CO2 reduction reaction requires proper combination of electrons for the reduction of CO2 and holes for the water splitting to produce protons with appropriate energy; those combinations will generate different products and related reactions are shown in Figure 1(b). Therefore, the choice



of photoanode and photocathode is very much important, because conduction band position of semiconductor photocatalyst should be perfectly matched or little bit more negative than that of the reduction energy level of CO2 for product selectivity and valence band position should be more positive than the oxidation of water molecule. Band positions of various semiconductor materials are presented in Figure 1(a). But it is difficult to find pristine semiconductor material with proper band energy position for both CO2 reduction as well as water oxidation reaction. Hence, it introduces different kind of PEC systems for CO2 reduction with solar light irradiation on photocathode, photoanode and both photocathode and photoanode (Figure 4). These systems with combination of two different semiconductors or combination of semiconductor and metal could achieve proper reduction and oxidation reaction. Basically, in the PEC system consisted of cathodic and anodic compartments separated by Nafion as proton exchange membrane, CO2 reduction reaction occur in cathodic compartment and water oxidation reaction occur in anodic compartment. Protons generated during water oxidation (2H2O + 4h+  O2 + 4H+) in the anode compartment will be transferred from anodic compartment to cathodic compartment through proton exchange membrane by concentration gradient driving force. The details about all these three systems are presented in text below with proper examples. CO2 reduction mechanism: CO2 is a highly stable molecule because carbon atom in CO2 has sp-hybridized and it will form two sigma bonds with oxygen atoms and other two nonhybridized p orbitals of carbon will form two pi bonding with p orbital of oxygen. It makes this molecule highly stable and need very high dissociation energy as  750 kJ mole-1.22 But proton coupled electron transfer reaction could reduce that energy; different number of protons and electrons forms different products of hydrocarbon


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fuels (Figure 2(b)). Possible reaction pathways to chemical fuels from CO2 reduction via PEC/EC/PC have been explored in the review articles.23-25 Different hydrocarbons fuels by electrochemical potential, which corresponds electrode potentials versus standard hydrogen electrode (SHE) in aqueous solution (pH = 7, at 25 °C, 1 atm, and 1.0 m concentration of other solutes) are listed up in Figure 2(b).26,27 Thermodynamically stable CO2 molecules can be converted to C1 and C2 hydrocarbon molecules, for example, formic acid (HCOOH), carbon monoxide (CO), formaldehyde (HCHO), methanol (CH3OH), methane (CH4), ethylene (C2H4) and ethanol (CH3CH2OH) between potential range of -1.90 V and -0.33 V.23 Such thermodynamic approach for hydrocarbon fuels can be realized by multiple electron-proton transfer and result in proposed reactor of PEC/EC CO2 reaction in Figure 2(a). Inherently, a composed mixture of liquid products (HCOOH, CH3OH, C2H5OH, etc.) and gaseous products (CO, CH4, etc.) can be evolved on the cathode in the PEC and EC cell. Such inevitable challenging lead to low efficiency and selectivity through CO2 reduction reaction path, requiring kinetic and thermodynamic understandings and control of sluggish CO2 reduction process. By the development of selectivity and efficiency depends on catalysts, based on the different reaction paths, different products can be formed. Please check Figure 2(c) and Figure 3 for reaction mechanism. Hori et al. have proposed that in the first step of CO2 reduction, a CO2 radical anion (CO2•−) could be stabilized on the catalyst surface by back donation of electrons from the highest occupied d orbital of catalyst to the lowest unoccupied antibonding (π*) orbital of CO2•−.28, 29 This pathways facilitate adsorbed CO2•− to be bent and result in lowers the energy for electron transfer and hydrogenation of the O atom. The evolution of adsorbed CO2•− anion radical is proposed to undergo rate



determining step (RDS) which is continuous reduction and protonation to hydrocarbon fuels. A coupled electrons and holes (protons) are required to be transferred to the adsorbed CO2 molecules ranging from 2e- (CO and HCOOH) up to 18e- (CH3CH2CH2OH) in the CO2 reduction reaction.30,31 Such a multi-electron-proton coupled transfer process is classified a C1 pathway for methane and a C2 pathway for ethylene.32,33 The C1 pathway facilitates that the CO intermediate is converted to methane through the formation of a *CHO or a *COH species. Comparatively, the C2 pathway includes that a CO dimerization is mediated by electron transfer to form a *C2O2− intermediate as the key C-C bondmaking step. The adsorbed CO dimer with negative charge is formed by proton transfer.10,34 This dimerization is the RDS for the reduction of CO. The pathways suggested by Hori et al. showed that the first electron transfer is the RDS for the formation of ethylene from CO is the RDS, whereas a proton transfer step is the RDS for methane after the first electron transfer.34,35 Given the fundamental research for CO2 reduction pathway, kinetic and thermodynamic aspects are expected to be considered for enhanced catalytic activity, higher product efficiency and selectivity. Different type of PEC systems Photoanode based PEC system: - PEC system consists of three important component electrodes such as reference electrode, counter electrode and working electrode. In this type of PEC system, working electrode and reference electrode should be in the same anodic compartment and counter electrode will be in cathodic compartment. A reference electrode and counter electrode will be


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Ag/AgCl and semiconducting photocatalyst, respectively. The choice of photoanode depends upon reaction parameters; photon absorption coefficient, electron/hole diffusivity, electron collection efficiency and band energies because the solar light is irradiated on the photoanode. Basically, photoanode as counter electrode and photocathode working electrode for CO2 reduction should be connected with external wire to supply external electrical energy by controlling bias potential to add more kinetic energy of photoproduced electron to be transported from photoanode to cathode. When light is incident on photoanode, electrons-holes are generated and water oxidation reaction occurs on the surface of photoanode with the produced holes; generated electrons will be transferred towards cathode for CO2 reduction reaction via external wire by the applied bias potential between photoanode and cathode electrode. P-type semiconducting photocathode have been usually unstable during CO2 reduction and their combination with counter electrode is proper to produce the two electron reduced products as gas product because of the performance of counter anode.37 As a photon absorber, many n-type photoanodes is low-priced and highly stable for producing proton from water oxidation. N-type semiconducting photocatalysts such as TiO2, ZnO, Fe2O3, BiVO4 and WO3 are favorable to produce large amount of proton for active CO2 reduction into liquid fuel products.17 Gong’s group describes a simple strategy using TiO2 nanorod photoanode to achieve stable aqueous CO2 reduction with a high Faradaic efficiency of 87.4% and a selectivity of 92.6% for carbonaceous products as shown Table 2.38 Few other examples are also presented in Table 2. Cheng et al. reported CO2 reduction reaction using anodized TiO2 nanotube and a Pt-modified TiO2 nanotube.39,40 Obtained CO2 reduction products are presented in Figure 5(b) & (d). In a photoanode driven CO2 reduction reaction, the carbon atom conversion rate markedly increased to 4340 nmol/h cm2 by optimizing CO2 reduction conditions in the PEC cell. The combination of the N-doped



TiO2 photoanode with the Cu cathode (Figure 5(a)) produced CH4 and CH3OH in addition to HCOOH, with the high applied bias potential (2.8 V) and the low faradaic efficiency (21%).41 Kang and his coworkers reported O2 evolution-favorable BiVO4 photoanode for the PEC CO2 reduction.42,43 Crystal facet engineered BiVO4 photoanode with (040) crystal facet exposure produced abundant proton to reduce CO2 molecules into liquid fuels such as HCHO, HCOOH, CH3OH, CH3COOH, CH3CHO and C2H5OH by reduction potential tuning (Figure 6(a)). Figure 6(b) shows the schematic of PEC cell used for CO2 reduction reaction. Its selective products were obtained by tuning the appropriate reduction potential, leading to faradaic efficiencies of 62% for formic acid (Figure 6(c)), 85% for formaldehyde (Figure 6(d)), 8% for MeOH (Figure 6(e)), and 6% for EtOH (Figure 6(f)) at different bias potential in NaCl electrolyte.43 CO2 reduction has been studied in a photoanode-driven photoelectrochemical system consisting of a WO3 photoanode and Cu or Sn/SnOx as the cathode (Figure 7(a)).44 The faradaic efficiencies of 67% for CH4 and 71.6% for all carbon-containing products were achieved (Figure 7(b)). With Sn/SnOx, a combined faradaic efficiency (CO + HCOOH) of 44.3% was obtained at +0.8 V. They reported that the 2-electrode potential between WO3 working electrode and the counter electrode was less than the lowest bias potential reported so far for conventional photocathode-driven systems. The results demonstrate that the intrinsically more stable photoanode-driven systems could accomplish the reduction of CO2 with higher efficiencies relative to the conventional photocathode-driven systems. Additionally, in the photoanode-driven CO2 reduction, oxygen evolution electrocatalyst would be tailored on composite photoanode. The carbonate-coordinated cobalt (Co-Ci)/BiVO4/WO3 (photoanode)–Cu (cathode) system produced exceptionally high photocurrent of 3.5 mA/cm2 at 1.23 VRHE under 1 sun illumination, and an onset potential of 0.2 V RHE for CO2 reduction into solar fuel production of C1-C2 hydrocarbons with 51.9% faradaic


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efficiency.45 Figure 7(d) & (e) shows bar diagram of faradaic efficiency as well as product of faradaic efficiency into charge with different photoanode combinations. Figure 7(c) shows the schematic representation of electron hole transfer process and product formation in case of (CoCi)/BiVO4/WO3 (photoanode)–Cu (cathode) system. Considering that semiconducting photoanode has been widely used for producing proton from water splitting, most of them with long term stability and higher performances are applied to PEC CO2 reduction. The introduction of FeOOH/NiOOH oxygen-evolution catalyst on the BiVO4 photoanode by Choi et al. has shown long term stability over 500 h by suppressing anodic photocorrosion (Figure 8(a)).46 In Figure 8(b), it is shown that until 300 h reaction time there is no much change in BiVO4 but after 500 h XRD measurement shows little reduction of peak intensity of BiVO4 (Figure 8(c)). Together with perovskite photovoltaics, unbiased photoelectrochemical tandem assembly has been reported by Park, which showed a high faradaic efficiency of 96.2% and an initial conversion rate of 2.4 mM h-1 without applied external bias potential (Figure 8(d)).47 A self-assembled nanocomposite photoanode (Figure 8(e)) consisted of an epitaxial BiVO4 matrix embedded with WO3 mesocrystals would be promising for photoelectrochemical CO2 reduction in the visible-light regime. The orientation of the crystal facet and interface provides a superior template to understand the intimate contact between the two constituent phases. The interfacial coupling of the WO3 mesocrystal and BiVO4 matrix improves the separation of photoexcited carriers and the properties of charge transfer, which resulted in a greatly enhanced PEC performance compared with their parent compounds.48 Photocathode based PEC system: - In this system working electrode as photocathode is photoactive and it belongs to p-type semiconductor. A counter electrode will be anode and it can be metal or metal alloy depending upon reaction condition. The detail examples are given in text



below. A standard Ag/AgCl electrode is used as reference electrode. All these three electrodes are located into electrolyte solution, various type of electrolyte solutions are available; this part has been explained separately in given text. In photocathode based PEC system, light is incident on photocathode and electron-hole pairs will be generated. Generated electrons are utilized for CO2 reduction reaction on cathode surface and oxidation reaction occurs on anode surface. The generated electrons will be used for CO2 reduction and more electrons will be added into photocathode by transferring via external wire by given external bias potential between anode and photocathode. In this case total number of electrons will be larger than photoanode based PEC system. TiO2 is a very common photocatalyst used as photoanode as well as photocathode in CO2 reduction system. Xu et al. reported modification of TiO2 photocathode, such as introducing Pd nanoparticles for proton capture, Eosin Y disodium salt to adsorb solar energy and four different amine ligands to capture and activate CO2 gas.49 It produces 43.6 M/h cm2 methanol from CO2 reduction with faradaic efficiency higher than the 100%. The reason of high faradaic efficiency and performance is presented in schematic mechanism Figure 9(a). Because of photoinduced electron transfer from dye molecule to conduction band of TiO2 and further it goes to Pd nanoparticles. Those electrons can generate hydrogen gas by transferring to the proton on Pd nanoparticles or it can be recombined with holes of surface TiO2. The amines will capture and activate CO2 on the surface of electrode and CO2 will get reduce into CO2-•, CO-•, CH3O-• and finally CH3OH by electrons and protons. Another combination with TiO2 has been introduced recently by Wang et al., it has been reported that nickel foam assisted amine functionalization of TiO2 (NH2/TiO2/Ni) as a photocathode in PEC CO2 reduction system. It produces methanol as a reductive product with the concertation of 153 M/h cm2. This system shows quantum efficiency of 1.2% which is two times better than the


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plants. However, they assumed that the faradaic efficiency as 100%. This reported photocathode shows better performance because of better light absorption and electron transportation due to the amine ligand which helps to adsorb CO2 on the surface of electrode. Figure 9(b) shows the mechanism of the CO2 reduction via PEC process.50 There are various combination of TiO2 with other materials reported so far. Ewelina et al. reported the mixed Cu2O nonstoichiometric TiO2 as a photocathode for CO2 reduction.51 In this report Ewelina et al. says that, not only pristine TiO2 but also defective TiO2 help to improve catalytic performance. A TiO2 treated with hydrogen flame to form structural defect (TiO2-x), those defects allow better charge mobility and suppresses electron-hole recombination rate, showed much higher CO2 reduction efficiency than pure TiO2. Hence, the photogenerated carriers do better performance in CO2 reduction. Recently Cardoso et al. suggested PEC system with new combination of metal-organic framework (MOF) based TiO2 nanotubes for CO2 reduction. The zeolite imidazole frame work-8 (ZIF-8) nanoparticles were deposited on TiO2 nanotubes using layer by layer process to be used as photocathode. The prepared photocathode shows CO2 reduction into methanol and ethanol fuels. The ZIF-8 is not only used for CO2 adsorption and activation but also it can be used as co-catalyst to transfer exited electrons to reduction site.52 In a previous several studies, TiO2 was used as protective layer for stability improvement. A Schreier et al. reported on the TiO2 protected and Cu2O assisted ZnO doped with alumina (Al:ZnO) photocathode with rhenium bipyridyl catalyst and showed a high efficiency of CO2 conversion into CO. The role of TiO2 was to increase stability and prepare stable electrode for CO2 reduction. In their study it was observed that 100% faradic efficiency for CO formation.53 Similarly Zeng et al. presented reduction of CO2 into methanol via GaP photocathode stabilizes by TiO2 layer. TiO2 layer improves stability of photoelectrode at the same time it helps to form



p-n junction with GaP and reduces electron-hole recombination rate. Futhermore it helps to lower the overpotential for CO2 reduction reaction.54 TiO2 is useful photocathode for CO2 reduction and there are some other materials shows more or less important and valuable performance in PEC CO2 reduction system. A p-type material Cu3Nb2O8 photocathode shows CO2 reduction towards carbon monoxide with faradic efficiency of 9% at -0.2 V vs Ag/AgCl.55 Another material reported from Yuan et al., Cus/CuO/CuInS2 photocathode shows better performance in methanol production during PEC CO2 reduction process.56 The performance is depending upon the In/Cu ratio, better performance comes from higher ratio. Duchene et al. suggested another system based on gold deposited p-type GaN (Au/pGaN) as a photocathode. In that case it was observed that Au/p-GaN photocathode shows better selectivity for carbon monoxide (CO) formation compared to H2 production in aqueous based electrolyte. It was concluded that non-equilibrium electron-hole pairs generated from Au surface plasmon and due to interfacial Schottky barrier at Au/p-GaN generated hot holes populated at valence band, detailed mechanism was presented in schematic diagram (Figure 9(c)).57 Very recently Rao et al. introduced silicon based photocathode with porous nanowire of SnO2 for CO2 conversion into HCOOH.58 It shows very high photocurrent of 10 mA/cm2 at -0.4 V vs RHE under solar light illumination and observed 60% faradaic efficiency for formic acid. A CuGaS2 as a new photocathode material was introduced by Ikeda et al. recently.59 However, this material is not highly stable, it needs to be improved, but this type of materials are helpful to reduce overpotential for CO2 reduction. Photoanode-photocathode based PEC system


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This system can be considered as an ideal system for CO2 reduction as same as photochemical process if this system does not have an external bias potential. It can be worked with external bias potential also, but the choice of photoanode and photocathode need to be considered with following important points. 1) Both electrodes should be highly stable in an electrolyte media for CO2 reduction, 2) band gap of both electrodes need to absorb visible range of light, 3) band position of both electrodes need to be matched with Z scheme so electron from anode can transfer to cathode with/without external bias and photoanode performs water oxidation reaction and photocathode performs CO2 reduction reaction simultaneously.37 These types of systems have been developed recently by different groups. A “Z” scheme system is basically considered as two electrode system and without bias potential. A PEC system consisted of p-type InP/Ru complexbased photocathode and TiO2 based photoanode showed that CO2 conversion rate into HCOO- was drastically enhanced without any bias potential (Figure 10(a)). It showed that product selectivity for formate (HCOO-) more than 70% and STF conversion efficiency was 0.03-0.04%.60 This is one of the good approaches for CO2 capture and reduction, but product selectivity and efficiency can be enhanced by optimizing other parameters such as conjugation confirmation, band positions of semiconductors, catalyst structure, etc. In the next experiment as following previous work, a system consisted of InP/Ru complex photocathode and SrTiO3 (r-STO) photoanode for CO2 reduction reaction without any bias potential shows that STF efficiency for formate was improved from 0.03% to 0.14%, moreover, it does not show any photodegradation of formate in aqueous medium (Figure 10(b)). Hence, the next study was done with very simple system without proton exchange membrane as wireless device of r-STO/InP/Ru complex in a one compartment as shown in Figure 10(c). It shows STF efficiency as 0.08%, which is higher than TiO2 based photoanode because no degradation of formate, but less than wire type system because, in wire type system,



electrochemical bias potential was given to transport produced electron from anode to cathode due to the lower potential at anode than cathode by the pH difference between two separate compartments. This pH difference was caused by CO2 bubbling in the cathode and Ar bubbling in the anode compartments, respectively.61 Lately, InP/Ru complex photocathode was replaced by ruthenium complex catalyst with multi-heterojunction structure of (TiO2/N, Zn-Fe2O3/Cr2O3) photocathode (Figure 11(b)) by keeping r-STO as photoanode. The whole system is shown in Figure 11(a), it shows STF efficiency of 0.15% without external bias potential which is higher than previously reported systems. The reason of high efficiency is easy charge transfer between photoanode and photocathode by proper band arrangement and heterojunctions in photocathode.62 The PEC system without external bias potential is an ideal one for photoanode-photocathode based system, but it is not easy to achieve. Hence, with external bias potential some photoanode photocathode based systems have been studied for CO2 reduction reaction. CO2 reduction to formate using Thiobacillus sp (TsFDH) biocatalyst was reported by PEC system consisted of three junction silicon based (3-jn-Si/ITO/CoPi) photoanode and hydrogen terminated silicon nanowire (H-SiNW) as photocathode under visible light illumination with external bias potential (Figure 12(a)).63 It showed that formate yield increased drastically from 0.069 to 0.26 with increasing external bias potential from 1.5 V to 1.8 V, respectively. It shows 16.18% faradaic efficiency of formate production at 1.8 V, however, lower potential (1.2 V) does not show production of formate (Figure 12(b)). This is known as biocatalytic PEC system and it’s totally NADH dependent FDH, which further assist reduction of CO2 to formate. In another example, the system consisted of NiORuRe photocathode and CoOx/TaON photoanode shows formation of carbon monoxide in visible light driven CO2 reduction at 0.3 V external bias potential.64 A schematic representation of this type of hybrid PEC cell with Z scheme was presented in Figure 12(c). A photocathode is consisted


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of Ru(II)-Re(I) supramolecular metal complex photocatalyst adsorbed on prepared NiO electrode. A photoinduced electron transfer from NiO to RuRe was contributed for CO2 reduction reaction for formate production. However, the total deposition amount of RuRe was estimated to be 9.7 nmol, but electrochemically active RuRe was measured to be 6.5 nmol, moreover, after PEC reaction for 5 hr, RuRe is reduced to 2.9 nmol by photocorrosion. However, lots of work has been published related to photoanode and photocathode, but a system based on the combination of photoanode-photocathode is rarely reported. This system has more opportunity for further development by considering various points such as band position tuning, proper product selectivity and multi-proton coupled multi-electron transfer to improve STF efficiency. Electrolyte solution Aside from the aforementioned photoelectrode materials, electrolyte also plays an important role in PEC reduction of CO2 into solar fuels. Chemically, an electrolyte is an ionic conductor consisted of ionic species in a specific solvent, providing ionic conductivity and thus facilitating charge compensation on each electrode in the PEC cell.65 Once an electrode is immersed in an electrolyte, it will form a Schottky junction with the electrolyte at the electrode/electrolyte interface. For example, in case of p-type semiconductors as the electrode material, when it is in contact with an electrolyte with a redox potential within the band gap of the semiconductor, electrons will be transferred from the redox potential of electrolyte to the Fermi level of the semiconductor and thus the energy of the Fermi level is raised until an equilibrium between the semiconductor Fermi level and the redox potential of the electrolyte is established. This results in formation of the Helmholtz layer (on the electrolyte side) at the electrolyte/electrode interface, and the depletion layer (on the



electrode side) at the electrode/electrolyte interface.66 This also causes the upward bending of the conduction and valence bands near the interface due to the in-built electric field in the depletion zone. On the other hand, in case of n-type semiconductors, the flow of electrons is from the semiconductor to the electrolyte, thus leading to a decrease in the Fermi level of the semiconductor and the downward band bending.67 It is obvious that selection of a solution as an electrolyte for the PEC reduction of CO2 is practically essential to obtain highly selective products and high Faradaic efficiency (FE). In other words, electrolyte compositions that were once thought to have no contribution to the overall PEC process of CO2 reduction may directly or indirectly influence on its product selectivity and conversion efficiency. Zanoni et al. obtained different pH-dependent products and CO2 conversion efficiency when conducting PEC reduction of CO2 on copper electrodes coated with copper oxide nanoparticles in a buffer solution of 0.1 M Na2CO3/ NaHCO3 as an electrolyte, with different pH value range of 8 - 11, at the applied potential of + 0.2 V (vs. Ag/AgCl).68 A 80% of CO2 was converted into methanol, ethanol, formaldehyde and acetaldehyde at pH 9 while lower conversion efficiency and higher selectivity were obtained at higher pH values, which is due to the change in the composition of the electrolyte at different pH values. Recently, a variety of solutions have been studied as electrolytes for PEC conversion of CO2 into solar fuels. Generally, they can be classified as aqueous, non-aqueous solutions consisting of sacrificial reducing agents or electron donors. Baker et al. successfully converted CO2 to acetate with 80% faradaic efficiency in 0.1 M NaHCO3 aqueous solution by using a CuO/CuFeO2 cocatalyst at -0.4 V (vs. Ag/ AgCl) under visible light irradiation.69 Rajeshwar et al. first reported that methanol was detected from the PEC reduction of CO2 on hybrid CuO/Cu2O nanorod arrays in 0.1 M Na2SO4 aqueous solution at -0.2 V (vs. SHE) with 95% faradaic efficiency.70 Recently, Cheng et al. showed 81% FE of PEC conversion of CO2


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into HCOOH on Pt/rGO/Ni foam photoelectrode immersed in a mixture of 1 M NaCl and 1 M NaHCO3 electrolyte solutions.71 With aqueous electrolytes, water serves as a sacrificial reducing agent as well as a plentiful proton source essential for the proton-coupled electron transfer (PCET) in the PEC CO2 reduction. However, the solubility of CO2 in water is relatively low (~ 35 mM at 1 atm),72 which limits the diffusion of CO2 to photocathode and largely suppresses the reduction process. Therefore, the use of non-aqueous solvents may overcome this limitation due to higher solubility of CO2 in non-aqueous solvents. For instance, it has been reported that the solubility of CO2 in acetonitrile is nearly 8 times higher than that in water under ambient conditions.73 Benefiting from the outstanding solubility, a variety of organic solvents such as methanol, acetonitrile, N,N-demethylformamide (DMF), etc. have been used as electrolytes for PEC reduction of CO2. Kaneco et al. investigated the PEC reduction of CO2 in a LiOH/methanol electrolyte at different metal-modified p-InP electrodes (Pb, Ag, Au, Pd, Cu and Ni).74 CO was the main product with the faradaic efficiency varied from 40% ~ 80%. The same product was also observed in CO2 conversion conducted in acetonitrile75 or DMF76 using different PEC systems. In those PEC systems, the proton source was absent, which results in the formation of CO as a predominant product of the CO2 reduction. Therefore, to sustain the PCET process, a sacrificial electron donor (e.g., triethanolamine, and triethylamine) must be added to consume photogenerated holes and supply protons.77,78 Water can also be added to non-aqueous solvents at a trace amount, and serves as a proton donor.79 In addition to organic solvents, ionic liquids have been reported to be emerging alternative electrolytes with the promising potential for PEC CO2 reduction due to their high ionic conductivity80 and superior ability to capture CO2 by making a complex with CO2 molecules.81 The ionic liquids have been widely used in electrocatalysis82 and photocatalysis.83 Recently, Wang et al. proposed that the utilization of an ionic liquid, namely



1‐aminopropyl‐3‐methylimidazolium bromide, can facilitate PEC reduction of CO2 with 86.2% faradaic efficiency at ambient conditions.84 The merit of the ionic liquids lies in their ability to lower the reduction barrier, mostly likely by complexation with CO2 molecules.85 Figure of merits: Recently there are many techniques which are used to identify performance of PEC system or performance of particular part of the system. Here we are reporting few methods which are highly used in PEC CO2 reduction system. Solar to fuel (STF) efficiency: Solar to fuel (STF) efficiency measurement is one the most important techniques to identify conversion efficiency of incident solar light under zero applied potential between working electrode and counter electrode.18 In a simple way it is a ratio of “output power” to the “input power”. Or it can be define as the ratio of “Generated chemical fuel” to the “incident solar light”.86 “Generated chemical fuel” is a multiplication of measured fuel (mmol/s) and change in Gibbs free energy of that particular fuel (kJ/mol); and “Incident solar light” is a product of power density of incident light (mW/cm2) and irradiated electrode area (cm2). Total formula is presented in Eq. 1. There are two approaches to calculate STF by considering output power measurement; one is related with direct measurement of fuel concentration by gas chromatography (GC) or GCMS (Eq 1). Another one is related with the measurement of product of current, voltage and faradaic efficiency (Eq. 2).

𝑆𝑇𝐹 =

𝑃𝑓𝑢𝑒𝑙(𝑚𝑚𝑜𝑙/𝑠)  𝐺𝑜(𝑘𝐽/𝑚𝑜𝑙) 𝑃𝑠𝑜𝑙𝑎𝑟(𝑚𝑊/𝑐𝑚2)  𝐴𝑟𝑒𝑎(𝑐𝑚2)

--------------------------------------(1)


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Pfuel is measured fuel production by GC or any analytic techniques; Go is the change in Gibbs free energy of CO2 to particular fuel formation; Psolar is a power density of incident light source and Area is electrode irradiated light area.

𝑆𝑇𝐹 =

𝐽𝑠𝑐(𝑚𝐴/𝑐𝑚2)  𝐸𝑜(𝑉)  𝐹𝐸 𝑃𝑠𝑜𝑙𝑎𝑟(𝑚𝑊/𝑐𝑚2)

------------------------------------------(2)

Jsc is the short circuit photocurrent density, Eo is the thermodynamic potential for fuel formation which are presented in Figure 1(b), FE is a faradaic efficiency and Psolar is the incident light power density. Current density: Current density is an important term to identify total charge generated by photoelectrode. Generally, chronoamperometry technique (I vs t) has been used to identify current density, according to definition of charge (Q, coulomb), it is product of current (A) and time (s). It is also helpful to calculate mole of electrons contributed during reaction by using Faradays constant (96485.33289 C = 1 mole of electrons). This can be further helpful to calculate Faradic efficiency (FE). Faradic Efficiency: - Faradic efficiency is a well-known with various names such as faradic yield, coulombic efficiency and current efficiency. It is explained by Eq. 3, moles of formed product (n) multiply by required number of electron (X) for CO2 conversion into particular product (Figure 1(b)) divided by charge (Q, current density) to Faradays constant (F, 96485C/mol). Finally, it will be multiplied by 100 to see output in easier form such as a percentage.

𝐹𝐸 =

𝑛(𝑚𝑜𝑙)  𝑋 𝑄(𝐶) 𝐹(𝐶/𝑚𝑜𝑙)

 100 --------------------------------------------------(3)



Moles of formed products (n) can be obtained from analytic techniques such as GC or MS. Few reported current density and FE values are presented in Table 2-5. Stability: Most of the semiconductor materials are not highly stable, hence, to check stability is an important part in this system. Stability can be checked via chronoamperometry such as “I vs t” plot with light irradiation; if photocurrent decreases with time then photoelectrode is not stable and need to improve its stability. Basically, lots of semiconductors are not stable because of photo corrosion, so those materials can be coated with another stable materials such as TiO2 or combine/coat with some hetero structure materials to minimize corrosion effect.46,53,54 Chang et al. reported that Cu2O corrosion occurs due photoengraved holes instead of electrons; therefore, it can be worked as dark cathode in a PEC system.38 Production rate: The main aim of PEC CO2 reduction system is to increase production rate of selective product. It is very well-known that production rate is depends on photocurrent density but, low photocurrent shows low production rate but that doesn’t mean high photocurrent always show high production rate because we need to consider product selectivity and FE for selective product. Sometimes high photocurrent can show various product (not particular one) as well as water splitting and back reaction, it reduces production rate due to high photocurrent. Hence, to calculate accurate production rate, quantification and product identification is important part such as liquid product (HCOH, CH3OH, HCOOH etc.) as well as gaseous product (such as CO, CH4, H2 etc.). The gaseous products mainly detect by using gas chromatography and liquid product from electrolyte solution can be detected by liquid chromatography and spectroscopy techniques such as nuclear magnetic resonance (NMR). Careful detection and quantification are most important part to identify accurate production rate. Production rate of specific products (in mole) is


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depending upon electrode area (cm2) and reaction time (h); hence unit of production rate is like (nmolh-1cm-2) or (mMh-1cm-2). Some examples from reference articles are presented in Table 2-5. Turnover number and turnover frequency: Another very useful and important point is turnover number (TON) and turnover frequency (TOF). It can be evaluated with following Eq. (4) and Eq. (5),22

𝑇𝑂𝑁 =

𝑇𝑂𝐹 =

𝑀𝑝𝑟𝑜𝑑𝑢𝑐𝑡 𝑀𝑐𝑎𝑡𝑎𝑙𝑦𝑠𝑡

………………(4)

𝑀𝑝𝑟𝑜𝑑𝑢𝑐𝑡 𝑀𝑐𝑎𝑡𝑎𝑙𝑦𝑠𝑡  𝑡

---------------(5)

TON is a formation of molar product with respect to molar formation of catalyst substrate and TOF is defined as TON per unit time. Reaction temperature and pressure Temperature and pressure have considerable impact on CO2 reduction process. It is mentioned that increasing temperature reduces CO2 solubility in the solvent, consequently it shows low production rate.22,87 Hence, few reports shows photocatalysis study with water cooling system.88,89 High pressure PEC system helps to increase concentration of CO2 resulting increasing PEC current density and it may increase production rate.90,91 PEC cell design Recently Castro et al. reported detail information related to PEC cell design for CO2 utilization.92 In general two types of PEC cell such as single compartment93 and double compartment36,43 cell was used for experimental purpose. Among them double compartment system is quite good for



CO2 reduction reaction. PEC cell is a most important part in CO2 reduction system, therefore, proper cell design needs to consider following few points. 1) Compartment for anode and cathode: It is important to have two compartments anode and cathode separated by high proton exchangeable membrane such as Nafion. It will help to identify product accurately as well it will avoid oxidation of formed CO2 product. 2) Size of each compartment: The main drawback of CO2 is that, it is low soluble in aqueous medium; hence compartment with low volume but high electrode surface area will be effective for CO2 interaction and reduction process. This type of system will be good to identify product precisely with qualitative as well as quantitatively. 3) Light window: The most useful light source in the PEC system is a 300 W Xenon arc lamp, this lamp will produce bright white light which is nearly match with natural sunlight. The incident light should be completely transferred through window and it should be reached on electrode surface if it is front illumination. Hence it is better to prepare window glass material with quartz glass, because it has high transmittance for complete solar light even UV light (

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