Frontiers in Photoelectrochemical Catalysis: A Focus on Valuable Product Synthesis

Photoelectrochemical (PEC) catalysis provides the most promising avenue for producing value‐added chemicals and consumables from renewable precursors. Over the last decades, PEC catalysis, including reduction of renewable feedstock, oxidation of organics, and activation and functionalization of C─C and C─H bonds, are extensively investigated, opening new opportunities for employing the technology in upgrading readily available resources. However, several challenges still remain unsolved, hindering the commercialization of the process. This review offers an overview of PEC catalysis targeted at the synthesis of high‐value chemicals from sustainable precursors. First, the fundamentals of evaluating PEC reactions in the context of value‐added product synthesis at both anode and cathode are recalled. Then, the common photoelectrode fabrication methods that have been employed to produce thin‐film photoelectrodes are highlighted. Next, the advancements are systematically reviewed and discussed in the PEC conversion of various feedstocks to produce highly valued chemicals. Finally, the challenges and prospects in the field are presented. This review aims at facilitating further development of PEC technology for upgrading several renewable precursors to value‐added products and other pharmaceuticals.


Introduction
Researchers have long been targeted at mitigating the ever-rising energy and environmental crisis that the extensive use of fossil Figure 1.The conceptual illustration of PEC catalysis for value-added chemical synthesis.PEC catalysis is a greener approach to transform readily available resources to valuable chemicals.The light source with energy (hv) greater than the band gap of photoresponsive material (Eg) will be shined to generate charge carriers (e − and h + ).The photogenerated charge carriers are separated by the applied bias, which is then involved in reducing/oxidizing the feedstocks.electrochemical, photochemical, and photoelectrochemical techniques to efficiently upgrade various precursors such as lignocellulose, [4] and industrial wastes, [5] have been the target of several studies.While there are still some technical and marketing challenges, the electrochemical synthetic strategy has shown comparatively promising progress, thus enabling the commercial production of high-valued chemicals. [6]For instance, the manufacturing of adiponitrile, acetoin, and 2,5-dimethoxy-2,5dihydrofuran has been realized through electrocatalytic synthesis schemes. [7]3b,8] Although at the early stage of development, interfacial photoelectrochemical tool has been emerged as versatile tool to derive a number of useful reactions in organic chemistry synthesis, which broaden the scope of visible light-driven catalysis for solar energy storage. [9]t present, solar-driven PEC has garnered several research interests due to its unique potential for converting solar energy and naturally available feedstocks into value-added chemicals. [10]y and large, the fundamental mechanism for the synthesis of fuels and chemicals over a solid semiconductor via PEC catalysis can be briefly described as follows: i) upon illumination with light, a semiconductor photocatalyst can capture the light, resulting in the generation of electrons and holes at the conduction band (CB) and valance band (VB), respectively; ii) the photogenerated charge carriers are then separated and transported to the photocatalyst surface with the aid of bias; and iii) these photoexcited electrons and holes are then utilized to drive reduction and oxidation reactions.From the point of thermodynamics, the position of CB and VB is very helpful in judging the suitability of a semiconductor photocatalyst to derive a particular reaction. [11]hile more rigorous study is still required to evaluate the practicality and economic benefits of employing this technology, it can be still regarded as a captivating platform to store those decentrally available resources in the form of chemicals, fuels, and consumables (Figure 1).The PEC synthesis of high-valued chemicals will gain impetus for at least four main reasons.First, it could be economically attractive since the process integrates sunlight, readily available resources (such as biomass, N 2 , and water), and catalysts to produce highly valuable chemicals, pharmaceuticals, and consumables; [12] second, compared with the thermochemical process being employed in the manufacturing of several chemicals, it can be operated at room temperature and pressure, which can avoid the extra cost required for heating and pressurizing the reactor system; third, the PEC process can offer the "greener approach" in synthesis chemistry (it is mostly carried out in an aqueous solvent, uses sunlight, and end with limited byproducts); lastly, the PEC catalysis offers the possibility to simultaneously tune the selectivity and kinetics of a reaction via manipulating the light illumination and electrode potential.These attributes may also be exhibited by a related technique known as PV-EC, where the electrochemical systems are coupled with a photovoltaic system.However, what makes the PEC system more advantageous is that it integrates the light illumination into the catalyst, and the photogenerated carriers are directly involved in the reaction system.This configuration ensures a simple and straightforward approach to obtaining valuable chemicals. [11]10a] Apart from this, the potential for employing Tandem configuration and the relatively reduced energy loss in PEC catalysis are additional benefits.By virtue of the aforementioned merits, PEC catalysis offers a landscape of various applications, which could be sought as an alternative route for the thermocatalytic processes.
Considering the utmost significance of chemical synthesis via PEC catalysis, this review will cover the recent development and challenges of employing the PEC technique to achieve highvalued chemicals over rationally designed thin-film photoelectrodes.Although several reviews related to PEC catalysis have been communicated in the last few years, [13] no equivalent coverage was given to the key research findings in the library of chemical reactions, and fundamentals related to PEC catalysis were lacking.As such, a comprehensive understanding involving the design and optimization of photoelectrochemical setups, enhancement of photoelectrodes' efficiency, and development of simple electrode preparation schemes was lacking in previous reviews.Hence, it is essential and timely to review the recent progress and challenges of PEC synthesis.Given the PEC catalysis for water splitting [13b,14] and CO 2 reduction [8a,15] have already been reviewed in detail, we will not discuss them here.This review is organized as follows: i) a general background for examining photoelectrodes, quantifying the reaction products and the analytical techniques employed during the value-added product synthesis; ii) the common electrode fabrication techniques employed to achieve a high-quality photoelectrode film; iii) a timely overview of the recent progress of some selected anodic and cathodic PEC synthesis; iv) current understanding of the selective C─H and C─C bond functionalization processes for realizing high valued chemicals and pharmaceuticals; v) perspective on the future outlook for the PEC process.The target is to put a significant footprint in revolutionizing PEC catalysis for valuable product synthesis.

Configuration of PEC Cell and Efficiency Metrics
Several crucial factors, including the optical response of the photoelectrode, the separation/recombination of carriers, the electrolyte/electrode interfacial carrier injection, and the mass transfer (including adsorption of reactant, desorption of product, and diffusion) affect the PEC synthesis.In particular, the direct PEC synthesis on-demand, evaluated by production rate, selectivity, and Faradaic efficiency for certain chemicals, is of importance.However, the methods employed to evaluate the photoelectrochemical conversions of various feedstocks to value-added products are not well-documented.The following sections aim to offer the basic methodologies employed for evaluating photoelectrodes, PEC reactions, product identification, and quantification.

Direct versus Indirect (Mediated) PEC Processes
Generally, the PEC processes reported until now can be carried out either directly on the electrode surface or utilizing a redox mediator.In the case of the direct PEC process, the photogenerated charge carriers directly oxidize/reduce the targeted feedstocks (Figure 2a).In this process, the reaction selectivity could be tuned by changing the applied potential and electrolyte pH. [16]n the other hand, an indirect (mediated) process involves redox mediators in which these mediators are first oxidized or reduced to give an active intermediate.The as-formed species would be then utilized to oxidize/reduce the targeted feedstocks to give the desired product (Figure 2b).
In the indirect PEC process, organic/inorganic compounds, molecules, or ions that own a specific property of being oxidized or reduced reversibly under certain conditions can be used as redox mediators (Table 1).Unlike the direct PEC process where the reaction takes place at the electrolyte-electrode interface, an indirect PEC process could extend the phenomenon in the bulk electrolyte, further expanding the space for the photoelectrochemical conversion. [17]Before choosing the redox mediator, its electrochemical behavior should be studied under the working conditions of the target reaction.When PEC catalysis is carried out in an aqueous electrolyte, the redox mediator may play several key roles.For instance, it could partly alleviate the problem associated with photoelectrode instability and minimize the possible competing/side reactions.
While the use of redox mediators in electrocatalytic and PEC synthesis of valuable chemicals is still ongoing research, several key factors are sought to influence the performances. [21]First, the alignment of the thermodynamic redox potential of the mediator with respect to the energy level of the semiconductor photoelectrode is essential, since it determines the possibility of the photogenerated electrons to be directly transferred to the mediator.Apart from this, the electrolyte pH, solubility of the mediator in the given electrolyte, and the reversibility of the mediator are very crucial factors that should be considered in choosing mediators.For further understanding of the nature and properties of common redox mediators, we direct the readers to refer to previously communicated reviews. [21]The PEC processes compiled in this review focus on the reduction or oxidation of several kinds of feedstocks involving both direct and indirect PEC processes.Phthalimide N-oxyl (PINO)/N-hydroxyphthalimide (NHPI) N-NHPI/PINO • C─H bond activation [18]   Tetrabutylammonium nitrate (Bu 4 NNO 3 ) Alcohol oxidation Abstraction of proton [19]   Halide redox mediators Br − /BrO − Alkene epoxidation Electron transfer [17a]   N-hydroxysuccinimide in the presence of pyridine a ) NHS − /NHS • Alcohol oxidation, cyclohexene to cyclohexanone, tetralin to tetralone Hole transfer [20]   a) Pyridine is added to abstract proton from N-hydroxysuccinimide.

Geometry of Light Illumination
Prior to any photoelectrochemical measurement, identifying which side of the photoelectrode could deliver higher photocurrent density is crucial. [22]Considering an n-type semiconductor photoelectrode as an example, the impact of the geometry of light illumination on the carrier migration can be briefly described as follows.When the electrode is illuminated from the front side (surface of catalyst), the light absorption occurs near the electrode surface, and less light reaches the back of the electrode since the light is absorbed through the thickness of the electrode film.As a result, a greater density of photogenerated charge carriers is formed near the surface of the electrode than on the backside.As clearly shown in Figure 3a, the photogenerated holes are closer to the semiconductor-electrolyte interface, and hence, the diffusion length for the holes does not need to be high.Instead, the photogenerated electrons created near the surface need to diffuse through the film to the back contact, where they are extracted and transported to the counter electrode for the corresponding reduction reaction.Hence, the electron diffusion length is greater than or equal to the film thickness in the case of front-side illumination.On the contrary, when light is shined from the backside, a higher density of photogenerated charge carriers will be created closer to the back contact (Figure 3b).As a result, the electrons need to travel only a short distance while the holes are required to diffuse through the bulk elec-trode to reach the electrode surface.Hence, it is very useful to identify the glass substrate side that can give better photocurrent density prior to further performance evaluation.For example, Mazzaro et al. showed that the direction of photoelectrode illumination could significantly affect the photocurrent behavior of Ti-doped hematite electrodes. [23]Owing to the inefficient hole transport process across the semiconductor, the undoped sample displayed a drop in its photocurrent when illuminated from the backside.PEC reactions in flow cell reactors usually fall on the option of using only back-side light illuminations.Hence, the photoelectrode design requires a proper optimization of electrode thickness.In a recent example by Gong et al., a Si photoanode passivated by amorphous Si was utilized to achieve an ultra-long minority carrier diffusion length and facilitate the separation of the light absorption region from the catalytic sites. [24]Such a decoupled structure allows the light to be illuminated on the photoelectrode from the anode side, which finally results in a high performance (38 mA cm −2 ) for CO 2 reduction.
Moreover, the direction of light illumination on the photoelectrode may make a difference in the quantum efficiency (QE).To quantify the QE of different illumination on photoelectrode, some theoretical models are established: i) For a photoelectrode illuminated with light from the front side, a simplified Gärtner-Butler expression can be applied  to estimate the quantum efficiency as follows: [25] QE where QE FS is quantum efficiency up on light illumination from the front side,  is the optical absorption coefficient, L p is the minority carrier diffusion length (electron in p-doped semiconductors and holes in n-doped semiconductors), and w is the width of the depletion layer in the p-n junction.The value of w can also be calculated from the following equation: where ɛ is the relative dielectric constant, ɛ o is the permittivity of free space, N D is the donor concentration, and e is the elementary charge.In this equation, the difference between the electrode potential (U) and the flat band potential (U FB ) corresponds to band bending.For a small value of optical absorption coefficient (), Equation (1) can be simplified based on Taylor series expansion as follows: ii) For the back-side illuminated electrode, the expression for estimating QE is given below.
where QE BS is quantum efficiency upon back-side illumination, and d is the thickness of the semiconductor.Moreover, it is very useful to measure the photocurrent as a function of the thickness of the photoelectrode film, which could be helpful to determine the minority-carrier diffusion length both theoretically and experimentally. [26]For a typical n-type semiconductor photoelectrode strongly illumi-nated from the back-side, the quasi-neutral region thickness of W > diffusion length, L p , the photocurrent can be given by: where R is the reflectivity of the surface, e is the charge on an electron (1.6021892 × 10 −19 C),  is the incident light flux,  is the absorption coefficient, and S is a dimensionless parameter (sL p /D p ) which describes the surface recombination velocity (s) at the backside of the PEC device relative to the minority-carrier diffusion coefficient (D p ). Equation ( 5) is useful to examine the characteristics of photocurrent as a function of electrode thickness.Accordingly, a reduced photocurrent will be achieved as a result of a high rate of surface electron-hole recombination at the photoelectrode's backside.Given that the efficiency of a photoelectrode can be affected by the geometry of photoelectrode illumination, it appears to deserve more comprehensive theoretical and experimental work to uncover the relationship between the semiconductor electrode and the carrier transport.

Evaluation of a Photoelectrode and Efficiency Metrics for PEC Synthesis of Valuable Chemicals
The capability of the photoelectrode to obtain the targeted reaction can be evaluated based on its current density-potential profile, namely J-V curve, in the presence and absence of the reactant under light illumination.For a typical PEC reaction in aqueous media, the possibility of oxidizing/reducing the targeted feedstocks can be evaluated from the onset potential linear sweep voltammetry (LSV), which should be different from the competitive reduction or oxidation of water.For an oxidation process, the photocurrent density shows a drastic enhancement upon the introduction of the reactant to be oxidized (Figure 4a,b).When recorded under aqueous conditions, a careful analysis of the LSV curve is required to compare the competitive water oxidation with the targeted oxidation reaction.The linear sweep voltammograms for the water oxidation and feedstocks oxidations must be evaluated to identify the proper potential window.Recently, many innovative PEC processes have been made available, in which the concurrent oxidation and reduction of the targeted feedstocks is carried out at a time.For such a process, the operating current could be obtained from the intersection of the LSV of the two half-reactions (Figure 4c).Apart from the simple assessments of the electrode on LSV, several efficiency metrics, including solar-to-fuel conversion (STF), incident photon-to-current conversion efficiency (IPCE), Faradaic efficiency (FE), external quantum efficiency (QE), applied bias photon-to-current efficiency (APCE), and absorbed photon-to-current efficiency (ABPE), have been used to evaluate the overall performance and stability of a particular photoelectrode.It should be noted that the FE and STF are two important factors that indicate the performance of the PEC chemical synthesis process, while the efficiencies metrics such as IPCE, APCE, and ABPE mainly provide an overall efficiency of the photoelectrode and they do not offer very crucial information on how well the photoelectrode reduce or oxidize the targeted feedstock. [27]ere, we summarize available efficiency metrics frequently used in PEC catalysis.

Solar-to-Fuel Conversion Efficiency
The solar-to-fuel (STF) conversion efficiency is quantified as the ratio of "chemical energy produced" to the "solar energy input."It is the most important of all efficiency measurements as it provides a comprehensive description of the overall efficiency of the PEC device when subjected to broadband solar illumination at Air Mass 1.5 Global (AM 1.5 G) conditions, without any external bias applied.In a given PEC reaction, STF can be calculated from the rate of fuel production (r fuel ) and the Gibbs free energy change (∆G) associated with that particular reaction (conversion). [10]F = r fuel (mmol of fuel) × ΔG • ( kJ mol −1 ) P solar (mW cm −2 ) × Area (cm 2 ) (6)   where P solar is the intensity of solar light.

Incident Photon-to-Current Conversion Efficiency (IPCE)
IPCE can be used to determine the contribution to the photocurrent of photons with different energies. [28]IPCE is usually obtained from a chronoamperometry measurement.In this system, a bias can be applied between the substrate (i.e., working electrode) versus a counter electrode (in the case of 2-electrode system) or a reference electrode (in the case of 3-electrode system) while measuring the current that arises as a result of illuminating monochromatic light at various wavelengths.It corresponds to the external quantum efficiency (EQE).In order to obtain IPCE as a function of wavelength, (nm), a calibrated monochromated light should be used.For a direct PEC catalytic process, the IPCE can clearly define how efficient the incoming radiation is in producing the target product.This implies that all electrons/holes are utilized to facilitate the targeted conversion (either via reduction or oxidation) instead of corroding the catalyst itself or deriving other side reactions.It can be calculated based on the following equation: [27] IPCE = hc e where J ph is the photocurrent density, (1239.8(V × nm)) represents the result of the multiplication of Planck's constant (h) by the speed of light (c), P mono is the intensity of incident monochromatic light, and  is the wavelength at which this illumination power is measured.

Faradaic Efficiency (FE)
The amount of product generated via PEC catalysis could be related to the charge passed during the galvanostatic or potentiostatic measurement.10a] FE = Number of holes∕electrons to oxidize∕reduce the substrate Number of all photogenerated holes∕electrons (9) For a particular reaction involving the electron/hole transfer process, the FE for target product can be calculated as: where e target product is the number of electrons or holes involved in the transformation of 1 mole of reactant to target product, N is Avogadro number, n target product is the yield of the target product, Q is electric charge, and e is elementary charge.

External Quantum Efficiency (QE)
The external quantum efficiency of the targeted product can be directly calculated from FE and IPCE as follows: QE = Number of electrones∕holes to convert the substrate to product Number of incident photon

Applied Bias Photon-to-Current Efficiency (ABPE)
When a bias is applied between the working and counter electrodes, the current drawn from the PEC device will increase.One should be mindful of the thermodynamic redox potential (∆E°) of the PEC reactions under consideration: if an applied bias is higher than the thermodynamic potential, it might be difficult to conclude if the PEC process offers an advantage over dark electrolysis.This parameter can be calculated from FE and J as follows:

Absorbed Photon-to-Current Efficiency (APCE)
The losses of photons resulting from both reflection and transmission are not excluded in IPCE and STF evaluations.Hence, the characterization of the inherent performances of the material should consider the subtraction of these losses.The APCE shows how efficiently the absorbed photons are converted into current.Therefore, it corresponds to the internal quantum efficiency (IQE).The APCE of a PEC system can be calculated from the ratio of IPCE to absorptance ( e−/h+ ).
where  transport and  interface are efficiencies corresponding to the charge transport to the solid-liquid interface and interfacial charge transfer, respectively.The value of absorptance,  e − /h+ , determines the fraction of electron-hole pairs obtained per incident photon flux.Note that the APCE value offer a versatile platform for understanding the property of a thin film electrode via optimizing the maximum photon absorption path length along with minimum effective e − /h + transport distance.

Product Analysis and Quantification
As for the analysis of the products, one or more analytical techniques are usually employed.UV-vis spectrophotometry, gas chromatography (GC), high-performance liquid chromatography (HPLC), gas chromatography-mass spectrometer (GC-MS), and nuclear magnetic resonance (NMR) are the most common analytical techniques utilized to quantify the amount of product formed during or at the end of the reaction.Based on the typical PEC synthesis, the analytical techniques employed for qualitative and quantitative analysis of products are summarized in Table 2.For a more precise quantitative estimation of some products, two or more techniques can be utilized.For instance, the amount of NH 3 produced from PEC catalysis of N 2 and NO 3 − precursors can be obtained from UV-vis spectrophotometry and NMR analysis.A detailed procedure for quantifying the reaction products described in this review can be found in previous reports elsewhere. [29]Meanwhile, more than one product may be detected during PEC synthesis, and some factors of conversion efficiency, production rate, and selectivity are very important to evaluate the process.For instance, the conversion of glycerol may yield several products such as 1,3-dihydroxyacetone (DHA), glyceric acid (GA), formic acid (FA), glycolic acid (GLA), and lactic acid (LA).Accordingly, the selectivity and production rate of the targeted product DHA can be calculated as follows; [10a,30] Selectivity target product = n target product where n target product is the yield of target product, n 1 , n 2 , n 3 , and n 4 are the yield of side products.C target product , C 1 , C 2 , C 3 , and C 4 are the target and side products concentrations in mol L −1 , respectively.
Similarly, the production rate of the target product can be calculated from the following equation: Production rate per unit area where V is the volume of the reaction solution in L, t is the reaction time in hour, and A is the area of photoelectrode in cm 2 .

Methods of Photoelectrode Fabrication
The electrode fabrication process has serious implications for the overall performance and stability of photoelectrodes. [37]Besides, it can be regarded as one important factor as it may determine the cost of a PEC device.For instance, a photoelectrode prepared under a complex procedure, harsh experimental conditions, and expensive inputs will increase the PEC device's overall cost, questioning the practicality of large-scale production.Hence, a simple and inexpensive electrode fabrication strategy is needed.To date, photoelectrodes can be either directly grown on the substrates of The digital image of black phosphorus (BP) and its photoelectrode film.d) Illustration of the electrochemical synthesis method for black phosphorus nanosheets and the spin coating method employed for BP electrode fabrication.Reproduced with permission. [40]Copyright 2020, Wiley VCH Weinheim.
interest (in situ preparation method) or cast on a piece of substrate once the catalyst is prepared (ex situ preparation method).
In this section, we provide a brief introduction to the existing photoelectrode fabrication strategies that have been employed in PEC cells.The methods described in Section 3.1 mainly focus on the ex situ preparation techniques where the semiconductor material is initially synthesized and cast over the substrate via different means.In fact, spin coating and spray pyrolysis are categorized as ex situ photoelectrode fabrication methods when only the final catalyst slurry is directly deposited on the specific substrate.However, when a precursor of the final product is coated via the above techniques to yield the final electrode, both fabrication methods would be considered as an in situ preparation technique.While the ex situ preparation techniques afford several advantages, the poor attachment of the catalyst on the substrate is considered a major limitation that affects the PEC system's performance.The methods categorized under in situ techniques, which are discussed in Section 3.2, can practically overcome these problems since the catalyst with the desired morphology can directly grow on the substrate of interest.In fact, both methods have their own merits and demerits, and a careful selection of electrode fabrication methods is required in the context of the type of semiconductor material to be prepared.

Drop Casting and Spin Coating
Drop casting and spin coating are the most straightforward techniques for fabricating photoelectrodes from the catalyst powder.
In a typical fabrication procedure, a semiconductor suspension is prepared via dissolving an appropriate amount of the catalyst in a suitable solvent, followed by ultrasonication of the solution to get a uniform suspension. [38]In the case of drop-casting, this suspension is directly transferred to a substrate, commonly indium tin oxide (ITO) or fluorine-doped tin oxide (FTO) dropwise, and the solvent is allowed to evaporate, resulting in the formation of the semiconductor film (Figure 5a).As for the spin-coating, the prepared suspension is added on a suitable substrate already attached to a spin-coater (Figure 5b).38b] After this process, postcalcination is needed to improve the adhesion of semiconductor particles on the substrate surface.Taking advantage of the centrifugal force during the spin-coating process, the method offers a comparatively uniform film thickness, which could be controlled by manipulating the rotating speed, the concentration of the A supramolecular complex containing melamine and bismuthiol was formed and finely ground.This mixture was blended into a paste along with ethylene glycol and graphene oxide (GO), and doctor-bladed onto an FTO substrate to get the photoelectrode films.c,d) DFT calculation depicting the interaction energy between melamine-bismuthiol and ethylene glycol (EG) and the melamine-bismuthiol supramolecular assemblies, respectively.e) Scanning electron microscopy (SEM) image of melamine-bismuthiol-graphene oxide 0.75 (MSG 0.75 ) films on FTO prior to calcination.42a] Copyright 2020, American Chemical Society.
catalyst slurry, and its viscosity.However, the combination of gas flow by spinning substrate and the centrifugal force could also result in a much faster drying kinetics compared with other conventional industrial coating procedures.In this context, the spin coating procedure appears difficult to facilitate a large-scale fabrication of PEC devices. [39]Yu et al. reported that an electrochemically exfoliated black phosphorous (BP) spin-coated on an indium tin oxide (ITO) substrate efficiently catalyzed the PEC nitrogen reduction reaction (Figure 5c,d). [40]During the electrochemical exfoliation, tetra-n-butylphosphonium was used instead of quaternary ammonium salt, which reduces interferences during ammonia detection.

Doctor Blade
The doctor blade is also a simple and common method for preparing semiconductor film on a relatively large surface area. [41]This method is widely employed for preparing photoelectrodes in dyesynthesized solar cells or chemoresistive gas sensors.A homogeneous catalyst ink/paste is first prepared in a typical fabrication procedure by dissolving the semiconductor in a suitable solvent.
Once the required ink/paste is prepared, it is applied on the conductive surface of the substrate, followed by thermal treatment under an air or argon atmosphere.As shown in Figure 6a, scotch tape is usually applied at the sides of the substrate, which acts as a spacer to maintain a relatively uniform film thickness.A very smooth and homogeneous layer can be achieved via blending the catalyst ink with additives such as glycerol or ethylene glycol. [42]e Shalom group utilized the doctor blade technique to rationally fabricate a carbon nitride photoanode that exhibits excellent electron diffusion length and hole extraction properties.Typically, a supramolecular film was first prepared from melamine and bismuthiol precursors.Then, the mixture was blended with ethylene glycol (Figure 6b).From density functional theory (DFT) calculations, it was speculated that the ethylene glycol additive interacts preferentially with melamine (E int , −48.62 kJ mol −1 ) than bismuthiol (E int , −44.41 kJ mol −1 ), as shown in Figure 6c,d.Besides, the relatively higher E int of 89.88 kJ mol −1 in a finely grounded mixture of melamine and bismuthiol suggests the probability of forming supramolecular structure.Finally, carbon nitride films were achieved after calcinating the as-prepared supramolecular films at 550 °C under N 2 atmosphere.Interestingly, graphene oxide was also blended in a similar fashion, drastically boosting the overall performance of the photoanode (Figure 6e).42a]

Finger Rubbing and Particle Transfer
The finger-rubbing method is widely employed for fabricating faceted semiconductor photoelectrodes since it can directly use catalyst powder without dissolving it in a solvent. [43]In a typical fabrication procedure, the semiconductor catalyst powder is manually rubbed on the surface of a planar substrate with the ease of a finger. [44]Before the rubbing procedure, a polyethylenimine (PEI) solution in alcohol is usually spin-coated over the Reproduced with permission. [45]Copyright 2013, Royal Society of Chemistry.b) Vacuum filter transfer method.Reproduced with permission. [46]Copyright 2019, Wiley VCH Weinheim.
substrate as the hydrogen-bonding mediator.For a similar purpose mentioned in the previous procedures, calcination is usually required to make a high-quality film.Two important factors likely govern the facile attachment of catalysts on substrates for this fabrication method.i) The "pressing" of the semiconductor against the substrate and ii) the forced surface migration of the crystals during the rubbing process.
In the case of the particle transfer method, the catalyst particles are first transferred onto a glass substrate using a suspension of photocatalyst powder in a suitable solvent (Figure 7a). [45]The resulting coated substrate is then dried under ambient conditions.Next, a contact layer is deposited on the resultant electrode (for instance using magneton sputtering or other thin film deposition methods), followed by forming a mechanically strong conductive layer over the contact layer.These layers are attached to a second glass plate using an epoxy resin, and then the primary glass plate is peeled off.The excess particles are finally removed by ultrasonication in water to obtain the desired electrode.

Vacuum Filter Transfer
Vacuum filter transfer is another alternative for fabricating photoelectrode film: i) the catalyst is dispersed in a suitable solvent to create a uniform slurry; ii) the slurry is subjected to vacuum filtration using a cellulose membrane, followed by transferring the film to a suitable conductive substrate; iii) a calcination or etching procedure is usually required to remove the cellulosic membrane prior to performance evaluation.This fabrication method appears simple, yet the poor adhesion of the as-obtained film with the substrate is the biggest limitation.As described in the aforementioned fabrication procedures, a post-calcination step is required to ensure proper attachment of the film and enhance the charge transfer of the semiconductor film and conductive substrate.In 2019, Sun and co-workers [46] successfully demonstrated seashell-inspired nanomaterials via assembling graphene and atomically thin 2D TiO 2 nanosheets layer-by-layer (Figure 7b).In a typical vacuum filter graphene or graphene oxide was firstly dispersed in N, N-dimethylformamide (DMF), or in water, respectively, and vacuum filtration was carried out to form a substrate layer.Next, the dilute 2D TiO 2 nanosheets' aqueous solution was slowly added into the filtering container to form a dense inorganic layer on the graphene substrate layer.

Spray Pyrolysis
The spray pyrolysis method has also been adopted to fabricate semiconductor or composite photoelectrodes. [47]In this technique, a precursor solution is first prepared and sprayed on a heated substrate where the constituents in the precursor solution react to form the desired product.This method affords several advantages, including its suitability to introduce dopant and composite structures in the target semiconductor, ease of operation, simplicity, and low cost.In 2017, Berglund and co-workers reported the facile fabrication of dense and homogeneous CuBi 2 O 4 photocathodes. [48]A precursor solution containing Bi 3+ and Cu 2+ in a mixture of acetic acid and ethanol was used as a spraying solution.The authors noted the rapid precipitation of the precursor solution, which was likely caused by the hydrolysis of Bi 3+ in water.Hence, additives, namely 5% triethyl orthoformate (TEOF) and 1% polyethylene glycol (PEG), were added to improve the stability of the precursor and the spreading behavior of the droplet, respectively.This method has widely been used to deposit electron transport layers, such as the dense blocking layer of TiO 2 , applicable to many PEC cells.Owing to the controllability of various process parameters such as substrate heating temperature, distance from the atomizer to the substrate, the concentration and content of precursor, and spray time and speed, this process allows the fabrication of diverse and large area semiconductor photoelectrode films.
The ex situ electrode preparations described above have their own advantages and limitations (Table 3).Hence, choosing the most suitable method requires understanding their merits.

In Situ Electrode Preparation Methods
In this section, we highlight the general in situ electrode fabrication techniques that are commonly employed to prepare photoelectrodes.

Electrochemical Fabrication Method
The electrochemical deposition method is among the most widely used electrode fabrication techniques which allows the synthesis of various semiconductor electrodes on the desired substrate.The electrodeposition can be carried out via manipulating the current (potentiostatic deposition) or potential (galvanostatic deposition) while keeping the other parameter constant.Under the conditions set for electrodeposition, the substrate should be electrochemically inert to avoid possible parasitic reactions.Hence, understanding the electrochemical behavior of the substrate is very useful, which is usually extracted from its CV curve.
An influential example of this technique is the anodic deposition of BiVO 4 film, which was reported by the Choi group in 2012 for the first time. [49]By utilizing Bi(NO 3 ) 3 and VOSO 4 as Bi and V precursors, respectively, the authors successfully synthesized the BiVO 4 photoelectrocatalyst.Later, this electrochemical deposition method was also adopted to dope Mo in BiVO 4 via simply introducing peroxomolybdates in the plating solution as a Mo precursor, indicating the versatility of the method. [50]Until now, the electrochemical deposition method has been widely employed to synthesize many binary and ternary photoelectrodes on different substrates. [51]ne feature of the electrochemical method is the facile deposition processes for target layers and protective layers on semiconductor photoelectrodes.When depositing a protective layer on synthesized photoelectrode, one should give attention to the stability of the photoelectrode in the plating solution.A careful examination of the chemical stability of the photoelectrode in the plating solution is always required before the deposition process.The LSV curve can be collected in the plating solution without the electrochemically active species, and a proper potential window where the photoelectrode is electrochemically inert should be identified for the electrodeposition process.To date, this method is well-acknowledged for depositing several kinds of protecting layers, including metal oxides, layered double hydroxides, [52] oxyhydroxides, etc.For a detailed understanding of the electrochemical deposition strategy, we direct the reader to refer to some seminal reviews communicated earlier, [51] which briefly discuss the selection and preparation of electrodes, plating solution, working parameters, deposition modes, etc., in the context of several kinds of photoelectrodes.

Hydrothermal/Solvothermal Method
The hydrothermal/solvothermal method is commonly defined as the synthesis or growth of crystals at an elevated temperature and pressure in a particular solvent. [53]It is especially used for substances that are not soluble under normal temperature and pressure conditions.The specific attributes can be modified by adjusting various experimental factors such as reaction temperature, reaction time, solvent type, surfactant type, precursor type, etc.This method is one of the most common and facile electrode preparation techniques employed to directly grow the catalyst on the substrate surface. [54]Until now, it has widely been adopted to design several traditional metal oxide catalysts, including BiVO 4 , WO 3 , ZnO, etc. [43b,55] It is worth noting that the compatibility of the resulting semiconductor catalyst with the target substrate is very crucial.For instance, semiconductors such as TiO 2 , [56] and Fe 2 O 3 [57] can be directly grown on FTO while oxides of Zn [58] and W [59] require a seed layer to facilitate the facile nucleation and thereby deposition of the oxide on the substrate.
In earlier pioneer work by Liu and Aydil, [60] a facile hydrothermal method was employed to directly grow oriented TiO 2 nanorod single crystals on a planner FTO substrate.The authors carefully studied the effects of titanium precursors, reaction time, additives, initial precursor concentration, and growth temperature on the length, diameter, and density of the as-obtained TiO 2 nanorods.It was noticed that a temperature less than 100 °C cannot facilitate the growth of TiO 2 nanorods on the FTO substrates, while the growth rate can be increased by a factor of 5 when tuning the temperature from 150 to 200 °C.Moreover, the diameter of TiO 2 nanorod grows faster when TiCl 4 precursor is used.Whereas utilizing titanium isopropoxide and titanium butoxide resulted in almost similar morphology under the same growth conditions.In another study, Feng and co-workers vertically grew tetragonal TiO 2 nanosheets with (001) exposed facets on the FTO substrates. [61]It was found that the addition of (NH 4 ) 2 TiF 6 , the ratio of H 2 O to HCl, the reaction temperature, and reaction time play significant roles in facilitating the growth of TiO 2 with (001) facet.The authors realized that lower growth temperature (<150 °C) and the absence of HCl cannot completely grow TiO 2 .Therefore, the hydrothermal/solvothermal method is the facile method for fabricating photoelectrodes enabling accurate control of size, shape/facet, crystalline structure, etc.  [66] Copyright 2020, Springer Nature.

Chemical Vapor Deposition
The chemical vapor deposition (CVD) is a synthesis process in which the chemical constituents react in the vapor phase near or on a heated substrate to form a solid deposit. [62]It is among the most widely employed electrode fabrication methods utilized for depositing thin film on different kinds of substrates.In general, the CVD process involves several key steps: the first step comprises the thermal evaporation and transportation of precursors/reactants and subsequent gas phase reaction.Next, the reactants will diffuse toward the substrate surface.After that, they adsorb and undergo surface diffusion toward the growth site where nucleation and surface chemical reactions take place.Finally, the desorption of fragmented products will take place.During the CVD growth process, an inert carrier gas is continuously fed into the heating chamber to facilitate the volatilization/transportation of the precursors.The CVD method has been employed to grow several kinds of metal oxide semiconductor photoelectrodes and their related heterostructures. [63]In previous work by Ariffin et al., an inert carrier gas (gaseous ethanol) was utilized to facilitate the ZnO precursor solution atomization into fine droplets. [64]fter the small droplets are transported to the heating chamber, their facile deposition takes place directly onto an FTO substrate.Apart from oxide-based semiconductors, sulfides of transition and post-transition metals can be also fabricated by this method. [65]65a] According to the authors, the loading of the SnS 2 nanosheets can be manipulated by controlling the growth temperature.In general, the most governing parameters to con-trol the fabrication of photoelectrodes in this technique are the temperature at various zones, flow rates of gases, pressure, type of precursors, type of substrate, substrate-precursor distance, etc.
Moving beyond the above two methods, the direct fabrication of a photoelectrode film on different substrates offers fair attachment between the catalyst and substrate, as well as ease of controlling morphology.For instance, the Shalom group utilized a facile route to synthesize carbon nitride (CN) film on an FTO glass substrate. [66]First, a film layer of thiourea was obtained by immersing a clean FTO substrate into a hot saturated thiourea solution; then, the obtained film layer was dried and calcined at 500 °C under N 2 atmosphere to get the carbon nitride film (Figure 8a,b).This method could be used to fabricate CN film on various substrates (carbon paper, glass slide, etc.), and the thickness of the film was controlled via a sequential dip-dry cycle.

Synthesis of Valuable Chemicals via Photoelectrocatalysis
This section discusses the recent development of photoelectrochemical strategies for the production of high-value chemicals.In particular, the photoelectrochemical processes involving the oxidation, reduction, bond activation, and functionalization of various organic and inorganic feedstocks, such as glycerol, HMF, benzyl alcohol, oxygen, nitrogen, N 2 , water (to H 2 O 2 ), nitrate, nitrite, etc. to high-valued chemicals are highlighted.Given the numerous reviews on PEC hydrogen evolution reaction [67] and CO 2 reduction, [15a,b,68] we do not include these two reactions in the present review.The Pt cathode is used to generate H 2 gas on the counter.Reproduced with permission. [80]Copyright 2022, American Chemical Society.

Oxidation of Glycerol
Glycerol is one of the main by-products of biodiesel production, and its purification process in wastewater is complex due to its viscosity.Glycerol has several oxidation products with industrial and economic importance in polymer synthesis and cosmetics, including 1,3-dihydroxyacetone (DHA, 150US$ per kg), glyceraldehyde (GLD, 40 US$ per kg), formic acid (0.4US$ per kg), glycolic acid (3.1US$ per kg), and lactic acid(1.7US$per kg). [69]In 2020, the annual production of glycerol has reached 3-4 Mt [70] and is projected to grow up to 6 Mt yr −1 by 2025. [71]34b,73] For example, a recent report from the Yang research group showed that Si photoanode can achieve a low onset potential (−0.05 V vs RHE) and high photocurrent density of 10 mA cm −2 at 0.5 V versus RHE bias during glycerol oxidation. [74]However, the selectivity toward a specific oxidation product is still a great challenge, and this emanates from the presence of two primary and one secondary hydroxyl functional group leading to complex oxidation paths (Figure 9a).As such, catalyst design strategies that favor the oxidation of glycerol toward the selective production of more valuable chemicals has become the focus of many researchers in this field.
The PEC transformation of glycerol over a semiconductor photoanode largely depends on several factors, including exposed facet, [75] morphology, [76] surface and interface of the catalyst, [77] and some other extrinsic factors like pH [31,70,78] and nature of electrolyte. [31]Until recently, several strategies involving the manipulation of photoelectrode or electrolyte composition have been tried to activate the middle hydroxyl functional group, thereby enhancing its selectivity toward DHA production.
Earlier work from Liu's group utilized nanoporous BiVO 4 photoanode to selectively oxidize glycerol in acidic media. [30]The oxidation product formation rate shows dependency on the applied potential and the selectivity toward DHA formation reached 63.6% at pH ≈2, while the overall FE in the liquid products was near ≈50%.Further, DFT calculations enabled the understanding of possible reaction pathways, in which the strong electrostatic force between Bi 3+ and oxygen at the terminal and middle hydroxyl group was predicted to favor the adsorption of glycerol on Bi 3+ .Compared with the terminal radicals, the tertiary radicals on the middle carbon were found stable, which supports the observed higher selectivity toward DHA production.Following this seminal report, a remarkable improvement in FE of DHA was achieved by tuning the acidity and composition of the electrolyte.For this purpose, Sayama and co-workers constructed Ta doped BiVO 4 /WO 3 composite photoanode as an acid resistant material, which was capable of oxidizing glycerol in an electrolyte containing 100 mm H 2 SO 4 and acetone to yield 96% FE. [70] In another recent study, Huang's group designed a metal-organic framework (MOF) derived N-doped carbon TiO 2 /CsPbBr 3 /TiO 2 photoanode for PEC glycerol oxidation. [79]In this work, the Ndoped carbon serves as a carrier transport channel to significantly enhance the charge transfer property of the electrode.The designed system showed promising performance and stability toward glyceraldehyde and DHA production under the optimized electrolyte composition and glycerol concentration.While these reports mainly focused on controlling the pH and content of the electrolyte, manipulation of the photocatalyst itself is found to trigger the adsorption of specific OH site.Very recently, Duans group highlighted that the selectivity of glycerol oxidation product can be switched from formic acid to DHA by incorporating Bi 2 O 3 on TiO 2 surface (Figure 9b). [80]On accounts of enhanced adsorption of the middle hydroxyl group of glycerol on Bi 2 O 3 , the designed Bi 2 O 3 /TiO 2 photoanode exhibited excellent selectivity (75.4% at 1.0 V vs RHE) compared with its TiO 2 counterpart (22.3%).Furthermore, a self-powered system constructed using Bi 2 O 3 /TiO 2 anode and Pt cathode enables the concurrent production of DHA and H 2 gas with a productivity of 11.5 μmol cm −2 h −1 and 0.32 mL cm −2 h −1 , respectively (Figure 9c,d).
77a] The designed architecture displayed a threefold improvement in the yield of glyceraldehyde and DHA during glycerol oxidation compared with H-WO 3 (Figure 9c).Evidence collected from the Kelvin probe force microscopy (KPFM) and photoluminescent (PL) imaging reflected the accumulation of charge at the interface within the defective heterostructure.Along with the positive surface photovoltage (SPV) signals of 140 and 40 mV for TiO 2 and H-WO 3 , respectively, the 0 mV SPV at the defective heterointerface has indicated the migration of electrons from TiO 2 to WO 3 and the holes from WO 3 to TiO 2 (Figure 10d,e).In contrast, the SPV value at the heterointerface of the defect-free WO 3 /TiO 2 (Figure 10f) was much higher (60 mV), indicating the slow charge separation process.The PL measurements further support the inhibition of car-rier recombination at the interface of a defective heterostructure (Figure 10g,h).The negligible fluorescence on the defective WO 3 surface has indicated the possible suppression of carrier recombination on the defective heterointerface.The intra-band state introduced due to this defective surface permits the reservoir of charge, facilitating the oxidation of glycerol (Figure 9i).Besides, the exposed facet of the photoelectrocatalyst also contributes to the PEC performance.Chiang and co-workers [82] unveiled the PEC transformation of glycerol promoted by monoclinic phase BiVO 4 photoanode with (010) dominated crystal facet.The authors rationally designed BiVO 4 with (010) and ( 121) exposed facets and investigated their relative performances toward the oxidation of glycerol.Owing to the preferential adsorption of glycerol on its surface, the (010) facet dominated BiVO 4 exhibited a higher yield of products when compared to BiVO 4 with the (121) dominated facet.Indeed, this study offers an insight on facet dependent glycerol oxidation, which could open new avenues for further study on numerous organic reactions.
Moving beyond crystal facet control and defect engineering strategies, existing modification techniques have also been employed to enhance glycerol oxidation in different media.Typically, the electrodeposition of layered double hydroxides on semiconductor photoelectrodes showed a positive influence during the oxidation of glycerol.Wang and co-workers electrochemically deposited trimetallic CoNiFe-layered double hydroxides (CoNiFe-LDHs) nanosheets on Ta 3 N 5 , which permits the enhancement of performance and stability of the semiconductor.The obtained photoelectrode exhibited 100% Faradaic efficiency for concurrent formate production from anodic glycerol oxidation and hydrogen production at the cathode along with marked improvement in solar energy conversion efficiency (0.56%).A similar benefit was also obtained through depositing NiO x (OH) y co-catalyst on W:BiVO 4 photoelectrode via atomic layer deposition (ALD).78a] Another study by Wang and collaborators highlighted the synergistic effect of Au and C 3 N 4 for the selective oxidation of glycerol to DHA. [83] By combining the theoretical and experimental results, the altered electronic structure of Au due to the strong electronic interaction with C 3 N 4 endows the Au site to preferentially adsorb and selectively oxidize the middle OH.Recently, a rationally designed ternary photoelectrode constructed from Ag, CoAl-LDH, and TiO 2 which enable DHA production from glycerol oxidation in neutral media is reported. [84]The hydroxyl radicals generated from the PEC water oxidation account for the observed selectivity.In this report, the LDH and Ag were found to play different roles: the LDH facilitated the selective adsorption and activation of the targeted OH in glycerol molecule and Ag accelerated the secondary OH oxidation, and their synergy resulted in higher selectivity toward DHA (72%).

Oxidation of Furan Based Feedstocks
Furfural and 5-hydroxymethylfurfural (HMF) are striking members of furanic compounds, which could be obtained via acid-catalyzed dehydration of pentose and hexose sugars, respectively. [85]The catalytic oxidation of furfural and HMF  and f) defect-free WO 3 /TiO 2 heterostructures, respectively.Spatially resolved PL images of g) defective and h) defect-free WO 3 /TiO 2 heterostructure.i) Schematic diagram depicting the charge separation across the defective WO 3 /TiO 2 nanointerface.77a] Copyright 2021, Elsevier BV.
results in the generation of industrially important chemicals, namely furanic acid (FA) and 2,5-furandicarboxylic acid (FDCA), respectively.While the previous studies largely focused on aerobic oxidation through heterogeneous catalysis, the extreme reaction conditions such as alkaline solution (pH ≥ 13), the need for highly pressurized O 2 or air as oxidant (3-20 bar), elevated temperatures (30-130 °C), and use of precious metals seriously affect the feasibility of the process.This derives the quest for an alternative technology that could produce chemicals and consumables from such bio-based refineries.In recent years, various scholars focused on converting biomass-based furanic compounds into useful chemicals and fuels through different strategies, including electrocatalytic, [4l] photo, and photoelectrocatalytic [86] transformations.Here, we briefly summarize the recent trends in the oxidation of the aforementioned furanic compounds via photoelectrocatalytic routes.
4j] The mechanistic studies have suggested that the PEC oxidation of HMF followed initial alcohol oxidation forming 2,5-diformylfuran (DFF) as an intermediate (Figure 11a).Moreover, the oxidation of 5-formyl-furan carboxylic acid (FFCA) to 2,5-furandicarboxylic acid (FDCA) is found to be the rate-limiting step during TEMPO-assisted oxidation of HMF.Following this inspiring work, the Li group reported a similar study employing TEMPO as a redox mediator. [87]They developed a heterostructured film composed of BiVO 4 and cobalt phosphate (CoPi) and demonstrated that the presence of CoPi can reduce the potential needed to oxidize TEMPO by 0.5 V.The as-deposited CoPi plays a crucial role in suppressing the recombination loss that could result from the reduction of TEMPO + to TEMPO, resulting in an overall improvement in the performance.The designed heterostructure was able to offer a nearly 88% yield of FDCA utilizing TEMPO as a redox mediator.While the redox mediator (TEMPO) used in the aforementioned studies has a crucial role in oxidizing the organic feedstocks, the overall PEC process still suffers from some limitations, such as the difficulty of separating the final product and stability of TEMPO itself during the catalytic process.Lhermitte and co-workers attempted the PEC oxidation of HMF in absence of such redox mediators utilizing WO 3 as photoanodes in acidic media (pH 4). [88]However, the yield of FDCA in this study was far too low (1%).Considering the industrial importance of HMF oxidation products, further study may be required to fully utilize bio-based resources and sunlight to produce value-added chemicals.
Apart from HMF, the oxidation of furan and its derivatives has recently gained enormous research interest due to the importance of their oxidation products in pharmaceuticals, agriculture, and other biorefineries. [89]In 2017, the Sayama group reported the PEC dimethoxylation of furan using a bromide redox mediator and BiVO 4 /WO 3 photoanode. [18]As high as 99% faradaic efficiency was achieved at an applied bias of 0. In a recent work by Wang and co-workers, [90] methylammonium lead bromide single crystal thin film (MAPbBr 3 SCTF) was used as a photoanode for the dimethoxydihydrofuran evolution from furan (Figure 12a-d).Like most photoanodes, the as-synthesized crystal faced a major challenge in its stability.Interestingly, by using an ultrathin Al 2 O 3 as a passivation layer, the trap-assisted nonradiative charge recombination was effectively suppressed, resulting in a twofold improvement of its surface charge carrier lifetime.Compared with the pristine MAPbBr 3 SCTF, the sample with the Al 2 O 3 passivating layer exhibited a nearly twofold enhancement of the SPV value (Figure 12e-h), affirming the crucial role of Al 2 O 3 in causing the accumulation of holes at the surface for further oxidation.Furthermore, the MAPbBr 3 SCTF photoelectrodes stability and performance toward oxidation of furan was improved via Ti layer deposition.The deposited Ti plays two basic roles: i) it isolates the halide perovskite from the electrolyte solution, which contributed to the robust stability; ii) it facilitates the desired oxidation reaction since the Ti 3+ can serve as a catalytically active site.Consequently, the photoanode exhibited excellent photoelectrochemical .Kelvin probe force microscopy (KPFM) images (e,g) and ΔCPD profiles (f,h) of pristine, MAPbBr 3 SC film and MAPbBr 3 SCTF/Al 2 O 3 .Reproduced with permission. [90]Copyright 2021, Springer Nature.
performance and stability, demonstrating 93% faradaic efficiency and robust stability for 6 h continuous operation.

Oxidation of Benzyl Alcohol
Akin to the valorization of biomass model compounds, PEC catalysis has also been used to derive the selective transformation of various aromatic-based compounds, including complex molecular structures. [20,23,91]91a] BAD, which is industrially produced via direct oxidation of toluene, has several applications, including in the production of dyes (like acridine and aniline), pharmaceuticals, antibacterial and antifungal preservatives, etc.In addition, it can be used as an intermediate for the synthesis of several organic compounds.While optimizing the catalyst surface and reaction parameters is still required to suppress the competitive and industrially less important OER, alcohol oxidation can be regarded as a good alternative anodic reaction for a complete PEC fuel production system. [92]In a very recent example by Zhao and co-workers, a photoelectrode based on a Bi 2 MoO 6 nanoparticle and a TiO 2 nanotube array was utilized to promote the anodic BA oxidation of BAD.91a] The search for an alternative photoelectrochemical system to derive alcohol oxidation continues to grow rapidly.Recently, Zhang's group utilized BiVO 4 photoanode coated with 2,2′bipyridine-based covalent organic framework containing single Ni sites (Ni-TpBpy/BiVO 4 ) to transform BA to BAD. [93] A radical-mediated reaction pathway followed in this system enables an efficient BAD conversion rate with the coated sample (Ni-TpBpy/BiVO 4 ) showing sevenfold improvement compared with pristine BiVO 4 photoanode (11.49vs 80.63 μmol h −1 ).Indepth experimental exploration elucidates that the surface-bound •OH radicals formed by the Ni-TpBpy served as the main reactive oxygen species (ROS) to efficiently catalyze BA oxidation.Moreover, Odobel and collaborators demonstrated a dye-synthesized photoelectrosynthesis system for upconverting para-methoxy BA to the corresponding aldehyde. [94]The authors fabricated TiO 2based dye-synthesized PEC cells that employ zinc porphyrin and TEMPO as sensitizers and organocatalysts, respectively.Such a PEC system showed a maximum of 82% FE for aldehyde formation using a borate buffer electrolyte (pH = 8).However, the catalyst performance decreased when acetonitrile was used as an electrolyte.The authors attributed the marked performance loss to emanate from the leaching of the photocatalyst from the TiO 2 electrode.Another recent study from the Reek group used dye-sensitized PEC (DSPEC) cells to compare the homogeneous and heterogeneous catalytic processes during the oxidation of BA to BAD. [91d] In this study, the authors fabricated a DSPEC that contains a photoanode of TiO 2 film sensitized with thienopyrroledione-based dye.For the sake of comparison, the authors immobilized TEMPO on the photoanode (denoted as a heterogeneous system) and directly added TEMPO to the DSPEC anolyte (in the case of the homogeneous system).They noted a marked decline in the photocurrent density when TEMPO was immobilized on the photoanode.
Utilizing nitrate salts as a mediator for the light-assisted oxidation of alcohols was found a crucial role in facilitating the rate of alcohol oxidation and improving its product yield. [19,95]In 2020, Bartlett and co-workers developed an indirect BA oxidation scheme employing tetrabutylammonium nitrate (Bu 4 NNO 3 ) as a mediator over BiVO 4 photoanode in acetonitrile solvent. [19]The nitrate ion (NO 3 − ) oxidation produces a reactive nitrate radical (NO 3 •) that can directly react with BA through the abstraction of a hydrogen atom.Shortly thereafter, the same group studied the base-assisted NO 3 − mediated oxidation of BA to BAD. [95b] In this study, a nearly 80% FE for BAD production was achieved using 0.25 m BA as the initial feedstocks.Kinetic experiments have shown that the electrochemical oxidation of NO 3 − is the rate-determining step that follows first-order reaction kinetics, whereas it is zero-order with respect to alcohol.In this work, the formation of oligomers derived from acetonitrile and the removal of ancillary C─H bond was pointed as grand challenges during NO 3 − mediated PEC alcohol oxidation, which requires further effort.A more experimental effort has also been made by the Santiago group, which aimed to examine the effect of light and atmosphere on the PEC oxidation of BA to BAD. [96] The authors unveiled the role of UV light-induced singlet oxygen in BAD production during PEC catalysis.Moreover, they noted that nonelectrochemical oxidation of BA over BiVO 4 photoelectrode could be possible through singlet oxygen sourced from UV light and oxygen.The examples above show that alcohol oxidation, which is a 2-electron transfer process, is still the ideal anodic reaction for developing a complete PEC fuel cell.However, further work is still required to improve the selectivity and yield of this reaction.

Miscellaneous PEC Reactions at the Anode
Apart from the oxidation of furan-based biomass precursors, glycerol, and others, the PEC technique has also been applied to derive a number of organic reactions ranging from simple to complex molecules. [97]In this section, we summarize the oxidation reactions, which are not highlighted in the aforementioned topics.
The conversion of methane to oxygenated products is economically attractive since methane is one of the most abundant and stable hydrocarbons found in different sources such as landfills, enteric fermentation, natural gas, and petroleum. [98]Albeit significant research effort has already been made for upgrading this resource, it has remained a challenge to design a highly selective catalyst for methane conversion to C 2+ products, and the over-oxidation of methane is still one of the issues in CH 4 catalysis.Moreover, the activation of a strong C─H bond (bond energy = 434 kJ mol −1 ) and its subsequent oxidation require high pressure and temperature (>700 °C). [99]In 2018, Wangs' lab reported the selective oxidation of CH 4 to CO using ALDgrown TiO 2 photoanode, and nearly 81.9% production yield was achieved. [100]The authors compared this result with those obtained with the commercial TiO 2 and found a significant difference.Experimental evidence revealed that such a difference in performance arose due to a higher concentration of Ti 3+ sites in ALD-grown TiO 2 photoanode.In another study, Amano and co-workers successfully demonstrated the gas phase PEC system to realize homo-coupling of methane under blue light illumination and achieved C 2 H 6 and H 2 at room temperature.However, the CH 4 % conversion (nearly 0.1%) and the selectivity of C 2 H 6 (54%, C-based) were not high, which shows the need of further improvement. [101]n an attempt to realize efficient ethylene glycol production from methane, the Xiaong group reported a facet-engineered WO 3 photoanode, which provides an optimized reactivity of hydroxyl radicals and efficient CH 4 conversion. [36]Hydrothermally synthesized WO 3 nanoplate arrays (WO 3 NP), WO 3 nanobar ar-rays (WO 3 NB), and WO 3 nanoflake arrays (WO 3 NF) possessing {010} exposed facets at their sides were employed as a photoanode for CH 4 PEC catalysis (Figure 13a).An obvious onset potential shift was observed for all obtained samples upon CH 4 gas introduction to the reaction system, indicating the preferential oxidation of CH 4 over the competitive water oxidation reaction.It was revealed that the {010} exposed facet in WO 3 played an important role.Accordingly, the WO 3 NP with the highest ratio of {010} exposed facet showed maximum PEC CH 4 conversion to ethylene glycol with the production rate and selectivity reaching 0.47 mmol cm −2 h −1 and 66%, respectively (Figure 13b,c).From an in situ diffuse reflectance infrared Fourier-transform spectroscopy (DRIFTS) study, a PEC duration-dependent peak corresponding to methylene (CH 2 ) symmetric stretching had gradually appeared at an applied potential of 1.3 V versus RHE, suggesting the production of EG (Figure 13d).Combining their experimental evidence, the authors proposed the mechanism as follows: Firstly, the •OH attached to the {010} facets abstract H from the C─H bond of CH 4 , resulting •CH 3 production.In the next step, the as-formed •CH 3 either combine with •OH or is coupled with •CH 3 to form CH 3 OH or C 2 H 6 .The EG was formed through the preferential attack of CH 3 OH by •OH on {010} facets of WO 3 NB (Figure 13d,e).
4j,18,20] Inspired by such seminal studies, the Sayama group used phthalimide Noxyl (PINO)/N-hydroxyphthalimide (NHPI) mediator to achieve an indirect PEC oxidation of cycloalkenes to cycloalkenones, realizing above 99% Faradaic efficiency.In this PEC system, photogenerated holes oxidize NHPI to PINO radical on the surface of the photoanode (WO 3 /BiVO 4 ).Subsequently, the PINO radical was directly involved in the process of C─H bond activation, facilitating the next bond functionalization process.The applicability of this PEC system was further investigated using a variety of Noxyl radical mediators and showed moderate to excellent faradaic efficiency for the oxidation of cycloalkenes. [102]he visible-light-driven PEC catalysis of lignin compounds has also been regarded as a potential strategy to provide economically competitive and valuable aromatic compounds, including phenol, benzaldehyde, and other feedstocks. [103]The Leems' group [104] designed dye-sensitized photoelectrochemical cells and investigated the selective transformation of benzylic alcohol moieties in different lignin model compounds.In this study, the PEC system was established using a TiO 2 -based photoanode surface decorated with Ru(II)-based photocatalyst as photoanode and hydrogen atom transfer (HAT) co-catalyst was involved in the electrolyte solution to derive the reaction.The designed photoelectrode displayed nearly 90% conversion efficiency (Figure 14a) and robust stability for nearly 80 h operation (Figure 14b).The authors noted a slight decrease in the conversion efficiency of the lignin model, which was attributed to the desorption or oxidative decomposition of RuC on the TiO 2 catalyst surface.Overall, experimental works have suggested that employing a mediator in the PEC system could have the following advantages: 1) mediators can efficiently minimize the influence of competitive water oxidation since the selected mediators can be easily oxidized compared to water oxidation.2) As  [36] Copyright 2021, Wiley VCH Weinheim.
a consequence of (1), the Faradaic efficiency could increase.3) The radicals formed as a result of mediator oxidation facilitate the formation of carbon radicals, allowing their selective oxidation.In spite of all these, the product separation and the additional cost incurred to design the PEC system with a mediator may pose a problem, which could limit its practical application in the future.In another study, the Park group attempted lignin valorization under visible light using an unassisted PEC cell. [105]hey used a PEC cell that consisted of three main components: a BiVO 4 photoanode, a triple-cation perovskite photovoltaic (PV) cell, and a carbon cloth cathode (Figure 14d).Here, the carbon cloth cathode was used to trigger the reduction of nicotinamide adenine dinucleotide (NAD + ) to NADH in the enzymatic redox process, which aids the reduction of CO 2 at the cathode.The lignin-fueled system presented a nearly threefold enhancement in formate synthesis rate compared with the reaction with wa-ter oxidation.Moreover, the negative control experimental results were obtained in the absence of PV cell, thiobacillus sp.NAD + , M, or CO 2 , which further confirmed the significant role of each component in this system during PEC catalysis (Figure 14e).A proportionally increased formate yield was observed as a result of an increase in the concentration of lignin (1-5 mg mL −1 ).Hence, the oxidation of biomass was found to be the rate-determining step which can also influence the reduction of CO 2 on the other side (Figure 14f).
Generally, PEC catalysis could be influenced by many factors.Beyond the intrinsic property of the semiconductor, the electrolyte is one essential factor that greatly contributes to surface catalysis.In broader terms, the composition and pH of electrolyte plays a crucial role in tuning the performance, stability, and selectivity toward the targeted product.Hence, selecting appropriate electrolytes is very important when developing electrolysis  [104] Copyright 2019, American Chemical Society.d) Schematic illustration depicting the transformation of lignin via PEC catalysis over BiVO 4 photoanode.e) Control experiment for unassisted PEC catalysis of lignin for formate production.f) Formate production rate under varying initial lignin concentrations.The reaction conditions were as follows: The cathode electrolyte contained NAD + (2.0 mm) and M (0.5 mm) in a phosphate buffer solution (100 mm, pH 6.5); the anode electrolyte contained 0-5 mg mL −1 lignin in a KHCO 3 buffer (10 mm, pH 8.2).Reproduced with permission. [105]Copyright 2020, Royal Society of Chemistry.
conditions in PEC cells.Until now, both aqueous and nonaqueous solutions have been utilized in PEC reactions beyond water splitting.One of the factors that impact an overall PEC process is the pH of the electrolyte solution. [30]A seminal work comparing the PEC glycerol oxidation product, DHA, under different pH led to the understanding of the crucial role of electrolyte pH on the selectivity of products. [30]The DHA appeared as a prominent oxidation product (with selectivity and overall faradaic efficiency reaching ≈50% and ≈30%, respectively) when the electrolyte pH was maintained at 2 (Figure 14).However, with increasing pH (pH = 12), glycerol was completely oxidized to formic acid, exhibiting a nearly 99% FE.
To summarize, solar PEC catalysis targeted at anodic reaction is a feasible strategy to replace the sluggish and industrially less important OER.The last few decades have shown many examples of PEC catalysis for transformative organic synthesis using several organic feedstocks, including alcohols, biomass-based aromatics, aliphatic hydrocarbons, and complex organic molecules.Despite this fact, little is known about the reaction mechanism and kinetics, and detailed theoretical work is still lacking.We suggest that a more comprehensive theoretical work could be helpful for understanding the reaction pathways and enhancing the conversion efficiencies.Moreover, the transformation of biomass-based precursors using a PEC cell would be more attractive yet challenging.Hence, proper optimization of catalyst surface and reaction conditions is still required to design a highly efficient PEC cell that could valorize the targeted biomass-based feedstocks.Therefore, there is still much hope for using PEC technology to produce various chemicals and pharmaceuticals.

PEC Synthesis at the Cathode
The PEC reduction of various organic and inorganic feedstocks to value-added chemicals is an attractive strategy for obtaining highly valuable chemicals from readily available precursors.This section focuses on the momentous progress of the PEC catalysis at the cathode that produces valuable chemicals from different resources.

Photoelectrochemical N 2 Reduction for NH 3 Synthesis
The industrial production of NH 3 has a century-old story, but its synthesis method still relies on the Haber−Bosch process. [106]his process is currently producing >200 million tons of ammonia, out of which 75-90% is used for fertilizer production. [107]s this NH 3 synthesis is energy-intensive (requires huge fossil fuel consumption) and operated under harsh reaction conditions (high pressure and temperature), there are critical concerns related to both energy and the environment.16a] In this regard, the PEC reduction of N 2 , which combines the sustainable sources (N 2 and solar light) under ambient conditions, is one elegant alternative to upgrading N 2 to value-added and industrially important products (NH 3 ). [108]Generally, the nitrogen reduction reaction (NRR) is an exothermic reaction that proceeds according to Equation (18).Owing to the high dissociation energy of N≡N bonds (9.80 eV per bond at 298 K) and the high ionization potential (15.0 eV), N 2 owns a characteristic inertness toward chemical reactions.This makes the production of NH 3 from the N 2 source difficult, and the process often requires mild reaction conditions to overcome the high first-bond cleavage energy (410 kJ mol −1 ). [109]The NRR involves several steps and reaction pathways (Figure 15).In the case of the dissociative pathway, the N 2 molecules are firstly adsorbed on the catalyst surface, and the cleavage of the N≡N bonds takes place before the addition of hydrogen atoms.Following the formation of NH 3 through hydrogenation of primarily dissociated N 2 , the desorption of NH 3 will take place.In the case of an alternative associative pathway, an adsorbed chemical species can be formed through a transfer of one proton and one electron from the aqueous environment and the catalyst, respectively.Then, the addition of hydrogen and electron on the adsorbed N 2 species will occur, resulting in the production and subsequent desorption of NH 3 .Another scenario is the so-called "distal pathway": once the preferential hydrogenation of distal N 2 occurs, the NH 3 is produced simultaneously, leaving the other adsorbed N species on the catalyst surface.This species can undergo hydrogenation and produce the second NH 3 as a product.Unlike the dissociative mechanism, the cleavage of N 2 is not necessary for the associative mechanism, which ultimately reduces the energy required for the overall process.
The last few years have seen a surge toward employing various strategies for achieving optimized NH 3 production via PEC catalysis. [110]The construction of composite structures and p-n junctions are among the key strategies employed to improve the NH 3 production rate from N 2 . [111]111a] In 2015, Ali and co-workers reported the PEC reduction of N 2 using plasmonenhanced black silicon (bSi) decorated with gold nanoparticles (GNPs) and Cr as the reduction catalysis sites and a hole-sink layer, respectively. [112]In particular, the designed architecture was successfully employed to upgrade atmospheric N 2 into NH 3 with a yield reaching 13.3 mg m −2 h −1 under 2 sun illumination.The authors noted that replacing bSi with pristine silicon caused a decreased NH 3 yield, with its value being only 11% of that obtained from the composite structure.It was pointed out that the bSi had multiple roles in effectively suppressing reflection, enhancing scattering and light absorption, and providing an extremely large surface area for decoration by the GNPs.In another study, Wu's group realized the conversion of N 2 to NH 3 using Ag nanoparticles supported on bSi, and they achieved a FE and NH 3 yield rate of 55.05% (at −0.1 V vs RHE) and 2.82 μmol h −1 cm −2 (at −0.2 V vs RHE), respectively. [113]A composite photocathode containing BiVO 4 , MnCO 3 amorphous film, and C has been proposed, which showed an eightfold improvement in the NH 3 production compared to the BiVO 4 counter component.The authors claimed that BiVO 4 plays a prominent role in capturing abundant photons, while MnCO 3 amorphous film and C play a significant role in suppressing carriers' recombination and facilitating carrier transport, respectively.Upon illuminating the photoelectrode with light, the photogenerated carriers travel to the MnCO 3 amorphous film and interact with the antibonding orbitals of N 2 , resulting in a weakening of the N≡N bond. [114]32b] The PEC system was shown to be promising, and the FE and NH 3 yield rate reached 28.91% and 7.72 μmol h −1 cm −2 , respectively.
One of the grand challenges in the PEC N 2 fixation to NH 3 is the adsorption and subsequent activation of N 2 on the  d,e) FDTD simulations depicting the electric field enhancement in TiO 2 .32a] Copyright 2018, Wiley VCH Weinheim.g) Bright-field scanning transmission electron microscopy (BF − STEM) image of the as-fabricated Au-NPs/Nb-SrTiO 3 interface.h) Illustration of the NH 3 synthesis device of the Nb-SrTiO 3 photoelectrode loaded with Au-NPs and a Zr/ZrO x thin film.i) Energy-level diagram demonstrating the device for plasmon-induced NH 3 synthesis.CB: conduction band; U: redox potential.Reproduced with permission. [116]opyright 2016, Wiley VCH Weinheim.32a] The spatial distribution of the electric-field intensity calculated from finite difference time domain (FDTD) simulation revealed a tenfold enhancement in the electric field intensity near the Au surface of the Au/TiO 2 sample (Figure 16c-e).Hence, the presence of Au promotes the generation rate of electron-hole pairs within TiO 2 , which is supported by the fact that the square of electric field intensity is directly proportional to the rate at which carriers are formed.Moreover, the hot electrons that can be injected from Au to the TiO 2 could directly reduce the adsorbed N 2 species, thereby improving the overall efficiency.In another important example by Misawa et al., the PEC transformation of N 2 into NH 3 was realized using a niobium-strontium titanate (Nb-SrTiO 3 ) photocathode modified with gold nanoparticles (AuNPs) and a zirconium/zirconium oxide (Zr/ZrO x ) thin film (Figure 16g,h).Particularly, localized surface plasmon resonance (LSPR) was employed to excite hot electrons and promote the separation of charge carriers.As a result, abundant electrons and holes could be involved in N 2 reduction and the counter-oxidation reaction, respectively.This PEC system showed a 6.5 nmol h −1 cm −2 NH 3 production rate using 10% ethanol as a sacrificial electron donor.The authors proposed that the observed NH 3 production was realized through a plasmon-induced charge separation scheme at the photocathode interface. [116]In a recent work by Park et al., -Fe 2 O 3 synthesized under an oxygen-depleted environment demonstrated an improvement of NH 3 production by 1.37-fold. [117]However, the reported performance in these works was quite low for practical application, which further requires careful optimization of Reproduced with permission. [118]Copyright 2019, Elsevier BV. f-h) Schematic depicting PEC catalysis over BP photoelectrode to produce NH 3 from N 2 (f) the corresponding potential dependent NH 3 production rate and FE (g) and stability test for 6 consecutive cycles.Reproduced with permission. [40]opyright 2020, Wiley VCH Weinheim.
catalyst surface area and morphology, and as well as harvesting the proportion of the incident solar energy flux could enhance the energy conversion efficiency.
Apart from the aforementioned challenges, the selectivity and NH 3 production rate in NRR is seriously influenced by the competing HER from the aqueous solution.To tackle this problem, Zheng et al. designed a unique aerophilichydrophilic heterostructured photocathode composed of Si, poly(tetrafluoroethylene) (PTFF) porous framework, and Au as a photoabsorber, gas diffusion layer, and active site, respectively (Figure 17a,b).The PTFE porous framework plays a vital role in offering an N 2 -rich environment on the photocathode surface, while the Au NPs on the PTFE framework efficiently reduce the energy barriers.Consequently, the designed heterostructure exhibited excellent performance with NH 3 yield rate and FE reaching 18.9 μg cm −1 h −1 and 37.8% at −0.2 V versus RHE (Figure 17c-e). [118]Moreover, research works have also been devoted to designing stable and promising photoelectrodes for ef-ficient NRR.Yu and co-workers have designed a photocathode based on black phosphorus (BP), which efficiently catalyzed NRR under ambient conditions (Figure 17f). [40]Such a PEC system showed an excellent NH 3 production rate (102.4μg h −1 mgcat.−1 ) and Faradaic efficiency (23.3% at −0.4 V) along with auspicious stability for 6 consecutive cycles (Figure 16g,h).
Recent works from the Xiong group have shown that doping B in Bi nanorolls with high curvature surface (BDB NR) could enhance the PEC NRR rate by facilitating the key step in NRR. [119]pecifically, the PEC system composed of BDB NR (cathode) and TiO 2 nanorod arrays (TiO 2 /FTO) plate (photoanode) was used to promote NRR.Interestingly, the photoanode employed here played a significant role in harvesting light and providing photogenerated electrons.This PEC system displayed excellent NRR performance with an NH 3 yield rate of 29.2 mg NH 3 gcat.−1 h −1 and FE of 8.3% at a bias of 0.48 V versus RHE, well exceeding pure Bi nanotubes (10.6 mg NH 3 gcat.−1 h −1 ) and nanosheets (9.1 mgNH 3 gcat.−1 h −1 ).A theoretical study further proved that  [122] Copyright 2022, Wiley VCH Weinheim.
doping with B in Bi matrix significantly reduces the energy barrier of the potential-determining step of NRR (N 2 → *NNH), while the high curvature surface of nanorolls facilitates the N 2 adsorption on the catalyst surface.

Photoelectrochemical Ammonia Synthesis from Nitrate
Nitrates are earth-abundant sources of nitrogen, which can be readily found in industrial wastes, livestock manure, and Ncontaining fertilizers.As part of the huge effort to develop an environmentally friendly route of NH 3 synthesis, the utilization of electrocatalytic and PEC techniques has attracted enormous research attention as it affords the dual advantage of balancing the global nitrogen cycle by synthesizing ammonia from wastewater and providing an alternative route for NH 3 synthesis to the energy-intensive Haber-Bosh process. [120]n 2022, the Amal group utilized a metal-organic complex derived, defect-rich TiO x co-catalyst engineered on a CdS/Cu 2 ZnSnS 4 photocathode (TiO x /CdS/CZTS) for selective transformation of NO 3 − to NH 4 + . [121]The TiO x was directly covered on CdS/CZTS using a spray-coating method.The ratio of defective Ti 3+ species in the TiO x layer was rationally tuned by adjusting the spray-coating temperature, and these species mainly contribute to the adsorption of reactants and intermediates (NO 3 − and *NO 2 ).Under an optimized condition, the overall system exhibits a reduced work function along with an improved carrier lifetime, which further enables photogenerated charge transfer with efficient surface reaction kinetics.The designed photocathode (TiO x /CdS/CZTS) shows an excellent performance with the FE for NH 3 reaching 89.1% at a bias of 0.1 V versus RHE.In another interesting work by Lee and co-workers, a highly efficient photocathode is designed based on gold nanoparticles decorated on silicon nanowires (O_SiNW/Au) utilizing a metal-assisted chemical etching process (Figure 18a-c). [122]The Au deposition presents a greatly improved onset potential and photocurrent density up on 1 sun illumination.This photocathode achieves an average FE and NH 3 production rate of 95.6% and 4.39 μg cm −2 h −1 , respectively, at 0.2 V versus RHE, which is more positive potential than the thermodynamic requirement.
Understanding the role of heterointerface formation on the PEC NO 3 − reduction performance is essential for future electrode design and optimization. [123]123c] By employing several characterizations, it was found that the presence of oxygen vacancies in CeO 2 improves the carriers' transfer and affords more accessible adsorption and active sites for NO 3 − , thereby improving the PEC NH 3 synthesis performance.More recently, Fan and co-workers constructed frustrated Lewis pair sites in carbon coated CeO 2 /BiVO 4 (CeO 2 −C/BiVO 4 ) for the PEC synthesis of NH 3 from nitrate.123b] Though an obvious improvement in PEC performance is seen in the aforementioned works, their FE is still lower, especially when compared with other electrocatalytic processes.Recent works in PEC NO 3 − catalysis utilize the integration of earth-abundant NiFe LDH with graphene/Si to form Schottky junction.The atomically thin layer of graphene deposited between NiFe LDH and Si imparts several advantages to the photoelectrode, including excellent conductivity, inherent chemical stability, and high optical transparency.Consequently, the designed photocathode showed efficient charge separation and large photocurrent density of (−31.9 mA cm −2 ) along with the highest FE to NH 3 of 92.5% at 0.15 V versus RHE, respectively. [124]imilar to most of the PEC reduction process at the cathode, the competing HER severely affects the PEC NH 3 synthesis from NO 3 − precursor. [125]Therefore, future catalyst development must take the competing side reactions into account.Apart from this, the design of photoelectrodes that comprise metal sites with optimal adsorption for NO 3 − and strong hydrogenation ability is essential.To fully understand the relationship between the structure and activity of the photoelectrode, detailed in situ and postmortem electrode characterization is required.In wider dimension, the PEC nitrate reduction could be fully exploited through coupling it with carbon-based organic feedstock to synthesize aliphatic amines, amides, and urea.

Photoelectrochemical H 2 O 2 Synthesis
The PEC production of H 2 O 2 from both oxygen reduction and water oxidation reactions is among the most active areas of research since H 2 O 2 has several applications in wastewater purification, bleaching agent in the paper manufacturing industry, medical disinfection, and organic synthesis. [126]Compared to the energy density of a compressed H 2 gas, which is near 2.8 MJ L −1 , aqueous H 2 O 2 (60%) could also provide an equivalent energy density (3 MJ L −1 ).Moreover, the fact that H 2 O 2 decomposition only produces water and O 2 in the fuel cell and the ease of transportation endows it to be an economically attractive carbon-neutral clean fuel.Up to now, the commercial manufacturing of H 2 O 2 relies on the oxidation of anthraquinone, which involves energy-intensive processes, substantial organic wastes, and expensive Pd hydrogenation catalysts.In this regard, the solar light-assisted PEC process has been regarded as an economically competitive strategy due to the possibility for on-site production with minimized energy consumption. [127]In this section, we highlight the latest development and challenges in developing a PEC cell for the synthesis of H 2 O 2 .
In 2018, Li et al. developed an unassisted PEC system that integrates water, oxygen, and light to efficiently generate H 2 O 2 and electricity. [128]This system successfully realized a two-sided H 2 O 2 synthesis (0.48 μmol min −1 cm −2 ) along with an opencircuit voltage and a maximum power density of 0.61 V and 0.194 mW cm −2 , respectively.Moreover, as high as 1.09 mA cm −2 short circuit current density was also achieved. [129]Alongside developing an efficient PEC system for H 2 O 2 synthesis, considerable effort has also been made toward designing a highly efficient photocathode capable of catalyzing O 2 reduction reaction (ORR).
Li and collaborators exploited metal-free polyterthiophene photocathode (pTTh) to obtain an H 2 O 2 concentration of 110 mm. [130]s shown in Figure 19a, the pTTh photocathode and BiVO 4 photoanode were parallelly configured which were also separated by the Nafion membrane.Using the pTTh photocathode solely in a three-electrode configuration, as high as 110 mmol L −1 H 2 O 2 was obtained after 11 h illumination along with >90% FE throughout the course of the reaction (Figure 19b).Further, the authors evaluated an unbiased PEC system for overall H 2 O 2 production, and maximum H 2 O 2 yield of 90 mmol L −1 was achieved after 14 h of illumination (Figure 19c).Interestingly, they conducted a rigorous theoretical investigation, which could help to understand the mechanism of PEC catalysis.The Gibbs free energies were evaluated for a series of ORR steps and transition states (TS) for the C─OOH and CO─OH bond cleavage steps (Figure 19d,e).Accordingly, the selectivity-determining step for the 2e-transfer process was found to be more than 200 times faster than that for the 4e-transfer pathway (Figure 19f).The Ma group recently published a study on using Gd 3+ doped CuBi 2 O 4 /CuO (CBO/CuO) heterojunction film cathode to catalyze ORR under ambient conditions.The Gd-doped sample showed a nearly 1.3 mm H 2 O 2 yield, and this yield was influenced by Gd 3+ ion concentration and pH of the solution. [131]More recently, Jang and colleagues developed an unassisted PEC cell to efficiently synthesize H 2 O 2 from single-step ORR. [132]They used a high-performance hybrid perovskite photocathode and an oxidized buckypaper as the H 2 O 2 electrocatalyst and a protection layer to achieve nearly 1.463% STC conversion and 100% selectivity toward H 2 O 2 .
It is well documented that a complete PEC cell could be constructed to simultaneously derive H 2 O 2 production and organic pollutant degradation. [133]In 2019, Quan et al. optimized the H 2 O 2 production over a PEC cell consisting of F-doped porous carbon (FPC) cathode and a WO 3 photoanode. [134]This system showed an efficient H 2 O 2 production rate and FE of 0.87 mmol L −1 h −1 and 75%, respectively.Moreover, by introducing Fe 2+ in the electrolyte system, more •OH could be produced, leading to the rapid degradation of organic pollutants present in the solution.
In an approach different from those discussed earlier, which can be envisaged as an innovative strategy, a number of studies have been communicated on efficient H 2 O 2 production from PEC catalysis.For example, Choi and collaborators reported the direct synthesis of concentrated H 2 O 2 (80 mm) using an electrolyte free PEC system. [33]This unique PEC system was constructed employing Ru catalyst decorated on TiO 2 as photoanode, anthraquinone-anchored graphite rods as cathode, and solid polymer as electrolyte (SPE).The authors described the working principle as follows: at the anode, H + could be produced as a result of water oxidation reaction, and at the cathode HO 2 − could be produced via a 2 electrons oxygen reduction.These two species could be selectively transported through the SPE to form H 2 O 2 .The designed three-component PEC system achieved promising H 2 O 2 production of 80 mm concentration over 100 h, at E = 0.However, this system suffered from limitations such as low SPE ionic conductivity and poor membrane stability during long-term operation.Moreover, the utilization of photo-electrobiochemical systems for the synthesis of commodity chemicals has received much research endeavor.In 2019, the Jang group developed a compartmented photo-electro-biochemical system  [130] Copyright 2020, Royal Society of Chemistry.
that integrates a photocatalyst (TiO 2 ), electrocatalyst (Co-based porphyrins), and biocatalyst (lignin peroxidase isozyme) to derive photovoltage generation, H 2 O 2 production, and lignin valorization, respectively (Figure 20a,c). [135]Interestingly, the photovoltage generated at the anode in such a system could produce H 2 O 2 .To alleviate the common stability problem in cobalt-based porphyrins, the authors atomically dispersed Co-N x sites which were created on carbon nanotubes (Co-N/CNT).Compared to the cobalt porphyrin molecular catalyst simply immobilized on the CNTs (CoTMPP/CNT), the synthesized Co-N/CNT showed a significantly lower overpotential of only 0.04 V (Figure 20d).In addition, the estimated working current obtained using Co-N/CNT in the integrated system was fairly higher (0.62 mA) than Co-based porphyrins, as shown in Figure 20c.Importantly, stable H 2 O 2 production could be achieved (Figure 20d,e) in a system that is freely diffused to the biocatalyst compartment to facilitate lignin depolymerization and biopolymer synthesis.Another recent study employed the same electrocatalyst (Co-N/CNT) in PEC cells to facilitate the epoxidation of propylene using an in situ-generated H 2 O 2 . [136]In the presence of propylene, the H 2 O 2 produced remained unchanged, indicating its continuous consumption to produce propylene oxide.This PEC system successfully demonstrated the production of propylene oxide via in situ generated H 2 O 2 .
To summarize, solar-driven oxygen reduction PEC catalysis has been regarded as an alternative strategy to generate valuable H 2 O 2 .It has already been pointed out that such a system requires an efficient and integrated platform that could save energy.While there is a potential to design a PEC cell that could simultaneously drive H 2 O 2 production and pollutant removal for the complete mineralization of wastewater, a more techno-economical study is required to study the feasibility of this process.In contrast, innovative strategies could be further employed to assess the indirect synthesis of other kinds of organic reactions.For instance, H 2 O 2 has many applications in organic synthesis.Hence, some kinds of organic reactions can be carried out with the assistance of an in situ generated H 2 O 2 , which could also solve one grand problem associated with the stability of H 2 O 2 during light illumination.Lastly, the selective reduction of some functional groups in a complex organic molecule could be more attractive to rationally synthesize complex structures, which is helpful in pharmaceutical industries.
Beyond NH 3 and H 2 O 2 synthesis, the reductive PEC upgrading of organic chemicals is a promising strategy for synthesizing several industrially relevant chemicals.For instance, the PEC synthesis of aniline from the reduction of nitrobenzene (NB) was reported by Mascaro and co-workers. [137]Similarly, Ohno et al. successfully synthesized aniline using a p-type Cu 2 ZnSnS 4 electrode.The authors found that the selective reduction of NB depends on the applied potential, and as high as 99% conversion of NB and >50% production of aniline was recorded. [138]he PEC catalysis targeted at reducing various feedstocks is still  [135] Copyright 2019, Springer Nature.
underexplored.Considering the availability of biobased precursors, this technology could be further explored to valorize the readily available precursors.An example of a PEC reaction targeted at reducing biomass is the seminal work by the Choi group.The authors used Ag and BiVO 4 as a cathode and photoanode in this work, respectively.Upon light illumination, the photogenerated holes at the BiVO 4 surface involve water oxidation, whereas the electrons migrate to the counter Ag for an efficient reduction of HMF to BHMF. [35]

PEC Reactions Involving C─H and C─C Bond Activation and Functionalization
The activation and conversion of C─H and/or C─C bonds is a highly value-added conversion, as it could offer a platform for synthesizing industrially and biologically important pharmaceuticals, natural products, and agrochemicals. [139]In 1980s, metal complexes and salts were used to facilitate the C─H bond activation through the oxidative addition mechanism. [140]owever, this strategy suffers from serious limitations since it requires an equimolar amount of organic feedstocks and metal source, and both of these sources were usually consumed at the end.As a result, the feasibility of the process is a major concern, limiting it from being commercialized at a large scale.The past decades have witnessed the de-velopment of this reaction via several strategies, including photocatalysis, [141] electrocatalysis, [142] photoelectrocatalysis, [143] electrophotocatalysis, [144] and Hofman-Loffer-Ferytang reactions. [145]The photoelectrochemical activation and conversion of C─H and C─C bonds present several advantages, such as ease of product separation and less energy consumption.
This section highlights the photoelectrochemical activation and conversion of C─H and C─C bonds for the high-valued transformation of organics.We would like to remind the readers that conversion of C─C/C─H bonds to the corresponding oxygenates is already highlighted in Section 4.1, and this section mainly focuses on the functionalization of non-oxygenate organics.
By and large, the selective functionalization of the C─H bond is challenging for organic chemists since the C─H bonds are kinetically inert.The high dissociation energy required to break the C─C bonds makes the process of C─C bond functionalization challenging.Despite all these facts, the activation and functionalization of these bonds have garnered much research interest due to their broad scope of application in manufacturing medicines, dyestuff and agrochemicals.In 2019, Hu and collaborators reported arenes' C-H amination using a hematite photoanode. [146]s shown in Figure 21a, upon illumination of the hematite electrode with blue LED light, an early onset potential of 0 V versus Fc/Fc + was observed, much lower than the dark (0.9 V vs Fc/Fc + ).The holes generated from photoanode illumination could  [146] Copyright 2019, Springer Nature.
oxidize an electron-rich arene to a radical cation.This radical, which is electrophilic in nature, further reacts with an azole, giving rise to intermediate formation.Next, deprotonation of the as-obtained intermediate and its subsequent oxidation could lead to the resultant amination product (Figure 21b).Interestingly, the authors demonstrated the potential of this process to derive the late-stage functionalization of various pharmaceuticals, including clofibrate, metaxalone, benzethonium chloride, with the product yield reaching 87% (Figure 21c-e).At the same time, the construction of C─P bond via dehydrogenation crosscoupling was reported by Wu et al. [147] The designed PEC system, which utilizes BiVO 4 photoanode, displayed good to excellent yield without the aid of metal catalyst and external oxidant.Moreover, this PEC system was compared with the electrocatalytic C─P bond construction system (which utilized glassy carbon anode), and has showed a greatly minimized external energy input by 90%.
More recently, Duan and co-workers have developed a strategy that utilizes oxygen-vacancy-rich TiO 2 photoanode (Figure 22a) to yield organic halides through PEC C-H halogenation catalysis. [148]By employing a series of experiments, the authors identified thermally treated TiO 2 -Ov at 400 °C as the best photoelectrode to catalyze the efficient synthesis of organic halides (Figure 22b).The authors noted that the photogenerated holes obtained upon irradiation of the photoanode could directly oxidize the halide ions (NaX, where X = Cl − , Br − , I − ) to produce the corresponding radical or dihalides (X 2 ).Importantly, understanding the reaction mechanism was made possible after investigating the EPR spectrum of TiO 2 -Ov at different times.After 5 min illumination, both chlorine and carbon center radical were observed.While the carbon-centered radicals remained after 20 min illumination, the chlorine radicals disappeared due to the quenching process after reacting with the C─H bond (Figure 22c).Interestingly, a practical PEC cell containing seawater, where  [148] Copyright 2021, Springer Nature.
halogenation reaction coupled with H 2 could be realized in one pot, was tested and showed a nearly 64.5 μmol cm −2 h −1 production rate and selectivity of 88.6%.Another PEC C─H bond activation reaction was reported by Sayama and co-workers, where cyclohexanol and cyclohexanone (KA oil) were produced from cyclohexane. [149]In this PEC cell, which employed a porous WO 3 photoanode, the partial oxidation, selectivity, and current utilization ratio were near 99% and 76%, respectively.Moreover, the IPCE at 400 nm was 40% which indicated the potential advantage of this PEC system to produce KA oil from cyclohexane.
Apart from the conversion and activation of C─H bonds through photoelectrochemical technologies, there has recently been a great deal of interest in using light and electricity to derive the activation and functionalization of C─H bonds in a slightly different way.In the process named electrophotocatalysis, the concept of homogeneous photocatalysis is integrated with electrochemistry to bring an intriguing reactivity.It should be noted that the concept employed in electrophotocatalysis is different from photoelectrocatalysis.For a general understanding of this process, we suggest the readers to refer some reviews communicated earlier. [150]n summary, although at its early stage of infancy, PEC catalysis has been acknowledged to derive several kinds of organic transformations.PEC processes involving the oxidation, reduction, bond activation, and functionalization of various feedstocks (organic and inorganic), such as furan-based feedstocks, waste glycerol, benzyl alcohol, and other readily available precursors (oxygen, nitrogen, water, and nitrate) to high-valued chemicals are discussed in this section.Until recently, the photoelectrode materials that have frequently been used for the oxidation of organic feedstocks are BiVO 4 , TiO 2 , WO 3 , and Fe 2 O 3 .These electrodes always suffer from corrosion when the reaction is carried out in an alkaline aqueous electrolyte.Hence, electrode passivation strategies, incorporation of metal and non-metals in their crystal structure, and heterointerface formation would partly alleviate their potential limitations.Another compelling aspect is the consideration of electrolytes (organic vs aqueous) in the PEC system.Organic electrolytes have been used in PEC reactions for some particular reasons.For instance, they play a critical role in minimizing or even eliminating the competitive HER (in the case of reduction) and OER (in the case of oxidation) processes.Apart from this, an aqueous medium induces high rates of photocorrosion and hence deteriorates the stability of the photoelectrodes. [20]In fact, there are attempts to suppress this photocorrossion by using protective layers such as CoO x , [151] NiO x , [152] and FeO x . [153]However, these layers compromise the rate of oxidation of the organic compound, favoring competitive OER.Overall, the research works described so far focus on unoptimized setups and reaction parameters, which further require future research attention.It appears that the design of flow-based reactors is rarely investigated in PEC organic transformations.Hence, by taking a lesson from the CO 2 PEC catalysis, further studies that focus on the design, cell configuration, and reaction parameter optimization are still required.

Conclusion and Future Outlook
Solar-light-driven PEC catalysis for valuable chemical synthesis from various organic and inorganic feedstocks has attracted enormous research endeavors due to its potential to store renewable resources in the form of a chemical bond.In the context of valuable product synthesis, the development of an efficient PEC system relies on several interlinked factors such as electrode preparation, the configuration of the photoelectrodes, the nature and pH of the electrolyte, the stability of photoelectrodes, etc.Moreover, a careful selection of an efficient photoelectrode material, electrode preparation method, and efficiency metrics are detrimental to revolutionizing the development of PEC technology and make the overall PEC system economically competitive.In this review, we have highlighted the emergence of PEC technology as an elegant platform for achieving an efficient chemical transformation under mild experimental conditions.
In general, the PEC oxidation of various feedstocks such as glycerol, HMF, furfural, benzyl alcohol, and lignin-based precursors to valuable oxygenates has been extensively investigated.Studies on the oxidation of biomass-based precursors to valueadded aromatics have demonstrated the potential of the PEC tool to efficiently utilize naturally available precursors without posing a huge environmental burden to the globe.Moreover, one or more valuable product has been achieved through PEC catalysis of various organic feedstocks.For instance, a valuable organic product such as FDCA and DFF could be achieved through the PEC oxidation of HMF.The anodic oxidation of organic precursors has a dual advantage.On the one hand, if a PEC system employing an efficient transformation of organics can be realized, it would be an appealing platform to supplement the energy-intensive thermochemical processes that are widely applied in chemical manufacturing industries.On the other hand, organic oxidation reactions at the anode could be regarded as an alternative reaction to replace the sluggish and industrially less important OER.
Furthermore, cathodic reactions that employ the PEC catalysis are still in progress.The NRR and ORR have shown astounding development, but the performances reported for these two reactions are still far from large-scale practical applications.Particularly, the NRR via PEC catalysis to synthesize NH 3 appears to be challenging due to the inertness of N 2 and its solubility in water.Hence, as an alternative to N 2 , nitrate (NO 3 − ) and nitrite (NO 2 − ) precursors could be tested and optimized via employing PEC catalysis.
To provoke future development in PEC catalysis, we propose the following issues to be considered in the forthcoming research directions.i) Developing an efficient photoelectrode.The type of photoelectrode and its method of preparation not only affect the performance of a PEC system but also determine the overall cost of the process.Until now, only a few photoelectrodes such as BiVO 4 , Ta 3 N 5 , WO 3 , TiO 2 , and Cu 2 ZnSnS 4 have been commonly used to derive a number of reactions in their pristine form as well as with other configurations including heterostructures and doped variants.Most of them usually require additional electrode processing procedures to enhance their stability and efficiency.Hence, designing an efficient and stable photoelectrode capable of catalyzing the targeted reactions is still a central challenge that needs to be addressed.Besides, studies related to photoelectrode degradation mechanisms and related strategies for improving their stability are vital.ii) Understanding the reaction mechanism in depth.In this regard, the design of experiment platforms for the real-time analysis of intermediates and products could be helpful to properly assess the reaction path of a particular reaction.Meanwhile, in situ characterization techniques such as X-ray absorption techniques, attenuated-total-reflection, Fourier transform infrared spectroscopy, and dynamic transmission electron microscopy, should also be developed.iii) Novel approaches for an integrated PEC system.In PEC catalysis, an innovative integrated system has been designed to derive some special reactions.In this system, a photoanode can be coupled with an electrode and biocatalytic system to achieve a transformative valuable chemical synthesis.For example, an integrated PEC cell consisting of photoelectrocatalyst, electrocatalysts, and biocatalysts in a three-compartmented system demonstrated efficient biomass valorization.Moreover, recent work has demonstrated the successful integration of a photoelectrocatalyst, electrocatalyst, and photocatalyst systems which efficiently catalyzed propylene epoxidation via an in situ generated H 2 O 2 .Such kinds of systems with novel reaction routes and PEC cell design could be further developed and optimized.iv) Scope of anodic and cathodic reactions.Taking a lesson from some successful electrochemical synthesis of organics, [154] numerous organic reactions can be tested and optimized.Moreover, only a few cathodic re-actions have been frequently investigated, and the scope of the reduction half-reaction beyond water can be taken into account.
PEC catalysis often combines two separate limitations of electrocatalysis (ohmic drop) and photocatalysis (light attenuation), which demand a careful design of PEC cells.10a] Notable examples of PEC CO 2 reduction reactions show that the use of flow-based reactors has great promise for improving overall energy utilization efficiency.For instance, a flow-based PEC cell that utilized silicon PV was able to produce syngas (H 2 + CO) without applying external bias. [155]In recent work by Gong et al. a PEC flow cell that efficiently catalyzes CO 2 reduction reaction to C 2+ on Cu catalyst is demonstrated, and the cell achieved a FE and STF of nearly 53% and 0.29%, respectively. [24]Driving lessons from the aforementioned works, a PEC cell that integrates electrochemical reduction of some readily available feedstocks with photoelectrochemical oxidation of water could be designed and optimized at high current densities.Meanwhile, integrated research on specific areas of flow chemistry and chemical engineering is urgently required to facilitate the future development of a PEC flow cell device.As such, the utilization of microreactors could lead to the controlled optimization of various parameters, further facilitating its rapid development.
In summary, though at the stage of infancy, employing PEC catalysis for valuable product synthesis could offer a promising avenue to store solar light in the form of chemical bonds.Despite the significant progress seen over the last few years, a number of challenges are still awaiting before applying this tool to real practical application.Along with the development of an efficient PEC system to achieve high efficiency, a techno-economical evaluation of this tool is still required.Finally, it is anticipated that more effort from the diverse field including material science and engineering, theoretical chemistry, and organic chemistry, will be put into action to facilitate the development of PEC tools in the future.

Figure 2 .
Figure 2. a) Photoelectrochemical catalysis via direct and b) indirect or mediated oxidation process.Fed and Med represent feedstock and mediator, respectively.

Figure 3 .
Figure 3. Schematics illustration of semiconductor photoelectrode under simulated solar light Air Mass 1.5 Global (AM 1.5 G) illumination.a) Front side illumination.b) Back side illumination.

Figure 4 .
Figure 4. a,b) Comparison of J-V curves for a typical PEC oxidation and reduction of a target feedstocks with competing HER/OER in an aqueous solution.c) Operating current selection in an integrated photo-electrochemical cell from the intersection of the LSV curves of the photoanode and photocathode.

Figure 5 .
Figure 5. Drop casting and spin coating electrode fabrication methods.Schematic illustration of electrode preparation by a) drop-casting and b) spin coating methods.The catalyst slurry is directly cast on fluorine-doped tin oxide (FTO) or indium tin oxide (ITO) substrates.c)The digital image of black phosphorus (BP) and its photoelectrode film.d) Illustration of the electrochemical synthesis method for black phosphorus nanosheets and the spin coating method employed for BP electrode fabrication.Reproduced with permission.[40]Copyright 2020, Wiley VCH Weinheim.

Figure 6 .
Figure 6.a) Schematic illustration of the doctor blade electrode fabrication method.b) Facile electrode fabrication scheme for carbon nitride photoelectrode.A supramolecular complex containing melamine and bismuthiol was formed and finely ground.This mixture was blended into a paste along with ethylene glycol and graphene oxide (GO), and doctor-bladed onto an FTO substrate to get the photoelectrode films.c,d) DFT calculation depicting the interaction energy between melamine-bismuthiol and ethylene glycol (EG) and the melamine-bismuthiol supramolecular assemblies, respectively.e) Scanning electron microscopy (SEM) image of melamine-bismuthiol-graphene oxide 0.75 (MSG 0.75 ) films on FTO prior to calcination.Reproduced with permission.[42a]Copyright 2020, American Chemical Society.

Figure 8 .
Figure 8. a) Schematic illustration for the synthesis process of carbon nitride film based on thiourea (CN T ) on FTO substrate.b) SEM image of thiourea on FTO substrate (inset: a cross-sectional image of three-layer thiourea film).c-f) Digital images of thiourea and CN T films on a different substrate.The images at the left correspond to thiourea, and CN T films at the right.Reproduced with permission.[66]Copyright 2020, Springer Nature.

Figure 9 .
Figure 9. a) Catalytic oxidation pathway for glycerol.b) Schematic depicting the role of Bi 2 O 3 in Bi 2 O 3 /TiO 2 nanorod array for preferential adsorption of the middle hydroxyl group of glycerol to facilitate its oxidation into 1,3-dihydroxyacetone (DHA).c) Schematic illustration and d) digital image depicting the construction of a practical PEC device for glycerol valorization.Time-dependent DHA and H 2 production in a paired electrochemical system.The PEC process is carried out under simulated solar light illumination of Bi 2 O 3 /TiO 2 photoanode in 0.5 m Na 2 SO 4 electrolyte (pH = 2) and 0.1 m glycerol concentration.The Pt cathode is used to generate H 2 gas on the counter.Reproduced with permission.[80]Copyright 2022, American Chemical Society.

Figure 11 .
Figure 11.a) Schematic depicting the reaction pathway for oxidation of HMF to FDCA.b) Photoelectrochemical and electrochemical TEMPO-assisted HMF oxidation.CB, conduction band; VB, valence band; EF, Fermi energy.c) Yield (%) of HMF oxidation products and their conversion change during TEMPO-mediated PEC oxidation.The applied potential was 1.04 V versus RHE in a 0.5 m borate buffer solution containing 5 mm HMF and 7.5 mm TEMPO.The photoelectrode was illuminated from the front side under AM 1.5 G illumination (100 mW cm −2 ).Reproduced with permission. [4j] Copyright 2015, Springer Nature.

Figure 13 .
Figure 13.a) Schematic illustration depicting the hydrothermally grown monoclinic WO 3 nanostructures with different {010} facet ratios on FTO substrate.b) Potential dependent production of ethylene glycol (EG) over WO 3 photoanode with different {010} facet ratio.c) Potential dependent selectivity of EG produced during PEC CH 4 catalysis over WO 3 photoanodes with different {010} facet ratios.The photoelectrocatalysis was carried out under 1 sun (100 mW cm −2 ) illumination.d) In situ diffuse reflectance infrared Fourier-transform spectroscopy for PEC CH 4 conversion on WO 3 NB.e) Schematic illustration of the proposed reaction mechanism for PEC CH 4 conversion into EG.Reproduced with permission.[36]Copyright 2021, Wiley VCH Weinheim.

Figure 14 .
Figure 14.a) Comparison of conversion efficiencies for the PEC oxidation of 1-ol to 1-one under different conditions.The standard conditions for PEC catalysis are 2.5 mm of the feedstocks (1-ol) and 5 mm NHPI/2,6-lutidine in 10 mL of acetonitrile with 0.1 m TBAPF 6 under 2 sun illumination (200 mW cm −2 white light, AM 1.5G).The applied bias for all PEC tests was 0.75 V versus SCE.b) Stability test for PEC oxidation of 1-ol to 1-one over FTO|TiO 2 −RuC photoanode.c) Proposed mechanism for the oxidative conversion of the lignin model.Reproduced with permission.[104]Copyright 2019, American Chemical Society.d) Schematic illustration depicting the transformation of lignin via PEC catalysis over BiVO 4 photoanode.e) Control experiment for unassisted PEC catalysis of lignin for formate production.f) Formate production rate under varying initial lignin concentrations.The reaction conditions were as follows: The cathode electrolyte contained NAD + (2.0 mm) and M (0.5 mm) in a phosphate buffer solution (100 mm, pH 6.5); the anode electrolyte contained 0-5 mg mL −1 lignin in a KHCO 3 buffer (10 mm, pH 8.2).Reproduced with permission.[105]Copyright 2020, Royal Society of Chemistry.

Figure 15 .
Figure 15.Proposed reaction pathways for NRR.The blue and light gray colors represent nitrogen and hydrogen atoms.

Figure 16 .
Figure 16.a) Comparison of NH 3 production over different photocathodes after 12 h reaction time.b) Schematic depicting single compartment PEC cell and c) the models of pristine TiO 2 and modified TiO 2 /Au structure used in the finite difference time domain (FDTD) simulation work.f) Comparison of Ti 2p XPS spectra of TiO 2 and TiO 2 /a-TiO 2 .d,e)FDTD simulations depicting the electric field enhancement in TiO 2 .Reproduced with permission.[32a]Copyright 2018, Wiley VCH Weinheim.g) Bright-field scanning transmission electron microscopy (BF − STEM) image of the as-fabricated Au-NPs/Nb-SrTiO 3 interface.h) Illustration of the NH 3 synthesis device of the Nb-SrTiO 3 photoelectrode loaded with Au-NPs and a Zr/ZrO x thin film.i) Energy-level diagram demonstrating the device for plasmon-induced NH 3 synthesis.CB: conduction band; U: redox potential.Reproduced with permission.[116]Copyright 2016, Wiley VCH Weinheim.

Figure 17 .
Figure 17.a) Schematics for the fabrication of Au/TS and hydrophobic poly(tetrafluoroethylene) porous framework with Au nanoparticle (Au-PTFE/TS) photocathode.b) The Field emission scanning electron microscopy (FESEM) image of the fabricated electrode.c) The potential-dependent NH 3 production rate (column diagrams) and the corresponding faradic efficiency (point plots) on Au/TS (orange) and Au-PTFE/TS (purple).The test was conducted for 4 h.d) NH 3 yield (green balls) and faradic efficiency (brown stars) over Au-PTFE/TS photoelectrode.e) Free energy diagram depicting the NRR steps on Au/TS and Au-PTFE/TS.The inset is the schematic illustration of distal mechanisms for NRR on the designed Au-PTFE/TS photoelectrode.Reproduced with permission.[118]Copyright 2019, Elsevier BV. f-h) Schematic depicting PEC catalysis over BP photoelectrode to produce NH 3 from N 2 (f) the corresponding potential dependent NH 3 production rate and FE (g) and stability test for 6 consecutive cycles.Reproduced with permission.[40]Copyright 2020, Wiley VCH Weinheim.

Figure 18 .
Figure 18.a,b) The SEM images of ordered silicone nanowires (O_SiNW) and gold decorated ordered silicone nanowires (O_SiNW/Au).In sets of (a) and (b) are the magnified images of their respective nanowires.c) The HAADF-STEM and corresponding EDS mapping of O_SiNW/Au sample depicting Au nanoparticles loaded on Si nanowires.d) Comparison of the current density-voltage profile of O_SiNW and O_SiNW/Au samples for nitrate reduction in Ar-saturated electrolyte (pH 3.5) containing 0.5 m K 2 SO 4 and 10 mm K 15 NO 3 .e) FE and f) production rate of 15 NH 3 using O_SiNW and O_SiNW/Au photoelectrodes under the bias of − 0.1 to 0.2 V versus RHE.Reproduced with permission.[122]Copyright 2022, Wiley VCH Weinheim.

Figure 19 .
Figure 19.a) Schematics of the unbiased H 2 O 2 production cell.The PEC cell consisted of metal-free polyterthiophene photocathode (pTTh) as photocathode and BiVO 4 -based photoanode in 0.1 m KOH and 1 m borate buffer electrolytes, respectively.b) Time-dependent H 2 O 2 production and the corresponding FE using pTTh photocathode (9 cm 2 ).c) Time-dependent H 2 O 2 generation and the corresponding solar to H 2 O 2 conversion efficiency.d) Energy profiles depicting the possible ORR pathways on pTTh.e) Free energy diagram of the branching point for 2e − and 4e − process selectivity of the ORR at pH = 13.The blue and red dashed line represents the O─O and C─O bond cleavage pathways, respectively.f) Proposed reaction cycles of PEC H 2 O 2 synthesis with the energetically most viable active sites.S 0 is the bare surface, S x (x = 1, 2, 3, 4, 5) are the structures of the intermediate states involved in the ORR.Reproduced with permission.[130]Copyright 2020, Royal Society of Chemistry.

Figure 20 .
Figure 20.a) Schematic showing a three-compartment photo-electro-biochemical system.b) Comparison of LSV curves of different catalysts.The gray, pink, and sky-blue colors correspond to the scans for photoelectrodes of carbon nanotubes (CNT), cobalt porphyrin molecular catalyst simply immobilized on the CNTs (CoTMPP/CNT), and atomically dispersed Co-N x sites on carbon nanotubes (Co-N/CNT), respectively.The dashed lines are the corresponding curves after 1000 cycles.c) J-V curves of H:TiO 2 photoanode (orange line) and Co-N/CNT cathode (sky blue line).The anode electrolyte is 0.1 m phosphate borate solution (PBS, pH 4.5), and O 2 -saturated PBS was used at the cathode.The photoanode was illuminated under simulated 1 sun (AM1.5 G). d) Amount of H 2 O 2 produced in a three-compartment integrated cell.e) The conversion and selectivity for H 2 O 2 over the course of the reactions in the PEC catalytic system.Reproduced with permission.[135]Copyright 2019, Springer Nature.

Figure 22 .
Figure 22. a) PEC C-H halogenation using sodium halide b) SEM images of TiO 2 -Ov-400.The scale bar is 200 nm.c) PEC conversion rate and selectivity during C-H halogenation catalysis over TiO 2 -Ov treated at different temperature.Reproduced with permission.[148]Copyright 2021, Springer Nature.

Table 1 .
Key redox mediators commonly used in PEC transformation of organic chemicals.

Table 2 .
Summary of the analytical techniques employed for qualitative identification and quantitative determination of products.

Table 3 .
Summary of pros and cons of various ex situ photoelectrode fabrication methods.
Method of electrode preparation Advantages Disadvantages Drop casting and spin coating Easy, cheap, and scalable Difficult to control film thickness and uniformity, a time-consuming process Vacuum filter transfer Simple Poor contact between the semiconductor film and the substrate Doctor blade and screen printing Possibility of scale-up Difficult to control film thickness below 1 μm for a short charge diffusion length