Production of High-Value-Added Chemicals on Oxide Semiconductor

Apr 11, 2018 - Biography. Kazuhiro Sayama is a Prime senior researcher and Team lead in the Advanced Functional Materials Team, Research Center for ...
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Production of High-Value-Added Chemicals on Oxide Semiconductor Photoanodes under Visible Light for Solar Chemical-Conversion Processes Kazuhiro Sayama* Research Center for Photovoltaics (RCPV), National Institute of Advanced Industrial Science and Technology (AIST), Central 5, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan ABSTRACT: This study reviews recent progress on the oxidative production of various high-value-added chemicals using simple and inexpensive oxide semiconductor photoanodes for technology associated with solar-to-chemical conversion processes. The photoelectrochemical production of oxidizing reagents can be highly economical and contribute to global warming by replacing the conventional processes. Various oxidizing reagents and useful chemicals, such as hydrogen peroxide (H2O2), hypochlorous acid (HClO), persulfates (H2S2O8), IO4−, Ce4+, and organic compounds, can be produced photoelectrochemically with excellent faradaic efficiencies. This process also allows H2O2 production via both photoanodic and cathodic reactions without external bias. These photoelectrochemical processes are key technologies enabling the expansion of solar energy utilization toward a sustainable and low-carbon society.

T

II) of natural photosynthesis and has been the main focus of study in the field of artificial photosynthesis in addition to the production of H2 (eq 5) and fuels.

he expansion of utilization methods capitalizing on clean and limitless solar energy is an important issue. The development of inexpensive, simple, and innovative technologies is required to enable using solar light energy, the disadvantages of which include low energy density and weather-related fluctuations. Artificial photosynthesis, which directly converts photon energy into chemical energy, is considered one of the few promising choices available to solve the global energy problems. Water splitting into H2 and O2 [solar hydrogen; eq 1] and CO2 fixation onto organic compounds and/or into CO [solar fuel; eqs 2 and 3] are examples of artificial photosynthesis reactions. 2H 2O → 2H 2 + O2

(1)

H 2O + CO2 → CO + H 2 + O2

(2)

2H 2O → O2 + 4H+ + 4e− E(O2 /H 2O) = +1.23 V vs RHE 2H+ + 2e− → H 2

(3)

These reactions are energy-accumulating or uphill reactions (ΔG > 0) that involve O2 evolution from water wherein the photon energy can be converted to chemical energy by photocatalysts and photoelectrodes using semiconductor materials or dyes. The resulting chemical energy can be accumulated stably over long periods and transferred over long distances. The O2 production from water as an electron donor (eq 4) is the most fundamental reaction in photosystem II (PS © XXXX American Chemical Society

E(H+/H 2) = 0 V vs RHE

(5)

However, the practical use of artificial photosynthesis exclusively as a producer of H2 and fuels (eqs 1−3) might be difficult economically over the short term because of the cost of deriving fuel or H2 from fossil energy as the competitor continues to be significantly cost-effective. Making a scenario supporting alternative targeting to bring artificial photosynthesis technology to the market economy or gain political framework support for such activity (e.g., the Paris Agreement) is very difficult. In the case of the photovoltaics field, they have intermediate applications in batteries for space satellites, watches, calculators, and toys, and finally, mega-scale solar power generation has been developed. As for the biomass energy field using natural photosynthesis, fundamental markets and/or intermediate practical applications such as food, medicine, and chemicals are present, and large-scale biofuel

4H 2O + 2CO2 → 2CH3OH (example of organic compounds) + 3O2

(4)

Received: February 24, 2018 Accepted: April 6, 2018

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DOI: 10.1021/acsenergylett.8b00318 ACS Energy Lett. 2018, 3, 1093−1101

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Cite This: ACS Energy Lett. 2018, 3, 1093−1101

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Figure 3 plots the CO2 emission factor and the price per electron of H2, O2, CO, methanol, and examples of high-valueadded chemicals produced in conventional methods mainly using fossil energy. The X-axis (kg-CO2/kmol-e−) indicates the impact on global warming, and the values of chemicals were calculated from the carbon-footprint database.13 The Y-axis (¥/kmol-e−, $1 = ca. ¥110) indicates the typical price per electron of chemicals in Japan, which is related to the economic benefit of producing chemicals when the conventional methods are replaced by a photoelectrochmical method using only solar energy. Both X and Y values associated with hydrogen peroxide (H2O2) and hypochlorous acid (HClO) are much larger compared with those of H2, O2, CO, and MeOH. These oxidizing reagents are produced worldwide in large quantities (4.5 million metric tons for H2O2 and 70 million metric tons for hypochlorite salt annually);14 the total CO2 emission resulting from their processing (24 million metric tons and 29 million metric tons, respectively) is also high. The CO2 emission factors per electron (kg-CO2/kmol-e−) of H2O2 and HClO are 90.1 and 15.5, respectively, which are much larger than those for H2 and O2 (2.5 and 1.1, respectively).13 In Japan, the price per electron for H2O2 and HClO is ∼140- and ∼250fold higher than that for O2 and ∼18- and ∼33-fold higher than that for H2. Therefore, there is a possibility that the realization of efficient photoelectrochemical production of these oxidizing reagents cannot only be highly economical but also contribute to alleviating global warming compared with the water splitting and the simple CO2 reduction. Moreover, other oxidizing reagents, such as persulfates, IO4−, and Ce4+, can also be produced photoelectrochemically,1,2,4 and the photoelectrochemical organic synthesis via oxidative reactions is also highly promising.11,12,15 The sum total production amounts of these various chemicals will become very large, even if each production amount of these special oxidizing reagents and organic compounds via the oxidative process is small.

production has been developed in the past several years. Therefore, new scenarios supporting various intermediate applications or alternative targets, except fuel production, should be created also for the artificial photosynthesis field.

New scenarios supporting various intermediate applications or alternative targets, except fuel production, should be created also for the artificial photosynthesis field. Signif icance and Impact on the Production of High-Value-Added Chemicals via Artif icial Photosynthesis. In the field of artificial photosynthesis, various oxidative products, except O2, have been scarcely paid any attention. Few studies on the oxidative production and accumulation of various oxidants via uphill reaction (ΔG > 0) have been reported;1−3 however, they focused on these reactions mainly based on general scientific interest. We previously investigated photoelectrochemical reactions for the efficient production of high-value-added oxidation reagents using porous oxide photoanodes prepared via simple processes4−12 and recognized the significance and impact of these reactions from the aspects of not only the basic science related to artificial photosynthesis reactions but also their useful and near-term applications in industries (Figure 1). Various chemicals are directly and indirectly produced by the consumption of fossil energy, and improving the energy efficiency and significantly reducing CO2 emissions from chemical industries are important. The efficient photoelectrochemical production of high-value-added chemicals using low voltage from solar energy directly is a desirable process; however, research and development of this activity remains scarce. It is difficult to obtain economic benefits exclusively by producing and selling electrochemically or photoelectrochemically derived H2 from water because the market price for H2 from conventional competitors using fossil energy is very low (Figure 2). Economical O2 production is also difficult because its market price is lower than that of H2. However, the capability of producing high-value-added chemicals that are significantly more expensive than H2 and O2 per electron (per photon) on photoanodes via oxidative reactions, in addition to H2 production on cathodes, makes the economic benefits associated with photoelectrochemical systems tremendously practical.

There is a possibility that the realization of efficient photoelectrochemical production of these oxidizing reagents cannot only be highly economical but also contribute to alleviating global warming compared with the water splitting and the simple CO2 reduction.

Figure 1. Purpose of producing high-value-added chemical reagents and H2 using solar light. (a) Conventional processes involved with chemical-reagent production. (b) Future vision of chemical-reagent production via sustainable renewable-energy sources. 1094

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Figure 2. Relation between the production cost of chemicals via cathodic and anodic reactions and the market price to obtain practical and economic performance from a photoelectrochemical system.

ΔEL (ΔEL values for O2/H2O and S2O82−/SO42− in the case of the WO3 photoanode are ca. 1.8 and 0.9 V, respectively). Therefore, the solar energy conversion and accumulation efficiency can be increased intrinsically, and the associated economic performance can be improved by simultaneously producing H2 and high-value-added chemicals. Moreover, pure O2 gas can be accumulated in one location using catalysts, such as Pt and Ag, in dark reactions through peroxide decomposition. The cathodic reactions, such as O2 reduction to H2O2 and CO2 reduction to organic compounds, can also be diversified. Various solutions containing oxidizing reagents can be applied to many environmental fields, including pollutant purification, disinfection, bleaching, cleaning, and oxidative organic synthesis. Small-scale and on-site production of disinfection solutions, such as H2O2 and sodium hypochlorite (NaClO), exclusively using solar energy is one of the most practical applications that can potentially occur in the near future. In the cases wherein conventional and large-scale central production are involved, solutions containing oxidizing reagents must be condensed when transporting over long distances with large amounts of CO2 emissions, whereas oxidizing reagent solutions can be diluted into a certain concentration at the consumption area. Therefore, distributed production supported by the ability to adjust reagents to target concentrations for consumption using solar energy will contribute to energy savings. The target concentration and purity of produced oxidizing reagents are different in each application. The balance between the requirement and the energy saving or cost should be considered. In some applications for bleaching and disinfection, the condensation and purification of the produced oxidizing reagent in the reaction solution is not always essential. Here, we present recent progress and review significant production advancements involving various oxidizing reagents processed via photoelectrochemical reactions using simple and inexpensive oxide semiconductor photoanodes under visible light. Oxide semiconductor photoanodes such as WO3 and BiVO4 films on conducting substrates can be prepared by wet coating and calcination processes, and these films have porous structures with small oxide particles. The porous structure has an advantage to improve the quantum efficiency due to the short diffusion length of the hole to the particle surface. Examples of Photoelectrochemical Production of High-ValueAdded Chemicals. Some papers on the photoelectrochemical

Figure 3. Plot of the CO2 emission factor and price per electron associated with the production of H2, O2, CO, methanol, and various high-value-added chemicals in Japan. $1 = ca. ¥110 (Japanese yen). Electron number: H2, CO, H2O2, HClO = 2, O2 = 4, methanol (MeOH) = 6.

The reaction mechanism associated with oxidizing reagent production on n-type semiconductor photoanodes is shown in Figure 4. Solar light is irradiated onto the photoanode, and e−

Figure 4. Principles of producing high-value-added chemicals on photoanodes.

and h+ are generated on the conduction and valence bands, respectively. The e− is transferred to the counter electrode, and water is reduced to H2. The h+ generally oxidizes water to O2; however, the energy loss (ΔEL) is very large when the valenceband potential is highly positive (Figure 4). However, there are many redox reactions with more positive potentials. For example, S2O82− produced from SO42− results in a very small 1095

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Table 1. List of High-Value-Added Chemicals Produced on Porous Oxide Photoanodes Using Visible Light product

photoanode

solution

light condition

potential (V)

W-halogen/Xe

0.8(SCE)

1 Sun 1 Sun 1 Sun W-halogen/Xe 1 Sun 1 Sun 1 Sun

1.4(NHE) 1.5(RHE) 1.5(RHE) 0.6(SCE) 1.23(RHE) 1.2(RHE) 0.22(Pt:O2-red)

1 Sun 1 Sun 1 Sun 550 nm light W-halogen/Xe 1 Sun 1 Sun 1 Sun

S2O82−

WO3

S2O82− S2O82− S2O82− Cl2(ClO−) Cl2(ClO−) Cl2(ClO−) Cl2(ClO−)

WO3 WO3 WO3 WO3 WO3:B RhO2/BiVO4:Mo BiVO4/WO3

3 M KHSO4 (pH 0.7) 1 M H2SO4 1 M H2SO4 1 M H2SO4 1 M HCl (pH 0) 0.5 M NaCl (pH 2) sea water 5 M NaCl

H2O2 H2O2 H2O2 H2O2 Ce4+ Ce4+ IO4− Cr6+

BiVO4/WO3 Al2O3/BiVO4/WO3 BiVO4 Ge-dye/TiO2 WO3 WO3 WO3 WO3

2 M KHCO3 2 M KHCO3 1 M NaHCO3 0.1 M Et4N+BF4− aq. 5 mM Ce3+ (pH 0) 1 M Ce(ClO4)3 0.2 M NaIO3 10 mM Cr2(SO4)3

photocurrent (mA/cm2) 0.6

1.2(RHE) 1.2(RHE) 1.2(RHE)  0.8(SCE)

remarks

85

ref 1

1.3 2.6 3.04 0.6 2.15 ∼2.6 0.25

85 100 100 76 ∼25 80

4 3.5 ∼3 0.007 0.35

54 79 100 99 56 40−50 ∼50 100

∼2

1.6(RHE)

FE (%)

ABPE = 1.3% ABPE = 2.2% ABPE = 2.45%

initial FE = ∼97% ABPE = 2.2%

2 4 8 1 3 21 10 5, 6 7 27 26 1 4 4 9

Figure 5. Time course of S2O82− production in H2SO4 aq. sol. on a WO3 photoanode. Constant photocurrent (1 mA).

(FE) was relatively high at ∼85%, but the photocurrent was low, even under intense light. Lewis et al.2 reported the value of the applied-bias photon-to-current efficiency (ABPE) for H2 and S2O82− production under simulated solar light (1 Sun: AM1.5, 100 mW/cm2). The ABPE and FE on WO3 photoanodes were 1.3 and 85%, respectively. The ABPE, which is also called the half-cell solar energy conversion efficiency, indicates the ability of solar energy conversion by one photoelectrode under an applied bias and is generally used in water splitting reactions to compare photoelectrode efficiency values (eq 7). The ABPE for H2 and S2O82− production and accumulation (eq 8) becomes intrinsically higher compared with that for the water splitting reaction involving H2 and O2 because of the highly positive redox potential of S2O82−/HSO4−.

properties of various photoanodes for the production of highvalue-added chemicals are summarized in Table 1. Important results from these papers and background on each chemical are introduced. Peroxydisulf uric Acid (H2S2O8). H2S2O8 (Na2S2O8, S2O82−) possesses a highly positive redox potential (eq 6) and exhibits the highest oxidizability among all conventional peroxides. 2HSO4 − → S2 O82 − + 2H+ + 2e− E(S2O82 − /HSO4 −) = +2.123 V vs RHE

(6)

H2S2O8 has been the subject of intense attention because peroxides are expected to be useful in a variety of chemical fields, including application to fuel sources using H2O2 easily generated by S2O82− hydrolysis,16−18 environmental purification fields for the removal of harmful substances,19 selective organic synthesis as typified by Elbs persulfate oxidation,20 etching, bleaching, and cleaning agents. During the process of etching resistors and metal films on semiconductor wafers, large amounts of H2S2O8 aqueous solution are produced via electrolysis on boron-doped diamond electrodes in H2SO4 aqueous solution in an electronics device factory using a high voltage and large amounts of electrical power. Graetzel et al.1 reported pioneering studies on the preferential production of peroxydisulfate (S2O82−) on a WO3 photoanode in H2SO4 aqueous solution. The faradaic efficiency

ABPE(H 2 , O2 ) = [Jopt × ((1.23 V − Eopt) × FE(O2 ))/Int] × 100

(7) 2−

2−

ABPE(H 2 , S2 O8 ) = [Jopt × ((2.123 V − Eopt) × FE(S2O8 ))/Int] × 100

(8)

These equations can be used as a function of the applied voltage at the optimal operating conditions between the working and RHE or counter electrodes in three- or twoelecrode setups (Eopt), the photocurrent density at Eopt (Jopt), and the intensity of irradiated simulated solar light calibrated to AM-1.5 (Int: 1 Sun condition, AM-1.5, 100 mW cm−2). 1096

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practical for niche uses without external electric power. Such small-scale systems can be used for sterilization and disinfection in food factories, stores, restaurants, healthcare facilities, public facilities, and bleaching processes for clothing items. A similar production apparatus for the creation of disinfection solutions, safe drinking water, water cleaners for emergencies, and use at sites, such as islands, remote areas, developing countries, and disaster areas, will be also realized in the near future. H2O2. H2O2 is a typical oxidizing reagent. H2O2, which produces only H2O or O2 after utilization, has been the subject of substantial attention as a versatile and clean redox agent for selective organic conversion, environmental purification, bleaching, cleaning agents, and as an energy source for H2O2 fuel cells.18,22−24 The major industrial application is pulp- and paper-bleaching and the manufacture of sodium percarbonate and sodium perborate, which are used as mild bleaches in laundry detergents. H2O2 is also used in the production of various organic peroxides, such as dibenzoyl peroxide, peracetic acid for polymerizations, and bleaching agents for flour and hair; it is utilized in wastewater treatment processes to remove organic impurities and reduce odor, as well as sterilization against viruses and bacteria. The anthraquinone process is widely used as an industrial and large-scale H2O2 synthesis method. This is a catalytic process that occurs after the production of H2O2 and an anthraquinone derivative by reacting an anthracene derivative with O2 and involves regeneration of the anthraquinone derivative to the anthracene derivative in the presence of H2 and a noble metal hydrogenation catalyst. However, this process has a multitude of serious problems, including a multistage synthesis process and a requirement for large amounts of harmful organic solvents and pressurized H2 gas. The H2 is generally formed by steam formation driven by fossil fuels, resulting in large amounts of CO2 gas emissions. In addition, the organic solvent and catalysts need to be subsequently and completely removed from the H2O2 solution. These problems can be overcome if we develop an efficient photoelectrochemical technology capable of producing H2O2 from H2O as a raw material via a clean, one-step synthesis process in an aqueous solution under solar light without H2 consumption or organic solvents (eq 12).

To improve the ABPE, the photocurrent density should be increased and the applied bias should be decreased. We prepared a thick and porous WO3 photoanode using a precursor solution of H2WO4 sol and polyethylene glycol, resulting in ABPE and FE for H2 and S2O82− production of 2.2 and ∼100% (Figure 5), respectively,4 with no observable O2 gas production. The solar energy conversion efficiency into H2 and S2O82− was 5.2% when dye-sensitized solar cells were used as the external bias in a standalone system. Recently, much thicker (>7 μm thickness) WO3 nanosponge photoanodes with 100% FE were prepared by a nanoparticle/solution hybrid dispersion method.8 The photocurrent and the ABPE for H2 and S2O82− production were >3 mA/cm2 at 1.5 V compared with a reversible hydrogen electrode (RHE) and 2.45%, respectively. HClO, Chlorine (Cl2), and NaClO. HClO, Cl2, and NaClO are typical oxidizing reagents. The fractions of Cl2, HClO, and ClO− change according to the pH in the aqueous solution (eq 9). They are used in large amounts for disinfecting drinking water in waterworks and swimming pool systems worldwide, as well as for pulp- and paper-bleaching, bleaches in laundry and household-use detergents, treatment of dye wastewater, food sterilization, and bacterial, spore, and viral disinfection. NaClO and Cl2 are industrially produced by electrochemical oxidation of Cl− in seawater (eqs 10 and 11); the produced Cl2 gas at the anode is mixed with NaOH produced at the cathode to form NaOCl and requires large amounts of electric power for electrolysis owing to a high applied bias with CO2 emission. Cl 2 + 2H 2O = 2ClO− + 4H+

(9)

2Cl−(aq) → Cl 2(aq) + 2e− E(Cl 2/Cl−) = +1.40 V vs RHE

(10)

Cl−(aq) + H 2O → HClO + H+ + 2e− E(HClO/Cl−) = +1.50 V vs RHE

(11)

It is difficult to preserve a NaOCl aqueous solution for long periods because NaOCl gradually decomposes over time to O2, even in the dark. Therefore, the distributed and on-site production of NaOCl aqueous solution is preferable at the consumption site. The combination of electrolyzer and photovoltaics is useful as a standalone system using solar energy; however, NaOCl production cost might become very expensive. Therefore, simple and effective photoelectrochemical production of NaOCl using solar light and inexpensive oxide photoanodes would be ideal. Graetzel et al.1 reported Cl2 production on WO3 photoanodes in HCl aqueous solution. The FE for Cl2 was relatively high at ∼76%, but the photocurrent was very low, even under an intense light. Augustynski et al.3 reported high photocurrent (>2 mA/cm2) on porous WO3 photoanodes under 1 Sun conditions in NaCl aqueous solution, with the FE for Cl2 production at ∼25%. Cl2 production on BiVO4 photoanodes in natural seawater was evaluated by Zou et al.,21 with the FE for Cl2 production at ∼25%. We previously reported the FE for NaOCl production on BiVO4/WO3 photoanodes at ∼80%, with an initial FE for NaOCl of around 97%.10 Moreover, the onset potential is capable of being negatively shifted when O2 is reduced at the counter electrode. Furthermore, the produced H2 gas at the counter electrode is not essential in a small standalone system, and the simple combination of NaOCl production and O2 reduction will make a standalone system

Oxidative H2O2 generation from H2O: 2H 2O → H 2O2 + 2H+ + 2e− E(H 2O2 /H 2O) = +1.77 V vs RHE

(12)

H2O2 is very unstable, and the oxidative reactions at anodes and/or photoanodes have not been effectively utilized. Experimental examples of quantitative H2O2 production and accumulation from H2O through oxidative processes involving photoelectrodes or photocatalysts are very rare, although H2O2 formation has been speculated upon and H2O2 has been observed as an intermediate only in very small amounts.2,25 This is because the O2 production from H2O [E(O2/H2O) = +1.23 V vs RHE)] and the successive degradation of H2O2 to O2 [E(O2/H2O2) = +0.68 V vs RHE)] take place easily compared with oxidative H2O2 generation from H2O via a twoelectron process according to the redox potential. Therefore, oxidative H2O2 generation from water is a very difficult reaction. Shiragami et al.17 reported H2O2 generation from water using a Ge-di(hydroxo)porphyrin dye-sensitized TiO2 photoanode 1097

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under visible light. The FE of H2O2 was very high (up to 99%), but the photocurrent was very low. In addition, we found a very unique bicarbonate (HCO3−) anion effect on oxidative H2O2 generation on BiVO4/WO3 composite photoanodes,5−7 with a FE of H2O2 at 54% and photocurrent of ∼4 mA/cm2 at 1.2 V versus RHE under solar-simulating light. The FE(H2O2) was improved to 79% by surface modification of the BiVO4/WO3 photoanode with porous Al2O3.7 Recently, a very high FE(H2O2) value (up to 100%) was reported using a BiVO4 photoanode in bicarbonate aqueous solution.27 H2 gas was formed at the counter electrode [eq 5, with the total reactions (eqs 12 and 5) shown in eq 13] via a two-photon process. The ABPE for H2 and the H2O2 reaction is advantageous owing to the highly positive potential relative to that of O2 evolution, with an ABPE value calculated at 2.2%.6 2H 2O → H 2O2 + H 2

(two‐photon process)

Figure 6. H2O2 production on both electrodes by solar light, water, and air without an external bias and an ion-exchange membrane via a two-photon process.

(13)

appeared to be ∼140%. The ion-exchange membrane was not needed between the electrodes. Moreover, H2O2 production using a Au-supported porous BiVO4 photocatalyst sheet or a Au-loaded BiVO4 powder photocatalyst suspension was also achieved in aqueous HCO3− solution.6 Therefore, oxidative H2O2 production from water on the photoanode via a twophoton process can offer several advantages compared with conventional photoelectrochemical systems and offers potential as a breakthrough technology as a “clean-energy and/or chemical-conversion process”. Furthermore, the development of distributed and standalone systems has many advantages with respect to improving total energy efficiency and simple utilization of solar energy conversion.

The reaction mechanism associated with the bicarbonateanion effect involves the BiVO4 photoanode possessing enough valence-band potential to oxidize HCO3− to a peroxide species comprising percarbonates, such as HCO4−, with these peroxides capable of immediately producing H2O2 and HCO3− by hydrolysis in aqueous solution.28−31 This indicates that percarbonates produced by oxidation of HCO3− on the photoanode could act as useful oxidative catalysts for producing H2O2 from H2O.6 Moreover, H2O2 is formed on a conductive glass loaded with small amounts of BiVO4 as an electrocatalyst under a highly anodic bias (>2 V) in bicarbonate aqueous solution in the dark,27,32 suggesting that BiVO4 in the BiVO4/ WO3 photoanode can function both as a semiconductor to absorb visible light and as an electrocatalyst to accelerate H2O2 generation on the surface. H2O2 can be also generated electrochemically by the reduction of O2, as shown in eq 14. Several studies investigating reductive H2O2 production through cathode reactions with a high applied voltage or through photocatalysis have been reported.22,33−36

Other Oxidative Reactions Using Photoanodes. Many oxidative reactions are available for the production of high-value-added chemicals. In the case of Ce4+ generated from Ce3+ [E(Ce4+/ Ce3+) = +1.7 V vs RHE], the FE on the WO3 photoanode was between 50 and 56%.1,4 As for periodate (IO4−) generation from IO3− [E(IO4−/IO3−) = +1.65 V vs RHE], the FE on the WO3 photoanode was ∼50%.4 In the case wherein Cr6+ is generated from Cr3+ [E(Cr6+/Cr3+) = +1.36 V vs RHE], the FE on the WO3 photoanode was almost 100%.9 Ce4+, IO4−, and Cr6+ are generally utilized as oxidizing reagents for organic synthesis. A variety of organic reactions have been widely investigated using electrochemical methods under dark conditions37 because a wide variety of reactive intermediates may be generated by electron transfer at the electrode surface without the need for handling sensitive, expensive, and toxic oxidizing reagents. However, a high applied potential and large amounts of electrical power are required to achieve conversion of organic compounds when using electrochemical methods. In contrast, the advantage is that the photoelectrochemical synthesis allows a dramatic decrease in applied potential owing to solar energy assistance. Many previous studies focused mainly on the oxidative degradation of toxic and/or harmful chemicals to harmless chemicals, including CO2, and there are only a few reports focusing on photoelectrochemical transformation of organic compounds oxidatively to useful and valuable chemicals. Choi et al.15 reported the photoelectrochemical oxidation of 5-hydroxymethylfurfural to 2,5-furandicarboxylic acid on a BiVO4 photoanode with 100% FE using 2,2,6,6tetramethylpiperidine-1-oxyl as a redox mediator. We developed a photoelectrochemical oxidation system for the methoxylation of furan mediated by Br+/Br− and using a BiVO4/WO3 photoanode,11 with the photoelectrochemical

Reductive H2O2 generation from O2: O2 + 2H+ + 2e− → H 2O2 E(O2 /H 2O2 ) = +0.68 V vs RHE

(14)

This reductive H2O2 generation at the cathode can be combined with oxidative H2O2 (eq 12) or O2 (eq 4) generation from water on photoanodes without an applied voltage. eqs 12 and 14: 2H 2O + O2 → 2H 2O2

(two‐photon process)

(15)

(four‐photon process)

(16)

eqs 4 and 14 × 2: 2H 2O + O2 → 2H 2O2

The reactions in eqs 15 and 16 look identical; however, the former processes via a two-photon process requires less solar light compared with the latter processed via a four-photon process. We reported that H2O2 could be produced on both the WO3/BiVO4 photoanode and the Au cathode with high selectivity without applied voltage in bicarbonate aqueous solution via eq 15, as shown in Figure 6,6 resulting in a FEanode(H2O2) of ∼50% and a FEcathode(H2O2) of ∼90%. Therefore, the apparent total current efficiency, FEtotal(H2O2), 1098

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Natural photosynthesis produces various valuable chemicals for human social activities, as well as fuels and energy, via uphill reactions (ΔG > 0) with solar light. Artificial photosynthesis should also have wider use in addition to targeting hydrogen and fuel generation (Figure 7). The “solar chemical” field focused on producing high-value-added chemicals will likely become more important economically compared with the “solar hydrogen” and “solar fuel” fields. Various chemicals, including organic compounds, produced reductively from CO2 have been widely investigated; however, oxidatively produced chemicals have received far less attention. This research field reviewed herein might be particularly referred to as “solar chemical upon oxidative reaction” or “solar oxidant” and should be investigated and developed much further. To accelerate research in this new field, it is important to clarify the importance and future prospects of such research. Here, we described the practical significance of the oxidative production of high-value-added chemicals on simple oxide semiconductor photoanodes as an inexpensive and innovative conversion and accumulation process of limitless solar energy into chemical energy. The production of valuable oxidizing reagents on photoanodes, and involving H2 and/or H2O2 production on cathodes, has the potential to reduce CO2 emissions accompanied by economic prospects, and it is strongly desired that the photoelectrochemical process will be implemented in the real world as soon as possible.

dimethoxylation of furan achieved in excellent FE (up to 99%). In addition, the photoelectrochemical oxidation of benzylic alcohol derivatives was achieved with excellent FE and yields of up to 99 and 97%, respectively, using a BiVO 4 /WO 3 photoelectrode.12 The FE for the oxidation of 1-phenylethyl alcohol via a Cr6+/Cr3+ redox reaction was also very high (>99%).9 These successful results suggested that the oxidative selectivity at the photoelectrode surface is an important factor in the improvement of FE and that oxidizing mediators play critical roles in avoiding both the overoxidation of products and unfavorable reactions. Summary and Future Outlook. There are some hurdles to overcome for the practical realization of valuable chemical production using a photoanode system. First, the ABPE and the solar energy conversion of the photoanode system should be improved further. New semiconductors that have a smaller band gap, higher quantum efficiency, and more negative conduction band potential should be developed. In this Perspective, oxide semiconductors are mainly introduced, but other materials such as nitride, sulfide, carbide, halide, and their mixed anion compounds are also applicable. A high-throughput screening method is useful for the development of new semiconductor materials by photoelectrochemical measurement.38 The screening speed by an automated system is generally an order of magnitude faster than that by hand. Second, the long-time stability of materials is also important. The stability of the photoanode can be improved by optimization of reaction solution conditions, surface modification, and loading of cocatalyst on the photoanode. Third, a total system for wide-scale practical applications should be designed. The development of a manufacturing process for large photoanodes with a collecting wire electrode and a flow system for the accumulation and purification of products is needed.

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Kazuhiro Sayama: 0000-0003-1847-5242

Artificial photosynthesis should have wider use in addition to targeting hydrogen and fuel generation. The “solar chemical” field focused on producing high-value-added chemicals oxidatively will likely become more important economically.

Notes

The author declares no competing financial interest. Biography Kazuhiro Sayama is a Prime senior researcher and Team lead in the Advanced Functional Materials Team, Research Center for Photovoltaics (RCPV), National Institute of Advanced Industrial Science and Technology (AIST). His research focuses on artificial photosynthesis reactions using semiconductor materials, solar hydrogen production, and dye-sensitized solar cells.

Figure 7. Expansion of concepts related to artificial photosynthesis to the areas of “solar chemical as well as solar H2 and solar fuel. 1099

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ACKNOWLEDGMENTS This study was partially supported by the International Joint Research Program for Innovative Energy Technology (METI) and JSPS KAKENHI Grant Number 17H06439 in Scientific Research on Innovative Areas “Innovations for Light-Energy Conversion (I4LEC)”.

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DOI: 10.1021/acsenergylett.8b00318 ACS Energy Lett. 2018, 3, 1093−1101