Yugo Miseki

Highly efficient photoelectrochemical water splitting using a thin film photoanode of BiVO4/SnO2/WO3 multi-composite in a carbonate electrolyte

The solar energy conversion efficiency considering the energy loss by the external bias for water splitting reached ca. 0.9 or 1.35% using single- or double-stacked photoanodes, respectively, of BiVO(4)/SnO(2)/WO(3) multilayers in a highly concentrated carbonate electrolyte aqueous solution.

Photocatalytic water splitting under visible light utilizing I3−/I− and IO3−/I− redox mediators by Z-scheme system using surface treated PtOx/WO3 as O2 evolution photocatalyst

Water oxidation reactions over a surface treated PtOx/WO3 photocatalyst in the presence of IO3− or I3− ions were investigated. When the IO3− ion was used as an electron acceptor, PtOx/WO3 treated by thermal treatment using Cs2CO3 aqueous solution followed by H+-exchange-treatment (PtOx/H-Cs-WO3) showed ca. 4 times higher activity compared with PtOx/WO3 without surface treatment. The apparent quantum yield at 420 nm was 20%. Moreover, when the I3− ion was used as the electron acceptor, the activity of PtOx/WO3 was negligible, while PtOx/H-Cs-WO3 showed a high activity for water oxidation. The cesium surface treatment also had an excellent effect for the improvement of photocatalytic performance of TiO2. The activity of a water splitting reaction under visible light by the Z-scheme system that combined Pt/SrTiO3:Cr/Ta with PtOx/H-Cs-WO3 was 3 times higher than that using PtOx/WO3 as an O2 evolution photocatalyst. Moreover, owing to the contribution from both IO3− and I3− ions as redox mediators, the water splitting reaction stoichiometrically proceeded in a wider pH region (pH 2–9).

Photoelectrochemical Reaction for the Efficient Production of Hydrogen and High-Value-Added Oxidation Reagents

A porous and thick photoelectrode of WO3 in the monoclinic phase was prepared to realize the recovery of H2 and high-value-added oxidation reagents with efficient solar energy conversion. The WO3 photoelectrode enabled the efficient production and accumulation of O2 , S2 O8 (2-) , Ce(4+) , and IO4 (-) as oxidation products. Most notably, S2 O8 (2-) , which possesses the highest oxidizability among all the peroxides, was generated with high applied bias photon-to-current efficiency (2.2 %) and faraday efficiency (≈100 %) upon irradiation from the back side of the photoelectrode. The design of a tandem photoelectrode system combining a dye-sensitized solar cell (DSSC) was also challenged for the realization of this photoelectrode system without external bias. A high solar energy conversion efficiency (5.2 %) was achieved in the tandem system comprising the WO3 photoelectrode connected to two DSSCs with a near-IR-utilizing dye in series for the production of H2 and S2 O8 (2-) .

Photoelectrochemical Hydrogen Peroxide Production from Water on a WO3/BiVO4 Photoanode and from O2 on an Au Cathode Without External Bias

The photoelectrochemical production and degradation properties of hydrogen peroxide (H2 O2 ) were investigated on a WO3 /BiVO4 photoanode in an aqueous electrolyte of hydrogen carbonate (HCO3- ). High concentrations of HCO3- species rather than CO32- species inhibited the oxidative degradation of H2 O2 on the WO3 /BiVO4 photoanode, resulting in effective oxidative H2 O2 generation and accumulation from water (H2 O). Moreover, the Au cathode facilitated two-electron reduction of oxygen (O2 ), resulting in reductive H2 O2 production with high current efficiency. Combining the WO3 /BiVO4 photoanode with a HCO3- electrolyte and an Au cathode also produced a clean and promising design for a photoelectrode system specializing in H2 O2 production (ηanode (H2 O2 )≈50 %, ηcathode (H2 O2 )≈90 %) even without applied voltage between the photoanode and cathode under simulated solar light through a two-photon process; this achieved effective H2 O2 production when using an Au-supported porous BiVO4 photocatalyst sheet.

High-efficiency water oxidation and energy storage utilizing various reversible redox mediators under visible light over surface-modified WO3

Tungsten trioxide (WO3) powder, treated with various metal salt solutions, was used for the photocatalytic oxidation of water into O2; the reaction was accompanied by the reduction of various redox oxidants. The photocatalytic activity of WO3 was remarkably improved by thermal treatment with alkali metal and silver salt aqueous solutions. Cs-treated WO3 showed the highest activity. The WO3 particles were covered with a very thin layer of a cesium tungstate species, and the ion-exchangeable sites on the WO3 surface were formed by Cs-treatment. The activity of Cs-treated WO3 was further improved by ion exchange of H+ and Fe2+ ions and was 24 times higher than the activity of WO3 without treatment. All methods of O2 evolution using IO3− and VO2+ on WO3 were also significantly improved with Cs-treatment, which suppressed the reverse reaction of the oxidation of the various redox reductants. The optimized WO3 in Fe(ClO4)3 aqueous solution had a high quantum yield (31% at 420 nm) and solar-to-chemical energy conversion efficiency (0.38%).

Photocatalytic water oxidation over PbCrO4 with 2.3 eV band gap in IO3−/I− redox mediator under visible light

PbCrO4 was prepared by a simple precipitation method under ambient conditions; its band gap was estimated to be 2.3 eV. Pt-loaded PbCrO4 exhibited high photocatalytic activity in aqueous NaIO3, promoting water oxidation to generate O2. The photonic efficiency (apparent quantum yield) at 420 nm was estimated to be 1.0%.

Efficient hypochlorous acid (HClO) production via photoelectrochemical solar energy conversion using a BiVO4-based photoanode

A BiVO4/WO3/FTO (FTO: fluorine doped tin oxide coated glass) multilayer photoelectrode was found to be suitable for photoelectrochemical oxidative hypochlorous acid (HClO) production from chloride ions (Cl−). HClO was efficiently produced in the electrolyte solution by irradiating the BiVO4/WO3/FTO photoanode with simulated solar light. It was expected that Cl− would be oxidized to chlorine (Cl2) by the photogenerated holes on the BiVO4, and then converted to HClO via the disproportionation reaction in an aqueous electrolyte solution. For the first 100 s, the current efficiency for HClO production was 97%, indicating that the photogenerated hole oxidized Cl− with high selectivity in preference to H2O. Moreover, the BiVO4/WO3/FTO multilayer photoanode enabled us to achieve significant diminution of the external bias required for HClO production under solar light irradiation.

WO3/BiVO4 photoanode coated with mesoporous Al2O3 layer for oxidative production of hydrogen peroxide from water with high selectivity

A WO3/BiVO4 photoanode coated with various metal oxides demonstrated high selectivity (faradaic efficiency) for hydrogen peroxide (H2O2) generation from water (H2O) under irradiation of simulated solar light in a highly concentrated hydrogen carbonate (KHCO3) aqueous solution. A mesoporous and amorphous aluminium oxide (Al2O3) layer significantly facilitated inhibition of the oxidative degradation of generated H2O2 into oxygen (O2) on the photoanode, resulting in unprecedented H2O2 selectivity (ca. 80%) and the accumulation (>2500 μM at 50C).