Simultaneous production of hydrogen and chlorine through overall brine splitting with a particulate photocatalyst.

Sunlight-driven photocatalytic water splitting shows promise for green H2 production. In an attempt to achieve seawater splitting, we constructed a new stoichiometric brine splitting system that produces H2 along with Cl2 instead of O2. Cl2-a more potent high-value-added oxidant than O2-was obtained with 100% selectivity over 10 h by adjusting the solution pH to acidic using a UV-light-driven Pt-loaded TiO2 photocatalyst. Our new photosynthesis system can permit economically feasible solar chemical production.

Surface-modified, dye-sensitized niobate nanosheets enabling an efficient solar-driven Z-scheme for overall water splitting.

While dye-sensitized metal oxides are good candidates as H2 evolution photocatalysts for solar-driven Z-scheme water splitting, their solar-to-hydrogen (STH) energy conversion efficiencies remain low because of uncontrolled charge recombination reactions. Here, we show that modification of Ru dye-sensitized, Pt-intercalated HCa2Nb3O10 nanosheets (Ru/Pt/HCa2Nb3O10) with both amorphous Al2O3 and poly(styrenesulfonate) (PSS) improves the STH efficiency of Z-scheme overall water splitting by a factor of ~100, when the nanosheets are used in combination with a WO3-based O2 evolution photocatalyst and an I3-/I- redox mediator, relative to an analogous system that uses unmodified Ru/Pt/HCa2Nb3O10. By using the optimized photocatalyst, PSS/Ru/Al2O3/Pt/HCa2Nb3O10, a maximum STH of 0.12% and an apparent quantum yield of 4.1% at 420 nm were obtained, by far the highest among dye-sensitized water splitting systems and comparable to conventional semiconductor-based suspended particulate photocatalyst systems.

Insights into the carbonate effect on water oxidation over metal oxide photocatalysts/photoanodes.

Photocatalytic/photoelectrochemical water splitting using metal oxide semiconductors is a promising technology for direct and simple solar-energy conversion. The addition of carbonate salts to an aqueous reaction solution has been known to promote stoichiometric O2 evolution and H2O2 production via H2O oxidation. To elucidate the effect of carbonates, density functional theory calculations are performed to study the photoinduced H2O and H2CO3 oxidation mechanisms on TiO2 and BiVO4. The oxidation reactions proceeded via peroxide intermediates, such as H2O2 for H2O, H2C2O6 for H2CO3, and H2CO4 for the coexistence of H2O and H2CO3 molecules. Regardless of the reactant and metal oxide, the free energy changes in the four proton-coupled electron-transfer (PCET) steps of the oxidation mechanism indicate that the first PCET requires the highest energy input and is the rate-limiting step. All PCET steps of the H2O oxidation, except the second one, are more endergonic than those of the H2CO3 oxidation. The H2O reactant requires a larger energy barrier at the highest energy profile, as well as at the final state, than the H2CO3 reactant. The computational results verify that the adsorbed H2CO3 molecule is easily photo-oxidized compared with the adsorbed H2O molecule, facilitating the formation of the peroxide intermediate and improving O2 evolution and H2O2 production.

Solar-to-Pharmaceutical Raw Material Production: Photoelectrochemical Naphthoquinone Formation Using Stabilized BiVO4 Photoanodes in Acid Media.

In the quest for efficient use of solar energy to produce high-value-added chemicals, we first achieved the photoelectrochemical (PEC) diketonization of naphthalene, using a BiVO4/WO3 photoanode, to obtain naphthoquinone, an important pharmaceutical raw material with excellent efficiency by solar energy conversion. In the electrochemical (EC) reaction using F-doped SnO2 (FTO) substrates and a 0.5 M H2SO4 H2O-acetone (60 vol %) mixed solution containing 5 mM naphthalene, we produced a small amount of naphthoquinone evolution in the dark. However, naphthoquinone (ηNQ)'s Faradic efficiency and its evolution rate at 1.7 VAg/AgCl were only 28.5% and 0.48 µmol·cm-2·h-1, respectively. The PEC reaction using a WO3 photoanode had very low efficiency for naphthalene diketonization, with low ηNQ and evolution rate values at 1.1 VAg/AgCl of 0.3% and 0.039 µmol·cm-2·h-1, respectively. In contrast, the BiVO4/WO3 photoanode strongly enhanced the PEC reaction, and the ηNQ and evolution rates at 1.1 VAg/AgCl were boosted up to 37.5% and 4.7 µmol·cm-2·h-1, respectively. The evolution rate of the PEC reaction in the BiVO4/WO3 photoanode was 10 times higher than that of the EC reaction with the FTO substrate regardless of the very low bias voltage. This result suggests that the BiVO4-based photoanode was very efficient for the selective oxidation of naphthalene even in acid media because of the acetone-mixed electrolyte's anti-photocorrosion effect and the multilayering of WO3 and BiVO4. At a naphthalene concentration of 20 mM, the naphthoquinone evolution rate reached its maximum value of 7.1 µmol·cm-2·h-1. Although ηNQ tended to decrease with the increase in the electric charge, it reached 100% at a low bias voltage of 0.7 VAg/AgCl. An intensity-modulated photocurrent spectroscopy analysis indicated the rate constant of charge transfer at the photoanode surface to the naphthalene molecules was strongly enhanced at a low bias voltage of 0.7-1.1 VAg/AgCl, resulting in the high ηNQ value. The acid-resistant BiVO4/WO3 photoanode functioned in acetone-mixed electrolytes enabled the realization of a new PEC oxidation reaction driven by solar energy to produce high-value-added pharmaceutical raw materials.

Acid-Resistant BiVO4 Photoanodes: Insolubility Control by Solvents and Weak W Diffusion in the Lattice.

We have revealed for the first time that BiVO4 photoanodes can be used even in strong acid media by mixing organic solvents into the electrolyte and depositing multilayers with a WO3 bottom layer. In general, the BiVO4 photoanodes are photocorrosive, especially in acid solutions. However, this shortcoming has been overcome using a combination of the two aforementioned modifications. We deduced that the contribution of each mixing organic solvent for the anti-photocorrosion of BiVO4 in sulfuric acid solutions can be evaluated on the basis of a new empirical indicator that incorporates molecular density, the Hansen solubility parameter, and molecular polarizability. Acetone and tert-butyl alcohol were especially promising solvents for stabilizing BiVO4 in acid media. We confirmed that the mixed organic solvents stabilized surface-emergent Bi oxide species as a passivation layer, which was generated via multilayering with a WO3 bottom layer. During heat treatment in the fabrication process, W weakly diffused into the BiVO4 layer and a Bi oxide layer was formed on the outermost surface because of the Bi segregation that arose from the charge compensation between W6+ and V5+ in the BiVO4 lattice. The surface Bi oxide layer, which was protected by the mixed organic solvents, steadily served as a passivation layer for anti-photocorrosion of the underlying BiVO4 layer. We have confirmed that the BiVO4/WO3 photoanodes in acetone-mixed aqueous sulfuric acid solution reliably functioned for a photoelectrochemical reaction under simulated sunlight illumination, and photoelectrochemical production of S2O82- ions was confirmed under light irradiation at λ > 480 nm. These results suggest that the BiVO4-based photoanodes have significant potential for use in acid media in conjunction with very straightforward modifications.

Electrochemical and Photoelectrochemical Water Oxidation for Hydrogen Peroxide Production.

Hydrogen peroxide (H2 O2 ), as a green fuel and oxidant, has drawn increasing attention in the energy and environmental research. Compared with the traditional anthraquinone process, the electrochemical (EC) and photoelectrochemical (PEC) syntheses of H2 O2 are cost-effective and environmentally friendly. In order to construct membraneless EC/PEC devices for the full H2 O2 synthesis, anodic H2 O2 production by water oxidation, which is less developed than cathodic H2 O2 generation, is highly desirable. Here, we review recent developments for the EC/PEC H2 O2 production by water oxidation, including fundamental aspects, benchmarking activity evaluation, material/catalyst selection, and strategies for increasing selectivity, efficiency, and accumulation. Furthermore, we discuss the challenges and outlook of water oxidation for H2 O2 production, especially device-level development, accumulation and stability, and industrial applications. Our review is intended to stimulate studies further improving EC/PEC H2 O2 production.

H2O2 production on a carbon cathode loaded with a nickel carbonate catalyst and on an oxide photoanode without an external bias.

Efficient H2O2 production both on a carbon cathode modified with various metal salts and on an oxide photoanode was investigated. The cathodic current density and faradaic efficiency for H2O2 production (FE(H2O2)) on a carbon cathode in KHCO3 aqueous solution were significantly improved by the loading of an insoluble nickel carbonate basic hydrate catalyst. This electrode was prepared by a precipitation method of nickel nitrate and KHCO3 aqueous solution at ambient temperature. The nickel carbonate basic hydrate electrode was very stable, and the accumulated concentration of H2O2 was reached at 1.0 wt% at a passed charge of 2500C (the average FE(H2O2) was 80%). A simple photoelectrochemical system for H2O2 production from both the cathode and a BiVO4/WO3 photoanode was demonstrated without an external bias or an ion-exchange membrane in a one-compartment reactor under simulated solar light. The apparent FE(H2O2) from both electrodes was calculated to be 168% in total, and the production rate of H2O2 was approximately 0.92 µmol min-1 cm-2. The solar-to-chemical energy conversion efficiency for H2O2 production (STCH2O2 ) without an external bias was approximately 1.75%.

Functions of MnOx in NaCl Aqueous Solution for Artificial Photosynthesis.

Photoelectrochemical water splitting has been intensively investigated as artificial photosynthesis technology to convert solar energy into chemical energy. The use of seawater and salted water has advantages for minimum environmental burden; however, the oxidation of Cl- ion to hypochlorous acid (HClO), which has toxicity and heavy corrosiveness, should occur at the anode, along with the oxygen evolution. Here, O2 and HClO production in aqueous solution containing Cl- on photoanodes modified with various metal oxides was investigated. The modification of MnOx resulted in the promotion of the O2 evolution reaction (OER) specifically without HClO production over a wide range of conditions. The results will contribute not only to the practical application of artificial photosynthesis using salted water but also to the elucidation of substantial function of manganese as the element for OER center in natural photosynthesis.

An Artificial Z-Scheme Constructed from Dye-Sensitized Metal Oxide Nanosheets for Visible Light-Driven Overall Water Splitting.

Sensitization of a wide-gap oxide semiconductor with a visible-light-absorbing dye has been studied for decades as a means of producing H2 from water. However, efficient overall water splitting using a dye-sensitized oxide photocatalyst has remained an unmet challenge. Here we demonstrate visible-light-driven overall water splitting into H2 and O2 using HCa2Nb3O10 nanosheets sensitized by a Ru(II) tris-diimine type photosensitizer, in combination with a WO3-based water oxidation photocatalyst and a triiodide/iodide redox couple. With the use of Pt-intercalated HCa2Nb3O10 nanosheets further modified with amorphous Al2O3 clusters as the H2 evolution component, the dye-based turnover number and frequency for H2 evolution reached 4580 and 1960 h-1, respectively. The apparent quantum yield for overall water splitting using 420 nm light was 2.4%, by far the highest among dye-sensitized overall water splitting systems reported to date. The present work clearly shows that a carefully designed dye/oxide hybrid has great potential for photocatalytic H2 production, and represents a significant leap forward in the development of solar-driven water splitting systems.

Two-Dimensional Perovskite Oxynitride K2 LaTa2 O6 N with an H+ /K+ Exchangeability in Aqueous Solution Forming a Stable Photocatalyst for Visible-Light H2 Evolution.

Undoped layered oxynitrides have not been considered as promising H2 -evolution photocatalysts because of the low chemical stability of oxynitrides in aqueous solution. Here, we demonstrate the synthesis of a new layered perovskite oxynitride, K2 LaTa2 O6 N, as an exceptional example of a water-tolerant photocatalyst for H2 evolution under visible light. The material underwent in-situ H+ /K+ exchange in aqueous solution while keeping its visible-light-absorption capability. Protonated K2 LaTa2 O6 N, modified with an Ir cocatalyst, exhibited excellent catalytic activity toward H2 evolution in the presence of I- as an electron donor and under visible light; the activity was six times higher than Pt/ZrO2 /TaON, one of the best-performing oxynitride photocatalysts for H2 evolution. Overall water splitting was also achieved using the Ir-loaded, protonated K2 LaTa2 O6 N in combination with Cs-modified Pt/WO3 as an O2 evolution photocatalyst in the presence of an I3 - /I- shuttle redox couple.

PINO/NHPI-mediated selective oxidation of cycloalkenes to cycloalkenones via a photo-electrochemical method.

The selective photo-electrochemical oxidation of cyclohexene to 2-cyclohexene-1-one was successfully performed with excellent Faraday efficiency (>99%) via indirect oxidation with a PINO/NHPI mediator and O2 on a BiVO4/WO3 photoanode under low applied bias.

Solar-light-driven photocatalytic production of peroxydisulfate over noble-metal loaded WO3.

The high-value-added chemical reagent peroxydisulfate (S2O82-) was produced photocatalytically over noble-metal loaded WO3 powder suspensions in aqueous H2SO4 under flowing O2 and simulated solar light irradiation. Pt cocatalyst showed the highest photocatalytic activity for S2O82- formation of the studied metals (Au, Pd, Rh, and Ru). Further study indicated that continuous accumulation of S2O82- was achieved only over the Pt/WO3 photocatalyst.

Photo-Electrochemical C-H Bond Activation of Cyclohexane Using a WO3 Photoanode and Visible Light.

The photo-electrochemical C-H bond activation of cyclohexane to produce cyclohexanol and cyclohexanone (KA oil) with high partial oxidation selectivity (99 %) and high current utilization ratio (76 %) was achieved in air at room temperature at atmospheric pressure. The production rate of KA oil was accelerated by applying a bias. The incident photon to current efficiencies at 365 and 420 nm were 57 % and 24 %, respectively.

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.

Efficient oxidative hydrogen peroxide production and accumulation in photoelectrochemical water splitting using a tungsten trioxide/bismuth vanadate photoanode.

An aqueous solution of hydrogen carbonate (HCO3(-)) facilitated oxidative hydrogen peroxide (H2O2) production from water on a WO3/BiVO4 photoanode with the simultaneous production of hydrogen (H2) on a Pt cathode even at an applied voltage far lower than the theoretical electrolysis voltage (+1.77 V vs. RHE) under simulated solar light. The unprecedentedly efficient simultaneous production and accumulation of H2O2 and H2 was achieved in 2.0 M KHCO3 at low temperature, and the maximum selectivity, accumulated concentration and turnover number (TON) of H2O2 generated reached ca. 54%, more than 2 mM and 108, respectively.

Discovery of Overcoating Metal Oxides on Photoelectrode for Water Splitting by Automated Screening.

We applied an automated semiconductor synthesis and screen system to discover overcoating film materials and optimize coating conditions on the BiVO4/WO3 composite photoelectrode to enhance stability and photocurrent. Thirteen metallic elements for overcoating oxides were examined with various coating amounts. The stability of the BiVO4/WO3 photoelectrode in a highly concentrated carbonate electrolyte aqueous solution was significantly improved by overcoating with Ta2O5 film, which was amorphous and porous when calcined at 550 °C. The photocurrent for the water oxidation reaction was only minimally inhibited by the presence of the Ta2O5 film on the BiVO4/WO3 photoelectrode.

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-) .

A comparative computational study on the interactions of N719 and N749 dyes with iodine in dye-sensitized solar cells.

The intermolecular interactions of the two most basic Ru(II) complex dyes for dye-sensitized solar cells (DSSCs), N719 and N749, with the iodine species are investigated using density functional theory (DFT). In addition to interactions with a single I2 molecule, multiple I2 interactions and simultaneous interactions of I2 and I(-) occur. N719 with two isothiocyanato (NCS) ligands interacts with two I2 molecules via the two terminal S atoms in the ground singlet electronic state, whereas N749 with three NCS ligands forms three S···I-I bonds. Irrespective of the NCS position and the number of I2 molecules, N749 has a stronger interaction with I2 than N719. Conversely, the interaction of I(-) with oxidized N749 via the terminal S atom of the NCS ligand is weaker than that with oxidized N719. However, simultaneous interactions of oxidized N749 with two I2 molecules promote the I(-) interaction, and the I(-) interaction with N749 becomes stronger than that with N719 bonded to both an I2 and I(-). The computational results of multiple interactions between the dye and iodine species suggest that the difference in DSSC performance between N719 and N749 dyes is explained by recombination related to the I2 interaction and regeneration of the oxidized dye by I(-).

Intermolecular interactions between a Ru complex and organic dyes in cosensitized solar cells: a computational study.

Intermolecular interactions in cyclometalated Ru complex dye (FT89) dimers, carbazole organic dye (MK-45 and MK-111) dimers, FT89-MK-45 complexes, and FT89-MK-111 complexes were investigated using density functional theory (DFT) and time-dependent DFT (TD-DFT) to elucidate the improvement mechanism of dye-sensitized solar cell (DSSC) performance due to cosensitization with FT89 and MK dyes. All of the dimers and complexes form intermolecular cyclic hydrogen bonds via the carboxyl groups. The FT89 dimer and complexes with the TiO2Na model system promote intermolecular interactions with I2via the NCS ligand of the FT89 monomer. The computational results verify that MK-111 behaves not only as a sensitizer but also inhibits FT89 aggregation by effectively serving as a coadsorbent similar to deoxycholic acid (DCA) in the dye solution, suppressing recombination of the injected electrons in TiO2 with I2, improving DSSC performance.