Toshihiro Takashima

Inhibition of Charge Disproportionation of MnO2 Electrocatalysts for Efficient Water Oxidation under Neutral Conditions

The development of Mn-oxide electrocatalysts for the oxidation of H(2)O to O(2) has been the subject of intensive researches not only for their importance as components of artificial photosynthetic systems, but also as O(2)-evolving centers in photosystem II. However, limited knowledge of the mechanisms underlying this oxidation reaction hampers the ability to rationally design effective catalysts. Herein, using in situ spectroelectrochemical techniques, we demonstrate that the stabilization of surface-associated intermediate Mn(3+) species relative to charge disproportionation is an effective strategy to lower the overpotential for water oxidation by MnO(2). The formation of N-Mn bonds via the coordination of amine groups of poly(allylamine hydrochloride) to the surface Mn sites of MnO(2) electrodes effectively stabilized the Mn(3+) species, resulting in an ~500-mV negative shift of the onset potential for the O(2) evolution reaction at neutral pH.

Regulating proton-coupled electron transfer for efficient water splitting by manganese oxides at neutral pH

Manganese oxides have been extensively investigated as model systems for the oxygen-evolving complex of photosystem II. However, most bioinspired catalysts are inefficient at neutral pH and functional similarity to the oxygen-evolving complex has been rarely achieved with manganese. Here we report the regulation of proton-coupled electron transfer involved in water oxidation by manganese oxides. Pyridine and its derivatives, which have pKa values intermediate to the water ligand bound to manganese(II) and manganese(III), are used as proton-coupled electron transfer induction reagents. The induction of concerted proton-coupled electron transfer is demonstrated by the detection of deuterium kinetic isotope effects and compliance of the reactions with the libido rule. Although proton-coupled electron transfer regulation is essential for the facial redox change of manganese in photosystem II, most manganese oxides impair these regulatory mechanisms. Thus, the present findings may provide a new design rationale for functional analogues of the oxygen-evolving complex for efficient water splitting at neutral pH.

Electrical Current Generation across a Black Smoker Chimney

In environments isolated from solar radiation, diverse microbial populations and ecosystems are sustained by the chemical energy supplied from Earth!s interior. The most outstanding example is a deep-sea hydrothermal vent, which discharges enormous amounts of reductive energy in the form of reduced sulfur compounds, H2, CH4, and reduced metals during magma degassing and hydrothermal reactions with hot rocks. 3] Hydrothermal vent chimneys are generated by mineralization in the mixing zone between hot, reduced hydrothermal fluid and cold, oxygenated seawater and provide an ideal habitat for chemolithotrophic microbial communities. As the chimney structures serve as an interface between the Earth!s reductive interior and oxidative exterior, sustained microbial redox metabolism is very feasible. Simultaneously, the energy potential mediated by the chimney structure leads to many hypotheses concerning chemical evolution in the prebiotic ocean and the early evolution of energy metabolisms and cellular functions in the ancient Earth. Black smokers are a type of hydrothermal vent with sulfide-rich emissions that precipitate to form sulfide mineral chimneys consisting mainly of chalcopyrite (CuFeS2) and pyrite (FeS2). The mineralogical and structural characteristics of black smoker chimneys have been extensively studied; however, electrical conduction and electrocatalysis of black smoker chimneys have never been examined. Although bulk crystals of CuFeS2 and FeS2 are typically considered poor conductors, it is predicted that a chimney structure composed of nanoand microsized crystalline particles would have quite a large surface area with the potential of mediating the efficient electron transport. Herein we therefore examine the electrochemical characteristics of the black smoker chimney and seek to estimate the redox potential between the hydrothermal fluid conduit and ambient seawater across the chimney wall. For the analysis, a black smoker sulfide chimney obtained from the Mariner hydrothermal field in the southern Lau Basin was characterized (Figure 1). The observed electrical conduction potential points to a possible new type of energy transport from hydrothermal fluid to seawater by electrical current generation in the sulfide chimney wall.

Biological iron-monosulfide production for efficient electricity harvesting from a deep-sea metal-reducing bacterium.

Micro-organisms inhabit a broad range of natural environments through the exploitation of multiple metabolic strategies and are able to perform most thermodynamically possible redox reactions to obtain energy for their growth. Present concerns about the depletion of natural resources has increased the need to understand inherent microbial ingenuities, and search for methodologies for utilizing (or replicating) their metabolism to harness renewable sources of energy. Herein, we report the biological production of single-phase iron monosulfide (mackinawite) for efficient harvesting of bacterial metabolized electrons in electrochemical cells. Iron sulfides (FeS), such as mackinawite, greigite, marcasite, and pyrite, are ubiquitous and abundant minerals in anoxic marine environments formed as by-products of microbial metabolism or a consequence of geothermal activity. Several biogeochemical processes associated with microbial interaction with FeSs have been reported, with the discovery of highly active biospheres around hydrothermal vents illustrating the significance of FeSs as an energy source for microbial activities in environments isolated from solar irradiation. Moreover, a number of biological functions of FeSs, including structural support, antimicrobial agents, and magnetic-, optical-, and gravity-sensing devices, have been identified; this highlights the diverse microbial mechanisms that exist for harnessing cell-surface-associated nanosized FeSs. Although the biomineralization of nanosized FeSs is a wellknown phenomenon, only a few previous studies have considered its biological function from the view point of electron-conducting properties. FeSs have a wide range of compositions and structures, fulfilling the diverse electrochemical functionalities required for solid-state batteries, electrocatalysis, photovoltaics, and other industrial applications. In an effort to further explore microbial ingenuity and understand the importance of FeSs in microbial metabolism and electron transfer, we examined the biomineralization of FeSs by the Fereducing bacterium Shewanella loihica PV-4, which was originally isolated from iron-rich microbial mats near a deep-sea hydrothermal vent (1325 m below sea level). This study demonstrated that S. loihica PV-4 has the ability to exploit the Fe 3d electrons of biologically synthesized iron monosulfide (FeS) as extracellular long-distant electron-transfer conduits. Self-organizing, electrically conducting cell–FeS assemblies enabled the generation of a microbial current two orders of magnitude higher than in cell cultures lacking the biogenic minerals. S. loihica PV-4 cells were cultured anaerobically at 25 8C in a medium containing both ferric ions (FeCl3) and thiosulfate (Na2S2O3) as terminal electron acceptors, and lactate as a carbon source and electron-donating compound. The cell suspension instantly turned from yellow to brown, and a black precipitate was generated after approximately 5 h of inoculation. The X-ray diffraction (XRD) pattern of the yellow precipitate present before the addition of cells showed a broad undefined region with no significant peaks (Figure 1, bottom). Upon incubation with cells, however, several peaks appeared in the XRD pattern, which were assigned to elemental sulfur (S), goethite (a-FeOOH), and mackinawite (FeS; Figure 1, 1 day). The peak intensities for both elemental sulfur and goethite decreased with increasing incubation times, while a concomitant increase in the peaks assigned to mackinawite was observed. SEM observations (Figure 2) combined with confocal microscopy on GFP-labeled cells (Figure S2) showed populations of rod-shaped cells surrounded by nanosized FeS colloids, with the nanocolloids serving to interconnect the cells (Figure 2, bottom). The black precipitate was not observed in the suspension lacking cells ; this confirms that the formation of mackinawite was a consequence of the microbial reduction of ferric ions and thiosulfate. Mackinawite is one of the major constituents of black sedimentary minerals in anoxic marine systems and has a struc-

A silver-inserted zinc rhodium oxide and bismuth vanadium oxide heterojunction photocatalyst for overall pure-water splitting under red light

We have prepared a solid-state heterojunction photocatalyst, in which zinc rhodium oxide (ZnRh2O4) and bismuth vanadium oxide (Bi4V2O11) as hydrogen (H2) and oxygen (O2) evolution photocatalysts, respectively, were connected with silver (Ag, ZnRh2O4/Ag/Bi4V2O11). ZnRh2O4/Ag/Bi4V2O11 was able to photocatalyze overall pure-water splitting under red-light irradiation with a wavelength of 700 nm. On the basis of the analogy with a solid-state heterojunction photocatalyst composed of ZnRh2O4, defective silver antimonate (Ag1−xSbO3−y), and Ag (R. Kobayashi et al., J. Phys. Chem. C, 2014, 118, 22450–22456), we consider that the overall water-splitting performance of the ZnRh2O4/Ag/Bi4V2O11 photocatalyst was derived from the photoproduced holes that were generated in the valence band (VB) of Bi4V2O11 contributing to O2 liberation and the photoexcited electrons that were generated in the conduction band (CB) of ZnRh2O4 contributing to H2 liberation. Importantly, Ag possibly acts as a solid-state electron mediator for the transfer of electrons from the CB of Bi4V2O11 to the VB of ZnRh2O4.

A heterojunction photocatalyst composed of zinc rhodium oxide, single crystal-derived bismuth vanadium oxide, and silver for overall pure-water splitting under visible light up to 740 nm

We recently reported the synthesis of a solid-state heterojunction photocatalyst consisting of zinc rhodium oxide (ZnRh2O4) and bismuth vanadium oxide (Bi4V2O11), which functioned as hydrogen (H2) and oxygen (O2) evolution photocatalysts, respectively, connected with silver (Ag). Polycrystalline Bi4V2O11 (p-Bi4V2O11) powders were utilized to form ZnRh2O4/Ag/p-Bi4V2O11, which was able to photocatalyze overall pure-water splitting under red-light irradiation with a wavelength of 700 nm (R. Kobayashi et al., J. Mater. Chem. A, 2016, 4, 3061). In the present study, we replaced p-Bi4V2O11 with a powder obtained by pulverizing single crystals of Bi4V2O11 (s-Bi4V2O11) to form ZnRh2O4/Ag/s-Bi4V2O11, and demonstrated that this heterojunction photocatalyst had enhanced water-splitting activity. In addition, ZnRh2O4/Ag/s-Bi4V2O11 was able to utilize nearly the entire range of visible light up to a wavelength of 740 nm. These properties were attributable to the higher O2 evolution activity of s-Bi4V2O11.

Efficient oxygen evolution on hematite at neutral pH enabled by proton-coupled electron transfer

The rate-determining step of the oxygen evolution reaction on hematite electrodes was switched from the sequential electron/proton transfer process to the concerted proton-coupled electron transfer (CPET) process by adding pyridine derivatives to the electrolyte. By inducing the CPET process, the overpotential for oxygen evolution at neutral pH decreased by approximately 250 mV.

Water Splitting Using Electrochemical Approach

For electrochemical water splitting, a number of bioinspired and biomimetic Mn-based materials have been developed; however, the catalytic performances markedly differ between natural and synthetic Mn catalysts. Based on the recent in situ detection of surface intermediates for the oxygen evolution reaction (OER) by MnO2, this chapter introduces the design rationale for the efficient OER catalysts, and discusses the evolutional origin of natural Mn4-clusters to provide a better understanding of the differences in activity between natural and man-made OER catalysts.

Acceleration of electrocatalytic CO2 reduction by adding proton-coupled electron transfer inducing compounds

Abstract. The induction of the concerted proton-coupled electron transfer (PCET) during electrochemical carbon dioxide (CO2) reduction was examined by introducing proton-donating compounds (acetic acid, malic acid, maleic acid, and pyridine derivatives) into an electrolyte. In the presence of proton-donating compounds, a copper (Cu) foil electrode showed enhanced CO2 reduction activity as well as the occurrence of the deuterium kinetic isotope effect, indicating that the concerted PCET promoted the reduction reaction. When a tin (Sn) foil was also subjected to electrocatalysis in an electrolyte containing a proton donor, enhanced activity was similarly observed, although the pKa range where the enhancement occurred was different from that observed for a Cu electrode. This discrepancy in the pKa range reflects a difference in the form of the intermediate species generated on Cu and Sn electrodes. Notably, these results demonstrate that the method presented here can be employed to induce the concerted PCET on a metal electrode irrespective of the reaction mechanism for CO2 reduction.

Development of optically transparent water oxidation catalysts using manganese pyrophosphate compounds.

One challenge in artificial photosynthetic systems is the development of active oxygen evolution catalysts composed of abundant elements. The oxygen evolution activities of manganese pyrophosphate compounds were examined in electrochemical and photochemical experiments. Electrocatalysis using calcium-manganese pyrophosphate exhibited good catalytic ability under neutral pH and an oxygen evolution reaction was driven with a small overpotential (η<100 mV). UV-vis diffuse reflectance measurements revealed that manganese pyrophosphates exhibit weak absorption in the visible light region while commonly used oxygen evolution catalysts exhibit intense absorption. Therefore, the efficient light absorption of a photocatalyst was retained even after surface modification with a manganese pyrophosphate, and photochemical oxygen evolution was achieved by using magnesium ferrite modified with manganese pyrophosphate nanoparticles under the illumination of visible light at wavelength of over 420 nm.