Utilization of solar light energy for water splitting and production of hydrogen, on the future industrial basis, will be performed in a mass-scale chemical plant involving lots of photocatalytic light-harvesting panels.  Within those panels, not only the photocatalysts but also many other functional mechanisms will be included in order to feed and circulate the reactant water, to eject and lead the product gases safely, and to maintain the conditions of the catalysts and water for stable operation.  All those functionalities consume energy, whereas the energy production is made only by the photocatalysts.  As an energy-producing plant, the production and self-consumption of energy should be balanced so as to gain a profitable amount of energy.

              We are now focusing on the operational technologies for our laboratory-class solar light-driven water splitting devices towards higher energy conversion efficiency and durability, taking this balance into our consideration.  Choice of the light-absorbing materials as well as co-catalysts for H₂/O₂evolution is the most important issue to realize satisfactory efficiency. Indeed most of our efforts are dedicated for research of materials suitable for light harvesting. To foster material research in this sense, we adhere to photocatalytic or photoelectrochemical devices without photovoltaics or external power supply as the main power source.

              We are intensively investigating several semiconductive candidate materials, such as oxides [1,2], nitrides, oxynitrides [3-5] and other chalcogenides [6-9] of transition metals for both photocathodes and photoanodes. The key issues in selecting the target materials are the light absorption spectrum in the solar region, the optimized photoelectrochemical current density, the onset potential of photoelectrochemical curves, and the durability after appropriate co-catalyst and protective coating. We chose pairs of photocathodes and photoanodes out of these materials of our own development, and assemble prototype water-splitting photo-cells.  A finite non-zero photo-electrolysis current is obtained when the cyclic photo-voltammetric curves of the cathode and the anode cross over each other under light irradiation in the same electrolytic solution.  This is a prerequisite of our water splitting device and is still a limiting issue for the choice of electrode materials for a highly efficient water-splitting device.  The content of operational solution and, in particular, the pH of the solution are also important in the dual electrode operation. Naturally the optimal pH for the photocathode alone is different from that for photoanode alone, and we must adjust the solution pH at the median of those two, as long as the electrodes are not deactivated.  The reactivity of co-catalysts are often dependent on the solutes.  Phosphate, sulfate and borate buffers are the typical choices.

              The durability of water-splitting devices is related to the maintenance of catalyst surface activity and stabilization of chemical and physical status of the aqueous solution near the catalyst surface.  To prevent or delay the chemical degradations of photocatalyst surfaces, we are developing protective layers that are electron/hole conductive and chemically inert. Physically, the sizes and arrangement of photocatalytic plates can facilitate the microscopic circulation of aqueous species near them. The rapidest decay of reactivity is usually due to building-up inhomogeneity of solute distribution between the two photoelectrodes during operation, often called “pH gradient”. This inhomogeneity will be cancelled by stirring, pumping or circulating the solution, or by applying the inverse voltage between the electrodes.  We are trying to work out how efficiently these mechanical/fluidic and electrochemical issues are solved in order to conserve the extra energy for those costs.

              In this talk some specific examples of materials and technologies from our group will be presented, and some benchmark devices with notable efficiency and durability for solar powered water splitting will be described.

References

[1]     M. Zhong et al., J. Am. Chem. Soc. 137 (2015) 5053.

[2]     Y.-B. Kuang et al., Adv. Energy Mater. 5(2015) 1501645.

[3]     T. Hisatomi et al., Energ. Env. Sci. 6 (2013) 3595.

[4]     T. Hisatomi et al., ChemSusChem, 7(2014) 2016.

[5]     K. Ueda et al., J. Am. Chem. Soc. 137(2015) 2227.

[6]     J. Zhao et al., Angew. Chem. Int. Ed. 53(2014) 11808.

[7]     G.-J. Ma et al., Chem. Commun. 51(2015) 4302-4305.

[8]     H. Kumagai et al., J. Mater. Chem. A 3(2015) 8300-8307.