In confrontation with worldwide energy problems and environmental issues, hydrogen production by photocatalytic overall water splitting attracts more and more attention in these days. However, for the development of the suitable photocatalysts for water splitting with actual high efficiency, there are many difficulties about the electronic structures of semiconductor materials. Namely, the materials must have proper band positions, where the bottom level of the conduction band must be more negative than the hydrogen evolution potential and the top level of the valence band must be more positive than the oxygen evolution potential. Furthermore, at the same time, water splitting also has kinetic difficulties that the photoexcited minority carriers must be used for 2-electron reaction of hydrogen evolution or 4-electron reaction of oxygen evolution. The photocatalyst materials, of course, have to be stable in water even under the irradiation. To avoid these difficulties, a new type of photoelectrochemical (PEC) cell with circulating redox reactions of metal-complexes between a photoanode and counter metal-electrode was proposed in this study. This cell may generate photovoltage by circulation of the redox reaction, and have possibility of generating adequate photovoltage for water electrolysis, over 1.23 V, with 1-step photoexcitation. Use of non-aqueous electrolytes will make it free from the limitation of potential window of water, and the photocatalytic materials, which are not able to be directly used for water-splitting because of their instability, can be applied for indirect water electrolysis by the photovoltage.

 In this research, cadmium sulfide (CdS), which is not suitable for overall water splitting due to the photocorrosion, was used as the photoanode. CdS is appropriate for confirmation of the concept of this PEC cell because the conduction band is relatively more negative than other semiconductors, single-crystal substrate of CdS is available, and non-aqueous solvent of this PEC cell may suppress the photocorrosion of CdS.

 It was found that the PEC cell with CdS photoanode generated higher open-circuit photovoltage than 1.23 V, which was higher than equilibrium potential of water-splitting. This photovoltage was obtained from the difference between the Fermi level of CdS surface and redox potential in the non-aqueous solution. The influence of the redox concentration on the cell performance was also discussed.