Photocatalytic Water Splitting for Solar Hydrogen Production Using the Carbonate Effect and the Z-Scheme Reaction

The development of innovative technologies for solar energy conversion and storage is important for solving the global warming problem and for establishing a sustainable society. The photocatalytic water-splitting reaction using semiconductor powders has been intensively studied as a promising technology for direct and simple solar energy conversion. However, the evolution of H-2 and O-2 gases in a stoichiometric ratio (H-2/O-2 = 2) is very difficult owing to various issues, such as an unfavorable backward reaction and mismatched band potentials. Two important findings have widened the variety of photocatalysts available for stoichiometric water-splitting, viz. the carbonate anion effect and the Z-scheme photocatalytic reaction using a redox mediator. The bicarbonate anion has been found to act as a redox catalyst via preferential peroxide formation and subsequent decomposition to O-2. As the Z-scheme reaction using a redox mediator mitigates band potential mismatches, it is widely applicable for various visible-light-active photocatalysts. This review describes the development of photocatalytic water-splitting for solar hydrogen production using the carbonate anion effect and the Z-scheme reaction. Moreover, recent developments in photocatalysis-electrolysis hybrid systems, an advanced Z-scheme reaction concept, are also reviewed for practical and economical hydrogen production.