Photoelectrolysis of water: Solar hydrogen--achievements and perspectives.

Thermodynamic analysis of energy conversion from light-to-chemical, light-to-electric and electric-to-chemical is presented by the case study of water photoelectrolysis on TiO(2) surface. It is demonstrated that at the current state-of-the-art energy conversion efficiency of water photoelectrolysis can be increased approximately 17 times by separating the processes of solar-to-electric and electric-to-chemical energy conversion and optimizing them independently. This allows to mitigate a high overvoltage of oxygen evolution reaction with respect to thermodynamic E(0)(O(2)/H(2)O) = 1.23 V potential as well as spectrally narrow absorbtivity of solar light by TiO(2) which determine the low efficiency (approximately 1.0%) of direct light-to-chemical energy conversion. Numerical estimates are provided illustrating practical principles for optimization of the solar energy conversion and storage processes.