ABSTRACT
Hydrogen can be evolved in either photocathode- or photoanode-based cells. In hydrogen-generating cells the quantum efficiency is determined by the ratio of the desired rate of hydrogen evolution to the sum of this rate and the undesired recombination and leakage rates. In a hydrogen-evolving solar cell ΔV is the equivalent of the photovoltage in a power-generating cell. In photoanode-based cells, hydrogen is evolved at their non-illuminated, catalyst-activated counter electrodes. For efficient solar-to-hydrogen conversion, the photogenerated charge carrier reaction rates must be fast relative to the rates of recombination and the rates of leakage across the barrier. The barrier height is increased and the external voltage bias required for electrolysis is reduced in macroscopic, catalyzed photocathodes upon hydrogen saturation of the catalyst. Although efficient solar electrolysis of HCl or water requires an external bias, a substantial part of the energy consumed in the generation of either hydrogen and chlorine, or of hydrogen and oxygen, can now be derived from sunlight.
