Hydrogen from the Solar Photolysis of Water

Photochemical methods of generating hydrogen from water by light of wavelengths greater than 185 nm are reviewed. Contrary to the commonly held opinion, solar water photodecomposition reactions are known. Both heterogeneous and homogeneous reactions have been described. Currently, the heterogeneous photolysis involving semiconductors as electrodes in photoelectrochemical cells is a very active field of research. Homogeneous - photoredox reactions are also known to result in water oxidation and reduction. Of these, the Eu2+ photolysis occurs in the sea level solar range of wavelengths with a reasonably high quantum yield. Energy is not stored in this reaction, however, and a means to reduce the photo-oxidized Eu3+ is not yet in hand. Thermodynamic considerations suggest that the direct photoreduction of aqueous ions is not likely to occur in the visible or neat ultraviolet range. Therefore, indirect routes must be sought for the half of the cycle that provides a reducing agent and generates oxygen, from water. This half of the cycle is probably the crucial one and represents a central problem in photochemical solar energy conversion.