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Abstract

This paper was stimulated by a publication of A. Fujishima and K. Honda, Electrochemical photolysis of water at a semiconductor electrode. Nature 238, 37-38 (1972), who had achieved photodecomposition of water by illumination of TiO2 electrodes with solar light. A year later, the first energy crisis created worldwide interest in alternative energy sources. Photoelectrolytic splitting of water has a close analogy to photosynthesis, with the important difference being that the energy carrier is hydrogen instead of carbohydrates, but the energy gained is nearly the same. This avoids the enormous difficulty of the reduction of CO2, which nature has achieved at room temperature with great perfection, while all technical processes for CO2 reduction require a large input of additional free energy. …