Thermodynamic and kinetic considerations about water splitting and competitive reactions in a photoelectrochemical cell

Considerations about thermodynamic and kinetic requirements for water splitting at n-type semiconductors are presented. A main point in water photooxidation concerns the catalytic role that the semiconductor must play in order to minimize the overpotential for oxygen evolution. On the basis of our previous results about water splitting at n-TiO/sub 2/ electrodes, and of the literature data on the electrocatalytic evolution of oxygen at RuO/sub 2/, the best metallic catalyst known up to date for this reaction, the minimum overpotential for water photooxidation is estimated to be of the order of 0.6 eV, which fixes the minimum semiconductor bandgap at about 1.8 eV. Implications of the model in photoreactions competing with water splitting are discussed.