In exemplary implementations of this invention, a photoelectrode includes a semiconductor for photocarrier generation, and a catalyst layer for altering the reaction rate in an adjacent electrolyte. The catalyst layer covers part of the semiconductor. The thickness of the catalyst layer is less than 60% of its minority carrier diffusion distance. If the photoelectrode is a photoanode, it has an OEP that is more than the potential of the valance band edge but less than the potential of the Fermi level of the semiconductor. If it is a photocathode, it has an RHE potential that is less than the potential of the conduction band edge but more than the potential of the Fermi level of the semiconductor. The absolute value of difference (OEP minus potential of valence band edge, or RHE potential minus potential of conduction band edge) is greater than zero and less than or equal to 0.2V.