In Search of the “Perfect” Inorganic Semiconductor/Liquid Interface for Solar Water Splitting

Abstract

Arguably, one would be hard-pressed to envision a more ideal renewable energy conversion system than the solar splitting of water. The energy-rich product, hydrogen, may be stored and used later on-demand for generating power either via combustion or in a fuel cell. In scenarios where dioxygen is needed for respiration (e.g., space travel), CO2 may be used instead of water as the reactant feed. Both these applications require a photon absorber for capturing sunlight, and an inorganic semiconductor fulfills this function. Therefore, a photoelectrochemical (PEC) system may be devised based on an n- or p-type semiconductor electrode in contact with the reactant fluid. On bandgap excitation of the photoelectrode, the generated holes or electrons respectively are used to drive the oxidation or reduction of the reactant species. In the case of water splitting, these are the OH- or H3O+ ions, respectively. In a CO2 photoreduction system, the corresponding species are OH- and (dissolved) CO2. In both cases, the analogy with a plant photosynthesis system is direct.