UNASSISTED SOLAR WATER-SPLITTING USING NANOSTRUCTURED OXIDE TANDEM CELLS

摘要

Based on the redox potential of water, a difference in potential of only 1.23 V is sufficient to split H2O into H2 and O2. Photoelectrochemically, this could be accomplished by a single semiconductor photoelectrode coupled to a metallic counterelectrode in the so-called S2 approach (single absorber, two photons). However in practice, finding a material with the optimized band gap and energy levels for S2 water splitting has remained a significant challenge. In a dual-absorber approach where each absorbed photon creates one excited electron-hole pair, four photons (two in each absorber) must be absorbed to create one molecule of H2. Following Bolton's convention, this is designated a D4 approach. Using an integrated tandem D4 approach, 22% solar-to-hydrogen efficiency η_(STH)) has been predicted to be possible based on reasonable assumptions. Previous attempts to construct devices mainly use the well-known band-gap engineering of III-V semiconductor systems to optimize light harvesting and energy levels. In particular, Turner and co-workers have investigated monolithic GaAs/GaInP2 (pn-p, pn-pn, or pn-pn-p) systems and attained η_(STH) up to 12.5%. While the conversion efficiencies reported with these systems are quite impressive, major concerns exist about the price (due to the requirements of high material purity and costly fabrication methods) and stability of these devices when they are used in contact with aqueous electrolyte.