Experimental demonstrations of spontaneous, solar-driven photoelectrochemical water splitting

Laboratory demonstrations of spontaneous photoelectrochemical (PEC) solar water splitting cells are reviewed. Reported solar-to-hydrogen (STH) conversion efficiencies range from <1% to 18%. The demonstrations are categorized by the number of photovoltaic junctions employed (2 or 3), photovoltaic junction type (solid-solid or solid-liquid) and the ability of the systems to produce separated reaction product streams. Demonstrations employing two photovoltaic (PV) junctions have the highest reported efficiencies of 12.4% and 18%, which are for cells that, respectively, do and do not contain a semiconductor-liquid junction. These devices used PV components based on III-V semiconductors; recently, a number of demonstrations with >10% STH efficiency using potentially less costly materials have been reported. Device stability is a major challenge for the field, as evidenced by lifetimes of less than 24 hours in all but a few reports. No globally accepted protocol for evaluating and certifying STH efficiencies and lifetimes exists. It is our recommendation that a protocol similar to that used by the photovoltaic community be adopted so that future demonstrations of solar PEC water splitting can be compared on equal grounds.