Energy & Environmental Science Experimental Demonstrations of Spontaneous, Solar-driven Photoelectrochemical Water Splitting † Broader Context

Laboratory demonstrations of spontaneous photoelectrochemical (PEC) solar water splitting cells are reviewed. Reported solar-to-hydrogen (STH) conversion efficiencies range from o1% 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 410% 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. There is significant recent interest in solar-driven photoelectrochemical water splitting to produce hydrogen as a potential carbon-neutral transportation fuel. Renewable energy technologies must provide a positive monetary and net energy balance over their lifetimes to be viable for large scale deployment. Techno-economic analyses have suggested that solar photoelectrochemical water splitting could provide hydrogen at a cost that is competitive with energy derived from fossil fuels. Thus, economical solar water splitting represents a goal with broad-reaching appeal. One specific implementation of this concept is an integrated or monolithic solar-to-fuel conversion device that operates spontaneously, without added external electrical bias. Experimental demonstrations of such systems date back to the early 1970s, when Fujishima and Honda first reported solar water splitting using single-crystal TiO 2. This inspired considerable research in the field and to-date there have been over 40 reported demonstrations of spontaneous, solar-driven photoelectrochemical water splitting. These have led to increased fundamental and functional understanding and to increases in the overall energy-conversion efficiency. Herein, we compile reported solar-to-hydrogen conversion efficiencies and longevities. This information can be used to evaluate progress in the field and to target technical areas for future development.