Laboratory demonstrations of spontaneous photoelectrochemical (PEC) solar water splitting cells are reviewed. Reported solar-to-hydrogen (STH) conversion efficiencies range from 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.

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Experimental demonstrations of spontaneous, solar-driven photoelectrochemical water splitting

Experimental Demonstrations of Spontaneous, Solar-Driven Photoelectrochemical Water Splitting

Photocatalytic decomposition of water at semiconductor electrodes

The efficiency of photoelectrolysis with solar radiation can be greatly enhanced by using simultaneously illuminated n‐ and p‐type electrodes. An energy level scheme for heterotype and homotype p‐n photoelectrolysis cells is presented which explains the enhanced efficiency. Data are presented for the homotype n‐GaP/p‐GaP and the heterotype n‐TiO2/p‐GaP systems which support the energy model. H2 and O2 are produced in the n‐TiO2/p‐GaP cell without the need of an external anodic bias.

p‐n photoelectrolysis cells

The faradaic efficiency for O2(g) evolution at thin-film WO3 photoanodes has been evaluated in a series of acidic aqueous electrolytes. In 1.0 M H2SO4, persulfate was the predominant photoelectrochemical oxidation product, and no O2 was detected unless catalytic quantities of Ag+(aq) were added to the electrolyte. In contact with 1.0 M HClO4, dissolved O2 was observed with nearly unity faradaic efficiency, but addition of a hole scavenger, 4-cyanopyridine N-oxide, completely suppressed O2 formation. In 1.0 M HCl, Cl2(g) was the primary oxidation product. These results indicate that at WO3 photoanodes, water oxidation is dominated by oxidation of the acid anions in 1.0 M HCl, H2SO4, and HClO4, respectively.

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A quantitative assessment of the competition between water and anion oxidation at WO3 photoanodes in acidic aqueous electrolytes

FUJISHIMA and Honda1 have described a novel photochemical cell with an open circuit emf of up to 05 V which they indicated had a capability for water photolysis The anode consisted of a TiO2 n-type single crystal which was illuminated by light of wavelength shorter than 415 nm This corresponds to the 30 eV band gap of TiO2 (refs 2–4)

G. M. Haas

Papers

Electricity from photosensitisation of titanium