Energy storage TiO2-WO3 photocatalysts, which retain anticorrosion and bactericidal effects, even in the dark, on the basis of reductive energy stored in WO3, are optimized in terms of apparent quantum yield, capacity and specific capacity (x in HxWO3) for photoelectrochemical charging of the film in the gas phase. The maximal apparent quantum yield (similar to8%) is obtained at a W/Ti ratio of 0.5, and a film thickness greater than or equal to 10 mum. The capacity is largest at W/Ti = 0.5, and increases with the film thickness. The specific capacity (similar to0.10) is almost independent of the parameters in the range examined. Although the charging rate and capacity are very low under dry conditions (humidity similar to10% RH), modification of the optimized TiO2-WO3 photocatalysts with phosphotungstic acid greatly improves these characteristics. (C) 2004 Elsevier B.V. All rights reserved.