Energy Storage of TiO₂−WO₃ Photocatalysis Systems in the Gas Phase

Abstract

Reductive energy generated at a TiO2 photocatalyst under UV light can be stored in WO3 by coupling them together, and the stored energy can be used after dark. However, the reduction of WO3 requires cation intercalation for charge neutralization. Thus, behavior of the TiO2-WO3 composite on an ITO electrode was examined in nonelectrolytic media. When the TiO2 and WO3 were close to each other (less than 1 mm), WO3 could be reduced even in pure water or humid air (relative humidity g25%), by irradiating the composite with UV light. In dry air, WO3 was not reduced efficiently, even if the TiO2 and WO3 nanoparticles were mixed well. These results suggest that protons generated at the TiO2 surface as a result of photocatalytic oxidation of water are intercalated into WO3, and therefore ionic conductivity of the medium or the composite film surface is important. The composite film charged in air exhibited almost the same electrode potential as that of the film charged in aqueous NaCl.

Energy Storage in Bifunctional TiO2 Composite Materials under UV and Visible Light

“…A TiO 2 coating is coupled with a WO 3 coating as an electron pool, in which the reductive energy can be stored. Snce then, TiO 2 /MoO 3 [23], SrTiO 3 /WO 3 [24], TiO 2 /phosphotungstic acid (PWA) [25], TiO 2 /Ni(OH) 2 [26] and several other TiO 2 /WO 3 [27][28][29] composite systems have also been developed by the groups of Tetsu Tatsuma and his collaborators. The energy can be stored either in reduced WO 3 ，MoO 3 ，PWA or in oxidized Ni(OH) 2 under UV light.…”

“…Based on their and our own work, we here introduce three representative optical energy storage mechanisms. Figure 1 shows the mechanism of reductive energy storage by TiO 2 /WO 3 composite system [27]. In this system, an excited electron and a corresponding hole are generated on TiO 2 under UV light:…”

“…(b) Proposed models of electron and ion transfer in the charging and self-discharging processes of the TiO 2 /WO 3 composite film in humid air [27].…”

Metal Oxides and Hydroxides as Rechargeable Materials for Photocatalysts with Oxidative Energy Storage Abilities

“…In order to retain its useful effects even after dark, we have developed photocatalysts with energy storage abilities. [3][4][5][6][7][8][9][10][11][12][13][14][15] They can store surplus charges from photocatalysts in rechargeable materials during the day, and the stored charges are used to carry on redox reactions during the night.…”

“…We have combined TiO 2 3-8 or SrTiO 3 9 as well as Au-TiO 2 , 10 which exhibits photocatalytic activity on the basis of plasmoninduced charge separation, [16][17][18][19] with a rechargeable material such as WO 3 , [3][4][5][6]9,10 MoO 3 , 7,10 or H 3 PW 12 O 40 (PWA), 8 which undergoes redox reactions at negative potentials. Those photocatalysts with reductive energy storage abilities retain some photocatalytic effects based on cathodic reactions, such as anti-bacterial 6 and anticorrosion 3 effects, even after dark.…”