Hydrogen production by photoelectrolytic decomposition of H/sub 2/O using solar energy. Final report, July 22, 1977-July 21, 1978

Abstract

The purpose of this program is to develop new semiconducting electrodes for the economical efficient photoelectrochemical decomposition of H/sub 2/O using solar energy. Due to the intimacy of the liquid-solid rectifying junction, this technique is particularly well-suited to inexpensive polycrystalline electrode fabrications. During the past year, work has concentrated on photoanode development, specifically anodically robust semiconducting oxides. It was found that some thin film oxides formed by a simple chemical vapor deposition (CVD) procedure perform quite well, with maximum solar power conversion efficiencies of 0.5 to 1.0% for CVD TiO/sub 2/ despite a band gap of 3.1 eV, which is a poor match for the solar spectrum. CuO, Fe/sub 2/O/sub 3/, WO/sub 3/ and some mixed oxide thin film electrodes were also prepared pyrolytically and showed good photoresponse. The argument is put forth, however, that no stable oxide photoanodes yet discovered have the combined properties of low electron affinity (EA) and optimal band gap required for the efficient photoelectrolysis of H/sub 2/O. Compounds like TiO/sub 2/ and SrTiO/sub 3/ have satisfactory EA's, but their valence bands lie too deep, rendering their band gaps impractically large. It is probable that only d-band oxides would have valence bands high enough in energymore » to give rise to this combination of properties. In order to produce such materials, over 60 new perovskites and rutiles containing d/sup n/ transition metals have been synthesized and evaluated as sintered disc photoelectrodes. Mostly 3d metal oxides were investigated. Sensitization of photocurrents to the visible portion of the spectrum was noted in several cases confirming this materials design philosophy. As a rule, photocurrent quantum yields were lower for the d/sup n/-substituted oxides than for the wide band gap, d/sup 0/-oxides like SrTiO/sub 3/ and TiO/sub 2/. This is ascribed partially to poor mobility of the d electrons.« less