New anode materials for photoelectrolysis

Abstract

The design strategy for a successful oxide anode for use in photoelectrolysis cells is reviewed. Conduction bands based on Ti⁴⁺ and Nb⁵⁺ can be made to possess sufficiently small electron affinities to allow the cell to operate without external bias. For example, data on SrTi_0.75Zr_0.25O₃ are presented to show that the Ti⁴⁺ conduction band can be fine-tuned to the required electron affinity using Zr⁴⁺ as dopant. The Zr⁴⁺ dopant also enhances remarkably oxygen evolution in the dark, and a mechanism for this observation is advanced. In order to reduce the bandgap in oxide titanates, incorporation of a second metal-ion sublattice appears to be essential. Three types of substituent cation that can, in principle, give rise to a valence band at the right energy are discussed: (i) transition metals giving rise to nd^m bands, (ii) lanthanides giving rise to 4f^m bands and (iii) reduced B-metals giving rise to ns² bands. Optical absorption studies of MnTiO₃ and NaCeTi₂O₆ indicate that the bulk Mn²⁺ 3d⁵ and Ce³⁺ 4f¹ levels are at about the right energy, but holes photogenerated in these levels become trapped in surface–polaron sites, as they only oxidise water slowly. Analysis of the transient photocurrents for MnTiO₃ shows that holes arrive at the surface in both the O^2–:2p⁶ band and the Mn²⁺:3d⁵ levels and the former oxidise water rapidly.