Combinatorial discovery and optimization of a complex oxide with water photoelectrolysis activity

A ternary oxide containing cobalt, aluminum, and iron and not previously known to be active for the photoelectrolysis of water was identified using a high throughput combinatorial technique. The technique involves ink jet printing overlapping patterns of oxide precursors onto fluorine-doped tin oxide conductive glass substrates. Subsequent pyrolysis yields patterns of mixed oxide compositions that were screened for photoelectrolysis activity by scanning a laser over the material while it was immersed in an electrolyte and mapping the photocurrent response. The composition and optimum thickness for photoelectrochemical response of the newly identified material was further refined using quantitative ink jet printing. Chemical analysis of bulk and thin film samples revealed that the material contains cobalt, aluminum, and iron in a Co3O4 spinel structure with Fe and Al substituted into Co sites with a nominal stoichiometry of Co3-x-yAlxFeyO4 where x and y are about 0.18 and 0.30, respectively. The material is a p-type semiconductor with an indirect band gap of around 1.5 eV, a value that is nearly ideal for an efficient single photoelectrode for water photoelectroylsis. Photoelectrochemical measurements indicate that the onset of hydrogen evolution is about 0.9 V positive of the thermodynamic value but the photocurrent is limited by slow kinetics for hydrogen evolution.