Ultrathin Hematite‐Hercynite Films for Future Unassisted Solar Water Splitting

Abstract Photoelectrochemical (PEC) water splitting requires stable, efficient, and cost‐effective photoelectrodes to enable future large‐scale solar hydrogen production. Ultrathin hematite‐hercynite photoanodes that meet all these criteria in an excellent way is presented here. Hematite‐hercynite photoelectrodes are synthesized in a self‐forming manner by thermal oxidation of iron–aluminum alloy films and characterized with regard to water splitting applications. Photoanodes fabricated from 17 wt.% Al at 493 °C for 8 h and 685 °C for 5 min exhibit, for instance, a photocurrent density of 1.24 and 1.53 mA cm⁻² at 1.23 V versus RHE, respectively, as well as superior light absorption in the visible range of the solar spectrum. The PEC performance improvement in comparison to pure hematite thin film electrodes is first achieved by adjusting the aluminum concentration with an optimum range of 12–17 wt.% and second by optimizing the annealing conditions. The resulting photocurrent densities are about a factor of three higher than those obtained from electrodes synthesized from pure iron thin films using the same synthesis conditions. Finally, it is shown that ultrathin hematite‐hercynite photoelectrodes enable even unassisted solar water splitting in a NaOH (1 m) electrolyte with a maximum solar‐to‐hydrogen conversion efficiency of 0.78%.