High Performance Silicon Photocathodes for Hydrogen Production Via Solar Water Splitting
Silicon is a particularly promising small band gap absorber material for application in a tandem PEC device due to its abundance, relatively low cost, excellent charger carrier transport, and near-ideal band gap. A number of recent studies have reported promising silicon photocathode structures.4-6 However, several challenges remain to be addressed to develop silicon photocathodes that are as efficient and stable as necessary for economical water splitting. The silicon must be combined with an active catalyst to reduce the kinetic overpotential necessary to drive the hydrogen evolution reaction (HER) at the electrode surface.4-6 The silicon must also be protected to prevent corrosion or oxidation, which can destroy device performance.6
We report our recent progress in designing highly active and stable silicon photocathodes. First, we discuss strategies for protecting silicon photocathodes against corrosion. We demonstrate that silicon electrodes covered with a thin surface coating of molybdenum disulfide show no loss in performance after 100 hours of operation.7 Transmission electron microscopy measurements reveal the atomic scale features of this electrode design that result in its excellent stability. While the MoS2 surface layer provides reasonable catalytic activity, the device performance is limited due to the low density of exposed catalytic sites. To further improve this cathode’s efficiency, we incorporate a second molybdenum sulfide nanomaterial, highly catalytically active [Mo3S13]2- clusters,8resulting in precious metal-free devices with photocurrent onset potentials within ~150 mV of the best reported Pt/Si photocathodes. Next, we discuss further improvements in the design of silicon photocathodes to maximize the photocurrent onset potential and device durability. These developments include the incorporation of new, highly active HER electrocatalysts as well as photocathode designs that reduce recombination within the silicon, maximizing the current-voltage characteristics of the device. Based on our findings, we propose strategies for further improving the performance of solar water splitting photocathodes.
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