Performance and stability of membranephotoelectrode assemblies with BiVO4 photoanodes for water splitting

Abstract

Membrane-photoelectrode assemblies are a promising device configuration to directly electrolyse liquid water or water vapour into hydrogen and oxygen using solar energy. We studied the performance and the stability of proton-exchange and anion-exchange membrane-photoelectrode assemblies with BiVO 4 photoanodes and CoPi co-catalysts on metallic felts. Upon illumination with simulated solar light photocurrent densities of 0.23 mA cm -2 at 1 V vs. RHE were obtained with the proton-exchange ionomer and 0.06 mA cm -2 with anion-exchange ionomer, all using liquid water at 30 °C. Operation with liquid water at 56 °C reduced the onset potential difference in light and dark due to more severe recombination of charges, independent on the choice of ionomer. The (photoelectro)chemical corrosion reactions resulted in the dissolution of Bi, V, Mo and Co, which was accelerated by temperature. The dissolved species formed solid particles mainly containing vanadium in the proton-exchange membranes. Operation of proton-exchange membrane-photoelectrode assemblies with water vapour using a humidified nitrogen flow resulted in 85% decrease of the photocurrent density produced at 1 V vs. RHE as the hydration of the ionomer was reduced. The more acidic local pH at the ionomer-photoelectrode interface (compared to liquid water operation) accelerated the dissolution of the photoactive material in time, resulting in a faster decrease of the saturation current density. Membrane-photoelectrode assemblies have demonstrated the promising capability to operate with both liquid water and humid air feeds, opening practical device configurations beyond conventional planar setups. The remaining performance gap with respect to planar BiVO 4 photoelectrodes highlights the gains that systematic optimisation of membrane-photoelectrode assemblies can still unlock.