Performance and stability of membrane-photoelectrode assemblies with BiVO₄ photoanodes for water splitting

Membrane-photoelectrode assemblies are a promising device configuration to directly electrolyse liquid water or water vapour into hydrogen and oxygen using solar energy. For the first time, we studied the performance and the stability of membrane-photoelectrode assemblies with BiVO₄ photoanodes and CoPi co-catalysts on metallic felt at different temperatures. 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 the anion-exchange ionomer, all using liquid water at 30 °C. Operation with liquid water at 56 °C reduced the onset potential difference under light and in the dark due to more severe recombination of charges, independent of 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 resulted in an 85% decrease in 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 in the saturation current density. Membrane-photoelectrode assemblies have been demonstrated to expand practical device configurations beyond conventional planar setups. The remaining performance gap with respect to planar photoelectrodes highlights the gains that systematic optimisation of membrane-photoelectrode assemblies can still unlock.