Membrane-photoelectrode assemblies, i.e. photoelectrodes on porous substrates coated with an ionomer and directly integrated with a polymeric ion-exchange membrane, are a promising design configuration to perform photoelectrochemical water splitting [1]. This compact and modular design inspired by commercial electrolyzers allows to feed the system with pure water (in liquid or gaseous phase) minimizing the products crossover.

The most common proton-exchange membranes (PEMs) absorb light only in the UV region (for λ< 400 nm) but they impose an acidic pH which causes the corrosion of the majority of the catalysts. The most used anion-exchange membranes (AEMs) are partially opaque in the visible range but the alkaline environment assures the chemical stability of a large variety of materials. Although multiple demonstrations of proton-exchange and anion-exchange membrane-photoelectrode assemblies have been reported [2], the effects of (photo)corrosion of the semiconductors and/or of the co-catalyst used were not investigated in-depth.

Here, we studied the dynamics and the effects of photocorrosion of molybdenum-doped bismuth vanadate (Mo:BiVO₄) photoanodes with cobalt phosphate (CoPi) co-catalyst integrated in proton-exchange and anion-exchange membrane-photoelectrode assemblies with commercial Pt/C cathodes. BiVO₄ suffers from (photoelectro)chemical instability in neutral and alkaline solutions, leading to homogeneous film dissolution [3]. We investigated for the first time the (photo)corrosion of the Mo:BiVO₄ photoanode with CoPi co-catalyst in contact with hydrated ionomers.

The photoanodes were deposited on porous metallic felts through a dip coating technique followed by annealing and the photoelectrodeposition of CoPi co-catalyst. The preliminary optimization of the dopant concentration, semiconductor loading and co-catalyst deposition time was done testing the performance of the coated felts in a simplified cell configuration with liquid electrolyte. The optimal photoelectrodes on metallic felts were placed in thin layers of ionomer solutions obtained by a doctor blade method. After the evaporation of the solvents, the ionomer-coated felts were hot pressed to form membrane-photoelectrode assemblies.        

Chronoamperometric durability tests of the membrane-photoelectrode assemblies with liquid water and humid air at different temperatures were interspersed with cyclic voltammetries and impedance spectroscopies to determine the degradation rates in these different operating conditions. Scanning electron microscopy, energy dispersive x-ray spectroscopy and x-ray photoelectron spectroscopy of the aged samples with inductively-coupled plasma mass spectroscopy of the liquid solutions were used to observe the effects of Mo:BiVO₄ and CoPi photocorrosion in PEM and AEM assemblies.

The methodology introduced and the setup developed allow to investigate the photostability of membrane photoelectrode-assemblies with other semiconductors and co-catalysts with the aim to develop efficient, sustainable but also durable materials and devices for the conversion of solar light into fuels.