Mechanically and photoelectrochemically stable WO3 | BiVO4 | NiFeOOH photoanodes synthesised by a scalable chemical vapour deposition method

The development of scalable, stable and high performance photoelectrodes remains the major bottleneck in up-scaling photoelectrochemical (PEC) water splitting systems. A photoanode structure of particular promise is WO3 | BiVO4, where the formation of staggered heterojunction between nanostructured WO3 and a thin layer of BiVO4 mitigates charge carrier mobility limitations present for BiVO4 alone and suppresses recombination. Although these electrodes remain prone to photo-corrosion, this effect can be mitigated through the application of water oxidation surface co-catalysts. An additional challenge that has rarely been addressed in the literature to date is the need for strong adhesion to the substrate and mechanical stability of these photoelectrodes, so that they can withstand flow-induced shear stress exerted by the electrolyte in continuous flow under operational conditions. Herein, we propose a scalable route to synthesising WO3 | BiVO4 | NiFeOOH photoanodes entirely by aerosol-assisted chemical vapour deposition (AA-CVD). The mechanical stability of the WO3 | BiVO4 heterojunction was optimised by tuning the morphology of the WO3 underlayer and improving its adhesion to the FTO transparent substrate. To address BiVO4 dissolution at the electrode | electrolyte interface, we fabricated a NiFeOOH co-catalyst by a novel AA-CVD method. This suppressed BiVO4 dissolution and enhanced the water oxidation performance of the photoanode, characterised by linear sweep voltammetry (LSV), photoelectrochemical impedance spectroscopy (PEIS) and chopped chronoamperometry. The photoanode materials were physically characterised by X-ray diffraction (XRD), UV-vis spectroscopy, scanning electron microscopy (SEM), high resolution transmission electron microscopy (HR-TEM), X-ray photoelectron spectroscopy (XPS), Raman spectroscopy and time-of-flight secondary ion mass spectrometry (ToF-SIMS). Our optimised photoanodes with 1 cm2 photoactive area delivered a stable photocurrent density of 1.75 mA cm-2 (at 1 simulated sun irradiance and 1.23 VRHE) during 24-hour testing in a continuous PEC flow reactor (operated at 0.5 cm s-1). Our method for growing WO3 | BiVO4 | NiFeOOH photoanodes is up-scalable, and therefore suitable for producing large-area demonstration devices, providing a pathway to commercial photoelectrochemical hydrogen production.