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

Abstract

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 V_RHE) during 24 hours 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.