Investigating Ti-Nb alloys as supports for iridium oxide water oxidation electrocatalysts

Abstract

The scarcity of Ir presents a major challenge for scaling up its use as a water oxidation electrocatalyst in proton exchange membrane (PEM) water electrolysers. Developing conductive and stable supports is an effective way to reduce iridium loading while maintaining performance. However, the influence of support conductivity and stability on Ir-based catalytic activity remains poorly understood. The behaviour of the support is often obscured in conventional membrane electrode assembly (MEA) systems because IrOx itself is both highly conductive and exceptionally stable. To decouple support conductivity and passivation effects from the intrinsic conductivity of IrOx, we demonstrate a screening platform by studying a series of Ti-Nb alloy thin films produced by sputter deposition and investigate their performance as supports for IrOx water oxidation electrocatalysts. A range of electrochemical tests including accelerated stress tests (AST) were carried out on these samples, where characterisation techniques, including X-ray absorption spectroscopy (XAS), X-ray photoelectron spectroscopy (XPS) and high resolution transmission electron microscopy (HRTEM), demonstrated the in situ formation of passivation layers on these supports during water oxidation. Our results suggest that a ~10 nm oxide passivation layer forms on metallic Ti-based supports. On alloying Nb with Ti metal, a more insulating rutile TiO2 phase forms during water oxidation whereas an anatase TiO2, with higher conductivity, is observed on the pure Ti support. Consequently, although alloying Ti with Nb improves the bulk conductivity, the structure of the oxide passivation layer results in a drastic decrease of conductivity and water oxidation activity. Our results demonstrate the importance of the structure and composition of surface oxide phases formed during water oxidation in controlling the overall stability and conductivity of support materials.