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

The scarcity of iridium-based water oxidation electrocatalysts limits the scaling up of proton exchange membrane (PEM) water electrolysers. Developing conductive and stable supports is an effective way to reduce iridium loading. However, it is very challenging to find materials that are both stable and conductive under the corrosive and oxidative oxygen evolution reaction (OER) conditions. Nb-doped TiO2 is a potential candidate with Nb dopant improving the conductivity of TiO2. However, its conductivity is still around 10 orders of magnitude lower than IrO2. Herein, we have fabricated a series of Ti-Nb alloy thin films with varied Nb content by sputter deposition and investigated their performance as a support for iridium oxide. Electrochemistry and accelerated stress test were conducted on these metal thin films and characterisation techniques including X-ray absorption spectroscopy, X-ray photoelectron spectroscopy and transmission electron microscopy were employed to characterise the in situ formed passivation layers during the OER. Our results suggested that a ~10 nm oxide passivation layer forms on Ti-based supports. On alloying Nb with Ti metal, a more insulating rutile Ti oxide phase forms under OER whereas an anatase Ti oxide, with higher conductivity, is observed on the undoped Ti. Consequently, although alloying with Nb improves the bulk conductivity, the structure of the oxide passive layer results in drastic decrease of conductivity, and OER activity for Nb-containing supports. Our results thus indicate the importance of the structure and composition of surface oxide phases formed during OER in controlling the overall stability and conductivity of candidate support materials.