Effect of Metal-Substitution within Ruthenium Oxide on Structure and Oxygen Evolution Activity and Stability

Abstract

The development and utilization of proton exchange membrane water electrolyzers (PEMWEs) is hindered by the cost, activity, and stability of the oxygen evolution reaction (OER) electrocatalyst. Iridium oxide (IrOx) is currently the go-to OER electrocatalyst, as it has been shown to have relative high activity and stability when compared to other OER active catalysts. However, iridium is one of the rarest elements in the Earth’s crust, and therefore cost is a major limitation of iridium-based electrocatalysts. Ruthenium oxide (RuO2) is much lower cost and more active than iridium oxide; however, RuO2 it is unstable in acidic media and undergoes degradation over time. We investigated substituting niobium, tantalum, and zirconium, which are OER-stable metals, into RuO2 to improve the OER stability. Our study explored the effects of different metals and varied concentrations within RuO2 (Ru1-xMxO2, M = Nb, Ta, and Zr) on the structure, morphology, OER activity, and stability. The structure and morphology were characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy, and scanning electron microscopy. Preliminary results from XRD showed observable phase separation at higher concentrations of Nb, Ta, and Zr and less phase separation at lower concentrations for Nb and Ta. There was no observable phase separation for Zr at lower concentrations. XRD peak shifts were observed and indicate the incorporation of the metal ions into the crystal structure of rutile RuO2. The OER activities and stabilities of Ru1-xMxO2 were measured using a rotating disk electrode configuration and compared with synthesized RuO2. Our preliminary results show that the OER activity and stability are strongly affected by the addition of the different metals and could be attributed to morphology and structural changes. Our findings help to further the development of lower cost, high activity, and increased stability OER electrocatalysts, which are crucial to the large-scale adoption of PEMWE’s.