Bimetallic Ir-Ru 25:75 Catalyst for PEM-WE: Uncovering the Origin of Its Activity by Operando Wide Angle X-Ray Scattering

Proton Exchange Membrane Water Electrolyzers (PEM-WEs) are entering the phase of commercial mass production. However, the issue of an iridium catalyst for the anode remains. Our previous work presented an iridium-ruthenium-based catalyst (25% Ir = 158 µg cm-2, 75% Ru) prepared as a thin film on the surface-enhanced PEM-WE-anode via magnetron sputtering. This catalyst demonstrated excellent activity and stability in single-cell PEM-WE (1 A cm-2 at 1.606 V, 80 °C, degradation rate – 1.3 µV h-1 at 1 A cm-2 for 500 hours). The origin of its outstanding electrochemical performance was, however, not known. We present a complete operando Wide Angle X-Ray Scattering (WAXS) study done at the European Synchrotron Radiation Facility (ESRF) using a dedicated PEM-WE cell. The structure of each sample is observed for several hours under PEM-WE conditions with increasing current density from 100 mA cm-2 to 600 mA cm-2. Consequently, we show the connection of the electrochemical performance of the Ir-Ru 25:75 catalyst to its structural properties and uncover the interplay between the face-centered-cubic iridium and hexagonal-close-packed ruthenium.