Nanostructural Engineering for Optimizing Performance and Durability of Sputtered RuO 2 Oxygen Evolution Catalysts in PEM Electrolyzers

Proton exchange membrane (PEM) water electrolysis is a key technology for sustainable hydrogen production but is constrained by the cost, complexity, and durability of the oxygen evolution reaction (OER) catalyst. Ruthenium oxide (RuO 2 ) is regarded as a promising alternative to the state-of-the-art IrO 2 catalyst, offering superior intrinsic OER activity at a lower material cost. However, structure–performance relationships in practical RuO 2 -based electrodes require deeper understanding, and their long-term stability in PEM electrolyzers is still unsatisfactory. In this study, RuO 2 thin films are fabricated by reactive magnetron sputtering and directly integrated as ionomer-free OER catalysts in porous transport electrodes (PTEs). By systematically varying deposition parameters, the nanostructure of the films is tuned over a wide grain-size range. Structural analysis by X-ray diffraction (XRD) and cross-sectional scanning electron microscopy (SEM) reveal a strong correlation between grain size and electrochemical behavior. Reduced grain size results in a higher electrochemically active surface area (ECSA), determined from evaluating the double-layer capacitance, which leads to an enhanced OER activity both in 3–electrode measurements in 0.5 mol L −1 H 2 SO 4 as well as in a PEM electrolyzer cell. Combining results from electrochemical impedance spectroscopy (EIS) and X-ray photoelectron spectroscopy (XPS) suggests that the higher ECSA in membrane electrode assemblies is presumably enabled by proton-conductive hydrated grain boundaries. Importantly, smaller grains also improve long-term stability in PEM electrolyzer tests. A cell with a low anode loading of 0.64 mg cm −2 operates for over 700 h at 1 A cm −2 , exhibiting a degradation rate of only 4 µV h −1 for more than 300 h. These results demonstrate that nanostructural engineering of sputtered RuO 2 enables efficient, durable, and scalable ionomer-free OER catalysts for PEM electrolysis.