Abstract
<jats:p> Proton exchange membrane (PEM) water electrolysis is a key technology for sustainable hydrogen production, yet it depends on scarce and expensive iridium-based oxygen evolution reaction (OER) catalysts. Therefore, substantial efforts are directed toward reducing the iridium loading, including (partial) substitution with other materials as well as thin-film fabrication strategies for ionomer-free catalyst layers. Ruthenium oxide (RuO <jats:sub>2</jats:sub> ) is a promising alternative because of its high intrinsic OER activity and lower material cost, however its long-term stability remains limited and structure–performance relationships are not fully understood, especially for ionomer-free thin films. In this study, reactive magnetron sputtering is used as a dry thin film fabrication method to investigate ionomer-free RuO <jats:sub>2</jats:sub> OER catalysts as porous transport electrodes (PTEs). By systematically varying deposition parameters, the nanostructure of the films is tuned over a wide grain-size range. X-ray diffraction (XRD) and cross-sectional scanning electron microscopy revealed a strong correlation between grain size and electrochemical behavior. Smaller grain sizes increase the electrochemically active surface area (ECSA), as determined by (pseudo) double-layer capacitance analysis, thereby enhancing the OER performance in both three-electrode measurements in 0.5 mol L <jats:sup>−1</jats:sup> H <jats:sub>2</jats:sub> SO <jats:sub>4</jats:sub> and PEM electrolyzer tests. Combining results from electrochemical impedance spectroscopy (EIS) and X-ray photoelectron spectroscopy (XPS) suggests that the increased ECSA in membrane electrode assemblies is enabled by proton-conductive hydrated grain boundaries. Notably, it was found that smaller grains also improve long-term stability in PEM electrolyzer operation. A cell with a low anode loading of 0.64 mg cm <jats:sup>−2</jats:sup> operates for over 700 h at 1 A cm <jats:sup>−2</jats:sup> , exhibiting a degradation rate of only 4 µV h <jats:sup>−1</jats:sup> for more than 300 h. These findings demonstrate that controlling the nanostructure of sputtered RuO <jats:sub>2</jats:sub> can enable efficient, durable, and scalable ionomer-free OER catalysts for PEM water electrolysis. </jats:p>