In-Plane Conductivity as a Descriptor of Apparent Durability of RuO₂ Anodes in PEM Water Electrolysis

The apparent durability of proton exchange membrane water electrolyzers (PEMWEs) employing RuO₂ anodes varies significantly, even though RuO₂ is often used as a benchmark for Ru-based catalysts. We find that the in-plane conductivity effectively indicates key microstructural characteristics that govern the apparent durability. Low in-plane conductivity (e.g., <10 S cm^–1 in this study) limits electron transport and catalyst utilization while increasing susceptibility to electrochemical degradation. Only when in-plane conductivity is sufficiently high (above ∼25 S cm^–1 in this study) does apparent device durability reflect the intrinsic electrochemical stability of RuO₂ catalysts. Increasing the isopropanol ratio in the catalyst ink or adding carbon black as a conductive additive markedly enhances in-plane conductivity, extending device durability from 10.7 to 63.5 h (5.9-fold) at 1 A cm^–2. This work clarifies the origin of apparent stability discrepancies among RuO₂ catalysts in PEMWEs, identifying in-plane conductivity as a key structural descriptor for reliable durability assessment.