Cation–Anion Co‐doping Enables Acid‐Stable MnO 2 Anodes for Proton Exchange Membrane Water Electrolysis

Developing acid‐stable, earth‐abundant oxygen evolution catalysts remains a central challenge for proton exchange membrane water electrolyzers (PEM‐WE), due to catalyst degradation via overoxidation under strongly anodic conditions. Here, we introduce a cation–anion co‐doping strategy that induces lattice‐level electronic stabilization in manganese dioxide (MnO 2 ) through incorporation of niobium (Nb 5+ ) and fluoride (F − ) into a γ/β‐MnO 2 framework. Structural and spectroscopic analyses, including operando X‐ray absorption spectroscopy, show that co‐doping stabilizes an electron‐enriched Mn environment and suppresses oxidation to unstable high‐valence states under OER conditions. This originates from charge redistribution across the Mn‐O‐Nb framework, enabling controlled Mn 3+ /Mn 4+ dynamics and mitigating overoxidation‐driven dissolution. As a result, MnO 2 ‐Nb‐F catalyst demonstrates an overpotential of 410 mV at 10 mA cm −2 in 0.5 M H 2 SO 4 , a Tafel slope of 90 mV dec −1 , and reduced charge transfer resistance compared to pristine MnO 2 . When integrated into a PEM electrolyzer (5 cm 2 , 60 °C), the catalyst achieves a cell voltage of 1.91 V at 1 A cm −2 and exhibits operational stability for 380 h at 0.4 A cm −2 . These results demonstrate that synergistic cation–anion engineering decouples the activity‐stability trade‐off in MnO 2 , establishing a viable design strategy for noble‐metal‐free PEM‐WE anodes.