Cation-Anion Co-doping Enables Acid-Stable MnO₂ 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 (MnO₂) through incorporation of niobium (Nb⁵⁺) and fluoride (F^-) into a γ/β-MnO₂ 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 Mn³⁺/Mn⁴⁺ dynamics and mitigating overoxidation-driven dissolution. As a result, MnO₂-Nb-F catalyst demonstrates an overpotential of 410 mV at 10 mA cm^-2 in 0.5 M H₂SO₄, a Tafel slope of 90 mV dec^-1, and reduced charge transfer resistance compared to pristine MnO₂. When integrated into a PEM electrolyzer (5 cm², 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 MnO₂, establishing a viable design strategy for noble-metal-free PEM-WE anodes.