Octahedral and tetrahedral site substitution in spinel Mn₃O₄ regulates oxygen evolution in acid

Designing cost-effective and acid-stable electrocatalysts for the oxygen evolution reaction is essential for advancing proton exchange membrane water electrolyzers. Here we report a dual-site substituted spinel Mn₃O₄ catalyst in which Ru partially occupies octahedral Mn sites to enhance Ru-O hybridization and intrinsic catalytic activity, while Co partially replaces tetrahedral Mn sites to stabilize the lattice and improve electronic conductivity. The catalyst requires an overpotential of 175 mV to reach 10 mA cm_geo^-2 and maintains operation for more than 650 hours at 100 mA cm_geo^-2 with a voltage degradation rate of 0.23 mV h^-1 in 0.5 M H₂SO₄. When used as the anode in a proton exchange membrane water electrolyzer, it delivers 1 A cm^-2 at a cell voltage of 1.69 V, demonstrating its potential for practical acidic water electrolysis. In situ characterization confirms that Ru and Co substitution alters the oxygen evolution pathway. These findings establish a site-specific cation substitution strategy for developing active, durable, and cost-effective Ru-based electrocatalysts under acidic conditions.