Polymorph γ-MnO2 with Optimal Phase Composition for Stable Oxygen Evolution Reaction in Acid

The development of non-noble-metal oxygen evolution reaction (OER) electrocatalysts stable in acid to replace rare and expensive iridium catalysts is crucial yet challenging for the large-scale application of proton exchange membrane (PEM) water electrolysis technology. Here, we show that increasing the Pr (pyrolusite ratio) in γ-MnO₂ from 73% to 84% results in about 10 times longer durability at a high working current density of 100 mA cm^–2 in 1 M H₂SO₄ for more than 1000 h. Dramatic stability enhancement is due to the inhibition of lattice Mn or O dissolution by leveraging the formation of more stable corner-shared pyrolusite MnO₆ octahedra with a higher oxidation state of Mn, fewer defects, a shorter Mn–O bond distance, an increase in stable mono-oxo-bridged Mn–O–Mn pyrolusite MnO₆ octahedra, and a decrease in di-oxo-bridged Mn–O2–Mn ramsdellite MnO₆ octahedra with more Mn–Mn tension. Because polymorph materials like γ-MnO₂ are quite common in nature, phase composition turning may serve as an effective strategy for the development of stable OER catalysts in acidic environments.