Polymorph γ-MnO<sub>2</sub> with Optimal Phase Composition for Stable Oxygen Evolution Reaction in Acid
Lin Liu, Shuang Kong, Yimeng Sun, Qinqin Hu, Pengfei Zhang, Ailong Li, Kiyohiro Adachi, Daisuke Hashizume, Ryuhei Nakamura, Taifeng Liu, Can Li, Hongxian Han
Abstract
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-MnO 2 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 H 2 SO 4 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 MnO 6 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 MnO 6 octahedra, and a decrease in di-oxo-bridged Mn–O2–Mn ramsdellite MnO 6 octahedra with more Mn–Mn tension. Because polymorph materials like γ-MnO 2 are quite common in nature, phase composition turning may serve as an effective strategy for the development of stable OER catalysts in acidic environments.