Crystal Phase-Dependent Performance of RuO_x/MnO₂ Nanorods for Alkaline Oxygen Evolution Reaction

MnO₂ is a widely studied non-noble metal electrocatalyst for the oxygen evolution reaction (OER) and has demonstrated phase-dependent performance. Among the various MnO₂ polymorphs, γ-MnO₂ has abundant defects and vacancies due to its disordered crystal structure of both β-MnO₂ and R-MnO₂ intergrowth, thus being a potential high-performance OER catalyst. However, γ-MnO₂ has been studied much less than other crystal phases of MnO₂, and γ-MnO₂-based heterostructures are rarely reported. In this study, it is discovered that γ-phase plays a unique role in RuO_x/MnO₂ heterostructured nanorods. Among the pristine α-, β-, and γ-MnO₂ polymorphs, α-phase shows the best OER activity; however, after loading RuO_x nanoclusters, RuO_x/γ-MnO₂ shows the largest enhancement and hence the best OER activity with an overpotential of 255 mV at 10 mA cm⁻² and excellent stability (> 300 h), which is much superior to the commercial RuO₂ catalyst. Furthermore, when tested in an anion exchange membrane water electrolyzer (AEMWE), it maintains excellent durability at 200 mA cm⁻² over 380 h. Mechanistic study shows that RuO_x/γ-MnO₂ exhibits the strongest electron transfer between Ru and Mn, which significantly weakens the Mn-O bond strength and reduces the interaction between intermediates and the MnO₂ surface, ultimately resulting in the lowest energy barrier for the reaction.