Ru-Substituted MnO₂ for Accelerated Water Oxidation: The Feedback of Strain-Induced and Polymorph-Dependent Structural Changes to the Catalytic Activity and Mechanism

Heteroatomic modulation of MnO₂ is an effective way to introduce and tailor the catalytically active sites for electrochemical water oxidation. While great efforts have been devoted to parsing the configuration and coordination of dopants in dictating the catalytic activity, less is considered about the feedback from the structurally adapted MnO₂ host to the intrinsic activity of catalytic sites. In this study, the topological effect on oxygen evolution reaction (OER) activity was systemically investigated for partially Ru-substituted MnO₂ of various polymorphs. We show that MnO₂ of different porosities responds differently to the Ru integration, thereby resulting in varied lattice strains and morphological changes. While the highly porous τ-MnO₂ undergoes amorphization upon Ru substitution, the closely packed β-MnO₂ suffers crystal splintering with drastically enhanced structural defects, which lends to a low OER overpotential of 278 mV at 10 mA cm^–2 and a high turnover frequency of 2022.2 h^–1 that is 19.6-fold higher than that of the commercial RuO₂ benchmark. Therefore, the integration of Ru does not simply append active sites to the relatively inert metal oxides but simultaneously modifies the crystal structure of MnO₂ to retroactively modulate the catalytic activity. We further show that OER on the Ru-substituted β-MnO₂ follows a lattice oxygen mechanism as a result of the adapted oxide substrate. This study furnishes a fresh and systemic view on the dopant–substrate interplay for modulating the electrocatalytic activity of tunneled MnO₂ structures.