Water oxidation through the Mn4Ca-oxo complex in photosystem II has fascinated many researchers because of its high efficiency and low energy input; therefore, it has triggered great interest in various polymorphs of manganese oxides for electrocatalysis. Herein we report a facial ionic liquid (IL)-assisted [IL: 1-butyl-3-methylimidazolium tetrafluoroborate (BMIM-BF4)] hydrothermal approach for tuning both crystallographic phase and nanostructure morphology of MnO2, enabling the excellent oxygen evolution reaction (OER) activity with an overpotential of 394 mV at 10 mA cm–2 and a Tafel slope of 49 mV dec–1. The roles of IL in the crystallographic and morphological transformation from β-MnO2 nanorods to α-MnO2 nanowires and in the OER are carefully scrutinized. TEM, EDX, FTIR, XPS characterizations all reveal the capping of IL cations on the surface of α-MnO2, where the amphiphilic nature, the electrostatic interaction, the steric hindrance, and the π–π stacking of IL cations collectively serve as entropic drivers for the templated growth of 2 × 2 tunnel structure incorporating K+ ions. This structure has been particularly beneficial for OER, owing to a concerted synergy from the nanostructured morphology, suitable tunnel structure with rich di-μ-oxo bridges, alkali-metal incorporation, as well as higher content of trivalent Mn3+. What’s more, our investigation indicates the surface-immobilized IL plays a crucial role toward efficient OER by facilitating the formation and stabilization of oxygen vacancies on the surface of α-MnO2 nanowires.

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Among the various polymorphs of manganese oxides, β-MnO2 has been long regarded as an inert electrocatalyst for oxygen conversion due to its high thermodynamic stability and consummate lattice structure. Herein, for the first time, we report the phenomenon of crystal splintering induced by interstitial atomic doping for drastically enhancing the activities of both oxygen evolution (OER) and oxygen reduction (ORR) reactions for β-MnO2. Ru-doped β-MnO2 exhibits an ultralow OER and ORR voltage gap of only 0.63 V, which is the best ever observed for β-MnO2 and surpasses many state-of-the-art bifunctional oxygen catalysts reported to date. Through advanced microscopic and spectroscopic characterizations, in conjunction with theoretical understandings, the drastically improved OER activity is attributed to the highly under-coordinated Ru–O sites exposed on the surface upon crystal splintering, while the enhanced ORR property originates from the strained M–O configuration with enriched Mn3+ content and oxygen vacancies.

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Heteroatomic modulation of MnO 2 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 MnO 2 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 MnO 2 of various polymorphs. We show that MnO 2 of different porosities responds differently to the Ru integration, thereby resulting in varied lattice strains and morphological changes. While the highly porous τ-MnO 2 undergoes amorphization upon Ru substitution, the closely packed β-MnO 2 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 19.6fold higher than that of the commercial RuO 2 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 MnO 2 to retroactively modulate the catalytic activity. We further show that OER on the Ru-substituted β-MnO 2 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 MnO 2 structures.