Phase and Morphology Transformation of MnO2 Induced by Ionic Liquids toward Efficient Water Oxidation

Water oxidation through the Mn 4 Ca-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-BF 4 )] hydrothermal approach for tuning both crystallographic phase and nanostructure morphology of MnO 2, 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-MnO 2 nanorods to α-MnO 2 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-MnO 2, 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 Mn 3+ . 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-MnO 2 nanowires.