Engineering the crystal facets of α-MnO₂ nanorods for electrochemical energy storage: experiments and theory

Crystal facet engineering is an effective strategy for precisely regulating the orientations and electrochemical properties of metal oxides. However, the contribution of each crystal facet to pseudocapacitance is still puzzling, which is a bottleneck that restricts the specific capacitance of metal oxides. Herein, α-MnO₂ nanorods with different exposed facets were synthesized through a hydrothermal route and applied to pseudocapacitors. XRD and TEM results verified that the exposure ratio of active crystal facets was significantly increased with the assistance of the structure-directing agents. XPS analysis showed that there was more adsorbed oxygen and Mn³⁺ on the active crystal facets, which can provide strong kinetics for the electrochemical reaction. Consequently, the α-MnO₂ nanorods with {110} and {310} facets exhibited much higher pseudocapacitances of 120.0 F g^-1 and 133.0 F g^-1 than their α-MnO₂-200 counterparts (67.5 F g^-1). The theoretical calculations proved that the {310} and {110} facets have stronger adsorption capacity and lower diffusion barriers for sodium ions, which is responsible for the enhanced pseudocapacitance of MnO₂. This study provides a strategy to enhance the electrochemical performance of metal oxide, based on facet engineering.