Ammonia-induced (111_) facet exposure of δ-MnO2 with enhanced electrochemical reactivity for high-performance supercapacitor

Abstract Crystal facet engineering is an effective way to modulate the intrinsic physicochemical properties of metal oxide materials. It is crucial to improve the ratio and site density of active facet to enhance the pseudocapacitance of electrodes. In this work, we present a simple method by using ammonia as a structure-directing agent to increase the exposure ratio of (11 1 ‾ ) active plane, and thus enhance the electrochemical performance of δ-MnO2. XRD and TEM results demonstrated that the presence of ammonia during synthesis can promote the exposure of (11 1 ‾ ) facet. As expected, the specific capacitance (192.0 F/g at 0.5 A/g) is increased by 68% compared with that of (001) facet-dominated MnO2 without ammonia. Theoretical calculations shown that the (11 1 ‾ ) plane of δ-MnO2 has more advantages in terms of lower adsorption energy and smaller ion diffusion barrier than the (001) plane during the faradaic reactions. Moreover, the specific surface area of the materials is largely increased with the presence of ammonia. Thus, both the increased active sites and enhanced electrochemical reactivity of the (11 1 ‾ ) plane enable the excellent performance of as-prepared δ-MnO2. The current findings will offer more choices for the design of metal oxide electrodes with enhanced electrochemical reactivity based on crystal facet engineering.