Cu²⁺ intercalation activates bulk redox reactions of MnO₂ for enhancing capacitive performance

Activating bulk redox reaction is important for enhancing capacitive performance. Here, Cu2+ intercalated delta-MnO2 is prepared through an ion exchange strategy to activate the bulk redox reaction. The sample shows an average Mn valence state of 3.01 and an increased electrical conductivity of 2.67 x 10(-4) S cm(-1). The bivalent cation intercalation structure shows higher diffusion-controlled contribution than the counterpart. Density functional theory calculation suggests that intercalated Cu2+ endows birnessite with metallic characters by enhancing the density of state around Fermi level. Bader charge analysis demonstrates that Cu2+ intercalation decreases the average Mn valence state, thus creating more redox-active sites in bulk MnO2. This work not only establishes a reliable method for activating bulk redox reactions for further improving the specific capacitance of MnO2-based electrodes, but also provides new insights on understanding the charge storage mechanism of MnO2.