Multifunctional MnO2−Carbon Nanoarchitectures Exhibit Battery and Capacitor Characteristics in Alkaline Electrolytes

We demonstrate that, when distributed as nanoscale coatings on the walls of carbon nanofoam substrates, manganese oxides exhibit voltammetric signatures in LiOH-containing alkaline electrolytes that are characteristic of either electrochemical capacitors or batteries, depending on the potential range investigated. Pseudocapacitance is observed for positive potential ranges, and ex-situ X-ray absorption spectroscopy confirms that the native layered birnessite MnOx structure is retained as the Mn oxidation state is toggled between 3.72 and 3.43. When the cycling range is extended to more negative potential limits, well-defined reduction and oxidation features are observed, with an associated reversible change in the Mn oxidation state of 0.71 after 25 cycles. For these deep-discharge conditions, high charge-storage capacities are facilitated by the reversible interconversion of birnessite and γ-MnOOH forms of the nanoscale MnOx coating. Solid-state 7Li NMR is used to investigate the role of Li+ from the alkaline electrolyte in enhancing the cycling stability of the MnOx−carbon nanofoam.