Electroless Deposition of Nanoscale MnO[sub 2] on Ultraporous Carbon Nanoarchitectures: Correlation of Evolving Pore-Solid Structure and Electrochemical Performance

Abstract

The self-limiting redox reaction of carbon nanofoam substrates with permanganate at room temperature in neutral-pH solutions produces conformal nanoscale MnO2MnO2 deposits throughout the macroscopic thickness (∼0.17mm)(∼0.17mm) of the nanofoam structure. The nanoscale MnO2MnO2 morphology ranges from ∼10nm∼10nm layered ribbons and rods for a 4 h deposition to ∼20nm∼20nm polycrystalline nanoparticles that form at long deposition times (20 h). The through-connected pore network of the carbon nanofoam is maintained at all deposition times (5 min to 20 h), although the average pore size shifts to smaller values and the cumulative pore volume decreases as the MnO2MnO2 coatings grow and thicken within the nanofoam structure. The electrochemical capacitance of the resulting hybrid electrode structure is dominated by the pseudocapacitance of the MnO2MnO2 and increases with MnO2MnO2 loading (a function of the exposure time in permanganate), particularly at low charge–discharge rates and at ac frequencies <0.1HzMnO2MnO2 mass loadings is accompanied by a modest increase in the Warburg resistance that develops as the pore size and void volume of the nanofoam substrate are reduced by internal MnO2MnO2 deposition.