Electroless deposition of nanoscale MnO2 on ultraporous carbon nanoarchitectures: Correlation of evolving pore-solid structure and electrochemical performance

The self-limiting redox reaction of carbon nanofoam substrates with permanganate at room temperature in neutral-pH solutions produces conformal nanoscale MnO2 deposits throughout the macroscopic thickness (similar to 0. 17 mm) of the nanofoam structure. The nanoscale MnO2 morphology ranges from similar to 10 not layered ribbons and rods for a 4 h deposition to similar to 20 nm 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 MnO2 coatings grow and thicken within the nanofoam structure. The electrochemical capacitance of the resulting hybrid electrode structure is dominated by the pseudocapacitance of the MnO2 and increases with MnO2 loading (a function of the exposure time in permanganate), particularly at low charge-discharge rates and at ac frequencies < 0.1 Hz. The significant enhancement in mass-, volume-, and footprint-normalized capacitance at high MnO2 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 MnO2 deposition. (C) 2008 The Electrochemical Society.