Spectroelectrochemical characterization of nanostructured, mesoporous manganese oxide in aqueous electrolytes

Manganese oxide (MnOx) xerogel and ambigel films were investigated as ultracapacitor electrodes in unbuffered aqueous electrolytes. These nanostructured, mesoporous gels, derived from the sol-gel reaction of NaMnO4 DT H2O and fumaric acid, were cast as continuous thin films on indium-tin oxide glass. Further processing of the wet gels produces birnessite Na8MnO2 DT xH2O nanoarchitectures in either a densified xerogel or a highly porous ambigel form. The pseudocapacitances of these mesoporous MnOx films in unbuffered aqueous electrolytes are characterized by spectroelectrochemical methods. Monitoring the changes in the optical absorbance during electrochemical cycling provides an in situ measure of the electronic state of the MnOx as the electrode is charged and discharged. Manganese oxide films prepared as highly porous ambigels exhibit a more capacitive-like character than xerogels, and also undergo less capacity loss with cycling.