The charge storage mechanisms in manganese dioxide (MnO2) electrodes in aqueous and organic electrolyte containing univalent (Li+ and Na+) and bivalent (Mg2+ and Ca2+) cations are investigated by inductively-coupled plasma optical emission spectroscopy (ICP-OES), X-ray photoelectron spectroscopy (XPS), and electron energy-loss spectroscopy (EELS). MnO2 nanowires in various morphologies were grown electrochemically using a template synthesis method which produced free-standing nanowires which required no binder or conductive additive which might interfere with elemental analysis. This work presents the stoichiometry of the metal cation and manganese in various charged states of MnO2 by ICP-OES elemental analysis technique for the first time. In aqueous electrolyte, the ICP-OES results show that the ratio of electrolyte cation and manganese is very small (< 0.2) and no change is observed between the charged and discharged states indicating that proton adsorption and insertion play a predominant role in aqueous electrolyte for MnO2 charge storage process. However, when the proton contribution is eliminated by using organic acetonitrile electrolyte, the amount of electrolyte cation that is inserted into MnO2  increases significantly. In addition, enlarging the potential window in acetonitrile increases the cation to Mn ratio to greater than 1.5, implying a possible larger charge storage capacity for MnO2 than previously reported values. XPS and EELS are used to support the ICP-OES results and they showed that in both aqueous and organic electrolyte the valence state of manganese was varying from +4 and +3 during the charge and discharge process.

Charge-storage mechanism investigation of manganese oxides using inductively-coupled plasma optical emission spectroscopy

246th National Meeting (2013)

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