A synchrotron X-ray powder diffraction and step potential electrochemical spectroscopy study on the change in manganese dioxide capacitive behaviour during cycling

In this work, the factors leading to the overall capacitance of manganese dioxide electrochemical capacitor materials, i.e. the double layer and pseudocapacitive behaviour, have been separated using Step Potential Electrochemical Spectroscopy (SPECS). In addition, variations in these parameters have been linked to regions of structural change in the material. In regions where ion diffusion into the bulk of a material is a facile process, the pseudocapacitive charge storage shows an increase. In addition, the double layer capacitance has been shown to increase with reduced protonation of the material surface. This has been attributed to variations in electronic resistance and also an increase in the number of sites that can accommodate electrostatic charge. Further, the origin of the residual current seen in previous experiments has been determined. In structures where ion diffusion into the bulk is a facile process, the residual currents are related to this process. Where there is no bulk ion diffusion, the residual currents have been assigned to the production of soluble Mn2+.