Investigating the charge storage mechanisms in electrochemical capacitors: separating faradaic and non-faradaic processes using step potential electrochemical spectroscopy

In this work a new method for characterising the charge storage mechanisms in electrochemical capacitors has been developed. This technique, step potential electrochemical spectroscopy (SPECS), has been used to separate the capacitive contributions from faradaic and non-faradaic charges storage processes in a range of electrode materials. Activated carbon electrodes in aqueous electrolytes were analysed and shown to exhibit significant pseudo-capacitive behaviour which can be attributed to redox active functional groups and decomposition of the electrolyte at the anodic or cathodic potential limits. The charge storage behaviour was examined as a function of scan rate and was found to transition from predominantly double layer capacitive behaviour at high scan rates to battery-like behaviour, characterised by significant pseudo-capacitance contributions, at low scan rates. A range of manganese dioxide electrodes with different material properties have been studied to examine the effects that material composition, structure and morphology have on the performance of these materials as electrochemical capacitor electrodes. The relationships between each of the contributions to total capacitance and the material properties have been identified. The charge storage mechanism has been shown to transition from predominantly pseudo-capacitive behaviour to predominantly double layer behaviour as the scan rate is increased. The relative contributions from each of these processes to the overall specific energy and power of each manganese dioxide samples has been analysed and related to the properties of each material.