The behavioural characteristics of two classes of electrochemical supercapacitors, based respectively on porous (ie activated) carbon electrodes and polypyrrole-coated carbon electrodes, have been compared by cyclic voltammetry in order to identify the electrode/electrolyte combinations suitable for high power density applications. It is shown that the large specific capacitances of activated carbon electrodes (in Fg(-1)) are not accessible during high rates of charge/discharge because ions from the electrolyte are slow to reach the internal surfaces of the carbon electrodes. Much larger charge/discharge rates (and hence power densities) are achievable with polypyrrole-coated carbon electrode/polymer electrolyte combinations. These concepts are now being incorporated into working prototypes for ultimate scaleup to meet the needs of electric vehicle applications.