Measuring time-dependent heat profiles of aqueous electrochemical capacitors under cycling

A calorimetric technique was developed for determining time-dependant heat profiles of electrochemical capacitors. The profiles were extracted from the temperature change of the capacitor during charge–discharge current cycles. The technique is described and its good performance is demonstrated. Experiments were carried out on symmetric and asymmetric aqueous capacitors prepared with activated carbon and manganese oxide as active materials. Heat in double-layer and hybrid capacitors, C–C and C–MnO2, varied with time, according to special profiles depending on storage processes. In carbon double-layer capacitors, the heat component was decomposed into a reversible and an irreversible term. Results suggested that irreversible heat was caused by the Joule loss through the porous structure and the reversible heat by the ion adsorption on the carbon surface. Moreover, endothermic and exothermic processes were observed on the MnO2 electrode of the asymmetric capacitor, over the charge and the discharge respectively, proportional to the charge rate. These results suggested that heat in the MnO2 electrode originated from the redox reactions and ion adsorption on the active material.