Adjustment of electrodes potential window in an asymmetric carbon/MnO2 supercapacitor

The electrodes mass ratio of MnO₂/activated carbon supercapacitors has been varied in order to monitor its influence on the potential window of both electrodes and consequently to optimize the operating voltage. It appeared that the theoretical mass ratio (R = 2), calculated considering an equivalent charge passed across both electrodes, is underestimated. It was demonstrated that R values of 2.5-3 are better adapted for this system; the extreme potential reached for each electrode is close to the stability limits of the electrolyte and active material, allowing a maximum voltage to be reached. During galvanostatic cycling up to 2 V, the best performance was obtained with R = 2.5. The specific capacitance increased from 100 to 113 F g⁻¹ during the first 2000 cycles, then decayed up to 6000 cycles and finally stabilized at 100 F g⁻¹. SEM images of the manganese based electrode after various numbers of thousands cycles exhibited dramatic morphological modifications. The later are suspected to be due to Mn(IV) oxidation and dissolution at high potential values. Hence, the evolution of specific capacitance during cycling of the asymmetric capacitor is ascribed to structural changes at the positive electrode.