Performance loss of aqueous MnO2/carbon supercapacitors at elevated temperature: Cycling vs. storage

Birnessite MnO2nanotubes of high specific surface area have been used as one electrode material in supercapacitors with a commercial-carbon-based second electrode ((NH4)2SO4(aq.) electrolyte). Assembled cells have been subjected to full electrochemical testing at temperatures ≤80 °C. At elevated temperatures, specific capacitance as high as 700 F g-1has been observed. The increase in specific capacitance with temperature has been found to be at a cost to cycling performance. Furthermore, the time spent at elevated temperatures "at rest" has been shown to have a major effect on device lifetime. It has been found that at 80 °C, without cycling, such devices lose all significant capacitance after 21 days. The findings herein are believed to have major implications for transport, storage lifetime and ultimate utilization of such systems.