Electrochemical capacitors (ECs) are high-power energy storage devices able to operate for thousands of cycles. Nevertheless, their calendar and cycle life is limited, especially when high voltage is applied. On the other hand, that parameter (voltage, U) significantly increases their energy values according to the equation E=0.5CU². Therefore, there is a particular need for the understanding of ageing and performance fade reasons in order to prolong supercapacitor operation.

Herein, the effect of ageing-related phenomena in water-based ECs is widely studied. LiNO₃ at various concentrations (1M and 5M) was utilized. According to the ageing criteria (IEC 62391-1) AC/AC systems were cycled at elevated voltages (1.5-1.7V) using floating test until 20% of initial capacitance fade. It has been observed that voltage increase by 0.1V interval results in a shortening of the operation time for about 50%, reflected by resistance increase and distorted voltammetry response. However, the utilization of highly concentrated LiNO₃ solution (5M) extends the electrochemical performance from 206h recorded for 1M LiNO₃ to 312h. In order to insightfully describe ageing phenomena, physicochemical carbon properties were monitored by post-mortem approach. It has been observed that remarkable S_BET decrease is observed on positive electrode only. The higher voltage applied the lower S_BET surface area of positive electrode, whatever the electrolyte concentration. TPD studies provided more the information about the surface of the AC electrode. Essentially, the oxidation of positive electrode was higher than the negative one Moreover, it is predicted that negative electrode suffers from some morphological transformations rather than surface functionality changes, likely caused by hydrogen evolution.

It seems that post-mortem approach which involves diversity of physio-chemical techniques can be definitely treated as an insightful approach for ageing research in the real capacitor system.