Ageing mechanisms in electrochemical capacitors with aqueous redox-active electrolytes

Abstract

This paper reports on the long-term performance of electrochemical capacitor operating in 1 mol⋅L−1 KI solution subjected to two ageing protocols, i.e., galvanostatic cycling and potentiostatic floating. Ageing tests have been performed at similar voltage conditions (1.5 V), estimated from the coulombic and energetic efficiency as well as the self-discharge analysis. The end-of-life criterion (80% of initial capacitance retained) was the same for both procedures. The influence of the ageing methodology on the structural and textural changes in electrode material has been discussed. The results obtained allowed for failure mechanisms description in the floating tests in comparison to traditional galvanostatic cycling. It has been stated that de facto the structural changes do not diminish the long-time performance. Notwithstanding, it has been found that galvanostatic cycling has a definitely stronger and detrimental impact on the carbon structure. Interestingly, both ageing methodologies affect the electrode porosity in a similar way. The nitrogen adsorption measurements indicated that the specific surface area decrease correlates with the capacitance fade. Changes observed in the pore size distribution allowed us to conclude that the porosity of both electrodes is blocked by adsorbed and/or deposited species. It looks that floating causes higher pore clogging especially for positive electrode.