Mechanisms of the performance fading of carbon-based electrochemical capacitors operating in a LiNO3 electrolyte

This paper reports on the ageing mechanisms in symmetric, carbon/carbon electrochemical capacitors operating with a 1 mol.L-1 LiNO3 aqueous electrolyte. The carbon electrodes were subjected to a constant polarization protocol (Le., floating) at various voltages and were analysed post-mortem by N-2 sorption at 77 K, Raman spectroscopy, Scanning Electron Microscopy (SEM), X-ray Photoelectron Spectroscopy (XPS), and elemental analysis. This approach allowed real capacitor performance deterioration to be simulated. Herein, the relationship between the electrochemical performance of high-voltage aqueous capacitors and the porosity of activated carbon cloth electrodes was investigated. Interestingly, two different ageing mechanisms, which mainly depended on the applied voltage, were identified. The oxidation of the positive electrode was the main reason for the capacitance fading. However, the oxidation of carbon was not the only reason, and it was postulated that solid-state deposits originating from CO and CO2 evolution block the electrode pores and induce capacitance fading. A proof-of-concept experiment presented in this paper confirmed a partial electrode surface recovery after ageing.