Strategy to assess the carbon electrode modifications associated with the high voltage ageing of electrochemical capacitors in organic electrolyte

This contribution presents a strategy to assess the electrode modifications leading to a fade of performance during high voltage operation of an electrical double-layer capacitor (EDLC) in organic electrolyte. To get rid of the commonly faced binder overlap in the post-mortem experiments, the activated carbon-based electrodes were here manufactured with a water washable carboxymethyl cellulose (CMC) binder, which was then eliminated by Soxhlet extraction from the aged (after ageing the EDLC by potentiostatic floating) and fresh electrodes. The washed fresh and aged electrodes were analyzed by temperature programmed desorption (TPD) and nitrogen adsorption/desorption to qualitatively and quantitatively determine the oxygenated functionalities created by ageing and to establish correlations with the pore volume modifications. By subtracting the TPD and gas adsorption data on the washed fresh electrode material from the results obtained on the washed positively and negatively aged electrode coatings, we could reveal that a relatively important amount of carboxylic, anhydride, carbonyl and quinone groups is created on the positive electrode due to its oxidation, contributing to reduce the volume corresponding to pores ranging between 1.2 and 2.6 nm. By contrast, under negative polarization, the functionalities initially observed for the fresh electrode are essentially transformed into more thermally stable ones, without any important change of the pore volume.