Recently, the research on carbon/carbon electrochemical capacitors (ECs) is essentially dedicated to increase their extractable energy which is directly related to the maximum operating voltage of the electrode/electrolyte system [1, 2]. Long-time operation at maximum voltage (so-called potentiostatic floating) accelerates ECs ageing, revealed by loss of capacitance, increase of resistance and internal pressure. The worsening of these three parameters is related to carbon surface oxidation and/or pore blockage by electrolyte decomposition products, together with ionic starvation in the electrode due to decrease of electrolyte concentration and gas evolution which entails weakening of the adhesion between the active mass and the current collectors [3, 4].
In aqueous medium, the most pronounced symptom of the degradation phenomena is H2 and CO evolution appearing at the negative and positive electrodes, respectively [5, 6]. Besides, the surface of the aged positive carbon electrode is modified by new oxygenated groups, while the aged negative electrode is also functionalized, although one could expect that it should not occur at this electrode under reductive conditions. Indeed, H2O2 which is first produced at the positive electrode as by-product of water electrolysis, migrates through the separator towards the negative compartment being then further involved in oxidation of the carbon surface .
In this context, it will be shown that it is possible to obtain information about all the realistic mechanisms of ECs ageing as well as to determine the relative impact of each process on the system degradation by using a combination of operando analyses consisting of electrochemical measurements, mass spectrometry and cell pressure records, together with post-mortem surface functionality analysis of individual carbon electrodes by temperature programmed desorption (TPD). On the example of an electrochemical carbon/carbon capacitor in aqueous Li2SO4, the approach is based on assigning the charge passing through the cell under potentiostatic floating (known as leakage current) to the charge: i) spent at each electrode for production of gases (taking into account operando EMS analysis and pressure records); ii) utilized to oxidize the electrodes surface (from post-mortem surface functionality analyses realized by TPD); iii) being redistributed throughout the electrochemical cell (taking into account the steady-state leakage current measurements).
5:00 PM–7:00 PM Apr 23, 2019 (US - Arizona)
PCC North, 300 Level, Exhibit Hall C-E