The recent research on electrochemical capacitors (ECs) is essentially dedicated to increase their extractable energy which is directly related to the maximum operating voltage. Since this voltage is limited by the electrochemical stability of the electrode/electrolyte system [1, 2], a detailed study of ECs degradation under high voltage hold is a cornerstone to validate the system. As already observed, the main ageing symptom of ECs, initiated by high voltage, is the formation of oxygenated functionalities and the evolution of volatile electrolyte decomposition products which may cause a partial blockage of carbon electrodes porosity, electrolyte depletion, worsening of the electrode-current collector contact and internal pressure increase, leading to resistance increase and/or capacitance loss [3, 4]. In aqueous medium, the most pronounced contribution is due to H₂ and CO evolution appearing at the negative and positive electrode, 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 oxidized, although one could expect that it should not occur under negative polarization of this electrode. Yet, H₂O₂ which is first produced at the positive electrode as by-product of water electrolysis, migrates through the separator towards the negative compartment, where it is further involved in oxidation of the carbon surface [7].

In this study, operando electrochemical on-line mass spectrometry (EOMS) has been used to quantitatively detect the gases produced during high voltage hold of ECs in aqueous electrolytes and to reveal the realistic degradation mechanisms of the system. The qualitative and quantitative analysis of the various reactions occurring at the carbon/electrolyte interface of the positive and negative electrodes is based on correlating the charge recorded as leakage current with the amount of charge spent at each electrode to: i) produce gases during potentiostatic floating taking into account MS signals and pressure records and ii) oxidize the surface of both electrodes taking into account post-mortem surface functionality analyses realized by temperature programmed desorption (see Figure 1). During continuous floating at 1.5 V, the leaking charge which passes through EC utilizing lithium sulfate electrolyte and carbon based electrodes is spent for water decomposition with formation of gaseous products, oxidation of carbon surface on both electrodes, and other parasitic reactions (ionic charge diffusion, formation of by-products during water electrolysis). Besides, it will be shown that system degradation is kinetically dependent on the amount of actives sites present on the electrodes surface. This strategy will then be extended to analyze the causes of performance degradation for ECs in organic electrolytes.

Acknowledgements

The Polish National Science Center (NCN) is acknowledged for supporting the OPUS project UMO 2014/15/B/ST4/04957.

References

1. Arulepp, J. Leis, M. Lätt, F. Miller, K. Rumma, E. Lust, A.F. Burke, J. Power Sources 162 (2006) 1460.
2. Ratajczak, K. Jurewicz, P. Skowron, Q. Abbas, F. Béguin, Electrochim. Acta 130 (2014) 344.
3. Azaıs, L. Duclaux, P. Florian, D. Massiot, M.-A. Lillo-Rodenas, A. Linares-Solano, J.-P. Peres,
4. Jehoulet, F. Béguin, J. Power Sources 171 (2007) 1046.
5. Ratajczak, K. Jurewicz, F. Béguin, J. Appl. Electrochem. 44 (2014) 475.
6. He, K. Fic, E. Frąckowiak, P. Novák, E.J. Berg, Energy Environ. Sci. 9 (2016) 623.
7. Batisse, E. Raymundo-Piñero, ACS Appl. Mater. Interfaces 9 (2017) 41224.
8. Berenguer, J.P. Marco-Lozar, C. Quijada, D. Cazorla-Amoros, E. Morallon, Carbon 47 (2009) 1018.