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This paper reports on the ion fluxes at the interfaces of various porous carbon electrodes/aqueous solutions of alkali metal cations (Na+, K+ and Rb+) and iodide anions, monitored by an electrochemical quartz crystal microbalance (EQCM). Different electrode material compositions as well as various electrolyte concentrations have also been considered. By tracking the ions during electrochemical polarization, we aimed to identify the reasons for the fading capacitor performance that occurs during long-term operation. The mass change profile suggests that hydroxide anions are responsible for counterbalancing the charge stored on the negative electrode. Furthermore, we found that iodide-based species are physically adsorbed on the carbon surface immediately after the electrodes come into contact with the electrolyte, regardless of the textural properties of the activated carbon used or electrode composition. Apart from the qualitative description, the mass change profiles allow the sequence of pore occupation to be determined. Additionally, the solvation numbers for alkali metals (Na+, K+ and Rb+) and hydroxide-based species have been determined. It is claimed that the solvation number is strongly affected by the electrolyte composition. Apparently, the concentration of water molecules available in the electrolyte cannot be neglected. The outcome of this research has fundamental and application context.

International audience; Various alkali metal (Li+, Na+, K+, Rb+, and Cs+) chlorides with Pluronic F127 were used as a soft-salt template for tuning the textural and structural properties of carbon. Highly conductive metal hydroxide solutions, where the cations are the same as those in the salt template, have been used as electrolytes. By increasing the size of the cation in the template, the textural properties of carbon, such as the specific surface area, micropore volume, and pore size, were remarkably enhanced. It directly translates to an increase in the specific capacitance of the electrode material. For a constant current charge/discharge at 0.1 A g–1, the electrode composed of LiCl-T and operating with 1 mol L–1 LiOH demonstrates the capacitance of 124 F g–1, whereas CsCl-T with the same electrolyte has a capacitance of 216 F g–1. Moreover, the materials show the highest capacitance retention (up to 75%) vs. the current regime applied when the cation used during synthesis matches the cation present in the electrolyte (i.e., LiCl-T with LiOH). Interestingly, capacitance normalized by specific surface area has been found to be the highest when LiOH solution is applied as an electrolyte. Thus, for this metric, the size of ions seems to be a crucial parameter. The importance of mesoporosity is highlighted as well by using materials with a similar fraction of micropores and with or without mesopores. Briefly, the presence of mesopore fraction proved to be essential for improved capacity retention (69% vs. 30%). Besides textural properties, the graphitization degree impacts the electrochemical performance as well. It increases among the samples, in accordance with cation-π binding energy, e.g., LiCl-T is the most “graphitic-like” material and CsCl-T is the most disordered. Thus, the more graphitic-like materials demonstrate higher rate capability and cycle stability.

This paper reports the successful attempts of symmetric aqueous electrochemical capacitor (EC) lifespan extension. A novel exploitation procedure, i.e., alternate floating, is herein proposed. Since only the positive electrode ages remarkably in EC systems with LiNO3 as the electrolyte, it is demonstrated that alternate polarization allows the properties of the negative electrode to be fully exploited. In our approach both electrodes play the role of the positive electrode and the negative one alternately; carbon electrodes were subjected to standard and alternate voltage-holding polarization. To observe the capacitor behavior under alternate floating at different conditions, various electrolyte concentrations of LiNO3 (0.2–5.0 mol L−1) were used. It was found that the utilization of alternate polarization during the aging test allows the lifespan of the capacitor to be noticeably prolonged. Moreover, the lifetime improvement strongly depends on the electrolyte concentration. A maximum improvement of 83% of capacitor operation time is achieved for 0.2 mol L−1 LiNO3, i.e., almost doubling the operation time. The aged electrodes were tested postmortem using SEM, TEM, XPS and N2 sorption at 77 K. The results demonstrated that the alternate floating protocol ages both electrodes almost equally.