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A comprehensive comparison of electrochemical capacitors (ECs) with various aqueous alkali metal sulfate solutions (Li2SO4, Na2SO4, Rb2SO4, and Cs2SO4) is reported. The EC with a less conductive 1 mol L–1 Li2SO4 solution demonstrates the best long-term performance (214 h floating test) compared to the EC with a highly conductive 1 mol L–1 Cs2SO4 solution (200 h). Both the positive and negative EC electrodes are affected by extensive oxidation and hydrogen electrosorption, respectively, during the aging process, as proven by the SBET fade. Interestingly, carbonate formation is observed as a minor cause of aging. Two strategies for optimizing sulfate-based ECs are proposed. In the first approach, Li2SO4 solutions with the pH adjusted to 3, 7, and 11 are investigated. The sulfate solution alkalization inhibits subsequent redox reactions, and as a result, EC performance is successfully enhanced. The second approach exploits so-called bication electrolytic solutions based on a mixture of Li2SO4 and Na2SO4 at an equal concentration. This concept allows the operational time to be significantly prolonged, up to 648 h (+200% compared to 1 mol L–1 Li2SO4). Therefore, two successful pathways for improving sulfate-based ECs are demonstrated., ACS Applied Materials & Interfaces, 15 (19), ISSN:1944-8244, ISSN:1944-8252

The point of zero charge (PZC) is crucial for investigating molecular level charging mechanisms in energy storage systems, as demonstrated in electrochemical capacitors. Three electrochemical techniques were studied: cyclic voltammetry (CV), staircase potentio electrochemical impedance spectroscopy (SPEIS) and step potential electrochemical spectroscopy (SPECS) for two activated carbons (ACs) with 0.1 mol L− 1 aqueous solutions of LiNO3, Li2SO4, and KI. The porous AC charging process in aqueous electrolytes is a complex phenomenon; the ion mixing zone covered a wide potential region. Inadequate PZC determination could lead to obscure data evaluation, which could further provide a misguided mechanism description at the molecular level. In the aqueous solutions studied, the adsorption of specific ions and active participation of all ionic species in the electrical double-layer formation were considered. The SPECS technique was determined to be the most beneficial for the PZC determination in electrochemical quartz crystal microbalance applications.

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.

The model activated carbon spheres (ACSs) were optimized to be used as negative electrode in asymmetric electrochemical capacitors (ECs). A microporous commercial activated carbon was used as positive electrode. Carbon spheres (CSs) were synthesized by precipitation polymerization at room temperature followed by pyrolysis and activation. By enhancing the carbon:KOH ratio from 1:2 to 1:4, an increase in the specific surface area from 380 m2 g−1 to 2835 m2 g−1 (1:4) was achieved along with an increase in pore volume/size. Consequently, the electrochemical performance in the aqueous electrolyte was improved. After activation, low-temperature-pyrolyzed polymer spheres (400 °C) result in random-like particles with macroporous structure, while intermediate- and high-temperature-pyrolyzed materials (550 °C and 700 °C) lead to partially and totally conserved spheres. The interaction between the electrode material and electrolyte is important and is related to the texture, morphology, and surface chemistry of ACS. In 1 mol L−1 H2SO4, capacitance and rate handling are mostly affected by electrode morphology and wettability with the electrolyte. In 1 mol L−1 Li2SO4 electrolyte solution, texture and surface chemistry of the electrode material are crucial to obtain high-performance EC. Regardless of morphology, poor wetting properties of synthesized materials were found in Li2SO4, compared to the H2SO4 solution., Journal of Power Sources, 542, ISSN:0378-7753, ISSN:1873-2755

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.