Volume 60, Nº 7 (2024)

The nanocomposite of a resorcinol-formaldehyde xerogel and carbon nanotubes after carbonation was obtained in the form of a composite carbon nanopaper (CCNP) with the thickness of 100–300 microns, the density from 0.1 g/cm³ to 0.5 g/cm³ and the electronic conductivity of more than 10 S/cm. The microporous structure of the nanopaper is formed by carbonized resorcinol-formaldehyde xerogel, and the mesoporous structure is formed by the nanotube framework. Previously, the characteristics of nanopaper electrodes in an aqueous electrolyte of 1 M H₂SO₄ were measured, where the maximum capacitance was 155 F/g (56 F/cm³). To work with an organic electrolyte, a method for activating CCNP with potassium hydroxide has been developed. In this paper the characteristics of electrodes made of activated nanopaper (a-CCNP) in an organic electrolyte 1 M 1,1-Dimethylpyrrolidinium tetrafluoroborate (DMPBF₄)/acetonitrile solution were measured. The capacitance in this electrolyte has been reached 70 F/g (27 F/cm³). According to measurements on a laboratory assembly of a symmetrical supercapacitor (SC) with electrodes made of CCNP, the characteristics are calculated when the SC operates in the mode of short pulse switching with an efficiency of EF = 95%. In an aqueous electrolyte of 1 M H₂SO₄ (U₀ = 1.0 V), the volumetric energy density was E_0.95,SC = 0.9 Wh/L and the volumetric power density was P_0.95,SC = 2.1 kW/L. In 1 M DMPBF₄/acetonitrile electrolyte (U₀ = 2.7 V), the design characteristics of the capacitor were: volumetric energy density E_0.95,SC = 3.8 Wh/L and volumetric power density P_0.95,SC = 2.0 kW/L. The specific characteristics of power SCs are compared with electrodes made of activated CCNP and of other carbon materials. In mass production, nanocomposite electrodes are estimated to be cheaper than activated carbon microfibers and significantly cheaper than graphene electrodes.

Express-method proposed recently for experimental determination of diffusion coefficients of electroactive ions inside a membrane and their distribution coefficients at the membrane/solution boundary (Russ. J. Electrochem., 2022, 58, 1103) is based on comparison of measured non-stationary current for the electrode/membrane/electrolyte solution system after a potential step with theoretical expressions for the current-time dependence. Application of this method for the study of bromide-anion transport across the membrane was performed in the previous work under the condition of the permselectivity of the membrane where the amplitude of the electric field inside its space was suppressed owing to a high concentration of non-electroactive counterions. Then, the coion (bromide anion) transport corresponded to the diffusional mechanism, for which the solution was available in an analytical form. This study considers for the first time a non-stationary electrodiffusional transmembrane transport of two singly charged ions (e.g. background cation М⁺ as the counterion and electroactive anion X^– as the coion) having identical values of their diffusion coefficients where the current passage induces a transient electric field in this spatial region, resulting in a deviation from predictions for the diffusional mechanism. It has been established that within the short time interval after a potential step from the equilibrium state of the membrane to the limiting current regime where the thickness of the non-stationary diffusion layer is significantly smaller than the thickness of the membrane, non-stationary distributions of the ion concentrations and of the electric field strength as a function of two variables (spatial and temporal ones, x and t) can be expressed via a function of one variable, Z(z), where z = x/(4Dt)^1/2, the form of which, depending on the ratio of the surface concentration of component X to the fixed charge density inside the membrane (X_m/C_f ) has been found by numerical integration. The limiting current varies with time according to the Cottrell formula (I ~ t^–1/2); dependence of the dimensionless current amplitude, i, on the ratio, X_m/C_f , has been found via numerical calculation; approximate analytical formula has also been proposed. In particular, it has been shown that the passing current is close to the diffusion–limited one for a low concentration of coions at the membrane/electrolyte solution boundary with respect to the concentration of immobile charged groups inside the membrane (X_m/C_f$\ll$1), whereas the migrational contribution to the ionic fluxes doubles the limiting current if the opposite condition (X_m/C_f$\gg$1) is fulfilled.