Abstract
Variations of the double layer capacitances (DLCs) at a platinum electrode with concentrations and kinds of salts in aqueous solutions were examined in the context of facilitating orientation of solvent dipoles. With an increase in ionic concentrations, the DLCs increased by ca. a half and then kept constant at concentrations over 1 mol dm−3. This increase was classically explained in terms of the Gouy–Chapman (GC) equation combined with the Stern model. Unfortunately, measured DLCs were neither satisfied with the Stern model nor the GC theory. Our model suggests that salts destroy hydrogen bonds at the electrode–solution interface to orient water dipoles toward the external electric field. A degree of the orientation depends on the interaction energy between the salt ion and a water dipole. The statistical mechanic calculation allowed us to derive an equation for the DLC as a function of salt concentration and the interaction energy. The equation took the Langmuir-type in the relation with the concentration. The interaction energy was obtained for eight kinds of salts. The energy showed a linear relation with the interaction energy of ion–solvent for viscosity, called the B-coefficient.
Highlights
Academic Editor: Masato SoneAccepted: 17 November 2021Double-layer capacitances (DLCs) used for an electric energy storage [1,2] have been developed to enhance the output energy, the power, and the cycle stability [3,4]
The experimentally conclusive results are that: (i) values of double layer capacitances (DLCs) increase with ionic concentrations and reach saturated values at most
Times of asthe large as the ion-free (ii) the values are not at all proportional to c1/2 by the GC theory; (iii) the Stern model is not valid in the context of the concentration dependence; (iv) ionic effects are caused by ion–solvent interaction rather than ion-ion interaction; (v) variation of C1 with c depends on ionic properties
Summary
Double-layer capacitances (DLCs) used for an electric energy storage [1,2] have been developed to enhance the output energy, the power, and the cycle stability [3,4]. The development has been directed to: Published: 18 November 2021 (a). Publisher’s Note: MDPI stays neutral (b) (c). Received: 1 October 2021 with regard to jurisdictional claims in published maps and institutional affiliations.
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