Electric Field-Dependence of Double Layer Capacitances by Current-Controlled Charge-Discharge Steps

Abstract

Voltage vs. time curves of double layer capacitances (DLCs) by current-controlled charge and discharge steps have been recognized to be composed of triangular waves. They are deviated slightly from triangles from the viewpoint of the time dependence or the constant phase element of the DLC. In order to evaluate the deviation, we measured DLCs of a platinum (Pt) electrode in KCl solution by current-control. Each time-voltage curve was convex rather a line, and was followed by the power law. Even if the time dependence was subtracted from each curve, the enhancement of the DLC was noticeable with an increase in the time well as the voltage. It can be attributed to the electric field effect, in which dipoles of solvents are oriented on an electrode so strongly that the DLC may be increased. The field dependence can be justified with the kinetic theory of interacting dipoles of solvents on an electrode through the observed linearity of the logarithmic DLC with the net voltage. This concept was applied to a commercially available super-capacitor to demonstrate a significant contribution of the field effect.

The Effect of Surfactants on Double-Layer Capacitance Induced by Electric Fields

“…Therefore, an external electric field or applied net voltage will have a nonnegligible effect on the orientation of the solvent dipoles on the electrode. Our previous work 9 found that the influence of the electric field is significant by removing the time dependence of capacitance, 10 and this behavior was also verified in a commercial supercapacitor. Zhu et al 11 reported voltage-dependent capacitance that forms an "ω" shape at the high-sweep-voltage range for a metal−insulator−metal (MIM) capacitor.…”

Double-Layer Capacitances Caused by Ion–Solvent Interaction in the Form of Langmuir-Typed Concentration Dependence

“…Values of λ obtained from the slopes (1 − λ) ranged from 0.09 to 0.11. Therefore, Equation (9) holds commonly for all the salts and the concentration ranges used here. Non-zero values of λ suggest the CPE behavior.…”

“…Especially, item (b) belongs to a simply chemical handling in examining various kinds of ions and their concentrations in appropriate solvents. It only deforms charge-discharge curves by the ohmic drop [9], but does not essentially improve DLCs because a DLC is caused mostly by orientation of solvent dipoles for relaxing the electric field in the Helmholtz layer [10,11].…”

Charging a electrochemical double layer supercapacitor exhibiting a distribution of relaxation times

“…In the literature, nonlinear charging curves have often been interpreted in terms of a voltage-dependent capacitance [25], which has been explained in terms of the perfectly smooth theory of electrical double layers [26] or due to voltage-dependent dipole orientation [27]. However, supercapacitors composed of micro and nanostructured carbon immersed in electrolyte do not have flat active surfaces, but are made of irregular surfaces supporting both dissipative and charge storage processes.…”