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

Ridong He,Koichi Jeremiah Aoki,Jingyuan Chen

doi:10.3390/electrochem1020015

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.

Highlights

A super-capacitor is a candidate back-up power supply because it can provide high current density in comparison with chemical batteries such as lithium ion batteries [1,2]
A capacitance generally exhibits time and voltage dependence [14,15,16,17,18], and a double layer capacitances (DLCs) was examined in detail
An example of iterative charge–discharge curves is shown in Figure 1A, where the constant current (0.7 μA) was applied to the Pt wire immersed into KCl solution for a second from the open circuit potential (0.21 V vs. Ag|AgCl) and the current direction was reversed

Summary

Introduction

A super-capacitor is a candidate back-up power supply because it can provide high current density in comparison with chemical batteries such as lithium ion batteries [1,2]. It belongs to a double layer capacitor (DLC) with a large net area of the electrode. The high current density [3] can be caused by physical processes of localization of charge, such as orientation of dipoles or heterogeneous distribution of ions rather than electrochemical reaction rates in chemical batteries. Experimentally charging curves are deviated from the linearity so that the slope of the tangent line gets small with an increase in voltage or the time [4]. A capacitance generally exhibits time and voltage dependence [14,15,16,17,18], and a DLC was examined in detail

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