Effects of electrode layer composition/thickness and electrolyte concentration on both specific capacitance and energy density of supercapacitor

Abstract

In this paper, the effects of several experimental conditions, such as electrode layer binder content, conducting carbon content, electrode layer thickness, as well as electrolyte concentration, on both the specific capacitance and energy density of a BP2000 carbon-based supercapacitor are investigated using both cyclic voltammetry and a galvanic charging–discharging curve. The electrode layer studied contains Super C45 carbon as the conducting additive, PTFE as the binder, and Na₂SO₄ as the aqueous electrolyte, respectively. With the purpose of optimizing the electrode layer structure, 15 wt% of Super C45 and 5 wt% of PTFE in the electrode layer with a thickness of 100 μm, are found to be the best composition in terms of improving both specific capacitance and energy density. Regarding the effect of electrolyte concentration in the range of 0.1–1.0 M, 0.5 M of Na₂SO₄ gives the best performance.

Introduction

In recent years, electrochemical supercapacitors (ESs) have been recognized as key energy efficiency devices for rapid energy storage and delivery. This is due to their high power density, long lifecycle, high efficiency, wide range of operating temperature, environmental friendliness, and safety, as well as serving as a bridging function for the power/energy gap between traditional dielectric capacitors (which have high power output) and batteries/fuel cells (which have high energy storage) [1], [2]. With these advantages, ES have become very competitive for applications such as electric hybrid vehicles, digital communication devices such as mobile phones, digital cameras, electrical tools, pulse laser technique, uninterruptible power supplies, and storage of the energy generated by solar cells [3]. However, several challenges such as low energy density, high cost, and high self-charge rate have limited its wider applications.

Regarding low energy density, one of the major limitations is induced by the low specific capacitance of the electrode materials used. At the current state of technology, the most practical material for ES electrode layer is nano-scaled carbon, such as activated carbon, carbon aerogels, carbon nanotubes, templated porous carbons and carbon nano fibers. Although some hybrid materials between carbon and metal oxides such as MnO_x and RuO_x have been used to construct EC electrode layers, they are still in a stage of research and development [3]. Therefore, nano-scaled carbons are still the preferred ES electrode materials.

In general, the amount of energy stored in nano-scaled carbon-based supercapacitors is determined by the specific capacitance of the carbon, the electrical/ionic conductivity of the electrode layer, the ionic conductivity of the electrolyte filled separator, as well as the voltage window of the electrolyte [4]. Regarding specific capacitance of the electrode layer, specific surface area, pore size, and electrode structure of electrodes are dominating factors [5], [6], [7], [8]. Two carbon surfaces exist in the electrode layer: one is the external surface and the other is the inner surface, and each surface can give their corresponding specific capacitances [9], [10], [11], [12]. It follows both surfaces make contributions to the total specific capacitance. From our experiments with Carbon black BP2000-based material, we observed that the contribution from the external carbon surface to the total capacitance is around 40% and the other 60% contribution came from the inner microporous surface. Barbieri et al. [13] studied the relationship between specific capacitance of various active carbon materials and their specific surface areas, and found that the specific capacitance could increase linearly with specific surface area within a specific range of surface areas. Shi et al. [5] also found a linear relationship between specific capacitance and surface area. These results indicate that carbon surface area plays an important role in increasing specific capacitance.

In addition to using carbon material with high surface area, for practical purposes, type of binder and its concentration, thickness of the electrode layer, as well as preparation procedure are also important in achieving high specific capacitance, and in turn high supercapacitor performance. The purpose of this work is to optimize the supercapacitor's performance by investigating the effects of these factors on the electrode layers.

In this paper, we report our attempt in the optimization of carbon based supercapacitor performance using in-house fabricated electrode layers and modified test cell apparatus. Several factors affecting electrode preparation were investigated including binder content, conducting carbon content, as well as electrode layer thickness. Electrochemical methods such as cyclic voltammetry (CV) and galvanic charging-discharging curve (GCC) were employed for testing electrode performance.