Enhancing Specific Energy and Power in Asymmetric Supercapacitors - A Synergetic Strategy based on the Use of Redox Additive Electrolytes.

Arvinder Singh,Amreesh Chandra

doi:10.1038/srep25793

Abstract

The strategy of using redox additive electrolyte in combination with multiwall carbon nanotubes/metal oxide composites leads to a substantial improvements in the specific energy and power of asymmetric supercapacitors (ASCs). When the pure electrolyte is optimally modified with a redox additive viz., KI, ~105% increase in the specific energy is obtained with good cyclic stability over 3,000 charge-discharge cycles and ~14.7% capacitance fade. This increase is a direct consequence of the iodine/iodide redox pairs that strongly modifies the faradaic and non-faradaic type reactions occurring on the surface of the electrodes. Contrary to what is shown in few earlier reports, it is established that indiscriminate increase in the concentration of redox additives will leads to performance loss. Suitable explanations are given based on theoretical laws. The specific energy or power values being reported in the fabricated ASCs are comparable or higher than those reported in ASCs based on toxic acetonitrile or expensive ionic liquids. The paper shows that the use of redox additive is economically favorable strategy for obtaining cost effective and environmentally friendly ASCs.

Highlights

The operating voltage window of an asymmetric cell is a convoluted effect of overpotential provided by the electrolytes and the difference of work functions of negative (Φn) and positive (Φp) electrodes i.e., Φn −Φp16,17,30
From the FESEM and TEM micrographs (Fig. 1a–f), it is clear that MWW comprises of WO3 nanostructures and partially disentangled Multiwall carbon nanotubes (MWCNTs) lying underneath these nano-plates
MWZ possessed nano-sized ZrO2 particles attached to the surface of MWCNTs (MW)

Summary

Introduction

The operating voltage window of an asymmetric cell is a convoluted effect of overpotential provided by the electrolytes and the difference of work functions of negative (Φn) and positive (Φp) electrodes i.e., Φn −Φp16,17,30. ASCs fabricated using TMOs with a large difference in their respective work functions and neutral aqueous electrolytes (having highly solvated ions) may be operated up to voltages as high as 2.2 V. Very few studies have been undertaken to explore the use of redox additives in the 3-electrode or symmetric cells[33,34,35,36] In these reports, galvanostatic charge-discharge curves are highly distorted exhibiting either a wide plateau region and/or non-linearity within a given discharge voltage range. Increase of ~105% in the specific energy value was observed with good cyclic stability even after 3,000 charge-discharge operations With such high specific energy and power values, the proposed ASCs have the capacity for large scale integration in applications such as portable electronics devices, back-up power supplies, hybrid electric vehicles and energy harvesting devices

Methods

Results

Discussion

Conclusion