Significant Performance Enhancement in Asymmetric Supercapacitors with Redox Additive Aqueous Electrolyte and Trade-Off Between Capacitance/Cycling at Elevated Temperatures

Abstract

Supercapacitors (SCs) are now competing with Li-ion batteries for large scale use in various niche technologies owing to their distinctive merits of rapid charging-discharging process, long lifespan, superior durability, high specific power and low maintenance. As we develop better understanding of the thermodynamics, kinetics and ion transport mechanism in electrode materials used in supercapacitors, rapid growth in this energy storage technology is envisaged. In addition to electrode materials, supercapacitor geometries and configurations will also have to be investigated to bring step change in device performance. For example, development of asymmetric supercapacitors (ASCs) has seen tremendous growth in recent times. Most used SCs today are mostly fabricated using nano-structured transition metal oxides (TMOs). TMOs such as MoO3, V2O5 and WO3 with higher work function or electron chemical potential act as hole-injection materials and hold great promise for application as negative electrode materials. In comparison, TMOs like ZrO2, MnO2 and SnO2, etc with low work function or electron chemical potential behave like electron-injection materials and are mostly suitable for positive electrodes. Such TMOs have rich redox chemistry (oxidation/reduction, intercalation/de-intercalation, chemisorption, etc.) but overcoming their limited specific power remains a challenge. As a result, composite of TMOs with multiwall carbon nanotubes (MWCNTs) is becoming popular. 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-Φp. Therefore, 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. The methodology of carefully unbalancing the device has also been recently proposed to increase the operating voltage window. In this paper, we show a novel strategy i.e., use of optimized concentration of redox additive electrolyte to bring significant enhancement in the specific energy whilst maintaining power of asymmetric supercapacitors. Very few studies have been undertaken to explore the use of redox additives in the 3-electrode or symmetric cells. It is shown that the charge-balanced ASCs can be operated up to 2.2 V leading to specific energy and power as high as ~65 Wh kg-1 and ~950 W kg-1, respectively. The specific energy value is significantly enhanced on addition of the optimized quantity of redox additive viz., potassium iodide (KI). More specifically, 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. In most of these applications, temperature effect of the storage device will be important. The fabricated ASCs show stable performance upto 70oC, which make them ideal for above mentioned applications. It is shown that the temperature mainly affects the capacitance fade during galvanostatic charging-discharging. The improvement in the specific capacitance at elevated temperatures is strongly governed by the activation energy of diffusions for ionic species and chemical potential of ionic species. Besides, the capacitance fade at higher temperatures is primarily attributed to (a) the total “time spent” at a given temperature during cycling and (b) a reduced kinetic barrier for iodine/iodide redox pairs at the positive electrode/electrolyte interface. This report provides significant information about the effect of temperature on the electrochemical performance of ASCs based on redox additive aqueous electrolytes.