Redox-active electrolyte for supercapacitor application

Abstract

This paper reports the electrochemical behaviour of supercapacitor carbon electrodes operating in different aqueous solutions modified by various redox-active species (hydroxybenzenes, bromine derivatives and iodide). Three dihydroxybenzenes with varying stereochemistry, i.e., -OH substitution, have been considered as electrolyte additives (0.38 mol L(-1)) in acidic, alkaline and neutral solutions. High capacitance values have been obtained, especially for the acidic and alkaline solutions containing 1,4-dihydroxybenzene (hydroquinone). Bromine derivatives of dihydroxybenzenes were also considered as the additive in alkaline solution for use as a supercapacitor electrolyte, and a significant increase in capacitance value was observed. The redox couple investigated next was an iodide/iodine system, where 2 mol L(-1) NaI aqueous electrolyte was utilized. In this case, the most promising faradaic contribution during capacitor operation was achieved. In particular, stable capacitance values from 300-400 F g(-1) have been confirmed by long-term galvanostatic cycling (over 100 000 cycles), cycling voltammetry and floating. The mechanism of pseudocapacitance phenomena was discussed and supported by electrochemical and physicochemical measurements, e.g., in situ Raman spectroscopy.

Redox Electrolytes in Supercapacitors

“…9 Furthermore, the effect of the substituent location of the hydroxyl groups on the electrochemical performance of redox electrolytes of dihydrobenzenes has been explained. 16 As expected, the wettability of the electrolyte on the electrode surface is very important in ensuring effective electro-sorption and transport of the redox species from the electrolytes. The roles of the pores of the carbon electrode in promoting the charge storage of the redox species has also been highlighted, because the various redox species from the electrolyte require optimal pore sizes to ensure effective electro-or chemisorption.…”

“…When involving simple or complex metal ions, redox reactions may be identified from the Pourbaix diagram. 16 However, thermodynamic data do not usually consider the influence of the environment in the pores on the redox reaction, such as de-solvation and adsorption. Therefore, the thermodynamically predicted electrode potential for a redox reaction may be altered when the reaction occurs inside the porous structure of the electrode.…”

“…11,16,[26][27][28][29] Research findings from these devices should however be analyzed with appropriate experimental and theoretical tools, and compared with that from batteries or other battery-like hybrid devices. In most cases, assessing the overall performance of SCs with redox electrolytes should not be based on capacitance or pseudocapacitance.…”

“…Still, it is neccesary to draw attention to some important issues inherent in the application of redox electrolytes in SCs. A number of these technical issues such as the existence of high charge capacities only at low (relatively impractical) voltages, 16 the irreversibility * Electrochemical Society Active Member. z E-mail: Chuang.Peng@nottingham.edu.cn; george.chen@nottingham.ac.uk of some redox species, 7,9 and the effects of the migration of redox species between electrodes, 17 have been addressed.…”

Quinone‐Decorated Onion‐Like Carbon/Carbon Fiber Hybrid Electrodes for High‐Rate Supercapacitor Applications

“…Motivated by the excellent electrical conductivity and fast ion electrosorption dynamics in OLC electrodes, OLC pseudocapacitors are ap romising technology.I np articular, three systems have been investigated:1 )adding redox-active metal oxidest oO LCs, [27,28] 2) functionalization with electrochemically active polymers like polyaniline, [29] and 3) decorating the onion surfacew ith redoxactive surface groupss uch as quinones. [15] The latter provides ap articularly attractive approach, as simple addition of quinones to the electrolyte [30] or dispersion of the carbon powder in quinone-containing solution can be used instead of more elaborate hydrothermal syntheses of metal-oxide particles. [15] The redox activity and high electrochemical reversibility of quinones has been studied before.…”