Electrochemistry Serving People and Nature: High‐Energy Ecocapacitors based on Redox‐Active Electrolytes

Abstract

Positive Poles: A new type of electrochemical capacitor with two different aqueous solutions, separated by a Nafion membrane is described. High capacitance values as well as excellent energy/power characteristics are reported and discussed. The neutral character of the applied electrolytes makes this capacitor an environmentally friendly, easy to assemble, and cost-effective device for energy storage.

Redox Electrolytes in Supercapacitors

“…Practically, the increased charge storage capacity of AC electrodes was demonstrated in the aqueous solutions of mixed FeSO 4 , CuSO 4 and H 2 SO 4 .…”

“…Depending on the surface properties of the electrode material, redox species could either be electroadsorbed in the electrode material (porous carbon electrodes), 6,7 or interact with the reactive edge sites and functional groups on the surface of the carbon material for example carbonyl groups (surface functionalized carbon materials). 3,4,26 Also, if the electrode material displays pseudocapacitance or non-capacitive activity, the electron transfer reactions occurring in the lattice of the electrode could be shuttled by the redox species of the electrolyte. 14,30 In order for the SC with a redox electrolyte to display satisfactory stability in repeated charging-discharging cycles, it is important for the redox specie to show fast and reversible electrochemical activity in the supporting electrolyte.…”

“…These surface oxy-groups can then be utilized to chemically adsorb metal cations and enable surface redox reactions. Examples of this mechanism include "(+) AC | CuSO 4 -FeSO 4 -H 2 SO 4 ", 3 "(+) functionalized carbon surface | VOSO 4 ", 4 and "(+) functionalized carbon surface | CuCl 2 -HNO 3 ", 26 and these are further discussed below.…”

“…However, in recent years, attention has been drawn to exploitation of the redox activity from the electrolyte in a bid to improve the performance of SCs. In this vein, redox active species, such as simple and complexes of transition metal ions, 3,4 halide ions, 5,6 quinones, 7 and phenylamide, 8 have been shown to be capable of undergoing fast redox reactions on the surfaces of some carbon or metal oxide based electrode materials. These reactions can then enhance the charge storage capacity of SCs.…”

“…10 There are literatures providing detailed and extensive discussions on the application of redox electrolytes in SCs. 9,11,12 The use of redox electrolytes has been claimed as a cheap means of improving the performance of SCs, particularly charge storage capacity, 9,11 promoting the use of more environmentally friendly aqueous electrolytes, 4 optimizing the low temperature performance of water based SCs, 13,14 and inhibiting the corrosion of current collectors, 15 just to mention a few. Still, it is neccesary to draw attention to some important issues inherent in the application of redox electrolytes in SCs.…”