Most methods for improving supercapacitor performance are based on developments of electrode materials to optimally exploit their storage mechanisms, namely electrical double layer capacitance and pseudocapacitance. In such cases, the electrolyte is supposed to be electrochemically as inert as possible so that a wide potential window can be achieved. Interestingly, in recent years, there has been a growing interest in the investigation of supercapacitors with an electrolyte that can offer redox activity. Such redox electrolytes have been shown to offer increased charge storage capacity, and possibly other benefits. There are however some confusions, for example, on the nature of contributions of the redox electrolyte to the increased storage capacity in comparison with pseudocapacitance, or by expression of the overall increased charge storage capacity as capacitance. This report intends to provide a brief but critical review on the pros and cons of the application of such redox electrolytes in supercapacitors, and to advocate development of the relevant research into a new electrochemical energy storage device in parallel with, but not the same as that of supercapacitors.

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Charge storage through electric double layer (EDL) charging of activated carbon (AC) and redox reactions of iodide and bromide ions in aqueous electrolytes and at the AC | electrolyte interface has been investigated by cyclic voltammetry and galvanostatic charging and discharging. Electrochemical experiments were carried out in both the three-electrode and two-electrode cells with the latter resembling the so-called supercapacitor-battery hybrid or simply supercapattery. By comparing the electrochemical behavior of bromide and iodide ions used as dissolved redox species (DRS), some observed features of the supercapattery are described and analyzed from the standpoint of the EDL charging of the AC electrodes, the thermodynamics and kinetics of the electrode reactions of the DRS, and the adsorption and transport of the charging reaction products. Furthermore, the effect of capacitance unequalization was explored for the adequate utilization of the charge storage from both the DRS and EDL contributions. It is also shown that counter-electrode oversizing has to be critically appraised for the design of optimal devices.

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Supercapattery is the generic name for various electrochemical energy storage (EES) devices combining the merits of battery (high energy density) and supercapacitor (high power density and long cycling life). In this article, the principle and applications of EES devices are selectively reviewed as the background for supercapattery development. The focus is on the engineering aspects for fabrication of two types of supercapattery: (i) by coupling a battery electrode with a supercapacitor electrode, or (ii) from materials that possess both the Nernstian and capacitive charge storage capacities. Fundamental rationales are discussed in relation with the designs, such as why the device is always asymmetrical, and what materials are suitable for making supercapattery. Whilst the key is how to optimize device performance in terms of energy capacity, power capability and cycle life, cost is also discussed on resource rich materials such as nanostructured composites and redox electrolytes.