(Invited) Storing Electrons and Holes in the Electrolyte - a New Opportunity for Supercapacitors

Abstract

Electrolytes are at the heart of batteries and supercapacitors and their primary role is always to conduct ions, even if their specifications are actually more complex: chemical stability, large cell voltage, high conductivity. However, depending on the design of the molecules that compose the cation and/or the anion, their function could be extending. In the field of supercapacitors, energy density is a technological limitation. The main orientation is to manufacture new electrode materials, with a purely capacitive character (EDLC) or with a pseudocapacitive character (i.e. a Faradaic phenomenon with a supercapacitor signature). In an innovative way, approaches are beginning to appear in the literature that consists in adding redox molecules to the electrolyte to participate to the charge storage. Despite this promise to increase energy densities (or apparent capacities), two limits are clearly identified: (1) self-discharge is important and (2) shuttle effects decrease Coulombic efficiency. We have recently shown that one of the opportunities is to use biredox ionic liquids. These molecules whose anion and cation are functionalized by redox molecules so that the cation and anion of the electrolyte contribute to charges storing. The announced advantage of this approach is that redox ionic liquid molecules are bulky molecules for which mass transfer becomes difficult after charging. This approach of inhibiting self-discharge and shuttle effects, through ion confinement, is interesting and represents a real alternative to storage using electro-actives electrolyte. However, achieving this confinement effect in a usual organic electrolyte such as acetonitrile is the real challenge. Moreover, studying the impact of the environment is fundamental to address this challenge. Indeed, succeeding in maintaining high performance but in an acetonitrile electrolyte (used in commercial devices) would allow these redox ionic liquids to be a credible alternative to the development of EDLC supercapacitors, while increasing their energy density (their apparent capacity). It is to this problem that we have addressed in this work: to find an alternative to redox electrolyte in supercapacitors by understanding the confinement of redox ionic liquid in pores.