Biredox ionic liquids: Electrochemical investigation and impact of ion size on electron transfer

Redox ionic liquids consisting of ions bearing redox moieties are receiving increasing interest in electrochemical applications, as they associate electroactive properties with the classical properties of ionic liquids. Here, biredox ionic liquid electrolytes are described in which both anion and cation are functionalized with anthraquinone and 2,2,6,6-tetramethylpiperidinyl-1-oxyl (TEMPO) groups, respectively. In-depth investigations based on crossed experimental and theoretical studies were carried out to elucidate how the bulkiness of ions bearing a redox moiety impacted electron and mass transfers, and accordingly the efficiency of electrochemical devices. The values of solvated radii of different redox ions, as well as the related kinetic constants, were extracted from cyclic voltammetry experiments. Reformulating the basic relations of electron transfer theory (based on Marcus-Hush theory) evidenced that in such redox species, with an unsymmetrical located redox centre, the electron transfer was not governed by the overall size of the solvated redox species, but rather by the radius of the redox active subunit, which takes preferential orientation towards the surface, thus allowing higher kinetic constants than what classical theory would predict. This vision opens ample opportunities for biredox ILs as electrolytes in electrochemical devices.