Faraday Discussions, 2017, 10.1039/C7FD00174F
Nowadays, commercial supercapacitors are based on purely capacitive storage and porous carbons are used as electrodes. Unfortunately their energy density is limited, so that many study investigated new materials that could improve the capacitance of the device. This new type of electrodes (e.g. RuO2, MnO2…) involves different mechanisms for energy storage (energy stored at the solid state in the material). In this latter case, redox reactions (Faradaic), that are slower than adsorption process (capacitive), occurs, explaining why such devices are called pseudo-capacitors. This is a real breakthrough to their development since, using this approach, capacity improvement inevitably involves important loss of power. A contrary, at the liquid state, even if Faradaic, redox reactions occur faster due to favourable environment. To design new devices allowing liquid dynamics with improved specific capacitance is very challenging. In this context, this paper presents a new approach to increase the specific capacitance, based on biredox ionic liquids, where redox species are part of the electrolyte, in contrast with classical pseudocapacitors proposed up to now. Anions and cations are functionalized with anthraquinone (AQ) and 2,2,6,6-tetramethylpiperidinyl-1-oxyl (TEMPO) moieties, respectively. Glassy carbon, carbon-onion, and commercial activated carbon electrodes that exhibit different double layer structures and thus different diffusion process (dynamics) were used to study electrochemical response of biredox ionic liquids simultaneously at the positive and the negative electrodes.