  Kinetics of electrochemical reactions are several orders of magnitude slower
in solids than in liquids as a result of the much lower ion diffusivity. Yet,
the solid state maximizes the density of redox species, which is at least two
orders of magnitude lower in liquids because of solubility limitations. With
regard to electrochemical energy storage devices, this leads to high-energy
batteries with limited power and high-power supercapacitors with a well-known
energy deficiency. For such devices the ideal system should endow the liquid
state with a density of redox species close to the solid state. Here we report
an approach based on biredox ionic liquids to achieve bulk-like redox density
at liquid like fast kinetics. The cation and anion of these biredox ILs bear
moieties that undergo very fast reversible redox reactions. As a first
demonstration of their potential for high-capacity / high-rate charge storage,
we used them in redox supercapacitors. These ionic liquids are able to decouple
charge storage from ion accessible electrode surface, by storing significant
charge in the pores of the electrodes, to minimize self-discharge and leakage
current as a result of retaining the redox species in the pores, and to raise
working voltage due to their wide electrochemical window.
