Outer Sphere Redox Chemistry for Rechargeable Flow Batteries

Abstract

Reversible redox species play central role as electron acceptors in state-of-theart flow batteries and fuel cells. At electrochemical interfaces, since the physical processes of mass transport occurs independently and in series to electron transport; the reaction velocity in reversible species is predominantly transport controlled due to their fast electron transfer events. Thus, the physical processes of bringing the reactants together play central roles in the electrochemistry of reversible redox molecules (because their extremely fast electron transfer events demand the reacting species to reach the reaction zone by simple diffusion). Even under convective fluid flow, spatial structuring of the solution forces simple diffusion to dominate the transport phenomena near the electrode, which in turn poses unique challenges to utilize the full potential of the molecules either by the electrode or fluid characteristics. This translates into extremely low volumetric energy density in flow batteries, and in the attempts to target it are mainly carried out by improving the solubility limits, whereas the root of the problem being their transport-controlled reaction velocity is often overlooked. We show that Coulombic force gated molecular flux almost doubles the volumetric energy density in reversible species-based flow batteries by generating an electrostatic current parallel to the diffusion current.