Electrochemical formation and dissolution of an iodine-halide coordination solid complex in a nano-confined space

Abstract

Iodide and iodine comprise a promising redox couple in aqueous energy storage systems (aqua-ESSs). However, the corresponding half-redox reaction on the cathode of an aqua-ESS has most often been considered as simply I-2 (or I-3(-))/I-. Here, we describe for the first time reversible electrochemical formation and dissolution of insoluble iodine-halide coordination networks, [(I-2)(n)center dot X-] (X- = Br- and I-), in confined nanopores with microporous carbon (micro-C) serving as a positive electrode in an aqua-ESS and using I- as the redox active electrolyte during charging. In an electrochemical cell without added Br-, the main half-redox reaction changed from I-2/I- to [(I-2)(n)center dot I-]/I- (n = 1 and 2) as charging and discharging accelerated (i.e., as current densities increased). When Br- was added to the electrolyte with I-, [(I-2)(n)center dot Br-] was formed by electro-oxidation of I-, which was stably encapsulated in nanopores of micro-C regardless of the charging/discharging rate. Our findings suggest that [(I-2)(n)center dot Br-]/I- half-redox reactions can produce superior energy and power densities in an aqua-ESS with porous carbon electrodes through the addition of Br- to their electrolytes compared with electrodes with I- only.