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 I₂ (or I₃⁻)/I⁻. Here, we describe for the first time reversible electrochemical formation and dissolution of insoluble iodine–halide coordination networks, [(I₂)_n·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 I₂/I⁻ to [(I₂)_n·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⁻, [(I₂)_n·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 [(I₂)_n·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.