Electrochemical oxidation of a highly soluble redox mediator in aqueous solution for energy conversion

An efficient and cost-effective electrochemical energy storage/conversion technique is needed to store electricity from intermittent renewable energy sources. Although electrochemical water splitting into hydrogen and oxygen is extensively studied, it suffers from kinetically-sluggish oxygen evolution reactions at extreme pH conditions. In this work, we investigated the oxidation of soluble redox species as an alternative anodic reaction, the kinetics of which have highly reversible characteristics on various electrode surfaces. The redox potential can be readily tuned by the ligand upon the formation of metal complexes, which provides various voltage options to develop electrochemical devices. The facile kinetics lead to diffusion overpotential being a major contribution of the electrochemical performance. Near neutral pH region was effectively chosen because it allows the versatile and safe operation with a reasonable voltage option. To overcome the diffusion issue, the solubility of redox species and the diffusion coefficient determined by temperature and the supporting buffer were considered. Fe2+/3+-(hydroxyethylethylenediaminetriacetic acid) (HEDTA) was identified as a suitable redox candidate due to its high solubility (> 1 mol kg−1) in a near-neutral pH range from 4 to 6. In the optimized conditions with a malonate buffer, the diffusion-limiting current density could reach over 100 mA cm−2 on a glassy carbon rotating disk electrode. The detailed insight presented in this paper will help to design an efficient electrochemical system for redox-mediated energy storage/conversion.