An Anionic Non-Aqueous Single Substance Redox Flow Battery Based on Triiodiden

A single-substance redox-flow battery (RFB) based on the iodide/triiodide and triiodide/iodine redox couples has been investigated. Stable charge-discharge curves were recorded under ambient air in a stirred PTFE batch cell. Current efficiencies were > 90 %. Current densities were kept low (33 mu A cm(-2)) due to high resistance (5.8 k Omega cm(2)) of the cation exchange membrane used. It is shown theoretically, that the open voltage potential E-oc of redox flow batteries with complex stoichiometry is concentration dependent. For comproportionation electrolytes, the increases with bulk concentration, which is proved experimentally for the I-/I-3(-)/I-2 system. The open cell voltage ranged from 0.36 V to 0.58 V for 1-80 mM solutions. The formal potential difference Delta E-0' was determined by cyclic voltammetry (0.655 V) and open cell voltages (0.69 V), respectively. Interestingly, the calculation of Delta E-0' required the evaluation of the open cell voltage at a state of charge of the inverse of the golden number. This is a consequence of the "golden" stoichiometric factors of the iodide/iodine comproportionation. To the authors' knowledge, this is the first report of a non-aqueous redox-flow battery utilizing an anionic catholyte and thus also the first where only anionic or neutral redox active species are employed.