Electroreduction of halogen oxoanions via autocatalytic redox mediation by halide anions: novel EC” mechanism. Theory for stationary 1D regime

Energy cycle based on a high specific energy aqueous flow battery and its potential use for fully electric vehicles and for direct solar-to-chemical energy conversion

“…5) in a similar manner. However, a more thorough analysis of these results (not shown) gives evidence against the applicability of the theoretical model [9] based on relations (1)-(5) to this system (i.e., predicting unrealistic values of the fitting parameters) containing a high concentration of a non-dissociated acid and a relatively low concentration of hydroxonium ions. We believe that the observed failure of the model is due to the effect of general acid catalysis in reaction (2) which adds to the kinetic Eq.…”

“…In this case, the bromine/bromide redox couple is not accumulated inside the diffusion layer and the current is determined by the transport of bromine from bulk solution, i.e., it is very low. In another limiting case, if the ratio of these layer thicknesses, x d and x k , is sufficiently large (the criterion has been derived in [9]), the comproportionation reaction takes place deeply inside the diffusion layer and most of its product (Br 2 ) goes to the electrode to generate the next portion of bromide. The concentrations of these components of the mediating redox couple may reach very high values, so that the experimentally measured current approaches the transport limit of bromate from bulk solution to the kinetic layer.…”

“…5 were analyzed using the theory developed in ref. [9]. The fitting was achieved via variation of two parameters:…”

“…V. Tolmachev in relation to the cathodic discharge process in high specific energy regenerative flow batteries utilizing highly soluble aqueous multi-electron oxidants (AMOs™), such as chlorates and bromates of lithium, magnesium, and calcium [5][6][7]. Fortuitously, the paths of Tolmachev and Vorotyntsev crossed around that time [8], resulting in their joint work on the development of theoretical models underlying the electroreduction of AMOs™ [9].…”

“…More specifically, as disclosed in Ftorion's patent applications [5][6][7], these highly soluble multi-electron salts have charge content, over 1000 Ah/kg of solution. Second, the autocatalytically accelerated electroreduction (1)-(2) of halogen oxoanions may deliver very large discharge currents (over 3 A/cm 2 ) and a very high areal power (over 2 W/cm 2 ) even when low-cost electrode materials such as carbon are used [9]. Additional benefits of AMO flow batteries comprise the use of cheap electrode materials (e.g., carbonaceous), abundant (see below), inexpensive, safe and environmentally benign reagents, as well as the anionic nature of the oxidant (halate) and of the final product of its reduction (halide) which prevents their crossover to the negative electrode when durable and commercially available cation-selective membranes (such as Nafion®) are used.…”