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Membranes 2022, 12, 1228 2 of 16 in these devices, the transmembrane fluxes of both the reagent, the bromate anion (BrO3−), and the final product, the bromide anion (Br−), are significantly suppressed due to perms- electivity effects [23–25]. At the same time, the concentration of a potentially hazardous Br-containing component in terms of its crossover, molecular bromine, Br2 (according to the study of K. Oh et. al. [18]), for which the membrane does not represent a permselective barrier, may be strongly diminished inside the catholyte. This occurs in the H2-Br2 system because this species is the principal reagent of the molar-range concentration, while in H2- BrO3− systems, molecular bromine is merely an intermediate component in the course of the bromate—bromide conversion (autocatalytic redox-mediator catalysis by the Br2/Br− couple, EC′′ mechanism) so that its concentration can be maintained at a sufficiently low level via properly selected combination of the catholyte supply rate into the electrode space and the intensity of the current generated. The principal reagent, the BrO3− ion, is non-electroactive at the electrode surface so that its reduction proceeds via a redox-mediator cycle: Br2 + 2 e− = 2 Br− at electrode surface; BrO3− +5Br− +6H+ =3Br2 +3H2O insolution (1) Global cathodic half-reaction: BrO3− + 6 H+ + 6 e− = Br− + 3 H2O Overall cell reaction: 3 H2 + BrO3− = Br− + 3 H2O This means that the current passage is only ensured by the discharge of bromine molecules, while the generated bromide ions, Br−, react with bromate ions in the solution phase, thus restoring solute bromine molecules which may react again at the electrode. The set of reactions detailed in Equation (1) might seem to represent a particular case of the well-known EC’ mechanism [26–43], Equation (2): Ox + n e− = Red at electrode surface; A + Red = P + Ox in solution (2) where an electrochemically inert principal reagent, A, is transformed into its inert prod- uct(s), P, owing to a catalytic redox-mediator cycle based of an Ox/Red couple. It is well- established that its rate under steady-state transport-control regime is proportional to the bulk-solution concentration of the catalytic species, Ox0, while the transport rate of species A across the diffusion layer does not limit the process. Theoretical analysis [44–49] of the bromate reduction process described by Equation (1) revealed its surprising features, which are radically different from those for the cycle in Equation (2). First, contrary to the conventional law for other reaction mechanisms, the highest rate of the process is ensured not by an intensive convection; quite oppositely, it is achieved for a sufficiently thick diffusion layer. Second, for a very low bulk-solution concentra- tion of the catalytic species, Br2, the passing current can become comparable with, or even exceed, the diffusion-limited one for the principal reagent, BrO3−, which is well over 1 A/cm2 for its molar concentration range. This difference in results for the bromate process, Equation (1), compared to those for the conventional EC’ mechanism, Equation (2), originates from the autocatalytic character of the former system. Namely, each passage of the redox cycle, Equation (1), transforms five Br− ions into three Br2 molecules which (under suitable conditions) may give six Br− ions at the electrode, i.e., the consumption of the principal reagent, BrO3−, leads to an increase in the number of catalytic species, Br2 and Br−, near the electrode surface. As a result, local concentrations of the latter species and, consequently, of the current may become so high that the process becomes limited by the bromate transport into the reaction layer near the electrode surface, which is proportional to the bulk-solution concentration of bromate. These theoretical predictions were fully confirmed later by experimental studies of bromate reduction for various electrochemical configurations [50,51], including the most intriguing conclusion of a drastic increase in the current for a less intensive convection.PDF Image | Hydrogen-Bromate Flow Battery
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