ИСТИНА

Redox flow batteries present numerous important advantages such as independent scaling of energy and power; high efficiency of energy transformation; cheap (carbon) electrodes without special catalysts, etc. However, energy density of reactants in reservoirs for most of actually employed systems of this type is relatively low. A new approach [1] is based on the use of aqueous solutions of multi-electron oxidants, first of all halate anions, XO3-, which would ensure a large redox capacitance owing to their reduction to halides, X-, e.g. 790 Ah/kg or 1400 Ah/dm3 for LiBrO3 and 1580 Ah/kg or 3100 Ah/dm3 for LiClO3 at 25oC. In combination with a suitable anodic process, e.g. H2 oxidation, these systems possess high energy densities: 750 Wh/kg for LiBrO3 and 1130 Wh/kg or 1150 Wh/dm3 for LiClO3 [2]. The principal problem of these systems originates from the absence of electrochemical activity of halate ions within a suitable potential range. It may be overcome via use of a proper redox mediator cycle. This approach is especially efficient for autocatalytic cycles, e.g. based on combination of electrochemical and chemical steps: X2 + 2e- = 2 X-; XO3- + 5 X- + 6 H+ → 3 X2 + 3 H2O (*) where each cycle leads to progressive accumulation of the catalytic species, X2 and X- [3]. We have carried out a theoretical analysis of this system under steady-state conditions via solving a coupled set of convective-diffusion transport equations for all components, XO3-, X-, X2 and protons, containing terms due to the chemical step (*), for the rotating disk electrode (RDE) configuration [3,4]. This study has revealed a crucial importance of the principal parameter, xdk, equal to the ratio of the thicknesses of the diffusion (zd) and kinetic (zk) layers: xdk = zd / zk. If its value is small (less or comparable to 1, corresponding to high electrode rotation rates, Fig. 1c) the catalytic cycle based on Eqs (*) is inefficient (consumption of XO3- is very small, Fig. 1a) and the presence of solute XO3- species does not affect the passing current (Fig. 1c) due to the discharge of X2 molecules diffusing from the bulk solution (Fig. 1b). For weaker solution agitation intensities (Fig. 1c) the maximal current increases rapidly, passing via a maximum at xdk close to 10, followed by a slow decrease for even lower electrode rotation rates (Fig. 1c). These very high values of the current (on the level of about 1 A/cm2, comparable to the diffusion-limited current for species XO3-) are due to accumulation of enormous amounts of X- (line b) and X2 (line c) near the electrode surface (Fig. 1b) [3,4]. Fig. 1. Steady-state transport for the halate reduction process via autocatalytic mechanism (*). Dimensionless concentration profiles for XO3- (lines a), X- (lines b) and X2 (lines c) for xdk = zd / zk equal to 3 (Fig. 1a) or 8 (Fig. 1b). Similar analysis has also been carried out for the microelectrode configuration [5] and for the 3D flow-through porous electrode [6]. These astonishing predictions have been confirmed experimentally [7-9]. For their practical applications in power-generating discharge devices it is of importance that such intensive currents were reached rapidly, even starting from the zero-current state and a very small bulk-solution X2 concentration (about 1 mM or lower). In this study the evolution of the halate process, Eq (*), between these states has been analyzed for the RDE configuration, via numerical solution of the non-stationary transport equations for all components of the system, XO3-, X- and X2. Three different stages of the process have been revealed (Fig. 1). Fig. 2. Temporal variation of dimensionless current, jk, as a function of dimensionless time, s = t / T, for various values (shown at each line) of the ratio of the steady-state diffisionf and kinetic layers' thicknesses, xdk = zd / zk, Within the initial stage the chemical step (t < T), cycle based on Eq (*) is inefficient so that the presence of the principal oxidant does not affect the process which consists in electroreduction of X2 species which diffuse from bulk solution to electrode surface. Thus, passing current is very weak due to a low X2 concentration, being close to its diffusion-limited current (Fig. 2a). If the value of the principal paramer, xdk, is sufficiently large, then after characteristic time moment, s ≈ 1, i.e. t ≈ T, determined by the product of the rate constant of chemical step (*) and the bulk-solution concentration of XO3- species, autocatalytic cycle (*) starts to generate extra amounts of X2 and X- species in the vicinity of the electrode leading to exponential increase of the current (Fig. 2b). Within an even longer evolution period, which takes place only for large values of the xdk parameter (over 10) the surface concentration of XO3- species decreases strongly, and the current drops after passing a maximum (Fig. 2b). For rapid chemical steps, e.g. for concentrated BrO3- solutions, the whole duration of the evolution is within the range of a few seconds. References [1] Y. V. Tolmachev, A. Pyatkivskiy, V. V. Ryzhov et al, J. Solid State Electrochem., 2015, 19, 2711 [2] M. A. Vorotyntsev, А. Е. Antipov, D. V. Konev, Pure Applied Chemistry, 2017, 89, 1429 [3] M. A. Vorotyntsev, D. V. Konev, Y. V. Tolmachev, Electrochim. Acta, 2015, 173, 779 [4] M. A. Vorotyntsev, A. E. Antipov, Electrochim. Acta, 2017, 246, 1217 [5] M. A. Vorotyntsev, A. E. Antipov, Electrochim. Acta, 2017, 258, 544 [6] M. A. Vorotyntsev, A. E. Antipov, Electrochim. Acta, 2019, 323, 134799 [7] A. D. Modestov, D. V. Konev, A. E. Antipov et al, Electrochim. Acta, 2018, 259, 655 [8] D. V. Konev, A. E. Antipov, M. M. Petrov, M. A. Shamraeva, M. A. Vorotyntsev, Electrochem. Comm., 2018, 86, 76 [9] A. D. Modestov, D. V. Konev, A. E. Antipov, M. A. Vorotyntsev, J. Solid State Electrochem., 2019, 23, 3075 [10] M. A. Vorotyntsev, D. V. Konev, submitted for publication Acknowledgements Supported financially by Russian Science Foundation (grant RSF20-63-46041).