ИСТИНА

Hydrogen-Chlorate Electric Power Source. Feasibility of Device, Discharge Characteristics and Modes of Operation D. V. Koneva,b, O. I. Istakovab, E. A. Rubana,b, M. A. Vorotyntseva a Frumkin Institute of Physical Chemistry and Electrochemistry, Russian Academy of Sciences, Moscow, Russia b Institute for Problems of Chemical Physics, Russian Academy of Sciences, Chernogolovka, Russia e-mail: mivo2010@yandex.com Redox-flow batteries on the basis of the halogenate reduction: XO3- + 6H+ + 6e- = X- + 3 H2O (*) as the cathodic process promise important advantages [1]. In particular, theoretical values of its specific redox capacity for the saturated aqueous LiClO3 solution are very large: 1580 Ah/kg, or 3100 Ah/dm3 of solution at room temperature, owing its high solubility and the 6-electron reduction process. Its combination with the hydrogen oxidation at anode gives 1.45 V for EMF and about 1150 Wh/dm3 for the theoretical estimate of the energy density of the H2-LiClO3 battery (assuming 700 atm for the H2 source). However, no chlorate-based power sources have been reported earlier since chlorate ion is non-electroactive within the needed potential range, even at specially modified electrodes. A way to overcome this problem might be based on a redox-mediator cycle where the Ox form of a redox couple is reduced at cathode into the Red form while the latter reacts chemically inside the solution phase with the principal oxidant, ClO3-, regenerating the Ox form which can participate again in the electrode reaction (EC' mechanism). This approach has got a serious disadvantage that its current (under conditions of the chlorate excess) is proportional to the bulk-solution concentration of the Ox component which has to be stored (besides chlorate) at a sufficiently high concentration. An alternative way has been discovered by us for an analogous process of the bromate reduction, X = Br in Eq. (*) [2], where the current can reach very high values owing to the autocatalytic redox-mediator cycle: Br2 + 2e- = 2Br- at cathode, BrO3- + 5Br- + 6H+ = 3Br2 + 3H2O in solution, even for negligibly low Br2 bulk-solution concentration. However, the rate constant of an analogous comproportionation step for ClO3- turned out to be several orders of magnitude smaller than that for BrO3-. Nevertheless, our study [3] has shown that the chlorate reduction process can proceed (for a sufficiently high acidity of solution) with the rate acceptable for its application in power sources, without addition of the Ox component of an extra redox couple. It has been demonstrated that the chlorate transformation takes place via the autocatalytic redox cycle where the key role is played by chlorine dioxide, ClO2, which is accumulated up high concentrations in the middle of the process, while the reduction product is mostly Cl-, in conformity with the global cathodic process (*). Our tests of the home-made H2¬ClO3- MEA cells [4] have shown at the room temperature for the initial 1M NaClO3 + (4-6)M H2SO4 solutions: up to about 0.8 A/cm2 for the discharge current density, 90% to 99% for the faradaic efficiency of global process (*), 44% to 48% for the degree of the chemical-to-electric energy transformation, 290 to 400 mW/cm2 for the maximal specific power (up to 500 mW/cm2 for 50oC), 145 to 350 mW/cm2 for the average specific power during the discharge process. Maximal characteristics are observed for 5M H2SO4 solution. In operando control via UV-visible spectroscopy of the catholyte has shown that the ClO2 concentration increases progressively within the initial stage of the cell operation (up to reaching a significant fraction of the initial amount of ClO3- which depends strongly on the acid concentration), with a monotonous diminution (up to its zero level) after passing its maximum. These data confirm a key role of ClO2 in the autocatalytic redox-mediator cycle which is responsible for the chlorate transformation to chloride (*). Financial support of the Russian Science Foundation is acknowledged (grant RSF 23-13-00428). References: 1. Yu.V. Tolmachev, A. Pyatkivskiy, V.V. Ryzhov, D.V. Konev, M.A. Vorotyntsev, J. Solid State Electrochem., 2015, 19(9), 2711. 2. M.A. Vorotyntsev, А.Е. Antipov, D.V. Konev, Pure Applied Chemistry, 2017, vol. 89, № 10, 1429. 3. D.V. Konev, O.A. Goncharova, Yu.V. Tolmachev, М.А. Vorotyntsev, Russ. J. Electrochem., 2022, 58, n. 11, 978 4. D.V. Konev, O.I. Istakova, E.A. Ruban, A.T. Glazkov, M.A. Vorotyntsev, Molecules, 2022, 27, 5638.