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Energies 2022, 15, 7397 8 of 20 does not need separation of the cathode and anode half-cells via proton exchange mem- brane. He also completed a thorough analysis of the catholyte complexation reaction in homogeneous conditions vs. the efficiency of such a flow battery system [134,135]. Re- cently Dr. Hou reported a chlorine redox flow battery that reversibly performs electrolysis of NaCl aqueous solution until Cl2 is generated and transformed into the carbonaceous tetrachloride (CCl4) [136]. Dr. Fisher and some other scientific groups thoroughly inves- tigated the complexation of bromine with different chemical agents (BCA) in aqueous electrolytes of halogen-containing flow systems. The problem of bromine vapor pressure reduction and halogen storage in the insoluble matter or solid material form is of interest in these works [137–139]. As a result, they showed that bromine can be stored in the fused salt of very high concentrations up to 13.6 M, rushing theoretical energy capacity up to 730 A h L−1 [137–139]. Recently this group also measured current densities in the diffusion-limited regime and corresponding current distribution mappings vs. the flow rate values for the H2-Br2 RFB [140]. Despite the recent developments in the H-Br2 battery, the solubility of bromine in water under normal conditions is about 0.21 mol l−1 [128]. Hydrogen-bromine systems usually use a mixture of bromine dissolved in water and hydrobromic acid (up to 8 mol l−1) [128], which makes it possible to achieve an energy capacity of about 170 W h kg−1 [141], much higher than most common vanadium flow batteries energy density of 20 to 50 W h kg−1 [19,34,76,142], nevertheless still much lower than, for example, for lithium-ion batteries [143]. Thus, it can be concluded that, despite promising performance in terms of power density, both vanadium-based compounds and halogens show insufficient solubility in water, which leads to limitations on the value of specific energy capacity. 3.3. Halate Hybrid Flow Batteries The solution to the problem of the insufficient energy capacity of hydrogen-halogen flow batteries was proposed in 2015 by Yu.V. Tolmachev in [15]—to abandon the use of dissolved halogens in favor of a new family of aqueous multielectron oxidants—solutions of halogen oxoacids. Lithium bromate LiBrO3, which should be converted at the cathode to lithium bromide LiBr, serves as a promising object of study in [15]. Due to the very high solubility of both substances in water (according to [15] even at room temperature, the concentration of a saturated solution of lithium bromate is about 9 mol l−1, and bromide—more than 10 mol l−1) and the transfer of 6 electrons for bromate-bromide transition results in very high charge densities, more than 1000 A h kg−1, which are more than an order of magnitude higher than similar indicators for other redox flow batteries systems proposed to date. Other advantages of this oxidizing agent, mentioned in [15], and demonstrated experi- mentally in [144], are its long-term chemical stability (at moderately low pH), low toxicity of the reagent and product, the utilization of commodity chemicals, low cost per unit of generated electrical energy (due to both a relatively inexpensive reagent and the absence of expensive catalysts for the cathodic reaction, in particular, noble metals), low self-discharge rate and the absence of fire and explosion hazards. Additionally, in addition to lithium bromate, the use of commercially available sodium bromates with water solubility up to 2 mol l−1, seems very promising [15]. The reason why the bromate anion (as well as other oxohalogenate ions) did not attract attention for applications in electrochemical energy earlier is its low electrochemical activity: on all the studied electrodes, including noble metals, its direct electroreduction reaction at high-rate proceeds at high overvoltages. For the first time, it has been experimentally confirmed that despite the direct electroreduction of bromate anion via heterogeneous reactions cannot be performed without the huge cathode overvoltage, nevertheless, it can be carried out via redox mediator autocatalysis (EC” mechanism) through a combination of the following bromine reduction reactions: Br2 + 2e− ↔ 2Br− (heterogeneous), (4)PDF Image | Halogen Hybrid Flow Batteries
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