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Energies 2022, 15, 7397 7 of 20 replacing it with another reagent (whereas the hydrogen reaction is stored at an anode) because such a replacement immediately eliminates the crucial problems of conventional fuel cells: the high cost of the system due to utilization of platinum and other catalysts, and dissolution of cathode catalyst [121–123]. However, an adequate choice of new oxidizing agent for a hybrid flow battery is an extremely difficult task, since several requirements must be met simultaneously: electrolyte must have a high energy density and its reduction reaction must be kinetically fast to ensure the high energy efficiency of the device. Additionally, all reagents and products of the cathodic reaction must be nontoxic, stable and non-flammable, preferably liquids or gases, since, e.g., solids, pastes and suspensions are often commercially less preferable in flow battery design. 3.2. Halogen Hybrid Flow Batteries Considering the above-mentioned requisites one can recall the hydrogen-halogen hybrid flow batteries, primarily due to rapid, reversible kinetics, which leads to excellent system performance and the use of inexpensive reagents [11,75,124–126]. In such a system a halogen oxidizer X2 (for example, Br2, Cl2, or I2) is used in the form of an aqueous solution and hydrogen (H2) is used as a reducing agent. The potential difference between the halogen and hydrogen electrodes V is equal to the EMF (from 0.54 V for I2 to 1.4 V for Cl2). It should be noted that F2, which is not considered here, has the largest potential difference relative to the standard hydrogen electrode (3.05 V), however, due to techno- logical difficulties in the separation of gaseous fluorine from a mixture of gases, its use is unreasonable due to energy considerations and the problem of materials’ compatibility with this reagent. In this battery the following reactions undergo on electrodes (from left to right—charge mode, from right to left—discharge): 2X− ↔ X2 + 2e− (at cathode), (1) 2H+ + 2e− ↔ H2 (at anode), (2) 2HX ↔ X2 + H2 (overall reaction) (3) The protons are transferred from the anode through the membrane into the catholyte (solution in contact with the cathode), whose composition changes inside the discharge unit from X2 to HX, and the HX solution enters the corresponding reservoir. The most promising halogens that can be used as oxidizing agents in such a system are Cl2 and Br2 because the standard potentials of the corresponding redox couples for these substances are the highest (1.36 V and 1.09 V, respectively, for Cl2/Cl− and for Br2/Br−). Recently, the interest of researchers has mainly shifted towards the hydrogen-bromine system due to its higher specific power with voltage efficiency of more than 90%, as well as a higher oxidant solubility in an aqueous solution. A serious disadvantage of hydrogen-halogen systems based on chlorine and bromine is the high toxicity and corrosiveness of the halogen oxidizer, whose concentrated solution must be stored in a tank. Another limitation for these systems is the risk of the catalytic layer degradation on the negative electrode due to halogen crossover through the separator, which should not only provide high selectivity but also have low internal resistance to minimize voltage losses. The transition from traditional vanadium-based redox flow batteries to hydrogen- bromine hybrid chemistry has led to a significant increase in the power density of flow systems. To date, specific power values of 1.4–1.5 W cm2 have been experimentally achieved for the H2/Br2 system [127,128], while for vanadium redox flow batteries, the highest peak power density for laboratory-scale devices reaches 1.3 W cm−2 [129–132]. The researchers develop new concepts and models: Dr. Wlodarczyk recently explored the bromine cath- ode thermodynamics for hydrogen–bromine chemistry in concentrated solutions [133]. Dr. Ronen proposed the perspective concept of the hydrogen-bromine flow battery, whichPDF Image | Halogen Hybrid Flow Batteries
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