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Energies 2021, 14, 5643 27 of 45 ZAFBs have an additional problem: the change of phase on the anode. Zinc is in a solid state and starts oxidizing to Zn2+ during operation, transforming it into an aqueous state. When the battery is recharged, zinc ions are reduced back into solid state, which can lead to the formation of dendrites that can easily puncture the membrane. The reduction of zinc ions can be influenced by many parameters. Using pulse current instead of direct current to recharge ZAFB (at 40 ◦C, a current if 1.4 A, frequency 50 Hz and 85% duty) has been shown to be an effective way to diminish the particle size of reduced zinc, and also, increase the amount that is reduced [240]. On the other hand, Yu et al. [241] assessed the advantage of using a flowing electrolyte in a ZAFB compared to a static zinc–air battery. They found that flowing the electrolyte improves the transfer of hydroxide and zincate ions, which translates to a 10% greater peak power density and 23% better specific discharge capacity. Lithium–air flow batteries (LAFBs) appeared as a solution to the problems that were noticed with lithium–air static batteries, e.g., pore clogs, high overpotentials, and low power density [232]. In 2015, Huang and Faghri developed a two-dimensional model for an aprotic LAFB, which was able to formulate two methods to increase the battery capacity; the dual layer cathode and alternating electrolyte flow, achieving 105% higher capacity and an increase of 3.7% of cathode capacity, respectively [242]. Y. G. Zhu et al. [243] showed a LAFB with soluble redox catalysts; however, the author’s solution did not solve the voltage hysteresis, which is noticeable in this type of battery due to the overpotentials. Later, the same group reported another study using the same strategy, however the soluble redox catalysts were changed. This study also proved to be unsuccessful since the catalysts used degraded over long cycles [244]. Ruggeri, Arbizzani, and Soavi proposed a lithium–air slurry flow battery, studying different weight percentages of Super-P® and Pureblack® carbons on the positive electrolyte. This report showed that by using this method the battery could be cycled for 120 cycles, i.e., 60 at 1.0 mA cm−2 and 60 at 0.5 mA cm−2 [245]. Even though MAFCs and MAFBs are promising technologies as energy storage sys- tems, they are still in a very early stage of development and far from being a commercial technology. MAFCs still have questions that have not been correctly and fully addressed, such as their implementation on a real system. How would the electrolyte be recharged in this situation? Would the addition of another system to recharge the electrolyte be a viable option? How would the second electrolyte used to recharge the main electrolyte be used? On the other hand, MAFBs are easier to implement than MAFCs. However, the challenge and the high cost to design electrodes that are active in the oxygen reduction reaction and in the oxygen evolution reaction must still be overcome. 3.3. Zinc–Bromine Flow Batteries Zinc–bromine flow batteries (ZBFB) are inserted in the electroplated flow battery category. This section will focus on the ZBFB, since this is considered one of the most representative type of electroplated flow battery, with a lot of research conducted to improve its performance. Moreover, this technology is one of the few RFB that has been commercially available and has large scale applications. When ZBFBs are fully charged, zinc is in the solid state, which gives an advantage in energy density to this type of RFB in relation to others. Additionally, ZBFB have a redox standard potential of 1.58 V. Even though the theoretical ZBFB specific energy is 440 Wh kg−1, commercial systems only reach 14–19% of this value. These batteries also suffer from other problems, e.g., zinc dendrite formation in the negative electrode, corrosion of the electrode, and the addition of expensive complexing agents to prevent the diffusion of bromine. The core materials used in ZBFB are cheaper than the ones used on other RFBs, however, the solutions to solve the problems previously explained make the commercial price of these batteries similar to other RFBs [3,10,246,247]. Zinc–bromine flow batteries (ZBFB) are inserted in the electroplated flow battery category. These batteries also suffer from other problems, such as zinc dendrites formation in the negative electrode, corrosion of the electrode and the addition of expensive complexing agents to prevent the diffusion of bromine. The core materials used in ZBFB are cheaper than the ones used on other RFBs; however, thePDF Image | PNNL Vanadium Redox Flow Battery Stack
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