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Energies 2021, 14, 5643 26 of 45 3.2. Metal–Air Flow Batteries (MAFB) and Metal–Air Fuel Cells (MAFC) MAFB are one of the answers for the low energy density of classical RFB and, as an added advantage, this technology also reduces the cost of materials. By reacting a metal, on the anode side, and oxygen, on the cathode side, it is possible to reduce the volume of the batteries, while also increasing the standard redox potential of the cell. These technologies are still in an early stage of their development; however, they show great potential as an energy storage system [3,232,233]. The main difference between MAFB and MAFC is that the latter are not rechargeable. Nevertheless, both technologies have a lot of similarities with RFBs, and they will both be discussed in this review. To date, most works published focus on the combination of vanadium, zinc, or lithium with air. This subchapter will focus on these technologies. Vanadium–air fuel cells (VAFC) are very interesting to achieve higher energy densities, when compared to VRFB. In these batteries, V (II) reacts with oxygen to produce V (III) and water. The utilization of air as one of the electrolytes removes the need to use two tanks to store energy but also unlocks the potential to increase the concentration of vanadium in the electrolyte operating at higher temperatures, since the V (V) solubility problems do not influence this technology. In 2019, a VAFC was reported by Risbud et al. [234]. They achieved a power density of 22 mW cm−2 at 40 mA cm−2 with a vanadium concentration of 3.6 M. With this concentration of vanadium, the VAFC has a theoretical energy density of 143.8 Wh L−1, almost 3.5 times greater than a VRFC with the same concentration of active species. The membrane is a very important component of this technology. Similarly to VRFB, the membrane needs to have a low permeability of vanadium ions, be chemically stable, and have a high ionic conductivity. In addition to this, low oxygen permeability through the membrane is required, as well as good water permeability to achieve good performance of the VAFC. Charvát et al. [235] studied the use of different cation-exchange membranes in a VAFC. They found that thinner membranes show better performance and stability under current load. The best assembly recorded a peak power density of 57 mW cm−2 and 75% coulombic efficiency with a discharge current density of 50 mA cm−2 and a charge current density of 150 mA cm−2. Zinc–air fuel cells (ZAFC) have the advantage of having a bigger standard potential (1.65 V) than VAFC (1.49 V). This technology still has a lot of parameters to be optimized. In 2019, Chen et al. [236] studied how the electrolyte flowrate affected the performance of a ZAFC, with zinc particles suspended in the solution. They reported a current density of 518.6 mA cm−2 at a potential of 0.8 V, which corresponds to a maximum power density of 427.7 mW cm−2 when using an optimum electrolyte flowrate of 100 mL min−1. The reaction of oxidation of zinc produces zincate, which can become zinc oxide if the solution becomes oversaturated. This factor hinders the discharge reaction in the ZAFC; however, Pei et al. [237] incorporated a filter on the hydraulic circuit of the electrolyte to remove the circulation of these particles and thus reduce the effects of this problem. They also optimized the flow field of the anode to reduce passivation of this component, achieving an electrolyte capacity of 1025 Ah L−1. On the other hand, Sageetha et al. [238] focused on the cathode side of the cell, testing different inlet air velocities, different oxygen ratios and pressurization of the inlet gases. Introducing air flow to the cell improved the cell’s performance, when compared to a cell with no air flow. Pressurizing the air also had a beneficial effect, since the content of O2 was larger, and pure oxygen showed even better results. The main difficulty for MAFB is finding cheap electrocatalysts that are active in the oxygen reduction reaction and in the oxygen evolution reaction. Moreover, these reactions are very sluggish, making the development of these electrocatalysts imperative to assure batteries with high efficiencies [3,232,233]. Zhang et al. [239] reported a zinc–air flow battery (ZAFB) that ran for 100 h without significant performance degradation. They designed a gradient hydrophilic/hydrophobic reactive interface in bifunctional oxygen electrodes, which reduced the overpotential at 50 mA cm−2 by 190 mV, increasing energy efficiency.PDF Image | PNNL Vanadium Redox Flow Battery Stack
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