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Energies 2021, 14, 5643 9 of 45 This review brings together the different types of RFBs that have achieved the most rel- evant advances and attention from the scientific community in the last few years. Moreover, a straightforward approach to RFBs variants is reported. Gathering all this information in one single manuscript, allows us to easily compare the level of development that each energy storage technology has reached so far. 2.1. Inorganic Aqueous 2.1.1. Vanadium Redox Flow Batteries (VRFB) The first publication on all-vanadium or VRFB electrodes was performed in 1987 by Rychcik and Skyllas-Kazacos and was focused on the carbon–polymer composite elec- trode [64]. Since then, several studies on electrodes for the VRFB have been reported. Organic electroactive molecules in both non-aqueous and aqueous RFBs can be considered to be in their infancy since no RFBs has been reported to replace the VRFB. Therefore, several efforts to develop more advanced RFBs with organic electroactive materials toward practical applications are being pursued and put in practice [52]. The vanadium electrolytes are a key component for VRFB due to their performance and the cost–benefit ratio involved. The electrochemical activity and the concentration and stability of vanadium ions determine the energy density and the reliability of the VRFB. Therefore, these factors contribute to electrolyte technology improvement and are still under optimization towards a more reliable and cost-effective system [65–68]. The development of new electrode components for VRFB systems will certainly increase in the short–medium term for many industrial and residential applications. Despite RFB having been explored both with aqueous and non-aqueous electrolytes, to date aqueous electrolytes are the only commercially viable option [69]. Water shows good performance as a solvent, mainly due to its high availability and therefore low cost. The cost involved is particularly relevant for a technology with relatively low energy density when compared to other battery technologies. Therefore, the electrolyte cost represents one of the major investment costs [69,70]. Moreover, aqueous electrolytes are highly conductive, have a lower viscosity, and can dissolve a wide range of transition metal salts, providing high ionic mobility [69]. The all-vanadium RFBs were proposed in 1988 by Skyllas-Kazacos to overcome prob- lems related to crossover, low reversibility, and self-discharge of the earliest proposed RFBs by taking advantage of the four oxidation states of vanadium. This technology became known as generation 1 redox flow battery (G1 RFB) [71,72]. Later, it was proposed to replace the V(IV)/V(V) pair in the positive side of a V/HBr/HCl electrolyte to increase the operational temperature range of the battery leading to the development of the generation 2 redox flow battery (G2 RFB) [73–75]. Finally, in 2010, Pacific North-Northwest Labo- ratories proposed the use of an acidic mixture (H2SO4 and HCl) strategy to increase the solubility of vanadium ions in electrolyte and to achieve superior performance, which was referred to as the generation 3 RFB (G3 RFB). Table 2 shows the main differences between the three generations of VRFB that will be discussed in the next sections. The information was collected from different references and organized to provide a quick picture of the main difference between the three generations of RFBs. Table 2. Comparison between the three generations of RFBs (G1, G2, and G3). G1 G2 G3 Name Positive Couple Negative Couple Supporting Electrolyte Vanadium Concentration (M) Temperature Range (◦C) Specific Energy (Wh/L) All-Vanadium V(III)/V(II) V(IV)/V(V) H2SO4 1.5–2 [76] 10–40 [76] 20–33 [77] Vanadium-Polyhalide V(III)/V(II) Cl−/ClBr2− HBr and HCl 2.0–3.0 [77] 0–50 [79] 35–70 [77] Mixed Acid Vanadium V(III)/V(II) V(IV)/V(V) H2SO4 and HCl 2.5–3.0 [78] −5–50 [4,78,80] 22–40 [4,78,80]PDF Image | PNNL Vanadium Redox Flow Battery Stack
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