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Energies 2021, 14, 5643 17 of 45 Year 2019 2020 Active Species [SiW12 O40 ]4− /[PV14 O42 ]9− (SiW12–PV14) [SiW12 O40 ]4− /[PV14 O42 ]9− (SiW12 –PV14 ) Li6P2W18O62/Li3PMo12O40 Cell Area (cm2 ) 25 1400 CE EE (%) (%) 96 64 99 86 Ref. [125] [125] [124] Table 4. Cont. E (WhL−1 ) NR j P (mAcm−2 ) (mWcm−2 ) NR NR 4 Table 3 shows that the number of publications in recent years, since 2013, on RFBs based on POMs is very limited. With the exception of Friedl et al. in 2019 [125], the remaining studies show, generally, a low cell area compared to what has been reported in VRFBs (≤5 cm2). Furthermore, although there are some interesting efficiency values, the current densities are still low. Additionally, in the studies mentioned many researchers have studied the electrochemical properties of POMs to apply to energy storage but these are still in a preliminary phase [119,121,137–142]. For instance, VanGelder et al. studied polyoxovanadate-alkoxide clusters as multi-electron charge carriers for symmetric NA-RFB [131]. Recently, the authors showed how molecular control over POV-alkoxide groups guides the formation of stable, multimetallic, electroactive groups in non-aqueous media. Researchers are currently focused on optimizing the physical and electrochemical properties of this class of POMs [132]. The application of POMs in NA-RFB is the main focus of research, mainly due to non-aqueous POM-based flow batteries, which have the potential to reach higher energy densities due to the abovementioned properties of POMs, e.g., solubility and existence of multi-electron redox pairs [124,131,132,135,139,143]. Others are also seeking to apply POMs to the remaining components of RFBs [120,122,130,141,144,145]. Among the described advantages for the use of POMs, it is worth mentioning the fact that they allow for the exchange of several electrons per reaction, have high kinetics, and their size prevents the existence of crossover. Since they have good solubility in non-aqueous solvents, they also have the advantage of being able to overcome the electrochemical window of water. However, currently they still face problems in reaching active areas higher than 5 cm2 and current densities competitive with current technologies. Despite the investment required to synthesize the POM-based electrolyte, POMs may eventually overcome the current RFBs considering all the advantages they can add to current energy storage. Current studies are still at an early stage, but if we invested in their optimization, they could lead to high-performance RFBs. 2.2. Organic Aqueous Aqueous organic redox flow batteries (A-ORFB) started to be studied as a solution for the aqueous inorganic RFB shortcomings. Organic molecules can be easily produced, and their precursors are abundant, which eliminates the problem of running out of raw materials to produce these active species and lowers their price. The organic active species are highly versatile and tunable, making personalization of active species for different applications a possibility, but, more importantly, the endless number of organic molecules that can be synthetized give the chance of finding a molecule that will fill all the require- ments to reach commercialization with ease. The characteristics appealing properties for the active species to be used in RFB are high solubility, high standard redox potential, electrochemical stability, high number of electrons to transfer, fast kinetics, and good re- versibility. In the past few years, great attention has been given to organic active species to find molecules that fulfill all these desirable characteristics and apply them in aqueous electrolytes. However, this goal has not been reached yet [25,146,147]. NR 95–100 ≈100 5.0 × 10−1 NR 2.55 E—energy density, CE—coulombic efficiency, EE—energy efficiency, j—current density, P—power density; NR—not reported.PDF Image | PNNL Vanadium Redox Flow Battery Stack
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