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Energies 2021, 14, 5643 23 of 45 Table 5 shows that, generally, the number of publications on NA-RFBs has been increasing since 2010. Although there is a trend towards higher efficiency in recent years, there is no direct relationship between the solvent, membrane, or active species used since the different studies differ in these aspects. Besides the redox pairs with application in a cell already mentioned in Table 5, over the last years several groups have emerged with new electrolyte ideas for application in NA-RFB and some interesting studies. In 2016, Sevot et al. delivered a promising metal coordination complexes (MCCs)-based anolyte candidate for NA-RFBs. Their main focus is on developing MCC-based anolytes with lower molecular weight per mole of electron transferred [168]. Other groups are studying about compounds such as Fc/FcBF4 [174] and cobalt and vanadium trimetaphosphate polyanions promising good potential to the future of NA-RFB [199]. Kwon et al. [200], in 2019, reported a multi-redox BMEPZ inspired by biosystems as a promising catholyte material with the highest energy density demonstrated for organic RFBs, but they also reported further engineering of redox active organic molecules (ROMs). Kosswattaarachchi and Cook studied different combinations of anolytes and catholytes, also reporting that these combinations and their concentrations influence cycling behavior and charge–discharge profiles [176]. Mirle et al. [201] proposed a carbazole-based cathode and a group of researchers from the University of Cincinnati proposed a NA-RFB based on all-PEGylated redox-active metal-free organic molecules [202]. Other than the mentioned groups, there have also been studies on ROM as a possible electroactive compound for NA-RFBs [203–207]. Another promising system in which higher cell voltages can be achieved are the thermally regenerative batteries. In such systems, thermal reactions induce a chemical reaction to charge the battery. Most of these types of batteries are generally based on silver or copper [208]. The main advantage of these systems is that in addition to heat-to-power conversion, they are also able to store energy. Recently, a net power density of ~30 W m−2 by a single cell was reported. The cell reported operated at a hydraulic retention time of 2 s (flow rate = 2 mL min−1) and showed a stable power production over 100 successive cycles [208]. Despite recent efforts by researchers to develop NA-RFBs, one of the great challenges is to develop membranes that meet all the requirements for their proper functioning, which include several properties such as high ionic conductivity and selectivity, low swellability, low cost, and high stability, both mechanical and chemical [209–211]. To this end, Yuan et al. in 2021 presented a set of radar plots summarizing the performance, advantages, and shortcomings of current membranes [63]. The analysis of the plots in Figure 7 allows us to observe why none of the current advances in membrane development are solving the problems of the technology. Nevertheless, it is possible to get an insight into which feature can be sacrificed to maximize another in each case. To use NA-RFBs on a large scale, membrane development is a problem that must be overcome. The literature found shows that to move towards advanced NA-RFBs it will be neces- sary to develop and/or optimize the anolytes and catholytes currently available. In this context, improved anolytes and catholytes with enhanced electrochemical performance particularly stability and reversibility will be developed. The analysis of the current state of the art also indicates that the expansion of the redox-active molecules for NA-RFBs started showing promising results of reaching the potential to become interesting for commercial- ization in the last three years. Moreover, for practical applications, the lifetime of redox electrolytes need to be further improved, namely their stability and species crossover.PDF Image | PNNL Vanadium Redox Flow Battery Stack
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