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Energies 2021, 14, x FOR PEER REVIEW 27 of 48 Energies 2021, 14, 5643 Elsevier. The major concern of this approach is the high crossover that leads to a coulombic efficiency of ca. 50% [33]. Despite several configurations being designed and tested to date in order to decrease the crossover effect, this has not been achieved [215–219]. Not only that, but in some cases, this effect is so high that is not possible to operate the cell in re- generative mode, and therefore the device is called a laminar flow fuel cell [215,216,218– 222]. Additionally, the diffusion layer between the two electrolytes thickens towards the channel, which increases the cell resistance [33]. An alternative to reduce this effect is to increase the flow rate; however, this also represents a limitation since the flow must be kept laminar in order to avoid the mixture of electrolytes [33]. Finally, since one of the major concerns about laminar flow membraneless cells is the reactant flow control, most of works were performed in microfluidic fuel cells. Despite 25 of 45 in Figure 9. chemical properties of electrolytes create an interface between the two phases, it is possible The laminar membraneless relies on hydrodynamic principles to keep a parallel lam- inar flow between the two electrolytes, creating an interface between them that is respon- this having a positive impact on the efficiency and current density of the device, it also Figure 8. Representation of a laminar membraneless RFB. Adapted from Ref. [33], Copyright 2017, siblienfcoreratshesitohneicheaxlclhenanggeet.oTuhpiscsatrleatehgisy tiescihllnuosltoragtyed[2i1n6,F2i1g9u,2re208,.222–225]. this having a positive impact on the efficiency and current density of the device, it also Figure 8. Representation of a laminar membraneless RFB. Adapted from Ref. [33], Copyright Figure 8. Representation of a laminar membraneless RFB. Adapted from Ref. [33], Copyright 2017, increases the challenge to upscale this technology [217,220,221,223–226]. Else2v0i1e7r., Elsevier. The immiscible membraneless RFB relies on thermodynamical principles to keep a parallel flow between two or more immiscible electrolytes. Therefore, since the very chem- The major concern of this approach is the high crossover that leads to a coulombic The immiscible membraneless RFB relies on thermodynamical principles to keep ical properties of electrolytes create an interface between the two phases, it is possible to efficiency of ca. 50% [33]. Despite several configurations being designed and tested to date a parallel flow between two or more immiscible electrolytes. Therefore, since the very design an electrochemical device without using a membrane. This concept is illustrated in order to decrease the crossover effect, this has not been achieved [215–219]. Not only that, but in some cases, this effect is so high that is not possible to operate the cell in re- to design an electrochemical device without using a membrane. This concept is illustrated in Figure 9. generative mode, and therefore the device is called a laminar flow fuel cell [215,216,218– 222]. Additionally, the diffusion layer between the two electrolytes thickens towards the channel, which increases the cell resistance [33]. An alternative to reduce this effect is to increase the flow rate; however, this also represents a limitation since the flow must be kept laminar in order to avoid the mixture of electrolytes [33]. Finally, since one of the major concerns about laminar flow membraneless cells is the reactant flow control, most of works were performed in microfluidic fuel cells. Despite this having a positive impact on the efficiency and current density of the device, it also increases the challenge to upscale this technology [217,220,221,223–226]. The immiscible membraneless RFB relies on thermodynamical principles to keep a parallel flow between two or more immiscible electrolytes. Therefore, since the very chem- ical properties of electrolytes create an interface between the two phases, it is possible to Figure 9. Schematic diagram of the working principle of an immiscible membraneless RFB. Adapted Figure 9. Schematic diagram of the working principle of an immiscible membraneless RFB. Adapted from Ref. [226], design an electrochemical device without using a membrane. This concept is illustrated from Ref. [227], Copyright 2017, John Wiley and Sons. Copyright 2017, John Wiley and Sons. in Figure 9. The major advantage of this kind of configuration in comparison with the laminar flow strategy is the reduction of reactant cross-over effect since the electrolytes are spontaneously separated due to their partition coefficients [227,228]. Additionally, it is easier to recirculate the electrolytes to operate in regenerative mode (charge–discharge cycles) [226–230]. However, as long as the anolyte and catholyte molecules on the interface are constantly in contact, this strategy still leads to higher indexes of self-discharge, which currently is one of the major drawbacks to be surpassed [228–230]. Despite there being some more advanced strategies to use the immiscible membraneless configuration, e.g., two non- aqueous phases separated by an aqueous phase, these devices are still on the laboratory scale due to their low energy density, solvent evaporation (which leads to unstable CDC), high ohmic resistance, and reliance on flammable organic solvents [231]. Finally, since the separation of electrolytes is provided by the partition coefficient, it also means that in order to select a redox pair for this battery configuration, one must not Figure 9. Schematic diagram of the working principle of an immiscible membraneless RFB. Adapted only combine good electrochemical stability and reversibility with a high concentration in from Ref. [227], Copyright 2017, John Wiley and Sons. their media but also an appropriate partition coefficient to guarantee the main advantages of this strategy [213,226,228–230].PDF Image | PNNL Vanadium Redox Flow Battery Stack
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