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Energies 2021, 14, 5643 12 of 45 doping with sulfur [94]. An interesting production method for a new material to be used as electrode was reported by I. Mustafa and colleagues. They produced a macro-porous carbon nano-foam by implementing a freeze-drying step in their tape casting fabrication method. Using these electrodes, their cell achieved an EE of ca. 80% at 50 mA cm−2 for 100 cycles [90]. The bipolar plates also contribute to the ohmic resistances; however, it is not enough to use a more conductive material, the electrical contact resistance between the bipolar plate and the electrode also needs to be reduced. Taking this into account, publications that focus on bipolar plates aim to lessen the ohmic resistances by reducing the electrical resistance of the material, the contact resistance, or both. Han et al. [87] produced bipolar plates with TiO2 nanotubes coated with IrOx. By using this material, it was possible to reduce the thickness of the bipolar plates when compared to the composite graphite typically used. Ca. 83% of EE at 40 mA cm−2 was reported and the battery was cycled for 100 cycles at the same current density without substantial degradation. A new low-carbon-content design based on the bridging effect of graphene for bipolar plates published by Liao and colleagues [89] achieved ca. 83% EE at 140 mA cm−2. The same group proposed another low-carbon-content bipolar plate, composed of graphene, carbon fibers and graphite powders, having reached ca. 81% EE at 140 mA cm−2 with these bipolar plates [93]. Jiang et al. [95] proposed a way to exclude the layer of resin on the surface of composite bipolar plates by using a surface treatment that grows cactus-like carbon nanofibers from catalyst cores. This technique resulted in an EE of ca. 80% at 160 mA cm−2. Membranes have an important influence on two factors: the ionic conductivity and vanadium crossover. Reducing crossover prevents self-discharge, however this is normally accomplished by reducing the ionic conductivity. For this reason, there needs to be a trade- off between these two parameters to assemble a battery that has a high ionic conductivity and low vanadium crossover. D. Zhang and colleagues studied a hybrid membrane using the solution casting method, where they embedded UiO-66-SO3H in a Nafion matrix to enhance the ion selectivity of the membrane. With this strategy, a vanadium permeation of one-third was achieved compared to a recasting Nafion membrane, an EE of 81% at 160 mA cm−2, and the cell could be cycled for 1000 cycles at 80 mA cm−2 without a no- ticeable performance change [97]. There was also an environmentally friendly method proposed to produce positively-charged membranes derived from pyridine-containing poly(aryl ether ketone ketone), where B. Zhang et al. [96] reduced the vanadium perme- ability to 3.4·10−7 cm2·min−1, achieving 80.1% EE at 180 mA cm−2 and stable cycling for 1000 cycles (ca. 600 at 120 mA cm−2 and 400 at 140 mA cm−2). Wan et al. [98] developed a composite membrane with a dense but thin polybenzimidazole layer and a thick but porous layer made with polybenzimidazole electrospun nanofibers. This novel technique of using polybenzimidazole demonstrated a vanadium permeability one order of magnitude lower than a Nafion 212 membrane, an EE of 82% at 150 mA cm−2, and stable operation for 200 cycles at 80 mA cm−2. In 2020, Kushner and colleagues studied the factors that impact the transport phenomena through the membrane in a VRFB. In this manuscript, it was found that there are few studies that analyze crossover at currents different from zero, which is a major flaw in these works, since migrating fluxes are increased at high current densities [99]. Shin et al. [100] suggest an innovative strategy to alleviate electrolyte imbalances, which are caused by crossover and water transport through the membrane. The proposed strategy is based on using different concentrations of sulfuric acid on the positive and negative electrolyte. VRFBs are one of the few RFB technologies that are in a very advanced phase of development, mainly due to the large boost of technological improvement given in the last few years. VRFB have outstanding properties for stationary applications and have one of the best performances in this sector; however, they still have room to grow. Moreover, it is expected that this technology will have a gigantic impact on the energy production transition. It will also impact on the share of the energy storage market in the next few years, since at present there are no alternatives good enough to take its place.PDF Image | PNNL Vanadium Redox Flow Battery Stack
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