Core component material
Skyllas-Kazacos et al. (2016) yielded a high concentration of vanadium electrolyte for use in the VRFBs by mixing V2O5 powder with sulfuric acid and then introduced SO2 gas for the reduction reaction to occur. In recent years, progress has been made to yield high purity vanadium electrolyte that not only solve the high cost associated with the production but also improves the performance of the VRFBs. This includes the use of a chemical reduction method to prepare high purity vanadium electrolyte. The general reduction method is to dissolve V2O5 in sulfuric acid and then add a reducing agent to reduce V(V) to V(IV) or V(III) or to mix the V2O5 with the reducing agent and sulfuric acid before dissolving the mixture during which the reduction happens (Guo et al., 2023; Ding et al., 2021).
Novel developments in electrolytes include the use of a regeneration cell connected to the VRFB system to reduce the imbalance of electrolytes which is important to improve the operation of the VRFB. Poli et al. (2021) used a method that rebalanced the proportions of V3+ and V4+ by 50/50. The regeneration cell was fed from the positive half-cell to reduce the concentration of V5+ ions in the catholyte. It successfully led to a rebalanced proportions of 50% electrolytes which was 50% V3+ and 50% V4+.
Another development in electrolytes for VRFBs includes the introduction of electrolytes that stay stable over a wide temperature range. For example, Du et al. (2025) developed V2+, V3+ and V5+ electrolytes prepared by electrolytic oxidation or reduction of the prepared V4+ electrolyte. To allow for controlled H+ content, no external hydrochloric acid was added as a chlorine source. The V3+ and V5+ electrolytes were stable over a wide temperature range of -5 oC to 50 oC and when the V3+ electrolyte was used in the VRFB, the energy efficiency of the VRFB was over 80% at 80 mA/cm2.
Novel aqueous ionic electrolyte solutions have also been developed such as by Chivukula and Zhao (2025). This electrolyte mixture containing 1-butyl-3-methylimidazolium chloride (BmimCl) and vanadium chloride showed a maximum theoretical energy density of approximately 44.24 Wh/L, a dynamic viscosity of 36.62 mPa, and an ionic conductivity of 0.201 S/cm at room temperature. The study mentioned that this novel electrolyte was promising for improving the energy density and operational efficiency of VRFBs.
Recent novel developments in membranes for VRFBs include the use of carbon and cross-linked materials. For example, Hu et al. (2024) recommended the use of carbon-plastic composites as a membrane which have good mechanical properties. The presence of graphite in the composites makes the membrane resistant to corrosion. Another example is the porous cross-linked polyamide membrane prepared using β-cyclodextrin by Xu et al. (2022). When used in the VRFB, this membrane displayed higher coulombic efficiency of 96.3-99.6% and energy efficiency of 67.9-88.9% compared to a commercial Nafion 212 membrane that had an coulombic efficiency of 74.3-94.8% and an energy efficiency of 67.7-77.5%.
Another example of a novel material in membranes for VRFBs is a sulfonated polyamide membrane that has a flexible decane chain and rigid tripcyene-based crosslinks (Jing et al. 2025). The rigid structure increased the chain spacing and hydrophilic-hydrophobic chain separation while the amine-functionalized network, also present in the membrane, formed hydrogen bonds with the sulfonated groups to conduct protons. The crosslinked network suppressed the permeation of the vanadium ions. When this novel membrane was used in a VRFB, the VRFB achieved a high energy efficiency of 80.39% at 160 mA.cm2 and over 400 cycles.
There have also been developments in electrodes for VRFBs. For example, Sun et al. (2021) developed a novel electrode made of microscale carbon fibers interweaved with highly porous carbon nanofibers. Due to their large macropores and lower tortuosity compared to carbon fibers, these electrodes had a high permeability and a large specific surface area for redox reactions to occur. When these novel electrodes were used in a VRFB, the VRFB had a 9.9% higher energy efficiency relative to the use of pure porous carbon nanofibers and 14.1% higher than with pure carbon nanofibers.
Besides the development of novel electrodes, advances in electrode technologies for VRFBs include coating the electrodes with metal alloys to enhance the performance of the VRFBs. An example is the use of graphite felt electrodes that had a coating of a mixed metal mixture containing vanadium, niobium, and molybdenum in a study by Tiwari et al. (2024). When used in a VRFB, the VRFB showed an energy efficiency of 80.1% at a current density of 100mA/cm2, which was 9.49% higher than when the VRFB used an unmodified graphite electrode.
Novel developments in bipolar plates for VRFBs also include using bipolar plates made of novel materials such as composite and thermoplastic materials. To improve the conductivity and the strength of the bipolar plates in a VRFB, Park et al. (2014) developed composite bipolar plates that had a maximum conductivity of 114S/cm and a flexural strength of 26 MPa. In another study, Onyu et al. (2022) studied the effects of bipolar plates made of thermoplastic vulcanizates, graphite, and woven carbon fibers. These bipolar plates exhibited higher corrosion resistance and had an electrical conductivity of 595.62 S/cm.
Other materials used for bipolar plates include polypropylene or polyvinylidene fluoride polymers supplemented with graphite particles of different sizes studied by Gupta et al. (2022). The results in this study showed that these compositions affect the surface properties and electrochemical behavior of the bipolar plates in a VRFB. The bipolar plates containing polyvinylidene fluoride and with average graphite particles of 75 µm sizes exhibited higher surface roughness and porosity than the bipolar plates with polypropylene and smaller graphite particles. Among the two materials, bipolar plates made with polyvinylidene fluoride polymers and graphite, due to their higher hydrophilicity and porosity, had lower electrochemical stability than the bipolar plates made with polypropylene and graphite.
Operation optimization: stack designs and flow field designs
Novel advancements in stack designs for VRFBs include evaluating the shunt current in different stack configurations. One such example is presented by Zhao et al. (2023) where circuit-based modeling was used to evaluate the shunt current according to different stack configurations having serial, parallel and mixed connections and thereafter, the coulombic efficiency was studied. Their results showed that shunt currents are more significant at the center cell of a stack than at the other cells in stack configurations and that shunt current is positively correlated with the battery state of charge. Furthermore, the VRFB had higher coulombic efficiencies and energy efficiencies when it consisted of parallel stack configurations than with series and mixed connected stack configurations. This was attributed to the absence of shunt current losses in the piping system in the VRFB with parallel stack configuration.
Developments in novel flow field designs include modification in serpentine flow fields and one example is the use of convection-enhanced serpentine flow field. Huang et al. (2023) showed that compared with serpentine flow field, their novel convection-enhanced serpentine flow field exhibited better mass transfer performance, reduced polarization during charging/discharging, and improved battery efficiencies. In another study, Huang et al. (2025) proposed a flow field design of bionic leaf veins akin to the leaf vein design in nature with the aim to enhance the uniformity of the electrolyte distribution and mass transfer capabilities in a VRFB. Their results showed that the energy efficiency of a VRFB using the flow field with the bionic leaf vein design was improved by 1.98% under the same conditions compared to the serpentine flow field.
System modeling
As the demand for energy efficiency continues, efforts have also been directed at improving the modeling of VRFB at the system level. This includes studies in both circuit and electrochemical models for VRFBs. Among these studies is the proposed comprehensive equivalent circuit model for VRFB by Yesilyurt et al. (2023). The proposed model included several characteristics, among shunt current, ion diffusion, and charge transfer resistance. The model was compared with experimental results and showed an accuracy of 3% under sample operating conditions.
In another study on circuit modeling, a non-linear equivalent circuit model was proposed that represented the non-linear characteristics of a VRFB under variable flow rates (Das et al., 2025). The validation of the model was studied by comparing the terminal voltage from it with the voltage from experimental output.
There have also been novel studies that have attempted to simplify electrochemical models because conventional models are sophisticated to apply. Liu et al. (2023) proposed a model that can be implemented in simulation studies. The hyperparameters were optimized by the honey badger optimization algorithm and the study mentioned this to be simpler to apply compared the conventionally sophisticated electrochemical models. Besides this direction, there has also been advances in physics-based approaches for electrochemical models such as the study by Rao et al. (2023). This physics-based electrochemical model of a VRFB can compute the variations in mass transfer potential over a wide temperature range.
Costs
Cost-effective approaches in the applications of VRBS have focused on developing and using lower-cost components. Examples include the use of a cost-effective composite membrane made of Nafion and lignin in the study by Ye et al. (2021). A composite membrane was made by adding to the Nafion 212 membrane and it was then tested in the VRFB. The composite membrane to which 5% lignin was added to the Nafion membrane exhibited the highest ion selectivity and long-term stability compared to the Nafion membrane without the lignin and with lignin at other proportions. This composite membrane also had lower cost because it had a 52.8% capacity retention after 1000 cycles in the VRFB, meaning was stable over a longer use compared to only 34.8% capacity retention after only 150 cycles for the Nafion 213 membrane.
Another example of cost-effective approaches for VRFBs include optimizing the metal ratio along with using a membrane to achieve a higher energy efficiency after more than 100 cycles as studied by Liu et al. (2025). In this study, vanadium ions were partially replaced with manganese ions to optimize the manganese to vanadium ratio. When this ratio and a polybenzimidazole membrane were used in a VRFB, the results showed an energy efficiency of 79.5% at 100 mA/cm2which was 1.6% higher than without this optimization and membrane use in VRFBs. After 100 cycles, the system retained 66.2% of discharge energy compared to 24.6% in VRFB without the optimization and the membrane use. The study mentioned that the economic analysis indicates that the manganese and vanadium electrolyte mixture as used here can reduce electrolyte costs by up to 45% compared to without their use.
Looking ahead
VRFBs have been used in the electrocoagulation of wastewater and have also been repurposed to CDIs for desalination. Although these technologies have proven to work, they have been met with limited success attributable to the limitations in the core components and operation and system modeling challenges that are compounded due to the high price of raw materials used in the VRFBs.
The recent progress in the development and use of novel materials to solve the challenges have led to significant improvement for VRFBs. These are centered on improving the energy efficiencies of the VRFBs through the use of electrodes that stay stable over a wide temperature range, novel ionic electrolyte solutions, and the use of membranes, electrodes, and bipolar plates composed of novel materials.
In addition to advancements in identifying stack and flow field designs, efforts have also been directed at system-level modeling of VRFBs, not only to improve the energy efficiency but to also simplify the electrochemical models over conventional, sophisticated models. The lower economic cost due to the use of composite materials and the recent progress for higher energy efficiency in VRFBs indicate that VRFBs are the future of energy generation particularly in wastewater electrocoagulation and desalination.
About the Author: This is the final article in a five-part series on Vanadium Redox Flow Batteries written by Dr. Saleha (Sally) Kuzniewski, Ph.D. Dr. Kuzniewski is a scientist and a writer. In addition to her work at the US Geological Survey on bioremediation and microbial ecology projects and her research in the field of environmental microbiology for the Virginia Department of Game and Inland Fisheries and the Salt Institute, she has also authored several scientific publications including in wastewater treatment and technologies. Dr. Kuzniewski received her BS degree in Biology from William Smith College, Master of Environmental Science degree from Memorial University of Newfoundland, and her Ph.D. degree from George Mason University.
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