Core component materials
The core component materials include electrolyte, membrane, electrode, and the bipolar plate. The basic properties of the VRFB are dependent on these core component materials. Specifically, the solubility of the electrolyte determines the energy density of the VRFB. The transfer of ions depends on the type of membrane used while the polarization of the VRFB depends on the electrode type and porosity. Electrolytes containing vanadium face stability and impurity issues. The stability issue arises due to the low stability of the vanadium ions in the operating temperature range of the electrolyte at 10oC to 40oC (Guo et al., 2023).
Additionally, the rate of mass transfer and other efficiency parameters in the VRFB depend on the presence of aluminum ions in the electrolyte. For example, the presence of 50 ppm Al3+ in the vanadium electrolyte has slowed down the mass transfer of the V4+/V5+ and V2+/V3+ redox reactions. As another example, Pahlevaninezhad et al. (2022) investigated the effects of Al3+ among other metal ions on the voltage efficiency and capacity retention of VRFBs. The results showed that the presence of 0.1M Al3+ decreased the voltage efficiency and the capacity retention of the VRFB.
Ion exchange membranes are used to transfer ions and separate the electrolytes. However, they could have problems in balancing the ionic conductivity and selectivity in a VRFB (Hu et al, 2024). The type of membrane used affects preferential water flow in VRFBs with amphoteric membranes showing the least preferential water transfer (Vlasov et al., 2022).
Membrane thickness also affects the VRFB’s energy efficiency as shown in a study by Huang et al. (2023). In this study, the membrane thickness was studied to determine its impact on the energy efficiency of the VRFB. The results showed that increasing the thickness of the ion conducting membrane allows the membrane to be effective in preventing cross-over of the vanadium ions. This study also analyzed the effects of four Nafion conduction membranes including Nafion 211 membrane on the energy efficiency of the VRFB. Due to Nafion 211 membrane’s thinness of only 25.4 µm, vanadium ions are able to cross the membrane at a higher flow rate compared to with other membranes of higher thickness and thus, the use of Nafion 211 yeilded the lowest energy efficiency in the VRFB compared to when other Nafion membranes of higher thickness were used.
Along with membrane thickness, membrane composition also affects the performance of VRFBs as shown by Noh et al. (2017). This study showed that meta-polybenzimidazole membranes have a much lower, negligible cross-over of V3+ and V4+ ions compared to the Nafion membrane due to the electrostatic repulsive forces between the vanadium cations and the positively charged PBI backbones and the molecular sieve effects of the nano-size pores in polybenzimidazole. The current efficiency was 99% for the meta-PBI membranes of 15 µm thickness compared to for the Nafion 212 that had a thickness of 50 µm and a current efficiency of 94%. Interestingly, the current efficiency increased for the Nafion membranes in the order of Nafion 212 (50 µm thickness) > Nafion 117 (175 µm thickness) while the opposite was observed for meta-PBI membranes which was PBI (15 µm thickness) > PBI (25 µm thickness) > PBI (35 µm thickness). The study mentioned that thick meta-PBI membranes require larger potentials to achieve the same charging current compared to thin membranes and recommended that membranes be thin for the VRFB to achieve high current efficiency.
The physical and chemical properties of the electrode also affect the performance of the VRFBs. The numerically calculated values showed that as the electrode thickness increased, the overall potential (voltage) of the VRFB decreased. The overall potential was the highest for the electrode with 1 mm thickness. The study also showed that the cell voltage was higher for electrodes of higher porosity. Specifically, the cell voltage was 1.35 V for an electrode of 0.9 porosity at 50 ml/min of electrolyte flow and this value was 1.31 V for an electrode of 0.3 porosity and the same electrolyte flow rate.
Jeong et al. (2021) demonstrated that the pore size of the electrode is an important aspect for the energy efficiency of the VRFBs by examining the pseudo-channel effects. They developed a method to enhance the performance of carbon paper electrodes in a flow-through type VRFB. The pseudo-channel effect is a name given to a flow phenomenon that has the same effect as the flow channel along the hole. The carbon paper with the holes of diameter 0.5 mm led to the highest voltage efficiency of 87.3% and relative to the carbon paper electrode without the holes, it led to an increase in energy efficiency by 10.41%. This was attributed to the large surface area due to the holes for contact with the electrolyte. The porosity patterns also affect the performance of the VRFBS. For example, Wang et al. (2022) developed novel electrodes with different patterned porosity using electrospinning technology and showed that the linear porosity variation of the electrodes mainly affects the local overpotential in the electrode.
The bipolar plates are an important component of the VRFB. Their conductivity is affected by the material used. For example, metal bipolar plates possess excellent mechanical characteristics but due to the acidic nature of the electrolytes, they could suffer from electrochemical corrosion (Lourenssen et al., 2019). In recent years, studies have explored the use of new materials including graphite for bipolar plates. However as Kim et al. (2021) pointed out, pure and expanded graphite swells and become brittle over time when used in VRFBs. The study also observed the effects of polytetrafluorethylene additives on graphite bipolar plates that were used in VRFBs and reported that the graphite bipolar plates had less swelling when PTFE was added to them. Swelling was negligible when 10% PTFE was used compared to when 3% and 6% PTFE were used. The resistivity of the graphite bipolar plate was also higher, 12.33 mΩ-cm2 compared to in the absence of PTFE which showed a resistivity value of 3.15 mΩ-cm2.
In addition to the material used, the width of the channels in the bipolar plates also affect the VRFB performance, namely current-voltage characteristics as shown by Slawinski et al. (2025). The study showed that channel widening of the bipolar plates reduces the flow resistance leading to higher efficiency of the electrochemical reactions in the VRFBs.
Operation optimization
Operation optimization in the VRFB includes optimizing the stack designs and flow field designs for current production. As Gundlapalli et al. (2018) noted, designing the stack in a VRFB is complicated due to the presence of numerous parameters including the cell size that affects the performance of the VRFBs. In their study, they arranged a short stack of four cells connected in series and evaluated their electrochemical performance. Increasing the cell size increased the gravimetric and volumetric efficiency of the stack in the VRFBs.
The module layout of the stacks affects the performance of the VRFB because it can magnify the impact of the electrical resistance. In their study on the effects of stack layout, Chen et al. (2019) showed that grouping stacks with similar resistance into the same branch can improve the module charging capacity and this can be further improved by optimizing the flow rate for the stack with the highest resistance.
Over the years, the flow field design of the VRFBs have evolved from simple, conventional models to complex and optimized configurations which is important to not only unlock the efficiency of the VRFBs but to also develop cost-effective energy storage solutions for the VRFBs. As an example, Liu et al. (2024) studied the effects of different types of flow field designs, namely the circular obstruction bionic flow field (CBFF), the rhombus obstruction bionic flow field (RBFF), and the triangular obstruction bionic flow field (TBFF). Among these flow field designs, the use of CBFF showed 1.3% higher power output than for RBFF. The CBFF demonstrated the best performance due to its structural characteristics. These best performance of the VRFB were specifically the uniform distribution of electrolyte concentration, velocity distribution, and reduced stagnant flow of the electrolyte.
System modeling
System modeling in a VRFB includes different types of circuit and electrochemical models. Xiong et al. (2019) developed a VRB model consisting of two similar circuits, an electrical circuit model where the electrochemical behavior of the VRB is modeled by a second-order RC network that considered the concentration of the vanadium ions and the electrochemical activation and a thermal equivalent network that modeled the heat transfer process in the VRB system. The results showed that the coupling of both models, called the electrothermal model, can enhance the VRB performance under various operating conditions.
As Munoz-Perales et al. (2022) noted, next generation redox flow batteries will benefit from the progress of continuum models that will allow the optimization of operational parameters without the need for expensive experimentations. They proposed a 2D steady-state isothermal model of a unit cell VRFB that included the sulfuric acid dissociation equilibrium and local mass transfer coefficients along with experimentally measured electrochemical kinetic parameters and electrolyte properties. The proposed model was then tested with parametric sweeps and risk maps were generated for crossover and self-discharge rates, and equilibrium ion concentrations. The model had the ability to perform predictions at the system level.
Commercial challenges
The high price of raw materials that are used in the different components of a VRFB are the main commercial challenges associated for research, development, and application of VRFBs.
The cost of the vanadium electrolyte used in VRFBs is high because the vanadyl sulfate crystals used in the production of vanadium electrolyte is high (Guo et al., 2023). To produce the vanadium electrolyte, VOSO4 is dissolved in sulfuric acid and in recent years, to find cheaper ways to produce the vanadium electrolyte, V2O5 became a substitute for VOSO4 (Martin et al., 2020; Ding et al. 2021). However, high purity V2O5 is required because the presence of metal impurities can negatively affect the performance of VRFBs. Notably, V2O5 attracts other metals due to its chemical structure and thus, high-purity V2O5 is costly because it is expensive to remove the tightly-bound metal impurities from it.
In the next segment
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. These developments for VRFBs will be discussed in the next segment.
About the Author: This is the fourth 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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