- Redox Flow Batteries (RFBs) have significant potential for grid-scale energy storage due to their unique ability to decouple power and energy density in the system.
- Vanadium RFBs have gained popularity as vanadium ions can be used in both the positive and negative half-cells, reducing the risk of cross-contamination.
- Significant development is still needed to drive down the cost of the membrane and electrolytes and to improve electrode activity before RFBs are ready for widespread commercial adoption.
The field of batteries is currently undergoing massive technological developments, particularly to electrify the transportation sector. Li-ion batteries have been instrumental over the past several decades in enabling energy storage for electric vehicles. However, when employed on a grid-scale, these systems often suffer from high costs and safety hazards. So, what kind of batteries can be employed on the grid-scale to power your house? One of the promising electrochemical energy storage technologies that can be operated on the grid-scale is the Redox Flow Battery (RFB) [1–3].
RFBs consist of positive and negative electrolyte reservoirs, an electrode-membrane assembly (called the stack) and pumps for flowing the electrolyte over the electrodes. The electrolyte reservoirs are large tanks of arbitrary size that contain electroactive components — generally transition metals or organic molecules — dissolved in aqueous or non-aqueous solvents. These electrolytes, which comprise the two half cells of the battery, flow over the stack where reversible oxidation and reduction reactions take place. The two half cells are typically separated using ion-selective semi-permeable membranes. A conversion between the oxidized and reduced forms of the redox couples leads to the charging/discharging of the battery. During charging, an external source of energy supplies the electrons to convert the discharged forms of the redox couples to the charged forms, which store the electricity in the form of chemical energy. During discharge, the electrolyte can be pumped back through the stack to convert the redox couples back to the discharged forms, thus regenerating the electrical energy. For instance, when a vanadium RFB discharges, V(V) accepts an electron and is converted to V(IV) at the positive electrode. Conversely, on the negative electrode, V(II) is converted to V(III); these reactions are reversed during charging. In this way, a redox flow battery interconverts between chemical and electrical energy.
Considerable research has been undertaken in the field of RFBs since their inception in the 1970s. One of the early demonstrations of RFBs was based on a Fe/Cr redox system. However, low energy densities, electrolyte contamination from the crossover of active species and expensive electrode materials motivated further research into other redox couples. Vanadium based system soon garnered increasing attention due to the ability of the vanadium ion to exhibit four different oxidation states of +2, +3, +4 and +5. This enabled the use of the same electroactive species in both half cells, leading to a significant reduction in contamination (as discussed below) and thus, extending the battery life. As a result, vanadium RFBs are some of the most widely commercialized systems to date. More recently, non-aqueous RFBs have been gaining traction due to their ability to span a wider potential range of two to three volts compared to the traditional aqueous systems which are limited to 1.23 V due to the hydrolysis of water.
RFBs demonstrate multiple advantages when employed on the grid-scale. They typically operate at energy conversion efficiencies of 70–85% and can be easily scaled from the kWh to MWh energy range. The energy storage capacity of an RFB can be increased by simply increasing the concentration of the electrolyte or by increasing the volume of the electrolyte. On the other hand, increasing the capacity of a traditional secondary battery, such as Li-ion, requires packaging multiple smaller units together. Thus, the cost associated with scaling up of RFBs is much smaller when compared to traditional secondary batteries, which makes them particularly attractive for grid-scale applications. In addition, RFBs possess a unique advantage in that they can independently scale the power and energy density of the system depending on the application. The energy density of RFBs is dependent on the volume and concentration of electrolytic species and the power density is dictated by the number of electrode assemblies and surface area of each electrode, thus decoupling the two parameters. Moreover, RFBs benefit from low degradation rates since the stack can be completely drained of the electrolyte when the system is not in operation. Thus, they exhibit long lifetimes, typically ranging from 10–15 years of operation. Finally, pumps constitute the sole moving parts in RFBs and can be easily disassembled for service, allowing for low maintenance costs.
Significant challenges still remain in the field of RFBs to enable their widespread commercial adoption. One of the major drawbacks of RFBs is their massive size due to the use of large electrolytic tanks. While the electrode assembly and other ancillary components are fairly compact, the bulky tanks make them less suitable in applications for which space is at a premium, such as electric vehicles. The high cost of the electroactive components and membranes is another major challenge. While vanadium RFBs are among the most commercialized systems, the cost of vanadium itself constitutes greater than 50% of the total RFB cost. Although other redox couples, such as Fe, can be employed to drive down cost, these systems are typically paired with a different redox couple in the other half cell. This requires the use of superior ion-selective membranes, which in turn increases the cost of RFBs. In addition, RFBs that employ two different redox couples in the two half-cells are susceptible to capacity fade occurring from the crossover of the active species. This also can be mitigated by developing superior membranes, but this again increases the cost of the system.
Current research in RFBs now revolves around improving performance metrics by developing novel electrode/electrolyte materials and improving cell design. For instance, improving the solubility of active species in the supporting electrolyte and developing multi-electron transfer redox couples are some of the aspects that are routinely targeted to achieve high energy density RFBs. The development of chemically stable redox couples that exhibit low degradation rates, high cell potentials, and negligible side-reactions in the corresponding solvents are also key research areas for emerging RFBs. In addition, improvements in electrode activity that can enable the operation of RFBs at much higher efficiencies and higher current densities form the basis of research for both developed and emerging RFB redox couples. Significant research in RFBs is also targeted towards the development of cost-effective semipermeable membranes and cell designs that result in lower transport losses, in an effort to improve the performance of the system.
Despite several challenges, RFBs have significant advantages over other electrochemical and potential energy storage systems that are currently employed on the grid-scale and can provide the excellent efficiency, cost, and capacity metrics needed for widespread commercial adoption in the future energy storage market.