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Redox Flow Batteries
Conventional Batteries vs. Flow Batteries
A flow battery, also known as redox flow battery, is a special type of electrochemical cells. They are batteries composed of two chemical components dissolved in appropriate solvents and stored in vessels separated using a membrane. The dissolved substances in the electrolytes (anolyte and catholyte) provide the chemical energy that is transformed to electrical energy. A pump is used to circulate the liquids in the system. Reactions occur at the current collectors sandwiching the ion-selective membranes. Ion exchange and electron transfer occur. The cell voltage of flow batteries typically range from 1 to 2.2 V. Redox flow batteries are also rechargeable. Once an external voltage is applied, the reactions are reversed, driving the regeneration of the fuels in the electrolytes. Aside from that, flow batteries can be instantly recharged simply by changing the liquid electrolyte.
Various chemistries and electrolytes have been explored for flow batteries, with an increasing trend among battery manufacturers to adopt vanadium as a fundamental material for both small-scale and large-scale applications. In the production of batteries, vanadium salts in sulfuric acid form the electrolyte. The utilization of vanadium in redox-flow batteries (VFBs) is gaining prominence as energy companies seek advancements in energy storage solutions.
The difference between conventional and flow batteries is that the chemical energy converted in flow batteries is stored as the electrolyte, whereas in conventional batteries, it is stored as the electrode material.
What makes fuel cells and flow batteries different? Flow batteries work in a similar way to fuel cells. In fact, they have been commonly referred to as regenerative fuel cells. The key difference is that fuel cells directly consume a fuel, such as hydrogen, while flow batteries store and release electrical energy through the redox reactions of liquid electrolytes. In terms of components, fuel cells often use expensive catalysts like platinum, while flow batteries rely on redox-active materials for electrolytes.
How do Vanadium Redox Flow Batteries Work?
The key strength of vanadium RFBs lies in the multiple valence states of vanadium (+2, +3, +4, and +5). This unique feature enables the use of vanadium as redox couples in both negative and positive electrodes. Unlike other RFBs using different electrolytes on each side, vanadium RFBs notably minimize cross-contamination concerns. In a vanadium RFB, the active vanadium species of different valence states are stored in electrolyte tanks. The negative tank contains the V3+/V2+ couple, whereas the positive tank has the V5+/V4+ pair. In both tanks, the vanadium species (1–3 M) are dissolved in sulfuric acid (1–2 M). The electrolytes are non-flammable, non-explosive and no toxic products are generated through cycling.
During discharge, V3+ is oxidized to V2+ at the negative side. The generated electron flows through the current collector, the load, and finally to the current collector at the positive side. Ions from the electrolyte (either H+ or SO42–) flow through the ion exchange membrane. Using the ions and generated electrons, VO2+ is then reduced to VO2+ on the positive electrode.
During charging, the reverse processes occur. In simpler terms, throughout the charge-discharge cycle, the active vanadium species in the electrolyte undergo reduction and oxidation, effectively converting chemical energy into electrical energy. Maintaining electroneutrality is seamlessly achieved by transporting ions within the electrolyte through the membrane. The presence of dissolved redox species in the recirculated solution distinguishes RFBs from other batteries that store energy within electrode structures, like lead-acid and lithium-ion batteries. The unique 'non-participating electrode' structure of VRBs allows for the independent customization of power rating and storage capacity.
Flow Battery Components
Conventional batteries, exemplified by lithium-ion batteries, employ fixed anodes and cathodes made of materials like graphite or lithium-containing compounds. They utilize either liquid or solid electrolytes to facilitate ion movement between the anode and cathode, separated by a porous material. A Battery Management System (BMS) is crucial for monitoring and managing various parameters during the charging and discharging processes.
In contrast, redox flow batteries (VRFBs) feature an innovative design. Both the anode and cathode active materials in VRFBs consist of salts dissolved in a liquid electrolyte, eliminating the need for distinct materials. The electrolyte is stored in external tanks, offering scalable energy capacity. An ion exchange membrane (IEM) separates the two electrolyte tanks, allowing protons to pass through while preventing mixing of ions. Multiple cells are combined in a stack, and a pump and flow system circulate the electrolyte between the tanks and through the stack during charging and discharging. This unique design provides advantages in scalability and long cycle life, particularly in applications like grid energy storage.
In vanadium redox flow batteries (VRFBs), the electrodes commonly used are primarily carbon-based. This includes materials like graphite felt, carbon felt, and carbon paper. These carbon-based electrodes serve as crucial components, facilitating the electrochemical reactions and the flow of vanadium ions during the battery's charging and discharging processes. The use of carbon materials contributes to the electrodes' conductivity and durability, making them integral to the overall performance of VRFB systems.
Electrodes for Redox Flow Batteries
In redox flow batteries, energy is stored in solutions that flow through the system, with their capacity being directly related to the volume of the solution tanks. This battery type is made up of two electrodes separated by a membrane. The energy storage and conversion processes occur on the surface of these electrodes, making the properties of the electrode materials critical to the battery's performance.
Carbon materials, such as graphite felt and graphitized carbon paper, are typically used as electrodes in redox flow batteries. These materials are chosen for their excellent conductivity, chemical stability, and high surface area. Several factors determine the performance of carbon electrodes in VRFBs:
The mass transfer of ions and protons in the pores of the carbon material, as well as the transport of electrons within the carbon structure, significantly affect the reaction rate. Efficient transport properties ensure that reactants reach the active sites quickly and that the generated electrons are efficiently collected and transported away from these sites.
Wettability
The wettability of the carbon electrode affects the penetration of the electrolyte into the pores, which in turn influences the accessibility of the active sites for the redox reactions. Improved wettability ensures better contact between the electrolyte and the electrode, enhancing the overall reaction kinetics.
Surface Area and Pore Structure
The surface area of the carbon material, along with its pore size distribution, directly impacts the number of active sites available for the redox reactions. A higher surface area provides more active sites, which can enhance the battery's capacity and efficiency.
Active Sites
The amount and nature of active sites on the carbon surface determine the charge transfer kinetics. An increase in the number of active sites can lead to a higher reaction rate, thereby improving the battery's charge-discharge efficiency and power density.
LINQCELL Graphite Felts are non-woven materials specifically engineered for use in redox flow batteries. GFP graphite felt series is made from high-quality polyacrylonitrile (PAN) fibers. These fibers are processed into a non-woven structure, which is then carbonized and graphitized. The result is a material with exceptional properties: it exhibits high electrical conductivity due to high-temperature treatment above 1000 °C, superior oxidation resistance, and excellent thermal stability. The non-woven structure of LINQCELL GFP provides flexibility and resilience, making it adaptable to various applications.
| Product | Felt Type | Thickness [mm] | Basis Weight [g/m2] | Voltage Loss [mV] | Roll Width [cm] |
|---|---|---|---|---|---|
| GFP2000 | PAN Graphite | 2 | – | <35 | >40 |
| GFP3000 | PAN Graphite | 3 | – | <35 | >40 |
| GFP6500 | PAN Graphite | 6.5 | 550±100 | – | >90 |
Ion Exchange Membranes for Redox Flow Batteries
Ion exchange membranes serve as a separator between the cathode and anode compartments within the redox flow battery stack. Ideally, it should only allow non-vanadium ions to pass through, maintaining electrolyte purity and ensuring charge balance in the system.
Basically, the ion exchange membrane in redox flow batteries needs to do two things:
ALLOW ONLY SPECIFIC IONS (LIKE H+) TO MOVE THROUGH
The IEM must facilitate the transport of protons (H⁺) or other charge carriers to maintain high ionic conductivity and enable efficient charge transfer between the cathodic and anodic compartments.
BLOCK UNWANTED IONS
The membrane must act as a barrier to prevent vanadium ions (V²⁺, V³⁺, VO²⁺, VO₂⁺) from migrating between the positive and negative electrolyte compartments. Vanadium ion crossover disrupts electrolyte balance, leading to capacity fade and self-discharge, which reduce battery efficiency over time.
An ideal IEM should have the following key properties to ensure optimal performance in redox flow batteries:
IEMs must have high ionic conductivity, efficiently conducting the target ions to facilitate efficient charge transfer. Area-specific resistance (ASR) or ionic conductivity can be used as indicators of this property, with lower values reflecting better performance.
The membrane should exhibit high ion selectivity, meaning it should allow the desired ions to pass through while effectively blocking vanadium ions, thereby preventing crossover and maintaining electrolyte balance. This is measured by the selectivity ratio (S), which compares proton conductivity to vanadium ion permeability. Higher selectivity ratios indicate better selectivity.
Chemical stability is crucial, as the membrane must withstand the highly acidic and oxidative environment of the VRFB without degrading, especially due to vanadium ion attack. If the membrane degrades, it can lead to increased ion crossover, reducing efficiency and causing premature failure. Additionally, the material must resist physical and chemical damage over time to ensure a longer operational lifespan and consistent performance under demanding conditions.
LINQCELL VRB-MEM Membranes are perfluorinated cation exchange polymers in the acid H+ form produced through solution casting. This chemistry and manufacturing method provide several benefits, including high acid capacity and conductivity, enhanced crystallinity and tensile strength, reduced linear expansion and anisotropy, and an extended service life. These characteristics make LINQCELL VRB-MEM Membranes highly recommended for use as ion exchange membranes in redox flow batteries, particularly in all-vanadium flow batteries.
LINQCELL VRB-MEM Membranes are available in two standard thicknesses—50 µm (VRB-MEM50) and 75 µm (VRB-MEM75)—and can also be custom-fabricated to meet specific requirements. The maximum roll width is 75 cm.
| Product | Type | Thickness [µm] | Conductivity [S/cm] | Available Acid Capacity [meq/g] | Water Content [%] |
|---|---|---|---|---|---|
| VRB-MEM50 | Cation exchange | 50 | 0.1 | 1 | 5 |
| VRB-MEM75 | Cation exchange | 75 | 0.1 | 1 | 5 |
Gaskets for Redox Flow Batteries
Redox flow batteries operate under challenging chemical conditions, typically between 40°C and 60°C, using an electrolyte of dissolved salts in sulfuric acid. These conditions demand that all components of the battery, including gaskets, meet stringent performance criteria.
Gaskets are crucial in RFB stacks, yet their importance is often underestimated. They play a pivotal role in ensuring the mechanical integrity of the battery stack. The selection of appropriate gasket materials is essential to prevent cracks in bipolar plates and to maintain the structural integrity of the membrane. A well-chosen gasket must seal the stack effectively to manage cumulative tolerance effects during both design and operation.
The primary functions of gaskets in an RFB stack include:
- Sealing and Leakage Prevention: Gaskets must prevent leaks in both the anode and cathode areas, as well as in cooling plates.
- Compensation of Tolerances: They must accommodate dimensional changes during stack assembly caused by interactions with different stack components.
- Mechanical Integrity: Gaskets need to support the stack without causing damage to other components.
Given the diverse requirements and specific operating conditions of different RFB systems, gaskets must be highly customized for each stack manufacturer. This customization ensures that the gaskets can effectively support the mechanical and chemical demands of the battery stack, thereby enhancing the overall efficiency and longevity of the system.
Several materials are commonly used for gaskets in RFBs, each with distinct properties:
EPDM, a synthetic rubber from the 1960s, excels in redox flow batteries. Its flexible ethylene, propylene, and diene monomer composition ensures tight seals, resisting chemical damage and gas leaks. With high-temperature stability and low gas permeability, EPDM enhances performance and lifespan in harsh conditions.
Fluorine Kautschuk Material (FKM)
FKM, or fluorinated rubber material, has high chemical resistance and high temperature stability (–20 °C to 250 °C). Made from fluorinated hydrocarbon polymers, FKM resists acids, bases, fuels, and solvents, ensuring reliability in demanding environments. Their flexibility and compression set resistance provide effective sealing under varying pressures and temperatures.
Viny Methyl Silicone (VMQ) Rubber
VMQ, a synthetic polymer made from silicon, oxygen, carbon, and hydrogen, offers superior flexibility, temperature resistance (–50° C to 230 °C), and sealing performance. Silicone rubber resists water, chemicals, and extreme temperatures, and its low compression set ensures long-term reliability. Additionally, it is compatible with a wide range of materials, preventing contamination and ensuring seamless integration in various applications.
| Flat Gasket Chemistry | Density [g/cm3] | Elongation at Break [%] | Tensile Strength [MPa] | Hardness [Shore A] | Operating Temperature [°C] | Voltage Loss (mV) |
|---|---|---|---|---|---|---|
| EPDM | 1.45–1.50 | 200 | 2.5 | 80 | –57 to 150 | 29.8 |
| FKM | 1.90–2.00 | 250 | – | 75 | –50 to 260 | 35 |
| PTFE | 2.10–2.30 | 150 | 21–28 | – | –180 to 260 | 27.6 |
| VMQ | 1.25–1.30 | 350 | 6 | 65–75 | –50 to 210 |
Frequently Asked Questions about Redox Flow Batteries
While metals can technically be used as electrodes in redox flow batteries (RFBs), the industry predominantly uses carbon-based electrodes due to several key advantages. Metal electrodes face practical limitations such as susceptibility to corrosion in acidic or alkaline environments, high costs for corrosion-resistant metals, and lower surface area compared to carbon materials.
Carbon electrodes, on the other hand, offer excellent corrosion resistance, high surface area for improved electrochemical activity, cost-effectiveness, and good electrical conductivity. These characteristics make carbon materials the preferred choice for RFBs, ensuring long-term stability, efficiency, and affordability for large-scale energy storage applications.
The thickness of electrodes in redox flow batteries varies depending on the design and application. Carbon felt electrodes typically range from 2 to 5 millimeters thick, offering high surface area and good conductivity. On the other hand, graphite plates or sheets can vary from 3 to 10 millimeters in thickness. The exact thickness is chosen to balance the need for sufficient surface area for redox reactions and minimal resistance to fluid flow, optimizing power density and system performance.
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