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Iron Flow Batteries


Lithium-ion batteries have dominated the ESS market to date. However, they have inherent limitations when used for long-duration energy storage, including low recyclability and a reliance on “conflict minerals” such as cobalt.

Iron flow batteries (IRB) or redux flow batteries (IRFBs) or Iron salt batteries (ISB) are a promising alternative to lithium-ion batteries for stationary energy storage projects. They were first introduced in 1981. Iron flow batteries are a type of energy storage technology that uses iron ions in an electrolyte solution to store and release energy. They are a relatively new technology, but they have a number of advantages over other types of energy storage, such as lithium-ion batteries. These cutting-edge batteries offer a new approach to storing and distributing electrical energy, promising efficiency, reliability, and sustainability.

An Illustration of Iron Flow Batteries
Source: Triple Pundit

How they Work

The Electrolyte

Unlike conventional batteries with solid electrodes, IRFBs employ a design where two chemical solutions, the electrolyte, is held in two tanks and are separated by a semi-permeable membrane. One of the two tanks contains iron ions in an oxidized state and the other contain iron ions in a reduced state. This membrane serves as a barrier between the two solutions, allowing ions to be exchanged while keeping the solutions physically apart.

The Chemical Reactions

The functionality of IRFBs lies in the electrochemical reactions that occur within these cells. When the battery is in operation, two key processes from which the battery technology derives the word 'redox', short for 'reduction' and 'oxidation' take place simultaneously:

1. Reduction

In one half-cell, a chemical reaction known as "reduction" occurs. During reduction, electrons are gained, resulting in a reduction of the chemical species present. In IRFBs, iron salt is typically used in this half-cell, and it undergoes a reduction reaction. When the battery is charging, the electrolyte solution is pumped through the tank containing the oxidized iron ions. The iron ions are then reduced, which stores energy in the battery.

2. Oxidation

In the other half-cell, an opposing chemical reaction called "oxidation" unfolds. When the battery is discharging, the electrolyte solution is pumped through the tank containing the reduced iron ions. The iron ions are then oxidized, which releases energy from the battery. During oxidation, electrons are lost, leading to the oxidation of another chemical species. Here, iron salt may also be employed, but in an oxidized state.

3. Ion Exchange

The semi-permeable membrane separating the two half-cells is crucial in this process. It allows ions to move from one half-cell to the other while preventing the mixing of the chemical solutions. As a result, ions migrate through the membrane to balance the charge generated by the reduction and oxidation reactions.

4. Production of Chemical Energy

As the reduction and oxidation reactions progress, chemical energy is produced within the IRFB. This energy is stored in the form of the transformed chemical species on either side of the membrane.

5. Electricity Generation

When there is a demand for electricity, the chemical energy stored in the IRFB can be harnessed. This is achieved by completing an electrical circuit that connects the two half-cells. Electrons flow through the external circuit from the half-cell undergoing reduction to the half-cell undergoing oxidation. This flow of electrons constitutes an electric current, which can be used to power electrical devices or be sent to the grid.

6. Recharging

When the battery is charged, the electron flow is reversed. Electrons are supplied to the half-cell undergoing oxidation, and the chemical reactions are driven in the opposite direction. This process restores the original chemical species, readying the battery for the next discharge cycle.

Iron flow battery deployed in container
They are deployed in common containers as shown in this picture.

The Pros

Sustainable Chemistry

Unlike some other battery types that rely on critical minerals like vanadium, lithium, or cobalt, IRFBs utilize earth-abundant materials such as iron, salt, and water. This not only reduces the environmental impact associated with the supply chain but also lowers their overall lifecycle greenhouse gas footprint.

High Efficiency

According to one of the major players in this niche, ESS, their IRFBs are designed for stationary applications and can achieve up to 70% round trip energy efficiency. Although other long-duration storage technologies like pumped hydro energy storage provide slightly higher efficiencies of around 80%, IRFBs offer an attractive balance between efficiency and sustainability.

Zero Degradation Over 20,000 Cycles

Also according to ESS, their IFB are engineered to experience zero degradation over a staggering 20,000 charge/discharge cycles. This longevity is achieved through a clever design: IRFBs circulate liquid electrolytes to charge and discharge electrons via a process called a redox reaction. Importantly, the same electrolyte is used on both the negative and positive sides, eliminating cross-contamination and ensuring stable chemistry for virtually unlimited deep-cycle charge and discharge cycles.

Safety and Sustainability

According to ESS, the electrolyte used in its IRFBs has a pH similar to that of soda or wine, making it safe to handle and reducing the need for extensive fire suppression equipment, secondary containment, or hazmat precautions. Additionally, IRFB systems are substantially recyclable at the end of their life, contributing to a more circular and eco-friendly energy storage solution.

Easy Sizing and Scalability

IRFBs offer remarkable flexibility in sizing and scalability. Unlike typical batteries packaged as fixed cells or modules, flow batteries, including IRFBs, have greater energy storage capacity. This flexibility allows users to precisely align both power output and energy storage capacity with the requirements of their projects, both now and in the future.

Enhanced Electrolyte Health Management

ESS says that their IRFBs incorporate an innovative electrolyte health management system that cleans and rebalances the electrolyte in real-time. This eliminates the need for frequent downtime required for recovery or rebalancing in other flow battery systems. Moreover, it works in tandem with a closed-loop plumbing system to prevent electrolyte evaporative losses and maintain near-atmospheric pressures, enhancing safety and scalability while eliminating the need for electrolyte augmentation during the product's life.

No Cooling Required

All-iron flow batteries also do not require cooling requirements unlike other flow batteries, making installation and maintenance processes easier as well as safer.

ESS Energy Warehouse. Source: ESS
ESS Energy Warehouse. Source: ESS

The Cons


IRFBs are not price-competitive at small scales. However, their per-unit cost of electricity drops as they increase in size. This is primarily because they can be expanded by simply enlarging the electrolyte tanks and increasing the volume of the electrolyte, which is relatively inexpensive. Conversely, lithium-ion batteries become progressively pricier when scaled up, as they require more cathode material, which is costly. Consequently, IRFBs are better suited for large-scale energy storage applications.

Inefficient for Short Discharge

IRFBs may not be the ideal choice for applications requiring short discharge times. Makers of vanadium flow batteries, for instance, expect their products to compete effectively with lithium-ion batteries when providing electricity for more than four hours. The cost competitiveness of IRFBs becomes more apparent as the discharge duration increases. Therefore, they are better suited for long-duration energy storage rather than applications with rapid discharge requirements.

Capacity Dependency on Iron Volume and Limited Scalability

Unlike some other types of flow batteries where the energy capacity can be easily adjusted by varying the electrolyte volume, IRFBs have a limitation. The capacity of IRFBs is not solely dependent on the electrolyte volume but also on the plating iron volume within the negative half-cell. This can make it more challenging to decouple the energy capacity from the stack size. In comparison, certain flow batteries allow for greater flexibility in adjusting capacity by manipulating the electrolyte volume.

Reduced Coulombic Efficiency and Precipitation Issues

During the charge reaction in IRFBs, hydrogen gas can evolve on the negative side of the cell. This can lead to a reduction in coulombic efficiency, meaning that some of the energy put into the battery during charging is lost in the form of gas production, which is undesirable.

Another issue arises from the pH increase within the battery during operation, which can cause the precipitation of insoluble iron hydroxide (Fe(OH)3), commonly referred to as rust. If not adequately managed, this precipitation can lead to cell degradation or failure. However, some IRFBs incorporate rebalancing systems to mitigate this concern and maintain the battery's health.

Auxiliary Components and Maintenance:

IRFBs, like all flow batteries, require auxiliary components such as pumps and valves to facilitate the flow of electrolyte. These components add complexity to the system and necessitate regular maintenance and upkeep. This is in contrast to some other battery technologies, like lithium-ion batteries, which are relatively self-contained and have fewer external components to manage.


IFBs are way more larger and bulkier than lithium-ion batteries. They often come in common containers. For this reason, they are typically used for largescale and heavy-duty grid energy storage or attached to power plants/electrical grids and are not suitable for non-stationary purposes like consumer electronics and electric vehicles.

IRBs on ESS' production floor facility in Wilsonville, Oregon. Source: Bloomberg.
IRBs on ESS' production floor facility in Wilsonville, Oregon. Source: Bloomberg.

American IRB pioneering company, ESS has been working to reduce the cost of IRFBs in a number of ways. First, ESS is developing new manufacturing processes that are more efficient and cost-effective. Second, ESS is recycling components from used IRFBs to reduce the need for new materials. Third, ESS is developing new applications for IRFBs that are more profitable than traditional applications. By developing new manufacturing processes, recycling components, and developing new applications, ESS is working to make IRFBs more competitive and viable for energy storage applications.

In addition to the above, ESS is also working on a number of other initiatives to improve IRFB technology. For example, ESS is developing new electrolyte formulations that can improve the performance and efficiency of IRFBs. ESS is also developing new electrode materials that can reduce the cost and improve the durability of IRFBs.

In September 2023, American Fortune 500 conglomerate and alternative energy capital giants, Honeywell, announced a collaboration with IRFB specialists, ESS, founded in 2011. The goal is to allow Honeywell to integrate ESS' IRFBs into its product offering. It is also expected to drive advancements in technology, cost reduction, and packaging of IRFB systems.

ESS Energy Warehouse
A Peep Into ESS' Energy Warehouse

Iron Flow Batteries are definitely a game-changer in the world of energy storage. Their sustainable chemistry, high efficiency, and exceptional durability make them a compelling choice for a wide range of applications, from renewable energy integration to grid stability. As the energy transition gathers pace, IRFBs and with further advancements and growing adoption, IRFBs are set to become a cornerstone of the global energy landscape. It is yet to be seen how or if IRBs and flow batteries in general can dethrone lithium. In the mean time, lithium reigns supreme.

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