VoltStorage (€65M for low-cost, temperature-resistant iron flow battery for renewable energy storage)

VoltStorage, a German energy storage startup founded in 2016, has developed vanadium redox flow battery technology for industrial and agricultural sectors to meet their energy requirements during periods of low wind and low sun. Meanwhile, the company develops a new cost-effective iron redox flow battery system with a 70% energy efficiency and 20-year lifespan. Its iron flow battery is also temperature-resistant, making it a promising energy storage solution that can be used globally to ensure renewable base load provision.

Challenges: Store renewable energy at a low cost

Redox flow batteries for renewable energy storage

By 2022, the world had about 1,185 GW of installed solar capacity and about 906 GW of installed wind capacity. Solar power has grown at a 24% annual rate over the last decade. The US solar industry installed 6.1 GW of capacity in the first quarter of 2023, 47% more than in the first quarter of 2022. On the other hand, the average power of new wind turbines installed in the US in 2022 was 3.2 MW, 7% higher than in 2021.

As more and more solar cells and wind turbines are installed, the ability to store this clean electricity would significantly increase the efficiency and reliability of these intermittent renewable energy sources.

Flow batteries store and release electrical energy through the movement of electrolytes. A typical flow battery consists of two half-cells separated by a membrane. Each half-cell contains an electrode and an electrolyte. The anolyte and catholyte contain redox-active species dissolved in an aqueous or non-aqueous solvent. They are stored outside of the battery. When the battery is charged or discharged, the electrolytes flow through the half-cells in contact with the electrodes via external pumps. This is very different from lithium-ion batteries.

The majority of flow battery systems are based on vanadium, Fe—Cr, and Zn—Br. They are simple systems and easy to scale up. They can have a long lifespan of more than 20 years. This makes them good candidates for storing renewable energy.

However, the high cost of vanadium salts, as well as the capacity decay and toxicity of the last two systems, has limited their application worldwide. The capital cost of flow batteries is around $800 per kilowatt-hour, which is more than twice as much as lithium-ion batteries.

Low-cost, eco-friendly iron flow batteries

The low-cost, eco-friendly iron flow battery uses iron and cheap aqueous electrolytes that are made of earth-abundant iron salts. The anolyte contains Fe²⁺ / Fe³⁺ redox couple and the catholyte contains Fe²⁺. To make the conductivity even better, the electrolytes also have chloride salts like potassium chloride (KCl), ammonium chloride (NH₄Cl), and sodium chloride (NaCl).

However, an iron flow battery generally has a parasitic reaction that releases hydrogen gas during its operation. This hydrogen gas evolution reaction causes an increase of the pH value of the catholyte and an imbalance in the ratio of iron oxidation states in the catholyte to anolyte. Specifically, the state of charge (SOC) of the anolyte is higher than the SOC of catholyte. The capacity and life of a battery go down when the pH changes and the SOC of both electrolytes is out of balance.

VoltStorage Technology

VoltStorage develops a new iron flow battery system that can rebalance the SOC of anolyte and catholyte and restore the aqueous electrolytes to their initial state using simple and inexpensive means. This iron flow battery system has a primary flow cell and a rebalancing flow cell. The primary flow cell operates in charge and discharge modes, whereas the rebalancing flow operates only in discharge mode to reduce the SOC of the anolyte when needed. To restore anolyte and catholyte, the iron flow battery system also has additional tanks of HCl solution, base solution, iron powder, and reducing solution.

VoltStorage iron flow battery

The diagram below depicts the VoltStorage iron flow battery system.

The system comprises a primary flow cell, a rebalancing flow cell, and storage tanks of anolyte, catholyte, acid solution, base solution, reducing solution, and iron powder, as well as pumps, valves, sensors, and controller (not shown).

Primary flow cell

The primary redox flow cell has two half-cells with the negative and the positive electrode separated by an ion-conducting membrane separator. The two half-cells are fed from the anolyte and catholyte tank by the electrolyte circulation pumps.

The primary flow cell operates in charge and discharge modes:

Negative electrode: FeCl₂ + 2e⁻ ⇄ Fe⁰ + 2Cl⁻

Positive electrode: 2FeCl₂ + 2Cl⁻ ⇄ 2FeCl₃ + 2e⁻

The charging of the primary flow battery has a parasitic hydrogen gas evolution reaction, which raises the pH of the catholyte. In this case, the pH can be lowered by adding concentrated HCl solution.

When necessary, the anolyte can be restored by adding concentrated HCl solution or basic solution. Any loss of iron (Fe²⁺) concentration in the anolyte due to undesired crossover of Fe²⁺ in the rebalancing cell can be compensated by adding iron (Fe⁰) powders. Iron reacts with Fe³⁺ species through the following reaction:

Fe⁰ + 2FeCl₃ → 3FeCl₂

Rebalancing flow cell

The rebalancing flow cell has the negative and positive half-cell electrode separated by an anion-exchange membrane. The anion-exchange membrane allows only chloride ions to pass between the two half-cells.

The positive half-cell of the rebalancing cell is fed by the anolyte electrolyte of the primary flow cell via a circulation pump.

The negative half-cell of the rebalancing cell is fed by a reducing solution containing a low-cost organic electroactive material of ascorbic acid. Over time, the reducing solution can be renewed from the reducing solution tank. When necessary, the pH of the reducing solution can be restored by adding HCl solution or basic solution.

This rebalancing flow cell only operates in discharge mode.

Negative Electrode: reducing agent → oxidizing agent + e⁻

Positive electrode: FeCl₃ + e⁻ → FeCl₂ + Cl⁻

The rebalancing flow cell reduces the SOC of anolyte to rebalance the SOC of anolyte and catholyte. It also reduces the concentration of chloride ions of the catholyte when HCl is added.

How VoltStorage iron flow battery works

The rebalancing flow cell’s reducing solution is first subjected to automated quality control (measurement of pH value, conductivity and thus determination of the oxidation state and possible contamination of iron particles). If quality control is negative, the reducing solution must be renewed.

If quality control passes, the SOC of the anolyte and catholyte of the primary flow cell are compared. The rebalancing cell is activated if the two values are not equal (SOC_anolyte > SOC_catholyte). The rebalancing flow cell operating in the discharge mode reduces the SOC of the anolyte until the balance (SOC_anolyte = SOC_catholyte) is restored. The rebalancing flow cell is then deactivated.

To prevent excessive self-discharge and efficiency loss due to high average oxidation state gradients, the SOC value of the anolyte must always cycle in a range below 80%. If the SOC of the anolyte rises above 80%, it must be electrochemically reduced in the rebalancing flow cell.

The pH of the anolyte is also measured. If the pH value is above 2, HCl solution is added until the pH is below 2 again.

The pH value of the catholyte electrolyte is determined. If the pH exceeds a threshold of pH=3.0-3.2, HCl solution is also added until the pH value drops, but does not fall below 2.

The addition of HCl solution results in an excess of chloride ions in the catholyte. These excess chloride ions can diffuse through the separator from the negative to the positive half-cell when the primary flow cell is charged. Therefore, the chloride ion concentration in the catholyte is automatically adjusted and only has to be influenced in the anolyte. The rebalancing flow cell reduces the concentration of chloride ions in the anolyte by transporting them through the anion-exchange membrane to the reducing solution which can be renewed when necessary.

VoltStorage iron flow battery performance

In 2020, VoltStorage built an iron-salt battery with a storage capacity of 10kWh. With the dimensions of a standard 20-foot ISO container, it can provide up to 9.4 MW of power. The battery is suitable for stationary energy storage applications with 10 to 100 hours power requirements. The battery has an efficiency of about 70% and a service life of more than 20 years.

VoltStorage iron flow battery cost

The Levelized Cost of Storage (LCOS) per MWh of the iron flow battery comes out at $159 (considering a roundtrip efficiency of 70% and maintenance cost over the lifetime of 25% of initial purchase price). This is much less than the majority of other battery technologies on the market yet.

VoltStorage Patent

EP4016681A1 Rebalancing system of a fe/fe redox flow battery
US10985395B2 Cell and cell stack of a redox flow battery, and method for producing said cell stack

VoltStorage Iron Flow Battery Applications

The VoltStorage iron flow battery is suitable for long-duration renewable energy storage. The battery is also temperature-resistant. It can be used even in climates as hot as 50 ºC.

VoltStorage Products

VoltStorage VDIUM C50

The VoltStorage VDIUM C50 is a vanadium redox flow battery. It has a storage capacity of 50 kWh and a system output of 10 kW. This product has a service life of more than 20 years. The battery can be easily integrated into an existing infrastructure.

VoltStorage Grid line

The VoltStorage Grid line is based on iron redox flow battery technology. It is specially developed for application as long duration energy storage and is suitable for stationary applications with a need for constant power throughout the year.