Quino Energy ($9M for low-cost aqueous organic redox flow battery for renewable energy storage)

Quino Energy, an American energy storage startup founded in 2021, develops redox flow battery technology using non-flammable, low-cost aqueous electrolytes. The redox active components of electrolytes consist of inexpensive organic quinones and ferrocyanide salts. The Quino Energy flow battery is suitable for renewable energy storage at the grid-scale.

Challenges: Store renewable energy at a low cost

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.

Flow batteries have an advantage over Li-ion batteries in terms of energy storage duration. By using larger tanks of anolyte or catholyte, it is possible to create longer duration batteries without spending money on an unnecessarily large flow battery stack.

Most 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 capital cost of flow batteries is around $800 per kilowatt-hour, which is more than twice as much as lithium-ion batteries. Therefore, it is necessary to develop cost-effective redox flow batteries.

Quino Energy Technology

Quino Energy has developed a cost-effective redox flow battery system with non-flammable aqueous electrolytes. The aqueous electrolytes are made of inexpensive quinones and ferrocyanide salts. But the quinones in the anolyte can be oxidized over time by reacting with the atmospheric oxygen that gets into the flow battery system through microscopic leaks. This decreases the state of charge (SOC) of anolyte and causes an imbalance between the SOCs of the anolyte and catholyte, thereby reducing the flow battery capacity.

The redox flow battery system developed by Quino features a rebalancing flow cell. This rebalancing flow cell can increase the SOC of the anolyte and rebalance the SOC of both anolyte and catholyte when necessary. Thus, the capacity of the redox flow battery is stable.

Quino Energy redox flow battery

The diagram below depicts the Quino Energy redox flow battery system.

The system comprises a primary flow cell, a rebalancing flow cell, and storage tanks of anolyte, catholyte, and supporting electrolyte, as well as pumps, valves, and a controller (not shown).

Primary flow cell

The primary flow cell functions in charging and discharging modes. It 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 electrodes of the primary flow cell are made of carbon cloth or carbon paper, such as thermally activated AvCarb carbon paper (EP-40).

The ion-selective membrane separates the negative and positive electrodes and prevents the anolyte and catholyte from mixing, while allowing small ions to flow through. The membrane of the primary flow cell can be FKE-50 membrane (cation-exchange membrane).

The aqueous anolyte contains a quinone, such as DCDHAQ (1,8- dihydroxy-2,7- bis(carboxymethyl)-9,10-anthraquinone). The aqueous catholyte contains sodium ferrocyanide and potassium ferrocyanide. Both electrolyte solutions have potassium hydroxide and/or sodium hydroxide to increase their electrical conductivity.

Rebalancing flow cell

The rebalancing flow cell only charges to increase the state of charge (SOC) of the anolyte when necessary and rebalance the SOC of anolyte and catholyte of the primary flow cell.

The rebalancing flow cell also has two half-cells with the negative and the positive electrode separated by a bipolar membrane, such as Fumatech FBM bipolar membrane. The two half-cells are fed from the anolyte tank of the primary flow cell and a supporting electrolyte tank by the electrolyte circulation pumps. The aqueous supporting electrolyte is concentrated potassium hydroxide and sodium hydroxide solution.

The negative electrode of the rebalancing flow cell is made of carbon paper, such as thermally activated AvCarb carbon paper (EP-40). The positive electrode comprises nickel/iron oxyhydroxide.

The bipolar membrane generates protons at the positive electrode side and hydroxide ions at the negative electrode side of the rebalancing cell.

How Quino Energy redox flow battery works

The primary flow cell functions in charging and discharging modes. Over cycling, the quinones in the anolyte are oxidized by oxygen and the SOC of anolyte reduces, which results in an imbalance of SOC between the anolyte and catholyte. This SOC imbalance decreases the flow battery’s capacity.

A rebalancing flow cell, which functions only in charge mode, can increase the SOC of the anolyte and rebalance the SOCs of the anolyte and catholyte.

When an external electrical potential is applied to the rebalancing flow cell, the oxidized quinones in the anolyte are reduced at the negative electrode (cathode) of the rebalancing flow cell:

Negative electrode: Oxidized quinone + e⁻ → Reduced quinone

The SOC of the anolyte is raised.

Meanwhile, hydroxide ions (OH⁻) in the supporting electrolyte solution are oxidized at the positive electrode of the rebalancing flow cell. Oxygen gas is generated, exits the half-cell, and is allowed to escape outside the system through the valve of the supporting electrolyte tank.

Positive electrode: 2OH⁻ → O₂ + 2H⁺ + 4e⁻

During the rebalancing process, hydroxide ions are accumulated in the anolyte solution and consumed in the solution of supporting electrolyte. This rebalancing process has the tendency to transfer water from supporting electrolyte into the anolyte through osmosis. This can be counteracted by using concentrated (over 2M) supporting electrolyte such that it has greater osmotic pressure than the anolyte solution.

Quino Energy Patent

WO2023091940A1 System and process for rebalancing flow battery state of charge
WO2022271456A2 System and process for anthraquinone functionalization

Quino Energy Redox Flow Battery Applications

Quino Energy’s flow battery based on low-cost, non-flammable aqueous electrolytes is suitable for grid-scale renewable energy storage, thereby facilitating the widespread adoption of renewable energy sources and increasing reliance on renewable energy electricity.