Flow battery for long duration energy storage: Development, challenges and prospects

The seriousness of global warming and the consumption of fossil fuels has become increasingly evident, prompting countries to take active measures to address this challenge, including setting carbon neutrality goals to tackle severe environmental issues. Decarbonizing energy is an inevitable measure to achieve dual carbon goals, and renewable energy is a key strategy to achieve energy decarbonization. However, renewable energy generation, such as solar and wind energy, faces challenges like volatility and intermittency, and direct integration into the power grid is extremely risky. Therefore, the long-term energy storage technology is needed to regulate power to achieve the effectiveness of renewable energy utilization. Among various long-term energy storage technologies, flow batteries have garnered considerable attention due to their rare advantages such as large capacity, high safety, and long lifespan. Flow battery technology consists of an electrochemical cell stack, electrolytes, and pumps, which are connected to each other through pipelines. The electrolyte is pumped into two chambers separated by the membrane for redox reactions, while the electrical energy is transported to the outside through the electrodes for power generation. The decoupling of flow battery energy and power makes it highly flexible and adjustable, and its modular design also greatly reduces maintenance costs. Since being first proposed in the 1970s, flow batteries have developed two systems: Inorganic and organic. The anolyte and catholyte of the inorganic system are inorganic materials, including all-vanadium flow batteries, zinc-based flow batteries, iron-based flow batteries, and halogen-based flow batteries, etc. Organic systems, on the other hand, have at least one side made of organic materials, including quinone flow batteries, viologen flow batteries, TEMPO flow batteries, and aza-aromatic flow batteries. Each system of flow batteries has its unique advantages, such as all-vanadium flow batteries with high power and high stability, zinc-based flow batteries with low cost and high energy density, and organic flow batteries with abundant molecular tunability and potential cost advantages. At present, technologies such as all-vanadium flow batteries, zinc-bromine flow batteries, and iron-chromium flow batteries have entered commercial application, and with the increase in demand for long-term energy storage facilities, their applications will be further expanded. Meanwhile, organic flow batteries are still at the laboratory scale and have not yet entered the stage of large-scale industrialization. This article reviews the cutting-edge research and commercial applications of various flow battery technologies in two fields: Inorganic and organic, analyzes the key issues faced by various flow battery technologies, and finally gives an overview of the long-term potential of flow battery technologies. Challenges and future developments in the energy storage field are proposed. In short, flow battery technology, as a key player in the field of long-term energy storage, can not only become a reliable energy storage solution for the energy system but also promote the large-scale application of renewable energy, providing strong support for solving environmental problems and achieving dual carbon goals. Although it still faces some challenges, it is believed that flow battery technology will have broader prospects with breakthroughs in key issues.