Flow battery is a type of battery that completes charging and discharging through electrochemical reactions on inert electrodes through soluble pairs of electricity. A typical flow battery cell structure includes ① positive and negative electrodes; ② electrode chamber surrounded by diaphragm and electrodes; ③ electrolyte tank, pump and piping system. Multiple battery cells are connected in series with bipolar plates to form a stack, and the stack is equipped with a control system to form a power storage system.
There are many subdivision types and specific systems for flow batteries. At present, there are only 4 types of flow battery systems that are truly in-depth researched in the world, namely, all-vanadium flow batteries, zinc-bromine flow batteries, iron-chromium flow batteries and sodium polysulfide/bromine flow batteries, and all have experience in commercial demonstration operation.
All vanadium redox battery (vanadium redox battery, VRB), referred to as vanadium battery, has the advantages of long life, large capacity, and frequent charging and discharging. At present, the global companies that can provide industrialized vanadium battery products mainly include Japan’s Sumitomo Electric, Beijing Pu Neng (acquired Canadian VRB Power in 2009), and Australia V-Fuel. Since the 1990s, Japan, Australia, and the United States have had some vanadium battery projects for demonstration operations. In the past two years, with the global attention to energy storage technology, vanadium batteries have successively obtained some project opportunities in China and the United States.
The reactive material of the zinc-bromine flow battery is zinc bromide. Zinc is deposited on the negative electrode when charging, and the bromine generated in the positive electrode will immediately complex the bromine complexing agent in the electrolyte into an oily substance. At present, the global zinc-bromide battery is still in the initial stage of industrial development, and research and development are mainly concentrated in the United States and Australia. In recent years, companies in China have also begun to engage in this area of development. As well-known zinc-bromine battery manufacturers, ZBB and Premium Power in the United States have developed products with different specifications (10~500kW). Recently, it has attracted the attention of the US government, the Department of Energy and the Electric Power Research Institute, and it is believed that it will continue to develop in the United States. In addition, Meidensha of Japan, Redflow of Australia, and Dalian Rongke of China are also actively developing research and development work.
The iron-chromium flow battery uses an acidic aqueous solution of CrCl2 and FeCl3 (usually a hydrochloric acid solution) as the battery’s negative and positive electrolyte and the battery’s electrochemical reaction active material, and an ion exchange membrane is used as the diaphragm. Although the iron-chromium flow battery system has the characteristics of cheap electrolyte raw materials, its negative hydrogen evolution is serious, the positive chlorine evolution is difficult to manage, and the system cycle life is short. Therefore, although Japan developed 1kW, 10kW, 60kW and even MW-class stacks in the 1980s, the research and development basically ceased in the late 1990s. In recent years, several companies in the United States have begun research and industrialization of iron-chromium batteries, including Deeya and Ktech.