Zinc-bromine flow cells

Simpson GD (1990) A simple model for a zinc/bromine flow cell and associated storage tanks. J Electrochem Soc 137 1843-1846. doi 10.1149/1.2086813... 

Evans XI, White RE (1987) A review of mathematical modeling of the zinc/bromine flow cell and battery. J Electrochem Soc 134 2725. doi 10.1149/1.2100277... 

Various materials have been used as separators in zinc—bromine cells. Ideally a material is needed which allows the transport of zinc and bromide ions but does not allow the transport of aqueous bromine, polybromide ions, or complex phase structures. Ion selective membranes are more efficient at blocking transport then nonselective membranes.These membranes, however, are more expensive, less durable, and more difficult to handle then microporous membranes (e.g., Daramic membranes).The use of ion selective membranes can also produce problems with the balance of water between the positive and negative electrolyte flow loops. Thus, battery developers have only used nonselective microporous materials for the separator. 

Flow cells (also redox flow cells, flow batteries) are similar to batteries, except that the electrodes are catalysts for the chemical reaction, which occurs as a microporous membrane allows ions to pass from one electrolyte solution to another. Among flow cells are types that use zinc and bromine, vanadium in two types with different states, or polysulfide and bromine as the pairs of electrolytes. The advantages of flow cells are that they are capable of a large number of cycles, and the electrolytes can be replenished. 

Flow cells are ideal for stors e systems in remote locations. Vanadium redox systems, for instance, deliver up to 500 kW for up to ten hours. Zinc-bromine systems have been produced for 50-kWh and 500-kWh systems to reinforce weak distribution networks or prevent power fluctuations. Hydrogen fuel cells can potentially do almost anything a battery can do provide backup power, perform power leveling, run handheld devices, and supply primary or auxiliary power to cars, trucks, buses, and boats. In many cases they are more efficient than petrochemical fuels. A hydrogen fuel cell in a vehicle that uses an electric motor, for example, can be 40 to 60 percent efficient, compared with the 35 percent peak efficiency of the internal combustion engine. 

The theoretical cell potential of a zinc-bromine battery is 1.82 V, and its practical energy density is around 65-75 Wh kg Both are better potential and energy level than those of VRBs systems. Meanwhile, more than 80% of the energy efficiency and low-cost chemical reactants made the zinc-bromine battery one of the most developed, commercially ready flow battery systems (Figure 7). 

In order to improve the bromine-storing capacity and hence the battery efficiency of a zinc-flow cell, knowledge of the structure and consistency of the complex phase during the entire charge-discharge cycle is an essential requirement. 

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