Redox Flow Batteries Trends—Chemistries and Materials

Fast growing grid-scale energy storage requires a portfolio of technologies tuned for intermittent renewable energy sources and capable to provide wide range of discharge times up to seasonal storage. Redox flow batteries (RFBs) can occupy an intermediate position with the storage time from several hours to tens of hours. The amount of stored energy can be adjusted depending on the customer’s need by changing the volume of electrolyte storage tanks for rated power, and the system cost could be competitive with other energy storage technologies.<br/>The cost reduction remains the major driver in current RFB development, which can be achieved by decreasing costs of the stack via increasing power density and using inexpensive stack components, energy storage media via employing inexpensive active materials and electrolytes, storage tanks via increasing solubility of active materials and BOP (separately or in combination). The former approach targets development of high-power cells based on mainstream vanadium chemistry. The least expensive option is the utilization of earth-abundant inorganic salts (e.g., iron or zinc) and water as solvent. These chemistries are limited by competitive reaction of water splitting. Another option is the use of water-soluble organic compounds, which potential and solubility can be tuned by adding functional groups to the core molecule. However, such tuning requires additional synthetic steps that increase the manufacturing cost and may negate the performance advantage. Non-aqueous RFBs have hypothetically higher (above water splitting threshold) cell potential and higher solubility of tunable active materials. However, this potential was not realized yet, and high cost, low conductivity and flammability of solvents render such RFBs not competitive with aqueous RFBs. Current trends, research needs and tradeoffs for RFBs with stationary (hybrid RFB) and liquid (conventional RFB) electrodes will be discussed.<br/>Development of novel ion-selective membranes and chemistries for RFBs was funded by ARPA-E via GRIDS, OPEN, IONICS and DAYS programs. Important lessons from completed projects will be also discussed.