Performance characterization and cost assessment of an iron hybrid flow battery

Abstract

Electrolyte solutions are a large percentage of the total cost of commercial flow battery systems. Decreasing the cost of the electrolyte has the potential to lower flow battery system costs. In this study, a design and corresponding cost model is developed for a 10 kW/20 kWh flow battery that uses an all-iron based electrolyte with a nominal open-circuit voltage of 1.2 V. Electrolyte costs for large-scale production of this battery are estimated to be 23 cents per liter (88 cents per gallon). Expected system costs are $1492/kW and $715/kWh for a production of 1000 units per year. A hypothetical scaled-up system is analyzed in a simulated area regulation application for one year of operations. Parallel studies were conducted on a small 50 cm2 cell with current densities from 20 mA/cm2 to 80 mA/cm2, and charge densities of 50 mA-hr/cm2 to 100 mA-hr/cm2. Symmetric electrolyte tests show reversible and repeatable reaction behavior on the positive electrode, with reactant utilization up to 67%. The iron flow battery can function with a microporous membrane, although electrolyte crossover problems were identified and the best results were achieved with a non-porous Nafion membrane. 56% energy efficiency was achieved at a current density of 50 mA/cm2. Coulombic efficiencies as high as 91% and voltaic efficiencies as high as 76% were observed.