SBIR Phase I: A Novel High Voltage, All-Solution, All-Iron Flow Battery (AIFB) for Long-Duration Energy Storage

The broader/commercial impact of this Small Business Innovation Research (SBIR) Phase I project is the potential development of a cost-effective and long-lasting all-iron flow battery (AIFB) suitable for long-duration energy storage (LDES). This type of battery is needed to facilitate the world-wide transition from a grid principally powered by fossil-fueled electricity generators to one powered by renewable electricity generators, solar cells, and wind turbines. Cost effective LDES would be a key enabler in this transition, since solar/wind generators are variable and, unlike fossil-fueled power plants, cannot be turned on or off to meet peak demand. In fact, the U.S. grid would need 225-465 gigawatts of LDES capacity by 2050, with a net investment of $ 330 billion. For short-term (≤ 10h) energy storage, the rapidly improving lithium-ion batteries are already practical, but flow batteries are needed for longer-term (≥ 10h) energy storage. The state-of-the-art flow battery technology is the vanadium redox-flow battery (VRFB), but the high cost and limited supply of vanadium restricts its application to shorter durations. The AIFB is based instead on iron as the active material, which is substantially cheaper and more Earth-abundant, thus offering the potential to approach more closely the levelized cost of storage (LCOS) target of $0.05/kWh that is needed to realize this vision. The intellectual merit of this project is the scientific and technological development of an all-iron, all soluble, high voltage, and cost-effective flow battery that would attain the LCOS target for long-duration energy storage. The development of such a flow battery is challenging because, unlike vanadium, which has four different oxidation states allowing for its use at both electrodes, soluble iron species come in only two oxidation states. Competing commercial all-iron-based batteries are typically hybrid batteries rather than flow batteries, requiring a large footprint, and providing a low cell voltage in an effort to avoid gas evolution. These scientific challenges are overcome in AIFB by suitable choice of ligands that form the soluble iron complexes for the posolyte and the negolyte, and by fine-tuning the pH, thus providing a large cell capacity via high solubility along with a high voltage. The specific objectives of this project include finalizing the electrolyte chemistry for a cell voltage exceeding the 1.5 V limits of aqueous batteries to avoid gas evolution, reducing the cell resistance to ensure a high round-trip efficiency, and establishing stable cyclic performance to ensure a long lifetime. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.