Engineered Reactor Components for Durable Iron Flow Batteries

All-iron redox flow battery (IRFB) is a promising candidate for grid-scale energy storage because of its affordability and environmental safety. This technology employs iron deposition/stripping process which governs the overall performance of the battery. Parasitic hydrogen evolution reaction (HER) occurring in parallel with iron deposition process, degrades the battery by increasing pH of electrolyte, leading to clogging of the porous flow-through electrode, fouling an ion conductive membrane, and increasing electrolyte resistance. In the present study we explored the effect of electrolyte composition on minimizing HER by varying FeCl
2
concentrations (1–4 M) and by introducing electrolyte additive (ascorbic acid, AA). The effects of FeCl2 concentration and presence of additives on deposition/ stripping process were first explored by monitoring mass change with Quartz Crystal Microbalance (EQCM) in 3-electrode cell and further investigated in full-cell battery tests. EQCM tests showed that high FeCl
2
concentrations and addition of AA help to maintain high faradaic efficiency (FE) of Fe deposition. The addition of 10 mM AA enabled dissolving precipitate-free 4 M FeCl
2
solutions at pH as high as 3.2. This electrolyte composition was selected for full-cell battery tests based on its superior performance in EQCM tests. Full-cell tests showed that while results for 2 M and 4 M FeCl
2
electrolytes without AA are identical, addition of AA to 4 M FeCl2 electrolyte improves FE by 7% and energy efficiency (EE) by 9% compared to the highest performance without AA. Determining the optimal concentration of AA additive will be the subject of our future work.