A low-cost sulfate-based all iron redox flow battery

Redox flow batteries (RFBs) are promising choices for stationary electric energy storage. Nevertheless, commercialization is impeded by high-cost electrolyte and membrane materials. Here, we report a low-cost all-iron RFB that features inexpensive FeSO4 electrolytes, microporous membrane along with a glass fiber separator. The addition of 0.1 м 1-ethyl-3-methylimidazolium chloride (EMIC) overcomes the low solubility of FeSO4 in water which is raised to 2.2 м. DFT calculations demonstrate that EMI+ can strengthen the interaction between sulfate anions and water molecules. This electrolyte composition also allows both anode and cathode reactions to operate without actively maintaining a pH gradient between them, thus eliminating the need for expensive ion exchange membranes. The all-iron RFB demonstrates stable operation at a current density of 20 mA cm−2 for more than 800 cycles via a simple, periodic regeneration process. Furthermore, the system cost of FeSO4/EMIC RFBs is projected to be $ 50 per kWh due to its low-cost active materials and the inexpensive microporous membrane. This low-cost, high-concentration all-iron RFB is a promising stationary energy‐storage system for storing renewable energy. •An all-iron aqueous flow battery based on 2 м FeSO4/EMIC electrolyte is proposed.•EMI+ improves FeSO4 solubility by strengthening the water-anion interaction.•EMIC improves the uniformity of iron metal deposition in carbon felt electrodes.•The system cost of the 2 м FeSO4/EMIC flow battery is estimated to be $ 50 per kWh.•The 2 м FeSO4/EMIC flow battery can cycle over 800 times with a regeneration process.