Multifunctional asymmetric bi-ligand iron chelating agents towards low-cost, high performance, and stable zinc-iron redox flow battery

Zinc‑iron flow batteries hold great potential as stationary storage due to their attractive cost and abundance of materials; however, they still suffer from precipitation problems. This work demonstrates a novel asymmetric iron complex with bi-ligand from pyridine and disodium 4,5-dihydroxy-1,3-benzenedisulfonate (TIRON) to synergically reduces precipitation, improves the mass transport of the redox-active species, and lowers the electron transfer resistance. The low activation barrier of mixed complexes calculated via the Density Functional Theory (DFT) explains the ease of redox reaction, which is reflected in the low overpotential voltage in the Fe//Fe symmetrical cell. Moreover, no change in Raman and Fourier-transform infrared spectroscopy peaks after symmetrical cell cycling confirms the stability of the mixed complexes catholyte. Contrary to other works, our strategy requires a lower molar ratio of complexes per iron, while still yields a superior capacity of 6.81 Ah/L. From its outstanding capacity utilization, the overall cost of the electrolytes for the mixed complexes system is only $26.3/kWh, which is lower than the non-complex iron system at $42.9/kWh. Additionally, the mixed complexes battery can be operated stably for >200 cycles. Our perspective on eco-friendly and low-cost catholyte design accelerates development towards reliable stationary storage with high performance and stability.