Effect of Spin-State and Molecular Clustering on Electrochemical Stability of Fe complexes in Aqueous Redox Flow Batteries

The development of low-cost and long-life energy storage systems (ESSs) is essential to address the increasing demands for energy and environmental concerns. Redox flow batteries (RFBs) are promising grid-scale ESSs due to their safety, cost-effectiveness, and decoupled energy and power density. While vanadium-based redox flow batteries are currently the most prevalent, offering high cell voltage, large capacity, and stability, the limited abundance of vanadium in the Earth's crust (0.012%) raises concerns about future supply shortages. In contrast, Fe-based transition metals are abundant in the Earth's crust and are significantly cheaper (5000 times) than vanadium metals, making them a potential solution to supply and cost issues.<br/><br/>Fe-based compounds in aqueous RFBs typically exhibit high redox potentials and are mainly used as posolytes. To develop an all-Fe RFB, negolyte materials with lower redox potentials are necessary. Phenolate moieties have been employed to reduce the redox potential of Fe complexes due to their π-donor and strong Lewis base properties. However, these ligands primarily form Fe complexes in the high-spin state with weaker binding affinity, leading to low electrochemical stability.<br/><br/>Here, we introduced a secondary amine group capable of forming strong σ bonding with the Fe metal center under aqueous conditions. Additionally, sulfonate groups on phenolate rings were utilized to facilitate intermolecular H-bonding with neighboring secondary amine groups, enhancing the σ bonding between amines and irons and promoting molecular clustering. We also investigated a Fe complex including hydroxyl groups on phenolate rings as a control group, which cannot form intermolecular H-bonding and molecular clustering, as confirmed by small angle X-ray scattering measurement. Evans method and Raman analysis confirmed higher effective magnetic moment values and weaker metal-ligand binding affinity for the hydroxylated Fe complex. Cyclic voltammetry and aqueous RFB measurements demonstrated superior electrochemical stability for the sulfonated Fe complex compared to the hydroxylated one, which decomposed rapidly. Furthermore, the sulfonated Fe complex exhibited high solubility in water at 0.7 M and demonstrated stable cycling of the RFBs for 300 cycles in a neutral aqueous electrolyte at a concentration of 0.5 M.