All-Soluble All-Iron Aqueous Redox-Flow Battery

The rapid growth of intermittent renewable energy (e.g., wind and solar) demands low-cost and large-scale energy storage systems for smooth and reliable power output, where redox-flow batteries (RFBs) could find their niche. In this work, we introduce the first all-soluble all-iron RFB based on iron as the same redox-active element but with different coordination chemistries in alkaline aqueous system. The adoption of the same redox-active element largely alleviates the challenging problem of cross-contamination of metal ions in RFBs that use two redox-active elements. An all-soluble all-iron RFB is constructed by combining an iron triethanolamine redox pair (i.e., [Fe(TEOA)OH](-)/[Fe(TEOA)(OH)](2-)) and an iron cyanide redox pair (i.e., Fe(CN)(6)(3-)/Fe(CN)(6)(4-)), creating 1.34 V of formal cell voltage. Good performance and stability have been demonstrated, after addressing some challenges, including the crossover of the ligand agent. As exemplified by the all-soluble all-iron flow battery, combining redox pairs of the same redox-active element with different coordination chemistries could extend the spectrum of RFBs.