The present invention reports iron complexes based on multidentate ligands with phosphonate coordination groups. These metal complexes are used as anolyte materials in aqueous redox flow batteries. Proposed all soluble iron flow batteries have a battery structure similar to conventional RFB technology, which consists of an anolyte tank and a catholyte tank, and a flow battery stack. Compared with the vanadium redox flow battery (VRFB), the main advantage achieved by the present invention is the use of a cost-effective iron complex-based electrolyte to replace the high-cost vanadium electrolyte, thereby further reducing the cost of RFB technology.