Hexacyanometalate-Based Redox Couple for High-Efficiency Iron-Chromium Flow Battery

Abstract

As increasing the importance of electrical grids and utilizing renewable energy sources, large-scale energy storage systems that can effectively store and supply electrical energy have attracted great research attention in recent times. Aqueous based redox flow batteries (RFBs) are promising candidates for large-scale energy storage due to their long cycle life, high safety, and high solubility of a catholyte and anolyte. Also, unlike other battery systems such as lithium-ion batteries, the energy density and power capabilities of RFBs can be decoupled, enabling flexibility for controlling capacity and power, and scalability as an industrial perspective. Iron-chromium (Fe-Cr) RFB is a good candidate for cost-effective systems comparing to all-vanadium RFB, which is well-known as the most developed RFB. However, sluggish Cr3+/2+ redox kinetics and hydrolysis of ferrous ion have become formidable obstacles to achieving high efficiency and stability. In particular, the poor kinetics of chromium ions make the hydrogen evolution reaction more dominant and cause the state of charge of the positive and negative side to being imbalanced.Herein, we have demonstrated a new RFB system based on hexacyanometalate materials ([M(CN)6]3-/4-, M = transition metal). The cyanide ligands linked through transition metal ions play a role in mitigating the hydrolysis in the aqueous environment and enhance the electrochemical kinetics. We have investigated the stability and electrochemical kinetic information of our redox couples comparing to conventional active materials. The superior kinetics of hexacyanometalate based-redox couples contributed to improving voltage efficiency. Besides, by use of a transition metal oxide type-catalyst on a carbon-felt current collector, this RFB system can achieve higher energy efficiency than that of the pristine carbon-felt electrode, suggesting that the employing catalyst can enhance the electrochemical kinetics of the active materials.