Cerium-containing complexes for low-cost, non-aqueous redox flow batteries (RFBs)

Abstract Redox flow batteries (RFBs) are currently of interest as energy storage devices with great potential. For non-aqueous RFBs, low-cost redox pairs with good reversibility and high solubility in the organic solvents are exploited to improve battery performance. In this work, two metal complexes, containing cheap and abundant cerium, were proposed with a very good electrochemical reversibility (Ce4+/Ce3+) in acetonitrile. As proof-of-principle, two cerium-based RFBs were designed: a Zn–Ce and a V–Ce RFB. For the Zn–Ce RFB, the Ce4+/Ce3+ and Zn2+/Zn redox pairs yielded an open cell potential of 1.86 V. In a RFB single cell, a coulombic efficiency of 94%, a voltage efficiency of 68% and an energy efficiency of 64% were achieved at 2 mA cm−2. Most importantly, the RFB is non-sensitive to moisture and oxygen, thus significantly decreasing the costs for practical applications. For the V–Ce RFB, a cell potential of 2.34 V was obtained. This high cell potential and the high solubility (more than 2.5 M) of [Ce(Py-O)8][Tf2N]3 in acetonitrile indicates that this type of battery is promising for high-energy density RFB applications. Hence, the cerium-containing complexes are very promising for developing RFB systems with low-cost and high-energy density.