Mechanism guided two-electron energy storage for redox-flow batteries using nickel bis(diphosphine) complexes†

The storage of multiple electrons per molecule can greatly enhance the energy density of redox-flow batteries (RFBs). Here, we show that nickel bis(diphosphine) complexes efficiently store multiple electrons through either sequential 1e⁻ redox waves or a concerted 2e⁻ redox wave, depending on their coordination environment. Mechanistic studies comparing ligand sterics (–Me vs. –Ph) and coordination of monodentate ligands (MeCN vs. Cl⁻) allow for selective control of the electron transfer pathway, steering electron storage toward the more favorable 2e⁻ wave. Continuous charge–discharge cycling experiments show more negative charge–discharge potentials and improved capacity retention in the presence of Cl⁻, thus improving the energy storage of nickel bis(diphosphine) complexes as anolytes in RFBs. This work shows how mechanistic understanding of 2e⁻ redox cycles for transition metal complexes can create new opportunities for multi-electron storage in RFBs.