A Stable Organo-Aluminum Analyte Enables Multielectron Storage for a Nonaqueous Redox Flow Battery

Redox flow batteries (RFBs) operate by storing electrons on soluble molecular anolytes and catholytes, and large increases in the energy density of RFBs could be achieved if multiple electrons could be stored in each molecular analyte. Here, we report an organoaluminum analyte, [(I₂P^-)₂Al]⁺, in which four electrons can be stored on organic ligands, and for which charging and discharging cycles performed in a symmetric nonaqueous RFB configuration remain stable for over 100 cycles at 70% state of charge and 97% Coulombic efficiency (I₂P is a bis(imino)pyridine ligand). The stability of the analyte is promoted by the kinetic inertness of the anolyte to trace water in solvents and by the redox inertness of the Al(III) ion to the applied current. The solubility of the analyte was optimized by exchanging the counteranion for trifluoromethanesulfonate (triflate), and the cell was further optimized using graphite rods as electrodes which, in comparison with glassy carbon and reticulated vitreous carbon, eliminated deposition of analyte on the electrode. Proof-of-principle experiments performed with an asymmetric NRFB configuration further demonstrate that up to four electrons can be stored in the cell with no degradation of the analyte over multiple cycles that show 96% Coulombic efficiency.