Impact of Pendent Ammonium Groups on Solubility and Cycling Charge Carrier Performance in Nonaqueous Redox Flow Batteries

The synthesis, characterization, electrochemical performance, and theoretical modeling of two base-metal charge carrier complexes incorporating a pendent quaternary ammonium group, [Ni(bppn-Me-3)][BF4], 3 ', and [Fe(PyTRENMe)][OTf](3), 4', are described. Both complexes were produced in high yield and fully characterized using NMR, IR, and UV-vis spectroscopies as well as elemental analysis and single-crystal X-ray crystallography. The solubility of 3 ' in acetonitrile showed a 283% improvement over its neutral precursor, whereas the solubility of complex 4' was effectively unchanged. Cyclic voltammetry indicates an similar to 0.1 V positive shift for all waves, with some changes in reversibility depending on the wave. Bulk electrochemical cycling demonstrates that both 3 ' and 4' can utilize the second more negative wave to a degree, whereas 4' ceases to have a reversible positive wave. Flow cell testing of 3 ' and 4' with Fc as the posolyte reveals little improvement to the cycling performance of 3 ' compared with its parent complex, whereas 4' exhibits reductions in capacity decay when cycling either negative wave. Postcycling CVs indicate that crossover is the likely source of capacity loss in complexes 3, 3 ', and 4' because there is little change in the CV trace. Density functional theory calculations indicate that the ammonium group lowers the HOMO energy in 3 ' and 4', which may impart stability to cycling negative waves while making positive waves less accessible. Overall, the incorporation of a positively charged species can improve solubility, stored electron density, and capacity decay depending on the complex, features critical to high energy density redox flow battery performance.