Electrochemical Stability of Bipyridylimino Isoindoline Metal Complexes As Multi-Electron Anolyte Materials for Non-Aqueous Redox Flow Batteries

Abstract

The development of grid scale energy storage will facilitate the integration of intermittent renewable electricity sources, such as wind and solar, into the grid. However, available technologies are limited by high cost or location dependence. Non-aqueous redox flow batteries (NAqRFBs) offer a scalable, non-geographically limited technology with the potential to provide a low cost, high energy density solution to the energy storage problem. Despite these promising attributes, the limited solubility and stability of current NAqRFB active species hinders implementation. In this work we explored a series of metal complexes of tridentate bipyridylimino isoindoline (BPI) ligands as anolyte materials for NAqRFBs. The goal was to evaluate the electrochemical stability of this class of materials and identify trends between the metal center and stability. Each complex exhibits 2 quasi-reversible redox events at -1.73 V and -1.83 V vs. Ag/Ag+. Bulk electrolysis through these 2 couples was used to quantify the electrochemical stability of each material and the following trend was identified: Co(BPI)2 = Ni(BPI)2 = Fe(BPI)2 > Zn(BPI)2 = Mg(BPI)2 > Mn(BPI)2. Co(BPI)2, Ni(BPI)2 and Fe(BPI)2 were all identified to be highly stable with no capacity fade observed over 50 cycles. This high stability is attributed to the tridentate, anionic ligand structure, which affords a stronger bond between the ligand and the metal center. Extended cycling experiments revealed that Ni(BPI)2 is stable through 2 negative electron transfers for over 200 cycles. Zn(BPI)2 and Mg(BPI)2 were both moderately stable as they have full valence shells while the instability of Mn(BPI)2 is attributed to rapid ligand exchange kinetics, which is common for Mn MCCs. For each of the unstable materials, post-cycling analysis revealed that ligand shedding is a prominent degradation mechanism. The developed stability trends can be used to guide the exploration and design of future NAqRFB active species to aid in the identification of electrochemically stable materials. Since solubility is also a concern, ligand functionalizations were identified that successfully enhanced the solubility of Ni(BPI)2 by three orders of magnitude, up to a maximum solubility of 0.7 M in acetonitrile. In conclusion, BPI MCCs exhibit remarkable stability, along with relatively high solubility, and we present the first demonstration of a stable 2 electron transfer for NAqRFB applications. These complexes can be paired with a suitable catholyte material to achieve a stable, high energy density, multi-electron transfer NAqRFB.