Tridentate Metal Coordination Complexes As Active Species for Non-Aqueous Redox Flow Batteries

Organometallic complexes with non-innocent ligands are excellent candidates for use in non-aqueous redox flow batteries because of their potential for multiple electron transfer. In this study, the electrochemical properties and solubility of 2,6-bis(1,2,3-triazol-4-yl)pyridine (BTP) and terpyridine (tpy) metal coordination complexes were investigated. A range of BTP ligands were synthesized with various functional groups to understand the impact of the ligand on the properties of Fe, Co, and Ni complexes. The results demonstrate that electron withdrawing groups on the pyridine rings lead to a higher number of redox couples and improved solubility. Additional modifications indicate further functionalization has a strong impact on solubility while only a minimal effect on the electrochemistry. This allows the tuning of these complexes to achieve desirable electrochemical properties and solubility. Based on its attractive combination of reversibility and solubility, 2,6-bis(di(3-(2-methoxyethyl)-1,2,3-triazol-4-yl)-4-butoxycarbonylpyridine)iron(II) tetrafluoroborate (BTP-1) was used to evaluate the cycling stability of the BTP complexes. Bulk electrolysis and charge/discharge experiments in a static H-Cell revealed that this complex had limited stability. Charge/discharge experiments with unsubstituted Fe(tpy)₂ show improved stability, but limited solubility. A tpy complex with analogous functionalization to BTP-1 was synthesized and evaluated to exploit both the stability of the terpyridine and the solubility of the BTP metal coordination complexes. This study demonstrates the potential of BTP and tpy metal coordination complexes for use as active species in non-aqueous redox flow batteries.