Multi-Week Cycling of a Nonaqueous Flow Battery Using Tris-Bipyridine Iron (II) Triflate without Additional Supporting Electrolyte

Tris-bipyridine iron (II) triflate was synthesized and used as an active species to demonstrate a symmetric disproportionation redox-flow-battery chemistry that works without a supporting electrolyte. Solutions of this coordination complex salt (0.1 M in acetonitrile), in which the cation provides the redox activity, were qualitatively characterized with cyclic voltammetry and used to perform extended full-cell charge/discharge cycling and impedance testing in reactors containing a porous Daramic 175 separator membrane. The cell, based on 10 ml reservoirs of active liquid, survived for more than eight hundred cycles, with charge/discharge cycling taking place over a period of more than two weeks. Four cycling protocols were evaluated to investigate the effects of applied current and depth-of-discharge on cell performance. The system allows for hundreds of cycles above 50% state-of-charge and is capable of exceeding 80% round-trip energy efficiency.