Influence of Linker Group on Bipolar Redox-Active Molecule (BRM) Performance in Non-Aqueous Redox Flow Batteries

Non-aqueous redox flow batteries (NARFBs) have garnered broad interest as a stationary energy storage device due to their high voltage operation compared to traditional aqueous flow batteries. Bipolar redox-active molecules (BRMs) have been recently employed in NARFBs to alleviate issues related to crossover which can hinder the performance of RFBs. In this work, BRMs comprised of ferrocene (Fc) and phthalimide (PI) active moieties were covalently linked with tethering groups of varying structure and length. The stability and overall performance of Fc-<i>n</i>-PI BRM-based NARFBs was greatly influenced by the length and steric shielding ability of the linker group. Primary sources of capacity loss are found to be BRM degradation for straight chain spacers &lt;6 carbons and membrane (Nafion) fouling. Fc-hexyl-PI exhibited the most stable battery cycling and coulombic efficiencies of &gt;98% over 100 cycles (~13 days). A NARFB using Fc-hexyl-PI as an active material exhibited high working voltage (1.93 V) and maximum capacity (1.28 Ah L^-1).