Influence of Linker Group on Bipolar Redox‐Active Molecule Performance in Non‐Aqueous Redox Flow Batteries

Redox flow batteries (RFBs) are an attractive choice for stationary energy storage of renewables such as solar and wind. Non‐aqueous redox flow batteries (NARFBs) have garnered broad interest due to their high voltage operation compared to their aqueous counterparts. Further, the utilization of bipolar redox‐active molecules (BRMs) is a practical way to alleviate crossover faced by asymmetric RFBs. In this work, ferrocene (Fc) and phthalimide (PI) are covalently linked with various tethering groups which vary in structure and length. The compiled results suggest that the length and steric shielding ability of the linker group can greatly influence the stability and overall performance of Fc‐n‐PI BRM‐based NARFBs. Primary sources of capacity loss are found to be BRM degradation for straight chain spacers 98 % over 100 cycles (~13 days). NARFB using Fc‐hexyl‐PI as an active material exhibited high working voltage (1.93 V) and maximum capacity (1.28 Ah L−1). Additionally, this work highlights rational strategies to improve cycling stability and optimize NARFB performance. A series of ferrocene‐phthalimide bipolar redox molecules with varying linkers are synthesized for non‐aqueous flow batteries. Cycling stability of the flow batteries is greatly improved via physical separation or steric shielding.