A higher voltage Fe(ii) bipyridine complex for non-aqueous redox flow batteries

Non-aqueous redox flow batteries (RFBs) offer the possibility of higher voltage and a wider working temperature range than their aqueous counterpart. Here, we optimize the established 2.26 V Fe(bpy)₃(BF₄)₂/Ni(bpy)₃(BF₄)₂ asymmetric RFB to lessen capacity fade and improve energy efficiency over 20 cycles. We also prepared a family of substituted Fe(bpyR)₃(BF₄)₂ complexes (R = -CF₃, -CO₂Me, -Br, -H, -^tBu, -Me, -OMe, -NH₂) to potentially achieve a higher voltage RFB by systematically tuning the redox potential of Fe(bpyR)₃(BF₄)₂, from 0.94 V vs. Ag/AgCl for R = OMe to 1.65 V vs. Ag/AgCl for R = CF₃ (ΔV = 0.7 V). A series of electronically diverse symmetric and asymmetric RFBs were compared and contrasted to study electroactive species stability and efficiency, in which the unsubstituted Fe(bpy)₃(BF₄)₂ exhibited the highest stability as a catholyte in both symmetric and asymmetric cells with voltage and coulombic efficiencies of 94.0% and 96.5%, and 90.7% and 80.7%, respectively.