N-(α-ferrocenyl)ethylphthalimide as a single redox couple for non-aqueous flow batteries

Abstract

Abstract To establish highly soluble single redox couple for non-aqueous flow batteries, N-(α-ferrocenyl)ethylphthalimide (α-FcEtPI) is derived from a promising single redox couple, N-ferrocenylphthalimide (FcPI), by inserting an α-methyl group between the ferrocenyl and phthalimide moieties. The resulting material (α-FcEtPI) shows the maximum solubility of 0.81\u202fM in 1.0\u202fM tetrabutylammonium tetrafluoroborate (TBABF4) in 1,3-dioxolane at 25\u202f°C. This notable increase in solubility (versus 0.3\u202fM for FcPI) is attributed to the disruptive effect of the α-methyl group on molecular packing in a solid state. α-FcEtPI exhibits two stable redox reactions at E1/2\u202f=\u202f0.01\u202fV and −1.97\u202fV (vs. Fc/Fc+), thus functioning as a single redox couple with working voltage (Ewk) of 1.98\u202fV. Considering the solubility and working voltage, the theoretical energy density of α-FcEtPI is calculated as 21.06\u202fW h L−1 (versus 7.8\u202fW h L−1 for FcPI). The cell data of 0.6\u202fM α-FcEtPI in 1\u202fM TBABF4 in 1,3-dioxolane electrolytes, indicates remarkably high Coulombic efficiency (97.8%) and excellent cycle retention.