Thienoviologen anolytes for aqueous organic redox flow batteries with simultaneously enhanced capacity utilization and capacity retention

Abstract

A series of thienoviologen derivatives ([(NPr)₂TV]Cl₄ and [(OHPr)₂TV]Cl₂) with a narrow bandgap, high solubility and two-electron transfer properties as anolytes for high-performance aqueous organic redox flow batteries (AORFBs) is reported. Compared with [(OHPr)₂TV]Cl₂ and other viologen anolytes, [(NPr)₂TV]Cl₄ showed a higher diffusion coefficient (D, 3.36 × 10⁻⁶ cm² s⁻¹) and electron transfer constant (k₀, 0.31 cm s⁻¹). Paired with (ferrocenylmethyl)-trimethylammonium chloride (FcNCl) as the catholyte, the specific capacity of the [(NPr)₂TV]Cl₄/FcNCl AORFB reached 4.62 A h L⁻¹, and the capacity utilization was up to 86.1%. Moreover, the system also maintained high stability over 300 cycles and delivered 87.9% capacity retention and 99.96% capacity retention per cycle. The simultaneously enhanced capacity retention and capacity utilization of [(NPr)₂TV]Cl₄-based AORFBs were attributed to the high D and k₀, resulting from the smaller molecular volume (583.38 ų) and appropriate dihedral angle (∼18.37°) between the pyridines.