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

A series of thienoviologen derivatives ([(NPr)(2)TV]Cl-4 and [(OHPr)(2)TV]Cl-2) 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 [(OHP)(2)TV]Cl-2 and other viologen anolytes, [(NPr)(2)TV]Cl-4 showed a higher diffusion coefficient (D, 3.36 x 10(-6) cm(2) s(-1)) and electron transfer constant (k(o), 0.31 cm s(-1)). Paired with (ferrocenylmethyl)-trimethylammonium chloride (FcNCl) as the catholyte, the specific capacity of the [(NPr)(2)TV]Cl-4/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 [(NPO2TV]Cl-4-based AORFBs were attributed to the high D and k(0), resulting from the smaller molecular volume (583.38 angstrom(3)) and appropriate dihedral angle (similar to 18.37 degrees) between the pyridines.