A tetradentate Ni(II) complex cation as a single redox couple for non-aqueous flow batteries

•Nickel(II)-1,4,8,11-tetraazacyclotetradecane is evaluated as a single redox couple for non-aqueous flow battery.•Ni(cyclam)[ClO4]2 shows a high solubility (0.4 M) in non-aqueous electrolyte.•Ni(cyclam)2+ offers a high working voltage (2.55 V) as a single redox couple.•Ni(cyclam)2+ is chemically and electrochemically stable.

Nickel(II)-1,4,8,11-tetraazacyclotetradecane is examined as a possible single redox couple for non-aqueous flow batteries. The nickel complex cation is reduced at −1.81 V (vs. Fc/Fc+) and oxidized at 0.74 V, showing that this tetradentate Ni(II) complex cation can be used as a single redox couple that offers a cell voltage of 2.55 V. The maximum solubility is 0.4 M in 1.0 M tetraethylammonium tetrafluoroborate dissolved in a mixed solvent of ethylene carbonate and propylene carbonate. When tested in a non-flowing cell, this single redox couple exhibits a very stable cycle performance.