Non-Aqueous Redox Flow Batteries with Nickel and Iron Tris(2,2′-bipyridine) Complex Electrolyte

An organic redox flow battery adopting tris(2,2′-bipyridine)nickel(II)tetrafluoroborate (Ni(Bpy)3(BF4)2) and tris(2,2′-bipyridine)iron(II)tetrafluoroborate (Fe(BPy)3(BF4)2)) in propylenecarbonate electrolyte is investigated. With cyclic voltammetry, the redox current of one-electron related Fe(II)/Fe(III) and two-electron related Ni(II)/Ni(0) appears at 0.65 V and −1.66 V (vs. Ag/Ag+), respectively and their cycleabilities are highly stable during 100 cycles. Their charge-discharge characteristics on the carbon-felt electrodes are evaluated with 0.4 M Fe(Bpy)3(BF4)2 and 0.2 M Ni(Bpy)3(BF4)2 which are adopted as catholyte and anolyte. Comparing with the aqueous system, the higher cell voltage of 2.2 V was achieved and initial coulombic and energy efficiencies were reached with 90.4 and 81.8%.