Abstract
Five metal complexes of the dithiolene ligand maleonitriledithiolate (mnt(2-) ) with M=V, Fe, Co, Ni, Cu were studied as redox-active materials for nonaqueous redox flow batteries (RFBs). All five complexes exhibit at least two redox processes, making them applicable to symmetric RFBs as single-species electrolytes, that is, as both negolyte and posolyte. Charge-discharge cycling in a small-scale RFB gave modest performances for [(tea)(2) V(mnt) ], [(tea)(2) Co(mnt) ], and [(tea)(2) Cu(mnt) ] whereas [(tea)Fe(mnt) ] and [(tea)(2) Ni(mnt) ] (tea=tetraethylammonium) failed to hold any significant capacity, indicating poor stability. Independent negolyte- and posolyte-only battery cycling of a single redox couple, as well as UV/Vis spectroscopy, showed that for [(tea)(2) V(mnt) ] the negolyte is stable whereas the posolyte is unstable over multiple charge-discharge cycles; for [(tea)(2) Co(mnt) ], [(tea)(2) Ni(mnt) ], and [(tea)(2) Cu(mnt) ], the negolyte suffers rapid capacity fading although the posolyte is more robust. Identifying a means to stabilize V(mnt) (3-/2-) as a negolyte, and Co(mnt) (2-/1-) , Ni(mnt) (2-/1-) , and Cu(mnt) (2-/1-) as posolytes could lead to their use in asymmetric RFBs.