Abstract
Five metal complexes of the dithiolene ligand maleonitriledithiolate (mnt2−) 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)2Vmnt], [(tea)2Comnt], and [(tea)2Cumnt] whereas [(tea)Femnt] and [(tea)2Nimnt] (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)2Vmnt] the negolyte is stable whereas the posolyte is unstable over multiple charge–discharge cycles; for [(tea)2Comnt], [(tea)2Nimnt], and [(tea)2Cumnt], the negolyte suffers rapid capacity fading although the posolyte is more robust. Identifying a means to stabilize Vmnt 3−/2− as a negolyte, and Comnt 2−/1−, Nimnt 2−/1−, and Cumnt 2−/1− as posolytes could lead to their use in asymmetric RFBs.
Highlights
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An assessment of the solubility of the complexes in MeCN showed that [(tea)2Cumnt] has a relatively high solubility of 0.91 m, which is promising for highdensity redox flow batteries (RFBs) with highly concentrated electrolytes, whereas [(tea)2Vmnt], [(tea)2Comnt], and [(tea)2Nimnt] have more modest solubilities of 0.53 m, 0.39 m, and 0.30 m, respectively. [(tea)Femnt] has poor solubility in MeCN (0.03 m) and is not suitable for application
Five transition-metal complexes of the dithiolene ligand mnt have been assessed for application as redox-active materials for single-species electrolytes in symmetric nonaqueous RFBs
Summary
As well as low viscosity (0.34 vs. 0.89 MPa s for water) and moderate dielectric constant (35.9 vs. 78.4 for water).[11]. An attractive and simple RFB system, which avoids crosscontamination of the two electrolyte solutions through membrane crossover, employs a single species electrolyte in a symmetric cell—that is, a battery that uses only one species as both negolyte (electrolyte that is reduced on battery charging, that is, anolyte) and posolyte (oxidized electrolyte, that is, catholyte) In this approach, the battery does not suffer from irreversible capacity loss and self-discharge through the mixing of electrolytes, and instead a rebalancing procedure to restore the original negolyte/posolyte composition can be performed, as is done in aqueous all-vanadium RFBs.[31] For this, the redoxactive species needs to have (at least) two redox processes and be stable across the three associated redox states. We extend the application of [(tea)2Vmnt] to flow cell experiments, as well as testing the wider family of bis-mnt complexes of [(tea)Femnt], [(tea)2Comnt], [(tea)2Nimnt], and [(tea)2Cumnt] as single-species electrolytes for nonaqueous RFBs (Figure 1)
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