Dithiolene Complexes of First‐Row Transition Metals for Symmetric Nonaqueous Redox Flow Batteries

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.

A Comparative Review of Metal‐Based Charge Carriers in Nonaqueous Flow Batteries

“…Numerous other complexes have been developed that do not meet the Darling criteria, exemplifying the difficulties facing the field, especially in the storage of high‐voltage electrons. For example, Hogue et al [30] . developed maleonitriledithiolate (mnt) complexes of V, Fe, Co, Ni, and Cu for symmetric RFBs, and while CV data were promising ( V cell >2 V) severe capacity fade was observed for all complexes.…”

“…Numerous other complexes have been developed that do not meet the Darling criteria, exemplifying the difficulties facing the field, especially in the storage of high-voltage electrons. For example, Hogue et al [30] developed maleonitriledithiolate (mnt) complexes of V, Fe, Co, Ni, and Cu for symmetric RFBs, and while CV data were promising (V cell > 2 V) severe capacity fade was observed for all complexes. Similarly, Berben et al [32] developed a bis(imido)pyridine aluminum complex ([(I 2 P À ) 2 Al]) with up to 4 electrons stored in the ligands; however, they were only able to achieve a maximum solubility of 0.26 m and redox potential of À 0.75 V vs. Ag/Ag + .…”

Characterisation of the ferrocene/ferrocenium ion redox couple as a model chemistry for non-aqueous redox flow battery research

“…Numerous metal-coordination compounds (MCCs) [10] and redoxactive organic molecules (ROMs) [11] have been applied thus far and have demonstrated excellent properties. Most noteworthy, several symmetric NA RFBs based upon MCCs [12][13][14][15][16][17][18][19][20] and ROMs [21][22][23][24][25] have been demonstrated which mitigate crossover limitations, akin to the VRFB.…”

Copper Coordination Complexes for Energy-Relevant Applications

“…The charge potential for the cell was 1.25-1.40 V, and the discharge potential was 0.85-1.05 V. The coulombic efficiency was 82%, while the energy efficiency was 49% at a current density of 2.5 mA cm −2 . Given the complex solubility, the maximum theoretical energy density for this cell is 28 W h L −1 , corresponding to a cell potential of 1.24 V. Finally, Toghill and co-workers investigated a RFB battery based on a copper dithiolene complex (Cu mnt , Figure 1) [8]. This complex has a maximum solubility in acetonitrile of 0.91 M, for a maximum theoretical energy density of 14 W h L −1 .…”

“…A tremendous research effort has been undertaken to develop green energy devices using organic optoelectronics since the 1980s. Compared to conventional inorganic semiconductors, organic/metalorganic small molecules have a higher degree of versatility and can be synthesized and fabricated by using different strategies to offer low-cost mass production over a span of applications, including organic light-emitting diodes (OLEDs) [1][2][3][4], energy storage (batteries) [5][6][7][8], hybrid photovoltaic cells [9][10][11][12] and artificial photosynthesis (catalysis, solar fuels) [13][14][15][16][17]. These properties triggered the fast-growing development of the field that involves the synthesis of new transition metal complexes, which have generated tremendous interest for different energy-related applications [18,19].…”