Electrochemical Characterization of Cobalt Complexes with Benzoate and Azole-Pyridine Ligands As Potential Electrolytes in Batteries

Abstract

The study of electroactive species with multiple redox events is of interest due to their applications as electrolytes in Redox Flow Batteries (RFBs). This multi-electron behavior can be observed in organic molecules and coordination compounds containing ligands with heteroatoms such as O and N in π-extended systems. In this context, a study of 3,5-dinitrobenzoic acid, its respective cobalt complexes (1), and azole-pyridine co-ligands (2) was conducted. Electrochemical characterization of the cobalt complexes was performed using cyclic voltammetry. Measurements were conducted in a three-electrode cell, using glassy carbon as the working electrode, Ag/AgCl as the reference electrode, and a Pt wire as the counter electrode. 1 mM solutions of the benzoate ligand, 0.6 mM of complex 1, and 0.7 mM of complex 2 were prepared in acetonitrile with 0.1 M TBAPF 6 as the supporting electrolyte. The 3,5-dinitrobenzoic acid ligand was studied in a potential window of -1.7 to 0 V. Two redox events were observed at potentials of -1.21 V and -0.79 V. Both events were quasi-reversible, with ΔE values of 88 mV and 53 mV, respectively. For complex 1, an exploratory window from -1.7 to 0 V was studied, and two quasi-reversible redox events were observed that appeared to coincide with those of the benzoate ligand. These events occurred at -1.24 V and -0.93 V. It is proposed that all these events are due to redox processes of the nitro groups present in the ligand, suggesting that cobalt may not be exhibiting redox activity. In contrast, complex 2 was studied in the potential window of 1 to 2.2 V, showing two irreversible oxidation events at around 1.73 V and 1.96 V. A decrease in peak current was observed over successive cycles, indicating a potential chemical process involving the oxidized species. To evaluate the stability of the species of interest, cyclic voltammetry over 20 cycles was performed It was observed that the 3,5-dinitrobenzoic acid ligand and its complex 1 presented more stable profiles, without significant changes in the peak currents or potentials. However, complex 2 exhibited instability because as the cycles pass, the current potentials decrease and the oxidation peaks disappear. Additionally, for the benzoate ligand and complex 1, cyclic voltammetries were carried out at different scanning speeds in order to obtain the diffusion coefficients of each species. In this order of ideas, both the 3,5-dinitrobenzoic acid ligand and complex 1 are the most suitable molecules to study their application in redox flow batteries. The benzoate ligand has the advantage of having a lower molecular weight, an important factor in energy density, but it has lower stability, while complex 1 has a higher molecular weight but provides greater stability. On the other hand, complex 2 does not present reversible redox events, which is why it is not applicable as an electrolyte in BFR. Figure 1