Investigation on a Ni(II)-2-Iminosemiquinone Radical Complex as Catholyte for Non-Aqueous Redox-Flow Batteries

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Abstract
The energy demand is steadily rising, while the availability of natural energy sources is gradually decreasing. The extensive use of these natural energy resources has significantly harmed the environment and human health by increasing carbon emissions. Thus, a system is needed that can mitigate energy demand while remaining environmentally friendly. In this note, the redox flow battery has emerged as a promising candidate. The efficiency of a redox flow battery depends heavily on the performance of the catholyte and anolyte, i.e., their redox potentials and stability. To investigate the reversible redox cycle of a metal ion-coordinated 2-iminosemiquinone radical for the development of a catholyte for a redox-flow battery, a Ni(II)-monoradical complex (1) was synthesized by reacting redox-active H2Rich(AP)N3 ligand with NiCl26H2O. The existence of the 2-iminosemiquinone radical in the complex was warranted by the X-ray single-crystal diffraction study and X-band EPR spectrum analysis. The complex was found to exhibit two oxidation waves and a reduction wave. The first reversible oxidation at 0.465 V vs. Ag/AgCl (Fc/Fc+ = 0.443 V vs. Ag/AgCl), and the second quasi-reversible oxidation at 1.168 V vs. Ag/AgCl were investigated in a 1:4 CH3CN:CH2Cl2 solvent mixture by galvanostatic charge-discharge (GCD) measurements in the potential range of 0 to 1.0 and 0 to 1.70 V vs. Ag/AgCl, respectively, in a static H-cell. A columbic efficiency (CE) of 89% and an energy efficiency (EE) of 80% for the first one-electron oxidation with 72 hours of running time were achieved. For the two successive one-electron oxidations, CE of 90% and EE of 40% were attained, confirming the applicability of complex 1 as a catholyte

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