Authors:

Md Musharraf Hossain

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The unique architecture of redox flow batteries (RFBs), which decouples energy density from power density, has encouraged scientists to consider them a promising renewable energy storage system for grid scale applications due to their safety, cost-effectiveness, and durability. In RFBs, redox- active molecules are used to store and release energy as anolytes and catholytes in solutions. The energy stored (ΔG = -nFEcell) depends on the number of electrons (n) transferred per molecule and the potential difference between catholyte potential (Ec) and anolyte potential (Ea) where Ecell = Ec - Ea. Increasing energy density and finding redox-active molecules of high stability are the two main challenges for practical use of RFBs. In this study, we investigated easily synthesized, highly stable, and soluble nickel(II) bis(diphosphine) complexes for anolytes in RFBs, which undergo electrochemically reversible two-electron redox reactions at notably negative reduction potentials in acetonitrile (MeCN) solvent. For example, [Ni(dmpe)2](BF4)2, where dmpe = 1,2-bis(dimethylphosphino)ethane, undergoes a reversible 1 x 2e- reduction to Ni(dmpe)2 at -1.36 V vs Fc+/0 and exhibits solubility about 1 M in MeCN. Our studies have shown that halide (X- = Cl⁻, Br⁻, I⁻) electrolyte conditions have significant impact on solubility, electrochemistry, and cycling stability of nickel(II) bis(diphosphine) complexes. It has been observed that the X⁻ ions increase the reduction potential to a more negative value, convert 2 x 1e- redox process to 1 x 2e- redox reaction, and enhance the charge-discharge cycling performance. We also found that addition of diphosphine ligands further improve the cycling performance of nickel(II) bis(diphosphine) complexes. Our focus will be the comprehensive understanding of redox mechanism of nickel (II) bis(diphosphine) complexes by analyzing data from scan rate dependent cyclic voltammetry (CV) studies, DigiElch modeling, UV-vis spectroscopy, and 31P NMR spectroscopy. The insights gained from this research will enhance our understanding of using multi-electron inorganic redox couples and design new complexes as suitable anolytes for practical RFB applications.

Source:

Auburn University / College of Sciences and Mathematics / 2025

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Co-authors:

Md Musharraf Hossain