Mechanistic Study of the Multi-Electron Redox Cycle of Nickel Dithiocarbamate and Dithiolate Complexes for Redox Flow Battery Applications
Type of DegreePhD Dissertation
Chemistry and Biochemistry
Restriction TypeAuburn University Users
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The necessity for new grid energy storage techniques, for example, redox flow batteries (RFBs), will be vital as consumption of renewable energy sources continues to increase. Nickel-based dithiocarbamate and dithiolate complexes are important for potential use as catholyte in non-aqueous redox flow batteries. The unique redox cycle of nickel dithiocarbamates (Ni(dtc)2) displays 2e- chemistry upon oxidation from Ni(II) → Ni(IV) but 1e- chemistry upon reduction from Ni(IV) → Ni(III) → Ni(II). The underlying reasons for this cycle lie in the structural changes that occur between four-coordinate Ni(dtc)2 and six-coordinate [Ni(dtc)3]+. Cyclic voltammetry and spectroscopic experiments show that these 1e- and 2e- pathways can be controlled by the addition of ancillary ligands such as pyridine derivatives and Lewis acids such as Zn(II). Nickel dithiolate complexes also show 2e- redox chemistry based on similar principles. Chapter 1 provides a general overview of the need for RFBs, how they function, and a description of electrochemical techniques which are employed in later chapters. Chapter 2 focuses on the mechanistic study of the addition of different pyridine-based ancillary N-donor ligands (L) to the Ni(dtc)2 solution. These studies show that 1e- oxidation of Ni(dtc)2 produces a mixture of five-coordinate [Ni(dtc)2L]+ and six-coordinate [Ni(dtc)2(L)2]+ intermediates which decay to [Ni(dtc)3]+ by parallel pathways. The equilibrium constants for L coordination were determined and found to increase with larger pKa values of the pyridine base. Chapter 3 reports how 2e- efficiency and reversibility of Ni(dtc)2 can be improved. The addition of Zn(II) to the electrolyte is shown to consolidate the two 1e- reduction peaks into a single 2e- reduction where [Ni(dtc)3]+ is reduced directly to Ni(dtc)2. The use of Zn (II) to increase the reversibility of 2e- transfer is a highly promising result which points to the ability to use nickel dithiocarbamates more effectively in RFBs. Chapter 4 discusses the synthesis and electrochemical characterization of two dithiolate-based ligands and their corresponding Ni(II) complexes. Finally, Chapter 5 discussed the importance of ionic exchange membranes in RFBs, revealing the best commercially available membrane for low-cost, robust, and conductive anion exchange in acetonitrile-based RFBs.