Mechanistic Study of the Multi-Electron Redox Cycle of Nickel Dithiocarbamate and Dithiolate Complexes for Redox Flow Battery Applications

Abstract

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.