Mechanistic Insights into Reversible Two-Electron Energy Storage for Redox-Flow Batteries Using Nickel Bis(diphosphine) Complexes

Abstract

Redox flow batteries (RFBs) have emerged as a promising energy storage technology for grid-scale integration of renewable energy. Low energy density remains a major barrier to their large-scale commercialization and broader application. The storage of multiple electrons per molecule can greatly enhance the energy density of redox-flow batteries (RFBs). Here, I show that nickel bis(diphosphine) complexes efficiently store multiple electrons through a concerted 2e– redox wave through the modulating their coordination environment. Continuous charge–discharge cycling experiments show more negative charge–discharge potentials and improved capacity retention in the presence of halide, thus improving the energy storage of nickel bis(diphosphine) complexes as anolytes in RFBs. This work shows how mechanistic understanding of 2e– redox cycles for transition metal complexes can create new opportunities for multi-electron storage in RFBs. Chapter 1 provides the general overview of the necessity of renewable energy storage systems, details description about redox flow batteries, electrochemical technique and nickel (diphosphine) complex chemistry. In chapter 2, the effect of substituents on diphosphine ligands and coordination of monodentate ligands (MeCN, Cl–) in the electrochemistry and their mechanistic studies has been discussed. Chapter 3 includes the detail mechanistic studies of sequential 1e– vs concerted 2e– redox couple and modulating the coordination conditions enable the selective 2e– pathway and achieving excellent capacity retention. In chapter 4, mechanistic and kinetic studies of nickel diphosphine complex have been carried out to aim high energy density.