A Study of Novel Nitrogen-Containing Quinoidal Electrolytes for Redox Flow Batteries

Abstract

Redox flow batteries (RFBs) with their decoupled power and capacity scaling are a promising energy storage technology. However, low energy density and high capital costs stymy such systems. For this reason, novel electrolytes are under active development. Previous electrolytes have involved a variety of metal-containing and organic systems. Organic systems can offer fast kinetics, high tunability and low cost. In this thesis, novel quinone derivatives were synthesised and investigated as potential electrolytes for organic aqueous RFBs. The first research chapter presents an investigation of quinones functionalised with aryl amines. 2,5-bis(4-carboxyanilino)-benzo-1,4-quinone was found to be the most promising example, retaining its electrochemical activity after reacting with hydroxide ions, the resultant molecule showing greater stability and reacting at a lower potential. Using NMR and EPR spectroscopy, insights into the charge delocalisation and chemical reactions of the molecule under battery operation were gained. The important conclusion from this study is that conjugate addition is not always detrimental. The next chapter involved screening several vanillin-derived quinones for potential viability for RFB use. Several candidates were synthesised and characterised and a flow battery based on 2,5-bis((methyl)aminoethanol)-benzo-1,4-quinone was constructed. These electrolytes were shown to be promising and chemically versatile, but solubility is still a major problem that will limit their application. The final research chapter explored several heteroaromatic quinones derived from tetra-amino-benzoquinone. Aqueous electrochemical characterisation was carried out and the most promising candidate, bis-triazolo-benzoquinone, was used as an anolyte in basic aqueous media. In situ spectroscopy was used to gain insight into the behaviour of the molecule under battery operation. Fused quinone systems of this type may be promising electrolytes but problems associated with solubility and long-term cycling are yet to be overcome. Overall, the thesis has (i) developed new protocols for the synthesis and derivatisation of important families of novel electrolytes for RFBs (based on simple chemical routes), (ii) introduced new methodologies for benchmarking electrolyte electrochemical performance, and (iii) exposed the compromise between solubilising and electrochemical functionality as a primary future challenge for organic RFB electrolyte development.