Organic Anolyte Materials for Novel Redox Flow Batteries

Abstract

The work presented in this thesis focusses on developing a high-energy all-organic membrane-free redox flow battery. Redox flow batteries are promising energy storage devices due to the decoupling of energy capacity and power. Low energy densities owing to using aqueous electrolytes, alongside costly inorganic redox materials and ion-exchange membranes hinder their mass commercialisation. Organic redox active materials and non-aqueous electrolytes are promising pathways to realising high energy density redox flow batteries at an affordable price. Additionally, manipulation of the liquid/liquid interface that forms between high concentration water-in-salt electrolytes and non-aqueous electrolytes presents a method of removing the expensive, and lifetime limiting membranes. The main body of this work concentrates on a novel redox active molecule for the negative half-cell, octafluoro-9,10-anthraquinone. Octafluoro-9,10-anthraquinone shows promising redox properties, with a highly negative redox potential, rapid mass transfer, and rapid kinetics. Instability of the highly reactive charged states when in high concentrations leads to rapid decomposition and passivates the electrode. Typical stabilisation techniques, such as hydrogen bonding, protonation, and more strongly supporting electrolytes did not stabilise the reduced states. The instability of the charged states of novel redox materials is a constant challenge in this field, as capacity loss is present in every non-aqueous organic redox material to date. Low concentrations of the quinone show promising battery performance in a membrane-free device. The membrane-free devices separate the half-cells by a liquid/liquid interface that forms from the spontaneous partitioning of high concentration water-in-salt electrolytes and acetonitrile. Through studying a selection of inorganic and organic catholyte materials, this study demonstrates a novel organic membrane-free water-in-salt based static battery. However, incomplete separation and subsequent reactions of the organic materials limits the system to only static function. Flowing the membrane-free device perturbs the interface and propagates self-discharge reactions, which destroys the promising performance seen in the static systems. The second focus of this work involves a separate study covering organic redox active, high concentration deep eutectic solvent-based electrolytes. The study discovers a redox active novel benzophenone/(2,2,6,6-tetramethylpiperidin-1-yl)oxyl deep eutectic solvent with a lower viscosity and density than previously reported systems. The work highlights the challenges of using highly concentrated electrolyte solutions for redox flow battery applications.

Date of Award	1 Aug 2023
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Robert Dryfe (Supervisor) & Mark Bissett (Supervisor)