Novel non-aqueous symmetric redox materials for redox flow battery energy storage

Armstrong, Craig and Toghill, Kathryn (2020) Novel non-aqueous symmetric redox materials for redox flow battery energy storage. PhD thesis, Lancaster University.

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Abstract

Redox flow batteries are promising energy storage devices for grid-scale applications due to their decoupled power and capacity. The utilisation of non-aqueous electrolytes, as opposed to conventional water-based electrolytes, is a promising pathway for achieving techno-economic targets via advancements in energy density. Herein a selection of novel redox materials were explored for application as symmetric redox flow battery electrolytes whereby the same compound is used in both the battery cathode and anode reactions. Metal coordination compounds such as Co(II) complexes with tridentate azole-pyridine ligands demonstrated good long-term stability and cell potentials in excess of 1.5 V, however low solubility due to their large size is problematic. Smaller metal complexes with bidentate dithiolene ligands gave promising redox properties however instability of charged oxidation states causes rapid decomposition of the inorganic electrolyte. Similar instability was observed for a new symmetric redox material, croconate violet, which arises from high reactivity of radical states. The instability of the charged oxidation states of novel redox materials remains a challenge for the research field, as capacity loss has been reported in practically every novel non-aqueous redox material. Indeed, by developing the ferrocene-ferrocenium ion redox couple for non-aqueous cell characterisation, a noteworthy capacity loss over extended battery cycling experiments was observed. The present work therefore highlights the challenges with identifying suitable non-aqueous redox materials for application.