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Towards a Fundamental Understanding of Aqueous Organic Redox Flow Batteries – A Study of Degradation, Aggregation, and Reactivity

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Hey, Dominic 


Renewable energy sources such as wind, solar, and hydropower are becoming more prominent in the global energy mix. To address intermittency and ensure a reliable energy supply, grid-scale energy storage systems are crucial. Redox flow batteries (RFBs) are a useful tool for grid-scale energy storage and are able to address the intermittent nature of renewable energy sources. RFBs are well-suited for grid-scale applications due to their scalability, long cycle life, flexibility, separation of power and energy, rapid response, and, most importantly, safety. The most widely used RFB is the all-vanadium system, which contains toxic and expensive metal ions. Thus, organic RFBs are a promising alternative to replace the all-vanadium system. To date, many different organic molecules, including quinones, viologens, phenazines, and alloxazines, have been investigated as potentially cheaper RFB active molecules. While a few molecules have shown good performance, most organic molecules considered for RFBs have lower energy density and generally experience degradation, reducing cell lifetime. Thus, fundamental insight at the molecular level is required to improve their performance.

In this work, different analysis methods were employed to understand the inter- and intramolecular interactions of the organic molecules in the negative electrolyte. In-situ methods based on various spectroscopic techniques were utilised: nuclear magnetic resonance (NMR), electron paramagnetic resonance (EPR); ultraviolet-visible (UV-Vis); and infrared (IR) spectroscopy.

In the first results chapter, a detailed understanding of degradation processes is developed for the negative electrolyte flavin mononucleotide (FMN), a redox-active biomolecule which is readily derived from vitamin B2. These findings were then used to improve performance dramatically. FMN hydrolysis products were identified via in-situ NMR and EPR, and it was shown that these products give rise to the additional plateau seen during charging of a FMN-cyanoferrate battery. The redox reactions of the hydrolysis product are not reversible. However, capacity retention was observed even after substantial hydrolysis, albeit with reduced Voltaic efficiency, with FMN acting as a redox mediator. Critically, degradation was mitigated, and battery efficiency was substantially improved by lowering the pH from 14 to 11.

Ex- and in-situ NMR analyses were then used to study 2,6-dihydroxyanthraquinone (DHAQ), a well-studied organic molecule in RFBs, and discussed in the second research chapter. The electrochemical degradation processes during cycling to high voltages and the electrochemical recomposition of decomposed DHAQ at low voltages are discussed. NMR was used to understand the previously unassigned electrochemical mechanisms, as well as the nature of the decomposition products. At high voltages, DHAQ decomposes to 2,6 dihydroxyanthrone (DHA) and its tautomer, 2,6-dihydroxyanthranol (DHAL). Suppression of the water solvent signal enabled the complete assignment of the obtained NMR spectra. Comparisons between deuterated water (D2O) and deionised water (H2O) were made as more NMR signals can be observed when H2O is used to avoid proton-deuterium exchange. DHAQ can then be electrochemically regenerated from the decomposition products through an oxidation to the dimer (DHA)24− followed by another low voltage oxidation. This electrochemical regeneration is a novel process that could only be identified by using the combination of ex- and in-situ NMR methods.

In the third chapter, intermolecular processes, such as aggregation, were studied via in-situ UV-Vis and related to the electrochemistry of DHAQ. The intermolecular interactions become stronger with higher concentration, and due to the drive towards using highly concentrated electrolytes in practical RFBs, an understanding of these interactions is crucial. The combination of non-negative matrix factorisation (NMF), a principal component analysis, and UV-spectroscopy enabled detailed characterisation of the inter- and intramolecular processes. A tendency for DHAQ to aggregate was identified via an ex- and in-situ UV-Vis concentration study coupled with ex-situ NMR experiments. This aggregate appears to directly influence the radical concentration during cycling and increases the reduction potential.

In the fourth research chapter, a novel operando IR cell was designed. With the cell, an analysis of the reaction kinetics and diffusion on the example system DHAQ is possible. The combination of microscopy and IR spectroscopy allowed for the direct targeting of carbon electrode fibres. Thus, the interfacial reactions of DHAQ on the carbon electrodes were better understood. Additionally, in combination with the line scan and mapping tools of the set-up, an estimation of the kinetics of the system, especially the diffusion, preferred reaction sites, and the rate of reaction of comproportionation – which has been to date unclear in the literature – was enabled.

In this doctoral thesis, new analysis tools were used to improve the fundamental understanding of chemical and electrochemical reactions in organic RFBs. By understanding degradation processes, aggregation, and interfacial processes, we are able to provide information for designing electrolytes more effectively, working with optimised parameters, such as pH and supporting electrolyte concentration, and provide valuable insight into the electrode structures and their reaction centres.





Grey, Clare




Doctor of Philosophy (PhD)

Awarding Institution

University of Cambridge