Organic Redox Flow Batteries: Active Materials and Reactor Performance

Doris Hoffmeyer

Research output: Book/ReportPh.D. thesis

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Abstract

Redox flow batteries are attractive as large-scale energy storage systems, as their working principle allows independent scaling of stored energy and power. High costs of the active materials in state-of-the-art systems are currently impeding widespread implementation, which has motivated the search for cheaper active materials. This thesis investigates the use of quinones, a subgroup of organic compounds, as the active materials for redox flow batteries, and the influence of experimental conditions on the reactor performance.

The molecular electrochemistry and stability of several benzo- and anthraquinones was investigated through three-electrode voltammetry and bulk electrolysis. These included both previously studied and in-house developed quinones. The ferri-/ferrocyanide couple was additionally characterised, as it was employed in cell cycling and as the model system for reactor performance studies. The investigated compounds generally displayed fast diffusion and electrode kinetics, with values in the same range as previously reported in the literature. These are desirable characteristics for redox flow battery electrolytes to avoid significant overpotentials. Three of the investigated quinones were found to be chemically unstable at pH 14, as seen from their convoluted electrochemistry, absence of an expected redox process, or loss of accessible capacity during bulk electrolysis.

Cycling of a single flow cell in both symmetric and full cell configurations enabled the study of the long-term stability of the investigated compounds. Symmetric cell cycling of ferri-/ferrocyanide at pH 7 resulted in capacity fade rates of 0.16 % d−1 and 0.008 % d−1 with a galvanostatic and potentiostatic protocol, respectively. The difference was attributed to an increasing membrane resistance and emphasises the importance of choosing an appropriate method for investigating capacity fade rates. Full cell cycling was performed with three different quinone negolytes against a ferrocyanide posolyte, and resulted in apparent capacity fade rates between 1.4 % d−1 and 21.2 % d−1. The experiments lasted between 9 hours and almost 7 days, but were generally of too brief duration to draw conclusions about the capacity fade. Furthermore, the fade rates were significantly higher than previously reported in the literature, which was ascribed to insufficient oxygen protection of the oxygen-sensitive quinones.

Reactor performance studies were conducted by recording the electrochemical impedance of symmetric single cells under the influence of changing experimental conditions. Modelling with an equivalent circuit allowed resolution of the impedance into contributions from ohmic, charge transfer, and diffusion processes. Pretreatment by thermal oxidation and compression of two carbonaceous electrode materials were shown to influence the ohmic resistance of the cell. The pretreatment furthermore increased the hydrophilicity, which was ascribed to a combination of changing surface groups and roughening of the carbon fibre surface. Pulsation dampening of the electrolytes and active temperature control were tested and found to be essential for the collection of stable impedance data of good quality. Pretreatment of the employed Nafion membranes was found to have a major influence on the ohmic resistance of the reactor. The accumulated knowledge was used to study the influence of electrolyte flow rate on the symmetric cell impedance of ferri-/ferrocyanide and two quinones. The finite diffusion resistance was found to consistently decrease with electrolyte flow rate for all three systems, in accordance with a decreasing average stagnant layer thickness with increasing flow rate. The ohmic resistance was furthermore found to be the main contributor to the total resistance for all three systems across all applied flow rates.
Original languageEnglish
Number of pages175
Publication statusPublished - 2021

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