Characterizing Degradation in Organic Redox Flow Batteries

Abstract

Organic redox flow batteries (ORFB)s are a promising energy storage technology that may facilitate the grid-integration of renewable energy. ORFB development is challenged by poor calendar lifetime and cycling stability. Currently, the most promising ORFB electrolytes are projected to last several years (whereas lifetimes must exceed 10 years to be feasible for grid-level storage) [1–3]. The majority of ORFB literature focuses on the development of stable redox molecules, which is crucial to extending cell lifetimes. However, electrode properties are known to significantly impact cell performance. Therefore, it is equally important to understand how the electrodes are affected by cell degradation.

This thesis aims to understand how electrode properties change with cycling in promising ORFB systems. We used a combination of electrochemical and spectroscopic methods to characterize changes to cycled cell components. These methods were developed on non-aqueous vanadium(III) acetylacetonate flow cells, which were used to validate our experimental set-up [4]. We employed this methodology to further examine an anthraquinone derivative (2,6-DPPEAQ) and potassium ferrocyanide as a model aqueous system, which was previously developed by Ji et al. [2]. We also examined a non-aqueous phenothiazine and viologen system, which was developed by Professor Odom and co-workers [5,6]. The temperature sensitivity of the non-aqueous system was also explored at 0, 25 and 40 oC.

Despite the exceptional chemical stability reported (and observed) for the aqueous electrolytes, a concerning loss in energy efficiency was observed with charge-discharge cycling. The performance loss is correlated with growth in charge-transfer resistance observed in full-cell impedance spectra during cycling. X-ray photoelectron spectroscopy (XPS) did not reveal significant foreign material on the cycled electrodes; however, systemic changes in oxygen bonding environment were observed, which may explain the observed performance loss.

In the non-aqueous system, stable cell capacity was observed at room temperature; whereas, rapid capacity loss was observed at 0 and 40oC. XPS revealed considerable accumulation of foreign material on electrodes cycled at 25 and 40 oC, including unexpected electrolyte impurities. Separator fouling was also observed, and correlated with reduced separator porosity and self-discharge current, which results in increased coulombic and energy efficiency with cycling.