Redox flow batteries have particular advantages for large-scale stationary energy storage to address the intermittency challenge of renewable energies. Conventional inorganic redox materials such as vanadium, iron or bromine are hampered by high chemical cost or long-term cyclability. Because of the structural diversity and tailorability, organic redox molecules can be rationally functionalized to improve solubility and stability, and thus have been extensively investigated for aqueous redox flow batteries. Particularly, a variety of organic anolyte molecules have been reported such as phenazine, viologen, anthraquinone, etc. However, there are much less successful examples of stable organic catholyte molecules, especially those having high redox potentials. This drawback has caused critical challenges to develop all-organic flow chemistries with comparably stable anolyte and catholyte molecules. In many cases, the ferri/ferrocyanide catholyte redox couple was used in studying anolyte molecules, but its stability and solubility are rather limited by the supporting ions especially in alkaline electrolytes. Therefore, there is an urgent need to unravel the decomposition mechanisms and develop stable organic catholyte molecules. The charged form of an organic catholyte molecule is often a radical cation for 1e– or a dication for 2e– reactions. Common decomposition pathways for the cations include hydrolysis, nucleophilic addition, and self-rearrangement. Here, we will report our investigations of the decomposition mechanisms of a catholyte organic molecule. The electrochemically charged radical cation form is monitored using electron paramagnetic resonance (EPR). Combined spectroscopic analysis unravels an evolution from an unstable to a more stable structure. The observation indicates the decomposition pathway but also suggests the key stability controlling factors, which has led to the development of a stable catholyte from this organic scaffold.

Decomposition mechanisms and stabilized organic catholyte molecules for aqueous redox flow batteries

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