Decomposition Mechanisms and Stabilized Organic Catholyte Molecules for Aqueous Redox Flow Batteries

Redox flow batteries have been widely recognized as promising stationary energy storage solutions for solving the intermittency issue of renewable energies. Owing to the high chemical cost, the state-of-the-art vanadium flow batteries are largely limited in market penetration. Organic redox materials have advantages of structural diversity and tunability, offering an opportunity to harvest exceeding materials properties such as solubility, stability and redox characteristics. In aqueous flow battery systems, organic anolyte molecules are extensively investigated and a variety of promising candidates have been reported such as phenazine, viologen, anthraquinone, etc. However, organic catholyte is an under-addressed area. The major limitation is the often limited chemical stability in aqueous electrolytes, especially for those having high redox potentials. Therefore, a ready urgency lies in understanding the decomposition mechanisms of chosen organic catholyte based on which to develop stable structures.<br/><br/>Here we report the mechanistic analysis of decomposition of TEMPO-based organic catholyte and the resulting development of a highly soluble, highly stable candidate. The unstable TEMPO derivative in the charged cation form is monitored temporally. Based on the evolutions of NMR peaks and pH values in its electrolyte, a decomposition pathway is proposed and a new TEMPO structure is accordingly designed so as to avoid the pathway. The new TEMPO analog exhibits a remarkably high solubility in water and an excellent stability in its charged form. Its solvate structure in water is analyzed and the preferential solvation interactions were suggested. The flow cells using this TEMPO catholyte demonstrate excellent cycling stability under practically relevant conditions. Our result represents a significant advancement in developing soluble, stable organic catholyte molecules to energy-dense, durable aqueous organic flow batteries.