Abstract
Redox active solution species are important in a variety of electrochemical energy storage systems. In lithium-air batteries, redox mediators are added as soluble catalysts that mitigate charge overpotentials as well as promote solution-phase reactions that improve the cycle life of a cell. Redox flow batteries are also dependent on solution redox species to carry charge and can attain high energy capacities by increasing the concentrations of these species. In both of these systems, crossover reactions, where the redox active species diffuse from one side of the cell to the other, result in capacity loss and parasitic side reactions. Here, we report a method to monitor crossover reactions using online nuclear magnetic resonance (NMR) and electron paramagnetic resonance (EPR) spectroscopy. 2,2,6,6-tetramethylpiperidinyloxyl (TEMPO) is a stable nitroxide radical used as a redox mediator in lithium-air batteries and charge species redox flow batteries. Radical concentrations can be probed and quantified during electrochemical cycling by flowing the electrolyte through the NMR and EPR spectrometers. The experimental results reveal severe crossover and self-reduction during electrochemical oxidation of TEMPO. Diffusion models performed support the in-situ data, highlighting that crossover is a major phenomenon in these systems. Figure 1
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