The nitroxide radical redox organic molecule, 2-phenyl-4,4,5,5-tetrame- thylimidazoline-1-oxyl-3-oxide (PTIO), was investigated for the first time in a deep eutectic solvent (DES)-like system consisting of a 1:4 molar ratio of choline chloride and ethylene glycol (Ch1EG4) as a redox flow battery electrolyte. PTIO is a single molecule with three oxidation states, and can provide both positive and negative redox couples for a flow battery. A flow battery using the PTIO/Ch1EG4 electrolyte demonstrated nearly 50% round trip efficiency with an approximately 1 V open circuit potential. Inefficiencies were primarily due to membrane resistance which can be significantly lowered with increased temperature. While PTIO appears stable over short periods (hours), the oxidized form is not stable in the DES-like electrolyte over longer times. Molecular modeling was performed to investigate the relative stability of PTIO in DES as compared to the previously studied 4-hydroxy-TEMPO (4HT). It was found that the oxoammonium cation 4HT+ exhibits a noticeably larger nucleophilic reactive cloud as compared to PTIO+, indicating a higher reactivity. This method to predict stability of the oxoammonium cation shows promise to inform the design and synthesis of promising redox systems based on nitroxide radicals in DES electrolytes to identify new chemistries for large scale energy storage.
Redox flow batteries (RFBs) possess multiple advantages as a flexible energy storage solution. However, RFB researchers are still facing many challenges in finding an appropriate electrolyte. Microemulsions have recently been proposed as a promising alternative RFB electrolyte because of their ability to accommodate organic redox species with fast electron transfer rates that are not soluble in aqueous phase, while still offering the high conductivity of an aqueous salt electrolyte. In this work, we focused on understanding the transport of ferrocene (Fc) in a toluene/Tween 20/1-butanol/water model microemulsion and studied the compositional influence on Fc diffusion. The results show that Fc redistributes among the oil, surfactant, and water microenvironments, and the corresponding diffusion and partition coefficients are quantified. Thus, a tortuous path diffusion model is proposed to describe the mass transport of Fc to an electrode surface. Diffusion coefficients are also obtained by pulsed-field gradient nuclear magnetic resonance (PFG NMR), while the values for Fc diffusion are substantially higher than those from electrochemical measurements, suggesting that they measure samples in different ways. The current contributions from each microenvironment indicate that the Fc permeability is much higher in the oil, even though the electron transfer reaction is likely occurring in the surfactant.
The Ag/AgCl reference electrode is commonly used in choline chloride based deep eutectic solvents. However, we found it undergoes significant potential shifts in electrochemical tests which previous reports largely ignored. In this work, we studied the degradation mechanism leading to its instability. Results show that due to the high Cl− concentration in ethaline, the AgCl film easily dissolves and forms AgCl2
− species causing a potential shift. Therefore, we suggest a [Fe(CN)6]3−/[Fe(CN)6]4− reference electrode based on the reversibility and low diffusivity of [Fe(CN)6]3−/[Fe(CN)6]4− redox couple in ethaline, which was demonstrated to be reliable and stable over weeks of operation.
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