Understanding Hydrogen Bonding Effects on the Reversibility and Redox Potential of Redox Organic Molecules in Deep Eutectic Solvents

Abstract

Deep eutectic solvent (DES) systems offer unique properties as electrolytes for redox flow batteries. They consist of hydrogen bond donor and acceptor pairs and often offer larger electrochemical windows than traditional aqueous electrolytes with lower vapor pressure than other non-aqueous systems like acetonitrile. They can contain a large concentration of charge carries and have the potential to be high energy density electrolytes1. When paired with redox organic species it is possible to tune both the electrolyte and redox actives to obtain desirable electrochemical properties such as the solubility of active species, kinetic and transport behaviors and even thermodynamic properties such as redox potential2. When attempting to understand these behaviors in DES it becomes important to understand the redox organic molecule’s interaction with the hydrogen bond network. For example, it has been show that altering the ratio of hydrogen bond donor to acceptor sites in a molecule can have dramatic effects on its solubility3. Hydrogen bonding between the DES and the redox organic in its varying charge states can dramatically impact many aspects of the electrochemical behavior.This work focuses on interactions that can have apparent effects on the kinetic and thermodynamic behavior of redox organics. We explore several molecules such as viologen derivatives, PTIO and N-methyl-phthalimide, some of which exhibit reversible behavior in both acetonitrile and a choline chloride and ethylene glycol based DES while other do not. Some see a dramatic shift in redox potential while other do not. We correlate these changes in behavior to influences of hydrogen bonding through the use of IR spectroscopy paired with voltammetry. Red shift in the stretching frequencies of redox active functional groups indicates the presence or lack of hydrogen bonding and this is related to changes in voltammetry as ethylene glycol, a hydrogen bond donor, is added to the redox organic in acetonitrile electrolyte system. It was observed that radical/cation reactions such as PTIO/PTIO+ or viologen were unaffected both kinetically and thermodynamically while reactions involving anionic species such as PTIO-/PTIO or N-methyl-phthalimide would either become irreversible or shift potential. This can be observed for PTIO in the accompanying figure where the cation reaction (more positive couple) does not change while the anion reaction (more negative couple) changes potential dramatically. We propose that some anionic species hydrogen bond more strongly at the redox active sites due to their higher electron density at a hydrogen bond critical point. Electron density has been strongly correlated to hydrogen bond strength4 and molecular modeling is being done to determine the comparative hydrogen bond strength of the anion and cation species as well as reversible and irreversible cationic species. These insights can lead to the intelligent design of highly soluble redox species that remain reversible and make use of the full electrochemical window of a given DES. E. L. Smith, A. P. Abbott and K. S. Ryder, Chem. Rev., 114, 11060–11082 (2014).N. S. Sinclair, D. Poe, R. F. Savinell, E. J. Maginn and J. S. Wainright, J. Electrochem. Soc., 168, 020527 (2021).B. Chen, S. Mitchell, N. Sinclair, J. Wainright, E. Pentzer and B. Gurkan, Mol. Syst. Des. Eng., 5, 1147–1157 (2020).G. Gilli and P. Gilli, J. Mol. Struct., 552, 1–15 (2000). Figure 1