Viologen Derivatives in Deep Eutectic Solvents for Energy Storage

Abstract

Redox flow batteries (RFBs) serve as one of the most promising battery technologies to meet the grid-scale energy storage requirements, owing to their scalable nature and acceptable redox chemistry. To achieve economical and sustainable energy storage, there has been a rapid transition from redox-active inorganic materials to the earth-abundant and tunable redox-active organic compounds for RFBs. Among explored redox-active organic materials, viologen derivatives show promise for battery applications due to their two successive redox reactions and low cost. Taking advantage of both redox reactions, however, may not be possible in aqueous electrolytes due to the insolubility of the charge-neutral viologen in aqueous solutions.1 Deep eutectic solvents (DESs) can offer an inexpensive electrolyte for energy storage which could potentially solve the solubility issues associated with these materials.2 Owing to their low vapor pressure, DESs can be also utilized at elevated temperatures for lower viscosity and enhanced conductivity.3 In an attempt to identify a high-concentrated and reversible redox-active DES, the electrochemical properties and solubility of methyl, ethyl, and benzyl viologens in choline chloride (ChCl) and ethylene glycol (EG) (1:4 molar ratio) mixture were investigated. To better qualify these redox-active molecules as an anolyte material for RFBs, the redox reactions were studied to assess the redox reversibility, the potential separation between the consecutive electron transfer reactions, and the redox kinetics. The electron-transfer rate constants were estimated using linear sweep voltammetry (LSV) at a glassy carbon rotating-disk electrode in a three-electrodes configuration cell. Diffusion coefficients were also calculated from the steady-state diffusion-limited current using the rotating-disk electrode as well as a microelectrode. Elucidating the kinetic parameters, diffusion coefficients, and transport properties of the viologen derivatives in DESs will bring insight into designing a concentrated redox-active DES for energy storage. B. Hu and T. L. Liu, J. Energy Chem., 27, 1326 (2018).B. Chen, S. Mitchell, N. Sinclair, J. Wainright, E. Pentzer, and B. Gurkan, Mol. Syst. Des. Eng., 5, 1147 (2020).N. S. Sinclair, D. Poe, R. F. Savinell, E. J. Maginn, and J. S. Wainright, J. Electrochem. Soc., 168, 020527 (2021).