Probing the molecular interactions and physicochemical properties of a cobalt-based redox electrolyte system for thermo-electrochemical cells

Abstract

Redox-active materials play a primary role in the high-performance electrochemical device research field. Their bulk ion dynamics and performances can be studied using different electrochemical analysis methods, but their molecular level interactions and dynamics on which these depend are often not well understood. Here, nuclear magnetic resonance (NMR) relaxation and double-stimulated echo pulsed field gradient (PFG) techniques have been used to gain insights into the molecular level interactions, exchange dynamics and self-diffusivity of the various species present in a cobalt-based redox active electrolyte system used for thermo-electrochemical applications, including how these factors depend on the oxidation state and concentration of the redox species. A series of liquid electrolyte samples consisting of a Co^2+/3+(bpy)₃(NTf₂)_2/3 redox couple (where bpy = bipyridyl and NTf₂ = bis(trifluoromethanesulfonyl)imide) in 3-methoxypropionitrile (MPN) have been investigated using NMR as well as viscosity and conductivity measurements carried out over a temperature range 293 to 353 K. The results provide insights into the mobilities and interactions between the various species present, including the exchange of the NTf₂⁻ anions between the solvation shells of the Co(bpy)₃ species. Such information will be useful in understanding the behaviour of these electrolytes in devices such as thermo-electrochemical cells.