Probing Redox Properties of Extreme Concentrations Relevant for Nonaqueous Redox-Flow Batteries

Abstract

Redox-flow batteries are an emerging energy storage technology that can pair with intermittent renewable energy technologies. There remains a need, however, to understand physicochemical relationships among the solvent, electrolyte salt, and redox-active molecules that comprise catholyte and anolyte solutions. To examine this relationship, we detail a systematic study wherein the concentrations of the redox-active molecule 2,2,6,6-tetramethylpiperidine 1-oxyl (TEMPO) and TBAPF6 electrolyte salts are varied over concentrations of 1 mM to over 1000 mM in acetonitrile. Three series were investigated: (1) varying the concentration of TEMPO while holding the concentration of TBAPF6 constant, (2) varying the concentration of TBAPF6 while holding the concentration of TEMPO constant, and (3) varying both the concentration of TEMPO and TBAPF6 with a 5:1 TBAPF6:TEMPO ratio. Cyclic voltammetry data from macro- and microelectrodes were used to quantify diffusion coefficients and heterogeneous electron transfer rates, and these metrics were connected to the conductivity and viscosity to develop clear trends over the entire concentration range. Fundamental chemical interactions that lead to changes in physical properties were implicated via vibrational spectroscopy and molecular dynamics (MD) simulations. Trends in conductivity and viscosity for systems were inversely related and correlated to trends in diffusion coefficients and heterogeneous electron transfer rates. Intuitively, faster diffusion and electron-transfer rates occurred with lower TEMPO concentrations and higher TBAPF6 concentrations, with the majority of conditions falling in the general proximity of literature values (k0 = 0.1–0.5 cm/s, D ≈ (2.0–4.0) × 10–5 cm2/s). At the highest TBAPF6 concentrations, vibrational spectroscopy and MD simulations show that intermolecular interactions were more nuanced, and solvation and ion-pairing effects begin to influence electrochemical and physical properties. This functional approach including electrochemical and physical characterization paired with MD simulations provides a template for methodically studying systems for redox flow battery applications.