Avoiding Pitfalls in the Determination of Reliable Electrochemical Kinetics Parameters for the Cu2+ → Cu1+ Reduction Reaction in Deep Eutectic Solvents

Deep eutectic solvents (DES), on account of their low-cost, non-flammability and electrochemical stability, are attracting attention for their potential use in applications such as electrodeposition and energy storage. In these applications, knowledge of the transport and electrochemical kinetics properties of DES is critically important. To date, attempts to measure the kinetics parameters of the Cu2+ + e <- Cu1+ reaction in ethaline DES have yielded a cathodic charge transfer coefficient (alpha) in the range of 0.2-0.3 suggesting an unexpected asymmetric polarization behavior. In the present work, we pursued a comprehensive study of the kinetics and transport properties of the aforementioned reaction. Using steady-state and transient polarization measurements on RDE and microelectrodes combined with diffusion-reaction modeling, we demonstrate that the Cu2+/Cu1+ transition exhibits a charge transfer coefficient in the range of 0.49-0.54 and an exchange current density in the range 1.72-1.88 mA/cm(2). Recommendations are provided for avoiding pitfalls in kinetics analysis of highly resistive DES electrolytes. These recommendations include the application of accurate IR Omega correction and the use of electrode configurations where the secondary current distribution is relatively uniform. (C) The Author(s) 2018. Published by ECS.