How Accurate Are Our Measurements? Evaluating Conventional Electrochemical Methods To Calculate Diffusion Coefficients in Ionic Liquids

The movement of a species from the bulk to the electrode surface is a fundamental process vital to many aspects of electrochemistry, including sensing, energy storage, and metal electrodeposition. For diffusion-controlled processes, several well-accepted methods to determine the rate of diffusion are available for conventional solvents; however, in more complex solvents such as ionic liquids (ILs), it is still unclear whether the same methods provide accurate measurements. In this work, we evaluated the accuracy of four electrochemical methods, namely, the Randles-Ševčík equation, the Cottrell equation, the Shoup and Szabo approximation, and the Levich equation, to determine the diffusion coefficient of ferrocene in two widely used ILs, using various working electrodes. The results show that the Randles-Ševčík equation yields reliable and accurate values on a glassy carbon (GC) electrode but lower than expected diffusion coefficients on Pt thin-film electrodes (TFEs). However, the Shoup and Szabo approximation was found to provide accurate values at a Pt micro disk electrode. The Levich method provided highly accurate diffusion coefficients for the two ILs tested; however, bubble formation prevented the diffusion coefficients from being measured in ILs with higher viscosities. The Cottrell equation was shown to generate inaccurate values, which may be due to a combination of slow rearrangement of the ions in the double layer and uncompensated ohmic resistance, leading to inflated currents after a large potential step. From these findings, it is suggested that multiple techniques should be used to confirm diffusion coefficient values in ILs, with the viscosity of the IL and the electrode material considered when choosing the best methods.