Kinetics of Charge Reversal in the Double Layer Studied with an Electrochemical Quartz Crystal Microbalance (EQCM-D)

Abstract

Normal pulse voltammetry on inert electrolytes was combined with a new electrochemical quartz crystal microbalance (EQCM) with millisecond time resolution, which allows to probe the kinetics of charge reversal in the diffuse double layer (DL) after a step in electrode potential. The QCM responds to the charge reversal because the local viscosity-density product depends on the type and the concentration of the ions. The time resolution was improved to 1 ms using multifrequency lockin amplification, which allows to study kinetic effects in dynamic electrochemistry, which were previously not accessible with gravimetry. The noise was lowered to less than 10 mHz by modulating the electrode potential and averaging Δf and ΔΓ over many modulation cycles.The overtone scaling was found to be the same as in gravimetry (-Δf/n » const. and ΔΓ/n < -Δf/n with n the overtone order), which, however, is not caused by a deposition of a rigid film in the Sauerbrey sense. A Sauerbrey-type QCM response is also caused by changes in Newtonian viscosity inside a layer with a thickness far below the penetration depth of the shear wave (such as the double layer).The frequency response in the electrolytes studied correlated well with the viscosity B-coefficients of the respective ions as employed in the Dole-Jones equation. The time constants inferred from gravimetry were similar to the time constants (the RC-times) from electrochemical impedance spectroscopy (EIS). There were small differences, which presumably go back to slow rearrangements inside the Helmholtz layer, which are not seen in EIS.The QCM response to double layer recharging in inert electrolytes is of great importance for the interpretation of QCM results on processes involving Faradayic charge transfer. Such a response is omnipresent as a background and might be misinterpreted as a gravimetric signal.[1] References[1] Leppin, C.; Peschel, A.; Meyer, F. S.; Langhoff, A.; Johannsmann, D. Kinetics of Viscoelasticity in the Electric Double Layer Following Steps in the Electrode Potential Studied by a Fast Electrochemical Quartz Crystal Microbalance (EQCM). Analyst 2021, 146 (7), 2160–2171. Caption: Shifts in frequency and bandwidth in response to potential steps (left). The gravimetric signal followed the steps with a delay of a few milliseconds. The amplitude and the response time depend on cation type (right). The counter ion was NO3 -. Both the amplitude and the response time correlate well with the viscosity B-coefficient. Figure 1