Combined Electrokinetic and Electrochemical Measurements to Study the Electric Double Layer at Electrode-Electrolyte Interface

Abstract

Electrochemical techniques, such as cyclic voltammetry and chronoamperometry are generally used to understand the reaction kinetics and structure of the electric double layer (EDL) formed at the electrode-electrolyte interface. While these techniques can effectively probe the Faradaic reactions, specific adsorption processes etc., they are limited in their applicability towards measuring potential at the outer Helmholtz plane (OHP). On the other hand, electrokinetic methods are broadly used to measure potential at the slip plane, Zeta potential, which determines the nature of the diffuse layer at the interface. Depending on the ionic strength of the solution (Debye length) Zeta potential and OHP potentials can coincide. In this study, we used the electrokinetic techniques, such as streaming-current and streaming-potential methods to measure Zeta potential of solid metallic surfaces and hence attempted to bridge electrochemical and electrokinetcs in understanding of the EDLs. Figure 1a shows a novel electrochemical cell designed by our group and adapted for the electrokinetic set-up. In conventional streaming current experiment, pressure difference between the two electrolyte reservoirs drive the ionic current mostly through the slip plane. By taking the slope of current vs pressure at a given pH a Zeta potential can be calculated. We report Zeta potential of a strip of Au, our model electrode, under various conditions (varied pH and electrolyte concentration), where the solid cell set-up measurements were validated against the more conventional Zeta potential measurements of colloidal particles with dynamic light scattering approach. By incorporating electrochemical set-up, we were able to vary applied potential on the electrode to probe potential of zero charge (pzc) of gold. The surface of gold electrode was cleaned beforehand in electrochemical three-electrode set-up using cycling voltammetry. Combining the data obtained from cyclic voltammetry and electrokinetics and applying the well-tested Gouy-Chapman theory of the EDLs, we were able to calculate the ion-distribution in the diffuse layer, the diffuse layer conductivity and the charge density on the surface of the electrode. We also attempted to comment on the possible relationship between the pzc and iso-electric point. The pzc of gold was found to be dependent on the pH of the electrolyte, and it varied between 200 mV to 500 mV on SHE. Next, we will extend the method to the other interfaces of interest to electrochemical community (such as Pt). Figure 1