Electrokinetic Streaming Current Method to Probe Polycrystalline Gold Electrode-Electrolyte Interface Under Applied Potentials

Abstract

We developed a method, by combining electrochemical and electrokinetic streaming current techniques to study ion distribution and ionic conductivity in the diffuse part of electrochemical double layer (EDL) of a metal-electrolyte interface, when potential is applied on the metal by a potentiostat. We applied this method to an electrochemically clean polycrystalline gold (poly Au)-electrolyte interface and measured zeta potential for various applied potentials, pH, and concentration of the electrolyte. Specific adsorption of chloride ions on poly Au was studied by comparing measurements of zeta potential in KCl and KClO4 electrolytes. In absence of specific adsorption, zeta potential was found to increase linearly with applied potential, having slope of 0.04–0.06. When Cl− adsorption occurs, zeta potential changes the sign from positive to negative value at ∼750 mV vs Ag/AgCl applied potential. Complementary cyclic voltammetry and X-ray photoelectron spectroscopy studies were conducted to determine a degree of chloride ion adsorption on a poly Au. A correlation was observed between the applied potential at which zeta potential is zero and potential of zero charge for poly Au. Ion-distribution and ionic conductivity in the diffuse layer were calculated from the measured zeta potential data using nonlinear Poisson-Boltzmann distribution.

Electric Double Layer at the Polycrystalline Platinum–Electrolyte Interface Probed by the Electrokinetic Streaming Current Method

“…Electrokinetic methods have significantly advanced in precision from the time when Frumkin and collaborators used these techniques, yet no attempt has been made to prove or disprove their claims of metal-charge reversal. Our earlier works presented a novel method to integrate an electrochemical three-electrode setup with a traditional electrokinetic setup to apply potential on the poly-Au surface and measure zeta potential, as well as charge in the diffuse layer at the metal–electrolyte interface. , Measured zeta potentials for various applied potentials were found to be in good agreement with the nonelectrokinetic methods, such as atomic force microscopy (AFM) studies. ,, The method correlated the known range of PZCs of poly-Au with the applied potential at which the zeta potential was found to be 0. As mentioned earlier, it was observed that even at potentials higher than PZFC, the net charge on the metal was negative due to the specific adsorption of Cl – ions on the metal.…”

“…Our previous study on the polycrystalline gold (poly-Au)–KCl interface revealed this phenomenon. It was found that the net charge on the poly-Au electrode in the nonadsorbing KClO 4 electrolyte was positive around 700 mV versus SHE, whereas it was negative in the adsorbing KCl electrolyte at the same pH and nearly the same applied potential . The applied potential is above the known range of PZFCs for Au .…”

“…The method of calculating zeta potential (ζ) from streaming current at the metal–electrolyte interface and subsequent analysis using Poisson–Boltzmann distribution to calculate net charge on the electrode were discussed in our previous studies. , In this study, we only provide a summary of the process and the most important equations to determine interfacial charges and ionic conductivities. Details of the experimental method are discussed in the Supporting Information, Sections S1 and S2.…”

“…From eq , ζ can be calculated using a modified version of the Helmholtz–Smoluchowski equation where the first equality is the modified Helmholtz–Smoluchowski equation. The second equality is introduced in our earlier work as I tot is the experimentally measured current, and the P dependence of I tot is only introduced through I S . In eq , L is the length of the microchannel, A is the cross-sectional area of the channel, η is the viscosity of the electrolyte, and ϵ r is the dielectric constant of the electrolyte.…”

“…As mentioned earlier, it was observed that even at potentials higher than PZFC, the net charge on the metal was negative due to the specific adsorption of Cl – ions on the metal. The zeta potential was found to reverse its sign and become negative when Cl – adsorbed strongly on the surface …”