Combined Electrokinetic-Electrochemical Probe to Understand Electric Double Layer at Metal-Electrolyte Interface: Application to Polycrystalline Gold and Platinum
Authors: Saha Prantik , Zenyuk Iryna V. 
Published: December 2020

We developed a novel technique combining electrochemistry and electrokinetics to understand the structure of electric double layers (EDLs) at metal-electrolyte interface, and we applied it to study polycrystalline Au (poly-Au)-electrolyte and polycrystalline Pt (poly-Pt)-electrolyte interfaces. Traditional electrochemical methods are used to study Faradaic charge transfer reactions and EDLs at a polarized metal-electrolyte interface. On the other hand, electrokinetic methods, such as streaming current/potential are used to study ion distribution and surface conductance of ions in the diffuse layer at the interface of unpolarized solid material and electrolyte. In traditional electrokinetic experiments, the solid material to be studied is mounted inside a microchannel. An electrolyte of known chemical composition (pH, concentration, specifically adsorbing ions etc.) is flown through the microchannel at various pressures. The diffuse layer formed at the interface contains mobile ions with a net positive or negative charge. These ions, flowing with the electrolyte constitute streaming current. Zeta potential is calculated from the streaming current vs pressure data.
We combined a 3-electrode electrochemical setup and a pressure regulated microfluidic setup to apply potential on a planar electrode mounted inside a microchannel and flow the electrolyte at various pressures at the same time. The equivalent circuit of our method is shown in Figure 1(a). As charge on the electrode surface strongly depends on the applied potential, different values of streaming currents are obtained for different values of applied potentials. Zeta potential at different applied potentials is calculated from the streaming current vs pressure data. These experimentally calculated zeta potential values are substituted into the exact analytical solution of Poisson-Boltzmann distribution in the diffuse layer to explicitly calculate ion density and ionic conductivity in this layer. The charging mechanism at the interface is studied using this data. We used the simplified Gouy-Chapman interfacial model in the capacitive region of the applied potential to compare the observations and gain insights about the charging behavior.
The method was applied to electrochemically clean poly- Au and Pt surfaces. Electrolyte of high concentration (thin diffuse layer) was used to minimize the iR drop. For poly Au-electrolyte interface, we observed an approximately homogeneous charging at the interface, in the applied potential window where Au is known to behave as an ideal capacitor, and zeta potential was found to linearly increase with the iR corrected applied potential (Figure 1(b). Applied potential at which the diffuse layer is completely neutral (0 zeta potential) is found to lie within the experimental range of potential of zero charge (PZC) of Au, implying low adsorption of charges at inner Helmhotz plane (IHP). At high applied potentials ( 750 mV vs Ag/AgCl), an onset of chloride anion adsorption in a chloride rich solution was observed on Au surface. As a consequence of that, we observed a drop in zeta potential from its linearly increasing trend.
For Pt, our initial experiments indicate a similar trend: zeta potential is found to increase in the capacitive region (Figure 1(c)). We observed an anomalous charging behavior in the H+ adsorption region (Vapp < PZC), where zeta potential decreased slightly. This drop is presumably a result of the chemisorbed H+ ions. For Pt EDLs, we observed the applied potential at which the diffuse layer is neutral is generally greater than the known PZC of 260 mV vs RHE. This is probably due to the pseudocapacitance which is caused by mild adsorption of ions at Pt IHP.
The method developed by us is general and applicable to a broad electrochemistry and electrokinetics community. Ion distribution in the diffuse layer at a polarized metal-electrolyte interface, and its dependence on applied potential and electrolyte chemical composition can be studied efficiently using this method. At present the technical limitation of this method is a high uncompensated resistance, preventing the application of high potentials to observe the effects of surface oxides on the double layer. In future, the knowledge of ion distribution and ionic conductivity calculated from this method will be applied to understand the ion-conduction mechanism inside the micropores of PEM fuel cells, where ionomers cannot penetrate and enhanced double layer conductivity can help the transport of H+ ions to Pt particles.

Figure 1