Monotonic Surface Charging at Polycrystalline Platinum-Electrolyte Interface in the Presence of Chemisorbed Species Using Streaming Current Method

Abstract

Surface charging characteristics at a polycrystalline Pt-electrolyte interface were studied using electrokinetic streaming current method integrated with in-situ electrochemical cell, which was developed by our group [1-3]. Experimentally, surface charging relations are determined from the potential-dependent capacitance data. For catalytic surfaces (e.g., platinum), this method is challenging to apply due to the strong chemisorption properties of the metal [4]. We developed a method to measure surface charge of a metal in presence of strong chemisorption of ions by incorporating electrokinetic streaming current technique with traditional electrochemical 3-electrode setup (Figure 1a).Generally, the metal charge increases monotonically with applied potential. Based on theoretical models, it was suggested that the Pt-electrolyte interface might deviate from this trend and have negative charge above the potential of zero charge (PZC) due to the metal’s strong chemisorption properties [5,6]. We attempted to experimentally determine the validity of this claim using our method. In addition, we also observed the effect of anion adsorption (oxide, chloride, sulfate, etc.) on surface charging. The experimental method also allowed us to observe the slow time-dependent growth of oxides on Pt surface.Since our first publication in 2019 [1], we were able to develop the setup to be more efficient by reducing noise, electrolyte resistance, current fluctuations, etc. that enabled us to apply higher potential on the metal, observe stronger ion adsorption effects on surface charging, and obtain more reliable data. Our recent experimental study shows that Pt charging remains monotonic and does not reverse due to chemisorption of oxides in the potential window where the charge reversal was speculated to occur (Figure 1b). Figure 1c shows that metal charge (at 1.2 V vs SHE) grows slowly over time to settle down to an equilibrium value. We hypothesize that it was caused by the slow growth of oxides at the interface. The time-dependent growth of oxides has already been observed [7]. The electrolyte used in our experiments was HClO4 where the anion (ClO4 -) is less adsorbing than Cl-, SO4 2-, Br- etc. The only adsorbing anions in this electrolyte solution are OH- and O2-. In this work, we will present a brief description of the improvements of the electrokinetic-electrochemical method and our experimental results showing monotonic charging behavior for Pt in the presence of ion adsorption and oxide growth.