Non-monotonic Surface Charging Behavior of Platinum: A Paradigm Change

Abstract

In electrochemical systems, metal surface charging phenomena dictate the strength of electrostatic interactions between the electrified electrode and ions in solution. These effects are of vital importance for electrochemical reactions in general. Historically, the potential of zero charge (pzc) of the metal has been employed to parametrize the surface charging relation. The structural model of the electrified interface presented in this article goes beyond the oversimplified pzc concept by accounting for the formation of surface oxide and the orientational ordering of interfacial water molecules. The analytical solution of the model reveals a peculiar non-monotonic charging behavior. The Pt surface exhibits a negative effective charge in a low potential region, a positive charge in an intermediate potential region, and a negative charge in a high potential region due to surface oxide dipoles. This non-monotonic behavior is in agreement with a seminal experimental work of Frumkin and Petrii [Electrochim. Acta197520347359] that had remained hitherto unexplained.

Ionic Conductivity over Metal/Water Interfaces in Ionomer‐Free Fuel Cell Electrodes

“… Pt features significant adsorbed hydrogen below 0.4 V vs. RHE, but above that value, the hydrogen should be largely desorbed. Above 0.75 V vs. RHE, the oxide coverage of the Pt develops, which introduces interfacial dipoles that leads to a more negative electrode charge The typical values of bare Pt PZC in the literature are in the 0.27–0.3 V range. …”

“…Above 0.75 V vs. RHE, the oxide coverage of the Pt develops, which introduces interfacial dipoles that leads to a more negative electrode charge …”

“…2) Above 0.75 V vs. RHE, the oxide coverage of the Pt develops, which introduces interfacial dipoles that leads to a more negative electrode charge. [21] 3) The typical values of bare Pt PZC in the literature are in the 0.27-0.3 V range. The high conductivity in the low potential region (0.2-0.4 V) where the surface charge should be small (bare Pt near the PZC) suggests that the conductivity is not due to protons in EDL.…”

“…However, the ORR activity of the extended Pt surface was not evaluated in their work. Modeling studies by Chan and Eikerling and Zenyuk and Litster suggested that the ionic conductivity of water‐covered regions can be strongly dependent on Pt surface charge, local pH, and a transport‐regulated ORR pathway, although, recent modeling results suggest the surface charge and these effects could be dramatically altered by surface oxides . Tu et al .…”

“…Modeling studies by Chan and Eikerling [19] and Zenyuk and Litster [20] suggested that the ionic conductivity of water-covered regions can be strongly dependent on Pt surface charge, local pH, and a transport-regulated ORR pathway, although, recent modeling results suggest the surface charge and these effects could be dramatically altered by surface oxides. [21] Tu et al [22] conducted CV scan rate studies on a Pt black microelectrode and provided evidence that the diffusion rate of strongly and weakly adsorbed hydrogen species are key factors limiting electrochemical reaction current density several micrometers away from the electrode/Nafion® interface. Thompson and Baker [23] measured ionic conductivity of Pt black ionomer-free electrode using electrochemical impedance spectroscopy (EIS) [24][25][26] at a fixed bias potential of 0.2 V. Their results showed that the ionic conductivity within the electrode is several orders of magnitude higher than pure water.…”