Intermediate stages of electrochemical oxidation of single-crystalline platinum revealed by in situ Raman spectroscopy

Abstract

Understanding the atomistic details of how platinum surfaces are oxidized under electrochemical conditions is of importance for many electrochemical devices such as fuel cells and electrolysers. Here we use in situ shell-isolated nanoparticle-enhanced Raman spectroscopy to identify the intermediate stages of the electrochemical oxidation of Pt(111) and Pt(100) single crystals in perchloric acid. Density functional theory calculations were carried out to assist in assigning the experimental Raman bands by simulating the vibrational frequencies of possible intermediates and products. The perchlorate anion is suggested to interact with hydroxyl phase formed on the surface. Peroxo-like and superoxo-like two-dimensional (2D) surface oxides and amorphous 3D α-PtO2 are sequentially formed during the anodic polarization. Our measurements elucidate the process of the electrochemical oxidation of platinum single crystals by providing evidence for the structure-sensitive formation of a 2D platinum-(su)peroxide phase. These results may contribute towards a fundamental understanding of the mechanism of degradation of platinum electrocatalysts.

Introduction

Platinum is one of the most fundamentally significant catalysts due to its widespread applications in heterogeneous catalysis and electrochemistry. The proton-exchange membrane fuel cell (PEMFC) is one of the most promising applications of Pt-based catalysts, offering a solution to the urgent energy problem as a stationary or automotive power source1,2. A PEMFC is usually composed of a Pt anode for the oxidation of fuel and a Pt-based cathode for the reduction of oxygen gas. The performance of PEMFC primarily relies on the electro-catalytic activity for the oxygen reduction reaction (ORR) of Pt catalysts2. Unfortunately, it has been found that the ORR activity decreases during the long-time running of PEMFC. Thus, many efforts have been devoted towards understanding the degradation of Pt catalysts during ORR3. Much of our understanding of the surface oxidation of platinum electrodes comes from detailed electrochemical experiments as summarized and exemplified in the works of Conway and colleagues4,5. More recently, the deployment of surface-sensitive techniques and well-defined surfaces has led to a more chemically and structurally detailed understanding of platinum surface oxidation. Cyclic voltammetry, ex situ X-ray photoemission spectroscopy characterization and in situ electrochemical scanning tunnelling microscopy studies of Pt(111) single crystals have identified majority species on the surface, such as OHads and Oads, and have shown that the well-defined terrace is damaged as steps and defects form at the onset of the formation of a three-dimensional (3D) oxide film, also known as the place exchange process6,7. In situ X-ray diffraction and energy dispersive X-ray absorption spectroscopy of supported Pt nanoparticles have illustrated this formation of a surface Pt oxide and its subsequent growth into a bulk oxide8. From a more practical point of view, ex situelectron microscopic observations have indicated that the Pt nanoparticles grow during operation and inductively coupled plasma mass spectroscopy coupled with voltammetry has shown that Pt dissolves into the electrolyte during the potential sweeping9,10,11. In spite of these significant advances, the nature of the surface species formed during the surface oxidation of Pt, and their dependence on surface structure, has remained ambiguous.

In this paper, we will explicitly identify the surface species formed during electrochemical oxidation of atomically flat Pt(111) and Pt(100) single crystals by in situ Raman spectroscopy. The recently developed method of shell-isolated nanoparticle-enhanced Raman spectroscopy (SHINERS) allows obtaining Raman spectra of surface species on single crystals covered by chemically inert silica-coated Au nanoparticles (Au@SiO2)12,13. Up to now, SHINERS has been successfully used to study the adsorption behaviour of small molecules on Au or Pt single crystals, such as pyridine, CO oxidation, room temperature ionic liquids and so on13,14,15,16,17. SHINERS also has been used to examine the surface composition of Ni-based alloys produced by electrochemical oxidation18. Previously, electrochemical surface-enhanced Raman spectroscopy has been used to reveal reaction intermediates on nano-structured surfaces19,20,21,22,23,24,25,26. The attempt in the present work to characterize electrochemical reaction intermediates on well-defined single crystals extends the application of Raman spectroscopy to the initial stages of surface oxidation of atomically defined catalytically relevant electrochemical interfaces. Our results will give new insights into the structure-sensitive surface oxidation of platinum single crystals in acid media, and provide evidence for a two-dimensional (2D) platinum surface (su)peroxide phase formed on Pt(111) before 3D platinum-oxide formation.

Results

Voltammetry and SHINERS of Pt(111) and Pt(100)

Figure 1a shows a cyclic voltammogram of Pt(111) in 0.1 M HClO4electrolyte. It exhibits the characteristic reversible peaks in the potential regions of 0–0.4 V and 0.6–0.9 V versus reversible hydrogen electrode as a reference (RHE). If the potential is scanned to more positive values, the voltammogram is no longer reversible. A sharp peak at 1.1 V is followed by a broad peak at 1.3 V in the forward scan, whereas a broad cathodic peak is observed at ca. 0.7 V in the back scan. The reversible plateau peak at the potentials of 0–0.4 V has been ascribed to the adsorption of hydrogen atoms (Hads) on the surface27,28. Previous spectroscopic evidence for the formation of Hads in this potential window comes from different infrared reflection techniques using Pt(111) or polycrystalline Pt (refs 29, 30, 31). The Raman spectra on Pt(111) to be described below were recorded at potentials ranging from 0.6 to 2.0 V, to reveal the surface species involved in the oxidation. The blank voltammetry of Pt(111) in the presence of the Au@SiO2 nanoparticles, as shown in Supplementary Fig. 1, shows that the main voltammetric features of Pt(111) are retained and that the nanoparticles block part of the surface but otherwise do not appear to influence significantly its electrochemical properties.

Figure 1: Cyclic voltammograms and potential dependent Raman spectra of Pt(111) and Pt(100).

(a) Voltammogram of Pt(111) in 0.1 M HClO4 electrolyte; scan rate 50 mV s−1. (b) SHINERS spectra of Pt(111) at the indicated potentials. (c) Voltammogram of Pt(100) in 0.1 M HClO4electrolyte; scan rate 50 mV s−1. (d) SHINERS spectra of Pt(100) at the indicated potentials. The SHINERS spectra were recorded with the potential stepped positively; every spectrum was collected in 55 s.