EC-XPS Analysis of Oxygen Species Adsorbed on Pt-Nanoparticle Model Electrodes

Abstract

The research and development of highly active cathode catalysts for the oxygen reduction reaction (ORR) is one of the most important subjects to achieve high efficiency in polymer electrolyte fuel cells (PEFCs). In order to understand the ORR mechanism and find clues to the design of high-performance cathode catalysts, it is essential to analyze surface the oxidation states of catalysts and the adsorbed oxygen species in the ORR. Recently, we have clarified the surface oxidation states of Pt polycrystalline and low-index single-crystal electrodes as a function of the electrode potential by using X-ray photoelectron spectroscopy combined with an electrochemical cell (EC–XPS).1 In practical PEFCs, however, Pt nanoparticles supported on carbon black have been employed as the anode and cathode catalysts. In the present research, we have prepared a model electrode of Pt nanoparticles supported on a carbon substrate and have examined the electronic structure of the Pt nanoparticles and the oxygen species adsorbed on the surface by using EC–XPS. Pt nanoparticles were prepared by a colloidal method,2 and dispersed on polished glassy carbon (GC). It was clarified by scanning electron microscopy (SEM) that Pt nanoparticles of ca. 3 nm diameter were well dispersed on the carbon substrate. We employed angle-resolved, grazing incidence X-ray photoelectron spectroscopy (AR-GIXPS), which is more surface-sensitive than conventional XPS, in order to analyze the small amount of Pt nanoparticles supported on the flat GC substrate. Figure 1 shows Pt 4f XP spectra of the Pt/GC model electrode after emersion from the 0.1 M HF solution at various electrode potentials. It was found that the peaks assigned to Pt 4f7/2 and 4f5/2 shifted to higher binding energies with increasing electrode potential. This peak shift can be ascribed to the surface core level shift induced by the surface oxidation of the Pt nanoparticles. At E > 1.1 V, peaks for bulk PtO, with binding energies of 73 and 77 eV, commenced to increase due to further oxidation of the Pt nanoparticles. Figure 2a shows O 1s XP spectra at the Pt/GC model electrode and a GC electrode (without any Pt) at 0.9 V, indicated by red and black lines, respectively. The O1s spectrum of the Pt/GC model electrode was found to include photoelectron signals attributed to oxygen species formed on the GC substrate, such as quinone and carboxyl groups. Then, we extracted the photoelectron signals originating from oxygen species adsorbed on the Pt nanoparticles by subtracting the O1s spectrum of the GC from that of the Pt/GC model electrode. Figure 2b shows the O1s difference spectrum for oxygen species adsorbed on the Pt nanoparticles. Thus, we succeeded in deconvoluting the difference spectrum into three components, H2Oad, OHad and Oad formed on the Pt nanoparticles. On the basis of the deconvoluted photoelectron intensities, it was found that H2Oad decreased with increasing electrode potential, while OHad increased. The Oad species was found to appear at E > 0.8 V. The onset potential of the OHad formation at the Pt/GC electrode was less positive than that for polycrystalline Pt, suggesting that the surface oxidation proceeded more easily at the Pt nanoparticle surfaces than at the polycrystalline surface. This work was supported by funds for the SPer-FC Project of NEDO, Japan References 1 M. Wakisaka, H. Suzuki, S. Mitsui, H. Uchida, and M. Watanabe, Langmuir, 25, 1897 (2009). 2 M. Watanabe, M. Uchida, and S. Motoo, J. Electroanal. Chem., 199, 311 (1986). Figure 1