Abstract
In situ high energy resolution fluorescence detection X-ray absorption spectroscopy (HERFD-XAS) was used to systematically evaluate interactions of H2O and O2 adsorbed on Pt and Pt3Co nanoparticle catalysts in different particle sizes. The systematic increase in oxidation due to adsorption of different species (H2O adsorption <O2 adsorption <O2 + H2O coadsorption) suggests that cooperative behavior between O2 and H2O adsorptions is responsible for the overpotential induced by hydrated species in fuel cells. From the alloying and particle size effects, it is found that both strength of O2/H2O adsorption and their cooperative effect upon coadsorption are responsible for the specific activity of Pt catalysts.
Highlights
In the past decade, polymer electrolyte fuel cells (PEFCs) have attracted increasing attention as highly efficient power sources[1, 2]
The Pt3Co samples have better electrochemical performance than Pt nanoparticles in their high Pt oxidization voltage and relatively high half wave values
The Pt L3 edge XAS spectra of the smallest Pt nanoparticle catalyst: Pt(A) measured with quick X-ray absorption fine structure (QXAFS) and HERFD-XAS methods for as-received and reduced conditions are shown in Fig. 3(a) and (b), respectively
Summary
Pt(A) and (B) as well as Pt3Co (A), (B), (C) nanoparticles as specified in Table 1, were loaded on a black carbon and characterized by TEM, electrochemical measurement and XRD as presented in Figs 1 and 2 and S1, respectively. The high energy resolution is definitely important for the precise peak fitting of the white line depending on the adsorbed molecules, which is essential to explain why the hydrated species increase the overpotential and decrease the energy conversion efficiency in PEFCs. Among the samples in this study, it is found that the strength of oxidation by O2 or H2O adsorption does not have simple correlation with the specific activity, while the suppressed oxidation by the coadsorption of O2 and H2O for the Pt3Co(A) and Pt3Co(B) catalysts has correlation with the high specific activity. Taking both alloying and particle size effects into account it is expected that both the strength of molecular adsorption (Sabatier principle) and the cooperative effect among those adsorptions, which is the origin of the overpotential by hydrated species, contribute to the specific activity of Pt catalysts
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