Role of Structural and Electronic Properties of Pt and Pt Alloys on Electrocatalysis of Oxygen Reduction An In Situ XANES and EXAFS Investigation

The electrocatalysis of the oxygen reduction reaction (ORR) on five binary Pi alloys (PtCr/C, PtMn/C, PtFe/C, PtCo/C, and PtNi/C) supported on high surface area carbon in a proton exchange membrane fuel cell was investigated. All the alloy electrocatalysts exhibited a high degree of crystallinity with the primary phase of the type Pt3M (LI2 structure with fcc type lattice) and a secondary phase (only minor contribution from this phase) being of the type PtM (LIo structure with tetragonal lattice) as evidenced from x-ray powder diffraction (XRD) analysis. The electrode kinetic studies on the Pt alloys at 95~ and 5 atm pressure showed a twoto threefold increase in the exchange current densities and the current density at 900 mV as well as a decrease in the overvoltage at i0 mA em -2 relative to Pt/C eleetrocatalyst. The PtCr/C alloy exhibited the best performance. In situ EXAFS and XANES analysis at potentials in the double-layer region [0.54 V vs. reversible hydrogen electrode (RHE)] revealed (i) all the alloys possess higher Pt d-band vacancies per atom (with the exception of PtMn/C alloy) relative to Pt/C electrocatalyst and (it) contractions in the Pt-Pt bond distances which confirmed the results from ex situ XRD analysis. A potential excursion to 0.84 V vs. RHE showed that, in contrast to the Pt alloys, the Pt/C electrocatalyst exhibits a significant increase in the Pt d-band vacancies per atom. This increase, in Pt/C has been rationalized as being due to adsorption of OH species from the electrolyte following a Temkin isotherm behavior, which does not occur on the Pt alloys. Correlation of the electronic (Pt d-band vacancies) and geometric (Pt-Pt bond distance) with the electrochemical performance characteristics exhibits a volcano type behavior with the PtCr/C alloy being at the top of the curve. The enhanced electrocatalysis by the alloys therefore can be rationalized on the basis of the interplay between the electronic and geometric factors on one hand and their effect on the chemisorption behavior of OH species from the electrolyte. The role of Pt/C and Pt alloys on the mechanism of the oxygen reduction reaction (ORR) has been investigated previously, 1-4 however the mechanism still remains elusive. One of the first investigations I of the ORR on Pt alloy electrocatalysts was in phosphoric acid; the effect of changes in the Pt-Pt interatomic distances, caused by alloying, was examined. The strength of the [M-HO2]aas bond, the intermediate formed in the rate-determining step of the molecular dioxygen reduction, was shown to depend on the Pt-Pt bond distance in the alloys. A plot of the electrocatalytic activity vs. adsorbate bond strength exhibited a volcano type behavior. 5 It was shown that the lattice contractions due to alloying resulted in a more favorable Pt-Pt distance (while maintaining the favorable Pt electronic properties) for dissociative adsorption of 02. This view was disputed by Glass et al. ~ in their investigation on bulk alloys of PtCr (the binary alloy at the top of the volcano plot) of different compositions. The latter investigation showed no activity enhancement for the ORR in phosphoric acid. This study therefore suggested the possibility of differences in electrochemical properties of bulk vs. supported alloy electrocatalysts (small particles of 35-85 A). A recent study on supported PtCo electrocatalysts ~ revealed the possibility that particle termination, primarily at the <100> vicinal planes in the supported alloy electrocatalyst, is the reason for the enhanced ORR electrocatalysis (i.e., vicinal planes are more active than ). Paffett et al., 3 attributed higher activities for the ORR on bulk PtCr alloys in phosphoric acid to surface roughening, and hence increased Pt surface area, caused by the dissolution of the more oxidizable alloying component Cr. In contrast to these findings on bulk alloys, the supported alloy electrocatalysts have been reported to retain their nonnoble alloying element in the electrode during long periods (6000-9000 h) of operation in phosphoric acid fuel cells (PAFCs) 6 and proton exchange membrane fuel ceils (PEMFCs). 7 Based on these previous investigations and in the context of the ORR mechanisms, the principle explanations for the * Electrochemical Society Active Member. d. Electrochem. Soc., Vol. 142, No. 5, May 1995 9 The Electrochemical Society, Inc. enhanced ORR activity could be enumerated as being due to (i) modification of the electronic structure of Pt (5 d-orbital vacancies); (it) changes in the physical structure of Pt (Pt-Pt bond distance and coordination number); (iii) adsorption of oxygen-containing species from the electrolyte on to the Pt or alloying element; and/or (iv) redox type processes involving the first row transition alloying element. 8 One most powerful analytical technique which in principle can elucidate these different explanations is in situ xray absorption spectroscopy (XAS). The application of XAS to electrochemical systems has been reviewed recently. 9 The spectra consists of two parts, the near-edge part XANES (x-ray absorption near-edge structure) which gives chemical information and EXAFS (extended x-ray absorption fine structure) which gives the structural information. The XANES (_+50 eV relative to the absorption edge) is comprised primarily of multiple scattering and transit ion-to-empty states in the vicinity of the Fermi level by low energy photoelectrons with relatively long meanfree paths. The XANES can provide information on the oxidation state from the size and shift in the edge-transition and on the coordination symmetry of ligands around the excited atom from the shape of the edge transition. For Pt, analysis of the XANES white lines at the L3 and L2 edge can yield information on the d-band vacancies. The EXAFS region is 40 to 1500 eu beyond the absorption edge and is caused by the modulation of the x-ray intensity due to backscattering by a small fraction of the backscattered photoelectron wave. This interference effect caused by single-scattering electrons with short mean-free paths provides information about the short-range atomic order (coordination number and bond distances). The present study focuses on the investigation of several Pt eleetrocatalysts alloyed with the first-row transition metals to elucidate the dependence of electrode kinetics of oxygen reduction on their electronic and structural properties. For this purpose ex situ or in situ electrochemical, XRD, and XANES and EXAFS techniques were used.