Article pubs.acs.org/JPCC
Comparative Study of Oxygen Reduction Reaction Mechanisms on the Pd(111) and Pt(111) Surfaces in Acid Medium by DFT Lihui Ou*,†,‡ and Shengli Chen‡ †
College of Chemistry and Chemical Engineering, Hunan University of Arts and Science, Changde 415000, China Hubei Electrochemical Power Sources Key Laboratory, Department of Chemistry, Wuhan University, Wuhan 430072, China
‡
ABSTRACT: DFT geometry optimization and minimum energy path calculations were used to investigate the mechanisms of oxygen reduction reaction (ORR) on the Pd(111) and Pt(111) surfaces, including the adsorption and dissociation of O2 molecule and the protonation of dissociated adsorbates. The results indicated that in the presence of a hydrated proton ORR on the Pd(111) surface proceeded through the adsorption and dissociation of O2 molecule, whereas ORR on the Pt(111) surface may involve in parallel the adsorption and dissociation of O2 molecule as well as the formation and dissociation of OOH species. During the entire fourelectron ORR, the protonation of adsorbed O atom to form OH is the rate-determining step (rds) on both of the Pd(111) and Pt(111) surfaces. Such a finding about the rds of ORR can well explain why Pt- and Pd-based catalysts that more weakly bind atomic oxygen have better ORR activity. Comparison of the ORR mechanisms on the Pd(111) and Pt(111) surfaces revealed that the adsorption and dissociation processes of O2 molecule more easily occurred on the Pt(111) surface and that the serial protonation of the dissociative product to form H2O molecule also more easily occurred on the Pt(111) surface than on the Pd(111) surface. Therefore, the difference between the catalytic activities for ORR between both metals was large, explaining why Pt can serve as ORR electrocatalysts and the inexpensive Pd cannot.
1. INTRODUCTION Proton exchange membrane fuel cells (PEMFCs) are highly promising sources of clean energy in the portable electronics and automobile industries1−4 because of their highly efficient direct energy conversion, high power density, and environmentally benign products. Extensive research has been conducted on the development of efficient and cost-effective PEMFCs; however, a key challenge to the ultimate commercialization of energy conversion devices is the development of robust, active, and low-cost catalysts.3,5 Over the past two decades, considerable research efforts have focused on developing improved electrocatalysts for oxygen reduction reaction (ORR) at fuel cell cathodes. ORR has long been used as a model reaction for electrocatalytic studies, but as a cathode reaction of PEMFCs, the sluggish kinetics of ORR has become one of the main bottlenecks in improving fuel cell efficiency. Platinum is the most efficient monometallic catalyst for ORR, but even Pt catalysts lack sufficient activity because of slow ORR kinetics. To address this challenge, Pt-based alloy catalysts for ORR are being studied.6−16 However, one of the major drawbacks of PEMFC commercialization is the high cost of Ptbased catalysts. Thus, alternative cost-effective catalysts must be developed to reduce or altogether eliminate the need for Ptbased ones. Recent studies on ORR electrocatalysis have attempted to replace Pt with less expensive materials.17 In the past five years, many Pd-based alloys have been used as catalysts for ORR.18−20 Alloying with Co, Fe, and Ni was found to increase strongly the catalytic activity of Pd, representing significant progress in the research on non-Pt catalysts for PEMFCs. For example, Adzic et al.21 studied © 2013 American Chemical Society
palladium alloy electrocatalysts and found that Pd/C nanoparticles modified with Co become very active for ORR. They also demonstrated the synthesis of a new class of electrocatalysts consisting of Pd−Fe alloys, in which Pd atoms migrate to the surface to form a pure Pd skin on the bulk alloys. They showed that an active ORR electrocatalyst can be devised without Pt, and its activity can surpass those of state-of-the-art carbon-supported Pt electrocatalysts.22 Other researchers have reported similar activity for sputtered Pd−Co films18 and Pd− Co−Au alloy nanoparticles.18,23 Further enhancement of the catalytic activity and stability of Pd for ORR is of considerable interest, and replacing Pt with the less-expensive, non-noblemetal Pd alloy can considerably decrease the operational costs, thereby facilitating the faster and broader application of fuel cells. Although promising performances have been preliminarily demonstrated, systematic work is required especially for comprehending the detailed mechanism of ORR on the Pd surface. Such works can also provide a scientific basis for designing Pd-based catalysts for O2 electrochemical reduction. Some studies have proposed thermodynamic guidelines for designing Pd-based alloy ORR electrocatalysts. Bard et al.23−25 suggested that for Pd−M alloys the reactive metal M constitutes the site for O−O bond breaking, forming Oads that migrates to hollow sites dominated by Pd atoms, where it is readily reduced to water. Based on this mechanism, the alloy surface should consist of a relatively reactive metal such as Co, Received: September 13, 2012 Revised: December 19, 2012 Published: January 2, 2013 1342
dx.doi.org/10.1021/jp309094b | J. Phys. Chem. C 2013, 117, 1342−1349
The Journal of Physical Chemistry C
Article
2. MODELS AND METHODS Calculations were performed in the framework of DFT on periodic supercells using the generalized gradient approximation of the Perdew−Burke−Ernzerhof functional32 for exchange-correlation and ultrasoft pseudopotentials33 for nuclei and core electrons. The Kohn−Sham orbitals were expanded in a plane-wave basis set with a kinetic energy cutoff of 30 Ry and charge-density cutoff of 300 Ry. The Fermi-surface effects were treated by the smearing technique of Methfessel and Paxton using a smearing parameter34 of 0.02 Ry. Calculations were carried out with spin polarization, which was essential to represent the electronic structure of molecular O2 properly. The PWSCF codes contained in the Quantum ESPRESSO distribution35 were used to implement all calculations, whereas the figures of the chemical structures were produced with the XCRYSDEN36−38 graphical package. In all calculations, we used (2 × 2) three-layer fcc(111) slabs with experimental equilibrium lattice constants of 3.92 Ǻ (Pt) and 3.89 Ǻ (Pd) to model the Pt(111) and Pd(111) surfaces, respectively. Brillouin-zone (BZ) integrations were performed with the special-point technique using (4 × 4) uniformly shifted k-meshes for the (2 × 2) slabs. Vacuum layers 12 Ǻ in thickness were added above the top layer of slabs in all cases. The Pt or Pd atoms in the bottom two layers were fixed at the experimental bulk positions, whereas the top layer on (2 × 2) three-layer slabs was allowed to relax; all other structural parameters were optimized to minimize the total energy of the system. Structural optimization was performed until the Cartesian force components acting on each atom were brought below 10−3 Ry/Bohr and the total energy converged to within 10−5 Ry with respect to structural optimization. The climbing image nudged elastic band (NEB) method39,40 was used to locate the saddle points and MEPs. The transition state images from the NEB calculations were optimized using the quasi-Newton method, which minimized the forces to find the saddle point. Geometry optimization was performed for each intermediate point in MEPs, in which the bottom two layers of metal atoms were fixed whereas the top layer of metal atoms and all other nonmetal atoms were allowed to relax.
and the atomic ratio of this metal should be 10%−20% such that sufficient sites exist for reactions of O−O bond breaking on M and for Oads reduction at hollow sites formed by Pd atoms. DFT calculations have indicated that one of the O atoms diffuses to the Pd hollow site, and the other is still adsorbed on the hollow site near Co after the dissociative adsorption of the O2 molecule.25 The second O2 can dissociate on Co with an O atom prebound on the hollow site near it. Balbuena et al.26,27 proposed a similar thermodynamic guideline for designing Pd alloy catalysts. For Pd with fully occupied valence d-orbitals, alloying with transition metals such as Co with unoccupied valence d-orbitals significantly reduces the Gibbs free energy both for the first charge-transfer step and for the steps involving the reduction of intermediates. However, whether O2 can still easily dissociate after the reactive metal centers are fully occupied by O is unclear. The reactive metals on the alloy surface are also unstable and rapidly leach out during electrochemical measurements. Thus, these arguments for ORR electrocatalysis on bimetallic surfaces cannot explain the relatively good stability of Pd−M alloys in acidic media.28 On the other hand, the Pd-enriched skin can account for both the good activity and stability of these alloys.29−31 Extensive experimental and theoretical studies aimed at elucidating the electrocatalytic mechanism of ORR have been conducted. However, this mechanism, even on the Pd surface, remains uncertain. Electrochemists still encounter challenges in formulating the entire steps for Pd-catalyzed ORR and in designing Pd-based alloys as electrocatalysts for ORR because of the complexity of ORR. Indeed, the possible elementary steps and mechanistic routes for the reduction of O2 to H2O are numerous; it involves four electron transfers, four proton transfers, and O−O bond cleavage. The adsorption of O2 and a wide spectrum of oxygenated adsorbed intermediates also probably occurs, further complicating the kinetic treatments of experimental data. Therefore, the main reason for the uncertainties in the design fundamentals of Pd-based alloy electrocatalysts lies in the uncertainties of the detailed mechanism of ORR, especially the rate-determining step (rds). Determining the ORR mechanism and rds can considerably aid in designing efficient Pd-based alloy ORR electrocatalysts, which is presently one of the major limitations in the progress of state-of-the-art fuel cell technologies. Theoretical studies have also indicated that the adsorption energies of O atom are nearly identical on the Pt and Pd surfaces, which are descriptors of catalysts design for ORR. Moreover, both metals belong to the same group elements, whereas the difference between the catalytic activities for ORR of these metals is large. Therefore, the ORR mechanisms on the Pt and Pd surfaces must be compared to understand why Pt can serve as an ORR electrocatalyst whereas the inexpensive Pd cannot. In this study, we conducted detailed DFT calculations on the geometric structures and minimum-energy paths (MEPs) for the adsorption and dissociation of O2 as well as on the protonation of the dissociation product O atom to form H2O on the Pt(111) and Pd(111) surfaces in the presence of hydrated hydronium ion. The possible reaction intermediates, reaction pathways, and rds were expected to be revealed, thereby providing a scientific basis for designing catalysts for O2 electrochemical reduction.
3. RESULTS 3.1. ORR Mechanism on the Pd(111) Surface. 3.1.1. Adsorption and Dissociation of O2 on the Pd(111) Surface. We used hydronium ion H3O+···(H2O)2 as a model of solvation in acid solution to model ORR at the Pt/electrolyte interface and evaluate the role of the proton. The rationalization of this model has been validated in our previous work41 that reported on a Pt(111)/O2/H7O3+ system. Similar to this system, physisorption, molecular chemisorption, and dissociative atomic chemisorption states were also identified for the Pd(111)/O2/H7O3+ system through geometry optimizations with different initial Pd−O2 distances z (the perpendicular distance between the center of O−O bond and the Pd slab surface) and O2 orientations. The physisorption state (Figure 1a) was trapped when O2 was placed above the Pd surface with a distance z > 3.00 Ǻ . When the initial Pd−O2 distance was
