Article pubs.acs.org/JPCC
Nature of the Selenium Submonolayer Effect on the Oxygen Electroreduction Reaction Activity of Ru(0001) Sergey Stolbov Physics Department, University of Central Florida, Orlando, Florida 32816, United States ABSTRACT: I present here results of first principles studies of the adsorption energetics of the intermediates of the oxygen electroreduction reaction (ORR) on a Se-modified Ru(0001) surface. The calculations were performed for 1/3 and 2/3 monolayer coverage of Se, as well as for clean Ru(0001) as a reference. The binding energies of O and OH on Ru(0001) are found to decrease significantly with the presence of Se, and this effect increases with the Se coverage. The Se surface modification is found not to change the Ru local density of electronic states noticeably. However, Se atoms accept electronic charge from the surface and thus become negatively charged. As a result, they repeal electrostatically the adsorbed negatively charged O and OH intermediates and in this way reduce their binding energies. This effect provides an alternative way of tuning the reactivity of the catalyst surfaces. Since for the Ru case reduction of the O and OH binding energies makes ORR energetically favorable, Se modification dramatically improves the ORR rate on Ru. Adsorption of O2 and coadsorption of the ORR intermediates and water have also been studied. The effect of coadsorption on the binding energies of O and OH are discussed.
1. INTRODUCTION Rational modification of the properties of transition-metal surfaces is among the major goals of surface science, because, if achieved, it will induce dramatic developments in many technological applications. One such application is proton exchange membrane fuel cells (PEMFCs) and direct methanol fuel cells (DMFCs), for which rational design of efficient electrocatalysts is of great demand. Indeed, the Pt-based catalysts, used in both electrodes of the fuel cell, make this very promising device unacceptably expensive. Furthermore, the performance of both PEMFCs and DMFCs suffers from a low rate of the oxygen reduction reaction (ORR) on the Pt cathode.1 Rational design of low-cost materials efficiently catalyzing ORR requires understanding of the microscopic mechanisms underlying this complex kinetic phenomenon, which involves electron and proton transfers between two electrodes with different Fermi levels. Fortunately, significant effort has been made in developing theoretical models describing ORR. Great progress was achieved with development of models which use the methods of conventional heterogeneous catalysis and gasphase reactions to describe this phenomenon by choosing a reference which links gas-phase and electrocatalytic quantities.2−8 It is important that the key parameters of these models can be calculated within the well-developed and widely used computational methods based on the density functional theory (DFT).9,10 For example, within the model proposed by Nørskov and co-workers,2 the free energies are calculated for each intermediate configuration involved in ORR to build the reaction free energy diagram, which is used to estimate such important characteristics of ORR as the electrode onset potential. Since the free energies essentially depend on the binding energies (Eb) of the reaction intermediates, revealing the relationship between these binding energies and the © 2012 American Chemical Society
composition and morphology of the electrocatalyst surface provides the basis for rational design of new efficient catalysts. The ORR is a multistep reaction with several possible pathways, which complicates requirements for optimal catalysts: its reactivity has to be high enough for O2 adsorption and OOH formation and, at the same time, low enough for atomic oxygen and hydroxyl removal.11,12 It has been shown for many materials, however, that Eb(OOH) and Eb(OH) are approximately proportional to Eb(O), which makes Eb(O) along a descriptor for ORR.13,14 The authors of refs 2 and 15 have built Eb(O)−Eb(OH)−Eb(OOH) diagrams (hypervolcano diagrams) for a number of transition and noble metals which show, for example, that an optimal catalyst for ORR has to have values of Eb(O) and Eb(OH) slightly lower than those for Pt. These findings suggest a way of searching for new electrocatalysts by modifying the material surface composition and morphology to tune Eb(O) to an optimal value. The Adzic and Mavrikakis research groups combining experimental studies and first principles calculations have made significant progress in this direction.16−20 The authors have found some Pt/M structures with ORR activity comparable to or even higher than that on bulk Pt. However, since such catalysts are mostly synthesized in the form of 2−5 nm nanoparticles, surface atoms (Pt) make a significant fraction of the volume of the nanoparticles. The Pt load is still high, which is a disadvantage of these systems. It is thus not surprising that much effort has been made to find efficient Pt-free electrocatalysts for ORR. One of the directions of this search is exploring Se−Ru systems. Since Alonso-Vante and Tributsch observed a high ORR rate for Mo4.2Ru1.8Se8,21 systems including Se and Ru are of high interest for the electrocatalytic community. More recent studies Received: July 30, 2011 Revised: January 24, 2012 Published: March 1, 2012 7173
dx.doi.org/10.1021/jp2072952 | J. Phys. Chem. C 2012, 116, 7173−7179
The Journal of Physical Chemistry C
Article
For all systems under consideration, the electronic structure, energetics, and equilibrium atomic configurations are obtained using the VASP5.2 code35 with projector-augmented wave potentials36 and the Perdew−Burke−Ernzerhof (PBE) version of the generalized gradient approximation (GGA) for the exchange and correlation functional.37 To maintain periodicity, we use supercells with a five-layer Ru(0001) slab and a vacuum layer of 15 Å. For all calculations the supercells have (3 × 3) inplane periodicity. The (4 × 4 × 1) k-point samplings in the Brillouin zone used in this work provide sufficient accuracy for the characteristics obtained by integration in the reciprocal space. A cutoff energy of 400 eV was used for the plane wave expansion of wave functions and a 600 eV cutoff energy was used for the charge density. To achieve structural relaxation, a self-consistent electronic structure calculation was followed by calculation of the forces acting on each atom. On the basis of this information, the atomic positions were optimized to obtain equilibrium geometric structures in which forces acting on the atoms do not exceed 0.015 eV/Å. To characterize the strength of bonding of the ORR intermediates (Int = O, OH) on the catalyst surface, we used the adsorption energy defined as follows:
show that the best electrocatalytic performance is achieved with RuxSey nanoparticles.22−30 Vogel et al.,22 Liu et al.,23 and Delacôte et al.24 have obtained similar nanostructures, including Ru hexagonal close-packed (hcp) core clusters covered with Se. It is not clear from those results, however, whether Se is just atomically adsorbed on the Ru core or forms some other, more complex, structures. The results of the anomalous small-angle X-ray scattering experiments performed by Zehl et al.25 suggest that their synthesized Ru−Se catalysts form Ru hcp clusters with an average size of 2.2 nm decorated with small Se islands. Zaikovskii et al.26 have synthesized similar structures; however, they assume that the Ru particles are covered not with metallic Se islands, but with Ru selenide clusters of
