Oxygen Reduction Reaction

DOI: 10.1002/ange.200504386

Changing the Activity of Electrocatalysts forOxygen Reduction by Tuning the SurfaceElectronic Structure**

Vojislav Stamenkovic, Bongjin Simon Mun,Karl J. J. Mayrhofer, Philip N. Ross,Nenad M. Markovic, Jan Rossmeisl, Jeff Greeley, andJens K. Nørskov*

The fuel cell is a promising alternative to environmentallyunfriendly devices that are currently powered by fossil fuels.In the polymer electrolyte membrane fuel cell (PEMFC), themain fuel is hydrogen, which through its reaction with oxygenproduces electricity with water as the only by-product. Tomake PEMFCs economically viable, there are several prob-lems that should be solved; the main one is to find moreeffective catalysts than Pt for the oxygen reduction reaction(ORR), 1/2O2 + 2 H++ 2e�=H2O. The design of inexpen-sive, stable, and catalytically active materials for the ORR willrequire fundamental breakthroughs, and to this end it isimportant to develop a fundamental understanding of thecatalytic process on different materials. Herein, we describehow variations in the electronic structure determine trends inthe catalytic activity of the ORR across the periodic table. Weshow that Pt alloys involving 3d metals are better catalyststhan Pt because the electronic structure of the Pt atoms in thesurface of these alloys has been modified slightly. With thisunderstanding, we hope that electrocatalysts can begin to bedesigned on the basis of fundamental insight.

[*] Dr. J. Rossmeisl, Dr. J. Greeley, Prof. J. K. NørskovCenter for Atomic-Scale Materials Physics, NanoDTUDepartment of PhysicsTechnical University of Denmark2800 Lyngby (Denmark)Fax: (+45)4593-2399E-mail: norskov@fysik.dtu.dk

Dr. V. Stamenkovic, Dr. B. S.Mun,Dr. K. J. J.Mayrhofer, Dr. P. N. RossMaterials Sciences DivisionLawrence Berkeley National LaboratoryUniversity of CaliforniaBerkeley, CA 94720 (USA)

Dr. N. M. MarkovicMaterials Science DivisionArgonne National LaboratoryArgonne, IL 60439 (USA)

[**] This work was supported by the Director, Office of Science, Office ofBasic Energy Sciences, Division of Materials Sciences, US Depart-ment of Energy (contract no. DE-AC03–76SF00098), by The DanishResearch Council through the NABIIT program, and by the DanishCenter for Scientific Computing (grant no. HDW-1103-06). J.G.acknowledges a H. C. Ørsted Postdoctoral Fellowship from theTechnical University of Denmark. N.M.M. acknowledges supportfrom the US Department of Energy under contract W-31-109-ENG-38.

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We studied polycrystalline alloy films of the type Pt3M(M=Ni, Co, Fe, and Ti). Although such alloys have alreadybeen considered,[1–4] the role of the 3d metals in electro-catalytic activity of Pt for the ORR remains elusive. Inprevious work,[1,3] one of the difficulties in illuminating theeffect of alloying components was that the kinetics of theORR was determined on poorly characterized Pt3M electro-des. However, we have recently studied Pt3M alloys that werethen annealed to 1000 K under ultrahigh-vacuum (UHV)conditions and subsequently characterized in detail.[5] Fromthe analysis of low-energy ion-scattering spectra, we foundthat the first surface layer of these alloys consists of pure Pt.The surface enrichment of Pt atoms occurs as a result of asurface segregation phenomenon, whereby one of the alloycomponents (in this case Pt) enriches the surface region.[6]

UHV experimental and theoretical analysis revealed that astrong enrichment of Pt in Pt3M alloy systems is counter-balanced by the depletion of Pt in the first two or three layers,resulting in a concentration profile that oscillates around thebulk value.[7–11]

Subsequent testing of these Pt overlayers in electrochem-ical environments showed significant differences in theactivity for the ORR. For the purpose of demonstratingcatalytic enhancement by alloying Pt with the 3d elements,two representative sets of polarization curves for the ORR onPt and Pt3Co, along with data on the corresponding produc-tion of peroxide, are summarized in Figure 1. Note that afterthese samples were removed from the electrochemical cellthey were again characterized in the UHV chamber, and thePt overlayer structure was found to be essentially unchanged.Although the rate of the ORR is significantly enhanced onPt3Co (Figure 1 a,b), equally small amounts of peroxide aredetected on the ring electrode on both surfaces, in agreementwith previous studies.[5] The rate of the ORR on Pt alloyed bythe 3d metals is dependent on the nature of the alloyingcomponent (see Figure 1c). While the Ni, Co, and Fe alloysshow a large improvement in activity compared to pure Pt,the enhancement is smaller for the earlier 3d metals. We thushave a series of well-characterized alloys showing an inter-esting volcano-shaped variation in the electrocatalytic activ-ity. As such, this result is an excellent basis for developing anunderstanding of the factors that control the kinetics of theORR.

A model of the ORR developed previously[12] was used asthe starting point for the analysis of the experimentallyobserved volcano-shaped dependence as the 3d metalchanges from Ni to Ti. By using density functional theory(DFT) calculations, the free energies of all intermediates ofthe ORR were calculated as a function of the cell potential(see Experimental Section). On this basis, a kinetic model wasdeveloped that gives the rate of the ORR at a given potentialas a function of a single parameter characterizing the catalystsurface—the oxygen chemisorption energy, DEO. Figure 2shows how this model leads to a volcano-shaped dependenceof the rate on DEO. For metals that bind oxygen too strongly(to the left of the maximum), the rate is limited by theremoval of adsorbed O and OH species; that is, the surface isoxidized and thus unreactive, as also suggested by Ross andco-workers.[13,14] For metal surfaces that bind oxygen too

weakly, the rate is limited by the dissociation of O2, or morelikely, the transfer of electrons and protons to adsorbed O2.These different rate-limiting steps are associated with differ-ent oxygen reduction mechanisms. Two such mechanisms,involving either O2 dissociation or proton and electrontransfer to molecular O2, were considered in our analysis.The mechanisms are both characterized by DEO, but they giverise to different slopes on the right side of the volcano-shapedplot (Figure 2). The volcano shape thus illustrates very wellthe Sabatier principle,[15] with the important new viewpointthat it is quantitative; thus, we know which bond energy givesthe maximum of the “volcano”.

Figure 1. a) Production of peroxide from the oxygen reduction reaction(ORR) detected on a ring electrode and b) polarization curves (kineticcurrent density vs potential) for the ORR on Pt and Pt3Co alloysurfaces on the disc electrode in 0.1 m HClO4 recorded at 50 mVs�1 at333 K (1600 rpm; RHE= reversible hydrogen electrode). Arrows indi-cate the values of the half-wave potential, which on Pt3Co is shiftedpositively by about 50 mV. c) Specific activity of Pt and Pt3M electrodesexpressed as a kinetic current density for the ORR at 0.9 V (0.1 m

HClO4, 333 K).

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One prediction of the model is that Pt binds oxygen a littletoo strongly, and finding a better electrocatalyst for the ORRthus amounts to finding a surface that binds oxygen moreweakly than Pt does by about 0.2 eV. To test this prediction,we calculated DEO for the (111) surfaces of a series of Pt3Malloys with pure Pt in the first layer and only 50 % Pt in thesecond layer to model the enrichment of Pt in the surfacelayer and the depletion of Pt in the next layer (the slabs thusretain bulk Pt3M stoichiometry). The composition of thesecond layer can vary modestly from one alloy to the next forthe different metals (see Reference [16] for a discussion of Pt–Fe, Pt–Co, and Pt–Ni alloy segregation). However, thecalculated binding energies are not highly sensitive to suchchanges; changing the composition of M in the second layerfrom 50 % to 25%, for example, changes binding energies byonly approximately 0.05 eV for M=Fe, Co, and Ni. Thus, oursimple stoichiometric model should be accurate for determin-ing the trends in activities. These data, plus the correspondingactivity predictions of the model, are shown in Figure 2.Clearly, the alloy surfaces form weaker bonds to oxygen thanpure Pt, and the predicted activities (relative to pure Pt) arecompatible with the experimental measurements that are alsoincluded in the figure. Experiment and theory agree almostquantitatively, but this excellent agreement in the relativerates is not the most significant point. Rather, we stress thatwe have a model of the ORR that accurately predicts trends

in the catalytic activity; when appropriate models areemployed, DFT calculations are known to be capable ofreproducing such trends quite well.[17]

As DEO is a good descriptor of the ORR activity of a givensurface, the question arises as to which property of thecatalyst determines it. It is well-established in the literature ofsurface science and heterogeneous catalysis that surface bondenergies correlate with the average energy of the d states onthe surface atoms to which the adsorbate binds (the d-bandcenter).[18–25] This conclusion also holds for bonding of oxygento the Pt3M alloys, as illustrated in Figure 3. Similar effects

have been observed for Pt(111) with a 3d transition metal inthe second layer, or for Pt(111) overlayers on 3d metals.[26,27]

The origin of this relationship is as follows: The variationin the oxygen–metal bond from one transition-metal surfaceto the next depends to a large extent on the strength of thecoupling between the oxygen 2p states and the metal d states.This coupling forms bonding and antibonding states asillustrated in Figure 3b–d. The bonding states are filled, andthe filling of the antibonding states, and thus the strength ofthe interaction, varies from surface to surface. In a metallicenvironment the filling depends on the position of the statesrelative to the Fermi level. An upward shift of the d statesrelative to the Fermi level must therefore result in an upwardshift of the antibonding states, leading to less filling and thusto a stronger bond. For Pt overlayers or other structuresinvolving bonding of oxygen to Pt, in which other factors thatdetermine the coupling strength are approximately the same,this effect leads to the relationship shown in Figure 3.

Unlike the oxygen bond energy, which is hard to measure,the d-band center is accessible experimentally. We measuredit for all of the alloys by using synchrotron-based high-resolution photoemission spectroscopy, a methodology pre-

Figure 2. Model of the activity (A=kBT ln(r), where r is the rate persurface atom per second) at a cell potential of 0.9 V shown (g, *)as a function of the adsorption energy of oxygen, DEO. On the left sideof the plot, the rate is limited by removal of adsorbed O and OHspecies; immediately to the right of the maximum, the rate is limitedby O2 dissociation; and on the extreme right, the limiting step isprotonation of adsorbed O2. Also shown in red are the measuredactivities relative to that of Pt. The activity of the experiment isA=kBT ln(i/iPt)+APt, where i/iPt is the current density relative to Pt,and APt is the theoretical value for the activity of Pt. It is assumed thatthe number of active sites per surface area is the same for Pt and allthe alloys. A factor of two in the preexponential factor would give riseto a difference of only �0.02 eV (kBT ln2) in the activity. Changes incoverage for the different alloys are not considered. See text for details.The predictions of the model for Pt3M (M=Ni, Co, Fe, Ti) alloys areshown relative to the predictions for Pt at the DFT-calculated values ofDEO.

Figure 3. a) The correlation between the d-band center and the oxygenadsorption energy. The d-band center was calculated (DFT) as theaverage for the Pt atoms in the two top layers. b) sp-broadened2p orbital for O(g), c) projected p density of states of oxygen atoms onPt(111), and d) projected d density of states of Pt(111).

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viously described.[28] Measurement of the d-band centersallows us to directly correlate the variations in the catalyticactivity for the ORR with the variations in the surfaceelectronic structure. As summarized in Figure 4, the activity

(A) versus d-band center position at 0.9 V exhibits a classicalvolcano-shaped dependence, which agrees very well with theactivity predicted from DFT calculations. For catalysts thatbind oxygen too strongly, the rate is limited by the removal ofsurface oxide, while for catalysts that bind oxygen too weakly,the rate is limited by the transfer of electrons and protons toadsorbed O2.

Note that a shift in the d states can also often be measuredas a core-level shift, as the d states and the core levels shifttogether.[20] This effect can explain the correlations betweenORR activity and the surface core-level shifts observed byWatanabe and co-workers.[29]

In summary, we have established how alloying Pt with3d transition metals tunes the electronic structure to changethe performance of an alloy for the ORR, and we have thusdeveloped a model for the electrocatalytic activity of a metalsurface for this process. The activity is given by the strength ofthe oxygen–metal bond interaction, which in turn depends onthe position of the metal d states relative to the Fermi level. Inthis way we have established a new approach for the screeningof new catalysts for the ORR, by looking for surfaces thatbind oxygen a little weaker than Pt or, specifically for Pt skins,by looking for surfaces with a down shift of the Pt d statesrelative to the Fermi level.

Experimental SectionDACAPO, the total energy calculation code,[30] was used in this study.For all calculations, a four-layer slab, periodically repeated in asupercell geometry with six equivalent layers of vacuum between anytwo successive metal slabs, was used. A 2 F 2-unit cell was employed,and the top two layers of the slab were allowed to relax. Adsorptionwas allowed on only one of the two surfaces exposed, and theassociated dipole was corrected for electrostatically.[31] Ionic coreswere described by ultrasoft pseudopotentials,[32] and the Kohn–Shamone-electron valence states were expanded in a basis of plane waveswith kinetic energy below 340 eV; a density cutoff of 500 eV was used.The surface Brillouin zone of close-packed metal surfaces was

sampled at 18 special Chadi–Cohen k points. In all cases, convergenceof the total energy was confirmed with respect to the k point set andwith respect to the number of metal layers included. The exchange-correlation energy and potential were described self-consistentlywithin the generalized gradient approximation (GGA-RPBE).[30] Theself-consistent RPBE density was determined by iterative diagonal-ization of the Kohn–Sham Hamiltonian, Fermi population of theKohn–Sham states (kBT= 0.1 eV), and Pulay mixing of the resultingelectronic density.[33] All total energies were extrapolated to kBT=0 eV. The d-band centers were calculated with an infinite cutoffradius.

On all metals and alloys (composition Pt3M) investigated in thisstudy, oxygen was found to adsorb at face-centered cubic (fcc) sites.Owing to the limited size of the unit cell, only two different fcc siteswere present, one of which is close to one of the M atoms in thesecond layer, while the other is close to two M atoms. The latter wasthe most stable for all the considered alloys.

Solvent and potential effects were treated with the same model asused in our previous study.[12] In that work, it was confirmed that abilayer of water has no effect on the binding energy of atomic oxygen.For the other species in the full model (not explicitly involved in thepresent calculations), the binding energies were corrected by theinteraction of a water bilayer with the appropriate species on Pt(111).Potential effects were included by adjusting the free energies ofadsorption (per transferred electron) by �eU. Electric-field effectswere not explicitly included, but these effects were found to changethe adsorption energies by less than 0.05 eV, and the effect wasessentially constant from one metal to the next.

Received: December 9, 2005Revised: February 6, 2006Published online: April 5, 2006

.Keywords: alloys · density functional calculations ·electrochemistry · oxygen · platinum

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