Platinum Monolayer Electrocatalysts for O 2 Reduction: Pt Monolayer on Carbon-Supported PdIr Nanoparticles

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1 DOI 107/s Pltinum Monolyer Electroctlysts for O 2 Reduction: Pt Monolyer on Cron-Supported PdIr Nnoprticles Seth L. Knupp & Miomir B. Vukmirovic & Prdeep Hldr & Jeffrey A. Herron & Mnos Mvrikkis & Rdoslv R. Adzic # Springer 2010 Astrct The kinetics of oxygen reduction ws investigted in cid solutions on Pt monolyers deposited on modified cron-supported PdIr nnoprticles using the rotting diskelectrode technique. Iridium is introduced into the Pd sustrte in order to fine-tune the Pt Pd interctions nd to improve Pd stility under operting conditions of the fuel cell. The kinetics of the oxygen reduction rection shows enhncement with the Pt monolyer on the PdIr nnoprticle surfces in comprison with the rection on Pt/C nd Pt monolyer on nnoprticles. The electrochemicl mesurements suggest tht reduced oxidtion of Pt monolyer on compred to Pt/C nd Pt monolyer on is the cuse of enhnced ctivity. Besides lignd effect induced to the Pt surfce y the presence of PdIr in the second sulyer of the nnoprticle, s suggested y our density functionl theory nlysis, Ir lso leds to Pd skin contrction, so the Pt monolyer on is compressed more thn on. Both effects led to further wekening of the Pt OH interction, thus incresing the ORR ctivity. The Pt-specific ctivity for is three times nd 25% higher thn tht of Pt/C nd respectively; the Pt-mss ctivity of S. L. Knupp : P. Hldr College of Nnoscle Science nd Engineering, University t Alny, Stte University of New York, Alny, NY 12203, USA M. B. Vukmirovic : R. R. Adzic (*) Chemistry Deprtment, Brookhven Ntionl Lortory, Upton, NY , USA e-mil: dzic@nl.gov J. A. Herron : M. Mvrikkis Deprtment of Chemicl nd Biologicl Engineering, University of Wisconsin-Mdison, Mdison, WI 53706, USA is more thn 20 times nd 25% higher thn tht of Pt/C nd, respectively. Keywords Core-shell. PEM fuel cell. ORR. Copper UPD Introduction Proton exchnge memrne fuel cells (PEMFC) re expected to ecome key source of power vi electrochemicl energy conversion in residentil/commercil power pplictions due to their unique properties nd ility to crete clen energy [1]. Their lower thn theoreticlly possile efficiency is predomintely dictted y the slow kinetics of the electroctlytic oxygen reduction rection (ORR). As result, high Pt (or Pt lloy) lodings in PEMFCs re needed for prcticl pplictions nd reserch hs focused on developing improved electroctlysts for the ORR in order to increse the overll fuel cell efficiency. Although there hve een considerle dvnces recently, the high cost of electroctlysts, e.g., Pt is still rrier to overcome nd impedes fuel cell commerciliztion [2]. Even with tenfold reduction in Pt loding in the lst two decdes from c. 4mg/cm 2 to 0.4 mg/cm 2, further reduction is still needed. Fst kinetics t the node (hydrogen oxidtion) hve mde it possile to chieve low lodings ( mg/cm 2 )compredto conventionl lodings (0.2 mg/cm 2 ) without ny fuel cell performnce reduction [3]. Conversely, t the cthode, reducing Pt loding is more chllenging. The ORR is highly irreversile rection with sluggish kinetics nd complex mechnism, which is still not well understood. Nørskov et l. used density functionl theory to ttriute the origin of high overpotentils to stle intermedites (e.g., dsored oxygen nd hydroxyl species t potentils close to equilirium) tht slow down the kinetics for ctivted proton

2 nd electron trnsfer processes [4]. This is in ccordnce with experimentl oservtions tht show Pt OH formtion in the V potentil region cusing reduction in the ORR ctivity [5, 6]. Pltinum lloys with trnsition metls hve een widely investigted nd hve shown to possess enhnced ctlytic ctivity for the ORR y decresing OH dsorption on such ltered Pt surfces [7 12]. However, the use of lloying, which is usully rich in Pt, is not n efficient method of significntly reducing Pt loding, deeming these processes unstisfctory [13]. A further reduction in Pt loding to c. 1/4 of the current stte-of-the-rt memrne electrode ssemly cthode ctlyst lyer, from out 0.4 to 0.1 mg/cm 2 without ny loss in cell voltge, is needed in order to chieve widespred commercil ppliction of PEMFCs [13]. A promising pproch ws estlished which mkes use of Pt monolyer ( ) on suitle metl nnoprticles, tht cn reduce Pt lodings nd surpss the ORR ctivity of the stte-of-the-rt cron-supported Pt electroctlysts [14, 15]. The deposition process involves the glvnic displcement of n underpotentilly deposited Cu monolyer on suitle sustrte y Pt [14]. The improved ctivity oserved for deposited on properly selected core metl cn e ttriuted to oth strin (geometric effects) nd electronic interction etween the nd its sustrte (lignd effect) [16 18]. The ltertions tht occur to the surfce Pt lyer reduce its rectivity nd decrese the strength of interction with vrious ORR intermedites. In turn, this cn significntly reduce the dsored Pt OH coverge nd therefore enhnce the ORR ctivity [19, 20]. To dte, deposited on cronsupported Pd nnoprticles ( ) hs een estlished s the stte-of-the-rt electroctlyst for the ORR [14, 15]. In this study, we present experimentl work including therml grvimetric nlysis (TGA), inductively coupled plsm spectrometry (ICP), trnsmission electron microscopy (TEM), cyclic voltmmetry (CV), nd rotting disk electrode (RDE) to gin fundmentl understnding of enhnced ORR ctivity for on PdIr nnoprticles ( ) where Ir is sumerged eneth the Pd surfce. The purpose of sumerging Ir under the Pd surfce is to influence the Pt Pd interction in order to improve the ORR ctivity of the lredy estlished highly ctive [14]. Also, ecuse the is not continuous, ddition of susurfce Ir cn enhnce Pd stility under operting conditions of fuel cell. The segregted Pd surfce lyer on PdIr nnoprticles exhiits different electrochemicl ehvior compred to tht of electrodes. To ssess the effect of Ir in susurfce lyers on the electroctlytic properties of electrode s surfce,we compred the electrochemicl ehvior nd the ORR ctivity for, Ir/C, Pt/C, Ir,,, Ir, nd nnoprticles. nd represent the nneled versions of Ir nd Ir, respectively; for detils see Experimentl section. Further, periodic self-consistent density functionl theory clcultions re performed to determine the fundmentl chnges to the ctlytic surfce properties which led to n enhnced ORR ctivity. The present work helps dvnce our understnding of -sustrte interction in order to improve ORR electroctlytic ctivity nd sustrte stility. Experimentl Reserch grde (20 wt.%), Pt/C (20 wt.%), nd Ir/C (20 wt.%) electroctlysts were otined from E-TEK, Somerset, NJ. ws first treted y scling up of known method involving the glvnic displcement of n underpotentilly deposited (upd) Cu monolyer on Pd sustrte y Ir [14, 15]. A 200 mg of ws dispersed nd sonicted in custom-mde glss cell filled with 200 ml of 50 mm H 2 SO 4 electrolyte to crete homogenous suspension. The cell for scle-up synthesis hs min ody consisting of glss; however, the ottom of the cell ws fit with thin cron film which cted s working electrode. It is corrosion-resistnt nd the upd of Cu is not tking plce on this surfce. Two seprte chmers, filled with electrolyte, on either side of the min glss cell hold two Pt lck counter electrodes, which were seprted with glss frits to void contct etween nnoprticles nd counter electrodes. An Ag AgCl, 3 M Cl electrode with slt ridge ws used s reference. After reducing Pd surfce oxides y pplying potentil cycles, n pproprite mount of CuSO 4 solution ws dded to djust concentrtion of Cu 2+ in the cell to 50 mm. Then the pproprite potentil ws pplied to underpotentilly deposit Cu nd ws held t tht potentil for 2 h. The solution ws occsionlly mgneticlly stirred to disperse the prticles in the electrolyte nd llowed to fll down on the cron electrode, therefore to ensure the formtion of upd Cu monolyers on the entire Pd nnoprticle surfces. The Cu upd process is crried out until current reches stedy vlue of ner zero. After the Cu upd formtion, n Ir 3+ solution (IrCl 3 in 50 mm H 2 SO 4 solution) ws slowly injected into the cell while the solution is vigorously stirred, to llow Ir toms to replce Cu monolyers on Pd surfces glvniclly. The Ir monolyer is not continuous due to differences in vlence sttes of Cu nd Ir. A 2/3 of monolyer is deposited even if efficiency of the process nd differences in tomic sizes of Cu, Pd, nd Ir re tken in ccount. The resultnt Ir electroctlyst ws wshed repetedly y filtering, then collected nd dried overnight in vcuum oven. Hlf of the Ir ctlyst ws then nneled t 350 C in tue furnce in n Ar environment to promote the segregtion of Pd to the surfce to form [21]. All solutions re purged with Ar gs thoroughly. A schemtic flow digrm of the entire process is shown in Fig. 1.

3 Fig. 1 Depiction showing the ctlyst preprtion (see text) Cu monolyer Ir 2/3 monolyer Anneling 350 o C Pd CuPd IrPd Cu monolyer Pt monolyer PdIr CuPdI PdIr Plldium Copper Iridium Pltinum A first step in preprtion of on (, IrPd/ C) ws to disperse certin mount of (, Ir) in wter nd then sonicte to form homogenous suspension. After tht, 10 μl of tht suspension ws displced on glssy cron electrode surfce (5 mm dimeter) nd dried under vcuum. Finlly, Pt monolyers were deposited on (, Ir) y glvnic displcement of upd Cu monolyer y Pt [14, 22]. After depositing Pt, the electrode ws coted with 10 μl of wt.%nfion solution(diluted with wter from 5% Nfion solution y Aldrich). Electrochemicl ehvior (CV) nd ORR kinetics (RDE) of the smples were otined in 0.1 M HClO 4 using custommde three electrode cell. RDE experiments were performed using Pine rottor. An Ag AgCl, 3 M Cl electrode with slt ridge, ws used s reference nd Pt flg ws used s counter electrode. All the potentils re given with respect to reversile hydrogen electrode (RHE). All electrochemicl curves were otined using Princeton Applied Reserch PARSTAT 2273 nd Rdiometer Anlyticl Voltl PGZ402 potentiostts. Pt/C,, nd Ir/C ctlysts were deposited on glssy cron surfce the sme wy s. The currents in voltmogrms were normlized y the geometric electrode surfce re ( cm 2 ) for ll CV plots. In order to chrcterize the metl loding of the electroctlyst, TGA ws crried out. A certin mount (~3 5 mg) of ctlyst ws plced into pltinum pn nd loded into TA Instruments thermo grvimetric nlyzer (TGA 2050). The smple ws then exposed to compressed ir t 100 cm 3 /min nd the temperture ws rmped from room temperture to 1,000 C t 5 C/min. Cron support undergoes grdul oxidiztion leving ehind unsupported metl nd the difference etween the initil mss nd finl mss defines the metl loding. The ws chrcterized for its morphology, prticle size, size distriution, nd dispersion using highresolution trnsmission electron microscope (JEOL JEM2010F). The microscope ws equipped with Schottky field emission source which ws operted t 200 kev using n ultrhigh resolution ojective pole piece nd post column gtn imging filter. The ttched energy dispersive X-ry spectrometer (EDS) ws used in order to otin informtion out the electroctlyst composition. Powder smples were prepred y dispersing smll mount of electroctlyst solution on n morphous holey cron film supported in Cu mesh grid smple holder. Theoreticl Self-consistent, periodic, density functionl theory clcultions (DFT) were performed with the PW91-GGA exchngecorreltion functionl using DACAPO [23, 24]. The ctlysts were represented y the (111) fcet of the fce-centered cuic ulk crystlline structure of the Pd sustrte, with the pproprite overlyers pseudomorphiclly deposited. For reference, the optimized ulk lttice constnts for Pd, Pt, nd Ir re 3.99, 4.00, nd 3.86Å, respectively. For comprison, the experimentlly determined lttice constnts for Pd, Pt, nd Ir re 3.92, 3.89, nd 3.84Å, respectively [25]. The OH inding energy ws clculted on unit cell, corresponding to 1/6 ML coverge. The metl sls consist of five lyers with the top three lyers relxed, nd n equivlent of five lyers of vcuum etween successive sls. The surfce Brillouin zone is smpled t 18 specil Chdi-Cohen k-points nd the Kohn Shm one-electron sttes re expnded in series of plne wves with n energy cutoff of 25 Ry. All clcultions re performed non-spin-polrized. More technicl detils on the clcultions cn e found elsewhere [26]. Results nd Discussion A representtive TGA plot is shown in Fig. 2, in which the temperture is rmped from 25 C to 1,000 C t 5 C/min.

4 Weight % T / o C Fig. 2 Therml grvimetric nlysis of 20 wt.% (lck) nd 23.5 wt.% (red) Exposing the ctlyst to ir t elevted tempertures cuses the cron support to e grdully oxidized leving ehind unsupported metl. Cron egins oxidizing t pproximtely 300 C nd is completely exhusted t 600 C. The smple weight for the commercil is 20.2 wt.% which is in good greement with the informtion provided y the vendor. After depositing Ir, the totl metl mss increses to 23.5 wt.% for the Ir. From this, we clculted the metl loding of Ir in Ir to e 4.1 wt.%. Also, it is clculted tht there is 11 t.% of Ir in IrPd. Knowing Pd nnoprticle size nd tht 2/3 Ir monolyer ws deposited, the Ir metl loding cn e estimted. For Pd nnoprticles with men dimeter of 5.6 nm (XRD dt not presented) surfce to ulk tomic rtio is 22.5% [27]. A 200 mg smple of 20.2 wt.% yields 40.4 mg of metllic Pd nnoprticles of which 9.09 mg eing on the surfce. Since the deposition of Ir only tkes plce on the surfce, ssuming sme tomic size of Pd nd Ir (difference ~1.6%), nd tht the process yields 2/3 of monolyer coverge, n Ir metl loding of 5.2 wt.% cn e otined which is in resonle greement with the mesured vlue. ICP mesurements were lso performed nd gree well with the vlue mesured vi TGA. Figure 3 shows the cyclic voltmmetry scns showing the electrochemicl ehvior of Ir (lue line), Ir/C (red line), nd (lck line) in de-erted 0.1 M HClO 4 t 25 C nd scn rte of 20 mvs 1. It is evident tht the peks for the hydrogen dsorption/desorption in the H upd region (<0.4 V vs. RHE) re more negtive for the Ir/C compred to ones. The surfce of the Ir/C lso oxidizes t much lower potentils thn the. In the cse of Ir, due to the fct tht the Ir monolyer is not continuous, the presence of Pd nd Ir on the surfce of the ctlyst is evident in the H upd region where chrcteristic peks of oth re present. Their positions re not chnged which suggests tht their ehvior in the H upd region is independent of ech other. Contrry to tht, the surfce oxidtion of n Ir tkes plce somewhere etween Pd/ C nd Ir/C indicting n Ir Pd interction. Figure 3 shows cyclic voltmgrms for Ir (lue line), (red line), otined fter nneling of Ir, nd (lck line). The surfce energy of Ir is higher thn tht of Pd cusing Ir to sumerge eneth Pd fter nneling [23, 28]. DFT clcultions of surfce segregtion energies for Ir Pd solute host system predict very strong ntisegregtion which results in migrtion of solute (Ir) into the ulk of the host (Pd) [29]. Proly, Pd is on the surfce of PdIr nnoprticles, while immedite susurfce lyers re mixed [21, 30 33]. This is cler from CV in H upd region where Pd nd Ir do not influence ech other. The ehvior of in j / ma cm -2 j / ma cm Ir Ir/C Ir (Not Anneled) (Anneled t 350 o C) Fig. 3 Voltmmetry curves showing the electrochemicl ehvior of (lck), Ir/C (red), nd Ir (lue). Voltmmetry curves showing the electrochemicl ehvior of Ir (lue) nd fter nneling (red) nd (lck). All curves were performed in de-erted 0.1 M HClO 4 t 20 mvs 1

5 j / macm Cu upd Figure 4 shows typicl curve for the underpotentil deposition of Cu on nnoprticles (red line) nd the (lck line). Cu upd ws performed using 50 mm CuSO 4 in 50 mm H 2 SO 4 under de-erted conditions. Severl peks re seen positive to the onset for ulk Cu deposition t ~0.4 V. The chrge ssocited with this region is 160 μc fter doule lyer correction. Figure 4 shows the voltmmetry curves for the (lck line), (red line), nd the deposited on ( ) (lue line). The chrge ssocited with the hydrogen desorption pek for is 73 μc fter doule lyer correction. This chrge is in good greement with Cu upd chrge (80 μc=160 μc/2 for two e process). The chrge for hydrogen desorption pek ws used to determine Pt loding (0.15 μg Pt ) nd the electrochemicl surfce re (231.7 m 2 /g Pt ) j / macm Fig. 4 Voltmmetry curves showing the electrochemicl ehvior of (lck) nd Cu underpotentil curve (red). Cyclic voltmmgrm showing the electrochemicl ehvior of (lck), fter depositing monolyer of Pt to form (red) nd (lue). All curves were performed in de-erted conditions. Cu UPD ws performed using 50 mm CuSO 4 in 50 mm H 2 SO 4.All other curves were performed in 0.1 M HClO 4 t 20 mvs the H upd region is similr to the one. Becuse metl monolyers supported on different sustrtes cn exhiit chemicl nd ctlytic properties very different from those of the surfces of individul metls, one cn expect tht Pd surfce lyer on PdIr ehves differently from Pd on Pd [34 37]. The influence of Ir is reveled in the oxidtion region. The strts to oxidize nd reduce t more positive potentils thn. This is in qulittive greement with DFT clcultions predicting weker Pd O interction on Pd/Ir (111) thn on Pd(111) [34]. Pd/Ir(111) is somewht similr to the system discussed here, ut more precise DFT results for the specific lloy system t hnd will e presented elow. Additionlly, reduced oxidtion cn improve PdIr stility under fuel cell working conditions. Counts Prticle Size (nm) Fig. 5 Low mgnifiction TEM microgrph showing the electroctlyst. Corresponding histogrm showing verge prticle size

6 Fig. 6 High-resolution TEM imge of. Energy dispersive spectrum focused on center of the ctlyst prticle (1 nm em dimeter). c Energy dispersive spectrum focused on cron support (1 nm em dimeter). Cu kα nd Cu kβ t 8.05 nd 8.94 kev, Pt Lα nd Pt Lβ peks re t 9.47 nd kev. Ir kα,ir kβ, nd Ir Lβ re t 2.07, 9.17, nd 10.7 kev, respectively, nd the Pd Lα,Pd kα, nd Pd kβ re t 2.89, 21.09, nd kev ville for ORR vi the ssocited chrge of 0.21 mccm 2 for polycrystlline Pt. It cn e seen tht surfce oxidtion of strts t more positive potentils thn which reflects the influence of the Ir underlyer on the electrochemicl properties. Figure 5 shows low-mgnifiction TEM microgrph of the while Fig. 5 shows the corresponding prticle size distriution histogrm otined from Fig. 5. The size distriution peks t out 7.0 nm. However, there re some lrger prticles tht cn e seen which re gglomertes of severl smll prticles. Figure 6 shows high-resolution TEM imge of the. It is difficult to distinguish the shell from the core prticle nd this is most likely due to the similrities in orienttion etween the core nd shell. TEM phse contrst is sensitive to differences in crystlline structure nd hence, pseudomorphic monolyer which exhiits such smll chnge in its lttice prmeter from the core mkes it difficult to differentite the from the core prticle. However, in Fig. 6, n EDS spectrum ws otined fter focusing 1 nm em on n electroctlyst prticle. All metls, Pd, Ir, Pt, nd C re prevlent in the spectrum. Figure 6c shows n EDS spectrum otined y focusing the em on the cron support. The C kα t 0.25 kev is the lrgest pek, which is due to comintion of the holey cron grid nd the cron support. The Cu kα nd Cu kβ t 8.05 nd 8.94 kev is due to the copper grid of the smple holder. This showed tht Ir nd Pt were not deposited on cron support. Figure 7 shows RDE mesurements for the ORR on plced on glssy cron electrode in oxygented 0.1 M HClO 4 t room temperture for vrious rottion rtes. The experiment demonstrtes the two chrcteristic regions of the ORR. In the potentil widow of V, the limiting current (j D ) is well defined nd ove 0.7 V is the mixed kinetic/diffusion control region. The ctivity of this surfce is considerle s indicted y the onset of O 2 reduction t low overpotentils ( V) nd the hlf-wve potentil of V. The Koutecky-Levich plots otined from Fig. 7 t different potentils re shown in Fig. 7. The linerity nd prllelism of these plots is sign of first-order kinetics with respect to moleculr oxygen [38]. Using the Koutecky-Levich eqution: 1 j ¼ 1 þ 1 j k j D ¼ 1 þ 1 nfkc O2 Bw 1=2 ð1þ Counts /.u. Counts /.u C k Ir k Pd L Cu k Cu k Ir k Pt L Ir LPtL Pd k Pdk Energy / kev c C k Cu k Energy / kev

7 I disk / ma 1/j / ma -1 cm rpm log j k / ma cm /2 1/2 ω / min Fig. 7 Polriztion curves for ORR on the surfce in oxygen sturted 0.1 M HClO 4 t room temperture. Scn rte, 10 mvs 1. Koutecký-Levich plot t vrious potentils nd the inset shows the Tfel plot t 1,600 rpm where j is the mesured disk current density, j k is the kinetic current density, j D is the diffusion current density, F is the Frdy constnt, k is the rection rte constnt, c O2 is the concentrtion of dissolved O 2, B (¼ 0:62nFAD 2=3 O 2 n 1=6 c O2 ) is constnt, nd ω is the rottion rte, one cn clculte the kinetic currents of O 2 reduction from the intercepts of the 1/j xis t 1/ω 1/2 =0. From the slopes of the Koutecky- Levich plots, i.e., the constnt B, the numer of electrons (n) exchnged in the reduction of n oxygen molecule cn e otined. The experimentl vlue for B, 99.4 μas 1/2, grees well with the theoreticl vlue 89.8 μas 1/2 clculted for four-electron reduction process using pulished dt for the concentrtion of dissolved O 2 in solution (c O2 ¼ 1: moll 1 ) [39], the diffusion coefficient for O 2 (D O2 ¼ 1: cm 2 s 1 ) [38], the viscosity of the electrolyte (ν= cm 2 s 1 )[38], the Frdy constnt (F), nd the electrode s geometric re (A). The inset in Fig. 7 displys the diffusion-corrected Tfel plot. It shows tht the Tfel slope chnges continuously, so it is difficult to determine single slope. The reson for this is tht the site locking nd electronic effect of the dsored OH ions vry with coverge over the potentil region of the mixed kinetic diffusion-controlled rection [5]. In Fig. 8, the voltmmetry curve of (red line) is compred to (lue line) nd Pt/C (lck line) in de-erted 0.1 M HClO 4, while Fig. 8 illustrtes set of polriztion curves for the ORR on the sme smples with the ddition of the not-nneled Ir (green line) t 1,600 rpm in oxygented 0.1 M HClO 4. The hydrogen dsorption/desorption chrge for (73 μc) is pproximtely 40% lower thn tht of the Pt/C (117 μc) fter correcting for the doule lyer nd this is most likely due to differences in prticle sizes. The Pt prticles re round 3.5 nm (XRD dt not presented) where the PdIr prticles re pproximtely 7.0 nm. The shows higher ORR ctivity thn Pt/C nd j / ma cm j / ma cm Pt/C 1600 rpm Ir Pt/C Fig. 8 Voltmmetry curves for (lue), (red), nd Pt/C (lck) in de-erted 0.1 M HClO 4 solution. Scn rte, 20 mvs 1. Polriztion curves for the ORR on Ir (green), (lue), (red), nd Pt/C (lck) t 1,600 rpm in oxygen-sturted 0.1 M HClO 4 solution. Scn rte, 10 mvs 1

8 Fig. 9 Sl model for PdIr ctlyst with nneling () nd without nneling () on left, with exploded views on right to revel the composition of susurfce lyers. Pt depicted in gry, Pd in lue, nd Ir in yellow. Surfce unit cell is shown in red nd projected onto the PdIr 2 susurfce lyer. System is denoted y sequence of lyers, strting from top s /Pd/PdIr 2 /Pd for () nd /PdIr 2 /Pd for (), respectively even though it hs lrger surfce re. The influence of the Ir sulyer on the ORR ctivity could e explined y the position of the d-nd center (ε d ). Previous DFT clcultions hve shown tht the ε d of the metl monolyer under compressive strin tends to downshift in energy, wheres tensile strin hs the opposite effect [18, 40]. A surfce chrcterized y higher-lying ε d, tends to ind dsortes more strongly, therey enhncing the kinetics of dissocition rections producing these dsortes. On the other hnd, surfce with lower-lying ε d tends to ind dsortes more wekly nd fcilittes the formtion of onds towrds lrger intermedites. The on Pd is compressed ut the position of the ε d for the depends oth on the strin (geometric effects) nd on the electronic interction etween the nd its sustrte (lignd effect) [18]. There is out 11 t.% of Ir in PdIr nd it is concentrted just eneth the Pd surfce s discussed ove. Introducing susurfce Ir will cuse smll contrction of the Pd surfce lyer due to the smller tomic size of Ir compred to Pd. As consequence, the put on top of the Pd surfce covering the PdIr susurfce will e further compressed cusing dditionl down-shift in ε d which is mnifested y weker Pt OH interction leding to the reduced oxidtion of this ternry system. Since the intrinsic ORR ctivity is lrgely determined y the inding energy of OH, weker Pt OH interction should result in enhnced ORR ctivity. Figure 8 indeed shows tht the oxidtion of the on nnoprticles is delyed compred to tht of the Pt/C nd nnoprticles, therey enhncing ORR ctivity, which is shown in Fig. 8. To directly demonstrte the destiliztion of OH on PdIr (s compred to Pd or Pt), density functionl theory clcultions were performed on representtive closepcked (111) fcets of the pproprite model systems, shown in Fig. 9. The results of these clcultions re shown in Tle 1. The clcultions show tht depositing monolyer of Pt on Pd sustrte destilizes OH inding (BE OH = 2.07 ev) s compred to pure Pt (BE OH = 2.17 ev) or Pd (BE OH = 2.29 ev). Since the lttice constnt for Pt nd Pd re quite similr, this difference is mostly mnifesttion of the lignd effect, lthough, in relity, there should e some compressive strin component s well. Tle 1 Binding energy (BE) of OH (in ev) on severl close-pcked (111) model surfces t 1/6ML coverge Pd Pt /Pd /Pd/PdIr 2 /Pd /PdIr 2 /Pd BE OH (ev) Zero of the energy scle corresponds to OH(g) nd the respective metl sl t infinite seprtion from ech other. /Pd/PdIr 2 /Pd represents the synthesized nneled ctlyst ( PdIr), wheres /PdIr 2 /Pd would e the expected ctlyst without nneling ( IrPd). The BE of OH on other relevnt model systems is provided for comprison

9 j s / ma cm -2 rel i m / A mg -1 Pt V Pt 20 /C 0.9V Pt 20 /C Fig. 10 Pt-specific ctivity, Pt-mss ctivities of Pt/C,, nd t 0.9 V vs RHE Introducing Ir into the second sulyer elow pure surfce nd pure Pd ML sulyer (representing the cse of the nneled PdIr) further reduces inding of OH on the Pt surfce (BE OH = 2.00 ev). If the ctlyst were not nneled prior to depositing the Pt overlyer, one would expect Ir mixed with Pd in 2:1 rtio in the first sulyer (see erlier discussion). In this cse, OH inding is enhnced (BE OH = 2.21 ev) compred to /Pd (BE OH = 2.07 ev) nd pure Pt (BE OH = 2.17 ev), trnslting into ORR performnce of the not-nneled IrPd inferior to tht of pure Pt, s one cn see in Fig. 8. Therefore, ccording to our clcultions, the nneled PdIr ctlyst possesses the wekest inding of OH of ll systems studied here, including the Pd system. Weker inding of OH leds to lower OH coverge, less poisoning for ORR, nd therey higher ORR ctivity. On the other hnd, the not-nneled IrPd system, hving Ir right elow the Pt surfce shows BE OH tht is etween tht of pure Pt nd Pd, suggesting tht the specific ctlyst would not represent n ttrctive lterntive to neither pure Pt nor pure Pd ORR ctlysts, s experimentlly demonstrted in Fig. 8. Our clcultions provide support to the reported distriution of Ir within the sulyers. In prticulr, the totl energies of the two PdIr model sls employed suggest tht Pd is more stle in the first sulyer (right elow the )thniris. This stiliztion is pproximtely 0.13 ev/ir tom. Therefore, upon nneling, Pd toms should e pulled from the ulk to form Pd-rich first sulyer right elow the Pt surfce monolyer, with n Ir-rich second sulyer found right elow the first Pd sulyer. Although we consider only these two models nd do not investigte whether the Ir my e pulled further into the ulk, we feel tht the specific models re ccurte representtions of the ctlytic surfces studied experimentlly. Additionlly, in the presence 1/6 ML of OH the nneled structure mintins stility y pproximtely 0.08 ev/ir tom. Another wy to express the ORR ctivity of n electroctlyst is y its Pt-specific nd Pt-mss ctivities. Figure 10 shows the Pt-specific ctivity (10) nd Pt-mss ctivity (10) t 0.9 V of Pt/C,, nd. The rel electrochemicl surfce re ws used to clculte specific ctivities. For, specific ctivity of ma cm rel 2 ws oserved presenting the eneficil effects of PdIr sulyer, enhncing Pt s resistnce to OH poisoning nd oxidtion, compred to (0.708 macm rel 2 ). The specific ctivity for the ws pproximtely three times higher thn wht ws oserved for Pt/C (0.301 ma cm rel 2 ). Similr oservtions for Pt/C were reported y Gsteiger et l. where specific ctivity of macm rel 2 ws oserved [13]. The Pt mss ctivity shown in Fig. 10 demonstrtes the improved performnce of (2.17 Amg Pt 1 ) compred to (1.64 Amg Pt 1 ). i m / A mg -1 i PGM / A mg -1 Pt V Pt/C 0.9V Pt 20 /C Fig. 11 Totl mss ctivity nd price djusted precious group metl (PGM) mss ctivities of Pt/C,, nd t 0.9 V vs RHE

10 The is more thn 20 times more ctive thn Pt/C (0.084 Amg 1 Pt ). Figure 11 shows the totl mss ctivities for the sme ctlysts. Although there is dditionl precious metl mss present in the cse of (2.17 μg) nd (2.56 μg) they still outperform Pt/C (2.02 μg). The PdIr hs totl mss ctivity of 0.13 Amg 1, hs Amg 1 nd Pt/C hs Amg 1. Figure 11 shows the price djusted totl precious group metl (PGM) mss ctivities for the sme ctlyst. The ctivities shown re djusted sed on their respective metl cost in terms of re pltinum. The prices for the precious metls used were sed on 5-yer verge using the Engelhrd Industril Bullion Prices from Jnury 2005 to Jnury 2010: pltinum ($1,250 troy oz 1 ), plldium ($309 troy oz 1 ), nd iridium ($375 troy oz 1 ). The PGM price djusted mss ctivity shown in Fig. 11 shows the improved performnce of (0.43 Amg 1 Pt ) compred to (0.37 Amg 1 Pt ). The is more thn five times more ctive thn Pt/C (0.084 Amg 1 Pt ) nd lso meets the Deprtment of Energy (DOE) trget for This finding emphsizes the possiility to design highly ctive electroctlysts tht cn significntly reduce Pt loding nd overll cost. Conclusions We prepred with the intent to influence the Pt Pd interction in order to improve the ORR ctivity of of the lredy estlished highly ctive nd to enhnce Pd stility under fuel cell operting conditions. The ws mde y firstly depositing n Ir lyer on vi the Cu upd method nd with susequent nneling t elevted tempertures which cused segregtion of Pd to the surfce. After tht, ws deposited vi the Cu upd method. The hs higher ORR ctivity compred to nd Pt/C nnoprticle electroctlysts. The electrochemicl mesurements point out tht the enhnced ORR kinetics oserved on PdIr/ C originted from decresed Pt OH coverge. This could e rtionlized y self-consistent DFT clcultions of the inding energy of OH on vrious relevnt model systems. Clerly, the existence of PdIr 2 monolyer locted right elow /Pd ML ilyer nd ove Pd(111) crystl leds to destiliztion of surfce OH compred to tht lredy shown on /Pd(111). The Pt-specific ctivity for is three times nd 25% higher thn tht of Pt/C nd, respectively; the Pt-mss ctivity of is more thn 20 times nd 25% higher thn tht of Pt/C nd, respectively. This new electroctlyst shows to e promising in the serch for vile cndidtes of core-shell ctlysts in fuel cell technology y simultneously reducing Pt loding s well s enhncing overll performnce. Acknowledgments Work t BNL ws supported y US Deprtment of Energy, Divisions of Chemicl nd Mteril Sciences, under the contrct no. DE-AC02-98CH Work t CNSE ws supported y New York Stte Foundtion for Science, Technology nd Acdemic Reserch. Work t UW-Mdison ws supported y DOE-BES, Chemicl Sciences. CPU time ws utilized t fcilities locted t ANL, PNNL, ORNL, nd NERSC, ll supported y the DOE. The uthors would like to thnk Dr. Junto Li nd Dr. Sei Hirigshi for their ssistnce otining TEM/EDS nd TGA dt respectively t CNSE. References 1. Vielstich W, Lmm A, Gsteiger H (2004) Hnd ook of fuel cells. John Wiley & Sons Inc, pp R. Bshym, P. Zeleny, Nture 443, 63 (2006) 3. H. Gsteiger, J. Pnels, S. Yn, J. Power Sources. 127(1 2), 162 (2004) 4. J. Nørskov, J. Rossmeisl, A. Logdottir, L. Lindqvist, J. Kitchin, T. Bligrd, J. Phys. Chem. B. 108(46), (2004) 5. J. Wng, N. Mrkovic, R. Adzic, J. Phys. Chem. B. 108, 4127 (2004) 6. R. Adzic, Frontiers in electrochemistry (VCH Pulishers, New York, 1998), p S. Mukerjee, S. Srinivsn, M. Sorig, J. McBreen, J. Phys. Chem. 99, 4577 (1995) 8. S. Mukerjee, S. Srinivsn, J. Electronl. Chem. 357, 201 (1993) 9. M. Min, J. Cho, H. Kim, Electrochim. Act. 45, 4211 (2000) 10. S. Koh, M. Toney, P. Strsser, Electrochim. Act. 52, 2765 (2007) 11. T. Tod, H. Igrshi, M. Wtne, J. Electronl. Chem. 460, 258 (1999) 12. U. Pulus, A. Wokun, G. Scherer, T. Schmidt, V. Stmenkovic, N. Mrkovic, P. Ross, Electrochim. Act. 47, 3787 (2002) 13. H. Gsteiger, S. Koch, B. Somplli, F. Wgner, Appl. Ctl. B. Environ. 56(1 2), 9 (2005) 14. J. Zhng, Y. Mo, M. Vukmirovic, R. Kile, K. Sski, R. Adzic, J. Phys. Chem. B. 108, (2004) 15. J. Zhng, F. Lim, M. Sho, K. Sski, J. Wng, J. Hnson, R. Adzic, J. Phys. Chem. Lett. B. 109, (2005) 16. J. Zhng, M. Vukmirovic, K. Sski, F. Urie, R. Adzic, J. Ser. Chem. Soc. 70(3), 513 (2005) 17. R. Adzic, J. Zhng, K. Sski, M. Vukmirovic, M. Sho, J. Wng, A. Nilekr, M. Mvrikkis, J. Vlerio, F. Urie, Top. Ctl. 46, 249 (2007) 18. J. Zhng, M. Vukmirovic, Y. Xu, M. Mvrikkis, R. Adzic, Angew. Chem. Int. Ed. 44, 2132 (2005) 19. M. Vukmirovic, J. Zhng, K. Sski, A. Nilekr, F. Urie, M. Mvrikkis, R. Adzic, Electrochim. Act. 52, 2257 (2007) 20. M. Prsnn, R. Srecstv, P. Strsser, J. Phys. Chem. C. 112, 2770 (2008) 21. N. As, M. Bowker, Surfce Science 310, 113 (1994) 22. S. Brnkovic, J. Wng, R. Adzic, Surf. Sci. 474, L173 (2001) 23. J. Greeley, J. Nørskov, Annu. Rev. Phys. Chem. 53, 319 (2002) 24. B. Hmmer, L. Hnsen, J. Nørskov, Phys. Rev. B 59, 7413 (1999) 25. N. Ashcroft, N. Mermin, Solid Stte Physics (Sunders College, Orlndo, FL, 1976), p A. Nilekr, M. Mvrikkis, Surf. Sci. 602, L89 (2008) 27. J. McBreen, S. Mukerjee, J. Electrochem. Soc. 142, 3399 (1995) 28. L. Vitos, A. Run, H. Skriver, J. Kollr, Surfce Science. 411, 186 (1998)

11 29. A. Nilekr, A. Run, M. Mvrikkis, Surfce Science. 603, 91 (2009) 30. V. Stmenkovic, T. Schmidt, P. Ross, N. Mrkovic, J. Eletronl. Chem. 554, 191 (2003) 31. V. Stmenkovic, B. Mun, J. Myrhofer, P. Ross, N. Mrkovic, J Am Chem Soc 128, 8813 (2006) 32. V. Stmenkovic, T. Schmidt, P. Ross, N. Mrkovic, J. Phys. Chem. B. 106, (2002) 33. Y. M, P. Bluen, Surfce Science. 602, 107 (2008) 34. M. Sho, T. Hung, P. Liu, J. Zhng, K. Sski, M. Vukmirovic, R. Adzic, R. Lngmuir. 22, (2006) 35. W. Zhou, X. Yng, M. Vukmirovic, B. Keol, J. Jio, G. Peng, M. Mvirkkis, R. Adzic, J Am Chem Soc 131, (2009) 36. L. Kiler, A. El-Aziz, R. Hoyer, D. Kol, Angew. Chem. Int. Ed. 44, 2080 (2005) 37. M. Sho, P. Liu, J. Zhng, R. Adzic, J. Phys. Chem. B. 111, 6772 (2007) 38. N. Anstsijevic, V. Vesocic, R. Adzic, J. Electronl. Chem. 229, 317 (1987) 39. D. Lide, R. Ed, CRC hndook of chemistry nd physics, 75th edn. (CRC Press, Florid, 1995) 40. B. Hmmer, J. Nørskov, Adv. In Ctlysis. 45, 71 (2000)