Octahedral PtNi Nanoparticle Catalysts: Exceptional OxygenReduction Activity by Tuning the Alloy Particle Surface CompositionChunhua Cui,† Lin Gan,† Hui-Hui Li,‡ Shu-Hong Yu,‡ Marc Heggen,§ and Peter Strasser*,†

†The Electrochemical Energy, Catalysis, and Materials Science Laboratory, Department of Chemistry, Chemical Engineering Division,Technical University Berlin, Berlin 10623, Germany‡Division of Nanomaterials and Chemistry, Hefei National Laboratory for Physical Sciences at Microscale, University of Science andTechnology of China, Hefei 230026, P. R. China§Ernst Ruska-Centre for Microscopy and Spectroscopy with Electrons, Forschungszentrum Julich GmbH, 52425 Julich, Germany

*S Supporting Information

ABSTRACT: We demonstrate how shape selectivity andoptimized surface composition result in exceptional oxygenreduction activity of octahedral PtNi nanoparticles (NPs). Thealloy octahedra were obtained by utilizing a facile, completelysurfactant-free solvothermal synthesis. We show that the choiceof precursor ligands controls the shape, while the reaction timetunes the surface Pt:Ni composition. The 9.5 nm sized PtNioctahedra reached a 10-fold surface area-specific (∼3.14 mA/cmPt

2 ) as well as an unprecedented 10-fold Pt mass based(∼1.45 A/mgPt) activity gain over the state-of-art Pt electro-catalyst, approaching the theoretically predicted limits.

KEYWORDS: Surface composition, PtNi octahedra, oxygen reduction reaction, ligand control

The sluggish kinetics of the oxygen reduction reaction(ORR) on costly platinum cathode electrocatalysts

represents a major obstacle to a more widespread use of thepolymer electrolyte membrane fuel cell (PEMFC). Recentrational design of geometric and electronic properties ofextended alloy catalyst surfaces have resulted in significantimprovements of the ORR activity.1−8 However, improving theORR activity further in practical nanoscale alloy catalysts is still agreat challenge.9 One of the most promising strategies is thedevelopment of shape and composition-controlled Pt-basedalloy nanoparticle (NP) catalysts.10−14 These NP catalysts withcontrolled shapes, that is, controlled exposed crystal facets, andcomposition profiles hold the promise of providing the sameideal surface electronic structure as extended surfaces, therebyrealizing their full activity advantage.15−17

Stamenkovic and co-workers reported a very active singlecrystal Pt3Ni(111) surface, which performed 10 times (10×)higher in ORR activity than a Pt(111) surface and 90× higherthan a commercial NP Pt/C catalyst.18 The enhancement wasattributed to the low coverage of hydroxyl species induced by thespecific electronic structure associated with an oscillatory near-surface compositional Pt and Ni profile across the 2−4outermost layers of the (111) surface.18−20 This was directexperimental evidence that the near-surface composition and itsatomic arrangement are key factors to improve the ORRactivity.21,22 Stamenkovic’s report on Pt3Ni(111) triggered aquest for shape-selective octahedral Pt alloy NPs, which wouldexhibit only the active (111) facets; if successful, this could makethe 90× catalytic activity gain a reality. Considering the typical

9−10× loss in activity when going from extended surfaces tocarbon-supported alloy nanoparticles,23 an activity gain of about10× is realistically expected for a carbon-supported Pt−Nioctahedron compared to a state-of-art carbon-supportedspherical Pt nanoparticle catalyst.Surfactant-directed synthesis of well-defined shape-selective

Pt−Ni catalysts was recently reported by Wu et al. and Zhang etal. using a careful choice of preparation techniques involvingdistinctly different surfactants, reducing agents, and sol-vents.10−13 The octahedral Pt−Ni particles achieved a 4−7×improvement in terms of the Pt surface area-specific activity butdisplayed a mere ∼4× improvement in Pt mass activity over Ptowing to residual capping molecules.10−12 Also, in all of thesestudies, a fundamental understanding of how success or failure toproduce shape selective particles depends on the applied reactionconditions has remained poorly understood. In particular, whilebasic Wulff-type particle shapes (octahedra, cubes) could bereproducibly produced and identified through atomic scalemicroscopy, their precise surface compositions on individualfacets, key for their catalytic reactivity, remained unexplored anduncontrolled.More recently, Snyder et al. presented a size-dependent

dealloying study on nonshape selective Pt−Ni NPs and showedthe formation of porous Pt−Ni NPs above a critical diameter of

Received: September 3, 2012Revised: October 3, 2012Published: October 12, 2012

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∼15 nm.24 Pt−Ni NPs ranging in particle size from 5 to 20 nmdemonstrate specific and mass ORR activity improvementfactors of 3−6× and 4−5×, respectively, due to their differentresidual Ni contents and surface morphologies after deal-loying.20,24,25 Carpenter et al. demonstrated 10× specific ORRactivity improvement for 12−15 nm shape-selective octahedralPt−Ni NPs, and their ORR-tested octahedra became porous,26

which is consistent with Snyder et al.’s conclusions. Despite theirfavorable specific activity, however, the Pt mass activity was only4−6× over Pt. In most of these PtNi studies, imperfections ingeometry and near-surface composition of the NPs were heldresponsible for the lower than expected ORR activities.Given our understanding of the electrocatalysis of octahedral

Pt−Ni nanoparticles outlined above, it is clear that a better fine-tuning of the near-surface composition of Pt−Ni octahedralparticles could have a great potential to further perfecting theORR activity gain over Pt. However, robust and facile syntheticstrategies to control the near-surface composition in octahedralNPs have remained elusive to date and represent a critical unmetneed in fundamental fuel cell alloy electrocatalysis.Here, we present a robust, facile, and surfactant-free

solvothermal synthesis of shape and size-selective octahedralPtNi NPs. The shape selective NPs show an exceptional ORRmade possible by their carefully tuned alloy particle surfacecomposition. We show that the choice of precursor ligandscontrols the shape selectivity, while we can use the reaction timeto tune the surface Pt:Ni composition and thus optimize theORR activity. We explain our findings in terms of a simplenucleation/growth model. At a surface composition of about 40at. % Pt, 9.5 nm-sized PtNi octahedra reached a 10-fold surfacearea-specific (∼3.14 mA/cmPt

2 ) as well as an unprecedented 10-fold Pt-mass based (∼1.45 A/mgPt) activity gain at 900mV/RHEand 5 mV/s anodic sweep rate over the state-of-art commercialcarbon-supported Pt electrocatalysts.We have utilized a simple, surfactant-free, low-temperature

(120 °C) solvothermal synthesis to prepare unsupported size-and shape-selective octahedral NPs. Figure S1 of the SupportingInformation shows the color change of the solvent from green toblack at 120 °C after 16 h indicating the formation of the PtNioctahedral NPs. Figure 1a reports the X-ray diffraction (XRD)patterns reflecting the bulk alloy phase structure of the PtNioctahedral NPs after three different reaction times, 16 h, 28 h,and 42 h (denoted as 16-PtNi, 28-PtNi, and 42-PtNi,respectively). The bulk composition of the three alloy NPs wasdetermined as Pt:Ni = 46:54 by inductively coupled plasma massspectrometry (ICP-MS) and energy-dispersive X-ray spectra

(EDX) regardless of reaction time (Figure S2). This is consistentwith the three basically overlapping pattern profiles indexed to aface-centered-cubic (fcc) phase. The octahedral morphology wasobserved by transmission electron microscopy (TEM) in FigureS3.Following earlier reports on solvothermal techniques at higher

temperatures (200 °C),26 the dimethylfomamide (DMF) solventacts as a complexing agent, solvent, and reducing agent.However, in this study, high heating rates (10 °C/min) and alower reaction temperature (120 °C) were utilized. A highheating rate resulted in a short induction time and highnucleation rates generating a large number of small seeds. Unlikeprevious reports,24,26 the low reaction temperature favors slowseed growth, keeping our particles small.24 We note that pure Ptand pure Ni precursors could not be reduced at this lowtemperature,26,27 suggesting a possible role of the exothermicheat of mixing during PtNi alloy seed formation.28−31 The initialseeds catalyzed the codeposition of Pt and Ni. Moreover, a lowerreaction temperature could favor the nanocrystal faceting duringgrowth in a colloidal solution.32

To obtain further insight in the formation mechanism ofoctahedral alloy nanoparticles, we interrogated the influence ofthe metal precursor ligands on the alloy particle shape selectivity.Use of Ni(acac)2 and Pt(acac)2 reproducibly resulted inoctahedral nanoparticles. When Ni(acac)2 was replaced withNi acetate, keeping all other synthesis conditions constant,uniform spherical alloy nanoparticles with ∼5 nm diameter wereobtained (Figure S4a). On the other hand, when the Pt(acac)2precursor was replaced with K2PtCl6, particle aggregates withsmaller mean size, limited shape-selectivity, and wider sizedistribution were observed (Figure S4b). These results suggestedthat it is not the interaction of DMF with the (111) facetsalone,33 which determines the formation of octahedra; theprecursor ligands, such as acetyl acetonate, had a critical influenceon the particle shape and size through a modification of thethermodynamic metal redox potential, possibly coupled to amodified kinetic metal ion reduction (metal atom production)rate. Both factors can have profound effects on the shape-selective growth.We then studied how the reaction time affected the octahedral

surface composition of Pt and Ni using X-ray photoelectronspectroscopy (XPS) (see Supporting Information). Based on theestimated sampling depth of ∼2 nm, the measured data isreferred to as “near-surface” rather than “surface” composition.34

It must be noted that the measured surface composition is anaverage value because the PtNi NPs are loaded on carbon and

Figure 1. (a) XRD patterns of PtNi octahedral NPs treated with different reaction times: (i) 16 h, (ii) 28 h, and (iii) 42 h. (b) Near surface compositionsof Pt (red bar) and Ni (green bar) measured by XPS (sampling depth of ∼2 nm); the red dotted line shows bulk Pt/Ni compositions measured by ICPand EDX.

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every part of the particles has the same chance to be exposed tothe X-ray. Our XPS data are shown in Figure 1b; it evidences thatthe reaction time directly controls the near-surface compositionof the resulting octahedral particles without affecting their size orshape. Raising the reaction time from 16 to 42 h, the near-surfacePt at. % increased from 30 to ∼41 at. % at identical shape, size,and bulk composition.To explain this, we emphasize that our experimental XPS data

provide direct evidence that the initial particle seeds catalyzehigher deposition rates for Pt than for Ni, consistent with theirrelative electrochemical deposition potentials.35,36 This induces acompositional gradient near the surface of the octahedra28−31,37

(see Figure 1b). As the dissolved precursors deplete, theoctahedra reach their time-stable final bulk composition, shape,and size. At this point, the reaction time acts like an in situannealing process, mainly smoothing out the near-surfacecompositional gradient by metallic interdiffusion.38,39

To monitor the particle size and shape changes with reactiontime, we performed transmission electron microscopy (TEM).The average particle size is∼9.0 nm for 16-PtNi,∼9.2 nm for 28-PtNi and∼9.5 nm for 42-PtNi (see Table 1, Figure S3 and Figure2a−c) suggesting that there is no size penalty with increasingreaction time. Owing to the uncompleted ripening process for16-PtNi material, the standard deviation of its particle sizedistribution is larger than those of 28-PtNi and 42-PtNi. After 42h reaction time, small particles have disappeared, and this is whythe particle size distribution became more uniform and thedeviation decreased to ±0.8 nm (Figure S3). High-resolutionTEM analysis in Figure 2c shows that the corresponding d-spacing for the (111) planes is 0.216 nm, which are indexed tothe octahedral PtNi NPs terminated with {111} facets. To turnthe unsupported NPs into a practical electrocatalyst, the NPswere supported on a high surface area carbon material. The TEMimage in Figure 2d evidence a fairly uniform distribution of thePtNi NPs on the commercial carbon support. As shown in Figure2e and f, the octahedral morphology and the atomic-scalecompositional distributions of Pt and Ni across an octahedronwere measured by probe corrected scanning transmissionelectron microscopy complemented with electron energy lossspectroscopy (STEM/EELS).To evaluate the electrocatalytic ORR activities of the

octahedral NPs, the octahedral PtNi/C NPs were loaded on aglassy carbon rotating disk electrode (RDE). Because thesecatalysts were synthesized in pure DMF solvent and no othersurfactants were used in the synthesis, any additional surfactant-removing step is not needed,26 a great practical advantage overthe polyol process applied previously to prepare Pt−Nioctahedra. The electrochemical active surface areas (ECSAs),evaluated using CO stripping,40 were 24.1, 36.7, and 50.0 m2/gPtfor the 16-PtNi, 28-PtNi, and 42-PtNi catalysts, respectively (see

Table 1 and Figure S5 of the Supporting Information). Theestimated CO stripping charge is somewhat larger but very closeto the estimated 2× Hupd charge, and calculated QCO/2QH iswithin the region of 1.04−1.12 (Figure S5). The increased valueof ECSA for 42-PtNi is consistent with the observed higher Ptsurface concentration.The effect of the initial near-surface alloy composition on the

ORR activity was studied in O2-saturated 0.1 M HClO4 solutionat room temperature (see Figure 3 and Table 1). Uponincreasing the reaction time from 16 to 42 h associated withthe increase in near surface Pt at. % from 30 to 41 at. %, the Ptmass activity increased by a factor of 3× at 0.9 VRHE. Theactually observed value at 16 h was 0.56 A/mgPt, rising toimpressive 1.45 A/mgPt for the 42 h catalyst at quasi-stationary 5mV/s scans. The Pt- surface area- specific activity increased byabout 50% from 2.35 mA/cm2

Pt to impressive 3.14 mA/cm2Pt.

For comparison, the state-of-art Pt/C catalyst exhibited 0.15 A/mgPt and 0.23 mA/cm2

Pt. We emphasize that these ORRactivities represent previously unachieved consistent 10×increases in both specific and Pt mass activity compared to astate-of-art commercial Pt/C electrocatalyst measured underidentical conditions. The 42-PtNi sample even performs superiorto extended polycrystalline Pt electrodes (∼1.2 mA/cm2

Pt).From the changes in mass activity and the 1.5× increase inspecific activity, we conclude that the number of catalytically

Table 1. Comparison of the ECSA, ORR Mass, and SpecificActivitiesa

reactiontimeb

particle sizec

(nm)ECSA(m2/gPt)

mass activity(A/mgPt)

specific activity(mA/cm2

Pt)

16 9.0 ± 1.1 24.1 0.56 ± 0.065 2.35 ± 0.2828 9.2 ± 0.9 36.7 1.02 ± 0.070 2.77 ± 0.2042 9.5 ± 0.8 50.0 1.45 ± 0.120 3.14 ± 0.24

aAll activities at 0.9 V/RHE in 0.1 M HClO4, 1600 rpm, 5 mV/s.Three independent synthesis/electrochemical tests. bReaction temper-ature was kept at 120 °C. cThe particle size is estimated by longestlength over two opposite ends.

Figure 2. (a and b) TEM and (c) HRTEM images of octahedral PtNiNPs after 42 h. (d) TEM image of octahedral PtNi NPs supported oncommercial carbon (Vulcan XC-72). (e) Cs-corrected HAADF-STEMimage of a selected 42-PtNi octahedron. (f) STEM-EELS line scansacross the octahedron (inset). Intensities are normalized by elementalscattering cross sections.

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active Pt surface sites roughly doubles. The remarkable 10×activity gains are likely a result of an improved Pt/Ni ratio in thesecond and third layers.18,41 EDX analysis of the active ORR-tested 42-PtNi electrocatalyst revealed that its final bulkcomposition changed to about Pt75Ni25 during the electro-chemical dealloying process, thus very close to the ideal bulkcomposition of the highly active extended (111) surface.However, this specific activity is still far short of that forextended Pt3Ni(111) crystals. This could be attributed to defectsand vacancies in the particle surface after mild dealloying in acidicelectrolytes. Moreover, TEM analysis showed that the particlesmaintained an octahedral shape. Being below the critical size of15 nm,24 the ORR-tested∼9.5 nm 42-PtNi catalyst did not showany indication of porosity, in line with earlier findings for Pt−Co,Pt−Cu, and Pt−Ni NPs.24,42 Preliminary data on the stability ofthe PtNi octahedra indicate that thermal postsynthesis annealingis very detrimental to the octahedral shape and ORR activity.Unannealed PtNi octahedra remained morphologically stableeven after tens of voltage cycles within the oxygen reductionreaction potential range. The long-term stability of the octahedraunder fuel cell conditions requires further scrutiny.In conclusion, we have synthesized octahedral PtNi NPs with

10× ORR activity gains in both Pt specific and Pt mass activityover a state-of-art Pt/C electrocatalyst. We attribute this highactivity to the octahedral shape and favorable surfacecomposition of the final electrocatalyst. To achieve this, wehave used a facile surfactant-free solvothermal method andshowed that the reaction time correlates with the near-surface Ptatomic composition of the octahedra without affecting their sizeor shape. Longer reaction times led to higher near-surface Pt-to-Ni atomic ratios, which resulted in higher intrinsic activity. Thereaction time effect was rationalized based on a nucleation/growth model that explained the near surface compositiongradient of Pt and Ni. The synthesis−structure−activity

relationships uncovered here provide ample room for newrational pathways to more active bimetallic alloy particles.

■ ASSOCIATED CONTENT*S Supporting InformationExperimental details, EDX spectra, TEM images, and COstripping of PtNi octahedra. This material is available free ofcharge via the Internet at http://pubs.acs.org.

■ AUTHOR INFORMATIONCorresponding Author*E-mail: pstrasser@tu-berlin.de.NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSWe thank Dr. Frederick T. Wagner for valuable discussions. Thiswork was supported by U.S. DOE EERE award DE-EE0000458via subcontract through General Motors. P.S. acknowledgesfinancial support through the cluster of excellence in catalysis(UniCat).

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Figure 3. ORR activity of the octahedral PtNi NPs with controlled surface alloy composition. (a) ORR polarization curves. Inset shows the cyclicvoltammograms of the catalysts in N2-saturated electrolyte. (b) Mass and (c) specific ORR activities. Insets show the activities of the referencepolycrystalline Pt and commercial Pt/C catalysts.

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■ NOTE ADDED AFTER ASAP PUBLICATIONThis paper was published ASAP on October 16, 2012. Thecaption of Figure 2 and the Supporting Information file havebeen updated. The revised version posted on October 19, 2012.

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