Journal of Catalysis 291 (2012) 69–78

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Journal of Catalysis

journal homepage: www.elsevier .com/locate / jcat

Enhanced electrocatalytic performance due to anomalous compressive strainand superior electron retention properties of highly porous Pt nanoparticles

Dae-Suk Kim a, Cheonghee Kim b, Jung-Kon Kim a, Jun-Hyuk Kim a, Ho-Hwan Chun c, Hyunjoo Lee b,Yong-Tae Kim a,⇑a School of Mechanical Engineering, Pusan National University, Busan 609-735, Republic of Koreab Department of Chemical and Biomolecular Engineering, Yonsei University, Seoul 120-749, Republic of Koreac Global Core Research Center for Ships and Offshore Plants (GCRC-SOP), Pusan National University, Busan 609-735, Republic of Korea

a r t i c l e i n f o

Article history:Received 24 December 2011Revised 31 March 2012Accepted 7 April 2012Available online 17 May 2012

Keywords:ElectrocatalystsExtended X-ray absorption fine structureHigh-resolution powder diffractionOxygen reduction reactionHighly porous Pt nanoparticles

0021-9517/$ - see front matter � 2012 Elsevier Inc. Ahttp://dx.doi.org/10.1016/j.jcat.2012.04.004

Abbreviations: HP-Pt, highly porous Pt; TTAB, tetbromide; ORR, oxygen reduction reaction; PEMEC, procell; HRPD, high-resolution powder diffraction; EXAFfine structure; XANES, X-ray absorption near-edge strof Energy; DI, deionized; TEM, transmission electronresolution TEM; CCD, charge-coupled device; XRD, XLight Source; XPS, X-ray photoelectron spectroscopRHE, reversible hydrogen electrode; CV, cyclic voltrotating disk electrode; ECSA, electrochemical surfacestep; 1NN, first nearest neighbor; RDF, radial distribWaller.⇑ Corresponding author. Fax: +82 51 514 0685.

E-mail address: yongtae@pusan.ac.kr (Y.-T. Kim).

a b s t r a c t

The shape and structure of electrocatalysts at the nanoscale level have a decisive effect on their activityand durability in low-temperature fuel cells. Herein, we report the discovery of unexpected structuralphenomena in exotic nanostructures: the anomalous compressive strain and superior electron retentionproperties of highly porous Pt (HP-Pt) nanoparticles synthesized using a weakly interacting organic cap-ping agent, tetradecyl trimethyl ammonium bromide. Even though the particle size of the HP-Pt nanopar-ticles was much larger than those of commercial electrocatalysts, bond length shortening occurredanomalously, and the downshifted d-band center eventually led to increased oxygen reduction reactionactivity. This is because the HP-Pt nanoparticles had a highly porous urchin-like dendritic structure,interestingly in the single-crystalline phase, despite the large particle size. In addition, their electronretention properties were superior to those of commercial samples, which led to drastically enhancedstability against Pt dissolution at high potentials.

� 2012 Elsevier Inc. All rights reserved.

1. Introduction devoted toward enhancing the ORR activity and durability of

One of the most serious hurdles to be overcome in the commer-cialization of proton exchange membrane fuel cells (PEMFCs) is theconsiderable activation loss in the cathode because of the slowkinetics of the oxygen reduction reaction (ORR), which in turn re-sults in a large amount of Pt usage for cathode electrocatalysts andthereby increases the production cost of such fuel cells [1,2].Furthermore, the rather fast deterioration of the cathode electro-catalysts, mainly due to both Pt dissolution and coalescence, makesit difficult to meet the US Department of Energy targets for durabil-ity: 5000 h for transportation and 40,000 h for stationary applica-tions [3,4]. For these reasons, considerable effort has been

ll rights reserved.

radecyl trimethylammoniumton exchange membrane fuelS, extended X-ray absorptionucture; DOE, US Department

microscopy; HRTEM, high--ray diffraction; PLS, Pohangy; FCC, face-centered cubic;ammetry; TF-RDE, thin-filmarea; RDS, rate-determining

ution function; DW, Debye–

Pt-based cathode electrocatalysts for PEMFCs.The most widely considered idea is alloy formation (with either

solid-solution or phase-segregated structures) with various transi-tion metals. Representative alloys reported to date include a Pt3Cosolid solution [5], Pt3Ni [6], Pt modified with Au clusters [4], a PtCuphase-segregated structure [7], and Pt-on-Pd nanodendrite sys-tems [8]. Although these alloys have shown enhanced ORR activity(due to the compressive strain effect caused by alloying with tran-sition metals having small atomic radii and the ligand effect causedby the overlapping of electronic structures), leaching of the transi-tion metals, which is considered to be one of the most seriousproblems in terms of durability, is inevitable in most alloy electro-catalysts (except for some phase-segregated alloys). Anotherchallenge in improving the ORR activity and durability is control-ling the shape of the electrocatalyst particles, which can exist asspheres [9], cubes [10], tetrahedra [11], cages [12], wires [13],and sheets [14]. In particular, dendritic nanoparticles have beensynthesized intensively [15] and evaluated for various electrocata-lytic reactions, including oxygen reduction [8] and small-moleculeoxidation [16], because these nanoparticles have a highly porousstructure, which leads to a large surface area per unit mass.

Lim et al. reported that Pd–Pt bimetallic nanodendrites synthe-sized by the seed-growth technique demonstrated an activity 2.5times higher than that of commercial Pt/C [8]. They concluded that

70 D.-S. Kim et al. / Journal of Catalysis 291 (2012) 69–78

the enhanced ORR activity was attributable to an increase in thesurface area caused by the dendritic shape and the facets likePt(111) preferred for the ORR. The degree of durability enhance-ment was relatively less remarkable than the activity increase,and the authors did not discuss in detail the mechanism of suchan improvement. Similarly, Peng and Yang demonstrated the en-hanced ORR activity and durability of Pt-on-Pd dendritic nanopar-ticles formed through a sequential synthetic method [15]. Theyelucidated that the origin of the ORR activity enhancement wasthe improved tolerance against OH poisoning, which was indicatedby the positive shift of the onset and peak potentials for hydroxylion adsorption during the forward sweep in cyclic voltammetry(CV). They also suggested that the durability enhancement couldbe explained by the favored interfacial structures between the Ptand Pd supports and the larger particle size of the Pt-on-Pd nano-particles. However, in most papers on dendritic nanoparticles, littlediscussion has been provided on the most essential origin of per-formance enhancement, such as the compressive strain effect forORR activity [7] and the electron retention property identified bythe d-band vacancy change at high potentials for durability [4,17].

In this study, we report the discovery of unexpected structuralphenomena in exotic nanostructures, that is, the anomalous com-pressive strain and superior electron retention properties of highlyporous Pt (HP-Pt) nanoparticles synthesized using a weakly inter-acting organic capping agent, tetradecyl trimethyl ammonium bro-mide (TTAB). To date, most studies on the formation of highlyporous or dendritic nanoparticles have employed surfactants orpolymers with strong interactions with metal surfaces, which weredifficult to remove and may have acted as surface poisons [18,19].However, since TTAB can be removed readily from the nanoparticlesurfaces, it was expected that enhanced electrocatalytic perfor-mance could be demonstrated. In order to elucidate the origin ofthe enhanced ORR activity and durability for HP-Pt nanoparticles,we performed a precise analysis of their fine structures and elec-tronic energy states. From various synchrotron studies, it wasfound that compressive strain was exerted on the lattice, influenc-ing the d-band center position; the d-band vacancy, which affectsthe oxidation potential, hardly changed at high potentials. For anin-depth analysis of the ORR activity enhancement resulting fromthe small compressive strain-induced change in the lattice con-stant, we employed fine-structure analysis techniques like high-resolution powder diffraction (HRPD) with Rietveld refinementand extended X-ray absorption fine structure (EXAFS) measure-ments with FEFF analysis using a synchrotron beam. Furthermore,in order to understand the origin of the stability against Pt dissolu-tion through examination of the d-band vacancy change, we inves-tigated the change in the white line in the Pt L3 absorption edge indetail by applying potential steps using the in situ X-ray absorptionnear-edge structure (XANES) technique; we used a specially de-signed homemade measurement cell for this analysis. In the study,the electrocatalytic properties of HP-Pt nanoparticles are comparedwith those of several commercial samples, such as Pt/C E-TEK, Pt/CE-TEK (300) (samples of Pt/C E-TEK heat-treated at 300 �C in ahydrogen atmosphere in order to maximize the durability), andPt/C Premetek.

2. Experimental section

2.1. Catalyst synthesis

In a typical synthesis, aqueous solutions of 20 mM K2PtCl4 (98%,Aldrich) solution 1 mL, 400 mM tetradecyltrimethylammoniumbromide (TTAB; 99%, Aldrich) solution 1.25 mL and 7.25 mL of DIwater were mixed in a 20-mL vial at room temperature. The mix-ture was heated at 90 �C for about 5 min until the solution becameclear. When the solution became clear, 120 mM L-ascorbic acid

(99 + %, Aldrich) solution 0.5 mL as the reducing agent was in-jected, and the solution was maintained at 90 �C for 7hrs. The totalvolume of the solution was maintained at 10 mL, and final concen-trations were 2 mM for K2PtCl4, 50 mM for TTAB, and 6 mM forL-ascorbic acid. The product was centrifuged at 3000 rpm for30 min. The supernatant solution was separated and centrifugedagain at 12,000 rpm for 10 min. The precipitate was collected andredispersed in 5 mL of deionized (DI) water by sonication. For thepreparation of carbon-supported 20 wt.% HP-Pt nanoparticles, car-bon supports (Vulcan XC-72R) were dispersed in DI water by son-ication at a frequency of 70 kHz for 10 min. After sonication, theprepared HP-Pt nanoparticles, equivalent to 20% weight ratio ofcarbon, were dispersed in the above solution by stirring for 5 h.The precipitate was filtered and washed several times with DIwater and ethanol and dried overnight in a vacuum oven at50 �C. Commercial Pt/C E-TEK, Pt/C E-TEK annealed at 300 �C for1 h (called Pt/C E-TEK (300)), and Pt/C Premetek with a metal load-ing of 20 wt.% were employed for comparison with the HP-Ptnanoparticles.

2.2. Catalyst characterization

The morphologies of the electrocatalyst samples were investi-gated using transmission electron microscopy (TEM, JEOL JEM-2011). Specimens for TEM observation were prepared by placinga drop of the particle-dispersed solution onto a copper grid. Themicroscope system was operated at an accelerating voltage of200 keV. Two hundred nanoparticles randomly selected from theTEM images were used to measure the average size of each sample.The indexes of the surface facets and lattice fringes of the Pt nano-particles were determined from high-resolution transmission elec-tron microscopy (HRTEM) and Fourier transform data. All theimages were recorded with a charge-coupled device (CCD) camera.

X-ray diffraction (XRD) experiments on the synthesized pow-ders were carried out using the synchrotron radiation of the 8C2HRPD beamline at Pohang Light Source (PLS). The incident X-rayswere monochromatized to a wavelength of 1.54950 Å by a dou-ble-bounce Si(111) monochromator. The diffraction pattern wasscanned in the 2h range of 10–151�, with a counting time of 4 sper 0.02�. Scherrer’s equation was used to estimate the particle sizefrom the XRD results. For this purpose, the (111) peak of the Ptface-centered cubic (FCC) structure at 2h = 40� was selected. TheXRD patterns were analyzed by the Rietveld refinement programFullprof [20] to determine the lattice parameters of the synthesizedpowders. From all the profiles in the Fullprof program, a pseudo-Voigt function was selected as the profile function.

X-ray photoelectron spectroscopy (XPS) experiments were per-formed at the PLS 8A1 undulator (U7) beamline equipped with avariable-included angle plane-grating monochromator. The inci-dent photon energy was 630 eV, and photoelectrons emitted fromthe surface of the Pt/C electrocatalysts, which were adhered to onthe carbon tape, were collected, and their energies were analyzedby an electron analyzer (Physical Electronics: Model PHI 3057 withan Omega lens and a 16-channel detector). Although Ar-ion bom-bardment cleans the sample surface, it also produces some nega-tive effects, such as variation of the surface topography andpreferential sputtering; hence, we did not clean the sample surfaceby Ar-ion bombardment. The binding energies of the photoemis-sion spectra were calibrated using the peak position of the Au 4fline at 84.0 eV from a standard Au sample located next to the sam-ple of interest. The XPS binding energies were analyzed using theXPS spectral data processing software XPS International to processthe distribution of the Pt species and the relative intensities of thesynthesized powder. Details of fitting analysis are provided inFig. S2 of Supporting information.

D.-S. Kim et al. / Journal of Catalysis 291 (2012) 69–78 71

Pt L3 edge (11,564 eV) X-ray absorption spectroscopy (XAS)spectra were recorded at the BL7C1 beam line of PLS with a ring cur-rent of 120–170 mA at 2.5 GeV. A Si(111) double-crystal mono-chromator was employed to monochromatize the X-ray photonenergy. The data were collected in the transmission mode withHe and N2 gas-filled ionization chambers as detectors. Higher-orderharmonic contamination was eliminated by using detuning to re-duce the incident X-ray intensity by ca. 40%. Energy calibrationwas completed using standard Pt foil. Every experiment on the Pt/C electrocatalysts was conducted using a homemade measurementcell with an aluminum holder for the XAS powder experiments. Thein situ XANES data were recorded using a potentiostat (BiologicVSP) via chronoamperometry measurements with potential stepsin N2-saturated 0.1 M HClO4 solutions. All the spectra wererecorded at room temperature. The spectra were processed usingthe program IFEFFIT [21] (IFEFFIT version 1.2.11, Copyright 2008,Matthew Newville, University of Chicago, http://cars9.uchica-go.edu/ifeffit/) with background subtraction (AUTOBK) [22] andnormalization. The phase shift and back-scattering amplitude werecalculated theoretically using the FEFF 9.03 code [23]. Fourier trans-form for the EXAFS spectra was performed in the range of approx-imately 2.5–16 Å�1 in k-space and 1–3 Å in the R window. Fromthese analyses, structural parameters such as the coordinationnumber (N), bond distance (R), Debye–Waller factor (Dr2

j ), andinner potential shift (DE0) were calculated.

2.3. Electrochemical measurements

All electrochemical measurements were performed in a three-electrode electrochemical cell connected to a potentiostat (BiologicVSP) at room temperature. A glassy carbon electrode (5 mm indiameter) was used as the working electrode; it was polished withdiamond solution and Al2O3 slurries and then washed in DI waterwith sonication before the experiments. HP-Pt/C powder (2 mg)was dispersed well in DI water (2 mL) using an ultrasonicator,and 20 lL of the dispersion was dropped onto the glassy carbonelectrode with a micropipette to afford a Pt loading of 20 lg/cm2

on the electrode. Uniform, thin HP-Pt/C films were prepared byevaporating the water at room temperature. Nafion solution(0.025 wt.%, 20 lL) was dropped onto a suspension-dried glassycarbon electrode and dried in a vacuum oven at 70 �C for 30 min.A Pt wire and a silver chloride electrode (Ag/AgCl sat. 3.5 M KCl)were used as the counter and reference electrodes, respectively.The Ag/AgCl reference electrode was calibrated on a daily basisusing a Pt disk electrode and H2-satuated 0.1 M HClO4 electrolyteto ensure that its potential did not drift. All potentials were con-verted to the reversible hydrogen electrode (RHE) scale. CV mea-surements were performed in O2-free 0.1 M HClO4 electrolyte(obtained by bubbling high-purity N2 gas through the electrolytefor 1 h). The electrodes were cycled in the potential range between0.05 and 1.15 V (versus RHE) at a scan rate of 20 mV/s after electro-chemical cleaning with a quick scan (scan rate: 200 mV/s) for 50cycles. After the CV measurements, the ORR was performed inthe same potential range in O2-saturated 0.1 M HClO4 at a scan rateof 5 mV/s with rotation speeds of 100, 400, 900, 1600, and2500 rpm. The durability tests were carried out at room tempera-ture in O2-saturated 0.1 M HClO4 solutions with a cyclic potentialsweep between 0.6 and 1.1 V (versus RHE) at a sweep rate of50 mV/s for 4000 cycles [8]. Then, CV measurements were per-formed in N2-saturated 0.1 M HClO4 electrolyte in the potentialrange between 0.05 and 1.15 V (versus RHE) at a scan rate of20 mV/s. Pt/C E-TEK, Pt/C E-TEK (300), and Pt/C Premetek wereused for the same set of electrochemical measurements underthe same conditions. All the current densities were normalized tothe geometric area of the rotating disk electrode, and all electro-chemical measurements were carried out at room temperature.

3. Results and discussion

3.1. Basic morphology and structure of HP-Pt nanoparticles

The morphology and lattice structure of the HP-Pt nanoparticleswere identified from the bright-field and dark-field TEM images. Asshown in Figs. 1a–c, the commercial samples are spherical with asize of about 3–4 nm. On the other hand, the HP-Pt nanoparticlesshown in Fig. 1d have urchin-like dendritic shapes and are highlyporous, with an average diameter of about 13 nm. Fourier transfor-mation of the dark-field image obtained by TEM allows us to imagethe reciprocal lattice points for the hkl planes and thereby enablesus to analyze the lattice structure and check the morphologysimultaneously. It is interesting to note that the reciprocal latticepoints, rather than the Ewald circle, are clearly shown in the Fou-rier-transformed dark-field image, as seen in the inset of Fig. 1e.This implies that the HP-Pt nanoparticles are not polycrystallinesecondary particles but primary particles with lattice structuresin the single-crystalline phase. Furthermore, the facets of the HP-Pt nanoparticles are identified to consist of Pt(111) and (110)rather than (100) or high-index planes, as observed in Fig. 1f.

This understanding is also supported by the HRPD patterns ob-tained using a synchrotron beam followed by Rietveld refinement.Fig. 2a shows the HRPD patterns and the reflection peaks, indexedas (111), (200), (220), (311), (222), (331), and (420), corre-sponding to face-centered cubic Pt with a lattice constant a =3.930 Å (PDF standard cards, JCPDS 10-1311, space groupFm3m[225]). The average particle size of the electrocatalysts wascalculated from the full-width at half-maximum of the Pt(111)peak using Scherrer’s equation:

d ¼ kkb cos h

ð1Þ

where d is the average particle size (Å); k, the shape-sensitive coef-ficient (0.9); k, the wavelength of the radiation used (1.5490 Å); b,the full-width at half-maximum (in radians) of the peak; and h,the angle at the position of the peak maximum (in radians). Thepeak corresponding to Pt(111) at around 2h = 40� was used in thecalculation of the Lorentzian function. The calculated crystallitesizes were 12.9 nm for HP-Pt nanoparticles, 3.3 nm for Pt/C E-TEK,6.2 nm for Pt/C E-TEK (300), and 3.8 nm for Pt/C Premetek, whichare in good agreement (within the margin of error) with the valuesevaluated from the TEM images (Table 1). In the Rietveld refine-ment results, it is interesting to note that the lattice parameter ofthe HP-Pt nanoparticles is the smallest among those of the fourkinds of electrocatalysts studied. In addition, as shown Fig. 2b, thepeak position corresponding to hkl (111) for HP-Pt/C was shiftedto a higher angle in comparison with that for the other commercialsamples. This indicated that a compressive lattice strain was ex-erted on the lattice, which could lead to a change in the electronicstructure and thereby in the electrocatalytic activity [7].

3.2. Investigation of ORR activity

In order to investigate the ORR activity of the HP-Pt nanoparti-cles, we conducted thin-film rotating disk electrode (TF-RDE) mea-surements [24]. Fig. 3 shows the results of electrochemicalmeasurements for the HP-Pt nanoparticles and several commercialsamples. The specific and mass activities of the samples weredetermined by calculating jk, the mass-transport-corrected kineticcurrent. For this calculation, the surface area employed was theelectrochemical surface area (ECSA) calculated from the hydrogenadsorption/desorption region in CV, as follows [25]:

ECSA ¼ Q H

L� qHð2Þ

Fig. 1. TEM images of (a) Pt/C E-TEK (average size: 2.6 ± 1.2 nm), (b) Pt/C E-TEK (300) (average size: 5.1 ± 1.1 nm), (c) Pt/C Premetek (average size: 2.8 ± 0.9 nm), (d) HP-Ptnanoparticles/C, and (e and f) HRTEM image of HP-Pt nanoparticles (average size: 13.1 ± 2.7 nm longest axis, 100% dendritic shapes) with the inset of Fourier transformpattern for a single HP-Pt nanoparticle.

72 D.-S. Kim et al. / Journal of Catalysis 291 (2012) 69–78

where L is the catalyst loading (lg/cm2) on the working electrode;QH, the charge for the hydrogen adsorption (mC/cm2), calculated asthe mean value of the amounts of charge exchanged during theelectroadsorption and electrodesorption of H2 on the catalyst sitesafter double-layer correction [24]; and qH, the charge required tooxidize a monolayer of H2 on Pt (0.210 mC/cm2) [26].

As seen in Table 2, both the specific and mass activities of theHP-Pt nanoparticles were higher than those of the commercialsamples. At 0.9 V (versus RHE), the specific activity obtained forthe HP-Pt nanoparticles was 680.22 lA/cm2

Pt, whereas those forPt/C E-TEK, Pt/C E-TEK (300), and Pt/C Premetek were 213.13,84.08, and 223.35 lA/cm2

Pt, respectively. While the trend for themass activity was almost same as that for the specific activity,the degree of activity enhancement was less appreciable. It shouldbe noted that in the case of the Pt/C E-TEK (300) samples (heat-treated at 300 �C in a hydrogen atmosphere), the limiting currentcould not reach the theoretical value based on the Levich equationat 1600 rpm. In fact, for Pt/C E-TEK (300) homogeneous thin-filmdeposition of catalysts on the glassy carbon (working) electrodecould not be effected by the aqueous catalyst solution droppingmethod; this was because unlike in the case of the other samples,the surface of the carbon support after annealing was too inert forthe formation of a homogeneous catalyst solution in water. There-fore, the geometric area of the glassy carbon electrode was incom-pletely filled with the catalyst particles, so that the limiting currentcould not reach the theoretical value calculated from the Levichequation. Detailed results for the ORR activity evaluation at variousrotation speeds are provided in Fig. S1 of Supporting information.

As shown in Fig. 3c, the electrode current densities measured atconstant potentials were used to produce a Koutecky–Levich plot[27], which represents the inverse current density (j�1) as a func-tion of the inverse square root of the rotation rate (x�1/2) and indi-cates the first-order kinetics with respect to molecular oxygen. TheKoutecky–Levich equation is written as

1j¼ 1

jkþ 1

jd¼ 1

jkþ 1

Bx1=2 ð3Þ

in which

B ¼0:62nFACO2 D2=3

O2

g1=6 ð4Þ

where j is the experimentally obtained current; jk, the kinetic cur-rent; jd, the diffusion-limited current; n, the number of electronstransferred; F, the Faraday constant (F = 96485.3399 C/mol); A, thegeometric area of the electrode (A = 0.19625 cm2); CO2, the O2 con-centration in the electrolyte (CO2 = 1.26 � 10�3 mol/L); DO2 the dif-fusion coefficient of O2 in HClO4 solution (DO2 = 1.93 � 10�5 cm2/s);and g, the viscosity of the electrolyte (g = 1.009 � 10�2 cm2/s) [28].The slope of the lines (j�1 versus x�1/2) is 1/B. The calculated valueof B was 0.0915 mA s�1/2 for the four Pt/C catalysts, and the numberof transferred electrons was calculated to be four for these fourkinds of Pt electrocatalysts.

The rate-determining step (RDS) in the ORR is the one-electron-transfer reaction shown below in reaction (5), for which the theo-retical Tafel slope is estimated to be �118 mV/decade on the basisof the assumptions that the onset potential is independent of thecoverage of surface oxygen species and that the cathodic transfercoefficient is 0.5 [29].

Sþ O2 þHþ þ e� ! S� O2H ð5Þ

The Tafel slope under acidic conditions, in particular in an aqueousHClO4 solution, in which the anion is hardly adsorbed on the elec-trode surface, is generally divided into two regions: �59 mV/decadeat higher potentials (about 1.0–0.8 V versus RHE) and�118 mV/dec-ade at potentials low enough for the formation of an oxide-free sur-face. It is well known that such a deviation in the Tafel slope at highpotentials in aqueous HClO4 solution arises from the Temkin condi-tions, where the potential is dependent on the surface coverage of

30 40 50 60 70 80 90 100 110 120 130

Pt/C E-TEK

Pt/C E-TEK (300)

Pt/C Premetek

(200

)

(111

) observed Rietveld refinement

Inte

nsity

(arb

itrar

y un

it)

2θ

2θ

HP-Pt/C

(420

)

(331

)

(222

)(311

)

(220

)

30 40 50 60

Pt/C E-TEK

Pt/C E-TEK (300)

Pt/C Premetek

HP-Pt/C

(200

)

(111

)

Diff

ract

ion

inte

nsity

(a)

(b)

Fig. 2. (a) HRPD patterns for Pt peaks with Rietveld refinement. The reflectionpeaks, indexed as (111), (200), (220), (311), (222), (331), and (420), correspondto face-centered cubic Pt (PDF standard cards, JCPDS 10-1311, space groupFm3m[225]). (b) Comparison of magnified peak of hkl (111).

D.-S. Kim et al. / Journal of Catalysis 291 (2012) 69–78 73

OH produced by water dissociation [30]. Density functional calcula-tions have shown that the ORR activation energy at the neighboringsites of an adsorbate molecule increases because the identical direc-tions of the induced dipole moments for the oxygen molecules andthe adsorbate (OH) species give rise to repulsive forces betweenthem [31]. Therefore, the potential varies with surface coverage inthis range, and the Tafel slope is not�118 mV/decade (the value ob-tained for the one-electron-transfer reaction (RDS), wherein there isno dependence of the potential on surface coverage) but �59 mV/decade. The surface oxide is gradually reduced with the cathodicscan, and eventually, the transition occurs at the point where thesurface OH coverage cannot have any effect on the shift of the poten-tial. In other words, it occurs at the potential of the transition fromTemkin to Langmuirian conditions. Hence, a catalyst with a more po-sitive transition potential can show faster ORR kinetics in aqueousHClO4 solution. As shown in Fig. 3d, the transition potential for theHP-Pt nanoparticles is 0.92 V (versus RHE), which is the highest

Table 1Average particle size and lattice parameter of catalysts from HRPD data shown i

Catalyst Average particlesize (nm)

Lattice par(Å)

Pt/C E-TEK 3.3 3.91Pt/C E-TEK (300) 6.2 3.92Pt/C Premetek 3.8 3.93HP-Pt/C 12.9 3.91

among those of all the samples tested. Therefore, it is once againproved that the HP-Pt nanoparticles show the fastest kinetics andare therefore the best electrocatalysts for ORR among these foursamples.

3.3. Discussion of enhanced ORR activity

It is well known that the catalytic activity is significantly af-fected by the surface strain that results from changes in the sizeor shape of catalyst particles or from alloy formation with a basemetal that has a different bulk Wigner–Seitz radius [32–34]. Thisis because the adsorption strength of the adsorbate in the transi-tion state, the key factor determining the catalytic activity, canbe tuned with the d-band structure of the catalyst particles, whichin turn varies markedly with surface strain. On the basis of the sur-face strain effect, we found a clue to the origin of the enhanced ORRactivity of the HP-Pt nanoparticles in the EXAFS measurements. Asseen in Fig. 4a, the peak at around 2.6 Å, corresponding to the Pt–Ptfirst nearest neighbor (1NN) of the radial distribution function(RDF) for the HP-Pt nanoparticles, was clearly shifted to a lower rvalue as compared to that of the commercial samples, indicatingthat compressive strain was exerted on the lattice of these HP-Ptnanoparticles. Indeed, the r value for the peak position in the EX-AFS RDF is incorrect because of the phase shift for the absorber–neighbor pair, and the exact bond length for the Pt–Pt 1NN shouldbe determined by fitting to the theoretical model using the widelyavailable FEFF code. The obtained bond length was 2.754 Å for theHP-Pt nanoparticles, which was about 0.013 Å shorter than that forthe Pt/C E-TEK commercial sample. The detailed EXAFS parameters,such as coordination number, energy shift, and Debye–Waller(DW) factor, can be found in Table 3. In general, it is widely recog-nized that a smaller particle has a shorter bond distance owing tothe increased surface tension [35,36]. Hence, there is a clear incon-sistency between our data and this general phenomenon. We be-lieve that this discrepancy is attributable to three factors: thehighly porous urchin-like morphology, the single-crystalline phase,and the lower degree of surface oxidation for HP-Pt nanoparticles.Even though the HP-Pt nanoparticle size is about 13 nm, the sur-face-to-volume ratio is quite higher than that of a spherical particlebecause of the highly porous urchin-like morphology. It has beenreported that a porous structure can lead to compressive strainin the lattice and thereby result in shorter bond lengths [37]. Inaddition, in spite of their somewhat complicated and porousshapes, which can readily lead to high structural disorder andcan be formed from the agglomeration of several primary particles,the HP-Pt nanoparticles are in the single-crystalline phase ratherthan a polycrystalline phase composed of primary particles. In gen-eral, the crystalline phase has both shorter bond lengths and lowerDW factors than the amorphous phase [38]. In addition, the DWfactor for smaller spherical nanoparticles with shorter bondlengths is usually higher than that for larger particles [39,40]; nev-ertheless, the DW factor for the HP-Pt nanoparticles is low, in spiteof their short bond lengths. This is because the HP-Pt nanoparticlesare in the well-ordered single-crystalline phase. Hence, we believethat the bond length of the HP-Pt nanoparticles is anomalouslysmall in spite of their relatively large size because of the synergistic

n Fig. 2.

ameter Reliability factors

Rp = 45.20%, Rwp = 40.10%, Rexp = 35.10%, S = 1.14Rp = 27.80%, Rwp = 24.40%, Rexp = 15.67%, S = 1.56Rp = 27.20%, Rwp = 25.70%, Rexp = 16.21%, S = 1.59Rp = 24.60%, Rwp = 23.20%, Rexp = 14.88%, S = 1.56

-1.0

-0.8

-0.6

-0.4

-0.2

0.0

0.2

0.4

0.6(a)

Pt/C Premetek HP-Pt/C

j /m

A c

m-2

E / V (vs RHE)0.0 0.2 0.4 0.6 0.8 1.0 1.2 0.2 0.4 0.6 0.8 1.0 1.2

-6

-5

-4

-3

-2

-1

0(b)

Pt/C E-TEK Pt/C E-TEK (300) Pt/C Premetek HP-Pt/C

j /m

A c

m-2

E / V (vs RHE)

-1 0 1

0.75

0.80

0.85

0.90

0.95

1.00

1.05

Pt/C E-TEK Pt/C E-TEK (300) Pt/C Premetek HP-Pt/C

(d)

-118 mV/dec

E/V

(vs

RH

E)

log (jk/mA cm-2)

-59 mV/dec

0.0 0.1 0.2 0.30.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

2 e-

(c)

Pt/C E-TEK Pt/C E-TEK (300) Pt/C Premetek Pt nanodendrites/C

j-1/(m

A c

m-2)-1

ω-1/2 / (rad/s)-1/2

4 e-

Fig. 3. (a) CV curves for Pt/C E-TEK and HP-Pt/C nanoparticle catalysts in N2-saturated 0.1 M HClO4 solution with a scan rate of 20 mV/s. (b) ORR polarization curves for Pt/C E-TEK, Pt/C E-TEK (300), Pt/C Premetek, and HP-Pt/C in O2-saturated 0.1 M HClO4 solution with sweep rate of 5 mV/s and rotation of 1600 rpm. (c) Koutecky–Levich plots forORR at 0.8 V (versus RHE). (d) Tafel plots for ORR (dashed lines indicate transition potentials from low Tafel slope to high Tafel slope).

Table 2Half-wave potential (at 1600 rpm), ECSA and ORR activities at 0.9 V (versus RHE) for commercial Pt/C catalysts and HP-Pt/C.

Catalyst Half-wave potential (V versus RHE) LPt (lgPt/cm2) ECSA (m2/gPt) Mass activity (lA/lgPt) Specific activity (lA/cm2Pt)

Pt/C E-TEK 0.85 20 65.07 144.28 213.13Pt/C E-TEK (300) 0.79 20 36.37 34.63 84.08Pt/C Premetek 0.85 20 51.08 119.52 223.35HP-Pt/C 0.88 20 27.89 204.05 680.22

74 D.-S. Kim et al. / Journal of Catalysis 291 (2012) 69–78

effect caused by the high surface-to-volume ratio, which in turn isdue to the exotic urchin-like morphology and the well-ordered sin-gle-crystalline phase. Another possible reason for the anomalousshort bond length of the HP-Pt nanoparticles is the low degree ofsurface oxidation resulting from the relatively larger particle size.It has been reported that a high oxidation state of Pt has the effectof increasing the Pt–Pt bond length [41]. Therefore, the high oxida-tion state of the Pt surface compared to that of the HP-Pt nanopar-ticles can result in a relatively longer Pt–Pt bond. A lower oxidationstate for the HP-Pt nanoparticles can also help in lowering thebond length, although this effect is not so critical. The compressivestrain exerted on the lattice of these HP-Pt nanoparticles is conse-quently thought to be the main origin of the enhanced ORR activ-ity. The increased overlap of the d-band electrons between themetal atoms, brought about by the compressive strain, results inbandwidth broadening, and in order to keep the d-band occupancyfixed, the energy of the d-band electron moves down from the Fer-mi level. The downshifted d-band center position weakens theadsorption of atomic oxygen, and the ORR activity is enhancedby the decreased overpotential [32–34]. In summary, the originof the enhanced ORR activity of the HP-Pt nanoparticles is their un-

ique, highly porous and urchin-like dendritic structure, which cangive rise to compressive strain owing to the high surface-to-vol-ume atomic ratio.

In addition to the compressive strain effect, the high proportionof advantageous facets for the ORR, such as Pt(111), in the HP-Ptnanoparticles can be considered another reason for the enhancedORR activity. It is interesting to note that while the mass activityfor the HP-Pt nanoparticles was only about 1.4 times higher thanthose of the commercial samples, the specific activity was overfour times higher, as shown in Table 2. This result can be attributedto the fact that the surface of the HP-Pt nanoparticles consistsmainly of facets that are advantageous for ORR, like Pt(111) orPt(110), even though the large particle size leads to a small ECSA.As shown in Fig. 3a, the shape of the CV curve in the hydrogenadsorption/desorption potential region is more rectangular forthe HP-Pt nanoparticles than for the commercial samples. This ischaracteristic of the Pt(111) facet showing the best ORR activityin an acid medium containing non-adsorbing anions [42]. The highproportion of Pt(111) can be also identified from the HRTEM im-age, as shown in Fig. 1f. Accordingly, while the increase in activityper Pt mass unit was not so high, a massive increase appeared in

11540 11560 11580 11600 11620

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8(b)

(a)

Pt/C E-TEK Pt/C E-TEK (300) Pt/C Premetek HP-Pt/CN

orm

aliz

ed a

bsor

banc

e (a

.u)

Energy (eV)

Fig. 4. (a) Fourier transforms and (b) XANES data of the Pt L3edge for Pt/C E-TEK, Pt/C E-TEK (300), Pt/C Premetek, and HP-Pt/C.

Table 3Best-fit structural parameters for Pt–Pt1NNs obtained from analysis of the Pt L3edgeEXAFS spectra.

Catalyst N Rj (Å) r2 (�10�6) (A2) r-Factor (�10�2)

Pt/C E-TEK 9.1 2.76 (3) 5.1 1.89Pt/C E-TEK (300) 10.5 2.77 (3) 4.6 1.67Pt/C Premetek 9.2 2.76 (3) 4.9 1.95HP-Pt/C 10.8 2.75 (4) 4.1 1.75

D.-S. Kim et al. / Journal of Catalysis 291 (2012) 69–78 75

the activity per Pt surface area unit. This is in good agreement witha previous report, which stated that the specific activity for ORR in-creases with the proportion of low-index planes and particle size[43]. Furthermore, Markovic et al. showed that the order of ORRactivity of the three low-index facets was Pt(111) P (110)�(100) in perchloric acid [42]. Therefore, we can conclude that thehigh proportion of advantageous facets like Pt(111) in the HP-Ptnanoparticles is another clear reason for their enhanced ORRactivity.

The small proportion of low-coordinated sites can be consid-ered the third origin of the enhanced ORR activity of the HP-Ptnanoparticles; this is because a small proportion of these sitescan result in decreased surface oxidation by OH adsorption as iden-tified from the XANES and EXAFS results depicted in Fig. 4. It iswidely recognized that for small particle sizes, the proportion oflow-coordinated sites increases on the surface, and such sites can

hinder the ORR activity by acting as blocking sites with very strongOH adsorption. Mayrhofer et al. reported that the ORR specificactivity decreases for small particles with high oxophilicity, be-cause the active sites for oxygen adsorption or O–O splitting areeffectively blocked by the strong OH adsorption at these low-coor-dinated sites [43]. As shown in Fig. 4b, the white line for the com-mercial samples is much higher and the edge energy (E0) is shiftedto the higher side as compared to that for the HP-Pt nanoparticles.Furthermore, there is a strong peak at around R = 1.6 Å in the RDF(without consideration of the phase shift in the EXAFS model), cor-responding to the Pt–O 1NN peak, for two types of commercialsamples (Pt/C E-TEK and Premetek), as seen in Fig. 4a. These resultsimply that the commercial samples with sizes of 3–4 nm have ahigh proportion of low-coordinated sites at which OH is prone tobe adsorbed by the dissociation of water in the air. However, afterthe samples were annealed in a reductive atmosphere, the strongpeak corresponding to surface oxidation in the EXAFS RDF de-creased in size because of the reduction of both the amount of sur-face hydroxide and the number of unstable low-coordinated sites.In the case of the HP-Pt nanoparticles, the number of low-coordi-nated sites is small because of the lowest-intensity white lineand the lowest absorption energy (E0) in the XANES spectra andthe absence of the oxidation peak in the EXAFS RDF. Hence, in sum-mary, the origins of the enhanced ORR activity for HP-Pt nanopar-ticles are the compressive surface strain leading to a downshift ofthe d-band center position, the high proportion of advantageousfacets like Pt(111), and the small proportion of low-coordinatedsites acting as ORR blocking sites.

3.4. Investigation of stability against Pt dissolution

Apart from electrocatalytic activity, the durability problem forelectrocatalysts is one of the most significant hurdles to be over-come in the commercialization of fuel cells. It is widely recognizedthat the degradation mechanisms of Pt-based cathode electrocata-lysts can be categorized into four types: Pt dissolution, Pt agglom-eration (sintering), carbon corrosion, and base-metal leaching foralloy electrocatalysts [44]. Among these, the issues most closely re-lated to this study are Pt dissolution and sintering, because theelectrocatalysts adopted for this investigation are Pt-only nanopar-ticles on the same carbon black support (Vulcan XC-72). Notably,many works have indicated that the growth of the catalyst particlesize after fuel cell operation is due to the Ostwald ripening causedby dissolution and redeposition, rather than coarsening throughmigration [45,46]. Because of this, we investigated in detail the sta-bility of the nanoparticles against Pt dissolution by accelerated po-tential cycling between 0.6 and 1.1 V (versus RHE). The stabilitywas determined by comparing the ECSA before and after cycling.As seen in Fig. 5, for the HP-Pt nanoparticles, 27.3% of the ECSAis lost because of Pt dissolution after 4000 cycles, while the lossesfor E-TEK, E-TEK (300), and Premetek are 47.8%, 35.7%, and 53.1%,respectively. This result indicates that the HP-Pt nanoparticleshave better durability as well as higher activity for the ORR thando the commercial samples tested.

3.5. Discussion of enhanced stability against Pt dissolution

It is well known that Pt dissolution is the most serious degrada-tion factor at intermediate or high (�0.8 V versus RHE) potentialsfor Pt/C electrocatalysts [47]. The following three reactions canbe considered as the origin of Pt dissolution [48]:

Pt! Pt2þ þ 2e� ðPt dissolutionÞ ð6ÞPtþH2O! PtOþ 2Hþ þ 2e� ðPt oxide formationÞ ð7ÞPtOþ 2Hþ ! Pt2þ þH2O ðPt oxide dissolutionÞ ð8Þ

0.0 0.2 0.4 0.6 0.8 1.0 1.2-0.5

-0.4

-0.3

-0.2

-0.1

0.0

0.1

0.2

0.3

0.4Pt/C E-TEK (300)

Initial state After 4000 cycles

(b)

35.7% loss of ECSA

j /m

A c

m-2

E / V (vs RHE)0.0 0.2 0.4 0.6 0.8 1.0 1.2

-2.0

-1.5

-1.0

-0.5

0.0

0.5

1.0

1.5 Pt/C E-TEK(a)

Initial state After 4000 cycles

j /m

A c

m-2

47.8% loss of ECSA

E / V (vs RHE)

0.0 0.2 0.4 0.6 0.8 1.0 1.2

-0.5

-0.4

-0.3

-0.2

-0.1

0.0

0.1

0.2

0.3 HP-Pt/C

Initial state after 4000 cycles

(d)

27.3% loss of ECSA

j/m

A c

m-2

E / V (vs RHE)0.0 0.2 0.4 0.6 0.8 1.0 1.2

-1.0

-0.8

-0.6

-0.4

-0.2

0.0

0.2

0.4

0.6

0.8Pt/C Premetek

Initial state After 4000 cycles

(c)

53.1% loss of ECSA

j /m

A c

m-2

E / V (vs RHE)

Fig. 5. CV Curves for (a) Pt/C E-TEK catalyst, (b) Pt/C E-TEK (300), (c) Pt/C Premetek, and (d) HP-Pt/C before and after 4000 cycles of accelerated durability tests.

11560 11580 116000.0

0.4

0.8

1.2(b)

HP-Pt/C

0.47 V 0.77 V 0.97 V 1.07 V

Nor

mal

ized

abs

orba

nce

Energy (eV)11560 11580 11600

0.0

0.4

0.8

1.2

Δμ 1

.07

V

0.47 V 0.77 V 0.97 V 1.07 V

Nor

mal

ized

abs

orba

nce

Energy (eV)

Pt/C E-TEK

Δμ 0

.47

V

(a)

(d)

0.4 0.6 0.8 1.0 1.2

0.00

0.05

0.10

0.15

0.20

0.25(c)

Δ(μ−

μ 0.47

V)/Δ

μ 0 .47

V

E / V (vs. RHE)

Pt/C E-TEK HP-Pt/C

Fig. 6. XANES spectra obtained with (a) Pt/C E-TEK and (b) HP-Pt/C nanoparticles at the Pt L3 edge at different potentials. (c) Comparison of the change of the absorption edgepeaks of the XANES spectra for Pt/C E-TEK and HP-Pt/C as a function of potential, obtained with electrocatalysts at different potentials in 0.1 M HClO4. (d) Homemademeasurement cell used for the in situ XANES experiments.

76 D.-S. Kim et al. / Journal of Catalysis 291 (2012) 69–78

70 72 74 76 78

70 72 74 76 78

Pt4f5/2

Binding Energy (eV)

Pt4f7/2

Pt/C E-TEK Pt/C E-TEK (300) Pt/C Premetek HT-Pt/C

Nor

mal

ized

Inte

nsity

Binding Energy (eV)

Fig. 7. XPS spectra for Pt4f peak. The inset plot shows the XPS spectrum fitting ofthe Pt4f photoemission from HP-Pt/C.

D.-S. Kim et al. / Journal of Catalysis 291 (2012) 69–78 77

Dissolved Pt is redeposited onto a larger neighboring particle byOstwald ripening or precipitated on the proton exchangemembrane by the hydrogen molecules permeating from the anode.Pt loss occurs through these processes, and thus the performance ofthe fuel cell gradually deteriorates.

The highly improved stability against Pt dissolution in the caseof the HP-Pt nanoparticles can be explained from two viewpoints:the electronic energy state of the surface and the unique morphol-ogy with large primary particle size. First, strong evidence for theelectronic energy state was obtained from the XANES experimentsperformed with potential steps applied using a specially designedhomemade cell (the details are shown in Fig. 6d). The white linein the XANES is a strong peak due to the dipole-allowed transitionof the core electron to the continuum state. Hence, the white linewould increase remarkably for a catalyst that is prone to ioniza-tion, or, in other words, is easily dissolved upon applying a poten-tial. Fig. 6 shows the change in the white line at the Pt L3

absorption edge; this change is induced upon applying a potentialto cause electronic excitation from the Pt 2p3/2 core orbital formedthrough spin–orbit coupling to the Pt d-band vacancy. It is interest-ing to note that the degree of white-line increase at high potentialsfor the HP-Pt nanoparticles is the lowest among that for all thesamples, implying that these nanoparticles are the most stable cat-alysts against Pt dissolution. That is to say, the HP-Pt nanoparticles

Table 4Distribution of Pt 4f7/2 species and relative intensities.

Sample Assigned chemicalstate

Binding energy(eV)

Relativeintensity (%)

Pt/C E-TEK Pt(0) 71.0 27.1Pt(II) 71.8 8.8Pt(IV) 72.7 20.3

Pt/C E-TEK (300) Pt(0) 71.0 30.0Pt(II) 71.8 13.6Pt(IV) 72.7 12.9

Pt/C Premetek Pt(0) 71.0 28.6Pt(II) 71.8 14.0Pt(IV) 72.7 6.4

HP-Pt nanoparticles/C Pt(0) 71.0 39.4Pt(II) 71.8 5.7Pt(IV) 72.7 8.1

require a much higher oxidation potential for dissolution, andtherefore, this is the decisive origin of their increased durability.

Such improved electron retention properties at high potentialscan be attributed to the surface conditions of the HP-Pt nanoparti-cles. As shown in Fig. 7 and Table 4, the proportion of fully reducedPt is much higher in the HP-Pt nanoparticles, which have fewerlow-coordinated sites on the surface, than in the other samples.This is in good agreement with the results found in the literature[44]; the surfaces with a high proportion of low-coordinated sitesshow a high oxidation state owing to an increase in the number ofinitially adsorbed oxygen species. This phenomenon can readily beidentified in the Pt/C E-TEK (300) heat-treated sample, for whichthe EXAFS RDF peak corresponding to the Pt–O 1NN disappearedafter heat treatment. Hence, the low proportion of low-coordinatedsites for HP-Pt nanoparticles can be considered to be the main ori-gin of their enhanced electron retention properties and the elec-tron-rich phase on their surface.

The above conclusion can be further supported by the fact thatthe shape change in the CV curve in the region of OH formation(around 0.6 V) is more distinct in the commercial samples, whichhave a higher proportion of low-coordinated sites such as step-edge, kink, and corner sites because of their smaller particle size,as indicated by the arrows in Fig. 5. This is because such low-coor-dinated sites are more prone to be dissolved during potential cy-cling than high-coordinated sites such as terrace sites. Hence, thepotential for OH formation is greatly shifted to the positive side.In contrast, the current profile change of OH formation for theHP-Pt nanoparticles is relatively smaller after cycling, demonstrat-ing that the nanoparticle surface is originally composed ofhigh-coordinated sites and can therefore be more tolerant todissolution.

The final state effect may be another significant reason for theenhanced electron retention characteristics observed at small pro-portions of low-coordinated sites. It is widely recognized that it ismuch more difficult for small particles to make up for the holesgenerated by photoelectron emission because of their discreteelectronic structure; therefore, the binding energy is shifted to ahigher value in X-ray photoemission spectra [49]. In the case ofthe HP-Pt nanoparticles, since the primary particle size is large,the final state effect can be minimized because of the absence ofsuch a discrete electronic structure; consequently, a high propor-tion of the electron-rich phase is obtained, as indicated by the Pt4f core peak.

4. Conclusions

The HP-Pt nanoparticles synthesized using a weakly interactingorganic capping agent, TTAB, showed enhanced ORR activity andstability against Pt dissolution. The origins of their enhanced ORRactivity and stability were identified by performing EXAFSmeasurements and in situ XANES studies. The high proportion ofsurface atoms (due to the urchin-like morphology) in the single-crystalline phase, despite the large primary particle size, led tothe generation of compressive strain through the high surface ten-sion; therefore, the ORR activity was enhanced by the downshiftedd-band center position. The unique urchin-like morphology alsoprovided a high proportion of facets advantageous for the ORR,such as Pt(111) and (110). In terms of durability, the stabilityagainst Pt dissolution was markedly enhanced because of the im-proved electron retention property and the decreased proportionof low-coordinated sites, which played key roles in the upshift ofthe oxidation potential in comparison with that of the sphericallyshaped commercial samples. From the above discussion, we canconclude that the highly porous urchin-like morphology is a prom-ising structure for ORR electrocatalysts used in PEMFCs.

78 D.-S. Kim et al. / Journal of Catalysis 291 (2012) 69–78

Acknowledgments

This research was supported by a National Research Foundationof Korea Grant funded by the Korean Government (2011-0003334,2011-0027954, and 2011-0030658) and a Grant (M2009010025)from the Fundamental R&D Program for Core Technology of Mate-rials funded by the MKE, Republic of Korea.

Appendix A. Supplementary material

Supplementary data associated with this article can be found, inthe online version, at http://dx.doi.org/10.1016/j.jcat.2012.04.004.

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