Defects do Catalysis: CO Monolayer Oxidation and ... - ACS Publications

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Defects do Catalysis: CO Monolayer Oxidation and Oxygen Reduction Reaction on Hollow PtNi/C Nanoparticles Laetitia Dubau,*,†,‡ Jaysen Nelayah,§ Simona Moldovan,∥ Ovidiu Ersen,∥ Pierre Bordet,⊥,# Jakub Drnec,∇ Tristan Asset,†,‡ Raphael̈ Chattot,†,‡ and Frédéric Maillard*,†,‡ †

Université Grenoble Alpes, LEPMI, F-38000 Grenoble, France CNRS, LEPMI, F-38000 Grenoble, France § Laboratoire Matériaux et Phénomènes Quantiques (MPQ), UMR 7162, CNRS & Université Paris-Diderot, Bâtiment Condorcet, 4 rue Elsa Morante, F-75205 Paris Cedex 13, France ∥ Institut de Physique et Chimie des Matériaux de Strasbourg (IPCMS), UMR 7504, CNRS-Université de Strasbourg (UdS), 23 rue du Lœss, Cedex 2 Strasbourg, France ⊥ Université Grenoble Alpes, Institut Néel, F-38000 Grenoble, France # CNRS, Institut Néel, F-38000 Grenoble, France ∇ European Synchrotron Radiation Facility, ID 31 Beamline, BP 220, F-38043 Grenoble Cedex, France ‡

S Supporting Information *

ABSTRACT: The catalytic performance of extended and nanometersized surfaces strongly depends on the amount and the nature of structural defects that they exhibit. However, whereas the effect of steps or adatoms may be unraveled with single crystals (“surface science approach”), implementing reproducibly in a controlled manner structural defects on nanomaterials remains hardly feasible. A case that deserves particular attention is that of bimetallic nanomaterials, which are used to catalyze the oxygen reduction reaction (ORR) in proton exchange membrane fuel cells (PEMFC). Point defects (vacancies), planar defects (dislocations and grain boundaries), and bulk defects (voids, pores) are likely to be generated in alloy or core@shell nanomaterials based on Pt and a transition metal due to the high lattice mismatch between the two elements. Here, we report the morphological and structural trajectories of hollow PtNi/C nanoparticles during thermal annealing under vacuum, N2, H2, or air atmosphere by in situ transmission electron microscopy and synchrotron X-ray diffraction. We evidence atmospheredependent restructuring kinetics, which enabled us to synthesize a set of catalysts with identical chemical compositions and elemental distributions but different morphologies, crystallite sizes, and lattice strain. By combining the results of Rietveld and pair-distribution function analyses and electrochemical measurements, we demonstrate that the structurally disordered areas located at the interface between individual crystallites are highly active for two reactions of interest for PEMFC devices: the electrochemical COads oxidation and the ORR. These results shed fundamental light on the effect of structural defects on the catalytic performance of bimetallic nanomaterials and should aid in the rational design of more efficient ORR electrocatalysts. KEYWORDS: proton exchange membrane fuel cell, oxygen reduction reaction, platinum−nickel alloy, hollow nanoparticles, structural defects, grain boundary, vacancy

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INTRODUCTION In both gas-phase and liquid catalysis, the reactivity of extended and nanometer-sized surfaces strongly depends on their crystallographic orientation, the average coordination number of the atoms, and the amount and type of structural defects that they exhibit.1−4 In that respect, the structure sensitivity of platinum (Pt) and platinum-based alloys (Pt-alloy) for the electrochemical oxidation of COads is particularly illustrative. This two-electron reaction can be performed either in the absence of CO in solution (“COads stripping” experiments) or in its presence (“bulk CO” electrooxidation experiments). © 2016 American Chemical Society

There are evidences that open, low-density Pt(110) and Pt(100) facets are more active than the close-packed Pt(111) surface in COads stripping experiments.5 The influence of structural defects is well-established for this reaction: on stepped Pt[n(111)x(111)] single crystals, the onset and the peak potential of COads electrooxidation shift toward negative potentials with an increase in the step fraction.6,7 This indicates Received: April 18, 2016 Revised: June 2, 2016 Published: June 7, 2016 4673

DOI: 10.1021/acscatal.6b01106 ACS Catal. 2016, 6, 4673−4684

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ACS Catalysis that (i) steps are the “active sites” where water is dissociated into oxygen-containing species (referred to as OHads in what follows), which are necessary to complete the COads electrooxidation, and that (ii) adsorbed CO molecules diffuse rapidly to these active sites. Note also that the structure sensitivity is maintained in the presence of CO in the electrolyte.8 At the nanometer scale, and in contrast to what could be expected on the basis of single-crystal studies, the onset and the peak potential in COads stripping voltammograms shift toward positive potential when the fraction of highly undercoordinated atoms increases (that is, when the size of the Pt nanocrystallites decreases).9−12 This is related to the decrease of the COads diffusion coefficient and of the rate of the COads + OHads recombination on Pt nanocrystallites with size less than 3−4 nm.9,13 Strikingly, the “negative” particle size effect levels off upon nanoparticle aggregation. The kinetics of the CO electrooxidation9−11,14 and of more complex molecules, including formic acid,15 methanol,14−16 and ethylene glycol,17,18 is significantly enhanced on “multi-grained” nanostructures formed by agglomerated Pt/C nanoparticles. The enhancement in catalytic activity has been ascribed to the structurally disordered areas formed at the interface between crystal grains (also referred to as “grain boundaries” in what follows). Interestingly, the same holds true when a more oxophilic metal, such as Ru or Sn, is alloyed to Pt: “multi-grained” Pt-alloy nanoparticles remain intrinsically more active than isolated Ptalloy nanoparticles for the electrooxidation of COads,19−22 methanol,20,23 and ethanol,18,24 confirming that grain boundaries play a key, yet underestimated, role in electrocatalysis. Structural defects also influence the rate of electroreduction reactions. On vicinal Pt surfaces, the kinetics of the oxygen reduction reaction (ORR) increase with an increase in the step density, irrespective of the step site symmetry.25−30 This effect was recently rationalized by Bandarenka et al.,30 who postulated that the OHads binding energy on (111) terraces is weakened when the step density increases. Similarly to what was reported for electrooxidation reactions, the catalytic activity for the ORR is depreciated when going from bulk Pt surfaces to nanoparticles (the specific activity decreases by a factor of 4 going from bulk Pt to 2 nm sized crystallites31,32). However, even if the beneficial effect of steps and kinks on the ORR rate has been documented for nanocatalysts,33,34 the impact of grain boundaries, vacancies, and cavities remains understudied. Recently, Wang et al.35 loaded carbon nanotubes with different Pt mass fractions (in the range of 10−93 wt %). The authors reported aggregation of the Pt/C nanoparticles at mass fraction greater than 20 wt % concomitant with a positive shift of the half-wave potential in ORR curves (enhanced catalytic activity). However, these results should be taken with caution, since the Pt loadings (the mass of Pt per geometrical surface area) were not identical in the different experiments. Using the same approach, Nesselberger et al.36−38 evidenced an increased specific activity for the ORR on highly loaded Pt/C nanoparticles. These findings were rationalized by considering the overlap of the electrical double layers between adjacent Pt nanoparticles and the associated change of the electrochemical potential at the compact layer,36,39 but the possible link between enhanced ORR electrocatalytic activity and grain boundaries was not considered. In addition to grain boundaries, recent ab initio calculations2,3,40 and experiments from Adzic’s group41 and from the authors42−45 underlined the beneficial effect of point (vacancies) and bulk defects (cavities; i.e., a cluster of vacancies) on the ORR kinetics. By means of density

functional theory (DFT) calculations, Calle-Vallejo et al.3 showed that cavities modify the average coordination number of Pt atoms and enhance their catalytic activity for the ORR by a factor close to 3 relative to a cavity-free Pt surface. This is in line with the DFT findings of Callejas-Tovar et al.40 that subsurface vacancies compress the Pt lattice parameter and reduce the affinity for oxygenated species, thereby enhancing the ORR kinetics on Pt-based catalysts. Here, hollow nanoparticles composed of a PtNi shell surrounding a hollow core were synthesized by a combination of galvanic replacement and nanoscale Kirkendall effects (a vacancy-mediated diffusion mechanism in binary alloys where one species diffuses faster than the other). Using highresolution transmission electron microscopy (HR-TEM), we show that the PtNi shell is highly disordered and composed of nanocrystallites interconnected by structurally disordered areas (“grain boundaries”). Thermal treatments under different atmospheres (vacuum, N2, H2, or air) were used to heal these structural defects in a controlled manner, resulting eventually in the collapsing of the hollow nanoparticles. This provided us with a set of catalysts featuring identical chemical compositions and elemental distributions but different crystallite sizes, lattice strains, and morphologies, which was used to shed light on the structure sensitivity of the electrochemical COads oxidation and the ORR.

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RESULTS AND DISCUSSION Synthesis and Characterization of Hollow PtNi/C Nanoparticles. Hollow PtNi nanoparticles supported on high surface area carbon with a Ni fraction of 15 at. % were synthesized via a one-pot method first introduced by Bae et al.46 During the synthesis, Ni/C or Ni-B/C nanoparticles are believed to form first47 and then act as sacrificial templates for the deposition of Pt2+ ions via galvanic replacement (a simple and spontaneous electrochemical process where the oxidation of Ni atoms provides electrons for the deposition of Pt2+ atoms). The continuous corrosion of Ni atoms progressively creates a gradient in chemical potential between the core (rich in Ni) and the external surface (where Ni atoms are leached out) and acts as a strong driving force for Ni atoms to diffuse outward. This in turn drives a flux of vacancies in the opposite direction, which coalesce in the center of the material (“nanoscale Kirkendall effect”) and ultimately results into hollow PtNi/C nanoparticles with a cavity similar in size/shape to that of the initial Ni or Ni-B/C nanoparticles.48−53 The scenario described above is supported by the high angle annular dark field in scanning transmission electron microscopy (HAADF-STEM) images and the X-ray energy dispersive spectroscopy (X-EDS) chemical maps displayed in Figure 1. In particular, the presence of a central void in the as-synthesized PtNi/C nanoparticles and the homogeneous distribution of Pt and Ni atoms in the outer shell are clear evidences that the nanoscale Kirkendall effect operated during the synthesis. Figure 1 also shows the morphological changes of hollow PtNi/C nanoparticles upon thermal annealing at T = 400 °C under N2, H2, or air atmosphere. Some chemical and structural parameters of the as-synthesized hollow and thermally annealed PtNi/C nanoparticles, in particular the chemical composition determined by X-EDS on hundreds of nanoparticles (lowmagnification TEM images) or on individual nanoparticles (high-magnification STEM-XEDS images), are displayed in Table 1. 4674


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size, indicating that the Pt atoms strained to accommodate the lattice of the Ni-rich/C sacrificial template. Post-synthesis thermal treatments were performed to control the concentration of these structural defects, following the approach described by Wang et al.54 for Pt3Co/C nanoparticles. The authors reported that (i) surface atoms relaxed at 300 < T < 400 °C and (ii) Pt and Co atoms segregate in the topmost and second surface layers at T > 400 °C. Similar results by other groups validated the thermal-annealing approach for various Pt-based alloys.54−58 Figure 4 illustrates the morphological changes of hollow PtNi/C nanoparticles upon annealing under vacuum. Minor changes in morphology were noticed for T < 400 °C. A nanoscale secondary void (indicated by arrows in Figure 4d) formed at T ≥ 400 °C, concomitant with a change in size and shape of the central cavity. This void then interdiffused to the outer surface at 500 °C and annihilated at 600 °C, in agreement with the Kirkendall theory.48,49 The hollow PtNi/C nanostructures collapsed between 600 and 700 °C, resulting in the formation of solid PtNi/C nanoparticles. During this process, the number of individual grains was divided roughly by a factor of 3, in agreement with the ex situ HR-TEM images displayed in Figure 2. The restructuring kinetics were found to be strongly dependent on the size of the central void: the hollow PtNi/C nanoparticles featuring the smallest central void collapsed at temperatures lower than those with larger voids (this is clearly visible if one compares Figure 4 and Figure S1 in the Supporting Information, where another series of STEMHAADF images is displayed). This suggests that the restructuring of hollow nanoparticles is thermodynamically driven (as the passage from two to one surface minimizes the total surface energy of the nanoparticle) but is kinetically limited by the amount of vacancies contained in the central void (that is, the size of the hollow core). Keeping the same idea in mind, we emphasize that the kinetic Monte Carlo simulations of Gusak et al.59 showed that the time to collapse for a hollow nanoparticle depends also on the temperature, the radius of the collapsed nanoparticle, the surface tension of the metal(s) considered, their atomic volume, and their interdiffusion coefficients. In addition, we noticed a significant influence of the gas atmosphere on the restructuring kinetics. As shown by Figure 5, the rearrangement of PtNi atoms initiated at 210 °C in the presence of oxygen and was complete at 280 < T < 350 °C.

Figure 1. HAADF-STEM images, line scan analysis, and elemental distribution in the hollow PtNi/C nanoparticles treated at 400 °C under different atmospheres. A HAADF image is displayed in (a) and STEM images in (b)−(d). In the elemental maps (middle) and in the line scans (right), the Pt atoms are represented in red and the Ni atoms in green. The X-EDS signals recorded at each point of the line scan (blue line) are displayed on the right-hand side.

Morphological Changes of Hollow PtNi/C Nanoparticles during Thermal Annealing. Conventional TEM images of the as-synthesized and thermally annealed PtNi/C nanoparticles are displayed in Figure 2a−d. They show that the fresh PtNi/C nanoparticles are polydisperse with respect to size (10−15 nm) and shape (spherical to ellipsoidal). Details on the average inner and outer diameters of the fresh hollow PtNi/C nanoparticles may be found in Figure S3 of ref 44. The HR-TEM images of Figure 2 indicate that the PtNi shell of the as-synthesized hollow nanoparticles is composed of individual nanocrystallites ca. 2−3 nm in size featuring different crystallographic orientations and being interconnected by structurally disordered regions. This polycrystalline nanostructure suggests that multiple Pt nucleation centers formed on the initial Ni or Ni−B/C nanoparticles grew and then contacted each other. It is important to stress that the structural disorder is not limited to the grain boundaries: the PtNi lattice is also distorted in the individual crystallites, therefore prohibiting any clear indexation of the lattice planes present (Figure 3). The interplanar distance measured for different crystallites was on average 2.18 ± 0.06 Å, suggesting (111) planes in a facecentered cubic (fcc) PtNi structure (Figure 3d,e). This distance is shorter than that in pure solid Pt nanocrystallites of the same

Table 1. Chemical and Structural Parameters of the Electrocatalysts Evaluated in This Worka X-EDS at. comp (%) from analyses on hundreds of nanoparticles (low magnification) solid Pt/C bulk Ni (PDF card no. 00-004-0850) fresh hollow PtNi/C hollow PtNi/C after thermal annealing under N2 hollow PtNi/C after thermal annealing under H2 hollow PtNi/C after thermal annealing under air

XRD

at. comp (%) from analyses on individual nanoparticles (high magnification)

lattice param (nm)

lattice param contraction vs Pt/C (%)

Pt100Ni0

Pt100Ni0

0.3933 0.3524

0 10.4

Pt86±2Ni14±2 Pt87±2Ni13±2

Pt88±2Ni12±2 Pt88±5Ni12±5

0.3869 0.3908

1.6 0.6

Pt87±2Ni13±2

Pt90±1Ni10±1

0.3886

1.2

Pt88±2Ni12±2

Pt89±1Ni11±1

0.3902

0.8

a

The atomic composition (at. comp) was determined by X-EDS on hundreds of nanoparticles (low-magnification TEM images) or on individual nanoparticles (high-magnification STEM-XEDS images). 4675


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Figure 2. Conventional TEM and aberration corrected HR-TEM images of the as-synthesized and thermally annealed PtNi/C nanoparticles: (a−d) assemblies and (e−h) HR-TEM images of the as-synthesized PtNi/C nanoparticles and after thermal annealing at 400 °C under different atmospheres. Zoom-in images of the fine nanostructure of the PtNi shell are shown in (i−l). The boundaries between the individual PtNi nanocrystallites are highlighted in color.

This result is ascribed to the decrease of the surface energy upon chemisorption and the enhanced diffusivity of Pt atoms under hydrogen or oxygen coverage. As shown on Pt single crystals, the motions of Pt surface atoms are much faster under a gas atmosphere,60,61 and facilitate the restructuring of the bulk of the nanoparticles.62 Convincing evidence of enhanced restructuring of Pt nanoparticles under gas atmosphere may also be found in the study by Cabie et al.63 The authors showed that exposure of cubic Pt nanoparticles to an O2 or H2 atmosphere promotes the development of (001) or (111) facets, respectively. The shape changes were found to be reversible and were also observed for Cu,64 Rh,65 and Pd66 nanoparticles. Interestingly, slight segregation of Ni or Pt atoms to the surface and subsurface layers was observed upon thermal annealing under air or H2 atmosphere by electron energy loss spectroscopy (EELS) elemental maps (Figure S2 in the Supporting Information) or local X-EDS analyses (Figure S3 in the Supporting Information). A slight enrichment in Pt in the surface and near-surface layers was observed upon thermal annealing under H2 by local X-EDS analyses (Figure S3), in agreement with the findings of refs 67−69. Similarly, after thermal annealing under an air atmosphere, the surface of the PtNi/C nanoparticles was slightly enriched in Ni, in agreement with refs 56 and 70. Note, however, that surface enrichments detected by X-EDS remained within the incertitude bar of the analyses (±1−2 at. %; see Table 1) and that X-EDS and EELS cannot provide quantitative insights into the composition of the surface and near-surface layers (these are bulk techniques). To better quantify the surface composition resulting from thermal annealing, X-ray photoelectron (XPS) measurements were performed. The results displayed in Figure S4 showed that the Ni content and the oxidation state of Pt and Ni atoms located

in the surface and near-surface region increased upon thermal annealing under hydrogen, most impressively after thermal annealing under air. We rationalized this by considering that, due to the acid leaching step performed in the last step of the synthesis, the fresh hollow PtNi/C nanoparticles feature a “skeleton” structure composed of a 2−3 monolayer thick Pt shell covering the mother PtNi alloy. Therefore, thermal annealing at T = 400 °C led to a more homogeneous distribution of Pt and Ni atoms, resulting in an increased Ni/Pt ratio at the surface (Figure S4A) with (Ni/Pt)air > (Ni/Pt)H2 > (Ni/Pt)fresh. Moreover, we found that thermal annealing under air atmosphere facilitated the formation of Pt and Ni oxides (Figure S4B,C), in agreement with former reports of Menning et al.71 and Ahmadi et al.56,70 on the PtNi system and of Xin et al.72 on PtCo nanoparticles, respectively. However, it is important to note that the electrocatalytic activity of the PtNi/C nanoparticles is only mildly affected by chemical changes in the surface and near-surface layers. Indeed, in acidic electrolyte and especially under ORR conditions, Ni atoms are leached out from the first 2−3 monolayers. This is clearly visible in Figure 1 if one compares the X-EDS line-scan profiles for Pt and Ni atoms. Convincing evidence was also provided by Durst et al.69 with the help of aberration-corrected scanning transmission electron microscopy (HR-STEM) and EELS elemental mapping that Co atoms are removed from the second and third atomic layers of Pt3Co/C nanoparticles upon exposure to an acidic environment. The change in structure was fast and independent of the configuration of the starting Pt3Co/ C nanoparticles: Pt3Co/C alloy or Pt skin nanostructure (Pt skin refers to a configuration in which the outermost surface layer is Pt rich, while the second layer is Ni rich). In other words, the surface and near-surface composition of the fresh or 4676


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Figure 3. Lattice distortions in the shell of the as-synthesized hollow PtNi/C nanoparticles: (a) HR-TEM image, (b, c) zoom-in images of the PtNi shell. The white boxes with dotted line indicate the regions over which the interplanar distances displayed in (d, e) have been measured. These distances are expressed in angströms.

atmosphere-dependent healing of point (vacancies), surface planar (grain boundaries), and bulk defects (voids, cavities). To separate the effects of crystallite size and microstrain (inhomogeneous interplanar distance) on the broadening of the XRD peaks, the synchrotron XRD patterns were refined with the Rietveld method after accounting for the instrumental broadening (Figure 6). Complementary to Rietveld refinement, an atomic pair distribution function (PDF) analysis of the synchrotron data was carried out. The measured and fitted XRD patterns may be found in Figures S8 and S9 in the Supporting Information, and the parameters used for the Rietveld and PDF analyses are given in Tables S1 and S2 in the Supporting Information, respectively. The results of the Rietveld analysis indicated that the coherent domain size varied by a factor of 3 during thermal annealing (Figure 6b) but always remained between 3.3 and 10.1 nm, i.e., much smaller than the diameter of the hollow nanoparticles. This confirms the HR-TEM images showing that the hollow PtNi/C nanoparticles are formed by several crystallites (grains), which have no structural coherence between them. In all of the samples, the PtNi nanocrystallites featured anisotropic shapes and were found elongated along the [111] direction. The anisotropy (difference between the longest and shortest directions) was close to ∼6−7 nm, independent of the average crystallite size. Finally, the Rietveld analysis revealed that microstrain significantly contributes to the broadening of the XRD peaks for the fresh and the N2-heat-

thermally annealed PtNi/C electrocatalysts under operando conditions is assumed to be Pt rich. Structural Changes of Hollow PtNi/C Nanoparticles during Thermal Annealing under Different Atmospheres. To gain insights into the structural changes occurring on the hollow PtNi/C nanoparticles during thermal annealing under different atmospheres, in situ laboratory X-ray diffraction (XRD) experiments were performed (the in situ XRD spectra taken at different holding temperatures are displayed in Figures S5−S7 in the Supporting Information), and the thermally annealed samples were then analyzed ex situ with synchrotron XRD radiation. A pronounced narrowing of the XRD peaks (signaling the growth of the PtNi crystallites) was observed during thermal annealing under H2 or air (Figure 6), thereby confirming the electron microscopy findings obtained under the same experimental conditions. A contrario, the XRD peaks remained broad for the fresh hollow PtNi/C nanoparticles before/after thermal annealing under N2. Along with these changes, the XRD patterns systematically shifted toward smaller 2θ angles upon thermal annealing: i.e., the PtNi lattice relaxed, whatever the gas atmosphere. Similarly to what was found for the changes in morphology, the relaxation of the PtNi lattice was gas dependent, being maximum under N2, less pronounced under air, and minimum under H2. Since the Ni content and the elemental distribution remained identical in the fresh and the heat-treated PtNi/C nanoparticles, this result is ascribed to 4677


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Figure 4. In situ snapshots of the structural changes occurring on hollow PtNi/C nanoparticles during thermal annealing under vacuum: (a, c) aberration-corrected HAADF-STEM and (b, d) bright-field-STEM (BF-STEM) images. The heating rate was 20 °C min−1. Thirty minute steps were performed at each temperature of interest. Note that the scale bar is different in the sixth column (700 °C) of (c, d) relative to the other columns. In these images, the magnification was changed to illustrate that the collapsed PtNi nanoparticles remained polycrystalline upon thermal annealing. Arrows indicate the formation, interdiffusion, and annihilation of a nanoscale secondary void at T ≥ 400 °C.

treated hollow PtNi/C samples and is reduced upon thermal annealing under H2 or air (Figure 6d), during which the average crystallite size increased markedly. The results of the PDF analysis were remarkably similar (note, however, that anisotropic size effects are not taken into account by this analysis, therefore explaining why it systematically yields smaller crystallite sizes). The PDF refinements showed that the thermally annealed PtNi/C nanoparticles maintained an fcc structure but that the lattice parameter relaxed significantly under the different gas atmospheres. We also noticed that the values of the atomic displacement parameter Uiso (determined by PDF refinement) and of the microstrain (determined by Rietveld analysis) were linearly correlated. Since Uiso reflects the distribution of interatomic distances from their average values and thus incorporates the effects of microstrain, this result confirms the variations of the microstrain for the different samples. As illustrated in Figure S10 in the Supporting Information, a large part of the microstrain arises from the nanometric dimension of the metal crystallites. This should be correlated to the 4υmγ/d term in the Gibbs−Thomson equation,73 which quantifies the increase of the chemical potential for nanomaterials relative to the bulk (υm is the molar volume, γ is the surface tension, and d is the crystallite size). However, due to the large concentration of structural defects in the hollow PtNi/ C nanocrystallites, the microstrain is more pronounced in

hollow nanoparticles relative to solid nanoparticles with identical crystallite size (Figure S10). Finally, we emphasize that no direct relation exists between the PtNi lattice relaxation and the microstrain broadening. This is because the microstrain represents the local deviation from the average lattice parameter (i.e., Δa/a, where a is the lattice parameter) caused by the structural defects. An illustration of this may be found in Figure 6c,d: the PtNi/C nanoparticles which were thermally annealed under an H2 atmosphere feature a more compressed lattice constant (by 1.2%) in comparison to those thermally annealed under air (0.8%), but both catalysts have similar microstrain. The same holds true for the PtNi/C nanoparticles in the fresh state and after thermal annealing under N2 (similar microstrains but different lattice constants). Structure−Catalytic Activity Relationships. Motivated by these very different structural characteristics, we investigated the catalytic activity of the fresh and thermally annealed PtNi/C nanoparticles for two reactions of interest for PEMFC devices: the electrochemical COads oxidation and the oxygen reduction reaction. Figure 7 shows a clear relationship between the concentration of structural defects and the catalytic activity of the PtNi/C nanoparticles for the COads electrooxidation. Two oxidation prepeaks at 0.66 < E < 0.70 V vs RHE dominate the reactivity of the fresh and the N2-annealed hollow PtNi/C nanostructures. In contrast, a peak located at 0.80 < E < 0.82 V vs RHE predominates in the COads stripping voltammograms performed on the solid Pt/C and the solid PtNi/C nano4678


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Figure 5. In situ monitoring of the structural changes occurring on hollow PtNi/C nanoparticles during thermal annealing under oxygen at atmospheric pressure: aberration-corrected (a, c) HAADF and (b, d) STEM images. The heating rate was 30 °C min−1.

Figure 6. Structural parameters of the fresh and the heat-treated hollow PtNi/C nanoparticles and the reference Pt/C material used in this study: (a) synchrotron XRD spectra after linear heating from room temperature to 400 °C at a rate of 2 °C min−1; (b) average crystallite size; (c) lattice parameter; (d) microstrain estimated by Rietveld analysis of the XRD spectral pattern.

Figure 7. Electrocatalytic activity of the hollow and solid PtNi/C and the reference solid Pt/C nanoparticles for the electrochemical COads oxidation: iR-corrected (a) base and (b) COads stripping voltammograms measured on the hollow and the solid PtNi/C nanoparticles and the reference Pt/C material. Conditions: Ar-saturated 0.1 M HClO4; v = 0.020 V s−1; 25 ± 1 °C; no rotation of the electrode; Pt loading 20 μgPt cm−2geo.

particles (which were thermally annealed in H2 or in air). The contribution of the COads electrooxidation prepeaks to the total electrooxidation charge was maximum for the fresh and the heat-treated under N2 hollow PtNi/C nanoparticles (featuring a high concentration of grain boundaries) and minimum for the PtNi/C nanoparticles thermally annealed under H2 or air and the reference solid Pt/C material.

A second structure−activity relationship appears if the catalysts are sorted into two distinct groupsthe hollow PtNi/C nanoparticles (fresh and thermally annealed under N2) 4679


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Figure 8. Electrocatalytic activity of the hollow and solid PtNi/C and the reference Pt/C nanoparticles for the oxygen reduction reaction. (a) Linear sweep voltammograms. (c) Tafel plots of ohmic and mass transport corrected linear sweep voltammograms displayed in (a). (b) and (d) are the specific and mass activities (MA0.95) for the ORR determined at E = 0.95 V vs RHE. Conditions: O2-saturated 0.1 M HClO4: potential sweep rate 0.005 V s−1; ω = 1600 rpm; positive-going potential sweep from 0.20 to 1.05 V vs RHE; temperature 25 ± 1 °C; Pt loading 20 μgPt cm−2geo. The error bars are the standard deviation of at least three independent measurements.

and the solid PtNi/C nanoparticles (thermally annealed under air or H2)and compared pairwise. Within both groups, the PtNi/C nanocatalysts featuring relaxed lattice parameters better catalyze the COads monolayer electrooxidation, as deduced from the shift of the onset and the peak potential to lower potential value (Figure 7b). These results can easily be understood in view of the higher affinity of relaxed PtNi lattices for OHads species,74,75 which are required to complete the electrooxidation of the COads monolayer. From a more global perspective, the results indicate that the CO ads monolayer electrooxidation is a fast and effective way to check the affinity of bimetallic Pt-alloy nanocatalysts for oxygenated species. An opposite structure−catalytic activity relationship was found for the ORR (Figure 8). In agreement with the literature,75,76 the electrocatalysts featuring the more compressed PtNi lattice were catalytically more active for the ORR. This may again be easily rationalized in the frame of the d band theory,77−79 which states that a compression of the Pt lattice results in a downshift of its average energy with respect to the Fermi level. This downshift in turn weakens the binding energy of ORR intermediates (OHads and OOHads) and accelerates the ORR kinetics. Strikingly, for nanocatalysts featuring similar lattice constants, the ORR rate was strongly enhanced in the presence of grain boundaries and vacancies (Figure 9). Since (i) no Ni was lost and (ii) the chemical composition of the topmost and nearsurface layers remains Pt rich under operando conditions (acidic electrolyte), the better electrocatalytic activity of the hollow PtNi/C nanocatalysts can only be related to their unique structural arrangement: i.e., to their hollow nanostructure. In our last paper,44 we ascribed the enhanced ORR kinetics on hollow PtNi/C nanoparticles relative to solid PtNi/ C nanoparticles to a combination of strain and ensemble effects and to their porous architecture. The present study shows that

Figure 9. Dependence of the specific activity for the ORR measured at E = 0.95 V vs RHE on the lattice parameter and the nanostructure/ concentration of grain boundaries and atomic vacancies of/in the nanoparticles.

structural defects, such as grain boundaries and vacancies, also contribute to the catalytic enhancement. We rationalize our findings in the framework of the recent DFT “coordinationactivity” plots of Calle-Vallejo et al.,3 which evidenced that the presence of subsurface cavities on a Pt(111) surface locally increases the mean coordination number of the surface atoms and enhances their ORR activity. Our results show for the first time that grain boundaries and vacancies are beneficial defects for ORR catalysis. We argue that the lattice distortions and different atomic packing densities in (or in close proximity to) these structural defects modify the electronic structure of Pt atoms and optimize the binding of ORR intermediates. In this framework, the synchrotron XRD results presented in this study unequivocally show that microstrain provides catalytic sites with compressed lattice constants, leading to ORR activity ascending toward the apex of the Sabatier volcano plot (PtNi catalysts with Ni fraction less than 25 at. % are located on the 4680


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ACS Catalysis strong-binding side of the volcano plot).55,75,76 Hence, the combination of a hollow nanostructure and of a Pt-alloy/C nanocatalyst is beneficial to ORR catalysis: first because the hollow-induced large microstrain allows accessing a large set of Pt−Pt bond distances and second because microstrain is stable unless a high-temperature annealing procedure is employed (Figures 4 and 5). This is in contrast with (but complementary to) conventional Pt-alloy/C catalysts for which strain/ligand effects are progressively vanishing due to dissolution of the alloying element in the harsh operating conditions of a PEMFC, leading ultimately to depreciated ORR kinetics.42,76,80−89

Synchrotron X-ray Diffraction Measurements. The synchrotron XRD measurements were performed at the ID31 beamline at the European Synchrotron Radiation Facility (ESRF) in Grenoble, France. Radiation from one undulator was monochromatized with a multilayer monochromator (bandwidth 0.3%) to the final energy of 60.0 keV (λ = 0.207 Å). The X-ray beam was focused with two transfocators to the final size of 5 μm × 20 μm (vertical × horizontal relative to the plane of the accelerator ring), and the flux at the sample position was 5 × 1012 photons s−1. The data were collected using a PerkinElmer 2D detector and azimuthally integrated using the pyFAI software package. The Rietveld refinements were carried out using FullProf.90 The XRD pattern of a CeO2 sample was used to determine the instrument resolution function. The refinements were based on the structure of a cubic close-packed metal (Fm3̅m, a ≈ 3.9 Å), including a polynomial description of the background. The best agreements were obtained with a description of peak shape using the TCH function,91 including an isotropic description of microstrain and a uniaxial anisotropic description of particle sizes, with the [111] direction as main axis. The refined parameters and agreement factor values are summarized in Table S1 in the Supporting Information and the refined plots in Figure S9 in the Supporting Information. The PDFs were obtained from the same patterns using PDFGetX3.92 The data were considered up to Qmax = 23 Å−1. The pattern from a capillary containing only the carbon phase used in the nanoparticle samples was subtracted from the data. The PDF of the CeO2 standard was used to determine the instrumental parameters (damping and broadening of PDF peaks due to experimental resolution), which were then fixed during the PDF refinements, carried out with PDFgui.93 The refinements were carried out for 1 ≤ r ≤ 40 Å on the basis of the same metal structure as for the Rietveld refinements. The refined parameters were a scale factor, the a cubic cell parameter, the δ parameter taking into account linear atomic correlations, the isotropic overall atomic displacement parameter (adp) Uiso, and the parameter D corresponding to the diameter of the isotropic coherent domain size. The results are shown in Table S2 in the Supporting Information, and the refinement plots are shown as Figure S8 in the Supporting Information. X-ray Photoelectron Spectroscopy Measurements. The XPS measurements were acquired in an ultra-high vacuum chamber (10−10 mbar base pressure, 10−8 mbar during the measurements) equipped with a hemispherical analyzer (VSW H100) and monochromatic X-ray source (SPEC XR-50) operating at 270 W (13.5 kV, 20 mA). The hemispherical analyzer was working in the constant pass energy mode (44 eV). Analyses were carried out at an angle of 60° between the sample surface and the analyzer axis. The XPS data signals were taken in increments of 0.25 eV with dwelling times of 10 s. The PtNi/C electrodes were prepared by depositing a nonquantified amount of powder on carbon scotch, which was then glued onto a molybdenum block and introduced into the XPS chamber within 3 h before the measurements. The XP spectra were analyzed using the CasaXPS software from CasaSoftware Ltd. using the Shirley background corrections, asymmetric peak shapes for Pt 4f doublets, and blend of Lorentzian and Gaussian peak shape for Ni 2p doublets. The reference was a partially oxidized Ni foil. Conventional TEM Measurements and X-EDS Mapping. The electrocatalysts were examined with a JEOL 2010 TEM instrument operated at 200 kV with a point to point

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CONCLUSION In conclusion, using thermal annealing under different gas atmospheres, we synthesized a series of PtNi/C nanoparticles with identical chemical compositions and elemental distributions but different nanostructures (hollow vs solid), crystallite sizes, and lattice strains. By combining synchrotron XRD, in situ TEM, and electrochemical measurements, we provided experimental evidence that structural defects such as grain boundaries and vacancies enhance the kinetics of two reactions of interest for PEMFC electrocatalysis: the electrochemical COads monolayer oxidation and the ORR. Our results shed fundamental light on the effect of structural defects on the catalytic performance of bimetallic nanomaterials and should ultimately aid in the rational design of electrocatalysts with enhanced activity for the sluggish ORR, the reaction limiting the electrical performance of PEMFC devices.

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MATERIALS AND METHODS Reference Electrocatalyst. A Pt/Vulcan XC72 catalyst with a weight fraction (wt %) of 20% was purchased from ETeK and used as reference material without any treatment. The number-averaged Pt nanoparticle size was 2.9 ± 0.6 nm. Synthesis of Hollow PtNi/C Nanoparticles. The hollow PtNi/C nanoparticles were synthesized by mixing 0.154 g of Pt(NH3)4Cl2.H2O (Alfa Aesar, Specpure) and 0.180 g of NiCl2 (Fluka, >98.0%) with 0.3 g of Vulcan XC72R (Cabot), 10 mL of ethanol, and 140 mL of deionized water (Millipore). An aqueous solution of NaBH4 (Aldrich 99.99%; 5.5 mmol, 0.22 M) was then added at a rate of 5 mL min−1 and the mixture stirred for 1 h with magnetic stirring at room temperature (20 ± 2 °C). The resulting mixture was filtered, thoroughly washed with deionized water, and dried for 45 min at 110 °C. The catalyst powder was then acid-treated for 22 h in a stirred 1 M H2SO4 solution at 20 °C. Laboratory X-ray Diffraction Measurements. The synthesized and reference electrocatalysts were analyzed using a PANalytical X’Pert Pro MPD vertical goniometer/diffractometer equipped with a diffracted-beam monochromator using Cu Kα mean radiation (λ = 0.15418 nm) operating at 45 kV and 40 mA. The 2θ angle extended from 10 to 125° and varied using a step size of 0.033°, accumulating data for 525 s. In situ XRD spectra on the hollow PtNi/C nanoparticles were recorded during thermal annealing from room temperature to 400 °C at a heating rate of 2 °C min−1. The heating assembly (Anton-Paar DHS 1100) consisted of a chemically resistant ceramics heating plate and a homemade stainless-steel crucible (sample holder). A domed-shape X-ray transparent window made of graphite, which was fixed on the heating assembly, allowed control of the gas atmosphere. 4681


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was denoised using standard principal component analysis implemented in the hyperspectral data analysis toolbox HyperSpy.95 Electrochemical Measurements. All the glassware accessories used in this study were first cleaned by soaking in a H2SO4/H2O2 mixture for at least 12 h and thoroughly washing with ultrapure water. The 1 M H2SO4 solution used for acid leaching was prepared with Milli-Q water (Millipore, 18.2 MΩ cm, total organic compounds 99.99%, Messer) by linearly sweeping the potential from 0.20 to 1.05 V vs RHE at a potential sweep rate of 5 mV s−1 and at different rotational speeds (400, 900, 1600, and 2500 rpm). The ORR specific/mass activity was determined by normalizing the current measured at E = 0.95 V vs RHE, after correction from the oxygen diffusion in the solution and the Ohmic drop, to the real surface area/mass of deposited Pt determined by COad stripping voltammetry, respectively.

resolution of 0.19 nm. The X-EDS elemental maps were acquired using a JEOL 2100F microscope operated at 200 kV and equipped with a SDD Centurio retractable detector. The X-EDS spectra were recorded on individual nanoparticles by scanning the beam in a square region adjusted to the particle size. The quantitative analyses were performed on Pt L and Ni K lines using the K factor provided by the JEOL software. In Situ TEM Measurements. The in situ measurements under vacuum or O2 environment were performed using a JEOL 2100F TEM/STEM electron microscope equipped with a Cs probe corrector. The images were acquired in the STEM mode using simultaneously both the HAADF (high angle annular dark field) and BF (bright field) detectors. For the in situ measurements under vacuum, a Gatan heating holder was used, and the PtNi/C electrocatalysts were deposited on a classical TEM grid covered by a carbon membrane. The sample was heated from room temperature to 700 °C at a rate of ca. 20 °C min−1, and held for 30 min at each temperature. For the in situ measurements under oxygen, a Protochips Atmosphere Gas environmental cell was used. The PtNi/C electrocatalysts were deposited on the Si3N4 membrane in contact with the SiC heating ceramic device (E chips) submitted to a pure oxygen environment at atmospheric pressure. The specimen was heated from room temperature to 450 °C at a rate of ca. 30 °C min−1 and held for 30 min at each temperature. Aberration-Corrected HR-TEM and Spectrum-Imaging EELS. Aberration-corrected HR-TEM images and two-dimensional spatially resolved EELS elemental maps were acquired with a JEOL-ARM 200F (JEOL) transmission electron microscope equipped with a cold-field emission gun operated at 200 kV and a CEOS aberration corrector of the objective lens.94 For the TEM studies, the PtNi nanoparticles were dispersed in dry conditions on holey film copper TEM grids (Agar Scientific Ltd.). The TEM investigations were undertaken on nanoparticles lying over the holes of the thin holey carbon films to eliminate the contribution of the latter to the HR-TEM and EELS signals. For spatially resolved EELS, the microscope was operated in STEM mode with the probe size setting “5c” and convergence and collection angles set at 32 and 100 mrad, respectively. EEL spectra were acquired using a Gatan Quantum ER imaging filter fitted with a fast CCD camera with a selected energy dispersion of 0.5 eV per pixel and a vertical binning of ×26. To analyze the Pt and Ni distribution within the nanoparticles, core-loss spectra over a large energy domain (∼300−1500 eV) containing both Pt-N3 (518 eV) and Ni-L2,3 (855 eV) were acquired in the spectrum-image (SI) mode. In this mode, the EELS data are captured sequentially in space by scanning in a controlled way the focused electrons at the surface of the sample and acquiring the loss signal over the energy range of interest at each probe position. In the present study, the coreloss EEL spectra were acquired over square or rectangular areas enclosing the nanoparticles with an acquisition time of 0.7−1 s per spectrum. To compensate for spatial drift of the sample during EELS acquisition, a drift correction was applied at the end of each row of the SI. Spectral drifts in the core-loss spectra were monitored and corrected by acquiring, in parallel to each core-loss SI, the corresponding SI of the zero-loss peak (with an acquisition time of 1 μs per spectrum) using the dual EELS capability of the imaging filter. Elemental maps were constructed by analyzing the intensities under the Pt-N3 and Ni-L2,3 edges after background subtraction using a power-law model. Prior to the generation of the elemental maps, each SI

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acscatal.6b01106. 4682


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ACS Catalysis

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In situ TEM images showing the morphological and structural changes during thermal annealing, local X-EDS analyses, XPS, laboratory, and synchrotron XRD spectra, and results of the Rietveld refinements and PDF analysis (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail for L.D.: [email protected]. *E-mail for F.M.: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was performed within the framework of the Centre of Excellence of Multifunctional Architectured Materials “CEMAM” No. ANR-10-LABX-44-01. The authors acknowledge the French CNRS and CEA-METSA network and financial support from the University of Grenoble-Alpes through the AGIR program (Grant No. LL1492017G) and from the French National Research Agency through the HOLLOW project (Grant No. ANR-14-CE05-0003-01).

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