Octahedral PtNi Nanoparticle Catalysts: Exceptional Oxygen


Oct 12, 2012 - Le Xin , Fan Yang , Somaye Rasouli , Yang Qiu , Zhe-Fei Li , Aytekin Uzunoglu , Cheng-Jun Sun , Yuzi Liu , Paulo Ferreira , Wenzhen Li ...
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Letter pubs.acs.org/NanoLett

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

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

ABSTRACT: We demonstrate how shape selectivity and optimized surface composition result in exceptional oxygen reduction activity of octahedral PtNi nanoparticles (NPs). The alloy octahedra were obtained by utilizing a facile, completely surfactant-free solvothermal synthesis. We show that the choice of precursor ligands controls the shape, while the reaction time tunes the surface Pt:Ni composition. The 9.5 nm sized PtNi octahedra reached a 10-fold surface area-specific (∼3.14 mA/ cm2Pt) as well as an unprecedented 10-fold Pt mass based (∼1.45 A/mgPt) activity gain over the state-of-art Pt electrocatalyst, approaching the theoretically predicted limits. KEYWORDS: Surface composition, PtNi octahedra, oxygen reduction reaction, ligand control

T

9−10× loss in activity when going from extended surfaces to carbon-supported alloy nanoparticles,23 an activity gain of about 10× is realistically expected for a carbon-supported Pt−Ni octahedron compared to a state-of-art carbon-supported spherical Pt nanoparticle catalyst. Surfactant-directed synthesis of well-defined shape-selective Pt−Ni catalysts was recently reported by Wu et al. and Zhang et al. using a careful choice of preparation techniques involving distinctly different surfactants, reducing agents, and solvents.10−13 The octahedral Pt−Ni particles achieved a 4−7× improvement in terms of the Pt surface area-specific activity but displayed a mere ∼4× improvement in Pt mass activity over Pt owing to residual capping molecules.10−12 Also, in all of these studies, a fundamental understanding of how success or failure to produce shape selective particles depends on the applied reaction conditions has remained poorly understood. In particular, while basic Wulff-type particle shapes (octahedra, cubes) could be reproducibly produced and identified through atomic scale microscopy, their precise surface compositions on individual facets, key for their catalytic reactivity, remained unexplored and uncontrolled. More recently, Snyder et al. presented a size-dependent dealloying study on nonshape selective Pt−Ni NPs and showed the formation of porous Pt−Ni NPs above a critical diameter of

he sluggish kinetics of the oxygen reduction reaction (ORR) on costly platinum cathode electrocatalysts represents a major obstacle to a more widespread use of the polymer electrolyte membrane fuel cell (PEMFC). Recent rational design of geometric and electronic properties of extended alloy catalyst surfaces have resulted in significant improvements of the ORR activity.1−8 However, improving the ORR activity further in practical nanoscale alloy catalysts is still a great challenge.9 One of the most promising strategies is the development of shape and composition-controlled Pt-based alloy nanoparticle (NP) catalysts.10−14 These NP catalysts with controlled shapes, that is, controlled exposed crystal facets, and composition profiles hold the promise of providing the same ideal surface electronic structure as extended surfaces, thereby realizing their full activity advantage.15−17 Stamenkovic and co-workers reported a very active single crystal Pt3Ni(111) surface, which performed 10 times (10×) higher in ORR activity than a Pt(111) surface and 90× higher than a commercial NP Pt/C catalyst.18 The enhancement was attributed to the low coverage of hydroxyl species induced by the specific electronic structure associated with an oscillatory nearsurface compositional Pt and Ni profile across the 2−4 outermost layers of the (111) surface.18−20 This was direct experimental evidence that the near-surface composition and its atomic arrangement are key factors to improve the ORR activity.21,22 Stamenkovic’s report on Pt3Ni(111) triggered a quest for shape-selective octahedral Pt alloy NPs, which would exhibit only the active (111) facets; if successful, this could make the 90× catalytic activity gain a reality. Considering the typical © 2012 American Chemical Society

Received: September 3, 2012 Revised: October 3, 2012 Published: October 12, 2012 5885

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

∼15 nm.24 Pt−Ni NPs ranging in particle size from 5 to 20 nm demonstrate specific and mass ORR activity improvement factors of 3−6× and 4−5×, respectively, due to their different residual Ni contents and surface morphologies after dealloying.20,24,25 Carpenter et al. demonstrated 10× specific ORR activity improvement for 12−15 nm shape-selective octahedral Pt−Ni NPs, and their ORR-tested octahedra became porous,26 which is consistent with Snyder et al.’s conclusions. Despite their favorable specific activity, however, the Pt mass activity was only 4−6× over Pt. In most of these PtNi studies, imperfections in geometry and near-surface composition of the NPs were held responsible for the lower than expected ORR activities. Given our understanding of the electrocatalysis of octahedral Pt−Ni nanoparticles outlined above, it is clear that a better finetuning of the near-surface composition of Pt−Ni octahedral particles could have a great potential to further perfecting the ORR activity gain over Pt. However, robust and facile synthetic strategies to control the near-surface composition in octahedral NPs have remained elusive to date and represent a critical unmet need in fundamental fuel cell alloy electrocatalysis. Here, we present a robust, facile, and surfactant-free solvothermal synthesis of shape and size-selective octahedral PtNi NPs. The shape selective NPs show an exceptional ORR made possible by their carefully tuned alloy particle surface composition. We show that the choice of precursor ligands controls the shape selectivity, while we can use the reaction time to tune the surface Pt:Ni composition and thus optimize the ORR activity. We explain our findings in terms of a simple nucleation/growth model. At a surface composition of about 40 at. % Pt, 9.5 nm-sized PtNi octahedra reached a 10-fold surface area-specific (∼3.14 mA/cm2Pt) as well as an unprecedented 10fold Pt-mass based (∼1.45 A/mgPt) activity gain at 900 mV/RHE and 5 mV/s anodic sweep rate over the state-of-art commercial carbon-supported Pt electrocatalysts. We have utilized a simple, surfactant-free, low-temperature (120 °C) solvothermal synthesis to prepare unsupported sizeand shape-selective octahedral NPs. Figure S1 of the Supporting Information shows the color change of the solvent from green to black at 120 °C after 16 h indicating the formation of the PtNi octahedral NPs. Figure 1a reports the X-ray diffraction (XRD) patterns reflecting the bulk alloy phase structure of the PtNi octahedral NPs after three different reaction times, 16 h, 28 h, and 42 h (denoted as 16-PtNi, 28-PtNi, and 42-PtNi, respectively). The bulk composition of the three alloy NPs was determined as Pt:Ni = 46:54 by inductively coupled plasma mass spectrometry (ICP-MS) and energy-dispersive X-ray spectra

(EDX) regardless of reaction time (Figure S2). This is consistent with the three basically overlapping pattern profiles indexed to a face-centered-cubic (fcc) phase. The octahedral morphology was observed by transmission electron microscopy (TEM) in Figure S3. Following earlier reports on solvothermal techniques at higher temperatures (200 °C),26 the dimethylfomamide (DMF) solvent acts as a complexing agent, solvent, and reducing agent. However, in this study, high heating rates (10 °C/min) and a lower reaction temperature (120 °C) were utilized. A high heating rate resulted in a short induction time and high nucleation rates generating a large number of small seeds. Unlike previous reports,24,26 the low reaction temperature favors slow seed growth, keeping our particles small.24 We note that pure Pt and pure Ni precursors could not be reduced at this low temperature,26,27 suggesting a possible role of the exothermic heat of mixing during PtNi alloy seed formation.28−31 The initial seeds catalyzed the codeposition of Pt and Ni. Moreover, a lower reaction temperature could favor the nanocrystal faceting during growth in a colloidal solution.32 To obtain further insight in the formation mechanism of octahedral alloy nanoparticles, we interrogated the influence of the metal precursor ligands on the alloy particle shape selectivity. Use of Ni(acac)2 and Pt(acac)2 reproducibly resulted in octahedral nanoparticles. When Ni(acac)2 was replaced with Ni acetate, keeping all other synthesis conditions constant, uniform spherical alloy nanoparticles with ∼5 nm diameter were obtained (Figure S4a). On the other hand, when the Pt(acac)2 precursor was replaced with K2PtCl6, particle aggregates with smaller mean size, limited shape-selectivity, and wider size distribution were observed (Figure S4b). These results suggested that it is not the interaction of DMF with the (111) facets alone,33 which determines the formation of octahedra; the precursor ligands, such as acetyl acetonate, had a critical influence on the particle shape and size through a modification of the thermodynamic metal redox potential, possibly coupled to a modified kinetic metal ion reduction (metal atom production) rate. Both factors can have profound effects on the shapeselective growth. We then studied how the reaction time affected the octahedral surface composition of Pt and Ni using X-ray photoelectron spectroscopy (XPS) (see Supporting Information). Based on the estimated sampling depth of ∼2 nm, the measured data is referred to as “near-surface” rather than “surface” composition.34 It must be noted that the measured surface composition is an average value because the PtNi NPs are loaded on carbon and 5886

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Table 1. Comparison of the ECSA, ORR Mass, and Specific Activitiesa reaction timeb

particle sizec (nm)

ECSA (m2/gPt)

mass activity (A/mgPt)

specific activity (mA/cm2Pt)

16 28 42

9.0 ± 1.1 9.2 ± 0.9 9.5 ± 0.8

24.1 36.7 50.0

0.56 ± 0.065 1.02 ± 0.070 1.45 ± 0.120

2.35 ± 0.28 2.77 ± 0.20 3.14 ± 0.24

a

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

every part of the particles has the same chance to be exposed to the X-ray. Our XPS data are shown in Figure 1b; it evidences that the reaction time directly controls the near-surface composition of the resulting octahedral particles without affecting their size or shape. Raising the reaction time from 16 to 42 h, the near-surface Pt at. % increased from 30 to ∼41 at. % at identical shape, size, and bulk composition. To explain this, we emphasize that our experimental XPS data provide direct evidence that the initial particle seeds catalyze higher deposition rates for Pt than for Ni, consistent with their relative electrochemical deposition potentials.35,36 This induces a compositional gradient near the surface of the octahedra28−31,37 (see Figure 1b). As the dissolved precursors deplete, the octahedra reach their time-stable final bulk composition, shape, and size. At this point, the reaction time acts like an in situ annealing process, mainly smoothing out the near-surface compositional gradient by metallic interdiffusion.38,39 To monitor the particle size and shape changes with reaction time, we performed transmission electron microscopy (TEM). The average particle size is ∼9.0 nm for 16-PtNi, ∼9.2 nm for 28PtNi and ∼9.5 nm for 42-PtNi (see Table 1, Figure S3 and Figure 2a−c) suggesting that there is no size penalty with increasing reaction time. Owing to the uncompleted ripening process for 16-PtNi material, the standard deviation of its particle size distribution is larger than those of 28-PtNi and 42-PtNi. After 42 h reaction time, small particles have disappeared, and this is why the particle size distribution became more uniform and the deviation decreased to ±0.8 nm (Figure S3). High-resolution TEM analysis in Figure 2c shows that the corresponding dspacing for the (111) planes is 0.216 nm, which are indexed to the octahedral PtNi NPs terminated with {111} facets. To turn the unsupported NPs into a practical electrocatalyst, the NPs were supported on a high surface area carbon material. The TEM image in Figure 2d evidence a fairly uniform distribution of the PtNi NPs on the commercial carbon support. As shown in Figure 2e and f, the octahedral morphology and the atomic-scale compositional distributions of Pt and Ni across an octahedron were measured by probe corrected scanning transmission electron microscopy complemented with electron energy loss spectroscopy (STEM/EELS). To evaluate the electrocatalytic ORR activities of the octahedral NPs, the octahedral PtNi/C NPs were loaded on a glassy carbon rotating disk electrode (RDE). Because these catalysts were synthesized in pure DMF solvent and no other surfactants were used in the synthesis, any additional surfactantremoving step is not needed,26 a great practical advantage over the polyol process applied previously to prepare Pt−Ni octahedra. The electrochemical active surface areas (ECSAs), evaluated using CO stripping,40 were 24.1, 36.7, and 50.0 m2/gPt for the 16-PtNi, 28-PtNi, and 42-PtNi catalysts, respectively (see

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

Table 1 and Figure S5 of the Supporting Information). The estimated CO stripping charge is somewhat larger but very close to the estimated 2× Hupd charge, and calculated QCO/2QH is within the region of 1.04−1.12 (Figure S5). The increased value of ECSA for 42-PtNi is consistent with the observed higher Pt surface concentration. The effect of the initial near-surface alloy composition on the ORR activity was studied in O2-saturated 0.1 M HClO4 solution at room temperature (see Figure 3 and Table 1). Upon increasing the reaction time from 16 to 42 h associated with the increase in near surface Pt at. % from 30 to 41 at. %, the Pt mass activity increased by a factor of 3× at 0.9 VRHE. The actually observed value at 16 h was 0.56 A/mgPt, rising to impressive 1.45 A/mgPt for the 42 h catalyst at quasi-stationary 5 mV/s scans. The Pt- surface area- specific activity increased by about 50% from 2.35 mA/cm2Pt to impressive 3.14 mA/cm2Pt. For comparison, the state-of-art Pt/C catalyst exhibited 0.15 A/ mgPt and 0.23 mA/cm2Pt. We emphasize that these ORR activities represent previously unachieved consistent 10× increases in both specific and Pt mass activity compared to a state-of-art commercial Pt/C electrocatalyst measured under identical conditions. The 42-PtNi sample even performs superior to extended polycrystalline Pt electrodes (∼1.2 mA/cm2Pt). From the changes in mass activity and the 1.5× increase in specific activity, we conclude that the number of catalytically 5887

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

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

active Pt surface sites roughly doubles. The remarkable 10× activity gains are likely a result of an improved Pt/Ni ratio in the second and third layers.18,41 EDX analysis of the active ORRtested 42-PtNi electrocatalyst revealed that its final bulk composition changed to about Pt75Ni25 during the electrochemical dealloying process, thus very close to the ideal bulk composition of the highly active extended (111) surface. However, this specific activity is still far short of that for extended Pt3Ni(111) crystals. This could be attributed to defects and vacancies in the particle surface after mild dealloying in acidic electrolytes. Moreover, TEM analysis showed that the particles maintained an octahedral shape. Being below the critical size of 15 nm,24 the ORR-tested ∼9.5 nm 42-PtNi catalyst did not show any indication of porosity, in line with earlier findings for Pt−Co, Pt−Cu, and Pt−Ni NPs.24,42 Preliminary data on the stability of the PtNi octahedra indicate that thermal postsynthesis annealing is very detrimental to the octahedral shape and ORR activity. Unannealed PtNi octahedra remained morphologically stable even after tens of voltage cycles within the oxygen reduction reaction potential range. The long-term stability of the octahedra under fuel cell conditions requires further scrutiny. In conclusion, we have synthesized octahedral PtNi NPs with 10× ORR activity gains in both Pt specific and Pt mass activity over a state-of-art Pt/C electrocatalyst. We attribute this high activity to the octahedral shape and favorable surface composition of the final electrocatalyst. To achieve this, we have used a facile surfactant-free solvothermal method and showed that the reaction time correlates with the near-surface Pt atomic composition of the octahedra without affecting their size or shape. Longer reaction times led to higher near-surface Pt-toNi atomic ratios, which resulted in higher intrinsic activity. The reaction time effect was rationalized based on a nucleation/ growth model that explained the near surface composition gradient of Pt and Ni. The synthesis−structure−activity



ASSOCIATED CONTENT

S Supporting Information *

Experimental details, EDX spectra, TEM images, and CO stripping of PtNi octahedra. This material is available free of charge via the Internet at http://pubs.acs.org.



AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] Notes

The authors declare no competing financial interest.



ACKNOWLEDGMENTS We thank Dr. Frederick T. Wagner for valuable discussions. This work was supported by U.S. DOE EERE award DE-EE0000458 via subcontract through General Motors. P.S. acknowledges financial support through the cluster of excellence in catalysis (UniCat).



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NOTE ADDED AFTER ASAP PUBLICATION This paper was published ASAP on October 16, 2012. The caption of Figure 2 and the Supporting Information file have been updated. The revised version posted on October 19, 2012. 5889

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