Electrocatalysis on Platinum Nanoparticles: Particle Size Effect on

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Electrocatalysis on Platinum Nanoparticles: Particle Size Effect on Oxygen Reduction Reaction Activity Minhua Shao,*,† Amra Peles,*,‡ and Krista Shoemaker† † ‡

UTC Power, South Windsor, Connecticut 06074, United States United Technologies Research Center, East Hartford, Connecticut 06118, United States

bS Supporting Information ABSTRACT: We determined the size-dependent specific and mass activities of the oxygen reduction in HClO4 solutions on the Pt particles in the range of 1 5 nm. The maximal mass activity at 2.2 nm is well explained based on density functional theory calculations performed on fully relaxed nanoparticles. The presence of the edge sites is the main reason for the low specific activity in nanoparticles due to very strong oxygen binding energies at these sites. Our results clearly demonstrate that the catalytic activity highly depends on the shape and size of the nanoparticles. KEYWORDS: Electrochemistry, fuel cells, heterogeneous catalysis, monolayers, density functional theory calculations

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he understanding of particle size effect of catalyst nanoparticles on their activity is one of the essential objectives in heterogeneous catalysis.1 5 Oxygen reduction reaction (ORR) is one of the most important reactions in electrochemistry,6 8 and its slow kinetics hinders the performance improvement of low temperature fuel cells.9 Enhancing the electrocatalytic activity of Pt based catalysts and consequently reducing the Pt loading is the essential task to commercialize the fuel cells.10,11 Herein, we report our study on the relationship between the Pt particle size and oxygen reduction activity in a HClO4 solution. Our results clearly show that both mass and specific activities depend on the Pt particle size. The specific activity increases rapidly by 4-fold as the particle size grows from 1.3 to 2.2 nm and increases slowly as particle size further increases. On the other hand, a maximum Pt mass activity was observed at 2.2 nm. We demonstrated that this particle size dependent behavior is associated with the oxygen binding energies on the different Pt sites accessible on cuboctahedral particles of various sizes. Density functional theory (DFT) calculations performed on nanoparticles showed that the {111} facets contribute to the high activity observed on the 2.2 nm Pt particle due to a proper oxygen binding energy. Much higher binding energy at edge sites and even the {111} facet on a smaller particle results in much lower oxygen reduction kinetics. The Pt particle size effect on ORR has been a long-standing problem that has yet to be solved.4,5,12 32 Some researchers observed the decrease of specific activity as particle size decreased.8,9,12 20,22 The mechanisms of such decrease in specific activity have not been fully understood and attracted different explanations, including a lower ratio of preferable crystal facets,12,17,33 stronger interaction between oxygen-containing species and Pt atoms,22,34,35 and lower potential of total zero charge (pzc).22,23 On the other hand, other researchers argued r 2011 American Chemical Society

that the specific activity does not depend on the particle size but on the interparticle distance.24 27 A recent study showed that the specific activity of Pt increased with decreasing particle size with the activity of a 0.9 nm cluster being more than 10 times higher than that of 2.5 nm.28 The main reason for the discrepancy may result from the fact that the activities of Pt were measured in different types of electrolytes and on different samples that may have different shapes and agglomeration degree. In this study, the Pt particle sizes were carefully controlled by layer-by-layer growth using a Cu UPD Pt replacement method.36 Pt nanoparticles with an average particle size of 1.3 nm supported on Ketjen Black (specially synthesized by Tanaka Kikinzoku Kogyo, 10 wt %) were used as the seeds for particle growth. Figure 1a shows the TEM image of as-received Pt/C indicating the Pt nanoparticles uniformly dispersed on carbon support. The narrow particle size distribution was confirmed by a small standard deviation (0.2 nm, see Figure S1, Supporting Information). Typical TEM images of Pt/C after depositing 2, 4, and 10 layers of Pt are also shown in Figure 1. The average particle sizes measured by TEM are 1.84, 2.46, and 4.65 nm for Pt particles shown in Figure 1b (2 layers), 1c (4 layers), and 1d (10 layers), respectively. All the samples have the standard deviations in the range of 0.2 0.3 nm indicating that the high monodispersity in particle size was maintained during the particle growth. Assuming a full Pt monolayer was deposited on the Pt particles in each Cu UPD Pt replacement cycle, the particle size would increase by ∼0.45 nm per layer. However, all the particle size values measured by the TEM are smaller than expected assuming a full Pt monolayer deposition by the UPD method. For instance, the particle size after 10 monolayer growth would be 5.8 nm, which is Received: May 23, 2011 Revised: July 28, 2011 Published: August 01, 2011 3714

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Figure 1. TEM images of Pt particles supported on carbon black: (a) as synthesized, (b) after 2 layers deposition, (c) after 4 layers deposition, and (d) after 10 layers deposition. The average particle sizes for (a), (b), (c), and (d) are 1.3, 1.84, 2.46, and 4.65 nm, respectively.

1.15 nm larger than that observed in Figure 1d. This discrepancy can be explained by the fact that the number of Cu atoms deposited during UPD is equal to the surface atoms of the Pt core, rather than the number required to fully cover the core (Figure S2, Supporting Information). The cyclic voltammograms of Pt/C for as synthesized and after depositing one, two, three, and four layers of Pt are shown in Figure 2a. As expected, the hydrogen adsorption/desorption current density increases gradually with an increase in Pt layers. Figure 2b shows the relationship between the electrochemical active areas (ECAs) and the Pt particle sizes. The ECAs were derived by integrating the hydrogen adsorption charges in Figure 2a assuming 210 μC cm 2,37 and the particle sizes were calculated by counting the atom numbers and confirmed by TEM. The theoretical ECA changes as a function of particle size assuming the Pt particles are cuboctahedron are also included for comparison.38,39 The experimentally measured surface areas are lower than the theoretical values, likely due to the underestimation of the ECAs using the hydrogen adsorption charges,23 and occluded by the carbon support and Nafion.38 Figure 2c displays the ORR polarization curves of Pt/C for as synthesized and after depositing one, two, three, and four layers of Pt in an O2-saturated 0.1 M HClO4 solution obtained using a rotating disk electrode (RDE) at 1600 rpm. The half-wave potentials for the Pt/C with one, two, three, and four extra layers of Pt are higher than that of the as received one by 24, 40, 56, and 59 mV, respectively. Figure 3 shows the mass and specific

activities measured at 0.93 V as a function of particle size. The reason to choose 0.93 V rather than 0.9 V to compare the activity is to minimize inaccuracy of the mass-transport correction on the calculation of kinetic currents based on the Koutecky Levich equation (see Supporting Information). As the Pt loading increases during particle growth, the current density at 0.9 V is higher than half of the limiting current in the RDE measurement (since third layer of Pt deposition, Figure S3, Supporting Information), the inaccuracy of using the Koutecky Levich equation to derive kinetic currents at this potential will be large.9 The mass and specific activities are obtained by normalizing the kinetic current to the Pt weight and electrochemical active area, respectively. The Pt mass activity increases with growth of the particles initially and decreases gradually with the maximum activity observed at ∼2.2 nm. The mass activity enhancement of 2.2 nm Pt particles compared with that of the 1.3 nm ones is about 2-fold. The specific activity increases sharply from 1.3 to 2.2 nm by ∼4 times and then very slowly from 2.2 to 4.65 nm. This trend is very similar to the change of the Pt oxide reduction peak potential as a function of particle size (Figure 2d). The ORR activities of commercial available state-of-the-art Pt/C catalyst from TKK (TEC10E50E, 46.7 wt %) with an average particle size of 2.5 nm (standard deviation = 0.4 nm) were also measured compared in Figure 3. The mass activity and specific activity of the state-of-the-art Pt/C at 0.9 V are 0.2 A mg 1 and 0.24 mA cm 2, respectively, and agree well with the literature data.9,40 At 0.93 V, its mass activity (0.1 A mg 1) and specific 3715

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Figure 2. (a) Cyclic voltammograms of Pt/C with different numbers of extra Pt layers in a N2-saturated 0.1 M HClO4 solution. Sweep rate = 50 mV s 1; room temperature. (b) The electrochemical active surface areas (ECAs) as a function of Pt particle size. The red squares are the experimental data and the blue diamonds are calculated values assuming cuboctahedral particles. (c) The polarization curves of Pt/C with different number of extra Pt layers in an O2-saturated 0.1 M HClO4 solution. Sweep rate = 10 mV s 1. (d) The peak potentials of Pt oxides reduction for Pt/C with different number of extra Pt layers obtained from the cyclic voltammograms in a N2-saturated 0.1 M HClO4 solution.

Figure 3. Size dependence of specific activity (blue diamond) and mass activity (red square) of Pt/C for oxygen reduction reaction at 0.93 V. The specific activity (open blue diamond) and mass activity (open red square) of state-of-the-art Pt/C from TKK (TEC10E50E, 46.7 wt %) with an average particle size of 2.5 nm were also included for comparison. The specific and mass activities were calculated by normalizing the kinetic current to the electrochemical active surface and the Pt weight on the electrode, respectively. The electrochemical active surface was derived by integrating the charge in the hydrogen adsorption region assuming 210 μC cm 2. The Pt weight was calculated by the Cu UPD charge.

activity (0.12 mA cm 2) are similar to those of Pt/C with a same particle size grown from the 1.3 nm Pt seed. This agreement also

validates our approach to examine the relationship between the activity and particle size. It is well-known that surface reactivity of the catalysts correlates well with its catalytic activity.41 43 For the oxygen reduction reaction, the too high reactivity of the surface impedes reaction by blocking the active sites with strong adsorbed intermediates, while too low reactivity hinders the dissociation of O O bond and charge transfer. As shown in Figure 3, the specific activity of Pt nanoparticles shows a 4-fold drop from 2.2 to 1.3 nm. Here we investigate the underlying reasons for this structural sensitivity of ORR and focus on the particle sizes in the range of 0.8 3 nm using DFT calculations. We are not aware of previous reports that use general particle description up to 3 nm and provide direct evidence of the particle size effects from the theoretical/ computational point of view. DFT calculations were performed on the cuboctahedral particle models that were fully relaxed and all possible adsorption sites on particles were considered systematically. Compared to Wang et al.’s study in which a semispherical particle model was implemented and only the top layer of Pt was allowed to relax by fixing the core atoms artificially,44 our approach is more general and provides full insight on the particle size and shape controlled reactivity since it does not impose any constraints on geometry. The dispersion, i.e., the ratio between the number of surface atoms and overall number of atoms in truncated octahedral particles, is plotted in Figure 4a for particles sizes up to 8 nm. Decreasing the particle sizes results in improved dispersion and an increase in the number of sites on the surface of the catalyst. 3716

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Figure 5. Averaged oxygen binding energy for fcc and all surface sites shown in Figure 4 as a function of particle size.

Figure 4. (a) Size dependence of dispersion and surface percentage of atoms on {111}, {100} facets and on the edges between the facets. (b) The geometry of a 2.6 nm particle showing the {111} and {100} facets and the coordination numbers of atoms comprising these facets and edges. (c) Calculated oxygen binding energy as a function of the particle size for various adsorption sites. Sites include hollow (face centered cubic (fcc) and hexagonal close packed) on {111} facets, bridge site on {100} facets and bridge sites on edges between different facets. Besides site notation, coordination numbers for atoms making sites are given in brackets.

The plot also shows the distribution of these surface sites per {111}, {100} facets and the edges of particles. The percentage of predominant (111) sites and also (100) sites decreases with a decrease in particle size. A sharp drop in the number of these sites is seen below 3 nm. The opposite trend is seen for the edge sites. Decreasing particle size increases the percentage of edge sites, which become predominant sites below ∼2 nm. We examine the surface reactivity of these different sites in particles by calculating the oxygen binding energies on the truncated octahedral particles between 0.8 and 3 nm having 38, 79, 116, 201, 314, 586, and 807 total Pt atoms, respectively. The details of the calculation can be found in the Supporting Information. Figure 4b shows the geometry of a 2.6 nm particle. The {100} and {111} facets are labeled with the coordination numbers for the atoms comprising these facets and edges between them. The coordination numbers are 9, 8, 7, and 6 for (111), (100), edge, and vertex atoms, respectively. Figure 4c shows the relationship between oxygen binding energy for possible adsorption sites on particles with sizes between 0.8 and 3 nm. For the 3 nm particle, only fcc sites were considered due to the computational intensity. The reactivity of the surface atom depends on particle size and its coordination number, reflected by the different oxygen binding energies. At an identical site, a decrease in binding energy with increase of particle size up to 2.1 nm is evident, while above 2.1 nm binding energies remain close or lower than that of a 2.1 nm particle. At the same particle size, decrease of the coordination number of atoms making up the adsorption site results in increased binding energy for adsorption sites on {111} and {100} facets. Further

lowering of the coordination number at edge sites results in an even higher increase in binding energy. However at 100/111 edge sites this is more pronounced even for the sites with the same coordination numbers as in 111/111 edges. We note that despite the same overall coordination number at edges, the geometry of sites is different in that the 100/111 edges have lower coordination to the neighboring surface atoms (5) than 111/111 (6) edge sites. These changes toward stronger binding with decrease of the coordination number can be understood through the profound changes in the electronic structure, in particular the energy of d-electrons of Pt. The lower coordination results in a narrower d-band and lower d-band filling,42 which consequently causes a stronger Pt O bond at these sites. The stronger Pt O bond is reflected with lower potentials for Pt oxides reduction peaks for smaller particles that have lower coordination numbers (see Figure S4, Supporting Information). The average oxygen binding energies of all surface sites, and fcc sites for particles with different sizes are shown in Figure 5. Due to the contribution from edge sites, the average oxygen binding energies of all surface sites are higher than that of fcc sites. In comparison with the calculated binding energy of extended Pt(111) surface ( 1.23 eV) to that in particles, the binding energy on {111} facets is lower by ∼0.17 eV on a 3 nm particle ( 1.06 eV). This is consistent with the change of the reactivity and shifts in the d-band centers due to the compressive surface strain observed in particles.44,45 Such a slight lowering of binding energy was found to improve the overall specific activity in the previous reports.40,43,46,47 Thus our results indicate that the specific activity in {111} facets of Pt nanoparticles in the range of 2 3 nm is higher than that of Pt(111) single crystal surface, in good agreement with the previous study using a semispherical model.44 However the contribution of {111} facets to overall specific acitivity becomes smaller in particles as their size decreases as shown in Figure 4a. Much higher binding energy at edge sites indicates that the rate-determining step (RDS) for these sites is desorption of intermediates. The observed slight increase in specific activity with increasing particle size above 2.2 nm can be attributed to the increase in the number of active sites in {111} facets. For example, a 4.7 nm particle has 11.4% more active sites than a 2.2 nm particle. Because of the strong reactivity of edge sites, it can be concluded that these sites are blocked by adsorbed intermediates during the oxgen reduciton reaction. As particle size decreases, the oxygen binding to Pt is expected to become weaker due to larger compressive strains.45 It is interesting 3717

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Nano Letters to note that all the sites including the {111} facets for particles below 2.1 nm have a very strong interaction with oxygen in Figure 4c. The stronger oxygen binding is likely due to the increase in percentage of the undercoordinated Pt atoms. These two opposing effects lead to a maximum mass activity around 2.2 nm and a sharp drop of the specific activity observed in Figure 3. In conclusion, we determined the size-dependent specific and mass activities of the oxygen reduction in HClO4 solutions on the Pt particles in the range of 1 5 nm. The mass activity increases by 2-fold from 1.3 to 2.2 nm and decreases as the particle size further increases. On the other hand, the specific activity increases rapidly by 4-fold as the particle grows to 2.2 nm and then slowly as particle size further increases. As opposed to low index single crystals, which are dominated by terrace atoms, the contributions of various sites cannot be neglected in the case of Pt nanoparticles. The maximal mass activity at 2.2 nm is well explained based on DFT calculations performed on fully relaxed nanoparticles. The weakest oxygen binding was found about 2.2 nm for all surface sites, with that on {111} facets being significantly weaker than others. The presence of the edge sites is the main reason for the low specific activity in nanoparticles due to very strong oxygen binding energies at these sites. For particles smaller than 2.2 nm, all the surface sites including the {111} show very strong oxygen binding energy, resulting in a very low specific activity. We conclude that both mass activity and specific activity are low for particle sizes smaller than 2.2 nm. We also confirmed that the state-of-the-art Pt/C catalyst with an average particle size of 2.5 nm has almost the maximum mass activity that conventional Pt nanoparticle can achieve. Our results emphasize the importance of facet sites in the particles for oxygen reduction reaction. We believe that our results can help in understanding the general role of different surface sites in nanoparticles during catalytic reactions and designing better catalysts, such as shape controlled nanocrystals.

’ ASSOCIATED CONTENT

bS

Supporting Information. Experimental procedures, DFT calculations, and additional figures. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Authors

*E-mail: [email protected]; [email protected].

’ ACKNOWLEDGMENT Part of the DFT calculations in this research used resources of the National Center for Computational Sciences at Oak Ridge National Laboratory, which is supported by the Office of Science of the Department of Energy under Contract DE-AC0500OR22725. ’ REFERENCES (1) Somorjar, G. A. Introduction to Surface Chemistry and Catalysis; John Wiley & Sons: New York, 1993. (2) Henry, C. R. Surf. Sci. Rep. 1998, 31, 231–325. (3) Yoo, S. J.; Jeon, T.-Y.; Lee, K.-S.; Park, K.-W.; Sung, Y.-E. Chem. Commun. 2010, 46, 794. (4) Hayden, B. E.; Suchsland, J.-P. Support and Particle Size Effects in Electrocatalysis. In Fuel Cell Catalysis: A Surface Science Approach;

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