Active and Stable Ir@Pt Core–Shell Catalysts for Electrochemical

Dec 28, 2016 - ... Daniel Jalalpoor , Ferdi Schüth , Karl J.J. Mayrhofer , Marc Ledendecker ... Ariel Jackson , Alaina Strickler , Drew Higgins , Tho...
0 downloads 0 Views 3MB Size
Download PDF
Active and Stable Ir@Pt Core−Shell Catalysts for Electrochemical Oxygen Reduction Alaina L. Strickler,† Ariel Jackson,† and Thomas F. Jaramillo*,†,‡ †

Department of Chemical Engineering, Stanford University, Stanford, California 94305, United States SUNCAT Center for Interface Science and Catalysis, SLAC National Accelerator Laboratory, Menlo Park, California 94025, United States

‡

S Supporting Information *

ABSTRACT: Electrochemical oxygen reduction is an important reaction for many sustainable energy technologies, such as fuel cells and metal−air batteries. Kinetic limitations of this reaction, expensive electrocatalysts, and catalyst instability, however, limit the commercial viability of such devices. Herein, we report an active Ir@Pt core−shell catalyst that combines platinum overlayers with nanostructure effects to tune the oxygen binding to the Pt surface, thereby achieving enhanced activity and stability for the oxygen reduction reaction. Ir@ Pt nanoparticles with several shell thicknesses were synthesized in a scalable, inexpensive, one-pot polyol method. Electrochemical analysis demonstrates the activity and stability of the Ir@Pt catalyst, with specific and mass activities increasing to 2.6 and 1.8 times that of commercial Pt/C (TKK), respectively, after 10 000 stability cycles. Activity enhancement of the Ir@Pt catalyst is attributed to weakening of the oxygen binding to the Pt surface induced by the Ir core.

O

intermediates bind to the Pt surface, a descriptor shown previously to correlate well with ORR catalytic activity.9−13 Density functional theory (DFT) calculations have shown that Pt has near-optimal oxygen binding strength; however, the ORR activity can be enhanced by slightly weakening the oxygen binding to the Pt surface.14,15 These types of oxygen binding energy calculations have helped to explain the observed enhancement in ORR activity of Pt alloys and overlayer structures,16−20 and have successfully predicted novel highly active materials, such as Pt3Y.21 To select a core material capable of enhancing ORR activity, we utilized a systematic approach for core−shell catalyst design developed in a previous study.22 In this recent study of the Ru@ Pt system, it was shown that the ORR catalytic activity can be significantly enhanced by selecting a core material, such as Ru, that is theoretically predicted to overweaken the oxygen binding strength calculated for a Pt monolayer on the flat core metal surface (Pt ML /Ru). The oxygen binding can then be strengthened toward the peak of the activity volcano by increasing the Pt shell thickness or through the presence of undercoordinated sites found in the nanoparticle morphology.23 The predicted enhancement of the Ru@Pt nanoparticle system over pure Pt nanoparticles was experimentally verified.22

ne of the most imperative challenges facing modern society is the development of a sustainable energy economy. The replacement of traditional fossil fuel sources with environmentally benign, efficient, and inexpensive energy conversion and storage devices is essential toward this end. As carbon-free energy conversion devices, polymer electrolyte membrane fuel cells (PEMFCs) are widely considered promising clean energy technologies, with commercialization in various sectors including transportation and stationary power.1 Improvements in device efficiencies and catalyst materials will continue to reduce technological and economic barriers and further enable the widespread deployment of fuel cell technology.2,3 Thus considerable research has focused on developing more active, inexpensive electrocatalysts for the kinetically slow oxygen reduction reaction (ORR) at the fuel-cell cathode.4−6 Because of the acidic and oxidizing environment in a PEMFC, precious metal platinum remains the ORR catalyst material for commercial fuel cells despite its high cost and insufficient performance.2,7 Recently, many researchers have looked to enhance activity and reduce Pt loading via core−shell nanostructures, whereby a Pt shell forms an overlayer structure on a core composed of a different metal or alloy.8 In addition to increasing Pt mass activity, the core material can increase the per Pt site activity (specific activity) by altering the electronic properties of the surface.9 Ligand and strain effects induced by the core material serve to tune the strength at which oxygen © XXXX American Chemical Society

Received: November 7, 2016 Accepted: December 22, 2016

244

DOI: 10.1021/acsenergylett.6b00585 ACS Energy Lett. 2017, 2, 244−249

Letter

http://pubs.acs.org/journal/aelccp

Letter

ACS Energy Letters Utilizing this same design concept, we hypothesize that other active core−shell systems for the ORR can be identified. Previous DFT work has shown that a Pt monolayer on Ir (PtML/Ir) has slightly more favorable binding energetics than PtML/Ru.24 The Ir@Pt nanoparticle system should thus have similar if not superior performance compared to Ru@Pt for ORR catalysis. Additionally, unlike many nonprecious metal cores previously investigated,25−27 Ir is among few materials that are thermodynamically stable and not predicted to surface segregate under ORR conditions.28−31 Although Ir is a precious metal and does not have the additional benefit of substantially decreased material cost, the potential activity and stability enhancement of the system for the ORR would be a significant achievement toward reducing overall precious metal usage in fuel-cell technologies. Extending upon the few reports that have investigated the Ir@ Pt system,32−36 this work explores the impact of varying Pt shell thickness and particle size uniformity. Herein, we report a higher activity Ir@Pt nanoparticle catalyst synthesized via a facile, scalable, one-pot polyol synthesis method. The highest performance 2:1 Pt:Ir nanoparticles were found to outperform the commercial standard Pt catalyst in both mass and specific activity and demonstrated excellent stability. Enhanced activity is attributed to the well-defined core−shell structure that tunes the oxygen binding energies by strategically coupling multiple Pt overlayers with nanostructure effects (i.e., undercoordinated surface atoms). This approach offers a reliable way to overcome conventional challenges facing Pt nanoparticle catalysts for the ORR by allowing one to achieve high Pt surface area without the diminished specific activity that accompanies undercoordinated sites. The Ir@Pt core−shell nanoparticles were synthesized using a two-step, one-pot wet chemical method similar to that previously reported for [email protected] In brief, an Ir precursor was quickly reduced at elevated temperatures in ethylene glycol to form the monodisperse Ir nanoparticles. A Pt precursor was then slowly reduced onto the existing Ir cores via a slow temperature ramp to form the core−shell nanoparticles (see Supporting Information for details). Material and electrochemical characterization for 2:1 Pt:Ir are presented here, while those for the additional compositions are presented in the Supporting Information. Transmission electron microscopy (TEM) micrographs of synthesized nanoparticles are shown in Figure 1 along with their respective particle size distributions. Ir-only and Ir@Pt have tight size distributions with average diameters of 1.4 ± 0.4 and 2.9 ± 1.1 nm, respectively. The Ir nanoparticle synthesis was found to be remarkably robust with monodisperse sub-2 nm particles easily achievable regardless of experimental conditions. Agreeing with previous studies, it was found that small, uniform Ir nanoparticles can be achieved without the use of any capping or stabilizing agent, while this is not the case for Pt.38 Despite efforts to modify the synthesis, larger Ir particles were not achieved, likely due to the relatively small barrier for homogeneous nucleation of Ir nanoparticles compared with heterogeneous growth.39 The Ir@Pt particles have a bimodal distribution, indicating that a small portion of uncoated Ir cores remains after Pt coating. It follows that the actual average size for the coated Ir@Pt particles corresponds to the peak at a greater diameter of 3.4 ± 0.6 nm (ca. 4.4 Pt monolayers). Dark-field scanning TEM (STEM) micrographs and corresponding energy-dispersive spectroscopy (EDS) spectra are shown in Figure 2. Because of the proximity of the Pt and Ir peaks, the small particle size, and the substantial drift that occurs while imaging, achieving both sufficient signal and accurate

Figure 1. TEM images of (a) Ir and (b) Ir@Pt. (c) HR-TEM of Ir@Pt. (d) Particle size distributions shown as a normalized frequency.

Figure 2. STEM image (a) and STEM-EDS (b) taken at the center of an Ir@Pt particle. STEM image (c) and STEM-EDS (d) taken at the edge of the same Ir@Pt particle. The hollow circles in (a) and (c) depict where the point scans in (b) and (d) were performed, respectively. EDS data was smoothed using adjacent averaging. The Cu peak is due to the Cu mesh TEM grid.

spatial resolution is problematic. Here a series of point scans with drift correction were performed to maintain reliable spatial resolution (see the Supporting Information for details). Low Xray counts remain an unfortunate consequence of these challenges; however, sufficient signal was acquired to yield reliable composition quantification. Orange hollow circles on the STEM images indicate where the scans were performed. Point scans revealed that the particle centers and edges had a Pt:Ir ratio of 1:1 and 8:1, respectively. Pt enrichment and the absence of a 245

DOI: 10.1021/acsenergylett.6b00585 ACS Energy Lett. 2017, 2, 244−249

Letter

ACS Energy Letters

Figure 3. Electrochemical characterization of commercial Pt/C (TKK) (black), Ir (blue), and Ir@Pt (green) nanoparticles. All experiments were performed in 0.1 M HClO4 electrolyte solution at 20 mV/s and 1600 rpm using a reversible hydrogen reference electrode (RHE). Catalysts have a mass loading of 10 μgPt/cm2 (10 μgIr/cm2 for Ir-only particles). (a) Cyclic voltammograms in N2-saturated electrolyte. Inset depicts 30 mV anodic *OH desorption shift of Ir@Pt compared to Pt/C (TKK). (b) Background-corrected anodic sweep cyclic voltammograms in O2-saturated electrolyte. Mass activity (c) and specific activity (d) of the catalysts.

known to exhibit HUPD.41 ECSA calculations were performed after electrochemical cleaning, when the majority of uncoated Ir cores were removed (see Supporting Information for details), to avoid significant overestimation of Pt surface area. With the same Pt loading, commercial Pt/C (TKK) had a higher ECSA (67 ± 6 m2/gPt) compared with Ir@Pt (37 ± 5 m2/gPt), an effect of differences in particle size and agglomeration. At potentials above 0.7 V, *OH adsorption and desorption is observed, whereby the shift in the *OH desorption peak can describe trends in the Pt−O binding energy.42 From the inset of Figure 3a, it was found that Pt/C (TKK) has an *OH desorption peak at 0.774 V, while Ir@Pt has a peak at 0.804 V vs RHE. This 30 mV anodic shift indicates that the Ir@Pt catalyst has, on average, a weakened Pt−O bond compared to Pt/C (TKK). Correspondingly, a shift toward lower potentials in the H adsorption region for Ir@Pt compared with Pt/C (TKK) was also observed (Figure S10). This destabilization of adsorbed H has been previously correlated with increased ORR activity.16 Weakening of the *OH and *H adsorbate binding is an expected result of the Ir core. A similar shift in *OH desorption was also observed for Ru@Pt nanoparticles, reflecting the similarities between the two systems.22 The slightly smaller shift of Ir@Pt relative to Ru@Pt is consistent with theoretical predictions that PtML/Ir binds oxygen slightly stronger than PtML/Ru, as discussed above. Anodic linear sweep voltammograms of synthesized Ir-only, Ir@Pt, and Pt/C (TKK) in oxygen-saturated electrolyte are shown in Figure 3b. The Ir@Pt catalyst is highly active with an onset and half-wave potential similar to Pt/C (TKK). The ORR

clear Ir peak at the particle edge are indicative of a core−shell structure. No Pt-only particles were observed, although some uncoated Ir particles were found, consistent with the bimodal particle size distribution previously discussed. The successful coating of the Ir cores is a result of the slow temperature ramp used during Pt shell formation. This discourages homogeneous nucleation of Pt-only particles by avoiding supersaturated conditions and providing a surface for heterogeneous growth. The fact that uncoated Ir cores remain could indicate that Pt experiences preferential or accelerated growth onto itself rather than Ir, despite the similar lattice spacing of the two metals (0.222 and 0.226 nm for Ir(111) and Pt(111), respectively). Advanced shell coating synthetic procedures can likely yield improved coating uniformity and Pt utilization. Electrochemical characterization was performed using a rotating disk electrode (RDE) with a loading of 10 μgPt/cm2 in 0.1 M perchloric acid electrolyte. Mass loadings are based on the amount of material introduced during synthesis and thus represent a lower bound for mass-based activities as losses such as incomplete yield are neglected. Synthesized materials were electrochemically cleaned prior to testing (see Supporting Information).40 Reported ORR activities and Pt-based surface areas are averages of five independent measurements. Figure 3a shows a typical cyclic voltammogram of Ir@Pt after electrochemical cleaning compared with a state-of-the-art platinum catalyst Pt/C (TKK) in a nitrogen-saturated environment. Electrochemically active surface areas (ECSAs) were determined from the hydrogen underpotential deposition (HUPD) region (0−0.4 V vs RHE). It should be noted that like Pt, Ir is also 246

DOI: 10.1021/acsenergylett.6b00585 ACS Energy Lett. 2017, 2, 244−249

Letter

ACS Energy Letters

Figure 4. Electrochemical stability of commercial Pt/C (TKK) (black) and Ir@Pt (green) nanoparticles. Catalyst stability was analyzed by cycling between 0.6 and 1.1 V vs RHE at 125 mV/s without rotation in oxygen-saturated 0.1 M HClO4 electrolyte solution. (a) Cyclic voltammograms in N2-saturated electrolyte, (b) background-corrected anodic sweep cyclic voltammograms in O2-saturated electrolyte, mass activity (c), and specific activity (d) of the catalysts before (solid lines) and after 10 000 stability cycles (dashed lines). (e) Mass activity (left) and specific activity (right) of Pt/C (TKK) and Ir@Pt at 0.9 V vs RHE initially (solid green bars) and after 10 000 stability cycles (striped purple bars). The mass and specific activity of the Ir@Pt catalyst improve after stability testing and are greater than the commercial Pt/C (TKK) catalyst.

pure Pt was not observed, here we find that the use of multiple Pt layers on Ir@Pt nanoparticles leads to a significant increase in Pt intrinsic activity. A single Pt monolayer on Ir overweakens oxygen binding to the Pt surface due to compressive strain and ligand effects, resulting in a decrease in specific activity relative to pure Pt.45 In contrast, the Ir@Pt nanoparticle system explored here takes advantage of nanostructuring and multiple Pt layers to counterbalance weakened binding induced by the Ir core. This allows the tuning of the oxygen binding toward the optimum resulting in an increased specific activity over pure Pt. In accordance with this theory, a recent report has found that multiple Pt monolayers deposited on a single-crystal Ir(111) substrate indeed result in substantially increased intrinsic activity over pure Pt(111).46 The mass activities of Ir@Pt and Pt/C (TKK) at 0.9 V vs RHE are 0.35 ± 0.02 A/mgPt (0.24 A/mgPGM) and 0.31 ± 0.04 A/mgPt, respectively. Ir@Pt provides a 13% increase in Pt-based mass activity over that of state-of-the-art Pt/C (TKK). Becaue of its high specific activity and core−shell structure, it is very likely that optimized shell thickness and particle dispersion can lead to even further enhancement in Pt mass-based activities for Ir@Pt. To evaluate long-term stability of the Ir@Pt catalyst, an accelerated stability test (AST) consisting of 10 000 cycles between 0.6 and 1.0 V vs RHE (125 mV/s) was performed. Figure 4 compares the electrochemical performance of Pt/C (TKK) and the highest performance Ir@Pt sample before and after the AST. The performance retention after AST averaged for three individual samples is compared with Pt/C (TKK) in Figure S11. From Figure 4, it can be seen that after cycling, both catalysts show a significant decrease in HUPD current, indicating that Pt surface area is being lost. For the Ir@Pt sample, some of

activity of Ir-only particles is shown before electrochemical cleaning as these particles were found to be unstable upon cycling (see Figure S3). Ir alone is clearly a very poor ORR catalyst. The unexpected instability of the Ir-only nanoparticles despite predicted bulk thermodynamic stability is likely a result of dissolution of the small Ir particles. A similar phenomenon was demonstrated for Pt, whereby small sub-4 nm nanoparticles were found to undergo direct electrochemical dissolution in acid following the Gibbs−Thomson equation.43 This instability indicates that uncoated Ir core particles are quickly removed and are not significantly contributing to the observed activity for Ir@Pt. To directly compare the catalysts, Pt-based mass and specific activities are shown in Figure 3c,d, respectively. At 0.9 V vs RHE, the specific activity of Ir@Pt (0.90 ± 0.03 mA/cm2Pt) far exceeds that of Pt/C (TKK) (0.47 ± 0.05 mA/cm2Pt). Although Ir@Pt particles are slightly larger than Pt/C (TKK), the size effect44 alone cannot explain the enhanced activity as the Ir@Pt specific activity also exceeds that of larger Pt nanoparticles (see Supporting Information). The ∼2 times enhancement in specific activity of Ir@Pt can be partially attributed to the electronic effects of the Ir core. The Ir core and slightly larger particle size serve to weaken the oxygen binding energy to the Pt surface, as evidenced by the anodic shift of the *OH desorption peak, leading to intrinsically more active Pt sites compared with Pt/C (TKK). The Ir@Pt specific activity also exceeds that of Ru@Pt (0.63 mA/cm2Pt)22 which is reflective of its more favorable oxygen binding energetics. In contrast with previous reports of single Pt monolayer systems, for example, Ir@PtML nanoparticles33 and PtML/Ir(111) single-crystal films,45 where specific activity enhancement over 247

DOI: 10.1021/acsenergylett.6b00585 ACS Energy Lett. 2017, 2, 244−249

ACS Energy Letters

■

the loss of current in this region may be attributed to the removal of uncoated Ir cores. Despite the apparent loss of Pt surface sites, the ORR mass activity of the Ir@Pt catalyst increases to 0.42 A/ mgPt (0.29 A/mgPGM) after AST, while that of Pt/C (TKK) falls to 0.24 A/mgPt. The specific activities of both samples increase after stability testing, with Ir@Pt substantially increasing to 1.28 mA/cm2Pt and Pt/C (TKK) slightly increasing to 0.49 mA/ cm2Pt. For Ir@Pt, an increase in specific activity along with a loss in Pt ECSA indicate that either low activity Pt sites are being removed or surface rearrangement is revealing a greater number of high activity Pt sites. Because the mass activity is also increasing, the latter must be occurring to some extent. XPS analysis affirms that some Ir is lost during initial electrochemical cleaning (Figures S6−S9) as uncoated Ir cores undergo dissolution; however, the Ir content is stable with continued testing (see Supporting Information for details). TEM micrographs acquired before and after the AST indicate that isolated particles maintain their approximate size and morphology while clustered particles tend to agglomerate during operation (see Figure S12), consistent with the observed decrease in Pt ECSA (Figure S11). TEM-EDS data before and after the AST agree with XPS results, as both Pt and Ir are present after the AST. The relative concentration of Ir compared with Pt appears to decrease in the TEM-EDS data, indicative of the loss of uncoated Ir cores and potentially other Ir loss mechanisms. Overall, the TEM micrographs and TEM-EDS data measured after the AST indicate that the Ir@Pt catalyst system is dynamic; subtle reconfigurations of Ir and Pt on atomic length-scales could be responsible for the substantial improvement in specific activity during AST, which ultimately helps to maintain a high mass activity as well despite losses in ECSA. These results suggest that by increasing coating uniformity and optimizing Pt shell thickness and particle dispersion, a durable Ir@Pt catalyst with even higher activity can be achieved. In this study, Ir@Pt core−shell catalysts with different compositions have been successfully synthesized via a scalable, inexpensive, one-pot polyol synthesis method. TEM, STEMEDS, and XPS verified that the most active 2:1 Pt:Ir catalyst is composed of 3.4 ± 0.6 nm Ir@Pt nanoparticles in a core−shell structure. Electrochemical analysis reveals that the Ir@Pt catalyst is highly active for the ORR with mass and specific activities of 0.35 A/mgPt (0.24 A/mgPGM) and 0.90 mA/cm2Pt, respectively. These compare favorably to the commercial standard Pt/C (TKK) with activities of 0.31 A/mgPt and 0.47 mA/cm2Pt. The Ir@Pt catalyst exhibits excellent stability with its mass and specific activity increasing to 0.42 A/mgPt (0.29 A/mgPGM) and 1.28 mA/cm2Pt, respectively, after 10 000 accelerated stability cycles. The 2.6-fold enhancement in specific activity of Ir@Pt over Pt/C (TKK) is attributed to the weakening of the Pt−O bond, which was evidenced by the anodic shift of the *OH desorption peak and predicted theoretically. Further enhancements in mass activity of the Ir@Pt catalyst can be realized through increasing particle dispersion, increasing shell coating uniformity, decreasing shell thickness, and optimizing particle morphology. Lastly, this work has shown that the generalized approach to core material selection developed in our previous work22 can be extended to successfully predict enhanced performance ORR core−shell catalysts.

Letter

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsenergylett.6b00585. Details of catalyst preparation, material and electrochemical characterization, performance of additional Ir@ Pt compositions, and XPS analysis (PDF)

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]; jaramillogroup.stanford.edu. ORCID

Thomas F. Jaramillo: 0000-0001-9900-0622 Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This work was supported by the U.S. Department of Energy, Basic Energy Sciences, through the SUNCAT Center for Interface Science and Catalysis. We thank Jonathan Snider for assistance with STEM, Drew Higgins for insightful discussions, and Jakob Kibsgaard for the graphical abstract figure. A.S. acknowledges fellowship support from the National Science Foundation Graduate Research Fellowship Program (NSFGRFP). Any opinions, findings, and conclusions or recommendations expressed in this material are those of the authors and do not necessarily reflect the views of the National Science Foundation.

■

REFERENCES

(1) Curtin, S.; Gangi, J. The Business Case for Fuel Cells 2015: Powering Corporate Sustainability; Fuel Cell and Hydrogen Energy Association: 2015 (2) Gasteiger, H. A.; Kocha, S. S.; Sompalli, B.; Wagner, F. T. Activity Benchmarks and Requirements for Pt, Pt-Alloy, and Non-Pt Oxygen Reduction Catalysts for PEMFCs. Appl. Catal., B 2005, 56, 9−35. (3) Markovic, N. M.; Ross, P. N. New Electrocatalysts for Fuel Cells from Model Surfaces to Commercial Catalysts. CATTECH 2000, 4, 110−126. (4) Pedersen, C. M.; Escudero-Escribano, M.; Velázquez-Palenzuela, A.; Christensen, L. H.; Chorkendorff, I.; Stephens, I. E. L. Benchmarking Pt-Based Electrocatalysts for Low Temperature Fuel Cell Reactions with the Rotating Disk Electrode: Oxygen Reduction and Hydrogen Oxidation in the Presence of CO (Review Article). Electrochim. Acta 2015, 179, 647−657. (5) Guo, S.; Zhang, S.; Sun, S. Tuning Nanoparticle Catalysis for the Oxygen Reduction Reaction. Angew. Chem., Int. Ed. 2013, 52, 8526− 8544. (6) Debe, M. K. Electrocatalyst Approaches and Challenges for Automotive Fuel Cells. Nature 2012, 486, 43−51. (7) Rabis, A.; Rodriguez, P.; Schmidt, T. J. Electrocatalysis for Polymer Electrolyte Fuel Cells: Recent Achievements and Future Challenges. ACS Catal. 2012, 2, 864−890. (8) Oezaslan, M.; Hasché, F.; Strasser, P. Pt-Based Core−Shell Catalyst Architectures for Oxygen Fuel Cell Electrodes. J. Phys. Chem. Lett. 2013, 4, 3273−3291. (9) Kitchin, J. R.; Nørskov, J. K.; Barteau, M. A.; Chen, J. G. Modification of the Surface Electronic and Chemical Properties of Pt(111) by Subsurface 3d Transition Metals. J. Chem. Phys. 2004, 120, 10240−10246. (10) Nørskov, J. K.; Rossmeisl, J.; Logadottir, A.; Lindqvist, L.; Kitchin, J. R.; Bligaard, T.; Jónsson, H. Origin of the Overpotential for Oxygen 248

DOI: 10.1021/acsenergylett.6b00585 ACS Energy Lett. 2017, 2, 244−249

Letter

ACS Energy Letters Reduction at a Fuel-Cell Cathode. J. Phys. Chem. B 2004, 108, 17886− 17892. (11) Kitchin, J. R.; Nørskov, J. K.; Barteau, M. A.; Chen, J. G. Role of Strain and Ligand Effects in the Modification of the Electronic and Chemical Properties of Bimetallic Surfaces. Phys. Rev. Lett. 2004, 93, 4− 7. (12) Strasser, P.; Koh, S.; Anniyev, T.; Greeley, J.; More, K.; Yu, C.; Liu, Z.; Kaya, S.; Nordlund, D.; Ogasawara, H.; et al. Lattice-Strain Control of the Activity in Dealloyed Core-Shell Fuel Cell Catalysts. Nat. Chem. 2010, 2, 454−460. (13) Zhang, S.; Zhang, X.; Jiang, G.; Zhu, H.; Guo, S.; Su, D.; Lu, G.; Sun, S. Tuning Nanoparticle Structure and Surface Strain for Catalysis Optimization. J. Am. Chem. Soc. 2014, 136, 7734−7739. (14) Stephens, I. E. L.; Bondarenko, A. S.; Grønbjerg, U.; Rossmeisl, J.; Chorkendorff, I. Understanding the Electrocatalysis of Oxygen Reduction on Platinum and Its Alloys. Energy Environ. Sci. 2012, 5, 6744−6762. (15) Hammer, B.; Nørskov, J. K. Theoretical Surface Science and Catalysis  Calculations and Concepts. Adv. Catal. 2000, 45, 71−129. (16) Escudero-Escribano, M.; Malacrida, P.; Hansen, M. H.; VejHansen, U. G.; Velazquez-Palenzuela, A.; Tripkovic, V.; Schiotz, J.; Rossmeisl, J.; Stephens, I. E. L.; Chorkendorff, I. Tuning the Activity of Pt Alloy Electrocatalysts by Means of the Lanthanide Contraction. Science 2016, 352, 73−76. (17) Vukmirovic, M. B.; Zhang, J.; Sasaki, K.; Nilekar, A. U.; Uribe, F.; Mavrikakis, M.; Adzic, R. R. Platinum Monolayer Electrocatalysts for Oxygen Reduction. Electrochim. Acta 2007, 52, 2257−2263. (18) Huang, X.; Zhao, Z.; Cao, L.; Chen, Y.; Zhu, E.; Lin, Z.; Li, M.; Yan, A.; Zettl, A.; Wang, Y. M.; Duan, X.; Mueller, T.; Huang, Y. HighPerformance Transition Metal − Doped Pt3Ni Octahedra for Oxygen Reduction Reaction. Science 2015, 348, 1230−1234. (19) Stamenkovic, V. R.; Fowler, B.; Mun, B. S.; Wang, G.; Ross, P. N.; Lucas, C. A.; Marković, N. M. Improved Oxygen Reduction Activity on Pt3Ni(111) via Increased Surface Site Availability. Science 2007, 315, 493−497. (20) Adzic, R. R.; Zhang, J.; Sasaki, K.; Vukmirovic, M. B.; Shao, M.; Wang, J. X.; Nilekar, A. U.; Mavrikakis, M.; Valerio, J. A.; Uribe, F. Platinum Monolayer Fuel Cell Electrocatalysts. Top. Catal. 2007, 46, 249−262. (21) Greeley, J.; Stephens, I. E. L.; Bondarenko, A. S.; Johansson, T. P.; Hansen, H. A.; Jaramillo, T. F.; Rossmeisl, J.; Chorkendorff, I.; Nørskov, J. K. Alloys of Platinum and Early Transition Metals as Oxygen Reduction Electrocatalysts. Nat. Chem. 2009, 1, 552−556. (22) Jackson, A.; Viswanathan, V.; Forman, A. J.; Larsen, A. H.; Nørskov, J. K.; Jaramillo, T. F. Climbing the Activity Volcano: CoreShell Ru@Pt Electrocatalysts for Oxygen Reduction. ChemElectroChem 2014, 1, 67−71. (23) Viswanathan, V.; Hansen, H. A.; Rossmeisl, J.; Nørskov, J. K. Universality in Oxygen Reduction Electrocatalysis on Metal Surfaces. ACS Catal. 2012, 2, 1654−1660. (24) Greeley, J.; Nørskov, J. K. Combinatorial Density Functional Theory-Based Screening of Surface Alloys for the Oxygen Reduction Reaction. J. Phys. Chem. C 2009, 113, 4932−4939. (25) Hasché, F.; Oezaslan, M.; Strasser, P. Activity, Stability, and Degradation Mechanisms of Dealloyed PtCu3 and PtCo3 Nanoparticle Fuel Cell Catalysts. ChemCatChem 2011, 1805−1813. (26) Chen, S.; Gasteiger, H. A.; Hayakawa, K.; Tada, T.; Shao-Horn, Y. Platinum-Alloy Cathode Catalyst Degradation in Proton Exchange Membrane Fuel Cells: Nanometer-Scale Compositional and Morphological Changes. J. Electrochem. Soc. 2010, 157, A82−A97. (27) Hasché, F.; Oezaslan, M.; Strasser, P. Activity, Structure and Degradation of Dealloyed PtNi3 Nanoparticle Electrocatalyst for the Oxygen Reduction Reaction in PEMFC. J. Electrochem. Soc. 2012, 159, B25. (28) Tsai, H. C.; Yu, T. H.; Sha, Y.; Merinov, B. V.; Wu, P. W.; Chen, S. Y.; Goddard, W. A. Density Functional Theory Study of Pt3M Alloy Surface Segregation with Adsorbed O/OH and Pt3Os as Catalysts for Oxygen Reduction Reaction. J. Phys. Chem. C 2014, 118, 26703−26712.

(29) Ramírez-Caballero, G. E.; Ma, Y.; Callejas-Tovar, R.; Balbuena, P. B. Surface Segregation and Stability of Core-Shell Alloy Catalysts for Oxygen Reduction in Acid Medium. Phys. Chem. Chem. Phys. 2010, 12, 2209−2218. (30) Wang, L.-L.; Johnson, D. D. Predicted Trends of Core-Shell Preferences for 132 Late Transition-Metal Binary-Alloy Nanoparticles. J. Am. Chem. Soc. 2009, 131, 14023−14029. (31) Pourbaix, M. Atlas of Electrochemical Equilibria in Aqueous Solutions; National Association of Corrosion Engineers: Houston, TX, 1974. (32) Karan, H. I.; Sasaki, K.; Kuttiyiel, K.; Farberow, C. A.; Mavrikakis, M.; Adzic, R. R. Catalytic Activity of Platinum Monolayer on Iridium and Rhenium Alloy Nanoparticles for the Oxygen Reduction Reaction. ACS Catal. 2012, 2, 817−824. (33) Kuttiyiel, K. A.; Sasaki, K.; Choi, Y.; Su, D.; Liu, P.; Adzic, R. R. Bimetallic IrNi Core Platinum Monolayer Shell Electrocatalysts for the Oxygen Reduction Reaction. Energy Environ. Sci. 2012, 5, 5297−5304. (34) Sasaki, K.; Kuttiyiel, K. A.; Su, D.; Adzic, R. R. Platinum Monolayer on IrFe Core-Shell Nanoparticle Electrocatalysts for the Oxygen Reduction Reaction. Electrocatalysis 2011, 2, 134−140. (35) Chen, D.; Chen, R.; Dang, D.; Shu, T.; Peng, H.; Liao, S. High Performance of Core−shell Structured Ir@Pt/C Catalyst Prepared by a Facile Pulse Electrochemical Deposition. Electrochem. Commun. 2014, 46, 115−119. (36) Kuttiyiel, K. A.; Choi, Y.; Sasaki, K.; Su, D.; Hwang, S.-M.; Yim, S.D.; Yang, T.-H.; Park, G.-G.; Adzic, R. R. Tuning Electrocatalytic Activity of Pt Monolayer Shell by Bimetallic Ir-M (M = Fe, Co, Ni or Cu) Cores for the Oxygen Reduction Reaction. Nano Energy 2016, 29, 261. (37) Alayoglu, S.; Nilekar, A. U.; Mavrikakis, M.; Eichhorn, B. Ru-Pt Core-Shell Nanoparticles for Preferential Oxidation of Carbon Monoxide in Hydrogen. Nat. Mater. 2008, 7, 333−338. (38) Bonet, F.; Delmas, V.; Grugeon, S.; Herrera Urbina, R.; Silvert, P. Y.; Tekaia-Elhsissen, K. Synthesis of Monodisperse Au, Pt, Pd, Ru and Ir Nanoparticles in Ethylene Glycol. Nanostruct. Mater. 1999, 11, 1277− 1284. (39) Xia, X.; Figueroa-Cosme, L.; Tao, J.; Peng, H.-C.; Niu, G.; Zhu, Y.; Xia, Y. Facile Synthesis of Iridium Nanocrystals with Well-Controlled Facets Using Seed-Mediated Growth. J. Am. Chem. Soc. 2014, 136, 10878−10881. (40) Jackson, A.; Viswanathan, V.; Forman, A. J.; Norskov, J.; Jaramillo, T. F. Effects of a New Electrochemical Cleaning Protocol on Ru@Pt Core-Shell ORR Catalysts. ECS Trans. 2013, 58, 929−936. (41) Jerkiewicz, G. Electrochemical Hydrogen Adsorption and Absorption. Part 1: Under-Potential Deposition of Hydrogen. Electrocatalysis 2010, 1, 179−199. (42) Stephens, I. E. L.; Bondarenko, A. S.; Perez-Alonso, F. J.; CalleVallejo, F.; Bech, L.; Johansson, T. P.; Jepsen, A. K.; Frydendal, R.; Knudsen, B. P.; Rossmeisl, J.; Chorkendorff, I. Tuning the Activity of Pt(111) for Oxygen Electroreduction by Subsurface Alloying. J. Am. Chem. Soc. 2011, 133, 5485−5491. (43) Tang, L.; Li, X.; Cammarata, R. C.; Friesen, C.; Sieradzki, K. Electrochemical Stability of Elemental Metal Nanoparticles. J. Am. Chem. Soc. 2010, 132, 11722−11726. (44) Shao, M.; Peles, A.; Shoemaker, K. Electrocatalysis on Platinum Nanoparticles: Particle Size Effect on Oxygen Reduction Reaction Activity. Nano Lett. 2011, 11, 3714−3719. (45) Zhang, J.; Vukmirovic, M. B.; Xu, Y.; Mavrikakis, M.; Adzic, R. R. Controlling the Catalytic Activity of Platinum-Monolayer Electrocatalysts for Oxygen Reduction with Different Substrates. Angew. Chem., Int. Ed. 2005, 44, 2132−2135. (46) Todoroki, N.; Watanabe, H.; Kondo, T.; Kaneko, S.; Wadayama, T. Highly Enhanced Oxygen Reduction Reaction Activity and Electrochemical Stability of Pt/Ir(111) Bimetallic Surfaces. Electrochim. Acta 2016, 7−12.

249

DOI: 10.1021/acsenergylett.6b00585 ACS Energy Lett. 2017, 2, 244−249