Tetrahexahedral Pt Nanoparticles: Comparing the Oxygen Reduction

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Tetrahexahedral Pt Nanoparticles: Comparing the Oxygen Reduction Reaction at Transient vs. Steady State Conditions Yu-Jia Deng, Gustav Karl Henrik Wiberg, Alessandro Zana, Shi-Gang Sun, and Matthias Arenz ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.6b02201 • Publication Date (Web): 15 Nov 2016 Downloaded from http://pubs.acs.org on November 17, 2016

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Tetrahexahedral Pt Nanoparticles: Comparing the Oxygen Reduction Reaction at Transient vs. Steady State Conditions Yu-Jia Deng1,2, Gustav Karl Henrik Wiberg3, Alessandro Zana1, Shi-Gang Sun2 and Matthias Arenz1,3,*

1. Nano-Science Center, Department of Chemistry, University of Copenhagen, Universitetsparken 5, DK-2100 Copenhagen Ø, Denmark 2. Department of Chemistry, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen, 361005, China 3. Department of Chemistry and Biochemistry, University of Bern, Freiestrasse 3, 3012 Bern, Switzerland Corresponding author: M. Arenz ([email protected]) Phone: +41 31 631 53 84; Fax: +41 31 631 39 94 Abstract In this work, we have synthesized tetrahexahedral (THH) Pt nanoparticles (NPs) enclosed with {730} high index facets using a one-step square wave potential procedure. The catalytic activity of the THH NPs towards the oxygen reduction reaction (ORR) is studied at both transient and steady state conditions. As benchmark, the ORR activity is compared with those of polycrystalline Pt and a commercial Pt/C catalyst. The results show that at transient conditions, the catalytic performance of the THH Pt NPs and Pt/C are approximately the same and about 2 times lower than polycrystalline Pt. However, at steady state conditions the THH Pt NPs perform considerably better than Pt/C. At steady state THH Pt NPs are even slightly more active than polycrystalline Pt. Key words THH; Pt; Nanoparticles; oxygen reduction reaction; steady state conditions

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1. Introduction In the past decades, proton exchange membrane fuel cells (PEMFCs) have gained considerable attention as clean and efficient devices for the conversion of chemical into electric energy. Especially their application in the automotive sector is often discussed, but had been frequently delayed 1. Despite these delays, tremendous advancements have been achieved and recently Toyota and Hyundai started the market introduction of fuel cell electric vehicles with their models Toyota Mirai and Hyundai Tucson Fuel Cell 2. Nevertheless, for a mass production enormous challenges lie ahead. These are mainly related to the fact that the state of the art catalyst used in PEMFCs is based on Pt. Especially the amount of Pt that is used at the cathode side, where the oxygen reduction reaction (ORR) takes place, is too high in relation to the available Pt resources. Therefore the activity of the ORR catalyst based on the amount of Pt (or other platinum group metals (PGM)) becomes the bottleneck for the commercialization of fuel cell cars at large scale 3. In order to alleviate this problem different strategies have been, and still are, explored to increase the catalytic activity of Pt-based catalysts. These include alloying of Pt with less noble elements and core shell Pt NPs to increase the performance (turn-over rate of the ORR) and at the same time reduce the amount of Pt in the catalyst 4. Another important factor of PEMFC catalysts is their stability towards degradation. Although stability concerns both electrodes (anode and cathode), the research focus is on the improving the degradation resistance of the cathode 5. In the presented work we investigate shape controlled Pt nanoparticles (NPs) consisting of high index facets as ORR model catalysts. These catalysts serve as link between studies on well-defined Pt single crystals 6 and industrial type, carbon supported catalysts 3a, 7. In an electrolyte with weakly adsorbing anions, such as diluted HClO4, the ORR activity on Pt low index has initially been determined to be in the order of {110} > {100} > {111} 8, whereas later work suggested an order of


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{110} > {111} > {100} 9. Independent of the exact order, both studies demonstrate that Pt planes with more open structures than {111} or {100} are favorable and the catalytic function of open surface structures like high index facets is intensively discussed. Recent investigations point towards a promoting effect of surface steps and increased activity of optimal surface sites, such as cavity sites on Pt(111)

6a, 6b, 10

. Such findings motivate the controlled synthesis of well-defined Pt

NPs with high index facets. The first breakthrough for the synthesis of such NPs was achieved in 2007 by Tian et al. 11. They developed a two-step square wave potential procedure and successfully synthesized the tetrahexahedral (THH) Pt NPs enclosed by {730} facets, which showed considerably high catalytic activity towards the electrooxidation of formic acid and ethanol 12. Since then considerable work has been devoted towards the synthesis and testing of shape controlled NPs on two dimensional substrates as well as in colloidal form

13

. For shape controlled NPs on two

dimensional substrates Tian et al. simplified their method further to a one-step procedure. High index faceted Pt and Pd NPs and even alloy Pd-Pt THH NPs were successfully synthesized by this procedure 13b, 14. Here we employed the one-step square wave potential procedure to synthesize THH Pt NPs and investigate their catalytic performance towards the ORR. For this purpose, for the first time THH Pt NPs were deposited directly onto a glassy carbon (GC) tip of a rotating disk electrode (RDE). This required a careful adjustment of the deposition parameter employed in previous work. Depositing the THH Pt NPs directly onto the GC tip of a RDE is crucial for the investigation of reactions (like the ORR) that critically rely on well-defined mass transport and the RDE approach. We establish the ORR activity in transient curves, as is standard in the field, and compared the results to steady state conditions where the surface is at a quasi-equilibrium. The investigation of the ORR at steady state conditions is motivated by the fact that previous studies on polycrystalline Pt surfaces have revealed that a transient activity determination can lead to an overestimation of the extracted ORR


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activity value as compared to the value obtained at steady state conditions, which is a more typical operation condition of PEMFCs. Furthermore, the proper determination of the oxide (or OHad) coverage via standard cyclic voltammetry is extremely difficult as different species coexist on the surface at transient conditions. Steady state ORR measurements in combination with the previously introduced stripping procedure allow a more careful correlation between oxide coverage and activity 15. 2. Experimental 2.1 Electrochemical preparation of THH Pt Nanoparticles The electrochemical preparation of THH Pt NPs was carried out in a home-made three electrode polytetrafluoroethylene (PTFE) cell 16. A saturated calomel electrode (SCE) was used as reference electrode. The working electrode was a GC disk (Ø=5mm) embedded into a PTFE holder of a RDE tip. This allows to investigate catalytic reactions on the THH Pt NPs under well-defined mass transport conditions, which is crucial for studying the ORR. To our knowledge, this is the first time that THH Pt NPs were directly deposited onto a RDE tip, which required careful optimization of the deposition parameters (deposition times and potentials as well as rotating of the electrode) and cell geometry (placement of working, counter and reference electrode in the setup). Prior to the electrodeposition of the THH Pt NPs, the GC disk of the RDE tip was mechanically polished using alumina powder of 1 and 0.3 mm in size. Subsequently the electrode was cleaned in an ultrasonic bath with Millipore water (18.2 MΩ cm). The THH NPs were electrodeposited onto the GC disk in a 0.4 mM K2PtCl4 + 0.1 M H2SO4 electrolyte solution using programmed potential stepping 14a, 17 controlled by the EC4TMDAQ software package of Nordic Electrochemistry ApS. In detail, the clean GC disk of the RDE tip was introduced to the electrolyte at 1.20 VSCE. Then the electrode was rotated at 100 rpm to ensure mass transport and afterwards subjected to a potential


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step from 1.20 to -0.10 VSCE for 0.1 s to generate Pt nuclei. The growth of the nuclei into THH Pt NPs was achieved by applying for 10 min a square-wave potential (f = 100 Hz) with a lower potential limit (EL) of 0.05 VSCE and an upper potential (EU) of 1.2 VSCE, respectively. The morphology and structure of the THH Pt NPs was characterized by scanning electron microscopy (SEM, FEI Quanta 200F operated at 15 kV), and transmission electron microscopy (TEM, Tecnai T20 operated at 200 kV). The SEM characterization was conducted by removing the GC disk with the THH Pt NPs from the Teflon sheath and attaching it directly onto the SEM sample holder. The TEM characterization was conducted by casting a drop of water on the particle covered GC disk, subsequently a razor blade was used to detach the THH Pt NPs from the GC surface. Afterwards, the GC was sonicated in order to evenly distribute the THH Pt NPs in the water layer. Finally, 10 µl water containing the THH Pt NPs was pipetted onto a TEM grid and left to dry at room temperature.

2.2. Electrochemical characterization and ORR activity measurements The electrochemical measurements for the basic characterization and the determination of the ORR activity were conducted in a separate, home built three electrode PTFE cell. The electrolyte was 0.1 M HClO4 solution prepared from concentrated HClO4 (Suprapur) and Millipore water. The cyclic voltammograms (CVs) were recorded in RDE configuration in argon saturated 0.1 M HClO4 electrolyte solution without rotation. For the ORR tests the electrolyte was saturated with O2 and a rotation rate of 1600 rpm was applied. The transient measurements are recorded at a scan rate of 50 mV s-1 evaluating the positive going scan. In order to correct for capacitive currents, the positive going scan of a CV recorded with the same scan rate in argon saturated electrolyte is subtracted from the ORR curve 7a. The steady state ORR activity is calculated from the current density after a potential hold for 100 s. As benchmark, the ORR activity of the THH Pt NPs is compared to the


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ORR activity of polycrystalline Pt and an industrial type, carbon supported Pt NP catalysts (TEC10E50E-HT from Tanaka TKK), indicated as Pt/C. The stripping measurements to evaluate the surface coverage of oxygenated species were conducted according to our previously reported experimental procedure

15b, 15c

. First the electrode potential is

held for 100 s at a certain deposition potential (ED) ranging from 0.6 to 1.3 VRHE. After the potential hold, starting from ED the potential is first swept to 0.05 VRHE with a sweep rate of 0.5 V s-1, called stripping sweep, and thereafter in positive direction to 1.1 VRHE or ED, whichever is higher. From the reduction current in the negative going stripping sweep the charge associated with the reduction of oxygenated species from the Pt surface (QOX) is evaluated; from the positive going linear sweep the charge associated with the desorption of adsorbed protons (under potentially deposited hydrogen) from the Pt surface (QH) is calculated. More details can be found in our previous work 15b. 3. Results and discussions

Figure 1. (a) Illustration of the square wave potential electrodeposition method for the preparation of tetrahexahedral Pt nanoparticles (THH Pt NPs). EN is a relatively low potential for quick nucleation and EU is the upper potential limit, while EL is the lower potential during the growth period. (b) SEM characterization of the shaped Pt NPs after an ORR measurement. The insets show a size histogram and a zoom in of the NPs. The average particle size is 127 nm. (c) TEM characterization of the shaped Pt NPs. Measuring the angles of the Pt tetra-hexahedron recorded along the [001] direction and comparing them with theoretical values, we


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can determine the facet as {730}. (d) The selected-area electron diffraction (SAED) of the particle demonstrates that the NPs are single crystalline.

Figure 1(a) illustrates the synthesis procedure for depositing THH Pt NPs onto the GC disk of a RDE tip. A nucleation pulse is followed by fast potential steps between EL and EU at 100 Hz, see experimental section for details. After ten minutes nicely shaped Pt NPs were obtained, which are evenly distributed over the GC surface, see SEM image in Figure 1 (b). The size histogram shown in the inset reveals an average particle size of 127 nm. The second inset shows a high magnification SEM image of the particles. One can see that the NPs are extremely well shaped and exhibit 24 facets. The base of the NPs is a cube and on each side of the cube there is a pyramid structure consisting of 4 triangular facets, see also the model depicted in Figure 1. The SEM image shown in Figure 1 was obtained after an ORR test. No difference could be observed between the THH Pt NPs after the ORR and the as prepared THH Pt NPs. The electron microscopy indicates that although the THH shape is not thermodynamically favored (in fact thermodynamically forming a single particle out of the NPs would be favored), there seems to be a “local minimum” of stability with high kinetic barrier for structural changes. This finding is in contrast to stability investigations of shaped Pt NPs prepared by colloidal methods, which report that the high index sites present on the shapecontrolled NPs undergo dissolution with few (200) potential cycles (in sulfuric acid electrolyte) forming at a positive potential limit of 1.2 VRHE polycrystalline NPs 18. The high stability observed for THH NPs may be related to the fact that they are prepared by a continuous, fast change between Pt dissolution and reduction, see experimental section. This preparation is significantly different from a colloidal approach and seems to lead to a stabilized structure. Nevertheless, it should be stressed that no degradation protocols were applied to investigate the shape stability in detail. We used TEM to characterize the facets of the Pt NPs. Figure 1 (c) shows the image of a representative single particle along the [001] zone axis. As illustrated by the THH model shown in the inset, in this orientation eight side facets parallel to the [001] axis can be imaged edge-on and


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form an octagonal projection. By measuring the angles of the crystal planes of the particle in this direction, we can determine the Miller indices of the exposed facets to be {730} according to the average values of α and β being 134.1° and 135.9°, respectively

11

. Figure 1 (d) shows the

corresponding selected-area electron diffraction (SAED) of the particle. The fourfold-symmetrical SAED pattern confirms that the particle is single crystalline.

Figure 2 Comparison of the cyclic voltammograms (CVs) of polycrystalline Pt and THH Pt NPs recorded in 0.1 M HClO4 electrolyte solution saturated with Ar. The scan rate was 0.1 V s-1.

The electrochemical characterization of thus prepared THH Pt NPs is shown in Figure 2. We compare the CV of the THH Pt NPs with the CV of polycrystalline Pt. Both CVs are recorded in 0.1M HClO4 electrolyte solution saturated with argon. The CV of polycrystalline Pt displays the well-known features of polycrystalline Pt or “standard” Pt NPs found in fuel cell catalysts 19. In the potential region from 0.05 to 0.4 VRHE, called the Hupd (underpotentially deposited hydrogen) region, the current correlates to hydrogen adsorption (negative scan direction) and desorption (positive scan


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direction), respectively. The Hupd features at 0.1 VRHE are more pronounced than the ones at 0.3 VRHE. This is in contrast to the CV of the THH Pt NPs where both features exhibit approximately the same height. The difference in the Hupd features between polycrystalline Pt and THH Pt NPs is discussed in the supporting information of the previous work by Tian et al. 11 and we do not repeat this discussion here. We only point to the fact that the more pronounced features at 0.3 VRHE in the CV of the THH Pt NPs as compared to the CV of polycrystalline Pt indicate a higher coverage of stronger bound Hupd, on the former. The Hupd features can also be used to obtain electrochemical active surface area (ECSA) by integrating to obtain the associated charge,.i.e. QH, and normalize the value to 210 µC cm-2 resulting in the so-called roughness factor Rf 20. The roughness factor is defined as the ratio of real surface area and geometric surface area. The roughness factors of the THH Pt NP sample and polycrystalline Pt were 1.14 and 1.88, respectively. In the potential region between 0.4 and 0.7 VRHE the currents are correlated to the charging of the double layer. The higher capacity of the THH Pt NP sample is most likely due to the properties of the GC substrate, which is not completely covered by the NPs. Between 0.7 and 1.1 VRHE, the current stems from the adsorption of oxygenated species. Also here differences between the CVs of the THH Pt NPs and polycrystalline Pt are apparent. When normalized to the ECSA, the current related to the adsorption/desorption of oxygenated species on the THH Pt sample is higher than the corresponding current on polycrystalline Pt. Integrating the current and constructing a (pseudo) adsorption isotherm therefore leads to a higher coverage of oxygenated species at a given potential. Thus, this finding can be associated with a stronger binding of oxygenated species to the surface 3a.


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Figure 3 Comparison between standard transient ORR polarization curve, i.e. positive going scan with 0.05 V s-1 and steady state (after 100 s potential hold) ORR currents on THH Pt in 0.1M HClO4 saturated with O2. The rotation rate was 1600 rpm.

It is well-documented that the ORR is structure sensitive due to the structure sensitive binding energy of the oxygenated reaction intermediates. Furthermore, anion adsorption from the supporting electrolyte is structure sensitive leading to different blocking of active Pt sites

6a, 21

. From the low

index planes the Pt (111) and (100) facets are predicted to be the most active surfaces 21, however recent studies report a promoting effect of surface steps and an increased activity of special surface sites

6b, 10

. Experiments for determining the ORR activity are usually performed at transient

conditions, i.e. positive going potential sweeps, whereas in applications usually less dynamic conditions are applied. Therefore, not much is known how structure sensitivity and its relation to the amount of adsorbed oxygenated species, OHad and Oad, compare at transient and steady state conditions. In Figure 3 we present such a comparison. As steady state ORR activity, we define the currents measured after a potential hold for 100 s at the indicated potentials. They are compared to the background corrected polarization curve of a potential sweep in positive direction recorded with a scan rate of 0.05 V s-1. The results demonstrate the well-known, but seldom discussed fact that the


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measured ORR activity strongly depends on the experimental conditions like scan speed etc.

3a, 22

.

The observed difference in ORR activity can be interpreted due to an increase of the surface coverage with oxygenated species at steady state conditions, leading to a blockage of active sites for the ORR. The distinction between transient and steady state ORR activity becomes important when comparing different catalysts. In Figure 4 the comparison of the specific ORR activity (SA), i.e., the measured kinetic ORR current normalized to the ECSA, of THH Pt NPs with polycrystalline Pt and a standard Pt/C catalyst (Rf of 7.45) from Tanaka TKK (see experimental part) is shown. From the Tafel plot in Figure 4a it is apparent that the transient ORR activity of the THH Pt NPs is considerably lower than the one of polycrystalline Pt. Roughly by a factor of 2. Its value is roughly the same as the ORR activity of the Pt/C standard (roughly a factor of 2.5 lower than the ORR activity of polycrystalline Pt). At large overpotentials some deviations are seen, however at such potentials the reduction current is almost diffusion limited and no proper activity evaluation is possible (we nevertheless show the full curve just for the sake of comparison with the steady state data). The lower ORR activity of the THH Pt NPs as compared to polycrystalline Pt can be correlated with the increased adsorption of oxygenated species observed in cyclic voltammetry. However, the relative ORR activity values change completely if we turn our attention to steady state conditions, see Figure 4b.


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Figure 4 (a) Comparison of polycrystalline Pt, THH Pt NPs, and TKK Pt/C, and at conventional ORR RDE polarization curves recorded at 50 mV/s and (b) at quasi steady state conditions (potential hold for 100 s) in 0.1M HClO4 saturated with O2, 1600 rpm.

At steady state conditions the ORR activity of the THH Pt NPs is considerably increased as compared to the Pt/C benchmark catalyst. The THH Pt NPs are even slightly more active than polycrystalline Pt. This indicates that not only the OHad formation during a transient potential scan is important, which determines the activity of Pt in standard ORR measurements, but the true equilibrium OH coverage. It has been reported that THH Pt NPs, which are formed by repeated oxidation and reduction are resistant to “massive” oxidation 11. In order to scrutinize the oxidation


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of the THH Pt NPs, we apply a recently developed stripping procedure, see experimental section and refs. 15b, 15c for more details.

Figure 5 QOX, QH, and EPP of THH Pt NPs determined by a stripping procedure performed in 0.1M HClO4 electrolyte solution saturated with Ar.

The results are summarized in Figure 5. Comparing them with our previous measurements on polycrystalline Pt, we find that the general behavior of the THH Pt NPs is quite similar. That is, we can divide four potential regions where different oxygenated species are predominant. More negative than 0.6 VRHE no significant charge (Qox) due to OHad reduction is observed. Above 0.6 VRHE Qox is detected and increases linearly with a simultaneous shift of the peak potential Epp. At about 0.95 VRHE the charges of Qox and QH are equal. Assuming that the Hupd and OHad coverage at the THH Pt NP surface are equal as well, this would imply the completion of a monolayer of OHad. However, in contrast to polycrystalline Pt

15b

the crossing points of Qox and QH do not exactly

correlate to the change in slope of Epp. This indicates that the Hupd and OHad coverage do not exactly correlate. This is also the case for Pt single crystal electrodes where non-uniform Hupd and OHad coverages are observed

23

. At higher potentials, the OHad layer gradually transforms first to Oad,

from which at higher potentials Pt-oxide is formed. To correlate the steady state ORR activity with 13 ACS Paragon Plus Environment

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the steady state coverage of oxygenated species in Figure 6 a plot of the Qox normalized to QH is shown. It can be seen that THH Pt NPs are more oxidation resistant than polycrystalline Pt. As ORR activity and OH coverage are correlated, this finding leads to the assumption that the high steady state ORR activity of the THH Pt NPs is might be a direct effect of its oxidation resistance.

Figure 6 Comparison of the ratio between QOX and QH of THH Pt NPs and polycrystalline Pt as function of the hold potential E. The curves are calculated from data shown in figure 5.

4. Conclusions By applying a one-step square wave potential procedure, we have successfully synthesized shaped THH Pt NPs onto the GC disk of a RDE tip. SEM and TEM analysis shows that the THH Pt NPs are enclosed with {730} high index facets. For the first time, such THH Pt NPs were investigated for their ORR activity under well-defined mass transport conditions. At transient conditions the catalytic ORR activity of the shaped THH particles is about 2 times lower than that of polycrystalline Pt. The ORR activity is roughly the same as that of a commercial Pt/C catalyst. However, at steady state conditions the THH Pt NPs show a considerably improved ORR activity as


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compared to the two benchmark catalysts. The ORR activity of the THH Pt NPs is even slightly higher than that of polycrystalline Pt. The findings are interpreted as follows. At transient conditions the formation of adsorbed oxygenated species is not at equilibrium and the THH Pt NP catalyst performs like conventional Pt/C fuel cell catalysts despite their larger particle size. However, at steady state conditions, the high oxidation resistance of the THH Pt NPs leads to a large improvement in their relative performance. This high oxidation resistance is also reflected by a relative high shape stability as demonstrated by the SEM images recorded after the ORR. Acknowledgements The research leading to these results has received funding from the Danish Council for Strategic Research (4M Centre). Y-J.D. acknowledges funding from the China Scholarship Council (CSC). References (1) (a) Debe, M. K., Nature 2012, 486, 43-51; (b) Corbo, P.; Veneri, O.; Migliardini, F., Hydrogen Fuel Cells for Road Vehicles. Springer London: 2011; (c) Wagner, F. T.; Lakshmanan, B.; Mathias, M. F., J. Phys. Chem. Lett. 2010, 1, 2204-2219. 2. Ball, M.; Weeda, M., Int. J. Hydrogen Energ. 2015, 40, 7903-7919. 3. (a) Gasteiger, H. A.; Kocha, S. S.; Sompalli, B.; Wagner, F. T., Appl. Catal. B 2005, 56 (1-2), 9-35; (b) Markovic, N. M.; Schmidt, T. J.; Stamenkovic, V.; Ross, P. N., Fuel Cells 2001, 1, 105-116. 4. (a) Wang, C.; Markovic, N. M.; Stamenkovic, V. R., ACS Catal. 2012, 2, 891-898; (b) Stamenkovic, V. R.; Mun, B. S.; Arenz, M.; Mayrhofer, K. J. J.; Lucas, C. A.; Wang, G. F.; Ross, P. N.; Markovic, N. M., Nat. Mat. 2007, 6, 241-247; (c) Aran-Ais, R. M.; Dionigi, F.; Merzdorf, T.; Gocyla, M.; Heggen, M.; Dunin-Borkowski, R. E.; Gliech, M.; Solla-Gullon, J.; Herrero, E.; Feliu, J. M.; Strasser, P., Nano Lett. 2015, 15, 7473-7480; (d) Gan, L.; Cui, C. H.; Heggen, M.; Dionigi, F.; Rudi, S.; Strasser, P., Science 2014, 346, 1502-1506; (e) Cui, C. H.; Gan, L.; Heggen, M.; Rudi, S.; Strasser, P., Nat. Mat. 2013, 12, 765-771; (f) Escudero-Escribano, M.; Malacrida, P.; Hansen, M. H.; Vej-Hansen, U. G.; Velazquez-Palenzuela, A.; Tripkovic, V.; Schiotz, J.; Rossmeisl, J.; Stephens, I. E. L.; Chorkendorff, I., Science 2016, 352, 73-76; (g) Hernandez-Fernandez, P.; Masini, F.; McCarthy, D. N.; Strebel, C. E.; Friebel, D.; Deiana, D.; Malacrida, P.; Nierhoff, A.; Bodin, A.; Wise, A. M.; Nielsen, J. H.; Hansen, T. W.; Nilsson, A.; Stephens, I. E. L.; Chorkendorff, I., Nat. Chem. 2014, 6, 732-738; (h) Stephens, I. E. L.; Bondarenko, A. S.; Gronbjerg, U.; Rossmeisl, J.; Chorkendorff, I., Energy Environ. Sci. 2012, 5, 6744-6762; (i) Chen, C.; Kang, Y. J.; Huo, Z. Y.; Zhu, Z. W.; Huang, W. Y.; Xin, H. L. L.; Snyder, J. D.; Li, D. G.; Herron, J. A.; Mavrikakis, M.; Chi, M. F.; More, K. L.; Li, Y. D.; Markovic, N. M.; Somorjai, G. A.; Yang, P. D.; Stamenkovic, V. 15 ACS Paragon Plus Environment

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Tetrahexahedral Pt Nanoparticles: Comparing the Oxygen Reduction

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