Octahedral PtNi Research Articles

Product distributions and efficiencies for ethanol oxidation at PtNi octahedra

An octahedral PtNi catalyst has been evaluated for direct ethanol fuel cells (DEFC) by measuring its performance and product distribution in a proton exchange membrane electrolysis cell. This avoids errors due to the chemical reaction between ethanol and oxygen that occurs in a DEFC. The performance of the PtNi/C catalyst at low potentials was similar to that of a commercial Pt/C catalyst, while its selectivity for complete oxidation of ethanol to carbon dioxide was significantly higher. For example, at 0.2 V vs. a hydrogen-producing cathode the faradaic yield of CO2 was 73% at PtNi/C at an average current of 5.6 mA, but only 55% at Pt/C, at 5.1 mA. Consequently, the PtNi/C catalyst would provide higher DEFC efficiencies than Pt/C, or commercial PtRu/C. The voltammetric behaviors of PtNi/C and Pt/C have also been compared in a liquid electrolyte at ambient temperature, where it was found that the activity of the PtNi/C for ethanol oxidation was improved greatly by heating in acetic acid. This removes surface Ni and capping agents and greatly improved CO tolerance. However, the superior CO tolerance of the cleaned PtNi/C relative to Pt/C was not reflected in polarization curves measured at 80 °C in PEM cells.

An ORR Identical Location STEM Investigation of the Stability of Pt3-Ni Octahedra Supported in Hollow Graphitic Spheres

The activity of a catalyst towards the oxygen reduction reaction (ORR) is determined by its surface composition and structure. Although careful synthesis strategies have managed to improve the ORR activity of Pt by a factor of 40 in recent years, the long-term stability of supported catalysts still poses a challenge.[1] One approach that can be used to supress degradation phenomena, such as particle migration and agglomeration, is incorporation of the active metal species in the pores of a conducting, porous graphitic matrix, in order to hinder the motion of the alloy particles. [2-4] Here, we describe the synthesis, characterization and stability of octahedral Pt3NiMo nanoparticle catalysts supported in hollow graphitic spheres. By combining electrochemically active surface area (ECSA) measurements, identical location scanning transmission electron microscopy (STEM) and measurements performed using an in situ electrocatalytic scanning flow cell coupled to an inductively coupled plasma mass spectrometer (SFC-ICP-MS), we demonstrate the stabilizing effect of the hollow graphitic spheres on the PtNi octahedra. After 10800 cycles, the octahedra are found to have retained their shape, with only a small loss of ECSA. In summary, we demonstrate the successful incorporation of shaped bimetallic nanoparticles into a porous support. This design strategy can be used for other catalytic reactions and opens new ways for achieving space confinement of faceted Pt alloy particles.

Surface Doping Treatment of Octahedral Ptni Alloy Nanoparticle Catalysts for Durable Fuel Cell Cathodes

Proton exchange membrane fuel cells (PEMFCs) convert chemical energy into electricity and uses hydrogen and oxygen as fuel. The sluggish kinetics of the oxygen reduction reaction (ORR) and harsh environment pose challenges on the development of cheap, active and stable catalysts. Shape-controlled bi- or tri-metallic Pt alloy nanoparticles, i.e. octahedral PtNi nanoparticles, have been reported to achieve extremely high mass activity in rotating disk electrode (RDE) experiments and for this reason are considered as a highly promising class of ORR catalysts.1 Despite the high mass activity, the low electrochemical surface area (ECSA), often below 50 m2/g, is considered not optimal for low-loaded PGM cathodes and possible to hinder their performance at higher current density in membrane electrode assemblies (MEA).2 A second challenge is their morphological stability under electrochemical testing. Ni loss and other degradation processes often result in shape changes into more “rounded” or concave structures with loss of the (111) facet and lower activity (Figure 1).3 Introduction of a third metal such as Co, Rh and Mo have been shown to have beneficial effects and is considered a promising step toward shape-stable PtNiX octahedral nanoparticles. In particular, an exceptionally high initial mass activity has been reported for Mo surface doped PtNi catalyst, which also showed good stability in an accelerated test in RDE.4 In this contribution we will show our recent efforts in improving the activity and especially the durability and morphological stability of PtNi based octahedral nanoparticle catalysts. A surface treatment involving the introduction of Mo is applied to octahedral PtNi/C. Activity and stability are evaluated from RDE measurements following a protocol developed inside the INSPIRE project.5 Degradation is monitored by in operando wide angle X-ray scattering (WAXS) performed at European Synchrotron Radiation Facility (ESRF). By this techniques is possible to follow the change in lattice parameter a during different stability tests. The results are correlated to Ni leaching from the crystalline alloy phase. P. Strasser, Science, 2015, 349, 379-380. A. Kongkanand and M. F. Mathias, J Phys Chem Lett, 2016, 7, 1127-1137. L. Gan, C. H. Cui, M. Heggen, F. Dionigi, S. Rudi and P. Strasser, Science, 2014, 346, 1502-1506. X. Q. Huang, Z. P. Zhao, L. Cao, Y. Chen, E. B. Zhu, Z. Y. Lin, M. F. Li, A. M. Yan, A. Zettl, Y. M. Wang, X. F. Duan, T. Mueller and Y. Huang, Science, 2015, 348, 1230-1234. S. Martens, L. Asen, G. Ercolano, F. Dionigi, C. Zalitis, A. Hawkins, A. Martinez Bonastre, L. Seidl, A. C. Knoll, J. Sharman, P. Strasser, D. Jones and O. Schneider, J Power Sources, 2018, submitted. The project leading to this application has received funding from the Fuel Cells and Hydrogen 2 Joint Undertaking under grant agreement No 700127. This Joint Undertaking receives support from the European Union’s Horizon 2020 Research and Innovation Programme and Hydrogen Europe and N.ERGHY. Figure 1. Life cycle of an octahedral PtNi nanoparticle showing a degradation pathway to loss of octahedral shape.3 Figure 1

Shape Stability of Octahedral PtNi Nanocatalysts for Electrochemical Oxygen Reduction Reaction Studied by in situ Transmission Electron Microscopy.

Octahedral faceted nanoparticles are highly attractive fuel cell catalysts as a result of their activity for the oxygen reduction reaction (ORR). However, their surface compositional and morphological stability currently limits their long-term performance in real membrane electrode assemblies. Here, we perform in situ heating of compositionally segregated PtNi1.5 octahedral nanoparticles inside a transmission electron microscope, in order to study their compositional and morphological changes. The starting PtNi1.5 octahedra have Pt-rich edges and concave Ni-rich {111} facets. We reveal a morphological evolution sequence, which involves transformation from concave octahedra to particles with atomically flat {100} and {111} facets, ideally representing truncated octahedra or cuboctahedra. The flat {100} and {111} facets are thought to comprise a thin Pt layer with a Ni-rich subsurface, which may boost catalytic activity. However, the transformation to truncated octahedra/cuboctahedra also decreases the area of the highly active {111} facets. The morphological and surface compositional evolution, therefore, results in a compromise between catalytic activity and morphological stability. Our findings are important for the design of more stable faceted PtNi nanoparticles with high activities for the ORR.

Impact of Heat Treatment on the Electrochemical Properties of Octahedral Pt-Ni Nanoparticles

Pt is still the most used catalyst for various electrocatalytic reactions. But its high cost and scarcity hinder its wide application. In order to address this problem, researchers found that alloying Pt with one or more transition metals such as Ni, Fe or Co can enhance its activity via maximizing the exposure of (111) facets.1 Octahedral Pt-Ni nanoparticles have been regarded as promising electrocatalyst for various reactions.2-4 It has been found that post heat annealing on the Pt-Ni octahedra plays a significant role in altering their activity and durability during oxygen reduction reaction. 5 In this study, the structure evolution and compositional segregation were systematically studied during annealing in various atmosphere (vacuum, N2, H2, Ar and air) by XRD, TEM, XPS and electrochemical techniques. The impact of the heat treatment on the activities toward oxygen reduction reaction, hydrogen evolution reaction, and ethanol electro-oxidation reaction were assessed. It is interesting to find that the carbon support might be unstable in certain atmosphere, for example, Ar, and can be deposited on the catalyst surface. Figure 1 compares the CVs of Pt-Ni treated in Ar and N2 at 400 °C with the unannealed one. It is clear that the surface area of Pt-Ni treated in Ar diminished due to carbon coating. In this talk, the composition and structural evolution result upon annealing will be presented. Their effects on various electrochemical reaction activities will be discussed. Figure 1. Comparisons of cyclic voltammograms of Pt-Ni annealed in N2 (blue), Ar (red) at 400°C for 30 min, and un-annealed one in an N2-saturated 0.1 M HClO4 solution. Reference: Stamenkovic, V. R.; Fowler, B.; Mun, B. S.; Wang, G.; Ross, P. N.; Lucas, C. A.; Marković, N. M. Science 2007, 315 (5811), 493-497. Sulaiman, J. E.; Zhu, S.; Xing, Z.; Chang, Q.; Shao, M. ACS Catalysis 2017, 7 (8), 5134-5141. Choi, S.-I.; Xie, S.; Shao, M.; Odell, J. H.; Lu, N.; Peng, H.-C.; Protsailo, L.; Guerrero, S.; Park, J.; Xia, X.Nano letters 2013, 13 (7), 3420-3425. Kavian, R.; Choi, S.-I.; Park, J.; Liu, T.; Peng, H.-C.; Lu, N.; Wang, J.; Kim, M. J.; Xia, Y.; Lee, S. W. Journal of Materials Chemistry A 2016, 4 (32), 12392-12397. Gan, L.; Heggen, M.; Cui, C.; Strasser, P.ACS catalysis 2015, 6 (2), 692-695. Figure 1

Ga-Doped Pt-Ni Octahedral Nanoparticles as a Highly Active and Durable Electrocatalyst for Oxygen Reduction Reaction.

Bimetallic PtNi nanoparticles have been considered as a promising electrocatalyst for oxygen reduction reaction (ORR) in polymer electrolyte membrane fuel cells (PEMFCs) owing to their high catalytic activity. However, under typical fuel cell operating conditions, Ni atoms easily dissolve into the electrolyte, resulting in degradation of the catalyst and the membrane-electrode assembly (MEA). Here, we report gallium-doped PtNi octahedral nanoparticles on a carbon support (Ga-PtNi/C). The Ga-PtNi/C shows high ORR activity, marking an 11.7-fold improvement in the mass activity (1.24 A mgPt-1) and a 17.3-fold improvement in the specific activity (2.53 mA cm-2) compared to the commercial Pt/C (0.106 A mgPt-1 and 0.146 mA cm-2). Density functional theory calculations demonstrate that addition of Ga to octahedral PtNi can cause an increase in the oxygen intermediate binding energy, leading to the enhanced catalytic activity toward ORR. In a voltage-cycling test, the Ga-PtNi/C exhibits superior stability to PtNi/C and the commercial Pt/C, maintaining the initial Ni concentration and octahedral shape of the nanoparticles. Single cell using the Ga-PtNi/C exhibits higher initial performance and durability than those using the PtNi/C and the commercial Pt/C. The majority of the Ga-PtNi nanoparticles well maintain the octahedral shape without agglomeration after the single cell durability test (30,000 cycles). This work demonstrates that the octahedral Ga-PtNi/C can be utilized as a highly active and durable ORR catalyst in practical fuel cell applications.

Differentiating the structure of PtNi octahedral nanoparticles through combined ADF\u2013EDX simulations

Advances in catalysis rely on the synthesis and characterisation of nanoparticles that have tailored structures and compositions. Although energy-dispersive X-ray (EDX) spectroscopy can be used to study local variations in the compositions of individual supported nanoparticles on the atomic-scale in the scanning transmission electron microscope, electron beam induced damage and contamination can preclude the use of long exposure times and tomographic approaches. Here, we perform simulations of EDX maps of seven different octahedral PtNi nanoparticles for a selection of crystallographic orientations and tilts, to evaluate which of them can be distinguished from elemental mapping performed in only one orientation.

Roles of Mo Surface Dopants in Enhancing the ORR Performance of Octahedral PtNi Nanoparticles.

Doping with a transition metal was recently shown to greatly boost the activity and durability of PtNi/C octahedral nanoparticles (NPs) for the oxygen reduction reaction (ORR), but its specific roles remain unclear. By combining electrochemistry, ex situ and in situ spectroscopic techniques, density functional theory calculations, and a newly developed kinetic Monte Carlo model, we showed that Mo atoms are preferentially located on the vertex and edge sites of Mo-PtNi/C in the form of oxides, which are stable within the wide potential window of the electrochemical cycle. These surface Mo oxides stabilize adjacent Pt sites, hereby stabilizing the octahedral shape enriched with (111) facets, and lead to increased concentration of Ni in subsurface layers where they are protected against acid dissolution. Consequently, the favorable Pt3Ni(111) structure for the ORR is stabilized on the surface of PtNi/C NPs in acid against voltage cycling. Significantly, the unusual potential-dependent oxygen coverage trend on Mo-doped PtNi/C NPs as revealed by the surface-sensitive Δμ analysis suggests that the Mo dopants may also improve the ORR kinetics by modifying the coordination environments of Pt atoms on the surface. Our studies point out a possible way to stabilize the favorable shape and composition established on conceptual catalytic models in practical nanoscale catalysts.

Recent advances in electrocatalysts toward the oxygen reduction reaction: the case of PtNi octahedra.

Designing highly efficient and durable electrocatalysts for the oxygen reduction reaction (ORR), the key step for the operation of polymer electrolyte membrane fuel cells (PEMFCs), is of a pivotal importance for advancing PEMFC technology. Since the most significant progress has been made on Pt3Ni(111) alloy surfaces, nanoscale PtNi alloy octahedra enclosed by (111) facets have emerged as promising electrocatalysts toward the ORR. However, because their practical uses have been hampered by the cost, sluggish reaction kinetics, and poor durability, recent advances have engendered a wide variety of structure-, size-, and composition-controlled bimetallic PtNi octahedra. Herein, we therefore review the important recent developments of PtNi octahedral electrocatalysts point by point to give an overview of the most promising strategies. Specifically, the present review article focuses on the synthetic methods for the PtNi octahedra, the core-shell and multi-metallic strategies for performance improvement, and their structure-, size-, and composition-control-based ORR activity. By considering the results achieved in this field, a prospect for this alloy nanocatalysts system for future sustainable energy applications is also proposed.

Tuning the Catalytic Oxygen Reduction Reaction Performance of Pt-Ni Octahedral Nanoparticles by Acid Treatments and Thermal Annealing

Shape controlled octahedral Pt-Ni alloy nanoparticles are promising oxygen reduction reaction (ORR) electrocatalysts for cathodes of low temperature Polymer Electrolyte Membrane fuel cells. Organic surfactants are used in order to control and tune particle composition, size, shape, and the distribution on the support material. Such methods request intense post-synthesis cleaning, or annealing procedures in order to remove the ligands, demanding for simpler cleaning and activation procedures. Here, we explore the effect of an acetic acid treatment of as-prepared Pt-Ni particles, applied prior to annealing. The resulting nanoparticles underwent an electrochemical surface characterization and were investigated in terms of their ORR activities and electrochemical long-term stability. After acid treatment the particles exhibit a Pt-rich surface, which changed slightly during annealing at 300°C but drastically to a more homogeneous alloy after annealing at 500°C due to Ni surface segregation. Besides changes in the (sub-)surface Pt and Ni composition, the octahedral shape did not survive the 500°C treatment. An improved ORR activity was obtained after annealing at 300°C. Our insights into effects and benefits of the described post-synthesis treatments aid our general understanding, but also may help improve the practical design of suitable treatment protocols of this class of ORR catalyst.

Gram-scale synthesis of highly active and durable octahedral PtNi nanoparticle catalysts for proton exchange membrane fuel cell

Proton exchange membrane fuel cells (PEMFC) are regarded as a promising renewable energy source for a future hydrogen energy society. However, highly active and durable catalysts are required for the PEMFCs because of their intrinsic high overpotential at the cathode and operation under the acidic condition for oxygen reduction reaction (ORR). Since the discovery of the exceptionally high surface activity of Pt3Ni(111), the octahedral PtNi nanoparticles have been synthesized and tested. Nonetheless, their milligram-scale synthesis method and poor durability make them unsuitable for the commercialization of PEMFCs. In this study, we focus on gram-scale synthesis of octahedral PtNi nanoparticles with Pt overlayers (PtNi@Pt) supported on the carbon, resulting in enhanced catalytic activity and durability. Such PtNi@Pt catalysts show high mass activity (1.24 A mgPt−1) at 0.9 V (vs RHE) for the ORR, compared to commercial Pt/C (0.22 A mgPt−1). Single-cell performance and electrochemical impedance spectroscopy (EIS) were also tested. The PtNi@Pt catalysts showed enhanced current density of 3.1 A cm−2 at 0.6 V in O2 flow while the commercial Pt/C had the value of 2.5 A cm−2. After 30,000 cycles of the accelerated degradation test (ADT), the PtNi@Pt still showed better performance than the commercial Pt/C in a single-cell system. The Pt layers deposition could enhance the catalytic performance and durability of octahedral PtNi nanoparticles.

Tuning the Electrocatalytic Oxygen Reduction Reaction Activity and Stability of Shape-Controlled Pt-Ni Nanoparticles by Thermal Annealing - Elucidating the Surface Atomic Structural and Compositional Changes.

Shape-controlled octahedral Pt-Ni alloy nanoparticles exhibit remarkably high activities for the electroreduction of molecular oxygen (oxygen reduction reaction, ORR), which makes them fuel-cell cathode catalysts with exceptional potential. To unfold their full and optimized catalytic activity and stability, however, the nano-octahedra require post-synthesis thermal treatments, which alter the surface atomic structure and composition of the crystal facets. Here, we address and strive to elucidate the underlying surface chemical processes using a combination of ex situ analytical techniques with in situ transmission electron microscopy (TEM), in situ X-ray diffraction (XRD), and in situ electrochemical Fourier transformed infrared (FTIR) experiments. We present a robust fundamental correlation between annealing temperature and catalytic activity, where a ∼25 times higher ORR activity than for commercial Pt/C (2.7 A mgPt-1 at 0.9 VRHE) was reproducibly observed upon annealing at 300 °C. The electrochemical stability, however, peaked out at the most severe heat treatments at 500 °C. Aberration-corrected scanning transmission electron microscopy and energy-dispersive X-ray spectroscopy (EDX) in combination with in situ electrochemical CO stripping/FTIR data revealed subtle, but important, differences in the formation and chemical nature of Pt-rich and Ni-rich surface domains in the octahedral (111) facets. Estimating trends in surface chemisorption energies from in situ electrochemical CO/FTIR investigations suggested that balanced annealing generates an optimal degree of Pt surface enrichment, while the others exhibited mostly Ni-rich facets. The insights from our study are quite generally valid and aid in developing suitable post-synthesis thermal treatments for other alloy nanocatalysts as well.

Octahedral Pt-Ni Nanoparticles for Electrocatalytic ORR – New Insights Using in-Situ Scattering, Spectroscopy and Microscopy

Shape-controlled octahedral Pt-Ni core shell nanoparticles have attracted much attention in recent years due to their promising oxygen reduction reaction (ORR) activities, presumably owing to their particular geometry and facet exposure. Yet much about their growth and degradation has remained insufficiently understood. Here, we present some recent work where we learned more about the relations between particle synthesis, morphology, bulk structure, surface composition and their catalytic ORR activity and stability using, among others, in situ high-temperature TEM and in situ electrochemical TEM. We follow the particle morphology in real time during annealing protocols, and, using real time imaging, we track and observe changes in the catalyst structure and particle shape during different electrochemical treatments Our study provides some interesting new insights in the real time behavior and degradation of shaped PtNi electrocatalysts, and may aid in out understanding of some of the parameters that will optimize the lifetime of the catalyst.

Simultaneous Improvements in Performance and Durability of an Octahedral PtNix/C Electrocatalyst for Next-Generation Fuel Cells by Continuous, Compressive, and Concave Pt Skin Layers

Simultaneous improvements in oxygen reduction reaction (ORR) activity and long-term durability of Pt-based cathode catalysts are indispensable for the development of next-generation polymer electrolyte fuel cells but are still a major dilemma. We present a robust octahedral core–shell PtNix/C electrocatalyst with high ORR performance (mass activity and surface specific activity 6.8–16.9 and 20.3–24.0 times larger than those of Pt/C, respectively) and durability (negligible loss after 10000 accelerated durability test (ADT) cycles). The key factors of the robust octahedral nanostructure (core–shell Pt73Ni27/C) responsible for the remarkable activity and durability were found to be three continuous Pt skin layers with 2.0–3.6% compressive strain, concave facet arrangements (concave defects and high coordination), a symmetric Pt/Ni distribution, and a Pt67Ni33 intermetallic core, as found by STEM-EDS, in situ XAFS, XPS, etc. The robust core–shell Pt73Ni27/C was produced by the partial release of the stress, ...

Structural Evolution of Sub-10 nm Octahedral Platinum-Nickel Bimetallic Nanocrystals.

Octahedral Pt alloy nanocrystals (NCs) have shown excellent activities as electrocatalysts toward oxygen reduction reaction (ORR). As the activity and stability of NCs are highly dependent on their structure and the elemental distribution, it is of great importance to understand the formation mechanism of octahedral NCs and to rationally synthesize shape-controlled alloy catalysts with optimized ORR activity and stability. However, the factors controlling the structural and compositional evolution during the synthesis have not been well understood yet. Here, we systematically investigated the structure and composition evolution pathways of Pt-Ni octahedra synthesized with the assistance of W(CO)6 and revealed a unique core-shell structure consisting of a Pt core and a Pt-Ni alloy shell. Below 140 °C, sphere-like pure Pt NCs with the diameter of 3-4 nm first nucleated, followed by the isotropic growth of Pt-Ni alloy on the seeds at temperatures between 170 and 230 °C forming Pt@Pt-Ni core-shell octahedra with {111} facets. Owing to its unique structure, the Pt@Pt-Ni octahedra show an unparalleled stability during potential cycling, that is, no activity drop after 10 000 cycles between 0.6 and 1.0 V. This work proposes the Pt@Pt-Ni octahedra as a high profile electrocatalyst for ORR and reveals the structural and composition evolution pathways of Pt-based bimetallic NCs.

Octahedral PtNi nanoparticles with controlled surface structure and composition for oxygen reduction reaction

Controlling the surface structure and composition at the atomic level is an effective way to tune the catalytic properties of bimetallic catalysts. Herein, we demonstrate a generalized strategy to synthesize highly monodisperse, surfactant-free octahedral Pt x Ni1−x nanoparticles with tunable surface structure and composition. With increasing the Ni content in the bulk composition, the degree of concaveness of the octahedral Pt x Ni1−x nanoparticles increases. We systematically studied the correlation between their surface structure/composition and their observed oxygen reduction activity. Electrochemical studies have shown that all the octahedral Pt x Ni1−x nanoparticles exhibit enhanced oxygen reduction activity relative to the state-of-the-art commercial Pt/C catalyst. More importantly, we find that the surface structure and composition of the octahedral Pt x Ni1−x nanoparticles have significant effect on their oxygen reduction activity. Among the studied Pt x Ni1−x nanoparticles, the octahedral Pt1Ni1 nanoparticles with slight concaveness in its (111) facet show the highest activity. At 0.90 V vs. RHE, the Pt mass and specific activity of the octahedral Pt1Ni1 nanoparticles are 7.0 and 7.5-fold higher than that of commercial Pt/C catalyst, respectively. The present work not only provides a generalized strategy to synthesize highly monodisperse, surfactant-free octahedral Pt x Ni1−x nanoparticles with tunable surface structure and composition, but also provides insights to the structure-activity correlation.

An Electrochemistry in Situ TEM Study on the Shape Evolution of Octahedral Pt-Ni Nanoparticles

In polymer electrolyte membrane fuel cell research, shaped nanoparticles have attracted much attention in the last years. For the cathode, octahedral PtNi nanoparticles show promise in their high oxygen reduction reaction (ORR) activities due to their extended availability of (111) facets. This group of materials has been studied extensively including methods like XPS, TEM, EDX and EELS to understand their growth and degradation1-4. However, nanoscale analysis of their degradation has not been observed in real time. Here, we present a novel in situ TEM electrochemistry study in acidic media on octahedral Pt-Ni nanoparticles synthesized via a solvo-thermal route. With real time imaging we were able to track and observe changes in the catalyst structure and particle shape during different electrochemical treatments5. For each electrochemical treatment, various potential sequences were applied in the TEM cell. We were able to confirm strong corrosive consequences on the catalyst structure and particle shape of an uncontrolled high open circuit potential (~ 1.6 V vs. RHE) after controlled electrochemical cycling. Additionally the impact on the material of stepwise increase of the upper potential (≤ 1.4 V vs. RHE) during electrochemical cycling in comparison with equivalent chronoamperometric profile was studied, showing increased degradation rates when high potentials were held. Furthermore we were able to observe the dissolution of nickel catalyst particles, even with relatively cautious electrochemical treatment (0.0 – 1.0 V vs. RHE). While electrochemical cycling up to 1.2 V vs. RHE is not harmful to the octahedral particle shape, it was still possible to observe limitations of catalyst stability by motion on the catalyst support. Our study allows us to gain deeper insight of the real time behavior and degradation of shaped PtNi electrocatalysts, and develop understanding of the parameters that will optimize the lifetime of the catalyst. 1. Arán-Ais, R. M.; Yu, Y.; Hovden, R.; Solla-Gullón, J.; Herrero, E.; Feliu, J. M.; Abruña, H. D. Journal of the American Chemical Society 2015,137, (47), 14992-14998. 2. Choi, S.-I.; Xie, S.; Shao, M.; Odell, J. H.; Lu, N.; Peng, H.-C.; Protsailo, L.; Guerrero, S.; Park, J.; Xia, X.; Wang, J.; Kim, M. J.; Xia, Y. Nano Letters 2013,13, (7), 3420-3425. 3. Gan, L.; Cui, C.; Heggen, M.; Dionigi, F.; Rudi, S.; Strasser, P. Science 2014,346, (6216), 1502-1506. 4. Beermann, V.; Gocyla, M.; Willinger, E.; Rudi, S.; Heggen, M.; Dunin-Borkowski, R. E.; Willinger, M.-G.; Strasser, P. Nano Letters 2016, 16, (3), 1719-1725.5. Holtz, M. E.; Yu, Y.; Gunceler, D.; Gao, J.; Sundararaman, R.; Schwarz, K. A.; Arias, T. A.; Abruña, H. D.; Muller, D. A. Nano Letters 2014, 14, (3), 1453-1459.

Tuning the Electrocatalytic Oxygen Reduction Reaction Activity and Stability of Octahedral Ptni Nanoparticles By Thermal Annealing: An Ex Situ and in Situ study

In fuel cell applications platinum is still the most used catalyst at the cathode for the oxygen reduction reaction (ORR). Nevertheless, it has been shown that both specific and mass activity can be enhanced by alloying Pt with one or more transition metals such as Ni, Fe, Co or Rh and by maximizing the exposure of (111) facets by the use of shape-controlled nanoparticles 1-4. Due to the remarkably high activities of platinum-based octahedral shaped nanoparticles, research on them has become more important during recent years. Several approaches have been reported describing methods to synthesize octahedral nanoparticles. Solvothermal methods using long-chain surfactants turned out to be well controllable in terms of nanoparticle composition and size, although they require intensive cleaning procedures 5, 6. To combine an effective cleaning procedure with the benefit of an improved alloy structure of the nanoparticles, we investigated the influence of a thermal post-treatment in different gas atmospheres at different temperatures in terms of the surface structure and composition, the electrochemical ORR activity and the stability. All catalysts in this study show a considerably high activity, but the one annealed at 300 °C in hydrogen exhibits an exceptionally high activity up to 2.7 A mgPt -1. We also observe a correlation between the annealing temperatures, the initial activities and the electrochemical stability after 4000 potential cycles. The particles were characterized by electrochemical CO oxidation and corresponding in situ FTIR experiments to gain insights into the nanoparticle surface composition as well as by transmission electron microscopy (TEM) to provide information on the particle shape evolution. Furthermore aberration-corrected scanning TEM in combination with energy dispersive X-ray analysis offers insightful information on the atomic distribution. To obtain detailed understanding about the processes taking place during heat treatment, in situ heating XRD and in situ heating TEM measurements were performed. The combination of these methods allows detailed atomic understanding involving important conclusions for the influence of the annealing temperature. Literature 1. Stamenkovic, V. R.; Fowler, B.; Mun, B. S.; Wang, G. F.; Ross, P. N.; Lucas, C. A.; Markovic, N. M. Science 2007, 315, (5811), 493-497. 2. Rudi, S.; Tuaev, X.; Strasser, P. Electrocatalysis 2012, 3, (3-4), 265-273. 3. Beermann, V.; Gocyla, M.; Willinger, E.; Rudi, S.; Heggen, M.; Dunin-Borkowski, R. E.; Willinger, M.-G.; Strasser, P. Nano Letters 2016, 16, (3), 1719-1725. 4. Kongkanand, A.; Subramanian, N. P.; Yu, Y.; Liu, Z.; Igarashi, H.; Muller, D. A. ACS Catalysis 2016, 6, (3), 1578-1583. 5. Choi, S.-I.; Xie, S.; Shao, M.; Lu, N.; Guerrero, S.; Odell, J. H.; Park, J.; Wang, J.; Kim, M. J.; Xia, Y. ChemSusChem 2014, 7, (5), 1476-1483. 6. Zhang, J.; Yang, H.; Fang, J.; Zou, S. Nano Letters 2010, 10, 638.

Theoretical Insights into the Effects of Oxidation and Mo-Doping on the Structure and Stability of Pt-Ni Nanoparticles.

Pt-Ni nanoparticles are promising catalysts for the oxygen reduction reaction but they suffer from Ni dissolution in oxidizing conditions. It has recently been shown that it is possible to stabilize octahedral Pt-Ni nanoparticles by doping them with a small amount of Mo. Using ab initio calculations and a quaternary cluster expansion, we provide atomic-scale explanations for the enhanced stability of Mo-doped Pt-Ni nanoparticles. We predict that for Mo-doped Pt3Ni nanoparticles with only a small amount of Mo doping (around 1.6% mole fraction) the equilibrium concentration of Ni atoms on the particle surface is greatly reduced, limiting the rate at which Ni atoms dissolve from the particles. Mo doping also increases Pt/Ni vacancy formation energies in the surface layer, which further stabilizes the nanoparticles against Ni dissolution and helps preserve the nanoparticle shape. Our calculations also reveal insights into the shape evolution of Pt-Ni nanoparticles: the preferential oxidation of edges can make (111) face sites more vulnerable to dissolution than edge sites, which may contribute to the observed formation of Pt-Ni nanoframes and nanoparticles with concave surfaces.

ChemInform Abstract: Engineering Multimetallic Nanocrystals for Highly Efficient Oxygen Reduction Catalysts

AbstractReview: 88 refs.