Stefan Rudi

Core–Shell Compositional Fine Structures of Dealloyed PtxNi1–x Nanoparticles and Their Impact on Oxygen Reduction Catalysis

Using aberration-corrected scanning transmission electron microscopy and electron energy loss spectroscopy line profiles with Ångstrom resolution, we uncover novel core-shell fine structures in a series of catalytically active dealloyed Pt(x)Ni(1-x) core-shell nanoparticles, showing the formation of unusual near-surface Ni-enriched inner shells. The radial location and the composition of the Ni-enriched inner shells were sensitively dependent on the initial alloy compositions. We further discuss how these self-organized Ni-enriched inner shells play a key role in maintaining surface lattice strain and thus control the surface catalytic activity for oxygen reduction.

Element-specific anisotropic growth of shaped platinum alloy nanocrystals

Morphological shape in chemistry and biology owes its existence to anisotropic growth and is closely coupled to distinct functionality. Although much is known about the principal growth mechanisms of monometallic shaped nanocrystals, the anisotropic growth of shaped alloy nanocrystals is still poorly understood. Using aberration-corrected scanning transmission electron microscopy, we reveal an element-specific anisotropic growth mechanism of platinum (Pt) bimetallic nano-octahedra where compositional anisotropy couples to geometric anisotropy. A Pt-rich phase evolves into precursor nanohexapods, followed by a slower step-induced deposition of an M-rich (M = Ni, Co, etc.) phase at the concave hexapod surface forming the octahedral facets. Our finding explains earlier reports on unusual compositional segregations and chemical degradation pathways of bimetallic polyhedral catalysts and may aid rational synthesis of shaped alloy catalysts with desired compositional patterns and properties. Platinum-rich phases that initially form create the edges and corners of octahedral nanoparticle alloys. Nanoparticle growth starts at the edges The high activity of precious metals such as platinum for reactions that occur in fuel cells can be enhanced by alloying with metals such as nickel and cobalt to form shaped nanoparticles, where platinum is concentrated at the corner and edge sites. Gan et al. used a combination of high-resolution imaging and modeling to follow the formation of octadedral nanoparticles of these alloys with increasing growth times. A platinum-rich phase with an extended morphology forms initially and becomes the edges and corners for the particles, and the alloying metals deposit to fill in the facets. Science, this issue p. 1502

Core–Shell and Nanoporous Particle Architectures and Their Effect on the Activity and Stability of Pt ORR Electrocatalysts

We review our recent progress in the development of Pt–Ni bimetallic electrocatalysts with both high sustained activity and sustained stability for oxygen reduction reaction (ORR). This was achieved by an atomic understanding and rational control of the core–shell compositional patterns and size-related nanoporosity within the bimetallic nanoparticles formed during chemical and electrochemical pretreatment and electrocatalysis. In particular, we reveal how the size of the nanoparticle directly influences the nanoporosity formation and thereby the near surface composition, catalytic activity and stability. Our atomic insights provide a clearer picture on how bimetallic nanoparticles should be tailored for optimal ORR performance.

Rh-Doped Pt–Ni Octahedral Nanoparticles: Understanding the Correlation between Elemental Distribution, Oxygen Reduction Reaction, and Shape Stability

Thanks to their remarkably high activity toward oxygen reduction reaction (ORR), platinum-based octahedrally shaped nanoparticles have attracted ever increasing attention in last years. Although high activities for ORR catalysts have been attained, the practical use is still limited by their long-term stability. In this work, we present Rh-doped Pt-Ni octahedral nanoparticles with high activities up to 1.14 A mgPt(-1) combined with improved performance and shape stability compared to previous bimetallic Pt-Ni octahedral particles. The synthesis, the electrocatalytic performance of the particles toward ORR, and atomic degradation mechanisms are investigated with a major focus on a deeper understanding of strategies to stabilize morphological particle shape and consequently their performance. Rh surface-doped octahedral Pt-Ni particles were prepared at various Rh levels. At and above about 3 atom %, the nanoparticles maintained their octahedral shape even past 30,000 potential cycles, while undoped bimetallic reference nanoparticles show a complete loss in octahedral shape already after 8000 cycles in the same potential window. Detailed atomic insight in these observations is obtained from aberration-corrected scanning transmission electron microscopy (STEM) and energy dispersive X-ray (EDX) analysis. Our analysis shows that it is the migration of Pt surface atoms and not, as commonly thought, the dissolution of Ni that constitutes the primary origin of the octahedral shape loss for Pt-Ni nanoparticles. Using small amounts of Rh we were able to suppress the migration rate of platinum atoms and consequently suppress the octahedral shape loss of Pt-Ni nanoparticles.

In situ study of atomic structure transformations of Pt-Ni nanoparticle catalysts during electrochemical potential cycling.

When exposed to corrosive anodic electrochemical environments, Pt alloy nanoparticles (NPs) undergo selective dissolution of the less noble component, resulting in catalytically active bimetallic Pt-rich core-shell structures. Structural evolution of PtNi6 and PtNi3 NP catalysts during their electrochemical activation and catalysis was studied by in situ anomalous small-angle X-ray scattering to obtain insight in element-specific particle size evolution and time-resolved insight in the intraparticle structure evolution. Ex situ high-energy X-ray diffraction coupled with pair distribution function analysis was employed to obtain detailed information on the atomic-scale ordering, particle phases, structural coherence lengths, and particle segregation. Our studies reveal a spontaneous electrochemically induced formation of PtNi particles of ordered Au3Cu-type alloy structures from disordered alloy phases (solid solutions) concomitant with surface Ni dissolution, which is coupled to spontaneous residual Ni metal segregation during the activation of PtNi6. Pt-enriched core-shell structures were not formed using the studied Ni-rich nanoparticle precursors. In contrast, disordered PtNi3 alloy nanoparticles lose Ni more rapidly, forming Pt-enriched core-shell structures with superior catalytic activity. Our X-ray scattering results are confirmed by STEM/EELS results on similar nanoparticles.

Comparative Study of the Electrocatalytically Active Surface Areas (ECSAs) of Pt Alloy Nanoparticles Evaluated by Hupd and CO-stripping voltammetry

This study intends to provide some insight in the up-to-date elusive assessment of a correct choice of method for estimating the active surface area of Pt alloy nanoparticle catalysts. Taking PtNi3 nanoparticles as an example, we have compared three types of electrochemically active surface area (ECSA) data, CO-ECSA, Hupd-ECSA, and Hupd/CO-ECSA, which were evaluated from CO stripping and underpotentially deposited hydrogen stripping steps applied at different times along a reference catalyst activity test protocol. Considering a total of six different detailed voltammetric test protocols, we address Pt alloy particle size effects, analyze the effect of the time of application of CO and hydrogen stripping, and study their effect on the Pt mass and Pt surface-specific activities for the oxygen reduction reaction (ORR). In a discussion of the ratio of CO charge to hydrogen charge, it is shown that this quantity is more complex than previously thought and not associated with a specific surface structure. The Hupd/CO-ECSA data are found to be a reasonable balance for the estimate of surface area normalized, so-called specific catalytic ORR activities.

Electrocatalytic Oxygen Reduction on Dealloyed Pt 1-x Ni x Alloy Nanoparticle Electrocatalysts

The synthesis, structural, and compositional characterization as well as the electrocatalytic oxygen reduction (ORR) activity of a number of carbon-supported PtxNi1−x (x = 1.00–0.14) nanoparticles in acidic electrolyte are reported. A number of different low-temperature colloidal synthesis routes were employed to prepare monodisperse, single-phase Pt-Ni alloy nanoparticles. The catalysts were characterized using XRD, TEM, and ICP-OES techniques, subsequently electrochemically dealloyed and, in their dealloyed state, tested for their Pt mass-based ORR activity, specific Pt surface area-based ORR activity. Additional 4,000 voltage cycles were applied to investigate the durability of the electrocatalysts in terms of their electrochemically active surface area and their final ORR activity. It is found that Pt-Ni alloys exhibit a distinctly different dealloying and ORR stability behavior compared to Pt-Co or Pt-Cu alloy nanoparticles. In particular, Pt-Ni alloys require longer cycling times to unfold their full ORR activity. A distinct ORR activity maximum was uncovered for Pt-Ni nanoparticle alloys with initial Ni contents in the neighborhood of 70–75 at.% consistent with results from dealloyed macroscopic Pt-Ni thin films.

Ni-Catalyzed Growth of Graphene Layers during Thermal Annealing: Implications for the Synthesis of Carbon-Supported PtNi Fuel-Cell Catalysts

Thermal annealing is an important and widely adopted step during the synthesis of Pt bimetallic fuel‐cell catalysts, although it faces the inevitable drawback of particle sintering. Understanding this sintering mechanism is important for the future development of highly active and robust fuel‐cell catalysts. Herein, we studied the particle sintering during the thermal annealing of carbon‐supported Pt1–xNix (PtNi, PtNi3, and PtNi5) nanoparticles, a reported recently class of highly active fuel‐cell catalysts. By using high‐resolution transmission electron microscopy, we found that annealing at an intermediate temperature (400 °C) effectively increased the extent of alloying without particle sintering; however, high‐temperature annealing (800 °C) caused severe particle sintering, which, unexpectedly, was strongly dependent on the composition of the alloy, thus showing that a higher Ni content resulted in a higher extent of particle sintering. This result can be ascribed to the solid‐state transformation of the carbon support into graphene layers, catalyzed by Ni‐richer catalyst, which, in turn, promoted particle migration/coalescence and, hence, more‐significant sintering. Therefore, our results provide important insight for the synthesis of carbon‐supported Pt‐alloy fuel‐cell catalysts.

The impact of the morphology of the carbon support on the activity and stability of nanoparticle fuel cell catalysts

This study explores how the morphology of nanostructured carbons impacts the morphological stability of supported Pt fuel cell nanoparticle catalysts under extended potential cycling. Using in situ small angle X-ray scattering (SAXS), we monitor the evolution of key structural parameters of four different Pt/carbon catalyst couples, involving carbons with vastly different porosity characteristics (hollow carbons, nanotubes, and carbon blacks). In line with the size of supported Pt nanoparticles, the intrinsic specific electrochemical oxygen reduction reaction (ORR) activities of all samples were comparable. However, counter to common sense, a non-monotonic trend between the carbon surface area and the ORR mass-based activity, coupled with a similar relative loss in the electrochemical surface area (ECSA), was observed. This is explained in terms of a varying effective ECSA, which is sensitively dependent on the morphology of the support. In situ SAXS monitoring revealed a mainly coalescence-based increase in mean particle size for the low surface area carbon nanotubes. In contrast, the highly microporous hollow carbons showed strongly enhanced particle stability where Ostwald ripening accounted for the observed coarsening. Altogether, our study provides new atom-scale insights into Pt/C fuel cell catalyst stability. Based on this study, supports of intermediate surface area provide the best compromise between activity and size stability, while highly graphitized or highly nanoporous supports are detrimental.