Strain Relaxation in Core-Shell Pt-Co Catalyst Nanoparticles

Surface strain plays a key role in enhancing the activity of Pt-alloy nanoparticle oxygen reduction catalysts. However, the details of strain effects in real fuel cell catalysts are not well-understood, in part due to a lack of strain characterization techniques that are suitable for complex supported nanoparticle catalysts. This work investigates these effects using strain mapping with nanobeam electron diffraction and a continuum elastic model of strain in simple core-shell particles. We find that surface strain is relaxed both by lattice defects at the core-shell interface and by relaxation across particle shells caused by Poisson expansion in the spherical geometry. The continuum elastic model finds that in the absence of lattice dislocations, geometric relaxation results in a surface strain that scales with the average composition of the particle, regardless of the shell thickness. We investigate the impact of these strain effects on catalytic activity for a series of Pt-Co catalysts treated to vary their shell thickness and core-shell lattice mismatch. For catalysts with the thinnest shells, the activity is consistent with an Arrhenius dependence on the surface strain expected for coherent strain in dislocation-free particles, while catalysts with thicker shells showed greater activity losses indicating strain relaxation caused by dislocations as well.Comment: 23 pages,7 figures, includes appendi

Independent polarisation control of multiple optical traps

We present a system which uses a single spatial light modulator to control the spin angular momentum of multiple optical traps. These traps may be independently controlled both in terms of spatial location and in terms of their spin angular momentum content. The system relies on a spatial light modulator used in a "split-screen" configuration to generate beams of orthogonal polarisation states which are subsequently combined at a polarising beam splitter. Defining the phase difference between the beams with the spatial light modulator enables control of the polarisation state of the light. We demonstrate the functionality of the system by controlling the rotation and orientation of birefringent vaterite crystals within holographic optical tweezers

Real-time imaging of activation and degradation of carbon supported octahedral Pt–Ni alloy fuel cell catalysts at the nanoscale using in situ electrochemical liquid cell STEM

Octahedrally shaped Pt–Ni alloy nanoparticles on carbon supports have demonstrated unprecedented electrocatalytic activity for the oxygen reduction reaction (ORR), sparking interest as catalysts for low-temperature fuel cell cathodes. However, deterioration of the octahedral shape that gives the catalyst its superior activity currently prohibits the use of shaped catalysts in fuel cell devices, while the structural dynamics of the overall catalyst degradation are largely unknown. We investigate the time-resolved degradation pathways of such a Pt–Ni alloy catalyst supported on carbon during cycling and startup/shutdown conditions using an in situ STEM electrochemical liquid cell, which allows us to track changes happening over seconds. Thereby we can precisely correlate the applied electrochemical potential with the microstructural response of the catalyst. We observe changes of the nanocatalysts’ structure, monitor particle motion and coalescence at potentials that corrode carbon, and investigate the dissolution and redeposition processes of the nanocatalyst under working conditions. Carbon support motion, particle motion, and particle coalescence were observed as the main microstructural responses to potential cycling and holds in regimes where carbon corrosion happens. Catalyst motion happened more severely during high potential holds and sudden potential changes than during cyclic potential sweeps, despite carbon corrosion happening during both, as suggested by ex situ DEMS results. During an extremely high potential excursion, the shaped nanoparticles became mobile on the carbon support and agglomerated facet-to-facet within 10 seconds. These experiments suggest that startup/shutdown potential treatments may cause catalyst coarsening on a much shorter time scale than full collapse of the carbon support. Additionally, the varying degrees of attachment of particles on the carbon support indicates that there is a distribution of interaction strengths, which in the future should be optimized for shaped particles. We further track the dissolution of Ni nanoparticles and determine the dissolution rate as a function of time for an individual nanoparticle – which occurs over the course of a few potential cycles for each particle. This study provides new visual understanding of the fundamental structural dynamics of nanocatalysts during fuel cell operation and highlights the need for better catalyst-support anchoring and morphology for allowing these highly active shaped catalysts to become useful in PEM fuel cell applications.TU Berlin, Open-Access-Mittel - 201

Pinning Susceptibility: The Effect Of Dilute, Quenched Disorder On Jamming

We study the effect of dilute pinning on the jamming transition. Pinning reduces the average contact number needed to jam unpinned particles and shifts the jamming threshold to lower densities, leading to a pinning susceptibility, χp. Our main results are that this susceptibility obeys scaling form and diverges in the thermodynamic limit as χp∝|ϕ−ϕ∞c|−γp where ϕ∞c is the jamming threshold in the absence of pins. Finite-size scaling arguments yield these values with associated statistical (systematic) errors γp=1.018±0.026(0.291) in d=2 and γp=1.534±0.120(0.822) in d=3. Logarithmic corrections raise the exponent in d=2 to close to the d=3 value, although the systematic errors are very large

Pt-Richcore/Sn-Richsubsurface/Ptskin Nanocubes As Highly Active and Stable Electrocatalysts for the Ethanol Oxidation Reaction

Direct ethanol fuel cells are one of the most promising electrochemical energy conversion devices for portable, mobile and stationary power applications. However, more efficient and stable and less expensive electrocatalysts are still required. Interestingly, the electrochemical performance of the electrocatalysts toward the ethanol oxidation reaction can be remarkably enhanced by exploiting the benefits of structural and compositional sensitivity and control. Here, we describe the synthesis, characterization, and electrochemical behavior of cubic Pt–Sn nanoparticles. The electrochemical activity of the cubic Pt–Sn nanoparticles was found to be about three times higher than that obtained with unshaped Pt–Sn nanoparticles and six times higher than that of Pt nanocubes. In addition, stability tests indicated the electrocatalyst preserves its morphology and remains well-dispersed on the carbon support after 5000 potential cycles, while a cubic (pure) Pt catalyst exhibited severe agglomeration of the nanoparticles after a similar stability testing protocol. A detailed analysis of the elemental distribution in the nanoparticles by STEM-EELS indicated that Sn dissolves from the outer part of the shell after potential cycling, forming a ∼0.5 nm Pt skin. This particular atomic composition profile having a Pt-rich core, a Sn-rich subsurface layer, and a Pt-skin surface structure is responsible for the high activity and stability.This work has been supported by Fundación Cajacanarias (project BIOGRAF) and the Ministry of Economy and Competitiveness (MINECO) through the projects CTQ2011-28913-C02-02 and ENE2014-52158-C2-2-R (cofunded by FEDER). We acknowledge the SEGAI services of Universidad de La Laguna for important technical assistance, and R.R. acknowledges the funding received from MINECO (EEBB-I-16-11762) to carry out a predoctoral stay in a foreign R&D center. E.P. acknowledges support from an electron microscopy facility supported by the NSF MRSEC program (DMR 1120296) and an NSF MRI grant (DMR 1429155). J.S.G. acknowledges financial support from VITC (Vicerrectorado de Investigación y Transferencia de Conocimiento) of the University of Alicante

Guiding Development of Fuel Cell Catalysts with Statistically Robust Transmission Electron Microscopy

Hydrogen fuel cells in fuel cell electric vehicles (FCEVs) are a promising technology to reduce, and eventually eliminate, carbon dioxide emissions from transportation. The Pt nanoparticles used to catalyze the fuel cell’s electrochemical reactions are an important limiting factor because at present levels, the cost of the Pt catalyst will prevent widespread adoption of FCEVs. Catalysts must be developed to reduce the amount of Pt while meeting vehicle power demands even after many years of use. Strategically improving catalysts requires detailed and statistically robust characterization of their microscopic structure to understand the connections between catalyst synthesis, structure, performance, and durability. This dissertation presents the development and application of (scanning) transmission electron microscopy ((S)TEM) techniques to guide advancement of catalysts through nanostructural characterization. We develop a robust strain mapping technique for complex catalyst specimens. We deploy a new exit wave power cepstrum (EWPC) transform to nanobeam electron diffraction (NBED) patterns to enable precise, high-throughput, dose-efficient strain measurement. This approach is suitable for statistically representative measurements of many particles without special requirements such as zone-axis orientation. We apply this strain mapping technique to core-shell Pt-Co nanoparticles in combination with a continuum elastic theory model and demonstrate two mechanisms contributing to the relaxation of strain at the catalyst surface: lattice dislocations and Poisson expansion due to the spherical geometry. Comparison with electrochemical measurements suggests that the geometrical Poisson relaxation accounts for the activity of catalysts with thin shells, but catalysts with thick shells experience additional activity loss from dislocation-driven relaxation. We then turn to the larger-scale catalyst structure, investigating the impact of porous carbon support morphology, local reactant transport, and catalyst durability. Using statistical analysis of STEM images, we compare Pt and Pt-Co catalysts on porous and solid carbon supports. Comparison of 3D tomographic images and electrochemical accessibility measurements indicated that carbon pores prevent ionomer adsorption for particles embedded within them, improving the catalyst activity, while allowing proton access through condensed water. By comparing images and composition maps before and after electrochemical stability tests, we find that porous carbon supports suppress Pt particle coalescence, accounting for improved overall durability