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

The Priority and Challenge of High-Power Performance of Low-Platinum Proton-Exchange Membrane Fuel Cells

Substantial progress has been made in reducing proton-exchange membrane fuel cell (PEMFC) cathode platinum loadings from 0.4–0.8 mg_Pt/cm² to about 0.1 mg_Pt/cm². However, at this level of cathode Pt loading, large performance loss is observed at high-current density (>1 A/cm²), preventing a reduction in the overall stack cost. This next developmental step is being limited by the presence of a resistance term exhibited at these lower Pt loadings and apparently due to a phenomenon at or near the catalyst surface. This issue can be addressed through the design of catalysts with high and stable Pt dispersion as well as through development and implementation of ionomers designed to interact with Pt in a way that does not constrain oxygen reduction reaction rates. Extrapolating from progress made in past decades, we are optimistic that the concerted efforts of materials and electrode designers can resolve this issue, thus enabling a large step toward fuel cell vehicles that are affordable for the mass market

Record activity and stability of dealloyed bimetallic catalysts for proton exchange membrane fuel cells

We demonstrate the unprecedented proton exchange membrane fuel cell (PEMFC) performance durability of a family of dealloyed Pt-Ni nanoparticle catalysts for the oxygen reduction reaction (ORR), exceeding scientific and technological state-of-art activity and stability targets. We provide atomic-scale insight into key factors controlling the stability of the cathode catalyst by studying the influence of particle size, the dealloying protocol and post-acid-treatment annealing on nanoporosity and passivation of the alloy nanoparticles. Scanning transmission electron microscopy coupled to energy dispersive spectroscopy data revealed the compositional variations of Ni in the particle surface and core, which were combined with an analysis of the particle morphology evolution during PEMFC voltage cycling; together, this enabled the elucidation of alloy structure and compositions conducive to long-term PEMFC device stability. We found that smaller size, less-oxidative acid treatment and annealing significantly reduced Ni leaching and nanoporosity formation while encouraged surface passivation, all resulting in improved stability and higher catalytic ORR activity. This study demonstrates a successful example of how a translation of basic catalysis research into a real-life device technology may be done.DFG, SPP 1613, Regenerativ erzeugte Brennstoffe durch lichtgetriebene Wasserspaltung: Aufklärung der Elementarprozesse und Umsetzungsperspektiven auf technologische Konzep

Circumventing Metal Dissolution Induced Degradation of Pt-Alloy Catalysts in Proton Exchange Membrane Fuel Cells: Revealing the Asymmetric Volcano Nature of Redox Catalysis

One of the major obstacles to the commercialization of proton exchange membrane fuel cells (PEMFCs) is the usage of scarce platinum in the cathode for the oxygen reduction reaction (ORR). Although progress has been made in reducing Pt usage by alloying with transition metals M (M = Co, Ni, Cu, etc.), practical applications of Pt-M/C catalysts are impeded by their insufficient durability under the highly corrosive conditions at a PEMFC cathode. Herein, we reconcile the durability difficulty by demonstrating that the high mass activity of the dealloyed PtNi₃/C catalyst with low nanoporosity further increases after 30k voltage cycles in PEMFCs. A novel method has been developed to implement an in situ X-ray absorption spectroscopy study of these PEMFC-cycled catalysts under operating conditions to understand the unusual activity trend. We reveal that the ORR activity of PtNi₃/C catalysts with varied nanoporosities exhibits a Sabatier volcano curve as a function of the strain governed by Ni content, and the volcano is skewed toward the Pt–O weak binding leg owing to the asymmetric site-blocking effect. The Ni dissolution during PEMFC operation, which was previously believed to be detrimental, becomes beneficial for the solid PtNi₃/C catalysts located on the Pt–O weak binding leg because it leads to the activity ascending toward the apex, and meanwhile the activity remains high throughout the long-term operation owing to the minimal site-blocking effect. More generally, the fundamental insights into the universal asymmetric volcano curve of redox catalysis will potentially guide the rational design of a broad variety of catalytic materials

Boosting Fuel Cell Performance with Accessible Carbon Mesopores

Boosting Fuel Cell Performance with Accessible Carbon Mesopore