Feihong Nan

The role of vacancies and defects in Na0.44MnO2 nanowire catalysts for lithium–oxygen batteries

Na0.44MnO2 nanowires were acid leached in nitric acid, and dehydrated by heat treatment to induce controllable defect formation as monitored by high resolution TEM studies. The charge–discharge tests using these materials as catalysts (or “promoters”) in rechargeable lithium–oxygen batteries (in non-carbonate electrolytes) showed that a high defect concentration results in a doubling of the reversible energy storage capacity up to 11 000 mA h g−1, and lowered overpotentials for oxygen evolution. The role of the defects/vacancies in determining oxygen reduction behavior is highlighted.

Scanning transmission electron microscopic tomography of cortical bone using Z-contrast imaging.

Previously we presented (McNally et al., 2012) a model for the ultrastructure of bone showing that the mineral resides principally outside collagen fibrils in the form of 5 nm thick mineral structures hundreds of nanometers long oriented parallel to the fibrils. Here we use high-angle annular dark-field electron tomography in the scanning transmission electron microscope to confirm this model and further elucidate the composite structure. Views of a section cut parallel to the fibril axes show bundles of mineral structures extending parallel to the fibrils and encircling them. The mineral density inside the fibrils is too low to be visualized in these tomographic images. A section cut perpendicular to the fibril axes, shows quasi-circular walls composed of mineral structures, wrapping around apparently empty holes marking the sites of fibrils. These images confirm our original model that the majority of mineral in bone resides outside the collagen fibrils.

Compositional and Morphological Changes of Ordered PtxFey/C Oxygen Electroreduction Catalysts

Changes in the O2 reduction activity (ORR) and structure of carbon‐supported catalysts upon electrochemical stress testing are investigated. Focus is placed on two alloy catalysts of nominal Pt3Fe/C and Pt3Fe2/C compositions. Energy dispersive X‐ray spectroscopy (EDXS) spot and line analyses reveal a dependence of the Fe composition on the particle size, particularly for the two as‐prepared catalysts. The catalyst particles are shown to have a Pt‐enriched shell and a PtxFey alloy core. Larger (>≈10 nm) particles are shown to have a higher Fe content that approaches the nominal composition, which suggests that the smaller (<≈6 nm) Pt catalyst particles are more difficult to alloy. High‐angle annular dark‐field scanning transmission electron microscopy (HAADF‐STEM), XRD, and SEM with EDXS show that Fe is lost gradually from the catalyst particles as a result of extensive potential (E)‐cycling. Changes upon E‐cycling are observed most clearly for the small (<3 nm) particles, in which Fe is almost entirely depleted. However, the catalytic ORR activities remain constant over an extensive cycling period for the PtxFey/C catalysts and the mass ORR activities decrease proportionally with Pt surface area (APt). The histograms before and after cycling are compared to observed changes in APt and are discussed in comparison to E‐holding experiments. It is concluded that the dissolution of Pt is a strong contributor for the observed decrease in APt and mass ORR activity for the PtxFey/C catalysts. The continuous transition between Pt oxide formation and its reduction to Pt metal is suggested to play a major role in the degradation of the PtxFey/C catalysts studied in this work.

Structure, ordering, and surfaces of Pt–Fe alloy catalytic nanoparticles from quantitative electron microscopy and X-ray diffraction

The current challenge in catalyst development is to produce highly active and economical catalysts. This challenge cannot be overcome without an accurate understanding of catalyst structure, surfaces and morphology as the catalytic reactions occur on the surface active sites. Transmission Electron Microscopy (TEM) is an excellent tool for understanding the structures of the nanoparticles down to the atomic level in determining the relationship with the catalysts performance in fuel cell applications. This paper describes a detailed structural characterization of Pt-Fe nanoparticles using aberration corrected TEM. Detailed analysis regarding the morphology, structural ordering, facets, nature of the surfaces, atomic displacements and compositions was carried out and presented in the context of their electrochemical performances. In addition, the effects of electrochemical cycling in terms of morphology and composition evolution of the nanoparticles were analyzed. Lastly, along with data from X-ray diffractometry, two different crystallographic models of the unknown Pt(3)Fe(2) nanoparticle phase are proposed. The detailed characterization by TEM provides useful insights into the nanoparticle chemistry and structure that contributes to catalyst development for next generation fuel cells.

STEM HAADF Tomography of Molybdenum Disulfide with Mesoporous Structure

A highly ordered mesoporous molybdenum disulfide has been developed for catalysis in heavy oil refining. The morphology, structure, and composition of the material have been systematically characterized with advanced electron microscopy techniques. Scanning transmission electron microscopy with high‐angle annular dark field tomography has been used to investigate the porous structure to give spatial information on the nanometre scale, and offer a direct view of individual porous particles in three‐dimensions. The pore‐size distribution, connectivity of the pores, and the mesoporous surface area have also been analyzed and offer useful information towards catalyst design.