Qianping He

Impact of oxidation on nanoparticle adhesion to carbon substrates

Carbon supported platinum (Pt/C) catalysts are of vital importance to todays fine chemical industry. However, both Pt and its carbon support surface undergo oxidation during operation. There is a lack of information on how surface oxidation affects the durability of Pt/C catalysts. In this study, we report the impact of oxidation of Pt/C on the binding energy and nanoparticle adhesion force. Classic molecular dynamics simulations are performed on systems containing PtO nanoparticles of 4 nm and graphite surfaces oxidized with either epoxy or hydroxyl groups at several oxidation extents (10%, 25% and 50%). Appropriate for service in polymer electrolyte membrane fuel cells (PEMFC), the effect of a thin Nafion film (1 nm thick) at different hydration levels (λ = 3, 6, 9 and 15) is also included in the system to study the impact of oxidation on the interface structure of the electrolyte and the electrode. This study shows that the adhesion of both the Nafion film and the PtO nanoparticle is drastically affected by the type and degree of oxidation on the carbon surface. The oxidation with hydroxyl groups on the graphite surface enhanced the binding energy between the polymer electrolyte and the carbon electrode, while oxidation with the epoxy group beyond a certain amount caused delamination of the film. The adhesion of the PtO nanoparticle was similarly enhanced by the hydroxylated surface and diminished by the epoxidized surface. The binding energies and adhesive forces are reported for all systems studied.

Choice of Specimen Thickness in Axial Bright-Field STEM Tomography of Cells

Axial bright-field electron tomography of biological specimens using scanning transmission electron microscopy (STEM) provides 3D reconstructions of cells from stained plastic sections that are around 1 μm in thickness, which enables visualization of large cellular structures in their entirety [14]. This is achievable because in STEM there are no imaging lenses after the specimen so that chromatic aberration effects can be ignored in the presence of strong multiple inelastic scattering. Previously, we have shown that bright-field STEM tomography gives higher spatial resolution than dark-field STEM tomography for thick specimens [5, 6]. However, due to elastic scattering, the practical thickness range depends on the concentration of heavy atoms in the specimen, i.e., the fixation and staining protocols. Here, we describe a simple way to assess the usable thickness range.

Electron beam induced radiation damage in the catalyst layer of a proton exchange membrane fuel cell.

Electron microscopy is an essential tool for the evaluation of microstructure and properties of the catalyst layer (CL) of proton exchange membrane fuel cells (PEMFCs). However, electron microscopy has one unavoidable drawback, which is radiation damage. Samples suffer temporary or permanent change of the surface or bulk structure under radiation damage, which can cause ambiguity in the characterization of the sample. To better understand the mechanism of radiation damage of CL samples and to be able to separate the morphological features intrinsic to the material from the consequences of electron radiation damage, a series of experiments based on high-angle annular dark-field-scanning transmission scanning microscope (HAADF-STEM), energy filtering transmission scanning microscope (EFTEM), and electron energy loss spectrum (EELS) are conducted. It is observed that for thin samples (0.3-1 times λ), increasing the incident beam energy can mitigate the radiation damage. Platinum nanoparticles in the CL sample facilitate the radiation damage. The radiation damage of the catalyst sample starts from the interface of Pt/C or defective thin edge and primarily occurs in the form of mass loss accompanied by atomic displacement and edge curl. These results provide important insights on the mechanism of CL radiation damage. Possible strategies of mitigating the radiation damage are provided.