Zhiyong Xie

Anti-oxidation Behavior of Chemical Vapor Reaction SiC Coatings on Different Carbon Materials at High Temperatures

To protect carbon materials from oxidation, SiC coatings were prepared on carbon/carbon(C/C) composites and graphite by chemical vapor reaction. SEM and XRD analyses show that the coatings obtained are composed of SiC grains and micro-crystals. The influence of different carbon substrates on oxidation behavior of coated samples was investigated, and then their oxidation mechanisms were studied. Oxidation test shows that the SiC coated graphite has a better oxidation resistance than SiC coated C/C composites at high temperatures (1 623 K and 1 823 K). In the oxidation process, the oxidation curves of SiC coated C/C composites are linear, while those of SiC coated graphite follow a quasi-parabolic manner. The oxidation mechanism of the former is controlled by chemical reaction while the latter is controlled by oxygen diffusion based on the experimental results. The variation of oxidation behavior and mechanism of SiC coatings on two kinds of carbon substrates are primarily contributed to their structure differences.

Improved Properties of Carbon Fiber Paper as Electrode for Fuel Cell by Coating Pyrocarbon via CVD Method

The fabrication of a pyrocarbon coated carbon paper and its application to the gas diffusion lay (GDL) of proton exchange membrane (PEM) fuel cell were described. This carbon paper was fabricated by using conventional carbon paper as the precursor, and coating it with pyrocarbon by pyrolyzing propylene via the chemical vapor deposition (CVD) method. For comparison, conventional carbon paper composites were also prepared by using PAN-based carbon fiber felt as the precursor followed by impregnation with resin, molding and heat-treatment. SEM characterization indicates that pyrocarbon is uniformly deposited on the surface of the fiber in the pyrocarbon coated carbon paper and made the fibers of carbon felt bind more tightly. In contrast, there are cracks in matrix and debonding of fibers due to carbonization shrinkage in the conventional carbon paper. Property measurements show that the former has much better conductivity and gas permeability than the latter. In addition, current density-voltage performance tests also reveal that the pyrocarbon coating can improve the properties of carbon paper used for electrode materials of fuel cell.

Edge-rich and (N, S)-doped 3D porous graphene as an efficient metal-free electrocatalyst for the oxygen reduction reaction

A novel edge-rich and (N, S)-doped 3D porous graphene was synthesized by chemical vapor deposition (CVD) and chemical corrosion. The hybrid material as a metal-free electrocatalyst exhibited a four-electron pathway, stronger alkaline tolerance and excellent catalytic activity for oxygen reduction reaction due to the edge effect and heteroatom synergistic effect.

Influence of Preparation Technology on the Structure and Phase Composition of MoSi2-Mo5Si3/SiC Multi-coating for Carbon/Carbon Composites

Oxidation protective MoSi2-Mo5Si3/SiC multi-coatings for carbon/carbon composites were prepared by chemical vapor reaction and slurry-sintering method. The influence of preparation technology on the structure and phase composition of the coating was investigated by scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS) and X-ray diffraction (XRD) analyses, and then their relationship was discussed. The results indicate that the Si/Mo ratio of the slurry and sintering processing were two main factors that significantly affected the structure and phase composition of the multi-coating. Appropriate sintering process and relatively high Si/Mo ratio were essential for preparing the multi-coating with dense structure and favorable phase composition. After being sintered at 1723 K for 2 h and with the Si/Mo ratio of the slurry being 4.5 (weight ratio), a dense structure accompanied by favorable phase composition of the coating can be obtained. When heat treated at 2373 K for 1 h, this coating became more compact and continuous. Oxidation tests (performed at 1623 and 1823 K) demonstrated that both of these two obtained multi-coatings exhibited better anti-oxidation property than single layer SiC coating.

The synthesis and electro-catalytic activity for ORR of the structured electrode material: CP/Fe-N-CNFs

The structured electrode has the advantages of polymer binder-free, non-precious-metal and without multiple and tedious manual assembly, exhibiting superior electro-catalytic activity for oxygen reduction reactions (ORR), compared with the traditional ink-based electrode. The structured CP/Fe-N-CNFs (Fe and N containing carbon nanofibers (CNFs) in-situ grown on carbon paper (CP)), has been one-step synthesized by chemical vapor deposition (CVD). In this paper, it can be concluded that the structured CP/Fe-N-CNFs with 0.30 at.% Fe-Nx moieties exerts the most positive onset-potential (−0.05 V), peak potential, and largest peak current density. The measured current density of the structured Fe-N-CNFs at −0.8 V is increased by 56.3% compared to that of the traditional Fe-N-CNFs. The traditional Fe-N-CNFs exhibit stronger alkaline tolerance comparing with commercial Pt electrode. That is, the pronounced catalytic activity of the structured Fe-N-CNFs might attribute to the homogeneous and undiluted active sites compared to that of the traditional Fe-N-CNFs.

Reactant concentration difference induced microstructure transformation of large-area vapor-grown carbon nanofibers

We explore the effect of mass transport on the microstructure formation of chemical-vapor-deposited carbon nanofibers (CNFs), considering the decrease in carbon source gas concentration along the large-area substrate surface. The microstructures of the CNFs grown along the substrate are transformed from porous, to platelet, to herringbone, and exhibit increased graphite ordering. A model for CNF growth under different reactant concentrations is suggested. The results indicate the critical role of mass transport on the CNF growth mechanism and suggest that adjusting the reactant concentration is an easy way of controlling the CNF microstructures during large-area synthesis.

Wear behavior of SiC/PyC composite materials prepared by electromagnetic-field-assisted CVI

Abstract Silicon carbide/pyrolytic carbon (SiC/PyC) composite materials with excellent performance of self-lubrication and wear resistance were prepared on SiC substrates by electromagnetic-field-assisted chemical vapor infiltration (CVI). The composition and microstructure of the SiC/PyC materials were investigated in detail by XRD, SEM and EDS, etc. The effects of the deposition temperature on the section features and wear resistance of the SiC/PyC were studied. The results show that the PyC layers were deposited onto SiC substrates spontaneously at a lower deposition temperature. The SiC substrates deposited with PyC can significantly reduce the wear rate of the self-dual composite materials under dry sliding condition. The wear tests suggest that the SiC/PyC composite materials own a better wear resistance property when the deposition temperature is 800 °C, and the wear rate is about 64.6% of that without the deposition of PyC.

(B,N)-Doped 3D porous graphene–CNTs synthesized by chemical vapor deposition as a bi-functional catalyst for ORR and HER

Novel (B,N)-doped three-dimensional (3D) porous graphene–carbon nanotubes (CNTs) can be used as an excellent alkaline and acid tolerant electrocatalyst for both the oxygen reduction reaction (ORR) and the hydrogen evolution reaction (HER). Based on density functional theory, the H and O atoms’ pre-and post-adsorption energy can effectively reduce the reaction energy barrier.

The in situ grown of activated Fe-N-C nanofibers derived from polypyrrole on carbon paper and its electro-catalytic activity for oxygen reduction reaction

The Fe-N-C nanofibers (NFs) exhibit excellent electro-catalytic activity for oxygen reduction reaction (ORR), which is essential for energy conversion system. Compared with the traditional ink-based electrode, we synthesized the structured Fe-N-C ANFs/CP (activated NFs derived from polypyrrole(PPy) doped with iron atoms in situ grown on carbon paper) by electrochemical polymerization. Herein, we investigate the effect of the iron concentration on the electro-catalytic activity for ORR, revealing that the as-prepared 0.05-Fe-N-C ANFs exhibit superior electro-catalytic activity than 0.01-Fe-N-C ANFs, 0.03-Fe-N-C ANFs as well as 0.10-Fe-N-C ANFs and excellent durability in alkaline solution. It is shown that the better electro-catalytic activity of 0.05-Fe-N-C ANFs for ORR could be ascribing to the fluffy fibrous structure, higher Fe-Nx concentration, and well-dispersed catalytic active sites.