Xuan Cheng

Catalyst Microstructure Examination of PEMFC Membrane Electrode Assemblies vs. Time

A series of single-cell, hydrogen-air proton exchange membrane fuel cells (PEMFCs) was operated for different lengths of time, namely, 200, 500, 700, and 1000 h. A group of reproducible and identical membrane electrode assemblies (MEAs) was used for those tests. Cell performance was studied by examining the cell polarization curves. After various lifetime tests, each MEA was cross-cut and characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscopy, and Raman techniques to investigate any changes in catalyst structure and morphology, as well as particle size and chemical composition. The average particle size of the catalysts was calculated from XRD results and was found to increase with cell operating time. In addition, the agglomeration in nanometer-sized catalyst particles was observed from TEM analysis after prolonged cell operation. Ruthenium oxide was identified from Raman spectra of the anode catalyst from the tested MEAs, while no oxides were found on the cathode catalyst at the cell operating voltage. It is possible that the formation of metal oxides at the surface of the anode catalyst led to larger particles and ultimately resulted in the decrease of catalyst activity. This might be responsible for the slightly degraded cell performance following 700 h of operation.

In-situ Raman spectroscopic studies on the oxide species in molten Li/K2CO3

Many concerns have been raised about the mechanism of cathode reaction in molten carbonate fuel cell (MCFC). The chemical behavior of oxide species at cathode in molten carbonate is a key for understanding the process of cathode reactions. In this paper, the variety and role of the oxide species in both bulk and thin-film of basic molten carbonates were investigated by using a novel in-situ Raman spectroscopy. The results indicated that the dominant oxide species under basic conditions was peroxide ion, and it was possible to transform into the oxygen of crystalline lattice during the lithium-doped process. It was demonstrated that in-situ Raman spectroscopic technique was a promising tool to elucidate the mechanism of electrode reaction in molecular level in the MCFC condition.

Achieving high capacity by vanadium substitution into Li2FeSiO4 for Li ion battery

Conference Name:Symposium on Rechargeable Lithium and Lithium Ion Batteries Held During the 220th Meeting of the Electrochemical-Society (ECS). Conference Address: Boston, MA. Time:OCT 09-14, 2011.

Effect of potential steps on porous silicon formation

Porous silicon microstructures were fabricated by applying potential steps through which both anodic and cathodic potentials were periodically applied to silicon wafers. The electrochemical behaviors of porous silicon layers were examined by performing polarization measurements, followed by analyzing the open-circuit potential (Eocp) and the reaction rate in terms of corrosion current density (jcorr). The surface morphologies and surface products of porous silicon were characterized by scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS). It was found that the values of Eocp and jcorr varied more significantly and irregularly during different polarization stages when the potentials were continuously applied to the wafer surface, while virtually unchanged after 2 min of periodic potential application. In addition, slower reaction rates were observed with applying potential steps, as indicated by smaller values of jcorr. The enhancement on refreshment of silicon surfaces by periodic potential polarization significantly accelerated the growth of porous silicon. The microstructures became more uniformed and better defined due to the improved passivating nature of wafer surfaces. # 2002 Elsevier Science Ltd. All rights reserved.

Effects of electrical characters on electrical fatigue behavior in PLZT ferroelectric ceramics

Electrical fatigue tests have been conducted on PLZT ferroelectric ceramics. It was found that the higher the applied electrical field magnitude, the faster the Pr decreases; the samples under the same electrical field magnitude but with square wave have higher fatigue rate than that of sine wave. It was observed that at low frequency of applied field (50Hz and 500Hz), the polarization decreased very quickly and dropped below 70% of the original value within 106.2 switching cycles, however, when fatigue test were conducted at high frequencies of 100kHz and 300kHz, the polarization did not change significantly even after 109 cycles. The SEM analysis indicated that the facture mode was mainly of trans-granular for the virgin non-fatigued sample, while of inter-granular for the fatigue sample. The quantitative information on the 90° domain switching was measured in situ using X-ray diffraction. The effects of electrical characters on electrical fatigue behavior were explained according to the mechanism of electrical fatigue in PLZT ferroelectric ceramics.

Controllable fabrication of PS/Ag core-shell-shaped nanostructures

In this paper, based on the previous steps, a facile in situ reduction method was developed to controllably prepare polystyrene/Ag (PS/Ag) core-shell-shaped nanostructures. The crucial procedure includes surface treatment of polystyrene core particles by cationic polyelectrolyte polyethyleneimine, in situ formation of Ag nanoparticles, and immobilization of the Ag nanoparticles onto the surface of the polystyrene colloids via functional group NH from the polyethyleneimine. The experimental parameters, such as the reaction temperature, the reaction time, and the silver precursors were optimized for improvement of dispersion and Ag coat coverage of the core-shell-shaped nanostructures. Ultimately, the optimum parameters were obtained through a series of experiments, and well-dispersed, uniformly coated PS/Ag core-shell-shaped nanostructures were successfully fabricated. The formation mechanism of the PS/Ag core-shell-shaped nanostructures was also explained.

Rheological Measurement of In Situ Preheating Polycarbosilane Melt

Rotational rheometer has been widely used in studying rheological properties of materials. However, the conventional rheological measurements may not be applicable for some non-traditional materials with characteristic properties. Polycarbosilane (PCS) is an oligomer and in the form of solid state at room temperature. A large amount of air bubbles are evolved during the heating process of PCS, which is extremely harmful for rheological measurements of PCS melt and detrimentally influences the accuracy of rheological data. In this work, a series of PCS raw samples were pretreated with different heating processes by the advanced rheometric expansion system at 315°C before the rheological measurements at 295°C. The softening point temperatures and molecular weight distributions of different PCS samples were obtained before and after the preheating processes. The experimental data collected with the in-situ preheating and ex-situ preheating for 1 h were compared.

Determining Zero Shear Viscosity of Polycarbosilane Melts

As an important precursor in preparation of silicon carbide ceramic fibers by polymer precursor-derived method, it is very difficult to find the zero shear viscosity through the conventional steady test for the oligomer-like polycarbosilane (PCS) because of the possible oxidation and a prolonged time required to attain a steady state of the samples. In this work, a steady test and a series of transient tests were performed for the PCS melts. The limitation of the steady test is discussed. An averaging method based upon data from the transient tests is developed to determine the zero shear viscosity for the melt. The reliability of the results from the method is discussed and the zero shear viscosity of the pretreated PCS melt was obtained.

A Comparative Study of Co-PPy-MWCNTs Prepared differently as a Non-noble Catalyst for Fuel Cells

The cobalt-polypyrrole (Co-PPy) composites adsorbed on the multiwall-carbon nanotubes (MWCNTs) were prepared by three methods. The structural, chemical and electrochemical properties of Co-PPy-MWCNTs samples were characterized by using XRD, SEM/EDX, element analysis, TG, FTIR, XPS, and cyclic voltammetry (CV). The results revealed that the Co-PPy was deposited on the MWCNTs surface and the ratios of Co/N for the three samples prepared differently varied. The conductivities and electrochemical catalytic activities of Co-PPy-MWCNTs samples were also affected by the different interactions between Co and PPy presented when using three preparation methods.

Comparative studies of adsorbed CO and methanol electrooxidation on carbon supported Pt and PtRu catalysts in acid solution

Abstract Physicochemical characterization of commercial Pt/C and PtRu/C catalysts was carried out by X-ray diffraction, X-ray photoelectron spectroscopy, and transmission electron microscopy. Comparative studies of methanol electrooxidation on carbon supported Pt and PtRu catalysts in acid solution were performed by means of an electrochemical method. The experimental results showed that PtRu/C not only had a higher electrocatalytic activity for the electrocatalytic oxidation of methanol than Pt/C, but also for the electrocatalytic oxidation of adsorbed CO. Alloy formation of Ru with Pt modified not only the characteristics of H 2 adsorption on the surface of catalyst but also the characteristics of CO adsorption on the surface of catalyst. The interaction of CH 3 OH with Ru was a temperature-activated process requiring elevated temperature.