Chien-Ming Lai

On the Accelerating Degradation of DMFC at Highly Anodic Potential

A convenient method of investigating the degradation of a direct methanol fuel cell (DMFC) was carried out at highly anodic potential (i.e., 0.8 V). Accelerating degradation of a DMFC was investigated by electrochemical methods, X-ray diffraction (XRD), transmission electron microscopy (TEM), electron probe microanalysis (EPMA), and X-ray photoelectron spectroscopy (XPS). The degradation was preliminarily diagnosed and predicted with electrochemical impedance spectroscopy and verified with microscopic examination. The EPMA and XPS results showed that the sulfonic acid vanished in the broken anodic layer compared to the original one, which results in an increase of internal resistance (Rs). The increase of interfacial (R if ) and electrochemical reaction resistance (R rxn ) in the degraded cell might be a result from catalytic degradation. Ru dissolves from the anodic catalyst to decrease the catalytic activity then reduced near the cathode. The evidence for Ru dissolution was determined by CO stripping combined with EPMA and XPS analyses. Its reduction near the cathode was observed through TEM and EPMA. The catalysts in both the anode and cathode aggregated to decrease the catalytic activity in the degradation process by XRD and TEM. On the basis of the electrochemical study and related analysis, we proposed a mechanism for this accelerating degradation of DMFC.

Effect of Ethanol on the Photoelectrochemical Fabrication of Macroporous n-Si(100) in HF Solution

The effect of ethanol on the photoelectrochemical fabrication of macroporous n-type Si(100) (pore diameter > 50 nm) in 2.0 M hydrofluoric acid was investigated. A cross-sectional scanning electron microscope examination revealed the formation of rough bigger pores (diameter ≈ 7-8 μm) in the absence of ethanol but smooth smaller ones (diameter ≈ 3-4 μm) in the presence of ethanol when the silicon was etched at 0.250 V (vs saturated calomel electrode) under 50 W illumination for 3 h. Characteristic electrochemical properties, such as limiting current density (i limit ), half-wave current density (i 1/2 ), transition potential (E trans ), and half-wave potential (E p/2 ) were derived from dc polarization. Electrochemical impedance spectroscopy conducted at E trans and E p/2 was helpful to illustrate the kinetics of the photoelectrochemical reaction. An additional inductive loop in the Nyquist plot occasioned at low frequencies in the presence of ethanol was attributed to the relaxation of the adsorption of ethanol in the pores. Addition of ethanol in the etching solution led to a decrease of contact angle between the solution and the silicon. Wetting behavior of ethanol plays an important role in the formation of smooth and small macropores.

Type-II Zn1-xMnxSe/ZnSe1-yTey quantum wells

Abstract Zn 1− x Mn x Se/ZnSe 1− y Te y ( x =0.03, y =0.08) multiple-quantum-well structures were grown on GaAs substrates by molecular-beam epitaxy. Strong photoluminescence associated with iso-electronic Te traps was observed. A type-II band alignment was proposed for this class of quantum-well structures. Electrons were confined in the Zn 1− x Mn x Se layers. Holes were localized in the ZnSe 1− y Te y layers. The respective valence and conduction band offset were determined as 350±50 and 205±50 meV.