Performance decay of proton-exchange membrane fuel cells under open circuit conditions induced by membrane decomposition

The degradation in performance of proton-exchange membrane fuel cells (PEMFCs) under open circuit conditions was investigated. The oxygen reduction reaction (ORR) kinetic current density at 0.9 V was found to decrease from 36 to 4 mA cm⁻² (geometric) without significant crossover increase or loss in the electrochemically active surface area. Cyclic voltammograms for the electrodes show characteristic changes, e.g. appearance of peaks at ∼0.2 V and shift of the onset of platinum oxide formation to higher potentials. It was identified that the large ORR kinetic decay has its origins in the reduction of available Pt sites due to adsorption of anions, which are postulated to be membrane decomposition products such as sulfate ions. Procedures carried out to condense water in the fuel cell led to the expulsion of anions out of the membrane electrode assembly (MEA) resulting in the partial recovery of ORR kinetic current density to 15 mA cm⁻². In order to attain complete performance recovery of the catalyst, a more effective and practical method to flush out the anions is desirable.     

Analysis of heterointerface recombination by Zn1−xMgxO for window layer of Cu(In,Ga)Se2 solar cells

An adjustment of a conduction band offset (CBO) of a window/absorber heterointerface is important for high efficiency Cu(In,Ga)Se₂ (CIGS) solar cells. In this study, the heterointerface recombination was characterized by the reduction of the thickness of a CdS layer and the adjustment of a CBO value by a Zn_1−xMg_xO (ZMO) layer. In ZnO/CdS/CIGS solar cells, open-circuit voltage (V_oc) and shunt resistance (R_sh) decreased with reducing the CdS thickness. In constant, significant reductions of V_oc and R_sh were not observed in ZMO/CdS/CIGS solar cells. With decreasing the CdS thickness, the CBO of (ZnO or ZMO)/CIGS become dominant for recombination. Also, the dominant mechanisms of recombination of the CIGS solar cells are discussed by the estimation of an activation energy obtained from temperature-dependent current-voltage measurements.     

Liquid-phase epitaxy of 2H-SiC film on a (0001) 4H-SiC substrate in Li-Si melt

We first applied a liquid-phase epitaxy (LPE) method to the growth of 2H-SiC to obtain an oriented 2H-SiC layer. The LPE growth was performed on a (0001) 4H-SiC substrate in a Li-Si-C melt at 850 °C. A LPE layer grew on the carbon face of the (0001) 4H-SiC substrate with the area of about 1 × 1 mm². The thickness and morphology of the LPE layer were confirmed by scanning electron microscope (SEM), and the polytype of the LPE layer was determined using a high-resolution transmission electron microscope (HR-TEM) and a Raman spectroscopy. SEM measurements revealed that the thickness of the LPE layer was 50 μm and that this layer developed with a hexagonal shape. HR-TEM observations and Raman spectral analysis showed that 2H-SiC layer grows on 4H-SiC substrate with the relationship of (0001) 2H-SiC (0001) 4H-SiC. In this study, we first succeeded to grow <0001>-axis-oriented 2H-SiC films and concluded that the LPE technique is applicable to the growth of oriented 2H-SiC films in the Li-Si melt.     

Synthesis of nanocrystalline GaN from Ga2O3 nanoparticles derived from salt-assisted spray pyrolysis

Gallium nitride (GaN) nanoparticles were successfully produced from nano-sized gallium oxide (Ga₂O₃) particles under a flow of ammonia gas. The gallium oxide nanoparticles were prepared by salt-assisted spray pyrolysis (SASP). Highly crystalline Ga₂O₃ nanoparticles with an average diameter of approximately 10 nm were obtained at various temperatures when a flux salt (LiCl, 5 mol/l) was added to the precursor solution. The effects of the crystallinity of the Ga₂O₃ particles and nitridation time on transformation to GaN were characterized using X-ray diffraction and scanning/transmission electron microscopy. Highly crystalline GaN nanoparticles with a mean size of 23.4 nm and a geometric standard deviation of 1.68 nm were obtained when Ga₂O₃ nanoparticles with relatively low crystallinity were used as the starting material. The resulting GaN nanoparticles showed a photoluminescence peak at 364 nm under UV excitation at 254 nm.     

URANS Computations of Shallow Grid Turbulence

This paper describes the unsteady Reynolds-averaged Navier–Stokes (URANS) computations of a quasi-two-dimensional (2D) grid turbulence in shallow open-channel flows, generated downstream of multiple piers aligned at regular intervals over the channel width. In shallow open-channel flows, the vertical confinement of the flow generally suppresses the three dimensionality and attains two-dimensional features with up-cascading of turbulent kinetic energy from small-scale toward large-scale structures. In this study, 2D depth averaged and 3D Reynolds-averaged equations with linear and nonlinear URANS turbulence models are applied to a shallow open-channel flow downstream of multiple piers and numerical results are discussed through a comparison with the experimental results performed by Uijttewaal and Jirka in 2003. We employed 0-equation models and k-ε models for the 2D and 3D computations, respectively. In 2D computations, vortices downstream of the grid occurred synchronously in the computation with both the linear and nonlinear 0-equation models. In the 3D computations, vortex merging and up-cascading of the kinetic energy were captured when artificial disturbance is added at the inlet. The measured decay of the turbulent kinetic energy in the streamwise direction, with a slope of ?1.3, was well captured by computation with the 3D models with inlet disturbance. The flow sensitivity on the inlet disturbance was rather small in the wide range of the disturbance ratios.     

Formation of bromo-substituted triclosan during chlorination by chlorine in the presence of trace levels of bromide

The side reactions of triclosan (2,4,4'-trichloro-2'-hydroxydiphenyl ether, TC) and chlorine in the presence of sodium chloride were investigated. In the absence of sodium chloride, three chloro-derivatives of TC, 2',3,4,4'-tetrachloro-2-hydroxydiphenyl ether (3-Cl-TC), 2',4,4',5-tetrachloro-2-hydroxydiphenyl ether (5-Cl-TC), and 2',3,4,4',5-pentachloro-2-hydroxydiphenyl ether (3,5-Cl(2)-TC) were formed, whereas in the presence of sodium chloride, 3-bromo-2',4,4'-trichloro-2-hydroxydiphenyl ether (3-Br-TC), 5-bromo-2',4,4'-trichloro-2-hydroxydiphenyl ether (5-Br-TC), (3 or 5)-bromo-2',4,4',(5 or 3)-chloro-2-hydroxydiphenyl ether ((3,5)-(BrCl)-TC), and 3,5-dibromo-2',4,4'-trichloro-2-hydroxydiphenyl ether (3,5-Br(2)-TC) were additionally formed. Radiochemical neutron activation analysis indicated that 1g of commercially available sodium chloride contained 73 microg of bromide and the bromide ion was determined to be the source of the side reactions. The rate of decrease of TC due to reaction with chlorine was greatly accelerated by the presence of bromide ion in the system: the rate with only 1 x 10(-5) M bromide ion was three times the rate in the absence of bromide.     

Evaluation of energy saving and CO2 emission reduction technologies in energy supply and end‐use sectors using a global energy model

Assessments of global warming mitigation technologies are important for achieving the Kyoto target and planning post‐Kyoto regimes. Regional differences in energy resources, growth in energy consumption, current technology diffusions, etc., should be considered in the assessments. A global energy systems model, DNE21+, with high regional resolution had treated the energy supply sectors in a bottom‐up fashion and the end‐use sectors in a top‐down fashion, which was expressed by using long‐term price elasticity. However, the assessments of technological options in the end‐use sectors are currently more important, particularly for the near and middle terms. In order to evaluate the technological options not only in the energy supply sectors but also in the end‐use sectors for energy savings and CO₂ emission reductions, DNE21+ has been modified for treating two energy‐intensive end‐use sectors, i.e. steel and cement sectors, in the bottom‐up fashion. The results reveal that the cost‐effective global CO₂ emission reductions in 2030 for stabilizing the atmospheric CO₂ concentration at 550 ppmv in comparison with that in the reference case would be approximately 68 MtC/yr and almost zero in the steel and cement sectors, respectively. The cost‐effective options include next‐generation coke ovens and coke dry quenching (CDQ) in the steel sector. Copyright © 2007 Institute of Electrical Engineers of Japan. Published by John Wiley & Sons, Inc.     

Order-Disorder Conformation Change of Succinoglycan in Aqueous Sodium Chloride as Studied by Static and Dynamic Light Scattering

Summary Static and dynamic light scattering measurements have been made on a sodium salt sample of succinoglycan in 0.01 M aqueous NaCl at different temperatures between 25 and 75°C where the polysaccharide undergoes a thermally induced change from an ordered (helical) to disordered conformation with raising temperature T. The weight-average molecular weight M _w, the z-average radius of gyration, and the hydrodynamic radius sharply decrease in a relatively narrow T range (around 55°C) in which the specific rotation was previously found to change sigmoidally with T. In particular, the value of M _w (4.55 × 10⁵) in the ordered state at 25°C is twice as large as that (2.27 × 10⁵) in the disordered state at 75°C, giving decisive evidence that the helical structure of the polysaccharide in aqueous NaCl is composed of paired chains. It is concluded that this structure is a double-stranded helix and breaks directly into two disordered chains with increasing T. Received: 6 July 2001 / Accepted: 24 July 2001     

Entrap-immobilization of invertase on composite gel fiber of cellulose acetate and zirconium alkoxide by sol–gel process

We prepared a composite gel fiber by the gel formation of cellulose acetate and zirconium tetra-n-butoxide. Gel fiber is stable in common solvents, phosphate solution, and electrolyte solution. Invertase was entrap-immobilized on the gel fiber. The immobilization was easily performed under the mild conditions. The apparent Michaelis constant (K_m) and maximum reaction velocity (V_max) were estimated from Eadie–Hofstee plot for immobilized invertase. The K_m of immobilized invertase was larger than that of native invertase, while the opposite tendency was observed for the V_max. The activity for the immobilized invertase became higher with increasing fiber diameter. It indicates that the hydrolysis of sucrose occurs in the neighborhood of the fiber surface. The thermal stability of the immobilized invertase was higher than those of its native counterpart. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 81: 2084–2088, 2001     

Two-group interfacial area transport equations at bubbly-to-slug flow transition

In relation to nuclear reactor accident and safety studies, experiments on hot-leg U-bend two-phase natural circulation in a loop with a relatively large diameter pipe (10.2 cm inner diameter) were performed for understanding the two-phase natural circulation and flow termination during a small break loss of coolant accident in LWRs. The loop design was based on the scaling criteria developed under this program and the loop was operated either in a natural circulation mode or in a forced circulation mode using nitrogen gas and water. Various tests were carried out to establish the basic mechanism of the flow termination as well as to obtain essential information on scale effects of various parameters such as the loop frictional resistance, thermal center and pipe diameter. The void distribution in a hot-leg, flow regime and natural circulation rate were measured in detail for various conditions. The termination of the natural circulation occurred when there was insufficient hydrostatic head in the downcomer side. The superficial gas velocity at the flow termination could be predicted well by the simple model derived from a force balance between the frictional pressure drop along the loop and the hydrostatic head difference. The bubbly-to-slug flow transition was found to be dependent on axial locations. It turned out that the formation of cap bubbles in the large diameter pipe caused the increased drift velocity, which would affect the prediction of the void fraction in the hot leg.