Numerical investigation of the effect of cathode catalyst layer structure and composition on polymer electrolyte membrane fuel cell performance

The effect of the cathode catalyst layer's structure and composition on the overall performance of a polymer electrolyte membrane fuel cell (PEMFC) is investigated numerically. The starting point of the sub-grid scale catalyst layer model is the well-known flooded agglomerate concept. The proposed model addresses the effects of ionomer (Nafion) loading, catalyst (platinum) loading, platinum/carbon ratio, agglomerate size and cathode layer thickness. The sub-grid scale model is first validated against experimental data and previously published results, and then embedded within a two-dimensional validated computational fluid dynamics code that can predict the overall performance of the fuel cell. The integrated model is then used to explore a wide range of the compositional and structural parameter space, mentioned earlier. In each case, the model is able to correctly predict the trends observed by past experimental studies. It is found that the performance trends are often different at intermediate versus high current densities—the former being governed by agglomerate-scale (or local) losses, while the latter is governed by catalyst layer thickness-scale (or global) losses. The presence of an optimal performance with varying Nafion content in the cathode is more due to the local agglomerate-scale mass transport and conductivity losses in the polymer coating around the agglomerates than due to the amount of Nafion within the agglomerate. It is also found that platinum mass loading needs to be at a moderate level in order to optimize fuel cell performance, even if cost is to be disregarded.     

Bioaccumulation patterns of methyl mercury and essential fatty acids in lacustrine planktonic food webs and fish

Organisms of the planktonic food web convey essential nutrients as well as contaminants to animals at higher trophic levels. We measured concentrations of methyl mercury (MeHg) and essential fatty acids (EFAs, key nutrients for aquatic food webs) in four size categories of planktonic organisms - seston (10-64 microm), micro-(100-200 microm), meso-(200-500 microm), and macrozooplankton (>500 microm) - as well as total mercury (THg) and EFAs in rainbow trout (Oncorhynchus mykiss) in coastal lakes. We demonstrate that, in all lakes during this summer sampling, MeHg concentrations of planktonic organisms increase significantly with plankton size, independent of their taxonomic composition, and that their MeHg accumulation patterns predict significantly THg concentrations in rainbow trout (R2=0.71, p<0.05). However, concentrations of total EFAs do not follow this pattern. Total EFAs increased from seston to mesozooplankton but decreased in the largest zooplankton size fraction. Moreover, concentrations of individual EFA compounds in rainbow trout are consistently lower, with the exception of docosahexaenoic acid, than those in macrozooplankton. The continuous increase of MeHg concentrations in aquatic organisms, therefore, differs from patterns of EFA accumulation in zooplankton and fish. We interpret these contrasting accumulation patterns of MeHg and EFA compounds as the inability of aquatic organisms to regulate the assimilation of dietary MeHg, whereas the rate of EFA retention may be controlled to optimize their physiological performance. Therefore, we conclude that bioaccumulation patterns of Hg in these aquatic food webs are not controlled by lipid solubility and/or the retention of EFA compounds.     

Pore morphology and pore surface roughening in rocks: a small-angle neutron scattering investigation

Pore morphology and pore-matrix interface roughening in some metamorphosed sedimentary rocks, sandstones and igneous rocks have been investigated using small-angle neutron scattering (SANS), in the length scales of 20–1000 nm., which reveal the fractal nature of the rock-pore interfaces. Surface fractal dimension of the metamorphosed rocks and the sandstones has been estimated to be 2.8 while, that for the igneous rocks has been found to be 2.3. An attempt has been made to explain the relatively high surface fractal nature of the former rocks with the help of a computer simulation model based on the formation mechanisms of these rocks. SANS data indicate some ideas about the upper cut-off of the fractal geometry for the igneous rocks as well as for the sandstone, but no unambiguous cut-off value has been obtained for the metamorphosed rocks in the accessible length scale. The multiple scattering effect in these rock specimens has also been looked into by performing the SANS experiments for the two thicknesses on each specimen.     

Nonequilibrium synthesis of nbai3 and Nb-Ai-V

In this study, the isothermal oxidation behavior of laser-clad NbAl₃ has been investigated in the temperature range between 800 °C and 1400 °C in air. The effect on oxidation of vanadium microalloying, used to increase the ductility of the otherwise brittle NbAl₃ and discussed in Part I, [1] has also been considered. Bulk and surface oxide chemistry has been investigated using X-ray diffraction and X-ray photoelectron spectroscopy (XPS), respectively. Oxidation kinetics have been determined from weight gain data. The XPS and X-ray diffraction data show that NbAl₃ does not exclusively form a protective A1₂O₃ layer when oxidized in air. The oxidation products at 800 °C are a mixture of Nb₂O₅ and A1₂O₃, while at 1200 °C, a mixture of NbAlO₄, Nb₂O₅, and Al₂O₃ is formed. At 1400 °C, a mixture of NbAlO₄, A1₂O₃, NbO₂, NbO_2.432, and Nb₂O₅ forms. Upon addition of vanadium, the oxidation rate is found to dramatically increase and may be related to the formation of (Nb, V)₂O₅ and VO₂, which grows in favor of protective A1₂O₃. Consequently, although vanadium may be a good additive in terms of its potential for improving ductility in NbAl₃, it is not in terms of its deleterious effects on oxidation.     

EFFECT OF LOADING WITH AIM OIL AEROSOL ON THE COLLECTION EFFICIENCY OF AN ELECTRET FILTER

Loading of an electret filter changes the distribution of electrical field in the filter from its preloading condition, and, therefore, affects the filtration efficiency of the filter. Liquid droplets collected on electret filters cause degradation of the electrostatic enhancement of filtration efficiency because of charge neutralization and the formation of a dielectric coating over die charged fibers. In this study, calculations were made for the penetration of aerosol particles through a spun-type electret filter as a function of the particulate loading. An assumption was made that each charge collected neutralized one charge of opposite polarity on the fibers of the filter. It was also assumed that the electrostatic charges present on the particles followed the Boltzmann equilibrium charge distribution. The decrease in fiber charge and resulting increase in penetration were calculated as a function of time and of total particulate loading on the filter. The calculated penetrations were compared with experimental measurements of loading on a spun fiber electret filter challenged with monodisperse liquid droplets of bis-Ethylhexyl Sebacate with equilibrium charge distribution and with zero charge. The rate at which the penetration increased was found to be the same for particles with zero charge as for particles with equilibrium charge distribution. For 1 um diameter droplets with equilibrium charge the theory predicted complete discharge of the filter at a loading of around 200 g/m². Experimentally, only about 0.3 g/m² was required. This difference indicates the presence of additional mechanisms for the discharge of the fibers.     

Extended solid solution and nonequilibrium phase diagram for Ni-Al alloy formed during laser cladding

Due to rapid solidification in laser cladding, the composition of the solute in the cladding alloy often exceeds the solid solubility limit far beyond that expected from the equilibrium phase diagram. Also, the effects of various process parameters on the composition of the cladding alloy depend on the sign of the slope of the equilibrium phase diagram at the nominal composition of the cladding powder. This paper uses a mathermatical model to determine the composition of metastable alloys and to study the effects of various process parameters on the composition of extended solid solution in the cladding alloys of Ni-Al for which the slope of the equilibrium phase diagram is positive at the nominal composition of the Ni-Al cladding powder. The model considers concentration dependent freezing point and nonequilibrium partitioning of solute at the solid-liquid interface. Because of this, nonequilibrium phase diagram can be obtained by following the methodology of this paper.     

Chemical trapping of ternary complexes of human immunodeficiency virus type 1 integrase, divalent metal, and DNA substrates containing an abasic site. Implications for the role of lysine 136 in DNA binding

We report a novel assay for monitoring the DNA binding of human immunodeficiency virus type 1 (HIV-1) integrase and the effect of cofactors and inhibitors. The assay uses depurinated oligonucleotides that can form a Schiff base between the aldehydic abasic site and a nearby enzyme lysine epsilon-amino group which can subsequently be trapped by reduction with sodium borohydride. Chemically depurinated duplex substrates representing the U5 end of the HIV-1 DNA were initially used. We next substituted an enzymatically generated abasic site for each of 10 nucleotides normally present in a 21-mer duplex oligonucleotide representing the U5 end of the HIV-1 DNA. Using HIV-1, HIV-2, or simian immunodeficiency virus integrases, the amount of covalent enzyme-DNA complex trapped decreased as the abasic site was moved away from the conserved CA dinucleotide. The enzyme-DNA complexes formed in the presence of manganese were not reversed by subsequent addition of EDTA, indicating that the divalent metal required for integrase catalysis is tightly bound in a ternary enzyme-metal-DNA complex. Both the N- and C-terminal domains of integrase contributed to efficient DNA binding, and mutation of Lys-136 significantly reduced Schiff base formation, implicating this residue in viral DNA binding.     

Nonequilibrium synthesis of NbAl3 and Nb-Al-V

NbAl₃, like many ordered intermetallic compounds, appears to have great potential for application as high-temperature structural material because of the high melting point and stability in high- temperature oxidizing environments. However, the D0₂₂ (tI8) lattice structure shows limited ductility at room temperature. Addition of group IVB, VB, and VIB elements to A1₃X (D0₂₂ type of compounds) as partial replacement of X is a method being applied by various researchers to promote ductility in these materials by activation of numerous dislocations and ordered twin- ning. In this study, V was added in different amounts to improve the ductility of NbAl₃. How- ever, this has an adverse effect on the oxidation response of the system, as is explained in Part II of this article. This article deals with the microstructure evolution, phase identification, and characterization of laser-clad (rapidly solidified) nominal NbAl₃ and also studies the effect of V additions on the final microstructure in this system. Laser cladding of nominal NbAl3 produces two metastable unreported phases [body-centered tetragonal (bct) and base-centered orthorhombic phases]. Addition of V suppresses the primary phase (NbAl₃) in the Nb-Al-V system. on leave from the Department of Metallurgy on leave from the Department of Metallurgy