bluuebunny/crossref_metadata_2025 huggingface dataset:
Effect of PTFE Content in Microporous Layer on Water Management
Authors: Park Sehkyu , Lee Jong-Won , Popov Branko
Published: September 2007

The effect of hydrophobic agent (PTFE) concentration in the microporous layer on the PEM fuel cell performance was investigated using mercury porosimetry, water permeation experiment, and electrochemical polarization technique. When air was used as an oxidant, a maximum fuel cell performance was obtained for a PTFE loading of 20 wt%. The polarization study indicated that the optimized PTFE content reduces oxygen transport limitation in the catalyst layer as well as in the gas diffusion layer by controlling liquid water saturation in the membrane-electrode assembly.
Capillary Pressure-Saturation Behavior of Carbon Paper Fuel Cell Diffusion Media: A Validated Approach
Authors: Kumbur Emin C. , Sharp Kendra V. , Mench Matthew M.
Published: September 2007

This study is motivated by the need to develop a validated capillary pressure-saturation relationship that can precisely describe the capillary transport characteristics of the fuel cell diffusion media (DM). A series of capillary pressure-saturation measurements were performed for SGL 24 series DMs tailored with various degrees of PTFE loadings (from 5% to 20% of wt.) over a wide range of operating conditions (i.e. at different temperatures and compressions). The benchmark data were then utilized to deduce an appropriate form of Leverett approach that can precisely determine the capillary pressure of the tested fuel cell DMs as a function of hydrophobic additive content, operating temperature, liquid saturation, compression pressure and uncompressed porosity of the DM.
A Novel Flow Field Design for the Effective Water Transport in Polymer Electrolyte Fuel Cells
Authors: Amiruddin Shabab , Li Bing , Prakash Jai
Published: September 2007

A novel flow field for the effective utilization of reactant gasses has been proposed in this work. The proposed flow field consists of a spiral pattern as compared to the already exiting serpentine flow channels. This study exemplifies the use of modeling in the design of flow channels for a proton exchange membrane fuel cell (PEMFC). This model is based on charge and mass balances in the gas diffusion electrodes and the catalytic regions in fuel cells. A two dimensional (2D) and a three dimensional (3D) models will be used to study the velocity distribution, water management and current density distribution. A comparison will be made between the proposed spiral flow fields and the existing serpentine flow fields.
Predicting Water-Droplet Detachment from GDL/Channel Interfaces in PEM Fuel Cells
Authors: Chen Ken S
Published: September 2007

An analytical model is presented for predicting the critical air-flow velocity at the onset of water-droplet detachment from the GDL/channel interfaces in PEM fuel cells. Our model is based on the force balance between pressure drag that tends to detach the droplet and surface tension that tends to hold the droplet in place. In the present work, we consider the flow regime in which pressure drag, which arises from inertia effects, dominates over viscous shear - this is the flow regime of interest in real-world PEM fuel cell applications, both automotive and stationary. Our analytical model predicts that the critical air-flow velocity varies inversely (to the 2/3 power) with water-droplet size. It further predicts that making the GDL surface more hydrophobic, decreasing contact-angle hysteresis, and shrinking channel height reduce the critical air-flow velocity. Model predictions are compared with experimental data available from the literature and reasonably good agreements are obtained.
First Principles Investigation of PEMFC Catalyst Alloy Surface Hydration and Oxidation
Authors: Opalka Susanne M. , Vanderspurt Thomas , Kandoi Shampa , Protsailo Lesia
Published: September 2007

Density functional theory predictions were made of atomic O adsorption on Pt, Pt3Co native, and Pt3Co Pt skin (111) surfaces without and with a water monolayer at the PEMFC operating conditions of 80 {degree sign}C and 1 atmosphere. In the absence of water, the Pt and Pt3Co Pt skin surfaces were predicted to have equilibrium coverages of 0.5 and 0.25 monolayers (ML) O, respectively. On the Pt3Co native surface, the strong surface Co-O bonding interactions enabled favorable accommodation of up to a full O ML. Water adsorption was predicted to yield similar surface free energies and stable coverages of up to 1 ML on all three surfaces. Overall, atomic O bonding was weaker in the presence of 1 ML H2O, with the projected equilibrium coverages of 0.25 ML for the Pt and Pt3Co Pt skin (111) surfaces, and 0.75 ML for the Pt3Co native surface. Thus, hydrated layers may free up more concerted surface sites for ORR, but will also increase their susceptibility to dissolution in acidic media.
First-Principles Study on Segregation and Surface Structures of Pt-Rh Alloys
Authors: Yuge Koretake , Seko Atsuto , Koyama Yukinori , Kuwabara Akihide , Oba Fumiyasu , Tanaka Isao
Published: September 2007

Atomic arrangements of (111) surfaces of Pt25Rh75 and Pt50Rh50 alloys were investigated by the conjunction of the first-principles calculation, the cluster expansion method, and the Monte Carlo statistical-mechanics simulation. The ECIs of points, pairs and multi-body clusters were different in the top two layers from those in the bulk, while they got converged in the third layer to those in the bulk. This indicates that the top two layers are the surface in terms of the ECIs. Segregation of Pt was observed in the top layers of the Pt25Rh75 and Pt50Rh50 alloys, and that of Rh was done in the second layers. A (sqrt(3)xsqrt(3))R30-type in-plane ordered structure was found in the second layer of the Pt50Rh50 alloy. The arrangement was unique to the surface, not observed in the bulk.
The Effect of S Contaminants on Hydrogen Interaction with Ni(111) and Pt(111) Surfaces: A First Principles Study
Authors: Pillay Devina , Johannes Michelle
Published: September 2007

Interactions between hydrogen and sulfur on Ni(111) and Pt(111) were investigated using density functional theory. Hydrogen adsorption on Pt(111) indicates that the potential energy surface is smooth while on Ni(111) hydrogen prefers three fold hollow sites. Sulfur adsorption on both surfaces is energetically favored at the three fold hollow fcc site. The adsorption energies of H and S individually are greater on Ni(111) than Pt(111); this correlates well with their respective d-band center positions. On both surfaces, an increase in S coverage shifts the local d-band center towards a lower energy and gives the expected decrease in adsorption energy. The magnitude of the d-band shift as a function of S coverage was found to be non-linear and nearly identical on Pt and Ni surfaces, but the overall adsorption energy decrease for coadsorbed H was significantly greater on Ni(111). This indicates that coadsorbed species in close proximity interact directly (electrostatically) and not only through the electronic structure of the surface.
Molecular Dynamics Simulations of Proton Diffusion in the Short-Side-Chain Perfluorosulfonic Acid Ionomer
Authors: Hristov Iordan I. , Paddison Stephen J. , Paul Reginald
Published: September 2007

This computational study seeks to understand why low equivalent weight short-side-chain (SSC) perfluorosulfonic acid membranes exhibit higher proton conductivities than Nafion when hydrated at similar water contents. The diffusion of protons in the SSC ionomer at water contents of 3, 6, and 13 H2O/SO3H were investigated through classical molecular dynamics simulations. We developed a unique force field set based on torsion profiles derived from extensive electronic structure calculations of a two side chain fragment of the SSC ionomer and simulated a system consisting of a single 40 repeat unit of the polymer (F3C-{[CF2-CF(OCF2CF2SO3H)]-(CF2CF2)3}40-CF3) corresponding to an equivalent weight of 578 at each of the three water contents. After an equilibration time of approximately 450 ps at 300 K where the density was fixed at 1.67 g/cm3, structural information was collected from 2 ns production runs at 315C. We calculated proton (as a hydronium ion) diffusion coefficients of 2.83×10-7, 1.37×10-6, and 3.87×10-6 cm2/s at the three water contents which show excellent agreement to experimentally measured diffusion coefficients at the lower water content only.
Object-Oriented Fuel Cell Model Library
Authors: Davies Kevin L. , Moore Robert M.
Published: September 2007

The PEMFCSim library is primarily a simulation structure designed to facilitate the simulation of Polymer Electrolyte Membrane Fuel Cells. PEMFCSim uses methods that are physical, dynamic, and component-based. It is written in Modelica, an object-oriented, physical modeling language. The library, as currently implemented, offers one-dimensional and quasi-two dimensional fuel cell simulations and is based on existing models from the literature. The Modelica object-oriented modeling approach allows the PEMFCSim library simulations to use intuitive and physically representative component boundaries and connections. The discussions and examples presented in this paper are based upon an initial set of models selected from the literature, but additional assumptions and characterizations can easily be adopted within the overall simulation structure without changing other elements of the simulation. This flexibility makes the PEMFCSim library ideal for collaboration among researchers focusing on different aspects of fuel cell modeling and simulation.
Effect of Sulfur-Containing Compounds on Fuel Cell Performance
Authors: Imamura Daichi , Hashimasa Yoshiyuki
Published: September 2007

The effect of sulfur-containing compounds on polymer electrolyte fuel cells (PEFC) was evaluated. The decrease in PEFC performance caused by the hydrogen sulfide contained in the hydrogen could not be recovered by the supply of pure hydrogen, but it was revealed that by holding the open circuit voltage with supplying pure hydrogen, the sulfur content poisoning the Pt catalyst disassociated as hydrogen sulfide and close to the initial performance was recovered. Holding the open circuit voltage and potential cycles with supplying neat air were also effective against the drop in performance due to the sulfur-containing compounds (sulfur dioxide, hydrogen sulfide) contained in the air, but the recovery in performance was limited. Further, it was revealed that the concentration of fluoride ions in the cathode effluent water increased after supplying sulfur-containing compounds with air. It is believed that when the air contains sulfur-containing compounds, not only are the active sites of the platinum catalyst reduced due to poisoning, but also the electrolyte membrane or ionomers simultaneously decompose and therefore an irreversible drop in performance is caused.
Impact of SO2 on the Kinetics of Pt3Co/Vulcan Carbon Electrocatalysts for Oxygen Reduction
Authors: Garsany Yannick , Baturina Olga , Swider-Lyons Karen
Published: September 2007

The poisoning of the oxygen-reduction reaction (ORR) by adsorbed sulfur-containing species is quantified for platinum cobalt alloy fuel-cell materials using rotating ring disk electrode (RRDE) methodology. Electrodes of Pt3Co on Vulcan carbon (Pt3Co/VC) are contaminated by submersion in SO2-containing solutions. The initial sulfur coverage of the Pt is determined from the total charge consumed as the sulfur is oxidized from S0 at 0.05 V (vs. RHE) to water-soluble sulfate (SO42-) at > 1.3 V. Initial sulfur coverage (θS,i) is higher on Pt3Co/VC compared to Pt/VC electrodes for poisoning done in the same SO2-containing solution. Electrodes are then evaluated for their ORR activity. Significant (99.8%) loss in initial Pt mass activity is measured when approximately 54% of the Pt3Co/VC surface has adsorbed the sulfur-containing species. When 96% of the Pt surface is covered with sulfur, the reaction pathway of the ORR on the Pt/VC catalyst changes from a 4-electron to 2-electron process reaction for peroxide.
Impact of Micromolar Concentrations of Externally-Provided Chloride and Sulfide Contaminants on PEMFC Reversible Stability
Authors: Steinbach Andrew J. , Hamilton Clayton V. , Debe Mark K.
Published: September 2007

In this paper, we evaluate the reversible performance stability of 3M membrane electrode assemblies containing either Nanostructured Thin Film Catalyst (NSTFC) or carbon-supported Pt (Pt/C) electrodes. Micromolar concentrations of chloride and sulfide ions, provided via the humidification DI water, were found to cause rapid but highly recoverable performance loss. The impact of chloride on the decay rate depends strongly on catalyst type, surface area, and cell temperature. The effect of these externally-provided species remains for several hours after they are removed from the humidification streams. The performance loss due to these species is largely recovered through a special shutdown and restart procedure, where liquid water is injected into the cell as it cools to room temperature.
In-Situ, In-Layer De-Alloying of Pt-M Intermetallics for High Performance PEMFC Electrode Layers: MEA Activity and Durability Studies
Authors: Mani Prasanna , Srivastava Ratndeep , Yu Chengfei , Strasser Peter
Published: September 2007

A novel class of electrocatalyst for oxygen reduction reaction (ORR) in polymer electrolyte membrane fuel cell (PEMFC) was prepared by electrochemically dealloying Pt25Cu75 precursor alloys in a membrane electrode assembly (MEA) cathode. The precursor was prepared by a novel liquid impregnation-freeze drying route. The performance of the electrochemically de-alloyed catalyst was compared to 3 other catalysts, in particular (i) a chemically dealloyed Pt25Cu75, (ii) a precursor Pt-Cu with an atomic ratio of Pt:Cu identical to that of the acid dealloyed catalyst, and (iii) a commercial 30wt% Pt catalyst. The mass activity of the electrochemically dealloyed catalyst @ 0.9V-IR-free is superior to acid dealloyed or as prepared Pt-Cu catalysts and significantly better than reported values of state-of-the-art MEA activities (1). It is observed that electrochemical dealloying of Pt25Cu75 leads to Pt enriched surfaces, results in very Pt-like voltammetric behavior, however, shifts the water activation to more anodic electrode potentials suggesting delayed chemisorption hindering oxygenated species.
Effects of Pressurizing Operation on the Degradation of Membrane Electrode Assemblies in PEFCs
Authors: Asano Koichi , Mugikura Yoshihiro , Watanabe Takao
Published: September 2007

The correlation between the degradation of membrane electrode assembly (MEA) and the operation pressure during the load cycling test was studied. The precipitated Pt, obtained as Pt band, and the thickness of the membrane were observed in the section of the tested MEAs. The precipitated ratio of Pt was determined by a method that involved image processing. An increase in the relative volume of precipitated Pt and a reduction in the thickness of the membrane depending on the operation pressure were observed.
The Effect of Transient Potential Exposure on the Electrochemical Oxidation of Carbon Black in Low-Temperature Fuel Cells
Authors: Gallagher Kevin G. , Wong David T. , Fuller T.
Published: September 2007

The electrochemical oxidation of carbon black catalyst supports is recognized as a durability challenge to the commercialization of low-temperature fuel cells in the transportation sector. An investigation in the effect of the transient potentials experienced in this environment is presented. The exposure of carbon black to square wave potential cycling results in an oxidation process different than the constant potential hold historically used to characterize this corrosion process. The differences are characterized using the liquid half cell and single fuel cell testing coupled with electrochemical techniques and inline gas analysis. Although cyclic voltammetry and XPS are unable to quantify significant differences in the surface oxide chemistry, inline gas analysis measurements demonstrate different corrosion activities.
Mechanism for Electrochemical Oxidation of Highly Oriented Pyrolytic Graphite during Potential Cycling in Sulfuric Acid Solution
Authors: Choo Hyun-Suk , Kinumoto Taro , Iriyama Yasutoshi , Abe Takeshi , Ogumi Zempachi
Published: September 2007

We investigated the electrochemical oxidation of highly oriented pyrolytic graphite (HOPG) during potential cycling in sulfuric acid solution to clarify the oxidation mechanism of carbon materials under dynamic operation conditions in polymer electrolyte fuel cells (PEFCs). The redox current peaks corresponding to hydroquinone-quinone reaction appeared on CVs following cycling between 0.6 and 0.8 V vs. NHE, suggesting that HOPG was partially oxidized to form surface functional groups, while it was hardly oxidized below 0.6 V. Ex-situ SEM observations show that the surface of HOPG became roughened with cycling between 0.6 and 0.8 V with formation an oxidized layer, whereas this oxidized layer became less stable in the course of gasification reaction involving CO2 and/or CO evolution in the potential ranges above 0.8 V. We discuss oxidation mechanism for HOPG corresponding to various potential ranges.
Characteristics of Performance Degradation and CO2 Emissions in Starved Operation of DMFC
Authors: Wang Zhen , Kotake Tomohito , Takagi Yasuo
Published: September 2007

The fatal performance decay caused by potential reversal in a single cell of a DMFC stack under methanol starved operation was investigated. As a result, it was made clear that the decay could be attributed to carbon corrosion in anode catalyst which depended on the oxygen which was made by the anode reaction of water electrolysis in starved operation. In this study, water oxidation and carbon corrosion was confirmed by measuring O2 and CO2 emissions in anode off-gas. The active surface area loss of the anode catalyst was also detected by cyclic voltammetry (CV) and the way to minimize MEA performance degradation is proposed.
Chemical Aspects of Membrane Degradation
Authors: Cipollini Ned E.
Published: September 2007

Membrane failure results from a complex interplay of mechanical stress and chemical attack; each have multiple origins. A picture of degradation is emerging where the polymer is attacked by hydroxyl-radicals generated on Pt particles within the membrane. We extend this picture to include several reactions of hydroxyl radicals with peroxide and the PFSA polymer. This extended picture allows us to explain three differences in behavior exhibited by membrane-degradation rates in Fenton's Reagent tests and in-cell tests regarding correlation of rate with end groups, spatial distribution of polymer loss and existence of induction periods. We postulate a 3-step degradation scenario: 1) hydroxyl radicals attack polymer end groups and side chains, 2) hydroxyl radicals are converted to peroxyl radicals which only attack polymer end groups, and 3) loss of polymer volume and membrane embrittlement leading to mechanical failure depending on the extent of degradation.
Degradation Mechanism of the PFSA Membrane and Influence of Deposited Pt in the Membrane
Authors: Endoh Eiji , Hommura Satoru , Terazono Shinji , Widjaja Hardiyanto , Anzai Junko
Published: September 2007

The degradation mechanism of the perfluorosulfonic acid (PFSA) membrane was investigated by a new accelerated test method, in which the membrane was exposed to H2O2 gas. It was clarified that the degradation of the PFSA polymer proceeds via a novel degradation mechanism. Subsequently, the influence of Pt particles, which deposited in the membrane during accelerated tests, on the degradation of the membrane was investigated. It was concluded that the Pt particles will not contribute to the degradation of the membrane.
H2O2 Formation under Fuel-Cell Conditions
Authors: Chen Cheng , Fuller T.
Published: September 2007

Using a multilayer MEA, H2O2 concentrations in Nafion® membranes were quantitatively measured by a spectrophotometric method. The effects of relative humidity and oxygen partial pressure on H2O2 concentration are discussed. H2O2 formation was found to be dependent on membrane thickness. H2O2 emission rates were measured in anode and cathode only MEAs to separately study H2O2 formation at each electrode. Based on the experimental results, H2O2 formation, diffusion and decomposition model under fuel-cell conditions is first proposed in this paper.
Preferential Thinning Behaviors of the Anode-Side of the PEM Under Durability Test
Authors: Shim Jae Young , Tsushima Shohji , Hirai Shuichiro
Published: September 2007

Polymer electrolyte membrane fuel cell (PEMFC) have attracted much interest as energy conversion systems, especially in automobiles and portable applications, in response to mounting environmental problems caused by fossil fuels. Since PEMFC durability is currently an important issue, it is of great importance to closely analyze the membrane of PEMFCs for sign of degradation. In this work, we have performed accelerated degradation tests of a PEMFC using over 100 hours of open circuit operation with both dry and humidified inlet gases. It was of interest to determine which side of the membrane presented the most severe degradation since this indicates the type of degradation mechanism that was occurring. After disassembly of the cell, cross sectional images of the membrane were obtained with scanning electron microscopy. It was found that the anode side showed more significant deterioration than the cathode side in terms of overall membrane thinning.
Stability of Cations for Anion Exchange Membrane Fuel Cells
Authors: Einsla Brian R. , Chempath Shaji , Pratt Lawrence , Boncella James , Rau Jonathan , Macomber Clay , Pivovar Bryan
Published: September 2007

The hydrothermal stability of quaternary ammonium hydroxides was evaluated to better understand the degradation of anion exchange membranes used in alkaline fuel cells. Benzyltrimethylammonium hydroxide and phenyltrimethylammonium hydroxide were examined as representative cations for membrane materials. The benzyltrimethylammonium hydroxides displayed much better stability than the phenyltrimethylammonium hydroxides under similar conditions. Additionally, as the concentration of the ammonium hydroxides in water increased, the stability of the cation decreased.
Analysis of Membrane Degradation Behavior During OCV Hold Test
Authors: Ohma Atsushi , Yamamoto Shinji , Shinohara Kazuhiko
Published: September 2007

A detailed analysis was made of the relation between the degradation behavior of a membrane electrode assembly (MEA) and deposited Pt (Pt band) in the membrane during an open circuit voltage (OCV) hold test. Molecular-structural changes in the membrane were investigated by micro-Raman spectroscopy. When the Pt band was not significantly observed in the membrane after the test, the membrane was relatively stable. In contrast, when the Pt band was clearly formed, the membrane around it was intensively degraded. The magnitude of the fluoride ion emission rate (FER) of the effluent water from the anode and the cathode was also consistent with the location of the Pt band. It was verified that the Pt band is one of the factors accelerating membrane degradation.
Dissolution Mechanism of Platinum in Acidic Media
Authors: Mitsushima Shigenori , Koizumi Yuki , Uzuka Shunsuke , Ota K.
Published: September 2007

The platinum solubility in a polymer electrolyte would be a key issue in order to improve and guarantee the durability of PEFCs. In this study, the platinum solubility has been determined in acidic electrolytes as a function of temperature and pH. The solubility was ca. 3 x 10-6 mol dm-3 at 25oC in 1 mol dm-3 of H2SO4, HClO4, and CF3SO3H under air, and increased with temperature and with the decrease in pH. The platinum solubility in oxygen was slightly larger than that in air. However, the solubility in nitrogen was much smaller than the mixtures. Based on these results, the dissolution of platinum in an acidic medium would follow an acidic dissolution mechanism.
Pt Redistribution within PEMFC MEAs and its Consequence on their Performances
Authors: Chatenet Marian , Guilminot Elodie , Iojoiu Christina , Sanchez Jean-Yves , Rossinot Elissabeth , Maillard Frédéric
Published: September 2007

In this paper, we studied the durability of PEMFC MEAs after on-site operation in stationary conditions. Harsh anode and cathode active layer degradation was monitored for Pt, C, F elements by chemical (EDS, XPS) and physical (SEM, TEM) techniques. At the cathode, Pt corrodes into Ptz+ species yielding severe redistribution within the MEA. Ptz+ dissolution and transport is favoured by activators/ligands (F or S-containing species resulting from the MEA polymers ageing, especially at the anode), which possibly act as counter ions. The presence of Ptz+ ions in the membrane favours chain-cutting and induces physical cross-links which modifies its thermo-mechanical, water uptake and conductivity properties. As a result, the risk of pinhole formation is greatly increased.
The Impact of Cycle Profile on PEMFC Durability
Authors: Uchimura Masanobu , Kocha Shyam S.
Published: September 2007

This paper reports on the effect of various simplified potential-cycle profiles (square and triangle) measured at 80oC under normal fuel cell operating voltage ranges (0.95-0.60V) in subscale fuel cells. Accurate measurements of the electrochemical surface area as well as hydrogen-oxygen I-V polarization curves were employed out to measure the change in ORR specific activity (is), mass activity (im) of commercial Pt catalyst supported on high surface area carbon per unit time as well as per cycle. Based on the measured in-situ loss of surface area and cathode catalyst activity over the first 15-30k cycles, some hints of the dissolution mechanism can be inferred. In addition, control and operation of fuel cells with lowered Pt dissolution rates can be deduced.
PtCo, a Durable Catalyst for Automotive PEMFC?
Authors: Ball Sarah C. , Hudson Sarah L. , Theobald Brian R. , Thompsett David
Published: September 2007

Carbon supported Pt and PtCo alloy cathodes have been evaluated under the DoE proscribed accelerated MEA durability protocol (0.7 - 0.9 V iR free, square wave cycle, H2/air). The Pt MEA showed reasonable stability up to 2500 cycles, after which significant performance loss was observed. Post mortem analysis indicated that this was due to Pt sintering and dissolution. The PtCo MEA showed similar stability up to 2500 cycles showing a mass activity of twice that of the Pt MEA. However, after this, activity loss was observed to a value similar to pure Pt, without significant loss in electrochemical area. The MEA was tested to 83,000 cycles with only a gradual further activity loss. Post-mortem analysis showed only modest Pt sintering and loss of Pt into the membrane. This early loss of PtCo activity has been correlated with surface loss of Co through activity testing of acid pre-leached catalysts.
Supportless Pt and Pt-Pd Nanotubes with High Durability and Activity Towards Oxygen Reduction Reaction for PEMFCs
Authors: Chen Zhongwei , Waje Mahesh , Li Wenzhen , Yan Yushan
Published: September 2007

A new class of cathode catalysts based on supportless Pt nanotubes (PtNTs) and Pt alloy nanotubes with remarkable durability and high catalytic activity have been developed. Due to their unique combination of dimensions at multiple length scales, the PtNTs can provide high Pt surface area by their nanometer-sized wall thickness and have the potential to eliminate or alleviate most of the degradation pathways of the commercial carbon supported Pt catalyst (Pt/C) and unsupported Pt black as a result of their micrometer-sized length. The PtNTs asymptotically approaches a maximum of ~20% Pt surface area loss in durability test, while the commercial PtB and Pt/C loses about 51% and 90% of their initial surface area, respectively. Moreover, the PtNTs and PtPdNTs show much higher specific activity than commercial Pt/C.
XAS Study for Degradation Mechanism of Pt/C Catalyst During Potential Cycling Test
Authors: Yoshida Hiroaki , Kinumoto Taro , Iriyama Yasutoshi , Uchimoto Yoshiharu , Ogumi Zempachi
Published: September 2007

Degradation of Pt particles on Pt/C catalysts under potential cycling tests was investigated by X-ray absorption spectroscopy (XAS) to obtain information upon the electrochemical stability as well as to clarify the degradation mechanism. Inherently features of Pt particles as structural parameters, e.g. electronic and local structures, depend on the particle size. Under the potential cycling test, the structural parameters of smaller particles became to be similar to the larger particles. The details of change in the electronic and local structures of Pt upon the degradation test and particle size effects on the electrochemical stability are discussed.
Design of Thin-Layered Membrane Electrode Assenbly Prepared by Sputtering Method for Direct Methanol Fuel Cells
Authors: Sudoh Masao , Nakase Katsuki , Tauchi Masashi , Makino Koji
Published: September 2007

Superior performance of direct methanol fuel cells (DMFCs) may be obtained by localized catalyst loading on reaction sites and reduction of charge transfer and diffusion resistances. These may be realized by thin-layered membrane electrode assemblies (MEAs) for which new component materials and a new catalyzed method are used. In this study, gas diffusion electrodes consisted of sputter-deposited Pt as a catalyst layer and "carbon silk" as a diffusion layer are evaluated as cathodes for DMFCs. A cathode prepared by sputter-deposited Pt on spreading the mixture of Nafion, carbon and butyl acetate (NCB) solution showed an increase in current density and mass activity for the oxygen reduction reaction. Consequently, effects of NCB on cathodic polarization are explained by supplying protonic and electronic conduction paths to electrochemically inactive Pt with oxygen diffusion path. Mass activities using NCB were found about five times higher than a sputter-deposited Pt Cathodes without NCB
Water and Fuel Crossover in a Direct Methanol Fuel Cell Stack
Authors: Jiang Rongzhong , Chu Deryn
Published: September 2007

Water and fuel crossover in a direct methanol fuel cell (DMFC) stack under constant voltage discharge was quantitatively determined by a fuel circulation method. The water permeation across the electrolyte membrane can be split into three portions, including proton osmotic-drag, methanol migration-drag, and spontaneous water crossover. Among these three types of water crossover, spontaneous water crossover is the most significant. Water crossover rate increases significantly with increasing operating temperature. As dry air was used as oxidant in this experiment, there was a significant portion of water evaporation at the cathode outlet, which indicates that water recycle-ability from the cathode is directly dependent on the relative humidity. The rates of methanol crossover across the electrolyte membrane in a DMFC stack under constant voltage discharge are experimentally determined by accurately measuring the methanol concentrations within specific time periods. The rate of methanol crossover increases significantly with operating temperature, but only slightly with increasing discharge voltage.
Development and Integration of Controls for a PEMFC Powered Aircraft
Authors: Stroman Richard O. , Kahn Aaron
Published: September 2007

The sensitivity of air vehicle performance to weight dictates the use of propulsion systems of high specific power. Proton exchange membrane fuel cells (PEMFCs) have advanced to the point of satisfying this requirement. The development and integration of PEMFC controls for a 330Wmax unmanned air vehicle (UAV) is the subject of this paper.
Fundamental Mechanisms of Gas Purge in a Polymer Electrolyte Fuel Cell
Authors: Sinha Puneet , Wang Chao-Yang
Published: September 2007

Gas purge intended to minimize residual water in the membrane, porous electrodes, and gas diffusion layers is critical for successful startup of a polymer electrolyte fuel cell (PEFC) from subfreezing temperatures. A basic understanding of the physical phenomena involved in gas purge, required to establish effective and energy-saving purge protocols, is currently absent in the literature. In this work we delineate underlying physics of water removal from a PEFC during gas purge. Various stages of GDL and membrane drying are characterized and the variations along the flow direction are considered and emphasized. The predicted GDL drying time constant is compared with the tomographic experiments with good agreement. Detailed experimental validations are carried out and good match with experimental membrane HFR variation with purge time is obtained.
Boron Nitride Nanotubes: Recent Breakthroughs and Challenges
Authors: Golberg Dmitri , Zhi Chunyi , Costa Pedro , Bai Xuedong , Tang Chengchun , Huang Qing , Mitome Masanori , Bando Y.
Published: September 2007

Herein we describe recent achievements in the synthesis and multi-purpose analysis of multiwalled boron nitride nanotubes prepared at high-yield at the National Institute for Materials Science (NIMS), Tsukuba. The latest results with respect to novel BN nanotube-containing polymeric composites and electromechanical property measurements on individual BN nanotubes are presented
ZnO Nanowires for Sensing and Device Applications
Authors: Tien L , Norton D , Kang Byoung Sam , Wang Hung-Ta , Ren Fan , Lin J , Pearton Steve
Published: September 2007

We have fabricated many types of ZnO nanowire devices, including MOSFETs, diodes, UV sensors and pH sensors. ZnO has been effectively used as a gas sensor material based on the near-surface modification of charge distribution with surface-absorbed species. The large surface area of the nanorods makes them attractive for gas and chemical sensing, and the ability to control their nucleation sites makes them candidates for high density sensor arrays.
Nitrogen-Doped Tungsten Oxide Nanowires and Their Devices
Authors: Chang Mu-Tung , Chou L.-J.
Published: September 2007

1-D tungsten oxide nanowires exhibiting distinctive electrochromic, gaschromic properties; in this study, we demonstrate an innovative way to fabricate the tungsten oxide nanowires on silicon substrate. The suggested growth mechanism is Vapor-Solid (VS) growth, while, the nitrogen-doping tungsten oxide nanowires were fabricated via the ammonia decomposition during the formation process. Their nano-devices were successful fabricated and the variety devices characteristics were reported.
Self-Aligned Fabrication Process Based on Sacrificial Catalyst for Pd-Contacted Carbon Nanotube Field-Effect Transistors
Authors: Rispal Lorraine , Heller Rudolf , Hess Gisela , Tzschöckel Gerhard , Schwalke Udo
Published: September 2007

In this work, we present a novel very simple process to fabricate carbon nanotube field effect transistors (CNTFETs). It is based on chemical vapor deposition (CVD) growth of single-walled carbon nanotubes (SWNTs) using a 'sacrificial' aluminum/nickel catalyst. The SWNTs are grown uniformly across the wafer surface and subsequently contacted with palladium. The silicon substrate is used as back-gate electrode. The process contains neither complicated manipulations of the SWNTs nor multi-step lithography, avoiding the risk of misalignment. The catalyst is called sacrificial because it turns into an insulating layer after process allowing the gate field to penetrate and control the current flow within the SWNTs. The fabricated structures are unipolar PMOS-like CNTFETs with on/off ratios up to 10 000.
Storage Function of Carbon Nanospaces For Molecules and Ions
Authors: Noguchi Daisuke , Hattori Yoshiyuki , Yang Cheol , Tao Yousheng , Konishi Takehisa , Fujikawa Takashi , Ohkubo Takahiro , Nobuhara Yoko , Ohba Tomonori , Tanaka Hideki , Kanoh Hirofumi , Yudasaka Masako , Iijima Sumio , Sakai Hideki , Abe Masahiko , Kim Yong-Jung , Endo Morinobu , Kaneko Katsumi
Published: September 2007

Interfacial importance and nanopore structures of single wall nanocarbons such as single wall carbon nanotube (SWCNT) and single wall carbon nanohorn (SWCNH) are described in terms of intermolecular interaction theory. The relationships between the nanopore structure and nanoconfinement effect for molecules and ions are shown using examples on supercritical H2 adsorption on SWCNT, quantum molecular sieving effect of SWCNT for H2 and D2 below 77 K, water structure and hydration structure of Rb, Cs, and Sr ions in nanopores of activated carbon fiber and SWCNH, and capacitance storage of SWCNH. The SWCNT produced by laser ablation shows upward-curved adsorption isotherm of H2 at 77 K, suggesting the presence of the strong sites. The adsorption amount of D2 on this SWCNT at 77 K was clearly larger than that of H2 by about 20% at 77 K due to quantum effect. The hydration number around Cs and Sr ions in 1.1 nm slit pores of ACF reduced by more than 20 %. The capacitance depended on the opening of SWCNH and charging time for organic electrolyte (C2H5)4NBF4 in propyrene carbonate.
Effects of Strain Field of Si Bicrystal on the Formation of Nanoscale Silicides
Authors: Hsin Cheng-Lun , Wu Wen-Wei , Chen Lih J.
Published: September 2007

Regularly distributed dislocation network with a controllable spacing could be formed by wafer bonding. Different kinds of Si bicrystals were fabricated by (001), (111) and (110) Si wafers bonded with (001) SOI. NiSi2 formed on the bicrystal was found to be affected by the underlying dislocation arrays. The nanostructures were confined by the dislocation grids. It demonstrates that by means of controlling the surface stress arrangement, the shape of the silicide nanostructures could be controlled and a variety of nanostructures could be obtained. The observation is conductive to the basic understanding of the stress effect on the formation of the silicide nanostructures on Si bicrystals.
Enhanced Light Extraction from Gallium Nitride Optoelectronics on Si Substrates
Authors: Mastro Michael , Holm Ronald , Caldwell Joshua D. , Bassim Nabil , Henry Richard , Eddy Charles
Published: September 2007

This letter presents a technique to enhance the light extraction from a III-nitride light emitting diode on a Si substrate by inserting a high-reflectance distributed Bragg reflector below the active layer of the device to reflect the emitted light away from the opaque substrate. Additionally, the alternating layer sequence in the distributed Bragg reflector absorbs strain and filters dislocations generated at the GaN / Si interface. Theoretical results show that optimal placement of the light source is critical to achieve enhanced emission from the optical cavity.
S-MMIC InP HEMT Amplifiers Enabled by the Advanced InP Backside Processes
Authors: Zhou Joe , Mei Xiaobing , Elmadjian Raffi , Geiger Craig , Zeng Xianglin , Cheung Chi , Dai Fred , Johnson Roosevelt , Luo Ke , Wang J. , Uyeda Jansen , Warner Bryce , Barsky Mike , Olson Scott , Dean William , Radisic Vesna , Lai Richard
Published: September 2007

35 nm gate InP HEMT devices were fabricated with S-MMIC amplifier designs at Northrop Grumman Space Technology (NGST). A 3-stage, grounded coplanar, 2-mil substrate LNA design exhibited a peak gain of 12 dB at 340GHz, the highest gain above 300 GHz reported to date. There are three core areas for the success of product fabrication, epitaxial material growth using Molecular Beam Epitaxy (MBE), frontside processing with a 35nm gate and backside processing with final wafer thickness of 50 μm. In this paper, we focus on the advanced InP backside process and the evaluation of device performance before and after backside processing.
Overcoming Contact Hurdles for Investigating Lower Dimensional Structures in Undoped Heterostructures
Authors: Sarkozy Stephen J. , Das Gupta Kantimay , Sfigakis Francois , Beere Harvey , Farrer Ian , Harrell Ruth , Ritchie Dave , Jones Geb
Published: September 2007

Undoped GaAs/AlGaAs heterostructures are attractive systems for studying lower dimensional structures and electron-electron interactions, eliminating or reducing many of the problems that come with the more traditional modulation doped system. A major obstacle involved in using systems where the two dimensional electron gas (2DEG) was induced with a biased top gate was the inability to form a reliable ohmic contact to the 2DEG. We report here a systematic study of the contact system (Au-Ge-Ni), including a discussion of how a various diffusion barriers may prove beneficial, and how the etch depth of the recessed ohmic contact effects the window of success. Yield with the new process approaches 100%, compared to a catastrophically low rate before the system was understood.
Rapid Electrical Detection of Hg(II) Ions with AlGaN/GaN High Electron Mobility Transistors
Authors: Wang Hung-Ta , Kang Byoung Sam , Chancellor TJ , Lele Tanmay , Tseng Yiider , Ren Fan , Pearton Steve , Johnson Wayne , Rajagopal Pradeep , Roberts John , Piner Edwin , Linthicum Kevin
Published: September 2007

Bare Au-gated and thioglycolic acid functionalized Au-gated AlGaN/GaN high electron mobility transistors (HEMTs) were used to detect mercury(II) ions. Fast detection of less than 5 seconds was achieved for thioglycolic acid functionalized sensors. This is the shortest response time ever reported for mercury detection. Thioglycolic acid functionalized Au-gated AlGaN/GaN HEMT based sensors showed 2.5 times larger response than bare Au-gated based sensors. The sensors were able to detect mercury (II) ion concentration as low as 10-7 M. The sensors showed an excellent sensing selectivity of more than 100 for detecting mercury ions over sodium or magnesium ions. The dimensions of the active area of the sensor and the entire sensor chip are 50 µm × 50 µm and 1 mm × 5 mm, respectively. Therefore, portable, fast response, and wireless based heavy metal ion detectors can be realized with AlGaN/GaN HEMT based sensors.
Performance Evaluation of III-Nitride Avalanche Photodiodes Grown on SiC and GaN Substrates
Authors: Shen Shyh-Chiang , Zhang Yun , Yoo Dongwon , Limb Jae-Boum , Ryou Jae-Hyun , Dupuis Russell D. , Britt Matthew , Yoder Paul D.
Published: September 2007

We will evaluate device performance of state-of-the-art III-N avalanche photodiodes (APD) that were grown on either silicon carbide (SiC) or free-standing gallium nitride (GaN) substrates. Through improved device epitaxial growth techniques, and optimized fabrication processing and design, we demonstrated high-performance III-N PIN APD with a linear avalanching gain of greater than 3E4 for both 280 nm and 340 nm deep ultraviolet optical wavelength applications.
Monolayer Formation on GaN Surface via Self-Assembly
Authors: Ito Takashi , Forman Sarah , Cao Chundi , Eddy Charles , Mastro Michael , Holm Ronald , Henry Richard , Hohn Keith , Edgar James
Published: September 2007

This paper describes monolayer formation of octadecylphosphonic acid (ODPA) on GaN surfaces via self-assembly. Adsorption of a series of primarily substituted hydrocarbons (RX; -X = -OH, -NH2, -COOH, -SH, -PO(OH)2) from their toluene solutions was systematically studied on undoped, Ga-face GaN films prepared on sapphire (0001) substrates. ODPA forms a well-packed monolayer on GaN, especially when the GaN has an oxide surface layer, as suggested by the very high water contact angle of ODPA-coated GaN. In contrast, the other compounds did not give such high water contact angle. Atomic force microscopy images of GaN surfaces with and without ODPA are similar, suggesting the formation of a very thin and uniform film. The surface oxide layer and the monolayer formation of ODPA were assessed using X-ray photoelectron spectroscopy. These results indicate that an oxidized GaN surface can be modified with a well-packed monolayer of molecules having a phosphonate moiety.
Enhanced Light Extraction Efficiency in InGaN Light-Emitting Diodes Through Photoelectrochemical Sidewall Shaping Process
Authors: Lin Chia-Feng , Yang Chung-Chieh , Kuo H. C. , Wang Shing-Chung
Published: September 2007

The cone-shaped-sidewall InGaN-based light-emitting diodes (LEDs) with higher light extraction efficiency were fabricated through the photoelectrochemical (PEC) process. The process procedures were consisted with the PEC selective wet oxidation on n-type GaN without p-type GaN layer, removing Ga2O3 layer, crystallographic wet etching process, and forming cone-shaped sidewall structure. The stable crystallographic etching planes were formed as the p-type GaN { } planes and n-type GaN { } planes with a 27{degree sign} included angle. The InGaN/GaN multi-quantum-well (MQW) layers were located at the cone's tip nano-structure in mesa-edge region, and the micro-photoluminescence spectrums had the blue shift phenomenon from 457.8nm to 450.8nm caused by the quantum confinement effect and particle compress strain released.
Effect of the Combination of Spray Pyrolysis and Flame Oxidation Synthesis of Carbon Modified (CM)-n-TiO2 Thin Film Electrodes on Solar to Hydrogen Production Efficiency From Water
Authors: Shaban Yasser A. , Khan Shahed
Published: September 2007

Carbon modified n-type titanium oxide (CM-n-TiO2) photoelectrodes were synthesized by natural gas flame oxidation (sample 1) and also by spray pyrolytic deposition on Ti metal sheets (Alfa, 0.25 mm thick) followed by their thermal flame oxidation (Sample 2). Sample 1 and sample 2 generated photocurrent density of 9.17 mA cm-2 and 13.83 mA cm-2 for water splitting at the same measured potential of - 0.7 V/SCE. at illumination intensity of 100 mW cm-2 from a 150 Watt Xenon Lamp respectively. The total photocurrent density of 12.15 mA cm-2 obtained by integration of wavelength, λ dependent photocurrent densities for sample 2 generated a total photoconversion efficiency of 12.33 % under illumination intensity of 100 mW cm-2 from a solar simulator. Importantly, under global Air Mass of 1.5 sunlight (1 sun) illumination, total photoconversion efficiency of 10.48 % for water splitting was observed for sample 2. Light concentration by oxygen gas bubbles temporarily trapped on highly porous and thicker CM-n-TiO2 surface of sample 2 acting as tiny lens helped to have observed quantum efficiency or incident photon conversion efficiency (IPCE) higher by a factor of ten in the UV region compared to theoretical maximum of unity and consequently generated such high photoconversion efficiency under global sunlight illumination.
Electrodeposition of Tellurium Atomic Layer on n-GaAs(100) Substrate
Authors: Kim Jay Y. , Stickney John L.
Published: September 2007

This paper describes the electrodeposition of Tellurium (Te) atomic layer on n-type GaAs(100) substrates. As-received n-GaAs(100) substrates were treated in 10 % HF and ultraviolet (UV) ozone cleaned. The substrate was then transferred to the ultrahigh vacuum (UHV) electrochemistry chamber and cleaned by Ar+ ion bombardment. The clean substrate was then transferred into the attached electrochemistry ante-chamber, and immersed in a tellurite solution. A number of deposition potentials were investigated. The resulting Auger peak height ratios, Te/Ga, were plotted versus the Te deposition potential. From the Auger ratio plot, below -0.8 V, a reduction feature was observed, at −0.9 V, due to the reduction of Te to Telluride ion (Te0 + 2e− → Te2−). At potentials of -0.9 and below, only a surface limited atomic layer of Te was left on the GaAs surface. An initial investigation of In deposition on this Te coated GaAs surface was also performed.