bluuebunny/crossref_metadata_2025 huggingface dataset:
Polymer Electrolyte Membrane Fuel Cell Active Area Utilization Dependence on Relative Humidity Measured via AC Impedance High Frequency Resistance
Authors: Steinbach Andrew J. , Peppin Daniel M.
Published: October 2013

In this paper, we report experimental data and analysis which suggests that the observed performance reduction of PFSA PEMFCs as reactant relative humidity is reduced below saturation is due to a reduction in the utilized active area of the membrane electrode assembly (MEA), rather than the typically reported mode involving increased bulk ionic resistances of the PFSA PEM and cathode electrode. This hypothesis is supported by the observation that the large measured performance decrease with reduced RH reactants can be very largely explained by assuming that the increase in measured AC impedance high frequency resistance (HFR) is due to decreased PEM conductive area rather than increased PEM bulk resistivity, using the HFR to estimate the actually-utilized active area and renormalizing the measured current density to the HFR-estimated active area. A simple qualitative model, consistent with the experimental data and analysis, suggests that performance loss with decreased RH may be primarily due to decreased areal density of ionically-conductive connections between the PFSA PEM and cathode electrode, leading to decreased electrocatalyst utilization in the direction parallel to the PEM-electrode interface.
Pressure Drop and Voltage Response of PEMFC Operation under Transient Temperature and Loading Conditions
Authors: Banerjee Rupak , See Evan , Kandlikar Satish G
Published: October 2013

Transportation powertrains need to be versatile in power generation to meet the varying vehicular dynamic loads. The PEM fuel cell stacks within vehicles are therefore designed to meet this additional requirement under part-load and varying-load driving conditions. The size of the on-board energy storage systems can be reduced significantly if the fuel cell powertrain is able to generate the load faithfully in corresponding to the varying demand load. The voltage and pressure drop variations in response to changes in the temperature and load conditions are experimentally studied in the current investigation for characterizing the dynamic system behavior. Temperature of the cell was changed over 20°C - 80°C in short time periods (<600 seconds) and the load conditions were changed by 400 mA/cm2 in 300 seconds. It was observed that changes in load at lower temperatures have a significant impact on the pressure drop in the channels. Additionally, a rapid increase or decreases in temperature have an adverse effect on the cell performance. Rapid increase in cell temperature results in dehydration of the membrane, thereby reducing the performance; while rapidly decreasing the cell temperature may result in increased condensation.
Separate Measurements of Current under the Channels and the Land in a PEM Fuel Cell with an Interdigitated Flow Field
Authors: Luo Song , Liu Hongtan
Published: October 2013

Separate in situ measurements of the current densities under the inlet channel, the land and the outlet channel in a proton exchange membrane (PEM) fuel cell with an interdigitated flow field are conducted. Experiments are carried out by using partially-catalyzed-membrane electrode assembly (MEA) on the cathode side and full MEA on the anode side. The experimental results show that the current density under the outlet channel is much lower than that under the inlet channel. Further experimental study results show that cathode flow rate and cathode back pressure affect the local current distribution significantly.
Investigation of a Cathodic Bimetallic Catalyst Based on Platinum and Cobalt for Application in Direct Methanol Fuel Cells
Authors: D'urso Claudia , Baglio Vincenzo , Sebastian David , Stassi Alessandro , Aricò Antonino S.
Published: October 2013

A PtCo catalyst was synthesized by using a specific colloidal preparation route and a carbothermal reduction at 800°C. This treatment, caused the occurrence of a primitive cubic ordered (L12) phase, and a mean crystallite size of 3.3 nm, as well as a suitable degree of alloying. The electrocatalyst was subjected to the pre-leaching procedure to modulate the surface characteristics. The catalyst after the carbothermal reduction at 800°C was further investigated at the cathode of a direct methanol fuel cell (DMFC). A comparison with a commercial Pt catalyst was carried out. A higher performance in DMFC was obtained by using the bimetallic PtCo catalyst.
Reaction Pathways and Current Efficiency for Electrooxidation of Ethylene Glycol at Intermediate Temperatures
Authors: Stuve Eric M. , Spies Kurt A.
Published: October 2013

Electrocatalytic reforming (ECR) of ethylene glycol (EG) was studied in an effort to determine the feasibility of ECR for hydrogen production from biomass. Measurements were made in a proton exchange membrane (PEM) reactor with Pt/C anode and cathode and Nafion electrolyte. Electrooxidation of EG in aqueous phase was studied over the temperature range of 40--137°C. Reaction progress was followed by cyclic voltammetry, potential-step chronoamperometry, and chromatographic analysis of reaction products. Sustained reaction occurred at an overpotential as low as 0.49 V at 137°C. A variety of partial oxidation products were observed; glycolaldehyde, glycolic acid, and traces of oxalic acid; mostly at low temperatures. The results were interpreted according to the dual pathways mechanism, in which path A involves a series reaction through a CO intermediate, and path B is parallel route to CO2 that bypasses CO
ads
. Reaction progression towards complete oxidation increased with temperature, from an average number of electrons transferred of 5.1 at 40°C to 9.0 at 137°C. Activation energies of 18 and 43 kJ/mol were measured for EG conversion and CO2 production. The lower value is attributed to formation of partial oxidation products, and the higher value to CO
ads
electrooxidation in path A. The sustained reaction at low overpotential along with near complete oxidation, both of which occur at high temperature, show that ECR can be a viable process in biomass reforming, and that further studies must be conducted at higher temperatures for a more complete understanding of this process.
A Study of Au/C Nanoparticles with Pt Monolayer and Sub-Monolayer Electrocatalysts for Ethanol Oxidation Reaction
Authors: Loukrakpam Rameshwori , Brankovic Stanko R. , Strasser Peter
Published: October 2013

Effective utilization of the high energy density of ethanol as a fuel for polymer electrolyte fuel cells is contingency of complete oxidation of ethanol to carbon dioxide. Major problems associated with the development of electrocatalysts for ethanol oxidation are the high content of Pt and partial oxidation of ethanol to acetaldehyde and acetic acid instead of complete oxidation to carbon dioxide via the 12 electron process. Despite extensive work on ethanol oxidation catalysts, it is difficult to get C-C bond splitting without compromising on the Pt content. In this report, Au/C nanoparticles with Pt monolayer: Au/C@Pt(II) and sub-monolayer: Au/C@Pt(IV) were studied as electrocatalysts for ethanol oxidation reaction (EOR). Au/C@Pt(II) and Au/C@Pt(IV) were synthesized using a Cu mediation electrochemical method. Electrochemical testing of these electrocatalysts showed that Au/C@Pt(II) had substantially higher activity towards EOR than Au/C@Pt(IV). The effect of Pt cluster sizes on the Au surface and the tensile strain on Pt due to the underlying Au and their effect on EOR will be discussed. The mechanism of EOR on these electrocatalysts was also studied using in-situ FTIR to understand whether there is complete or partial oxidation of ethanol.
Pt-Decorated Graphene-like Foam for Electrochemical Oxygen Reduction with High Mass Activity
Authors: Liu Jianfeng , Sasaki Kazunari , Lyth Stephen Matthew
Published: October 2013

Gram-scale synthesis of graphene-like foam from low cost precursors is reported as a support material for platinum in fuel cell cathodes. The graphene-like foam was produced by combustion of sodium ethoxide, followed by washing and heat treatment in various gases. The BET surface area is higher than 1500 m2/g, although the material is highly defective. These defects result in an excellent distribution of platinum nanoparticles on the surface. The electrochemical performance of the resulting electrocatalysts are characterized using cyclic voltammetry and linear sweep voltammetry techniques, which reveal that this Pt-decorated graphene has large electrochemical surface area (101 m2/g) and high mass activity for the oxygen reduction reaction (176 A/gPt), making this graphene-like foam a potentially useful catalyst support for use in polymer electrolyte membrane fuel cells.
Development of Graphitic-Carbon Nitride Materials as Catalyst Supports for Polymer Electrolyte Fuel Cells
Authors: Mansor Noramalina , Belen Jorge Ana , Corà Furio , Gibbs Christopher , Jervis Rhodri , McMillan Paul F. , Wang Xiaochen , Brett Dan J. L.
Published: October 2013

Graphitic-carbon nitride materials (gCNM) are investigated for developing highly-durable catalyst supports for polymer electrolyte fuel cells (PEFCs). The gCNMs are more electrochemically stable compared to conventional carbon black (Vulcan XC-72) following accelerated corrosion testing. After 2000 scans, the gCNM shows a 133% increase in double-layer capacitance compared to 166% for Vulcan. Despite the low initial electrochemical surface area (ECSA), Pt/gCNM electrocatalyst exhibits improved durability over Pt/Vulcan, with 20.9% ECSA loss versus 41.0% for Pt/Vulcan after 1000 scans. Superior methanol oxidation activity is observed for Pt/gCNM on the basis of the catalyst ECSA
Pt-Based ORR Catalyst on Carbon-Supported Amorphous Niobium Oxide Support
Authors: Xu Chunchuan , Pietrasz Patrick , Yang Jun , Soltis Richard , Sun Kai , Sulek Mark , Novak Robert
Published: October 2013

This work uses a carbon-supported amorphous conductive metal oxide as Pt-based catalyst support for ORR in PEMFC. Amorphous conductive metal oxides have neither grain boundaries, nor the long-range atomic order to be easily transformed into insulating crystalline structure, i.e. amorphous niobium oxides are resistant to oxygen incorporation, thus preserving the structural stability and electric conductivity.
Chaos during H2/CO Electroxidation: Trends and Usefulness
Authors: Mota Andressa , Rocha Tairo A. , Gonzalez Ernesto Rafael
Published: October 2013

Kinetic oscillations are extensively reported in fuel cells fed with CO-containing hydrogen. Relaxation oscillations are fully explained by the electrochemical mechanism, but chaotic oscillations are not. Regardless the exact mechanism description, we are able to analyze the phenomenology of the experimental findings. Herein, chaos is reported to have a special trend, the predictability at short range, i.e. it is possible to predict how long will be the period of the next cycle given the spike (amplitude) of the previous cycle. Furthermore, other useful information could be obtained. Different chaotic states can be compared since the predictability at short range is linked to the CO2 production amount, as experiments with on-line mass spectrometry point out.
Reformate Hydrogen Fuel in PEM Fuel Cells: the Effect of Alkene Impurities on Anode Activity
Authors: Kortsdottir Katrin , Perez Ferriz Francisco J. , Lagergren Carina , Lindstrom Rakel Wreland
Published: October 2013

Reformate hydrogen contains many impurities, some are well known while others have been less studied. Hydrocarbons are possible impurities in reformate hydrogen and are among those less studied. This study if aimed at alkenes, with special focus on propene. Adsorption and desorption on the Pt catalyst is studied using stripping cyclic voltammetry combined with mass spectrometry. The results show that although the effect of propene in the presence of hydrogen is expected to be minimal, adsorption and blockage of catalytic sites cannot be ruled out. A small amount of ad-species is formed on Pt at low adsorption potentials, and in the presence of hydrogen, although suppression of the hydrogen desorption peak was minimal if hydrogen was adsorbed on the Pt catalyst prior to exposure.
Modeling Non-Precious Metal Catalyst Structures and Their Relationship to ORR
Authors: Holby Edward F. , Wu Gang , Zelenay Piotr , Taylor C. D.
Published: October 2013

In this manuscript, we discuss the application of periodic density functional theory to modeling non-precious metal catalysts with a focus on the potential energy surfaces of the oxygen reduction reaction on two likely active-site-structures. Determination of the thermodynamic PES and overpotential are estimated for these sites. The role of van der Waals and its effect on predicted overpotential are addressed. It is found that overbinding of O2 reactants and subsequent overbinding of *OH moieties lead to major sources of deviations from ideal thermodynamic behavior. As such, minor structural and chemical modifications are suggested to improve the studied active site candidates.
Effect of the Membrane Thickness on the Over-Potential Behavior of the Direct Formic Acid Fuel Cell
Authors: Tsujiguchi Takuya , Iwakami Takanori , Nakagawa Nobuyoshi
Published: October 2013

The power generation characteristics of the DFAFC having different membrane, Nafion 117, Nafion 115 and NR 212 were investigated by measuring the over-potentials of the anode and the cathode, individually. The highest performance was obtained when NR 212 was used since the ohmic resistance and the anode over potential of NR 212 was lower that of Nafion 115 and Nafion 117. From the over-potential measurements, it was found that the anode over-potential increased with increasing the membrane thickness. From these results, it is considered that the NR 212 is suitable as the electrolyte membrane of the DFAFC.
The Electrocatalytic Oxidation of Ethanol in a Proton Exchange Membrane Electrolysis Cell (PEMEC): A Way to Produce Clean Hydrogen for PEFC
Authors: Lamy Claude , Baranton Stève , Coutanceau Christophe
Published: October 2013

The electrocatalytic oxidation of ethanol was investigated in a Proton Exchange Membrane Electrolysis Cell (PEMEC) working at low temperature (20°C) on several Pt-based catalysts (Pt/C, PtSn/C, PtSnRu/C) in order to produce very clean hydrogen by the electrochemical decomposition of a biomass compound. The electrocatalysts used were characterized by several physico-chemical methods: TDA-TGA, XRD, TEM, EDX, etc. The electrocatalytic activity was determined by cyclic voltammetry and the rate of hydrogen evolution was measured for each catalyst at different current densities (j = 2 to 150 mA cm-2). The discussion will be focussed on the primary importance of the catalytic properties of the anode material and aims to show the feasibility of producing clean hydrogen from the electrochemical decomposition of ethanol chosen as a model compound from biomass feedstock.
Copper Delafossite Anode for Water Electrolysis
Authors: Hinogami Reiko , Toyoda Kenji , Aizawa Masato , Kawasaki Takashi , Gyoten Hisaaki
Published: October 2013

The design of highly active anode for water electrolysis is a critical issue. Here we report on copper delafossite oxide (ABO2; A = Cu) as a novel anode for the oxygen evolution reaction (OER). Among the copper delafossites we synthesized, copper rhodium delafossite (CuRhO2) showed the excellent cyclic voltammogram characteristic for OER in 1 M KOH (aq), comparable to that of Co3O4. We also confirmed that the eg orbital occupancy at the B site of delafossites, calculated from the bulk structure, had a good accordance with the catalytic activity of those materials. The CV characteristics of Cu delafossites were more accurately explained by considering the electronic conductivity.
In-situ Self-Assembly of Graphene Supported MnO2 Nanowires for Enhanced Water Oxidation in Both Alkaline and Neutral Solutions
Authors: Yuan Weiyong , Jiang San Ping
Published: October 2013

The sluggish water oxidation reaction represents a significant challenge in water splitting for energy storage using hydrogen. We herein reported a graphene supported MnO2 nanowires with an ultrasmall diameter and ultrahigh aspect ratio using a novel in-situ self-assembly approach. This nanostructure was investigated as water oxidation catalysts, showing an ultrahigh catalytic activity and good durability in both alkaline and neutral conditions.
High Catalytic Cu Electrode for CO2 Electroreduction from Combined Sphericity and Nanorod Surface Structure Induced by Thermal Process and Pre-Electroreduction
Authors: Jiang Ping , Qiao Jinli , Fan Mengyang , Ma Chengyu , Liu Jianshe
Published: October 2013

The high catalytic Cu electrode toward CO2 electroreduction was prepared by annealing Cu foil in air and then electrochemically reducing the resulting Cu oxide layers in H2SO4 at controlled potential. The Cu foil annealed at 500oC for 9h gave the best catalytic activity toward CO2 reduction, where the resulting Cu electrode performed a favorable current density and reached about 75 mA cm-2 at -1.5V vs Ag/AgCl. SEM analysis revealed that special surface structure of dense array of sphericity in the inner layer and the nanorods surrounded with kernels in the outer layer were obtained by further electrochemical reduction of Cu oxide films in 1M H2SO4 using the potential modulation method. These two surface structures show a strong synergistic effect for CO2 electroreduction. Also, the Cu electrode after modifications shows the excellent stability during the electrolysis time.
Numerical Investigation of the High Temperature PEM Electrolyzer: Effect of Flow Channel Configurations
Authors: Ruiz Diego De Haro , Sasmito Agus P. , Shamim Tariq
Published: October 2013

In this work, we aim to develop a mathematical and numerical framework for the HT-PEM electrolyzer that serves two main objectives: the first involves the study of the fundamental aspect of the HT-PEM electrolyzer and the associated transport and electrochemical processes. The second objective concerns with the development and integration of various flow channel design for high performance hydrogen production. Three different flow channel configurations, namely parallel, serpentine and multiple-serpentine were simulated and compared in term of hydrogen generation, parasitic load arising from pumping power and local temperature profiles. A single-domain model of a HT-PEM electrolyzer is implemented in the commercial CFD code ANSYS Fluent. The non-isothermal model considers conservation of mass, momentum, species, energy and charge (electronic and ionic). Good agreement between model prediction and experimental polarization curves is obtained within the experimental range. The established model is then extended to account for the effect of flow channel designs. The results suggest that multiple-serpentine channel design performs better in term of hydrogen production and uniformity of temperature with reasonable pressure drop.
Nickel-Cerium Electrodes for Hydrogen Evolution in Alkaline Water Electrolysis
Authors: Santos Diogo M.F. , Amaral Luis , Sljukic Biljana , Macciò Daniele , Saccone Adriana , Sequeira Cesar
Published: October 2013

The electrocatalytic activity of nickel-cerium (Ni-Ce) alloys towards the hydrogen evolution reaction (HER) in alkaline solution has been evaluated. Ni-Ce alloy electrodes with different low amounts of the rare earth element were prepared and tested in 8 M KOH aqueous electrolytes at temperatures ranging from 25 to 85 ºC. Polarization measurements are done to evaluate the HER on the Ni-Ce alloy electrodes and to determine several kinetic parameters, namely the Tafel coefficients, charge-transfer coefficients, and exchange current densities, allowing a direct comparison of the intrinsic HER activity of these electrodes with single Ni metal electrode prepared according to the same procedures
Stability of Air Electrode in Solid Oxide Electrolysis Cell
Authors: Kim Sun Jae , Choi Gyeong Man
Published: October 2013

A steam electrolysis using Solid Oxide Electrolysis Cell (SOEC) is re-emerging as the state-of-the art technology since water molecules are easier to split into hydrogen and oxygen molecules at high temperatures. In addition, SOEC and SOFC (Solid Oxide Fuel Cell) can be reversibly operated to generate hydrogen and electricity. Similar cell components of SOFC and SOEC may generally be used. However, the flow of oxygen in opposite direction requires the understanding of different electrochemical reaction. The air electrode such as (La,Sr)MnO3 (LSM) is stable in SOFC mode, however, it often delaminates from the electrolyte in SOEC mode. In this study, we have selected La0.6Sr0.4Co0.2Fe0.8O3-
δ (LSCF) as an air electrode and tested its possible use in both SOFC and SOEC modes. Gd0.2CeO2-x (GDC) was used interlayer between YSZ (8mol% yttria-stabilized zirconia) and LSCF.
The Effect of Particle Size on the Performance of LSFC Perovskite Anode of Proton Conducting Solid Oxide Fuel Cell (PC-SOFC)
Authors: Sun Yi-Fei , Yan Ning , Wang Guang-Ya , Luo Jing-Li , Chuang Karl T.
Published: October 2013

A promising and stable perovskite structure anode material La0.3Sr0.7Fe0.7Cr0.3O3-x (LSFC) prepared by a glycine nitrate process was applied as the anode catalyst of a proton conducting solid oxide fuel cell (PC-SOFC). The particle size of the anode material could be controlled by varying the calcination temperatures. The influence of particle sizes on electrochemical performances had been studied. It was shown that the higher calcination temperature resulted in larger LSFC particles, which decreased the PC-SOFC performance significantly.
Fundamental Investigation of Hydrogen Transport in Ferritic Stainless Steel for SOFC Interconnect Applications
Authors: Zimny Christopher I , Amendola Roberta , Gannon Paul E. , Berry Chaparral
Published: October 2013

Ferritic stainless steel (FSS) is widely used to separate different gaseous environments in many energy conversion systems such as turbines, heat exchangers, and intermediate temperature solid oxide fuel cell (SOFC) stacks. This so called “dual atmosphere exposure” at elevated temperatures (650-800°C) is commonly believed to facilitate accelerated and/or anomalous corrosion. In SOFC stacks, FSS interconnects (ICs) separate individual cells in a stack and are simultaneously exposed to the fuel (H2) on one side and the oxidant (air) on the other. This study focuses on the effects of SOFC operating conditions on FSS AISI 441, which has been observed to exhibit significant dual atmosphere effects on air-side corrosion. Subsequent 100 hours of dual atmosphere exposure (separating moist H2 from moist air at 800°C) FSS 441 sample surfaces and polished cross sections were analyzed using a field-emission scanning electron microscope (FE-SEM) equipped with energy dispersive x-ray spectroscopy (EDX) for elemental analysis. The study confirms the results of previous studies exploring secondary phase precipitation at grain boundaries and within the grains of FSS 441. Additionally, large Ti-rich nodules were observed on the cross-section surfaces. Ongoing efforts to investigate phase composition and texture using electron backscatter diffraction (EBSD) are discussed with the aim to better understand hydrogen transport mechanisms and attendant dual atmosphere corrosion of FSS 441.
Rational Design of Sulfur-Tolerant Anode Materials for Solid Oxide Fuel Cells
Authors: Cheng Zhe , Liu Meilin
Published: October 2013

The state-of-the-art nickel-yttria stabilized zirconia (Ni-YSZ) cermet anodes for solid oxide fuel cells (SOFC) are susceptible to poisoning by ppm-level sulfur contaminants (e.g., hydrogen sulfide, H2S) typically found in most hydrocarbon fuels, leading to severe performance loss or even cell failure. This problem calls for the development of new sulfur-tolerant anode materials for SOFC with high electrochemical activity. In light of such a research need and the “trial-and-error” nature of many previous studies in this area, this paper describes an effective approach to rational design of sulfur-tolerant anode materials for SOFC. First, our selection of potential sulfur-tolerant anode materials is guided by the insights about the sulfur poisoning mechanism of conventional Ni-YSZ cermet anodes. Further, thermochemical analysis and properly-designed experiments are used as a powerful tool to quickly screen and narrow down on candidates that are worth further investigations.
Development and Study of Self-Sustained Electrochemical Promotion Catalysts for Hydrocarbon Reforming
Authors: Huang Hao , Wang Zedong , Zhou Xiangyang , Liu Hongtan , Wei Yongsheng , Pramuanjaroenkij Anchasa , Bordas Alexi , Page Michael , Cai Sirui , Zhang Xuyang
Published: October 2013

A series of study on self-sustained electrochemical promotion (SSEP) catalysts are summarized. The SSEP catalyst enables more than 90% conversion and 90% hydrogen sensitivity at temperatures of 450 – 650 ºC for methane and n-pentadecane reforming. The conversion and sensitivity is up to two orders of magnitude greater than those over Ni based catalyst and a commercial Pt catalyst at 450-550°C. The kinetic models incorporating Langmuir-Hinshelwood model and SSEP effect successfully describe the major characteristics of hydrocarbons reforming over the SSEP catalyst. This concept of SSEP catalyst may provide a new route for developing high performance catalysts for hydrocarbons reforming.
Enhanced Carbon Dioxide Photoconversion Efficiency by 1D Structured Platinized TiO2 Films
Authors: Wang Wei-Ning , An Woo-Jin , Ramalingam Balavinayagam , Mukherjee Somik , Niedzwiedzki Dariusz M. , Gangopadhyay Shubhra , Biswas Pratim
Published: October 2013

A facile development of highly efficient platinized titanium dioxide (Pt-TiO2) thin films with unique one dimensional (1D) structure by using versatile gas phase deposition methods was established. The Pt-TiO2 thin films demonstrated excellent carbon dioxide (CO2) photoreduction efficiency with a maximum quantum efficiency of about 2.41%. Carbon dioxide was selectively converted into methane (CH4), with an optimal CH4 yield of over 1300 µmol/g-cat/hr, that is due to the synergistic effects of high surface area and minimized charge barriers by highly oriented single phase crystallinity of the TiO2 thin films, and efficient electron-hole separation by the ultra-small Pt NPs.
Effect of H2s On Ni-YSZ SOFC Anodes: A Combined In Situ Raman Spectroscopy-Impedance Spectroscopy Study
Authors: Mai Thi Hai Ha , Sergent Nicolas , Pagnier Thierry
Published: October 2013

A combined Raman spectroscopy-impedance spectroscopy study of a Ni-YSZ anode of a SOFC cell has ben performed at 500°C in open circuit potential and at 500 mV polarization. The effect of adding 200 ppm H2S to the Ar-3 %H2-3% H2O feeding gas have been observed. In OCP conditions, the Raman spectrum of Ni3S2 is observed only 5.5 hours after the introduction of H2S, while the electrode impedance was reduced. The impedance grew only when the Raman signal of Ni3S2 saturated. At 500 mV polarization, Raman spectra of Ni3S2 were also observed together with a huge increase of the electrode impedance. For temperatures higher than 500°C, Ni3S2 shows no Raman spectrum, but high temperature optical spectroscopy shows that the growth of the Ni3S2 crystals can be observed at 715°C.
A Rotating Electrolyte Disc (RED) for Operation in Liquid Metal Anode SOFCs
Authors: Toleuova Aliya , Yufit Vladimir , Simons Stefaan J. R. , Maskell William C. , Brett Dan J. L.
Published: October 2013

A novel Rotating Electrolyte Disc (RED) apparatus has been developed to determine parameters controlling mass transport of dissolved oxygen (and other gases) and oxidised species including metal oxide in liquid metal anode SOFCs. The validity of the concept and design for high temperature studies was first demonstrated by testing the setup in an aqueous system at ambient temperature (equimolar solution of ferrous and ferric chloride in KCl as supporting electrolyte). The diffusion coefficients have been determined for Fe2+ and Fe3+ by measuring the limiting oxidation and reduction diffusion-controlled currents. The obtained data have been compared with appropriate literature data. Preliminary testing of RED in molten tin electrodes is discussed.
Electrochemical Measurements on Cells, I: Simulation of Potential Distribution with an Embedded Probe
Authors: Zhang Lei , Liu Feng , Virkar Anil V,
Published: October 2013

Finite element analysis is used to calculate electrochemical potential distributions of charged species and chemical potential distributions of neutral species in solid electrolytes of electrochemical cells. Potential distributions of charged species follow the Laplace equation; while that of neutral species are related to the potentials of charged species through the local chemical equilibrium. Simulations with different cell geometries and transport parameters show that an embedded probe with electrically insulating coating along the probe surface (with tips uncoated) is required for the measurement of electrochemical potential of electrons within a solid electrolyte. Simulation also shows that by optimizing both electronic and ionic conductivities, cell stability can be improved.
New Structure of Cathode Function Layer of SOFC by Nanoporous Columnar YSZ Films
Authors: Jiang Huanhuan , Ming Li , Xie Bin
Published: October 2013

The Solid Oxide Fuel Cell (SOFC) is an electrochemical device operated at high temperature(500-900℃), it transforms the chemical energy stored in fuels into electrical energy and heat. The first element of fuel cell industry is reduce the SOFC to 400-600℃. We realize the nanoscale interface to enhance the conductivity effect by nanostructure of the electrode function, thereby we find a new way to explore new cathode functional layer materials and new structure.
Numerical Investigation of Amounts of Heat, Power and Temperature Distribution in IT-SOFC
Authors: Bakal Ahmet , Kaya Murat , Matc Mahmut D.
Published: October 2013

In this study, thermodynamic and heat transfer concepts were combined to understanding correlation of heat, power and temperature gradient on the cell. All of heat sources and amounts has been determined for electrolyte supported SOFC having 81 cm2 active area and thus temperature gradient due to each heat source can computed and total temperature distribution clearly has been shown. For this, electrochemical, ohmic, activation and concentration polarization heat sources have been computed by using the boundary conditions taken from the experimental data such as current-voltage-power and mol fraction. Then 80% of the heat generated at 0.7 V and 700°C was removed by the heat transfer and then the maximum temperature difference is found less than 6 °C. Also, it was found that the electrical and the heat generations are 25.41 and 23.03 W, respectively.
Effects of Humidified Gas Environments on the Electrode Behavior of La0.6Sr0.4CoO3-δ
Authors: McDonald TJ , Adler Stuart B.
Published: October 2013

The degradation of La1-xSrxCoO3-δ electrodes upon humidification of the gas environment remains a poorly understood phenomenon. Nonlinear Electrochemical Impedance Spectroscopy is used to measure the full nonlinear response of a La0.6Sr0.4CoO3-δ electrode in dry and humidified gas environments to determine the likely sources of degradation.
Unexpected Sources of Basal Plane Dislocations in 4H-SiC Epitaxy
Authors: Stahlbush Robert E , Mahadik Nadeemullah A
Published: October 2013

The suppression of degradation inducing basal plane dislocations (BPDs) in the critical drift layer of SiC power devices has occurred mainly by preventing BPDs in the substrate from propagating into the drift layer. As optimized epitaxial growth has produced drift layers free of BPDs over a large fraction of the wafer, other sources of BPDs have become important. Two alternate sources are discussed. The first is epitaxial inclusions, which mainly consist of grossly misoriented 4H-SiC. The local stress field around the inclusion introduces a cluster of BPDs. In low-BPD epitaxy, the outermost BPDs can glide centimeters forming half-loop arrays that have BPD segments along their whole length. The second source of BPDs not normally considered is BPDs that are converted to threading edge dislocation before reaching the drift layer. Experiments suggest that for high current levels and/or over time, device degradation is possible.
Properties of Al-SiO2-SiC(3C) Structures with Thermally Grown and PECVD Deposited SiO2 Layers
Authors: Przewlocki Henryk Maria , Gutt Tomasz , Piskorski Krzysztof , Borowicz Pawel , Bakowski Mietek
Published: October 2013

The 3C-SiC silicon carbide is a promising substrate material for MOS transistors. Parameters of MOS transistors are strongly dependent on the processing of the semiconductor-dielectric system. We study and compare the crucial features of the SiC(3C)-SiO2 systems with thermally grown and PECVD deposited SiO2 layers. For both types of systems, using the Al-SiO2-SiC(3C) capacitors as test structures, we determine and compare the following properties: The leakage currents and breakdown voltages of SiO2 layers, densities and distributions in energy of interface traps, band diagrams of the SiC(3C)-SiO2 systems, as well as structural properties and mechanical stresses in both types of SiO2 layers. Characterization of the above mentioned features is done using electrical, photoelectric and optical methods, including micro-Raman spectroscopy. The characterization results lead us to the conclusion, that PECVD deposition of SiO2 followed by wet oxygen annealing is more advantageous for MOSFET fabrication than the thermal growth of the SiO2 layer.
Comparison of SiC Epitaxial Growth from Dichlorosilane and Tetrafluorosilane Precursors
Authors: Song Haizheng , Rana Tawhid , Chandrashekhar M.V.S. , Omar Sabih U. , Sudarshan Tangali S.
Published: October 2013

As a novel Si-precursor in chemical vapor deposition epitaxial growth of 4H-SiC, tetrafluorosilane (TFS) is studied for both its advantages and disadvantages. Due to its high Si-F binding energy, TFS presents exceptional ability in the suppression of parasitic deposition and Si-droplet formation during SiC epitaxial growth. As compared with epigrowth using a chlorinated precursor, dichlorosilane (DCS), growth using TFS shows a lower growth rate most likely due to the TFS etching of SiC during growth and the lower reactivity of TFS, but the quality of the TFS grown epilayers is always superior. Epigrowth using TFS also shows better control of the uniformities of epilayer surface morphology and doping concentration, which offers excellent uniformity of SiC device performance. The influence of growth conditions including temperature, gas flow rate, Si/H2 ratio, and C/Si ratio on growth rate and epilayer properties is investigated in growth using TFS and compared with growth using DCS.
Characterization and Performance of D-Mode GaN HEMT Transistor Used in a Cascode Configuration
Authors: MacElwee Tom , Roberts John , Lafontaine Hugues , Scott I. , Klowak Greg , Yushyna Lyubov
Published: October 2013

Cascode configured D-mode GaN HEMT device performance is reported in this paper. The basic parameters of the D-mode HEMT will be covered as well as the integration of the cascode configuration into the PQFN package. Finally 500 volt 3.3 amp switching characteristics of the cascode will be discussed showing the excellent switching performance with voltage slew rates as large as 70 V/ns being measured.
Reliability of GaN HEMTs: Electrical and Radiation-Induced Failure Mechanism
Authors: Anderson Travis J , Koehler Andrew D , Tadjer Marko J , Hobart Karl D , Specht Petra , Porter Matthew , Weatherford Todd R , Weaver Brad , Hite Jennifer K. , Kub Fritz J
Published: October 2013

The reliability of AlGaN/GaN HEMTs is currently a limiting factor in the development of next-generation power amplifier technology. In this work, we use atomic-resolution transmission electron microscopy (TEM) to directly image the defects associated with device failure. Furthermore, we attempt to induce defects through electrical and radiation-induced stress on the device, and compare the mechanism for failure by TEM analysis.
Power Electronic Module Packaging at UA
Authors: Ang Simon S. , Mantooth H Alan , Balda Juan C
Published: October 2013

Power electronic modules incorporating silicon carbide (SiC) and gallium nitride (GaN) power semiconductors enable many power electronic system applications otherwise not possible for their silicon counterparts. These power electronic modules are desired to operate at junction temperatures of greater than 200oC. As such, the design and fabrication of power electronic modules become more challenging and involve several multi-disciplinary engineering expertise. This paper addresses the educational and research efforts in power electronic module packaging at the University of Arkansas.
GaN Power Transistors with Integrated Thermal Management
Authors: Eddy Charles R. , Anderson Travis J , Koehler Andrew D. , Nepal Neeraj , Meyer David J. , Tadjer Marko J , Baranyai R. , Pomeroy J. W. , Kuball Martin , Feygelson Tatyana I , Pate Bradford B , Mastro Michael A , Hite Jennifer K. , Ancona Mario G. , Kub Fritz J , Hobart Karl D
Published: October 2013

The concept for and design/fabrication of a GaN power transistor with integrated thermal management is presented. Key elements of the design, including those supporting enhancement-mode operation, high breakdown voltages and low on-resistance are described in detail. The importance of surface preparation and growth of high-k gate dielectrics using atomic layer deposition is summarized. Aspects of barrier design and surface passivation to promote low access resistance including lattice matched barriers and novel low-temperature AlN passivations are discussed. Finally, the integration of nanocrystalline diamond coatings on both the top-side and the back-side of the device for thermal management are described. Initial performance assessments of each of these components, as measured in the device operation, are presented and future efforts are highlighted.
1000V Vertical JFET Using Bulk GaN
Authors: Diduck Quentin , Nie Hui , Alvarez Brian , Edwards Andrew , Bour David , Aktas Ozgur , Disney Don , Kizilyalli Isik C.
Published: October 2013

Bulk GaN substrates with low defect density are now commercially available. This material enables the fabrication of vertical GaN devices with high breakdown voltage, and excellent electrical performance and power device figure of merit. In this paper, we present a 1000V Vertical JFET that has a positive threshold of 1V. The normally-off FET integrates a hetero-junction in the design to improve the transconductance and enable efficient switching operation into the 10s of MHz.
3D TCAD Simulations for More Efficient SiC Power Devices Design
Authors: Phung Luong Viêt , Planson Dominique , Brosselard Pierre , Tournier Dominique , Brylinski Christian
Published: October 2013

SiC devices become more and more prominent in the power semiconductor industry. Thanks to a technology that seems to be mature enough, SiC devices become more and more sophisticated. Therefore, they can be serious competitors to existing silicon devices in not so distant future at least for high temperature and high power applications. In addition to undoubtedly better electrical and thermal properties, SiC devices still require attention regarding their design. Indeed, the material is still more expensive than silicon and some limitations such as the inability to create deep p-n junctions prevent from re-using existing silicon design. Therefore, SiC devices should be designed so that the best trade-off between active area and breakdown voltage is achieved. In such a context, 3D TCAD can be become an interesting approach mostly thanks to modern computer farms.
Materials Issues for Vertical Gallium Nitride Power Devices
Authors: Williams Adrian D. , Moustakas Theodore D.
Published: October 2013

In this paper we are addressing some of the fundamental materials issues for the development of vertical GaN-based power devices. Major components of such device are the n+ GaN freestanding substrate on which a thick (~50 µm), low defect density and low carrier concentration (<1016 cm-3) n-GaN drift region is grown homoepitaxially. We show that the hydride-vapor-phase-epitaxy (HVPE) is a method capable of producing economically free standing n+ GaN substrates as well as the required thick and low defect and carrier concentration n-GaN drift region. The formation of freestanding GaN substrates by a natural separation mechanism from the sapphire substrate is demonstrated and effectively eliminates the need for post-growth processes such as laser liftoff, chemical etching or mechanical lapping to form freestanding GaN substrates. These results were accounted for by calculating the thermal stresses in the GaN film and substrate as a function of film thickness using Stoney’s equation and assuming that the GaN buffer undergoes decomposition at the growth temperature. The structure of these films was determined by x-ray diffraction and the dislocation density was measured to be as low as 5x106 cm-2. The lowest carrier concentration in these heteroepitaxially grown films was found to be 1017 cm-3. Furthermore, we have identified the origin of this n-type auto-doping and proposed method to reduce the carrier concentration to values 1016 cm-3 or lower.
Abrasive-Free Polishing of SiC Wafer Utilizing Catalyst Surface Reaction
Authors: Sano Yasuhisa , Arima Kenta , Yamauchi Kazuto
Published: October 2013

Catalyst-referred etching (CARE) is an abrasive-free chemical polishing method that utilizes a catalytic chemical reaction at the contact points of a wafer and a catalyst plate surface. Using a platinum plate catalyst and hydrogen fluoride aqueous solution, an atomically flat 4H–SiC (0001) wafer was obtained. The removal rate was found to be proportional to the rotational velocity and processing pressure, and it reached approximately 500 nm/h, which is comparable to chemical mechanical polishing methods. Furthermore, it was found that an atomically flat SiC surface can be obtained by CARE using only deionized water and a platinum catalyst, which is significant from the viewpoint of the widespread use of CARE.
PN-Diode P-Oxide-Semiconductor/N-SiC/N-Si Resistive Nonvolatile Memory for Cross-Point Memory Array
Authors: Sato Yoshihiko , Yamashita Atsushi , Tsukamoto Takahiro , Suda Yoshiyuki
Published: October 2013

We have demonstrated resistive nonvolatile memory devices with the pn-diode structures of p-AgO
x
/SiO
x
/n-SiC/n-Si(111) and p-CuO
x
/SiO
x
/n-SiC/n-Si(111) which have a good rectifying current-voltage (I-V) characteristic. The forward currents change between high and low, indicating a resistive nonvolatile memory. The I-V and capacitance-voltage (C-V) curve and the structural analyses suggest that existence and nonexistence of trapped electrons in the SiO
x
layers corresponds to the low and high forward current states, respectively. These memory devices have a well rectifying characteristic feature; they are expected to be suitable to the most theoretically dense cross-point array.
Effect of Intermetal Dielectric Layer On the Interpoly Dielectric Properties of Nonvolatile Memory Devices
Authors: Ryoo Jin-Yong , Sung Suk-Kang , Yim Yong-Sik , Song Jun-Eui , Shin Wang-Chul , Song Jai-Hyuk , Song Du-Heon , Choi Jeong-Hyuk , Roh Yonghan
Published: October 2013

This paper describes the influence of water-related species contained in intermetal dielectric layer on the interpoly dielectric properties of nonvolatile memory devices. P-TEOS layer used as an intermetal dielectric enhances degradation of interpoly dielectric of nonvolatile memory devices due to the water-related components contained in the layer.
Low-Dielectric Constant SiCN Charge Trapping Layer for Nonvolatile Memory Applications
Authors: Kobayashi Kiyoteru , Naito Shinji , Nakiri Shuichi , Ito Yoshina
Published: October 2013

We investigated the charge trapping property of SiOx-SiCN-SiO2 stacked films formed on silicon substrates. The stacked films captured charges and the trapped charge density was found to be roughly equal to that in a conventional SiOx-SiNX-SiO2 stacked film. On the other hand, the flat-band voltage shift induced by the trapped charge in the metal-SiOx-SiCN-SiO2-silicon capacitor was greater than that in the metal-SiOx-SiNX-SiO2-silicon capacitor. The larger flat-band voltage shift is attributed to the lower dielectric constant of SiCN film. The programming and erasing characteristics of metal-SiOx-SiCN-SiO2-silicon structures were also investigated.
CdSe Embedded ZrHfO Gate Dielectric Nonvolatile Memories – Charge Trapping and Breakdown Studies
Authors: Lin Chi-Chou , Kuo Yue
Published: October 2013

MOS capacitors composed of nc-CdSe embedded ZrHfO high-k gate dielectric stacks were fabricated and investigated on the charge trapping and breakdown mechanisms. The interface layer between the high-k film and Si wafer was transformed to a more SiO2-like HfSixOy group after the inclusion of nc-CdSe layer. The frequency-dependent conductance-voltage measurement result shows that part of the trapped charges were loosely trapped at the nc-CdSe/ZrHfO interface and easily tunneled back to the Si substrate under low frequency measurement condition. Compared with the control sample, the nc-CdSe embedded sample has a larger leakage current and a higher breakdown voltage due to the formation of new leakage paths and a larger physical thickness. The trapped charges can be retained in the nc-CdSe site after the breakdown of the high-k film.
(Invited) Recent Developments in ST-MRAM, Including Scaling
Authors: O'Sullivan E. J. , Gajek M. J. , Nowak J. J. , Brown S. L. , Gaidis M. C. , Hu G. , Sun J. Z. , Trouilloud P. L. , Abraham D. W. , Robertazzi R. P. , Gallagher W. J. , Worledge D. C.
Published: October 2013

Spin-torque Magnetoresistive Random Access Memory (ST-MRAM) is the subject of intense investigation since it extends MRAM technology to densities beyond those achieved with the earlier field-switched MRAM technology. This paper reviews recent developments in ST-MRAM MTJ device technology, including exciting progress in scaling MTJs down to dimensions approaching 20 nm. Fabrication issues relevant to development of ST-MRAM, including its integration with CMOS back-end-of-line (BEOL) processing, are also briefly discussed.
Evaluation of TaN as the Wet Etch Stop Layer during the 22nm HKMG Gate Last CMOS Integrations
Authors: Cui Hushan , Xu Jing , Gao Jianfeng , Xiang Jinjuan , Lu Yihong , Tang Zhaoyun , He Xiaobin , Li Tingting , Luo Jun , Wang Xiaolei , Tang Bo , Yu Jiahan , Yang Tao , Yan Jiang , Li Junfeng , Zhao Chao
Published: October 2013

In this work, PVD and ALD TaN were evaluated as wet etch stop layer (WESL) in order to selectively remove TiN & Ti in the high-k/metal gate (HKMG) gate last CMOS integrations. The selectivity of TiN & Ti towards PVD or ALD TaN is achieved at least 5.6 with an ammonia hydrogen peroxide mixture (APM). In case of PVD TaN, nitrogen content in the film has great impact on its chemical resistance. Both of PVD and ALD TaN have enhanced thickness loss by the APM in presence of TiN & Ti. Based on the C-V results of MOS capacitor (MOSCAP), it can be concluded that both PVD and ALD TaN can be introduced into the CMOS integrations as WESL. But, for p-type body MOSCAP (nMOS), ALD TaN thickness should not exceed 1nm. Thanks to excellent filling capability of ALD TaN, it can be extensively used in advanced CMOS technology as WESL.
Lanthanum Interaction with Surface Preparations
Authors: Garnier Philippe , Joseph Vincent , Krachewski Remi
Published: October 2013

In latest integrated circuits’ generation, lanthanum has been commonly introduced to cap hafnium based high k gate oxides. Indeed it enables strong threshold voltage modulation of the devices. Nonetheless its implementation is challenging, especially during the gate patterning, where hydrofluoric acid is used to remove the high k film on the active areas. On the one hand, lanthanum is highly corrodible. On the other hand, it easily generates non soluble salts. Lanthanum fluoride salts are among the most stable ones. Preventing their deposition is hereby discussed. Hydrochloric acid is one of the best solutions to solubilize lanthanum, before removing the high k with hydrofluoric acid.