bluuebunny/crossref_metadata_2025 huggingface dataset:
Oxygen Transfer from Metal Gate to High-k Gate Dielectric Stack: Interface Structure & Property Changes
Authors: Ha Jeong-Hee , AlShareef Husam , Chambers Jim , Sun Yun , Pianetta Piero , McIntyre Paul , Colombo Luigi
Published: September 2007

Photoelectron spectroscopy and transmission electron microscopy provide clear experimental evidence to show that oxygen initially present in the W gate layer of a metal/high-k gate stack is transferred to the substrate and increases the interfacial oxide during post-deposition thermal processing. Oxide interface layer growth was reduced, but still occurred, if a TaN barrier layer was used to cap the W gate metal, thus minimizing atmospheric oxygen incorporation. Our results suggest that oxygen is incorporated into the W layer (perhaps at grain boundaries and interfaces) during or immediately after its deposition, thus providing a source for subsequent oxidation of the underlying layers. This effect, which may be exhibited by other high workfunction metals besides W, may be an important contributor to the Vfb instability observed in scaled PMOS devices based on the metal/high-k gate stacks.
Instability and Defects in Gate Dielectric: Similarity and Differences Between Hf-Stacks and SiO2
Authors: Zhang Jian F. , Zhao Ce , Chang Mo , Zhang Weidong , Groeseneken Guido , Pantisano Luigi , De Gendt Stefan , Heyns Marc
Published: September 2007

To control gate leakage current, Hf-based dielectrics have been selected for replacing SiON. Hf-stacks generally suffer from more defects than SiO2. This work reviews the latest progress in the understanding of these defects and compares them with those in SiO2. Attention will be focused on the stress-induced negative charging and the process-induced positive charging. On the negative charging, difficulties for characterizing the highly unstable trapping in thin Hf-layers will be tackled. Key trap properties are determined, including capture cross sections, effective density and spatial location. It is found that the capture cross section of as-grown electron traps in Hf-stacks is similar to that of generated electron traps in SiO2. On process-induced positive charges, it will be shown that Hf-stacks and SiO2 also have common features, including a large sample-to-sample variation, mobile and fixed charges, and stability. Overall, Hf-stacks, especially nitrided, behave like an inferior or stressed SiO2.
Materials and Process Integration Issues in Metal Gate/High-k Stacks and Their Dependence on Device Performance
Authors: Callegari Alessandro , Babich Katherina , Zafar Sufi , Narayanan Vijay , Ando Takashi , Batson Philip E.
Published: September 2007

Electron mobility, work function instabilities and re-growth of the thin SiO2 interfacial layer are main concerns for integrating metal/high-k stacks in high performance CMOS. High electron mobility has been obtained with both high (T~1000oC) and low (T~400oC) temperature processing but still the exact mechanism of the metal interaction with the gate stack is not fully understood. Threshold voltage control is also an open issue since metal/high-k stacks exhibit mid-gap work functions after high temperature annealing. Fixes have been proposed which require the introduction of positive (negative) charges near the Si/SiO2 interface to produce devices operating at the Si band edges.
Screening of High-k Layers in MIS and MIM Capacitors Using Aqueous Chemical Solution Deposition
Authors: Van Elshocht Sven , Hardy An , Adelmann Christoph , Caymax Matty , Conard Thierry , Franquet Alexis , Richard Olivier , Van Bael Marlies K. , Mullens Jules , De Gendt Stefan
Published: September 2007

Alternative high-k materials are being researched for future dielectrics in various CMOS applications. We report on the aqueous chemical solution deposition (CSD) technique as an alternative material screening technique applied to ZrO2, Nd2O3, Pr2O3, Eu2O3, and Sm2O3. We used ZrO2 to explore the effect of the different process parameters on dielectric quality and found that proper optimization of these parameters can enhance the film quality to similar levels as for example atomic layer deposition. CSD can also be used to deposit dielectrics for MIM devices, but the preferred 600{degree sign}C anneal temperature to optimize the dielectric quality can be a limiting factor for metals that become insulating when oxidized.
Overview of Materials Processing and Properties of Lanthanum-Based High-k Dielectrics
Authors: Lichtenwalner Daniel J. , Jur Jesse S. , Inoue Naoya , Kingon Angus I.
Published: September 2007

Lanthanum-based high-K dielectrics remain attractive for gate dielectric applications, in the forms of either lanthanum oxide, lanthanum silicate, or as a lanthanum addition to other dielectrics. Lanthanum oxide and silicate MIS devices have been reported to have EOT values as low as 0.5 nm under low temperature processing. After 1000 {degree sign}C, 10 sec anneals consistent with dopant activation, lanthanum silicate is shown by cross-sectional TEM to remain in the amorphous phase, with no La diffusion into the Si substrate as measured by back side SIMS. Lanthanum addition to hafnium-based dielectrics is presently of interest for the purposes of increasing the dielectric constant, and for providing threshold voltage (Vt) control for obtaining band-edge nMOSFET device operation. A 400-500 mV reduction in Vt is obtained using a lanthana cap layer on HfSiON, achieving band edge nMOSFETs with minimal mobility degradation.
High-k Characterization by RFCV
Authors: San Andres Enrique , Pantisano Luigi , Roussel Philippe , Toledano-Luque Maria , Trojman Lionel , Severi Simone , DeGendt Stefan , Groeseneken Guido
Published: September 2007

In this paper we review some advances in High-κ characterization by means of capacitance measurement in the radio-frequency regime (widely known as RFCV). Firstly, we present a robust methodology for the gate impedance parameter extraction in short channel leaky devices, based on measurements from DC to the GHz range and fitting with a robust weighted algorithm. Secondly, we will present a novel RFCV technique which pushes the conventional split-CV measurement to the MHz range. This RF-split-CV is based on measuring with a 2-port network analyzer, as opposed to the conventional technique which uses a 1 port LCR meter. This improved technique is the basis for accurate mobility extraction as studied in the final part of the paper. This methodology takes parasitics fully into account: the RF-split-CV curves obtained are used for the accurate metallurgical channel length extraction. Combination of these Lmet results with conventional Ids Vgs curves measured in the linear regime leads to the source and drain resistance calculation. After all these corrections have been performed, the mobility is finally calculated.
On the Stability and Oxidation of Single Crystal (100) InAs Surfaces
Authors: Chang Chia-Lin , Shutthanandan Vaithiyalingam , Singhal Subhash C. , Ramanathan Shriram
Published: September 2007

We report on a method to prepare clean and smooth surfaces of InAs (100) along with in-situ high-resolution studies of the nanoscale oxidation of the pristine surface. A hydrogen molecular cleaning (HMC) technique has been developed that results in complete removal of native oxide. This has been verified in-depth by in-situ nuclear reaction analysis (NRA) using the 16O(d,p)17O reaction and X-ray photoelectron spectroscopy. Further, ion channeling studies have been performed to verify atomically smooth surfaces after post cleaning. We derive kinetic boundaries for oxide formation on cleaned InAs surfaces using NRA measurements.
In-Situ Deposition of High-k Gate Stack on InGaAs and GaAs for Metal-Oxide-Semiconductor Devices with Low Equivalent Oxide Thickness
Authors: Kambhampati Rama , Koveshnikov Sergei , Tokranov Vadim , Yakimov M. , Moore R , Tsai Wilman , Oktyabrsky S.
Published: September 2007

We have demonstrated that Fermi level pinning at the interface between InGaAs or GaAs and HfO2 gate dielectric can be prevented by an in-situ deposited amorphous silicon interface passivation layer. A fully in-situ process of III-V surface passivation and high-k gate stack deposition allows scaling of the Si passivation layer thickness thus resulting in significant reduction of equivalent oxide thickness (EOT) for InGaAs and GaAs Metal Oxide Semiconductor (MOS) devices. A low EOT of 1.6 nm was obtained for 10 nm HfO2 for both n-type and p-type GaAs and InGaAs MOS Capacitors. Good control of the drain current by gate voltage was observed on depletion mode a-Si passivated GaAs MOS transistors.
Nonvolatile Memories Based on Nanocrystalline Zinc Oxide Embedded Zirconium-doped Hafnium Oxide Thin Films
Authors: Lu Jiang , Lin Chen-Han , Kuo Yue
Published: September 2007

Memory devices containing nanocrystalline ZnO embedded Zrdoped HfO2 high-k dielectric films have been prepared and characterized. The memory effect was manifested by the large counterclockwise capacitance-voltage hysteresis, e.g., 1.22V at {plus minus}6V gate bias, and negative differential resistance region in the positive bias current-voltage range. The maximum trapped charge density of 6.43 ×1012cm-2 was obtained after -9V→ +9V→ -9V sweeps. The memory effects were mainly caused by electron trapping at low bias voltage. A large memory operation window, e.g., 0.90 V, with a long charge retention time, e.g., > 36,000s, was achieved under the proper gate stress voltage. It is a viable dielectric for future nano-size metal oxide semiconductor field effect transistors and capacitors.
Improvement of Multiple-Level Cell Endurance of NAND Flash Memories Using HfO2/ZrO2 Charge Storage Layer
Authors: Zhang Gang , Yoo Won Jong
Published: September 2007

The endurance reliability of multiple-level cell (MLC) using a Si3N4 and high-k HfO2/ZrO2 charge storage layers (CSL) has been investigated. It is found that the base level disturbance, which is resulted from over-erasure or under-erasure, is one of the major degradation factors of the endurance reliability for MLC using a Si3N4 CSL. Programming/erasing (P/E) offsets and thermally-induced base level disturbance significantly limit the Si3N4 MLC endurance reliability over the 104 - 105 P/E cycles. In contrast, HfO2 and ZrO2 CSLs effectively suppress over-erasure, resulting in a strong resistance to P/E offsets or thermally induced base level variation. The MLC endurance reliability is thus effectively improved, and level merge is prevented for 105 P/E cycles.
Dysprosium Scandate Thin Films Prepared by Metal Organic Chemical Vapor Deposition on Pt/TiOx/SiO2/Si Substrates for MIM Capacitor Applications.
Authors: Thomas Reji , Karan Naba K. , Ehrhart Peter , Waser Rainer , Katiyar Ram S
Published: September 2007

DyScO3 thin films having thickness between 8 and 38 nm were grown on Pt/TiOx/SiO2/Si(100) substrates by metal organic chemical vapor deposition. Thickness dependent crystallization was observed and thicker films were polycrystalline at 700 oC. Root mean square roughness of the films having thickness 20 nm was around 1-1.5 nm, comparable with the underlying Pt film. Experimentally extracted dielectric constant of the films was ~ 21{plus minus}1.5 which is comparable to the dielectric constant (24) of DyScO3 calculated from the Clausius-Mosotti equation using the molecular polarizabilites of Dy2O3 and Sc2O3. Electrical properties are promising: ~8 nm thick DyScO3 film reached an equivalent SiO2 thickness (EOT) of ~1.95 nm. Dielectric breakdown of the film was 2.3 MV/cm and the corresponding energy density is around 5 J/cm3. For the amorphous films leakage current density through the metal-insulator-metal stacks were in the range 10-4 to 10-7 A/cm2 at an electric field of about 1 MV/cm.
AVD and MOCVD TaCN-based Films for Gate Metal Applications on High k Gate Dielectrics
Authors: Karim Zia , Barbar Ghassan , Boissière Olivier , Lehnen Peer , Lohe Christoph , Seidel Tom , Adelmann Christoph , Conard Thierry , O'Sullivan Barry , Ragnarsson Lars-Aåke , Schram Tom , Van Elshocht Sven , De Gendt Stefan
Published: September 2007

Atomic Vapor Deposition (AVD®) and MOCVD of TaCN-based films are studied for high work function needed for PMOS Gate-first designs. Work functions as high as 4.85eV and 4.92eV are achieved on HfSiON and HfSiO high k dielectrics, respectively. The film composition was varied from pure Ta2N to TaC films which improve the work function as well as thermal stability. The high work function was different depending on whether different underlying high k dielectrics and N compositions were used. The approaches for obtaining high work function and the impact of underlying high k films are discussed. After detailing the physical and chemical characteristics of MOCVD and AVD® TaCN films, this paper correlates the deposition technologies to the high work function values obtained by extraction from flatband voltage / equivalent oxide thickness (EOT) measurements.
Ultra-Thin Ruthenium Films by Pulsed Chemical Vapor Deposition
Authors: Hendrix Bryan C. , Welch James J. , Roeder Jeffery F. , Wang Ziyun , Xu Chongying , Stawasz Michele , Baum Thomas
Published: September 2007

Ruthenium films have been deposited via a pulsed CVD process using tris(2,2,6,6-tetramethyl-3,5-heptanedionato) ruthenium (i.e., Ru(thd)3) , oxygen, and hydrogen at 280 and 300{degree sign}C. Film resistivity near theoretical values (16 μΩ-cm at 3 nm) was achieved with repeatability. Films were smooth and continuous down to 2 nm; a low density of pinholes could be observed on films with 1 nm thickness. Adhesive tape tests indicated good adhesion. Carbon content is low.
Structural and Morphological Properties of Nitrogen Doped Polysilicon for Advanced Gate Material
Authors: Woo Sang Ho , Kim Yil Wook , Um Pyung Yong , Kim Hai Won , Cho Sung Kil , Choe Hyung Su , Lee Dong Keun , Kim Chang-Koo
Published: September 2007

Deposition of N-doped poly-Si films from SiH4 and NH3 using a single wafer type LPCVD system was investigated to improve the grain size reduction and the grain size distribution. The deposition rate and surface roughness of N-doped Si were greatly affected by the NH3/SiH4 ratio such that they decreased with increasing NH3/SiH4 ratio. XRD and TEM measurements revealed that with increasing NH3/SiH4 ratio, the size of the grains was decreased and the grains size distribution got uniform. Finally, we successfully obtained N-doped poly-Si films having uniform grain size below 40 Aå.
Investigation of Conducting Oxide and Metal Electrode Work Functions on Lanthanum Silicate High-k Dielectric
Authors: Lee Hee Young , Lichtenwalner Daniel J. , Jur Jesse S. , Kingon Angus I.
Published: September 2007

The effective work functions (EWF) of thin transparent conducting oxide (TCO) interlayers, i.e. indium tin oxide (ITO) and indium zinc tin oxide (IZTO), on lanthanum silicate metal-insulator-semiconductor (MIS) devices were examined. The properties were compared to those obtained using conventional metal electrodes. It was found that the EWF of all metal gates investigated was lowered by ~0.3 - 0.4 compared to expected values, due to effects related to the lanthanum silicate dielectric stack. The presence of the TCO electrode layer yielded EWF values ranging from 4.05 to 4.45 eV depending primarily on the annealing condition, regardless of the TCO type. As it is known that ITO can have a work function ~4.75 eV, the presence of the oxide electrode still results in an EWF ~0.3 - 0.4 eV below expected values. These results indicate that EWF lowering in lanthanum silicate gate stacks is not strongly dependent on the top interface.
Determination of the Coulomb Scattering Parameter (Alpha) for ALD HfO2/TiN Gate n and p-MOSFETs Using Negative Bias Temperature Stress
Authors: Negara Muhammad Adi , Cherkaoui Karim , Majhi Prashant , Tsai Wilman , Bauza Daniel , Ghibaudo Gerard , Hurley Paul
Published: September 2007

The aim of this paper is to determine the Coulomb scattering parameter (α) for ALD HfO2/TiN gate n and p-MOSFETs using Negative Bias Temperature Stress. The charge pumping characteristics with increasing stress bias indicate interface state generation and an associated increase in bulk/interface positive charge. Accounting for both the fixed positive charge and interface state generation during Negative Bias Temperature Stress, values of 2565 Vs/C and 11100 Vs/C are determined as for n and p HfO2/TiN gate MOSFETs
Enhanced Channel Mobility Materials for MOSFETs on Si Substrates
Authors: Donnelly Joe , Shahredji Davood , Kelly David , Tutuc Emanuel , Banerjee Sanjay
Published: September 2007

MOS high-k/metal devices with Ge and GaAs semiconductors were used to fabricate PMOS and NMOS devices, respectively. Selective epitaxially grown (SEG) Ge PMOSFETs with a 2× enhanced IDsat and greater than 2× improvement in hole mobility compared to Si PMOSFETs. Buried-channel (BC) Ge1-xCx PMOSFETs showed a 2× enhancement in IDsat over Si control PMOSFET and ~1.5× improvement in effective mobility over universal curve for Si. The corresponding enhancement for surface channel (SC) Ge1-xCx PMOSFETs was 3× and ~2.5×, respectively. The Ion/Ioff ratio for (BC) Ge1-xCx was greater than 5×104. GaAs NMOS capacitors using two different chemical surface treatments NH4OH and (NH4)2S prior to ALD-Al2O3 deposition were examined. Both treatments remove As2O3/As2O6 upon Al2O3 deposition and unpinned the Al2O3/GaAs interface. However (NH4)2S has better frequency dispersion behavior and slightly smaller capacitance equivalent thickness (CET) than NH4OH treatment. Quasi-static CV measurement confirmed formation of a true inversion layer in GaAs.
Proton Conductive Composite Materials Using Functionalized and Crosslinkable VDF/CFTE Fluoropolymers and Proton Conductive Inorganics
Authors: Chung Mike , Zhang Zhicheng , Chalkova Elena , Wang Chunmei , Fedkin Mark , Komarneni Sridhar , Sharma Sanjeev , Lvov Serguei
Published: September 2007

The synthesis of crosslinkable P(VDF-CTFE) copolymer with controllable structure, molecular weight and terminal and side chain silane groups was developed. The chemistry of the synthesis was centered on a specifically designed functional borane initiator containing silane groups. Polymer electrolyte composite membranes were fabricated by dispersion of additive particles in the polymer solution and film casting, followed by thermocompression, which allowed for formation of crosslinking sites in 3-D polymer network. Two types of membranes were investigated: P(VDF-CTFE)/Nafion/Inorganic and P(VDF-CTFE) with incorporated sulfonated units. Different types of inorganic proton conductors were synthesized and tested in P(VDF-CTFE)/Nafion/Inorganic membranes. The highest conductivity was obtained using H3OZr2(PO4)3 particles. Optimization of the P(VDF-CTFE)/Nafion/H3OZr2(PO4)3 membrane composition allowed to substantially increase the membrane conductivity. Two avenues were explored to add sulfonic acid groups to P(VDF-CTFE) to obtain a desirable membrane conductivity without an addition of Nafion: introduction of sulfonated silicon dioxide and sulfonated SPETMS.
Multiblock Copolymers for Low Relative Humidity Fuel Cell Operations
Authors: Kim Yu Seung , Einsla Melinda L. , Hawley Marilyn , Pivovar Bryan , Lee Hae-Seung , Roy Abhishek , McGrath James
Published: September 2007

Fuel cell operation under reduced humidity is highly desirable for automotive applications. Unfortunately, sulfonated polymer electrolytes tend to lose proton conductivity with decreasing water activity. It has been observed in our studies that proton conductivity of polymer electrolytes is critically related to nanostructured (phase separated) morphology. In this study, we focus on morphological effects on proton conductivity, particularly at a low relative humidity. Two copolymer systems - . multiblock copolymers and traditional random copolymers were used in the study. Water sorption, proton conductivity and morphology of the systems are compared and H2/air fuel cell performance was evaluated at high temperature and low relative humidity.
Modified Polybenzimadazole Type Membranes Based on Polyphosphonic Acid for Pem Fuel Cells
Authors: Cabasso Israel , Yuan Youxin
Published: September 2007

Modified polybenzimadazole membranes have been developed for the PEM fuel cell. Modified PBI type polymers (MPBI) have been functionalized with both diethyl phosphonyl, and phosphonic acid groups. Diethyl phosphonylation yields alcohol soluble high temperature polymer; these PBIPs have been further hydrolyzed to yield phosphonic acid derivatives (PBIPOH). The PBIPOH exhibit very high ionic charge density (~10 meq/g) and proton conductivity (~0.5 S/cm2 at 95 {degree sign}C). PBIPOH, and its blends, have been cast into ion-exchange membranes for fuel cell applications. The membranes shows good performance at 80 - 100 {degree sign}C (at 30 psig) and give current density of ~ 2A/cm2 at 0.5 V with maximum power density 0.98 W/cm2. Short-term durability testing of the PBIPOH at temperatures ranging from 50O to 120 {degree sign}C with daily start-ups and shutdowns has been conducted for greater than 500 h without a reduction in the fuel cell performance.
Fabrication of Nano-Templated Proton-Exchange Electrolyte Membrane for Fuel Cell Application.
Authors: Jang Ju-Hee , Oleksandrov Sergiy , Lee Young-Kwan , Chung Chan-Hwa
Published: September 2007

Sulfonated nano-template/polystyrene (AAO/PSS) composite membranes were developed by changing amount of polystyrene inside AAO for direct methanol fuel cell (DMFCs) application. The mixture of styrene monomer, divinyl benzene, and benzoyl peroxide were inserted into nano-channel of AAO and then, polymerized at 80{degree sign}C cycle by cycle. In order to add sulfonic groups, membranes were treated with acetic acid, mercapto-propyltrimethoxysilane, and toluene solution, and then oxidized by an aqueous solution containing hydrogen peroxide and nitric acid. Ion conductivity and methanol permeability were studied for the membrane. It was found that with increasing a number of cycles, ion conductivity (0.683S/cm) was increased and methanol permeability (1.295×10-6cm2/s) was decreased slightly. These results would confirm the nano-template proton exchange electrolyte membrane prepared could be applied to DMFCs.
Composite Membranes for Hydrogen/Air PEM Fuel Cells
Authors: Pintauro P. , Mather Patrick , Arnoult Olivier , Choi Jonghyun , Wycisk Ryszard , Lee Kyung Min
Published: September 2007

Composite membranes containing sub-micron diameter electrospun fibers of proton conducting sulfonated poly(ether ether ketone) have been fabricated. Interconnected fibers were formed into a dense mat and then embedded with an inert polymer matrix to create a water/gas impermeable proton conducting fuel cell membrane. The fibers were composed of 1.65 mmol/g ion-exchange capacity polymer and the inert matrix polymer was a commercially available photocrosslinkable and solvent-less adhesive, NOA 63 (Norland Optical Adhesive). After electrospinning and before NOA embedding, the fiber mat was densified by heating and the fibers were welded to one another by exposure to ethanol solvent. The volume fraction of fibers in the resulting membranes was high (0.74) and the proton conductivity after embedding NOA 63 was 0.049 S/cm (for water equilibration at 25oC).
3DOM Silica Composite Membrane Including a Binary Polymer Electrolyte for DMFC
Authors: Munakata Hirokazu , Sasajima Keiji , Kanamura Kiyoshi
Published: September 2007

3DOM silica composite membrane including a binary polymer (PAMPS-SPEES) was fabricated, in which the pore surface was coated with SPEES and the vacant space was occupied by PAMPS. The methanol permeability of composite membrane was successfully reduced from 1.9 x 10-6 cm2 s-1 to 3.2 x 10-7 cm2 s-1 at 30 {degree sign}C by the stepwise filling of SPEES and then PAMPS. On the other hand, the proton conductivity was not significantly influenced by the filling stage, and relatively high values (6.1~7.3 x 10-2 S cm-1 at 30 {degree sign}C in 100% RH) were obtained. This result suggests that the interface region between silica and SPEES have high transfer of protons. Consequently, the PAMPS-SPEES-3DOM composite membrane exhibited more than ten times higher performance than Nafion® as DMFC electrolyte membrane.
Advances in Hybrid Organic-Inorganic Proton Exchange Polymer Membranes Incorporating Heteropoly Acids
Authors: Horan James L. , Aieta Niccolo V. , Leisch Jennifer E. , Kuo Mei-Chen , Dec Steven F. , Herring Andrew M.
Published: September 2007

We describe a new class of materials for proton conduction for proton exchange membrane (PEM) fuel cell applications based on the heteropoly acids (HPAs). HPAs are readily functionalized by a variety of moieties including polymerizable organic groups. These hybrid monomers are polymerized into polypolyoxometallates (polyPOMs). The materials are characterized by IR, NMR, and SAXS. Proton diffusion coefficients are measured by Pulse Field Gradient Spin Echo NMR and are observed to increase with temperature. In-plane proton conductivity measurements show that performance similar to a state-of-the-art PFSA ionomer is obtained at 80 {degree sign}C and 100% RH. Conductivities at lower RH or higher temperature are not yet sufficiently high. For these first generation materials many problems still need to be addressed in order to fabricate materials with all the required properties for a next generation PEM.
Novel Glass Electrolytes for Fuel Cells Operating at Low Temperature
Authors: Thanganathan Uma , Masayuki Nogami
Published: September 2007

A sol-gel method was used as a means of forming stable proton conducting glass materials constituted of heteropolyacids (HPA), i.e phosphotungstic acid (PWA) and phosphomolybdic acid (PMA) and silicaphosphate and these were subsequently investigated and reported on as electrolytes for H2/O2 fuel cells. These devices have attracted a considered amount of research effort in recent years, aimed at providing a better understanding of the factors affecting their performances. The performance of H2/O2 fuel cells based on glass composite membrane is dependent on the ability of the cell to transfer gases to the membrane electrode assembly (MEA) with various electrode thickness at various flow rates. In the present investigation, the performance of a fuel cell was characterized after optimization of its MEA. Thus, a membrane based thereof is a promising electrolyte, applicable to H2/O2 fuel cells operating at room temperature with various flow rates.
Fundamental Characterization of Fullerenes and Their Applications for Proton-Conducting Materials in PEMFC
Authors: Gasa Jeffrey , Wang Hengbin , DeSousa Ryan , Tasaki Ken
Published: September 2007

In the first section of the paper, AC impedance measurements for C60 and fullerene derivatives have been performed to investigate the fundamental proton-conducting characteristics. The unique difference has been observed among various fullerene derivatives in their proton-conducting characteristics. Discussion is given in terms of the differences in proton conducting mechanism. In the second half, applications of fullerene derivatives to fabricate composite membranes with poly(ether ether ketone) are discussed. Our measurements have shown that the fullerene derivative not only reduced the membrane swelling significantly, but increased the conductivity under low humidity conditions.
Titanate Nanotubes as Inorganic Fillers of Nafion Membranes for PEM Fuel Cell Operating at High Temperatures
Authors: Matos Bruno R. , Santiago Elisabete I. , Fonseca Fabio C. , Linardi Marcelo , Lacerda Rodrigo G. , Lavayen Vladimir , Ladeira Luiz O. , Ferlauto André S.
Published: September 2007

Nafion-trititanate nanotubes composites were investigated as electrolytes for proton exchange membrane fuel cells (PEMFC) operating at high temperatures (T). With the addition of 5-15 wt.% of nanotubes to the ionomer, PEMFC performance can be significantly sustained for T up to 130 {degree sign}C. This behavior reflects a competing effect between an increase in water uptake due to the extremely large surface area of the nanotubes and a decrease in proton conductivity of the composites.
Performance of Nafion-TiO2 Hybrids Produced by Sol-Gel Process as Electrolyte for PEMFC Operating at High Temperatures.
Authors: Santiago Elisabete I. , Matos Bruno R. , Fonseca Fabio C. , Linardi Marcelo
Published: September 2007

The fabrication and testing of Nafion-TiO2 hybrid membranes as the electrolyte of proton exchange membrane fuel cell (PEMFC) are reported. A low temperature sol-gel process was used to effectively incorporate the hydrophilic anatase nanoparticles into the Nafion matrix. Fuel cell testing at temperatures up to 130 {degree sign}C revealed that the hybrid membranes exhibit a higher ohmic drop when compared to Nafion. On the other hand, the hybrid membranes show an increased performance with increasing temperature and reduced relative humidity (RH). In addition, it was found that the inorganic phase confers stability to the polymer allowing for the operation at high temperature and reduced RH.
Performance of High-Density Acid Electrolyte and Analysis of Proton Conduction Mechanism
Authors: Hase Kohei , Nakano Tomohiro , Koiwai Akihiko
Published: September 2007

High-density acid model electrolyte was synthesized by sol-gel method and the effect of acid density on proton conductivity was investigated. Model electrolyte showed high proton conductivity (over 0.01 S/cm) at high temperature and low humidity (120 {degree sign}C, below 1 %RH). In model electrolyte, dense ion channel was formed and proton conduction by grotthuss mechanism became dominance than that by vehicle mechanism at low relative humidity.
Voltammetric Surface Dealloying of Pt Bimetallic Nanoparticles: A Novel Synthetic Method Towards more Efficient ORR Electrocatalysts
Authors: Strasser Peter , Koh Shirlaine , Yu Chengfei
Published: September 2007

We report on a novel strategy to modify surface electrocatalytic reactivity of Pt bimetallic nanoparticles for the oxygen reduction reaction (ORR). We voltammetrically dealloy Cu rich Pt-Cu precursor until we obtain a Cu free catalyst surface. This dealloyed Pt surface exhibited up to fivefold improved ORR activities compared to pure Pt nanoparticle catalysts (~ 0.53 A/mgPt and ~650 µA/cm2Pt). Surface area effects could be ruled out as the origin of the activity improvements. We investigate the early stages of the dealloying process for a Pt25Cu75, Pt50Cu50, and a Pt75Cu25 precursor and use a simple qualitative two layer alloy model in order to explain the resulting metal dealloying polarization curves. The model predicts that the critical corrosion (dealloying) potentials of Pt-Cu surfaces of distinct composition varies with Cu monolayers on pure Pt subsurface layers exhibiting the highest corrosion overpotentials. The model is consistent with our and previous experimental observations. The role of geometric and electronic mechanisms to explain the enhanced catalytic reactivity are discussed.
Preparation of Cubic Platinum Nanoparticles of Different Sizes and Their Electrochemical Propeties
Authors: Matsuzawa Koichi , Fukushima Tomoko , Yamada Hirohisa , Inaba Minoru
Published: September 2007

Pt nanoparticles with controlled shapes and sizes would be useful for fundamental researches on the catalytic activity for oxygen reduction reaction. In the present study, cubic Pt nanoparticles were prepared from a solution of K2PtCl4 in the presence of polyacrylic acid (PAA) as a capping reagent. The effect of temperature, pH, and PAA concentration on the shape, size, and stability of the nanoparticles were investigated, and cubic Pt nanoparticles of different sizes in the range of 4.4-10 nm were obtained. The cubic nanoparticles showed strong features of Pt {1 0 0} faces on their cyclic voltammogram under argon atmosphere. Rotating ring disk electrode measurements in O2-saturated H2SO4 solution revealed that the cubic nanoparticles have a high catalytic activity for ORR.
De-Alloyed Pt-M Nanoparticle Electrocatalysts for Efficient Electroreduction of Oxygen: Structure-Activity-Stability Relationships
Authors: Koh Shirlaine , Yu Chengfei , Strasser Peter
Published: September 2007

Contrary to general beliefs, transient instabilities (corrosion) of Pt-Cu system during chemical and electrochemical leaching of Cu leads to a more stable and Pt-richer partially de-alloyed catalysts that exhibit specific activity enhancements of about 3-5 x compared to supported pure Pt electrocatalysts. This study lays the groundwork for understanding the formation of the de-alloyed Pt-Cu catalyst using electrochemical methods the cyclic voltammetry (CV) and linear sweep voltammetry (LSV) for observing the initial dissolution of Cu from the alloyed catalyst. X-ray diffraction, XRD, was performed to obtain crystallographic phases and compositions and Energy Dispersive Spectroscopy, EDS, for elemental compositions before and after electrochemical dissolution of Cu to complement the observations made from electrochemical methods. Initial short term stability testing on the catalyst indicated probable stability with time. Geometric effects due to favorable Pt arrangement were speculated to be the origin of the catalyst's high activity.
Electrochemical Observation of Ligand Effects on Oxygen Reduction at Ligand-Stabilized Pt Nanoparticle Electrocatalysts
Authors: Pietron Jeremy J. , Garsany Yannick , Baturina Olga , Swider-Lyons Karen , Schull Terence
Published: September 2007

We have incorporated small Pt nanoparticles (1.7 {plus minus} 0.5 nm) stabilized with triphenylphosphine triphosphate (Pt-TPPTP) or triphenylphosphine oxide (Pt-TPPT-oxide) into composite polymer-Vulcan carbon thin films and measured the effects of the ligands on the ORR activity of the Pt nanoparticles using rotating disk electrochemistry (RDE). We measure ORR activities for TPPTP-stabilized particles of 0.21 A/mgPt at 0.9 V, which is less than the 0.25 A/mgPt that we measure for the "bare" Pt nanoparticle. However, when normalizing the ORR kinetic current for Pt microscopic surface area, the specific activity, is, for Pt-TPPTP is significantly higher than for the parent nanoparticles (0.41 mA/cm2real vs. 0.29 mA/cm2real), suggesting a kinetic enhancement of the ORR viz. a weakening of the Pt-O bond strength by the TPPTP ligand. Cyclic voltammetry also reveals a positive shift in Pt-oxide electrochemistry for Pt-TPPTP compared to that for bare Pt nanoparticles, which is consistent with the improved area-specific ORR kinetics.
Importance of Cathode Kinetics for Higher Temperature PEM Fuel Cell Applications
Authors: Kunz Harold R. , Mittal Vishal , Xu Hui , Choi Pyoungho , Fenton J.
Published: September 2007

In the evaluation of membranes for use in high temperature PEM fuel cells, the membrane conductivity at the higher temperature and an associated low relative humidity (RH) is a primary property of consideration. However, an electrolyte, or ionomer, must also be within the electrodes to provide for the electrochemical reactions. The properties of the membrane ionomer that influence the kinetics of the electrode reactions must also be considered. Just as low RH has a negative effect on the ionic conductivity of the membrane, low RH can also have a detrimental effect on the rate of the oxygen reduction reaction (1). The reactant permeability through the electrolyte is also an important property of the electrolyte that must be considered. Ultimately, a different ionomer might be used in the membrane and electrodes.
Highly Active Carbon Composite Electrocatalysts for PEM Fuel Cells
Authors: Nallathambi Vijayadurga , Wu Gang , Subramanian Nalini , Kumaraguru Swaminatha , Lee Jong-Won , Popov Branko
Published: September 2007

Highly active carbon composite catalysts were developed using a carbon based metal-free catalyst developed at USC as a support through catalyzed pyrolysis followed by chemical post-treatments. Materials characterization studies indicated that the nature of nitrogen functional groups on the carbon surface and the carbon nanostructures play a critical role in the activity and stability. The catalytic activity as high as 2.0 A cm-2 at 0.2 V was obtained for the carbon composite catalyst in the fuel cell, and no irreversible activity loss was observed during stability test.
New Platinum-Free Carbon Nitride Electrocatalysts for PEMFCs Prepared Using as Precursors PAN/M(CNCH3)x Complexes (M = Pd, Co, Au, Ni)
Authors: Di Noto Vito , Negro Enrico , Piga Matteo , Piga Luigi , Lavina Sandra , Pace Giuseppe
Published: September 2007

A new synthesis protocol aimed at the preparation of palladium-based, platinum-free carbon nitride electroctatalysts for PEMFCs is described. First, a metallorganic precursor is prepared by coordinating metal complexes with a nitrogen-ligand macromolecule; second, a suitable thermal treatment is applied to the precursor to obtain the final electrocatalyst materials. This protocol allows to prepare materials having a well-controlled chemical composition and including in their structure a significant amount of nitrogen. These systems proved to be more resistant to oxidative decomposition with respect to standard carbon-supported electrocatalysts. The temperature of the final thermal treatment tf is a crucial parameter to modulate in order to control the final electrochemical properties of these materials. A significant mass activity in the HOR and the ORR was observed, making this protocol feasible for the preparation of promising electrocatalyst materials for PEMFCs and DMFCs.
Establishing the Mechanism for Oxygen Reduction on Au/SnOx Using In Situ X-ray Absorption Spectroscopy
Authors: Gatewood Daniel , Ramaker David , Sasaki Kotaro , Swider-Lyons Karen
Published: September 2007

Gold nanoparticles supported on hydrous tin-oxide (Au-SnOx) below 0.5V vs. RHE are active for the 4-electron oxygen reduction reaction (ORR) in acid electrolyte. In comparison, gold on Vulcan carbon (Au/VC) is inactive for ORR and generates mostly peroxide (i.e. 2-electron reduction) rather than water. These results indicate a significant support effect from SnOx which enhances the activity of the Au. This metal support interaction was studied in this work using in situ X-ray absorption spectroscopy (EXAFS and XANES) at the Au L3 and Sn K edges in an electrochemical cell. The results show that a bifunctional mechanism plays the dominant role in the ORR reaction on Au/SnOx. The O2 adsorbs and dissociates on the SnOx surface, and then is reduced on the Au either directly or after conversion to O[H] groups. Enhanced O[H] adsorption occurs at potentials above 1.2 V on the Au; this adsorption is enhanced through a ligand mechanism involving the SnO support, but this has no apparent effect on the ORR occurring at lower potentials
Study on Degradation of the Pt-Ru Anode of PEFC for Residential Application (1): Degradation of the Anode Performance in Long-Term Operation of Single Cell
Authors: Yamazaki Osamu , Oomori Yasuharu , Shintaku Hideki , Tabata Takeshi
Published: September 2007

Durability tests of Membrane Electrode Assemblies (MEA) of Polymer Electrolyte Fuel Cell (PEFC) for more than 30,000 h have been carried out using a simulated reformed gas (SRG) without air bleed under the conditions of residential application. When SRG; CO(10ppm) / CO2(20%) / H2(balance) was used as fuel gas, relatively rapid and accelerated voltage decay was observed, while the voltage trend showed gradual decay in the case of H2. The trends of I-V curves, O2 gain and H2 gain indicate that not only the degradation of diffusivity of cathode but also the degradation of CO tolerance of anode is an important degradation factor for long term operation of the residential PEFC.
The Use of Heteropoly Acids as Supports for Low, Ultra-Stable Pt Electrocatalysts
Authors: Herring Andrew M. , Stanis Ronald J. , Kuo Mei-Chen , Turner John A.
Published: September 2007

We have successfully immobilized several HPA on carbons using covalent attachment. The morphology of these materials is still under investigation, but the hybrid HPA carbons can be used to stabilize nano-Pt particles. The fuel cell performance is not improved over a suitable control electrode. If the correct HPA is used it appears that significant advantages can be obtained in fuel cell catalyst layer stability. Stability over 4000 cycles has been achieved. Encouragingly the F- emission rates from the MEA can be dramatically reduced.
Comparative Investigation on Nano-Sized SiO2 as a Filler for Proton Exchange Membranes (PEM) Fuel Cells
Authors: Saccà Ada , Pedicini Rolando , Carbone Alessandra , Gatto Irene , Passalacqua Enza
Published: September 2007

A commercial hygroscope SiO2 was used as an inorganic filler in a Nafion® polymeric matrix. The influence on chemical-physical properties and electrochemical behaviour in Polymer Electrolyte Fuel Cells (PEFCs) was investigated in terms of amount and particle size of the introduced inorganic compound. Such SiO2 has got a good water retention capacity that improves the water uptake and contributes to improve the stiffness of the developed membranes, probably making more efficacy in the medium temperature operation. Current density values of about 720mW/cm2 and 640W/cm2 at 80{degree sign}C and 110{degree sign}C (0.6V and 100%RH), respectively were reached from the N-SiO2 membrane containing a 5%wt/wt of oxide with a particle size of 7 nm. Such result compared to a home-made bare Nafion membrane showed an important improvement, high lightening the benefit of the inorganic filler when PEFC operates at medium temperature.
Neutron Radiography Imaging of Simulated Non-Isothermal Start-up of a Polymer Electrolyte Fuel Cell
Authors: Fu Richard , Pasaogullari Ugur , Hussey Daniel S. , Jacobson David L. , Arif Muhammad
Published: September 2007

Neutron imaging of a simulated non-isothermal start-up of a fuel cell is performed using the Neutron Imaging Facility at the National Institute of Standards (NIST) Center for Neutron Research. The cell is started from 40 ºC and heated up to operating temperature of 80 ºC using a predefined cell temperature profile. To ensure artifact free imaging, the cell hardware (i.e. end plates, which also contain the flow field and acts as current collector) is made of gold plated aluminum alloy, and multiple (two per plate) cartridge heaters are used to ensure uniformity of cell temperature. In order to ensure a constant inlet relative humidity, the humidifier heaters are also controlled to maintain a desired dew point temperature. It is shown that a relatively constant inlet relative humidity is achieved. Our results show that there is a lag between the total liquid water volume and the temperature profile.
Imaging of Water Profiles in PEM Fuel Cells Using Neutron Radiography: Effect of Operating Conditions and GDL Composition
Authors: Mukundan R. , Davey John R. , Rockward Tommy , Spendelow Jacob S. , Pivovar Bryan , Hussey Daniel S. , Jacobson David L. , Arif Muhammad , Borup Rod
Published: September 2007

The performance of polymer electrolyte membrane (PEM) fuel cells as a function of cathode inlet relative humidity (RH) and gas diffusion layer (GDL) properties has been characterized. The performance of 50 cm2 fuel cells at high current densities was a strong function of the polytetrafluoroethylene (PTFE) content in the cathode GDL microporous layer (MPL). The voltage at a current density of 1.4 A cm-2 decreased at all inlet RHs as the PTFE content in the cathode MPL increased from 5 % by weight to 23 % by weight. This was associated with a corresponding increase in the mass transport resistance as measured by AC impedance. The low frequency resistance also increased with increasing cathode inlet RH. These results were validated by high-resolution neutron radiography on specially designed 2.25 cm2 cells that showed increased water content in the GDLs at high inlet RHs and high microporous layer PTFE content. High-resolution neutron imaging also revealed higher water concentrations at the outlets, cathode GDL, anode flow channel, and MEA/GDL above the land when compared to the inlets, anode GDL, cathode flow channel, and MEA/GDL above the channel respectively.
In-situ Measurement of Water Vapor and Oxygen Concentration in a PEMFC Channel by Tunable Diode Laser Absorption Spectroscopy Techniques
Authors: Fujii Yoshinobu , Tsushima Shohji , Hirai Shuichiro
Published: September 2007

n this study in-situ measurements of water vapor and oxygen concentration in gas flow channels in an operating polymer electrolyte membrane fuel cell (PEMFC) are obtained using Tunable Diode Laser Absorption Spectroscopy (TDLAS). Wavelengths of the diode laser are 1392nm for measurement of water vapor concentration and 760nm for measurement of oxygen concentration, respectively. It was successfully demonstrated that the optical remote sensing based on TDLAS techniques can detect local variations of oxygen and water vapor concentration in the channel of a PEMFCs under load.
Determination of Electrode Sheet Resistance in Cathode Catalyst Layer by AC Impedance
Authors: Liu Yuxiu , Murphy Michael , Baker Daniel , Gu Wenbin , Ji Chunxin , Jorne Jacob , Gasteiger Hubert A.
Published: September 2007

Proton conduction resistance in the cathode electrode with various ionomer/carbon weight ratios (I/C-ratios) was characterized using AC impedance at different relative humidities (RH) in a H2/N2 cell. AC impedance data were analyzed by fitting sheet resistance, Rsheet, sheet capacitance, Csheet, and the high-frequency resistance, HFR, to a simplified transmission-line model with the assumption that the proton resistance and the pseudo-capacitance are distributed uniformly throughout the electrode. For given types of model membrane electrode assemblies (MEAs), a percolation threshold for proton conductivity in the electrode was observed at I/C-ratios of approximately <0.6/1. The comparison to H2/O2 fuel cell performance shows that the ohmic loss in the catalyst layer can be quantified by this technique, and the parameter Rsheet extracted from the AC impedance spectra is indeed an intrinsic RH-dependent electrode property.
Pt Utilization Analysis Using CO Adsorption
Authors: Shinozaki Kazuma , Hatanaka Tatsuya , Morimoto Yu
Published: September 2007

Methods for estimating the Pt utilization of an MEA, the ratio of the electrochemically active Pt surface area of the MEA to the Pt surface area of the catalyst, were examined. The most reliable and accurate estimation was obtained by the method which uses CO as the probe for both the surface area. With this method, the Pt utilization of our conventional MEA was found ca. 90%, which is much higher than the values other widely-used methods predicted.
Performance of PEM Fuel Cells at Sub-Freezing Temperatures
Authors: Mukundan R. , Kim Yu Seung , Rockward Tommy , Davey John R. , Pivovar Bryan , Hussey Daniel S. , Jacobson David L. , Arif Muhammad , Borup Rod
Published: September 2007

The performance of single-PEM fuel cells at sub-freezing temperatures has been characterized. Fully humidified fuel cells that were subjected to multiple freeze/thaw cycles down to -40 oC showed degradation in performance associated with mass transport problems at the gas diffusion layer. The performance loss was negligible for cloth GDLs and slow cooling rates while fast cooling rates and paper GDLs showed significant degradation. When a single-fuel cell was operated at -10 oC at a constant voltage, the current first increased to a maximum value, then started decaying due to ice formation. This ice formation was monitored using neutron radiography and was found to be concentrated near the inlets, outlets and cell edges resulting in a significant increase in mass transport resistance as revealed by AC impedance spectroscopy. Cyclic voltammograms at -10 oC show no change in catalyst electrochemical surface area associated with this decreased performance. Finally the cell did not exhibit any degradation in performance at 80 oC even after 5 separate operations at -10 oC, where the cell was allowed to completely clog with ice each time until no further current was produced.