bluuebunny/crossref_metadata_2025 huggingface dataset:
(Invited) Study of Minority Carrier Lifetime Killer by Synchrotron X-Ray Topography
Authors: Goue Ouloide Yannick , Guo Jianqiu , Yang Yu , Raghothamachar Balaji , Dudley Michael
Published: September 2016

The lifetime maps for a 2” n-type 4H-SiC quarter wafer (sample 1) and a 4” diameter half wafer (sample 2) were recorded using microwave photoconductive decay (μPCD) measurements and correlated with the synchrotron X-ray topography (SXRT) map of structural defects. The lifetime map of sample 1 revealed a drastic reduction in carrier lifetime along the edges and inside one of its facets (facet 1), correlated with microcracks, BPD loops, overlapping triangular defects, high density of multi-layer Shockley stacking faults (SFs) and low angle grain boundaries (LAGBs). Similarly, sample 2 showed a reduced carrier lifetime along the edges corresponding to regions of high density of overlapping triangular defects, microcracks and BPD loops in the SXRT map. Shorter lifetime observed in the middle region of sample 2 correlated with networks of interfacial dislocations (IDs) and half loop arrays (HLAs) originating from 3C inclusions that were generated during epilayers growth. Further analysis of the SXRT maps revealed the SFs in facet 1 were double Shockley stacking faults (DSSFs) that have likely nucleated from scratches present on the substrate surface and LAGBs present in that region, which propagated during epilayer growth. We discuss the mitigated influence of numerous morphological defects observed on the surface of both epilayer samples.
Investigation of Penetration Depth and Defect Image Contrast Formation in Grazing Incidence X-ray Topography of 4H-SiC Wafers
Authors: Yang Yu , Guo Jianqiu , Goue Ouloide Yannick , Raghothamachar Balaji , Dudley Michael , Chung G , Sanchez E , Maning I
Published: September 2016

Techniques utilizing the grazing incidence geometry are particularly useful for discerning defects at different depths below the crystal surface particularly for 4H-SiC epitaxial wafers. The penetration depths calculated theoretically by just considering a dislocation as simple line defect are much smaller than those measured experimentally. In our study we have developed and compared two models involving different diffracting volumes to account for this discrepancy. In one model, the image contrast is formed by simulating the density map of the diffracted beam according to the local lattice plane tilt assuming the dislocation image formation is dominated by orientation contrast. In the second model, diffracting volume for kinematical diffraction is defined by the surface over which the effective misorientation associated with the dislocation is equal to the rocking curve width of dislocations with different Burgers vectors. By comparing between measured penetration-depths in grazing incidence geometry with these two approaches, our study show that both model play a role in dislocation contrast formation mechanism.
Evaluation of Different High Κ Materials for In situ Growth of Carbon Nanotubes
Authors: Keyn Martin , Krauss Tillmann Adrian , Kramer Andreas , Schwalke Udo
Published: September 2016

In this work we investigate the in situ growth of carbon nanotubes (CNTs) on different dielectric stacks for use in discrete field-effect transistor devices. While CNT growth is demonstrated on all stacks, only devices fabricated on atomic layer deposited aluminum oxide show sufficiently reliable gate dielectrics. Electrical burn pulses are applied to the devices to selectively remove undesired metallic CNTs in order to obtain proper transistor behavior with an on/off current ratio of five orders of magnitude.
(Invited) Compliance-Free Pulse Forming of Filamentary RRAM
Authors: Shrestha Pragya , Nminibapiel David Malien , Kim J. H. , Baumgart Helmut , Cheung K. P. , Campbell J P
Published: September 2016

Despite the overwhelming effort to improve the efficacy of resistive random access memory (RRAM), the underlying physics governing RRAM operation have proven elusive. A survey of the recent literature almost universally indicates that the remaining glaring issues center around variability as well as endurance. The initial filamentary “forming” process is often linked to these problems. This work details our recent efforts to bring the forming process under control and the resulting improvements in RRAM viability in hafnia-based devices. We track the forming process via a “forming energy” metric that allows for filament optimization. By removal of all current compliance elements, and their associated parasitics, a targeted forming energy is achieved using ultrashort voltage pulses. By tailoring the forming energy, we show remarkable endurance window control.
Impact of Etch Process on Hafnium Dioxide Based Nanoscale RRAM Devices
Authors: Beckmann Karsten , Holt Josh , Olin-Ammentorp Wilkie , Van Nostrand Joseph , Cady Nathaniel
Published: September 2016

Resistive Random Access Memory (RRAM) is a novel form of non-volatile memory that is expected to play a major role in future computing and memory solutions. We have developed memristive RRAM devices and integrated logic devices on a 300mm wafer platform with the IBM 65nm 10LPe process technology. The RRAM device consists of an inert tungsten bottom electrode, HfO2 based active switching layer, a Ti oxygen exchange layer and a TiN top electrode. The TiN/Ti capping layers were etched with reactive ion etch (RIE), whereas two approaches were tested for the HfO2 removal: a wet etch with diluted hydrofluoric acid, and a BCl3/O2 based RIE process. The impact on the RRAM devices for both etch processes was investigated with respect to the physical structure, the electrical characteristics, and yield. The electrical characteristics include simple I-V DC sweeps and pulse-based endurance measurements.
Resistance Switching in Individual Hydrogen Silsesquioxane (HSQ) Nanopillars
Authors: Ng Wing Hung , Buckwell Mark , Mehonic Adnan , Kenyon Anthony Joseph
Published: September 2016

The ability to fabricate nanometre-scale Resistive Random Access Memory (ReRAM) devices is of interest both to provide a fundamental understanding of resistance switching mechanisms and ultimate scaling limits, and also to precisely define the position of conductive filaments in devices. In this paper, we report the observation of resistance switching of individual hydrogen silsesquioxane (HSQ) nanopillars. The nanopillar array structure was based on a metal-insulator-metal (MIM) configuration and fabricated using electron beam lithography with Ti and Mo as top and bottom electrodes, respectively. The 130 nm diameter nanopillars were biased using a conductive atomic force microscope (CAFM) tip. Subsequent probing of the nanopillar array showed an increase of conductivity of individual pillars. This indicates that we are able to address a single nanopillar as a switchable element in a ReRAM array.
(Invited) Epitaxial Oxides on Silicon for CMOS and Beyond
Authors: Osten H. Joerg
Published: September 2016

A very promising way to realize advanced future devices is using single-crystalline, closely lattice matched oxides, which will be grown epitaxially on the substrate of choice. We present results for crystalline gadolinium oxides on silicon grown by solid source molecular beam epitaxy. The dielectric properties of such oxides are sensitive to small variations in structure and symmetry. For example, thin crystalline Gd2O3 films epitaxially grown on silicon exhibit dielectric constants above 20 although the known bulk value is only around 14. The reason for that “enhancement effect” is not fully understood yet. Here, we report about different investigations on strain-induced effects on dielectric properties. We explain these effects by severe strain induced structural phase deformations. Further, dielectric properties of epitaxial oxide thin films have been found to improve significantly by incorporation of suitable dopants. To achieve optimum electrical properties from such doped oxides it is important to understand the correlation between doping and the electronic structure of the material. Finally, we will demonstrate different approaches to grow Si nanostructures embedded into crystalline rare earth oxides. By efficiently exploiting the growth kinetics one could create nanostructures exhibiting various dimensions, ranging from three dimensionally confined quantum dots to the quantum wells, where the carriers are confined in only one of the dimensions.
Thermoelectric Properties of Highly Ordered Metal-Organic Framework Films
Authors: Chen Xin , Wang Zhengbang , Lin Pengtao , Zhang Kai , Baumgart Helmut , Redel Engelbert , Wöll Christof
Published: September 2016

In this work, quasi-liquid epitaxially highly oriented Surface Anchored Metal-Organic Framework (SURMOF) films and for comparison random polycrystalline MOF films loaded with tetracyano-quinodimethane (TCNQ) infiltration were fabricated and characterized. The horizontal Seebeck coefficient of the oriented SURMOF films and the random polycrystalline MOF films parallel to the sample surface was measured and has been discussed. The polycrystalline MOF films exhibit a high Seebeck coefficient of 949.9 µV/K at 290 K, while the horizontal Seebeck coefficient of oriented SURMOF films is practically around 0 µV/K. Because the quasi-epitaxial oriented SURMOF films are highly anisotropic, there is no measurable horizontal carrier transport parallel to the SURMOF surface. However in contrast to oriented SURMOF films, the electrical properties of random polycrystalline MOF films with sputtered Au contact pads could be measured. The high Seebeck coefficient of these random polycrystalline MOF films demonstrate a promising application potential of MOF films in future thermoelectric and electronic devices.
Enhanced Photocurrents in the Vertically Aligned Hybrid CdTe-Si One-Dimensional Nanostructures with High Surface Area
Authors: Lim Jinho , Hwang Sung-Hwan , Jung Hyunsung
Published: September 2016

One-dimensional hybrid CdTe nanostructures on Si nanowires in a wafer batch were synthesized by wet chemical processes. Si nanowires were vertically aligned by the employment of Ag nanoparticles as a catalyst of an electroless etching. The etched Si nanowires were utilized as sacrificial materials for the galvanic displacement reaction to Te nanostructures. The hybrid CdTe on Si nanostructures were created by the wet chemical reaction of the Te nanostructures prepared on aligned Si nanowires. The electrical and optical properties of the fabricated hybrid CdTe-Si nanostructures were characterized. Especially, the one-dimensional hybrid CdTe-Si nanostructures with high surface area demonstrated the enhanced optical current densities. The vertically aligned one-dimensional hybrid CdTe-Si nanostructures can propose an efficient route for the synthesis of hybrid hierarchical nanostructures with high electrical and optical properties in a wafer batch
(Plenary) Doing More with Less: Challenges for Catalyst Layer Design
Authors: Putz Andreas Michael Vincent , Susac Darija , Berejnov Viatcheslav , Wu Juan , Hitchcock Adam P , Stumper Jürgen
Published: September 2016

This paper discusses key elements supporting continuing progress towards the achievement of the automotive PEMFC commercialization targets, in particular the reduction of total precious metal (PGM) content. An approach for PGM loading reduction without associated increase in mass transport loss is proposed, recent progress in advanced CL structural characterization/imaging is reviewed and the reliability of loss breakdown as guide for further performance improvement is discussed. A list of bulk and interface functionalities for the cathode catalyst layer is defined and the availability of measurement tools for the associated properties reviewed.
Accelerated Degradation of Polymer Electrolyte Membrane Fuel Cell Gas Diffusion Layers: Mass Transport Resistance and Liquid Water Accumulation at Limiting Current Density with in operando Synchrotron X-ray Radiography
Authors: George Michael G , Liu Hang , Banerjee Rupak , Ge Nan , Shrestha Pranay , Muirhead Daniel , Lee Jongmin , Chevalier Stéphane , Hinebaugh James , Messerschmidt Matthias , Zeis Roswitha , Scholta Joachim , Bazylak Aimy
Published: September 2016

An SGL 29BC gas diffusion layer (GDL) was degraded (aged) in hydrogen peroxide. The impact of ageing was measured through deviations from pristine GDLs in terms of limiting current densities, oxygen mass transport resistances, and liquid water spatial distributions in the GDL. For dry and saturated cathode conditions (oxygen concentrations of 1% and 21%, respectively) the fuel cell containing the aged GDL reached limiting current densities that were up to 10.3% lower and experienced increases in oxygen mass transport resistance of up to 11.7% versus the pristine GDL. This performance reduction was attributed to a higher liquid water content (13% more at an oxygen concentration of 21%) in the aged GDL under the channel compared to the pristine GDL. In fuel cells built with aged and pristine GDLs, up to 200% more water was present in the GDL under the land compared to under the channel at limiting current operation.
The Effect of Nitrogen Cross-Over on Water Balance Measurements in Proton Exchange Membrane Fuel Cell Using Constant Temperature Anemometry
Authors: Al Shakhshir Saher , Berning Torsten , Kær Søren Knudsen
Published: September 2016

A novel method to obtain an ad-hoc and real time electrical signal of the PEMFC water balance by employing a constant temperature hot wire anemometry has been developed by our fuel cell research group. In this work, the effect of nitrogen-cross over on this method is experimentally demonstrated by introducing 1% of nitrogen concentration to the dry and humidified hydrogen flow simulating the PEMFC anode outlet. The hot wire voltage is measured with and without nitrogen and it was slightly lower with the presence of nitrogen. The effect of the voltage reduction on the measured water balance is small. This is due to the effect of 1% nitrogen on power law constant’s “m” which can be used in determining the water balance as explained in previous work was extremely low. Thus, the hot wire technique for measuring the PEMFC water balance is still accurate with the nitrogen cross-over.
Investigating the Equilibration Time of Catalyst Coated Membranes Using AC and DC Methods
Authors: Côté Philippe J. , Cloutier Caroline R. , Malevich Dzmitry , Pharoah Jon G.
Published: September 2016

Methods to quantitatively measure the time for rolls of catalyst coated membrane to equilibrate to a controlled temperature and relative humidity were investigated. It is important to ensure that the water content of a catalyst coated membrane roll has equilibrated before it is used in the manufacturing of a proton exchange membrane fuel cell stack. Equilibration is achieved when the water content in the catalyst coated membrane reaches equilibrium with controlled relative humidity and temperature conditions. Alternating current and direct current methods were compared to measure the through-plane ionic resistance and in-plane electronic resistance as the catalyst coated membrane equilibrated. It was found that both alternating and direct current methods showed strong correlations as the CCM equilibrated in response to a relative humidity change. The average equilibration time estimated from the through-plane ionic measurements was 14.5 hours and the average equilibration time estimated from the in-plane electronic measurements was 13 hours.
Molecular Dynamics Simulation of Oxygen Diffusivity, Solubility, and Permeability in Ionomer on Pt Surface
Authors: Kurihara Yuya , Mabuchi Takuya , Tokumasu Takashi
Published: September 2016

We analyzed the dependence of the structure properties and the permeation properties of oxygen molecules in ionomer on water content. We constructed a system in which oxygen molecules permeated ionomer on a Pt surface. The ionomer/gas interface, bulk region, and ionomer/Pt interface were defined based on the density distribution of ionomer at each water content. Furthermore, the oxygen diffusivity, solubility, and permeability in each region of the ionomer were evaluated to analyze the oxygen permeation properties. As a result, the oxygen permeability in the ionomer/Pt interface is the lowest, for all water content. Moreover, comparing the contributions of the oxygen diffusivity and solubility to the oxygen permeability, it was found that the oxygen solubility plays a dominant role in the oxygen permeability, indicating that the oxygen solubility in the ionomer/Pt interface is the dominant factor in the oxygen permeability in the ionomer.
Direct Simulations of Coupled Transport and Reaction on Nano-Scale X-Ray Computed Tomography Images of Platinum Group Metal-Free Catalyst Cathodes
Authors: Ogawa Shohei , Komini Babu Siddharth , Chung Hoon T , Zelenay Piotr , Litster Shawn
Published: September 2016

The nano/micro-scale geometry of polymer electrolyte fuel cell (PEFC) catalyst layers critically affects cell performance. The small length scales and complex structure of these composite layers make it challenging to analyze cell performance and physics at the particle scale by experiment. Herein, we present a computational method to simulate transport and chemical reaction phenomena at the pore/particle-scale and apply it to a PEFC cathode with platinum group metal free (PGM-free) catalyst. We numerically solve the governing equations for the physics with heterogeneous oxygen diffusion coefficient and proton conductivity evaluated using the actual electrode structure and ionomer distribution obtained using nano-scale resolution X-ray computed tomography (nano-CT). Using this approach, the oxygen concentration and electrolyte potential distributions imposed by the oxygen reduction reaction are solved and the impact of the catalyst layer structure on performance is evaluated.
Gas Diffusion Layer Coated with a Microporous Layer Containing Hydrophilic CNTs to Enhance PEFC Performance without Humidification Using Anode Gas Recirculation
Authors: Kitahara Tatsumi , Nakajima Hironori , Ishikawa Kenta
Published: September 2016

The present study was carried out to clarify the effect of anode gas recirculation on PEFC performance without humidification. Raising the gas flow velocity in the channel to 2 m s-1 by increasing the anode gas recirculation promotes water transport from the anode gas to the membrane. Decreasing the pore diameter of a microporous layer (MPL)-coated gas diffusion layer (GDL) at the cathode is effective at preventing membrane dehydration, which enhances PEFC performance without humidification. However, decreasing the MPL pore diameter significantly increases the water breakthrough pressure, reducing the ability of the MPL to reduce the flooding under high humidity conditions. When using an MPL containing hydrophilic carbon nanotubes, it is possible to decrease the pore diameter without raising the water breakthrough pressure. This is effective at enhancing PEFC performance under both no and high humidity conditions, compared with that obtained for a hydrophobic MPL-coated GDL.
Scale Model Experiment of Liquid Water Transport in Gas Diffusion Layer of PEFC and Optimization of the Structure
Authors: Sakaida Satoshi , Tabe Yutaka , Chikahisa Takemi
Published: September 2016

In a polymer electrolyte fuel cell (PEFC), it is important to control water transport in the cell to supply oxygen smoothly to the reaction zone. Produced water in the catalyst layer transports through micro-porous layer (MPL), gas diffusion layer (GDL), and then reaches to the channel. All of the layers are important for the water control, and this study focuses on the water transport behavior in the GDL, which has complex fiber structure. For observation of the motion, a scale model experiment was developed using silicon oil and water to keep Capillary number equivalent to the actual cell. The fiber structure of the GDL was fabricated with a 3D printer. With the scale mode, effects of GDL property such as wettability on the water behavior were investigated, and the results were compared to the numerical simulation with Lattice Boltzmann Method.
Acceleration of Liquid Water Removal from Cathode Electrode of PEFC by Combination of Channel Hydrophilization and Diffusion Medium Perforation
Authors: Nishida Kosuke , Kono Yudai , Sato Masahiko , Mizuguchi Daisuke
Published: September 2016

To accelerate through-plane water removal from catalyst layer (CL) to gas channel in polymer electrolyte fuel cells (PEFCs), this study presented the novel combinational structure with hydrophilized channel and perforated gas diffusion electrode (GDE). The effect of channel hydrophilization on the water droplet behavior in a PEFC was firstly investigated with cross-sectional visualization method. Furthermore, the performance enhancement induced by introducing the combinational structure was demonstrated under high-current and high-humidity operations. The perforated electrode structure was fabricated by electric discharge machining (EDM) and manual micro-drilling technique. The influence of wettability of penetration hole on the water discharge through the diffusion medium was also discussed. It was found that hydrophilic coating on the cathode channel surface positively sucks up water droplets emerging from the GDL surface. The combinational structure of channel hydrophilization and GDE perforation further promotes the through-plane water discharge, contributing to a reduction in liquid flooding at the CL.
Performance Benefits of Multiwall Carbon Nanotubes in the Polymer Electrolyte Membrane Fuel Cell Gas Diffusion Layer
Authors: Lee Jongmin , Banerjee Rupak , Ge Nan , Chevalier Stéphane , George Michael Gerald , Liu Hang , Shrestha Pranay , Muirhead Daniel , Hinebaugh James , Bazylak Aimy
Published: September 2016

A commercial gas diffusion layer (GDL) with a microporous layer (MPL) containing multiwall carbon nanotubes (MWCNT) exhibited better performance in a polymer electrolyte membrane (PEM) fuel cell than that with a conventional GDL. This performance benefit was attributed to improved oxygen mass transport in the cathode GDL. The operation of the fuel cell was visualized with synchrotron X-ray radiography to measure the liquid water saturation in the two GDLs. A higher liquid water saturation was measured in the operating fuel with the MWCNT-based MPL than with the conventional MPL. But, the MWCNTs induced higher effective porosity within the GDL, allowing for improved overall oxygen transport, and thus the cell performed better even with higher saturation of liquid water in the GDL.
Neutron Imaging Detector with Scintillation Light Magnification and Amplification
Authors: Hussey Daniel S , LaManna Jacob M , Baltic Elias , Jacobson David L
Published: September 2016

In order to apply neutron radiography to the study of the water saturation levels in the membrane electrode assembly of proton exchange membrane fuel cells, the spatial resolution of the method must reach 1 µm, an improvement of a factor of 10 over current state of the art. Gadolinium oxysulfide scintillators (also called phosphors) provide high spatial resolution of ~20 µm for neutron imaging due to the ~20 µm thick deposit required for about ~80% thermal neutron stopping power. However, since the conversion electrons carry about ~50 keV to 70 keV of the energy of the nuclear reaction, the light output of these scintillators is low compared to Li based materials. Using an image intensifier coupled to a low noise sCMOS camera we show that we improve the signal to noise ratio by a factor of more than 20. Further, by magnifying the scintillation light with a macroscope configuration, amplifying the light with an image intensifier and acquiring short exposure time frames, we are able to capture individual neutron capture events and perform centroid analysis to improve the spatial resolution of the scintillator by at least a factor of 3.
MRI Investigation of Local Diffusion Phenomena in PEFC Catalyst Ink
Authors: Kameya Yuki , Iriguchi Norio , Uemura Suguru , Sasabe Takashi , Yoshida Toshihiko , Hirai Shuichiro
Published: September 2016

The catalyst layer (CL) of polymer electrolyte fuel cells plays a critical role in their operation and also the production process of CL is of significant importance in terms of the cell performance as well as the system cost. Herein, we propose a novel approach to investigate the characteristics of catalyst inks by using magnetic resonance imaging (MRI) combined with nuclear magnetic resonance (NMR) spectroscopy. We developed a technique to acquire NMR spectra from a selected volume of a liquid sample and determine the local diffusion coefficients of component molecules. It was found that the diffusion coefficients of both water and n-propyl alcohol (NPA) remarkably changed depending on the mixuture composition, which indicates the change in molecular microstructure. In addition, we obtained MRI images of catalyst inks and the precipitated bed of carbon particles was visualized. We found the dependence of the local diffusion coefficient of water in the particle bed region on the diffusion time, which suggested that the molecular diffusion of solvent molecules were distracted by the closely-packed carbon particles. Furthermore, 19F-NMR spectroscopy of Nafion dispersions suggested the change in the dispersion state of Nafion molecuels. We demonstrated that our approach provided a unique capability to investigate the catalyst ink and its decal process. It will be proved that this technique is useful in investigating many samples prepared at various conditions and better understand the decal process.
Durability of PtCo/C Cathode Catalyst Layers Subjected to Accelerated Stress Testing
Authors: Macauley Natalia , Mukundan Rangachary , Langlois David A. , Neyerlin K.C. , Kocha Shyam S. , More Karren L. , Odgaard Madeleine , Borup Rod L.
Published: September 2016

In this paper we evaluate the durability of two PtCo/C alloycatalysts that have different initial particle sizes by usingaccelerated stress tests. The durability test results are comparedwith a commercially available Pt/C catalyst. The implications ofusing PtCo/C alloy catalysts are discussed, with results presentedfrom TEM, SEM, XRF, XRD and electrochemical testing. Theinitial ECSAs of the PtCo/C catalysts are lower than Pt/C, howeverby the end of the AST, the ECSAs are nearly identical as theparticle sizes all approach approximately 5.5 nm. The mass activityof PtCo/C catalysts is higher than Pt/C, thus leading to higherkinetic performance. The durability and activity of PtCo/Ccatalysts was found to be comparable or better than that of the Pt/Cbased catalysts. However, gradual Co leaching from the alloystructure continues to be an issue. Co was found to essentiallyequilibrate between the catalyst layer and the membrane.
Ultralow Degradation Rates in HT-PEM Fuel Cells
Authors: Rastedt Maren , Pinar Francisco Javier , Wagner Peter , García Héctor R. , Steenberg Thomas , Hjuler Hans A. , Paidar Martin , Bouzek Karel
Published: September 2016

One of the major advantages of polybenzimidazole (PBI) based high temperature polymer electrolyte (HT-PEM) fuel cells compared to the low temperature representatives of this type of fuel cell technology is the higher tolerance against impurities like CO. Nevertheless lifetime and durability are still an issue. In this work, an improvement in degradation rates and therefore an extension of lifetime of HT-PEM fuel cells will be presented. Extended long term tests have been performed at different facilities under similar test conditions. After 3,000 h of operation, an average degradation rate of -1.7 µV/h has been achieved. One of the tests is still under operation; this MEA already reached a lifetime more than 9,000 hours with an actual degradation rate of -3 µV/h.
Investigation of Degradation at the PEFC Cathode Layer under Higher Temperature Operation
Authors: Miyamoto Hidemasa , Kitamura Masahiko , Noda Zhiyun , Sasaki Kazunari , Hayashi Akari
Published: September 2016

Toward the development of electrocatalysts for future higher temperature PEFCs, extraction of critical issues for higher temperature PEFCs was aimed. Degradation phenomena and mechanisms at 105 oC were evaluated for both Aquivion® MEA and Nafion® MEA. The correlation between proton conductivity and changes in cathode structures on the acceleration degradation test was studied in detail. As a result, even though the macroporous structure of the cathode layer was homogeneous in Aquivion® MEA, larger pores almost intensely formed near the Nafion® membrane side. We have found that the carbon oxidation reaction is deeply related to H2O produced by a fuel cell reaction if it is under the low humidification condition. Therefore, when proton conductivity is low like Nafion® MEA, H2O is mainly generated at the interface between the cathode layer and Nafion® membrane and carbon oxidation intensely occurs at the interface.
An FIB-SEM Study on Correlations between PEFC Electrocatalyst Microstructure and Cell Performance
Authors: Okumura Makito , Noda Zhiyun , Matsuda Junko , Nishihara Masamichi , Hayashi Akari , Sasaki Kazunari
Published: September 2016

The electrocatalyst layer of the polymer electrolyte fuel cells (PEFCs) has a complicated porous microstructure. Since the porous nanostructure allows the transport of reactant gas, water vapor, protons, and electrons, cell performance depends strongly on their 3D microstructure. Understanding the correlations between the electrocatalyst microstructure and the cell performance is therefore essential for further improving the PEFC electrochemical performance. In this study, these correlations are quantitatively considered by using the focused-ion-beam coupled scanning electron microscopy (FIB-SEM) technique. In particular, while it is difficult to distinguish between the solid and the pore of the electrocatalyst layer due to the complicated porous nanostructure, this problem has been solved by the auto-image thresholding technique.
Non-Destructive Optical Measurement of Oxygen Concentration on PEFC Catalyst Layer
Authors: Uemura Suguru , Yoshida Toshihiko , Jao Ting-Chu , Sasabe Takashi , Hirai Shuichiro
Published: September 2016

Insufficient supply of oxygen (O2) to the cathode Pt catalyst layer can be caused by transport resistance, and is a major problem in proton exchange membrane fuel cells. To clarify O2 transport phenomena, a non-destructive technique for measuring the O2 concentration is required. In this study, the O2 concentration at the catalyst layer was measured under operating conditions, using a tiny optical fiber and oxygen indicator. The O2 concentration decreased continuously with decreasing cell voltage. It was considered that a decrease in size of the reaction area was caused by water accumulation under the rib. Local O2 concentration measurements suggested that power generation was concentrated at the channel region.
Model and Experimental Analysis of Transport Resistances of Oxygen in Various Structures of PEFC Catalyst Layer
Authors: Hayashi Tomoya , Tabe Yutaka , Chikahisa Takemi
Published: September 2016

For efficient use of Pt catalyst in polymer electrolyte membrane fuel cells, the effects of cathode catalyst layer (CL) structure on the cell performance were evaluated by experiments and model analysis for various I/C ratios. To distinguish oxygen transport resistance in the CL from other resistances, IV characteristics were measured with different total pressures, and the limiting current density method was applied. Pore size distributions of the CLs were measured by nitrogen physisorption method, and the CL structural parameters were estimated using the estimation model developed by the authors. Using the model analysis of oxygen transport resistances in CL with the estimated parameters, the contributions of various oxygen transport resistances in CL were evaluated in further detail, and we discuss the relationship between the oxygen transport resistances and the cathode CL structure.
(Invited) Development of a Hydrogen Energy System as a Grid Frequency Management Tool
Authors: Ewan Mitch , Rocheleau Richard , Swider-Lyons Karen , Devlin Peter , Virji Maheboob B.V. , Randolf Guenter
Published: September 2016

It has been postulated that, similar to a battery, the optimized use of an electrolyzer as a variable load may help regulate grid frequency and increase the penetration of renewable energy resources on the grid. The use of an electrolyzer in this way could provide an “ancillary service” to the grid that can be assigned a monetary value. This monetary value can be used to offset the cost of hydrogen production. The hydrogen in turn can be used in high value applications such as a transportation fuel. The Hawai‘i Natural Energy Institute (HNEI) is conducting research to assess the technical potential and economic value of using an electrolyzer-based hydrogen production and storage system as a demand response tool for grid management. This presentation will describe the Hydrogen Energy System (HES) configuration, plans for operation in the field, and results of initial tests to determine the dynamic response of the electrolyzer and overall system.
Development of Low Cost High-Temperature Polymer Electrolyte Fuel Cell Membrane-Electrode-Assemblies for Combined Heat and Power Plants in Single Family Homes
Authors: Schenk Alexander , Gamper Stefan , Grimmer Christoph , Pichler Birgit Elvira , Bodner Merit , Weinberger Stephan , Hacker Viktor
Published: September 2016

The stationary use of high-temperature polymer electrolyte fuel cells allows the efficient decentralized production of electricity and heat at customer site. In order to reduce the production cost of such devices, platinum-cobalt based catalysts were synthesized directly at the surface of gas diffusion electrodes with a lower overall precious metal loading than comparable, commercially available state-of-the-art electrodes without impairing the overall fuel cell performance. The prepared membrane-electrode-assemblies showed high durability in single cell and stack tests.
Effect of Operation Strategies on Phosphoric Acid Loss in HT-PEM Fuel Cells
Authors: Rastedt Maren , Pinar Francisco Javier , Pilinski Nadine , Dyck Alexander , Wagner Peter
Published: September 2016

One of the most important degradation mechanisms, which cause a reduction of lifetime and performance of high temperature polymer electrolyte membrane (HT-PEM) fuel cells, is phosphoric acid loss. This paper presents the effect of different operation strategies on performance and acid loss of HT-PEM fuel cells on single cell level. The applied operation strategies are fuel switching, Start/Stop-cycling and long term testing under constant load with different gas compositions. The membrane electrode assemblies (MEA) are investigated in-situ via electrochemical characterization and ex-situ by post-mortem micro-tomography imaging and analysis. These results are compared with the ante-mortem counterpart. The anode humidification causes a higher amount of product water but no increase of phosphoric acid loss. In addition the wet fuel results in unclosing of the pores of both gas diffusion layers.
Impact on Diffusion Parameters Computation in Gas Diffusion Layers, Considering the Land/Channel Region, Using the Lattice Boltzmann Method
Authors: Espinoza Mayken , Sundén Bengt , Andersson Martin
Published: September 2016

Diffusion phenomena through the gas diffusion layer (GDL) at the microscale are one of the most complex physical phenomena to be described in proton exchange fuel cell (PEFC) numerical models. Predicting transport parameter behavior in GDLs is a valuable stage to propose micro-architectural designs, which can improve the efficiency and performance of fuel cells (FCs). The purpose of this paper is to propose an expression to estimate parameters involved in the diffusion process such as gas-phase tortuosity and effective diffusion coefficient when the effects of the land/channel region are considered. Three-dimensional GDL approximations are generated considering real characteristics of micro-porous layers employed in FCs. The fluid behavior through the porous media is simulated using the Lattice Boltzmann method (LBM), and the mentioned parameters are studied. The incidence of the land/channel presence over the gas-phase tortuosity is determined, and its effects over the effective diffusion are estimated.
Review on Dimensionless Numbers Relevant for Polymer Electrolyte Fuel Cells
Authors: Andersson Martin , Li Tingshuai
Published: September 2016

With the concern about global warming, air pollution and energy security, the prospect of using polymer electrolyte fuel cells (PEFCs) in future sustainable and renewable energy conversion systems has achieved substantial momentum. Dimensionless numbers can be determined before construction of a model is started, to make simple estimations on the transport processes, for example, within the porous PEFC GDL. Microstructural parameters, such as tortuosities and contact angles, are frequently treated as fitting parameters used in the respective governing equations, such that unrealistic values could be assumed or properties could not be representative of the corresponding microstructure. This treatment can be avoided if the origin of the expression is clearly clarified and if geometric properties are not used for fitting. Instead, it is recommended to, when needed, introduce parameters, without a geometrical meaning used only for fitting the model to experimental data, such as the pre-exponential factor in the advanced microstructural approach.
Development of a Portable Polymer Electrolyte Membrane Fuel Cell System Using Metal Hydride as the Hydrogen Storage Medium
Authors: Davids Moegamat Wafeeq , Tolj Ivan , Jao Ting-Chu , Lototskyy M , Pasupathi S , Sita C
Published: September 2016

This paper describes the development of a Portable Polymer Electrolyte Membrane Fuel Cell / PEMFC system that was developed at HySA Systems Competence Centre. The system has a rated peak output power of 130W at 240Vac and 5Vdc USB output power. Hydrogen is supplied from Metal Hydride (MH) hydrogen storage canister which uses multi-component AB2 – type hydrogen storage hydride alloy. The in-house developed MH canister was made up of stainless steel tube with external aluminium fins and had a maximum hydrogen storage capacity of 90 NLH2. The improvement of H2 discharge dynamic performance of the MH bed, which is cooled down in the course of H2 desorption, was achieved by the placement of the MH canister in front of exhaust of air cooling system of the PEMFC stack. By the utilisation of the PEMFC waste heat, in combination with the external fins (increase of heat exchange area) and compacting the MH material with Thermal Expanded Graphite / TEG (improvement of the effective thermal conductivity in the MH bed), the MH canister allowed for >40 minutes-long stable operation at the stack power of 130 W (225 W/kg(MH)) with the utilisation of >90% of the stored H2.
(Plenary) Development of PFSA Ionomers and Their Use in Fuel Cells
Authors: Kinoshita Shinji , Shimohira Tetsuji , Watakabe Atsushi , Hommura Satoru , Saito Susumu , Tanuma Toshihiro , Yamada Kohta
Published: September 2016

Perfluorosulfonic acid (PFSA) ionomers are desired to have higher proton conductivity even at elevated temperatures (95-120°C) and low relative humidity (RH) conditions so that the fuel cell systems especially in fuel cell vehicles (FCVs) can be compact, simple and lower cost through easy heat removal and humidifier less. For the ionomer in the cathode electrode, besides transporting the protons, it has to deliver high flux of oxygen to the platinum surface when operating at high current density using low platinum loading membrane electrode assembly (MEA). And for the microporous layer (MPL) on gas diffusion layer (GDL), ionomer was used as binder instead of hydrophobic resin such as polytetra-fluoroethylene (PTFE). It showed good performances in elevated temperatures and low RH conditions as well as in wet conditions through good water control. The design concepts as well as their cell performances are presented.
Proton Exchange Membranes Based on Polymeric Ionic Liquids for Fuel Cell Applications
Authors: Ortiz Sainz De Aja Alfredo , Díaz Vejo Mariana , Ortiz Uribe Inmaculada
Published: September 2016

Proton Exchange Membrane Fuel Cells (PEMFCs) are being considered as a promising energy source due to their high energy efficiency and low environmental impact. The proton exchange membrane, which is the key component of a PEMFC, needs a considerable improvement regarding its performance without external humidification. Concerning this aspect, many authors are working incorporating ionic liquids as proton exchange components in PEMFCs. In this work, polymeric ionic liquids have been designed for their use as electrolytes without external humidification. For this purpose, membranes based on [HSO3-BVIm][TfO]/[HSO3-BMIm][TfO] and [SBMIm][SEM]/[SBVIm][SEM] were developed for H2/air proton exchange membrane fuel cells working without external humidification. Fuel cell tests and ionic conductivity experiments show the great potential of these ionic liquid-based polymers as proton exchange membranes.
Molecular Dynamics Study on Proton Transport in Supported Nafion Ionomer Thin Films on Lennard-Jones Walls
Authors: Aochi Joji , Mabuchi Takuya , Tokumasu Takashi
Published: September 2016

Molecular dynamic simulations were carried out to investigate the effects of wettability of carbon supports and the thickness of thin films on proton transport. The simplified Lennard-Jones (LJ) wall model was used as the support model, which allows us to reproduce different hydrophobic walls by just tuning the LJ parameters. The thickness dependence was investigated in the observed thickness range in a catalyst layer: 4 and 10 nm. The proton transport property was evaluated in terms of self-diffusion coefficients (D
H+) and water cluster structures formed in the thin films. The obtained D
H+ indicated that the D
H+ increases with increasing hydrophobicity of the wall. It is also indicated that, for the high hydrophobic wall case, the D
H+ decreases with increasing the thickness of thin films, while, for the low hydrophobic wall case, the D
H+ is almost the same. It was confirmed that these trends were associated with the water structures formed in the thin films, i.e., water-layer-like structures are formed when the D
H+ are larger.
Reflection Characteristics of Oxygen Molecule on Ionomer Surface
Authors: Nakauchi Masataka , Mabuchi Takuya , Kinefuchi Ikuya , Takeuchi Hideki , Tokumasu Takashi
Published: September 2016

We investigated the scattering phenomena of oxygen molecules on an ionomer surface, which plays a crucial role in oxygen transport in catalyst layers, using molecular dynamics simulations. The probability density functions (PDFs) of the translational energy and scattering angle of scattered oxygen molecules were both analyzed for a wide range of incident condition and water contents. The PDFs of the translational energy depends on the incident energy, but are insensitive to the incident angle and the water content. The PDFs of the scattering angle are independent of the incident conditions and the water contents. Two types of collision behaviors, i.e. single and multiple collisions, were observed in the trajectory calculations. The fraction of multiple collisions and the residence time of oxygen molecules on ionomer surface were also investigated. These results provide an insight into the atomic level dynamics of oxygen scattering on ionomer surface.
Water Transport in Ionomer and Ice Formation during Cold Startup with Supercooled State in PEFC
Authors: Wakatake Naoyuki , Tabe Yutaka , Chikahisa Takemi
Published: September 2016

At cold startup of polymer electrolyte fuel cells near 0 °C like –10 °C, the produced water remains as supercooled state before it freezes, and then the shutdown occurs with ice layer formation between the cathode catalyst layer (CL) and micro-porous layer (MPL). It was estimated from our previous study that water transport in CL occurs by concentration gradient in ionomer. This study investigated the water transport in ionomer and the ice formation behavior by measurement of cold start characteristic and observation of cross-section of membrane electrode assembly by a cryo-SEM. To evaluate the water transport, we measured change in polymer membrane resistance corresponding to water content during the supercooling release. Further, to improve the cold start characteristic, we introduced a hydrophilic MPL and confirmed that the hydrophilic MPL makes the startup period until the shutdown longer, and improves the ice distribution.
Time-Resolved Nanostructural Analysis of Thin-Film Formation Process from Nafion Solution by Synchrotron X-Ray Scattering
Authors: Konosu Yuichi , Masunaga Hiroyasu , Hikima Takaaki , Tokita Masatoshi , Matsumoto Hidetoshi , Sasabe Takashi , Yoshida Toshihiko , Shinohara Kazuhiko , Hirai Shuichiro
Published: September 2016

The dynamics of thin-film formation process from Nafion solution induced by solvent evaporation were investigated by in-situ time-resolved grazing-incidence small- and wide-angle X-ray scattering (GISAXS/GIWAXS) measurements utilizing synchrotron radiation. GISAXS/GIWAXS measurements revealed subnano- to nano-scaled structural development during thin-film formation process from Nafion solution. In the early stage, solvent evaporation decreased the distance between the scatter structures in Nafion solution. With the solvent evaporation, the ordered structures with a spacing of 3.3 nm appeared and finally solidified. In-situ time-resolved GISAXS/GIWAXS techniques provide a powerful tool to investigate the development of structures during thin-film formation process from polymer solutions including ionomer solution.
Mo-doped Shaped Nanoparticles based on PtNi-alloys – A Promising ORR catalyst?
Authors: Kühl Stefanie , Heyen Henner , Strasser Peter
Published: September 2016

Here, we present on the results of the successful synthesis development of Mo-doped PtNi nanoalloys. The obtained nanoparticles are in octahedral shape with an edge length of around 4-5 nm. Our results in ORR reveal that Mo has not a controlled beneficial effect on the catalytic performance of Pt-based octahedral nanoparticles as reported in recent literature. Besides, benzoic acid as structure direction agent was found to be advanteageous to the catalyst performance. Both mass as well as specific activity were increased with addition of benzoic acid to the synthesis of PtNi nanoparticles.
Surface Structures and Electrochemical Stabilities for Pt/Pd(111) Model Electrocatalysts
Authors: Todoroki Naoto , Bando Yohe , Tani Yuki , Kaneko Soma , Watanabe Hirofumi , Wadayama Toshimasa
Published: September 2016

We investigated surface structures and electrochemical stabilities for Pt/Pd(111) bimetallic surfaces fabricated using molecular beam epitaxy. The Pt/Pd(111) surfaces were prepared by depositing four monolayer (ML)-thick Pt onto Pd(111) at substrate temperatures of 300 °C. The oxygen reduction reaction (ORR) activity of the pristine 4ML-Pt/Pd(111) surface was ca. 4.5 times higher than that of clean Pt(111). Electrochemical stabilities were evaluated by applying square-wave potential cycles (PCs) between 0.6 and x V (x = 0.8, 0.85, 0.9 and 1.0; xV-PCs) in O2-saturated 0.1 M HClO4 at 80 °C. Although the 0.9 and 1.0 V PCs cause ORR deactivation, the pristine ORR activities remained nearly unchanged by the 0.8 and 0.85V-PCs even after 5000 PCs. The results suggest that the electrochemical structural stability of Pt/Pd core–shell type catalysts depends on the cell voltages used under the practical operating conditions of polymer electrolyte fuel cells.