bluuebunny/crossref_metadata_2025 huggingface dataset:
Quantitative Measurements of Cryobiological Characteristics of Mouse Dendritic Cells and Its Evaluation Using Commercialized Coulter Counter
Authors: Chen Hsiu-hung , Gao Dayong
Published: February 2010

Quantitative measurements of cell osmotic behavior and membrane transport properties is critical for the development of cell-type-specific, optimal cryopreservation conditions. A microfluidic perfusion system has been developed here to measure the kinetic changes of cell volume under various extracellular conditions, in order to determine cell osmotic behavior and membrane transport properties. The system is fabricated using soft lithographic techniques and is composed of inlets, outlets, micchannels and a perfusion chamber for trapping cells. In this study Dendritic cells (DCs) are using as a model to validate our microfluidic system with a commercialized Beckham Coulter Counter (Multisizer 3). DCs are antigen presenting cells that have been increasingly used in immunotherapy for the treatment of various diseases. Cryopreservation and banking of DCs is critical to facilitate flexible and effective immunotherapy treatment. Using mouse DCs (MDC), membrane transport properties were first investigated using our microfluidic perfusion system. Cells in the microfluidic system were perfused with 3x phosphate buffer solution. The kinetics of cell volume changes under the specific extracellular conditions were monitored by a digital camera and analyzed using a biophysical model to determine water and cryoprotectant transport properties of the cell membrane. DCs were later tested using Beckman Coulter Counter, where the kinetic osmotic behaviors of cells were quantified through the correlation between electric pulses and their corresponding cell sizes. It was shown from this study that the cryobiological characteristics of DCs determined using microfluidic perfusion system and Coulter Counter agreed well.
Biomechanical Comparison of Trans-Sacral and Triangular Osteosynthesis in a Vertically Unstable Pelvic Fracture Model
Authors: Zamorano David P. , Min Kyong S. , Wahba George , Garcia Ivan , Bahtia Nitin N. , Lee Thay Q.
Published: February 2010

Vertically unstable fractures of the pelvis are uncommon high-energy injuries. There exist various methods of fixation for posterior pelvic ring injuries such as anterior plating, tension band-plating, trans-iliac bars, spinopelvic fixation, and iliosacral (IS) screws. Recent literature supports that triangular osteosynthesis (spinopelvic fixation) provides superior fixation strength compared to traditionally placed IS screw fixation. The theoretical advantage of triangular osteosynthesis fixation is that this technique combines unilateral spinopelvic distraction osteosynthesis for vertical stabilization with ipsilateral iliosacral screw fixation for horizontal stabilization, thereby providing biplanar stability. While spinopelvic fixation provides biplanar biomechanical stability there are inherent risks of spinal instrumentation, including neurologic injury from malpositioned pedicle screws, hardware loosening or breakage, local infection, wound dehiscence, hardware prominence requiring removal, and predisposition of spinal arthritis. Iliosacral screws are thought to be advantageous in that they can be applied percutaneously or as an open procedure, potentially limit soft tissue dissection, and minimize blood loss. However, IS screw fixation alone has been associated with fixation failure in unstable vertical shear pelvic fractures. There exists a vertical shear repair technique that has been used in revisional and non-union cases, which utilizes trans-sacral screws to stabilize vertical shear fractures. This trans-sacral technique provides biplanar stability and can be implemented without spinal instrumentation. The purpose of this study was to compare the structural integrity of trans-sacral (TS) versus triangular osteosynthesis (TO) in an unstable sacral fracture model.
Biomechanical Effects of Kneeling With Posterior Cruciate Retaining and Posterior Cruciate Substituting Total Knee Arthroplasty
Authors: Hofer Jason K. , Gejo Ryuichi , McGarry Michelle H. , Lee Thay Q.
Published: February 2010

Kneeling is an activity that is important in certain occupations and recreational activities. Culturally, kneeling plays a large role in Middle Eastern and Asian countries, where activities of daily living require the ability to kneel and achieve deep knee flexion.
Shoulder Arthroplasty: Current State of Affairs and Future Directions
Authors: Gupta Ranjan
Published: February 2010

Shoulder arthroplasty is a reliable procedure used to treat glenohumeral arthritis thanks to the efforts of many orthopaedic surgeons and design engineers over the past thirty years. Surgeons such Drs. Charles Neer and Robert Cofield were instrumental to making clinical observations that were effectively translated into improving implant design and clinical outcomes. Yet, there remains much room for further growth and development. With the increase numbers of shoulder arthroplasties performed, new observations and problems have been recognized that remain unanswered.
Volumetric Image Alignment Utilizing Particle Swarm Optimization
Authors: Cheng Li-Chang , Meyer Joerg
Published: February 2010

In some cases an object can not be completely imaged with a single image. These cases call for taking multiple images and stitching the images together to form a complete view of the object. The images that are dealt with are taken with a confocal laser-scanning microscope. This produces a stack of two-dimensional images that represent the volume in the microscope’s field of view. While the microscope automatically creates a z stack, the positioning of the tiles in x and y is done manually. Therefore, the slices within each stack are assumed to be already aligned. Our technique will take multiple stacks of images and stitch them together to form a single volume. We will use a particle swarm optimization technique to calculate the proper transformations required to produce the final volume.
Demonstration of Efficient Water Recovery for Fuel Cell Power Systems
Authors: Izenson Michael G. , Rozzi Jay C.
Published: February 2010

Water recovery and recycling are key technologies for fuel cell power systems. This paper describes technology to recover and recycle water using a compact, efficient condenser to separate water from a fuel cell exhaust stream. The condenser uses an innovative, micromachined condensing surface to achieve very high condensation mass flux and enable very high water recovery efficiency from a compact system. The condenser is sized for a 5 kWe, solid oxide fuel cell (SOFC) power system, but can easily be scaled up for higher power systems. We demonstrated operation of the condenser using an input stream that simulated the exhaust from an SOFC power system. Our device condensed and recovered 97–99% of the water in the input stream while consuming very little power (about 50 W).
Effect of Bi-Layer Interconnector Design on the Current Density of Solid Oxide Fuel Cells
Authors: Chen Qiuyang , Zhang Jian , Wang Qiuwang , Zeng Min
Published: February 2010

The concentration gradient of fuel and oxidant gas is great in the plane normal to the solid oxide fuel cells (SOFC) three-phase-boundary (TPB) layer, especially in the porous electrode. We present a novel interconnector design, termed bilayer interconnector, for SOFC. It can distribute the fuel and air gas in the plane normal to the SOFC TPB layer. In this paper, we develop a 3D model to study the current density of the SOFC with conventional and novel bi-layer interconnectors. The numerical results show that the novel SOFC design Rib1 can slightly enhance the mass transfer in the porous anode and current density. The novel SOFC design Rib2 can improve the current density significantly under low electrical conductivity of interconnector.
In-Situ Measurement of Surface Deformation and Temperature of SOFC Button Cell as a Function of Current Density
Authors: Guo Huang , Iqbal Gulfam , Kang Bruce S.
Published: February 2010

An experimental technique is developed that can measure in-situ surface deformation and monitor surface temperature of a solid oxide fuel cell (SOFC) anode, along with its electrochemical performance. In this research, a NexTech Probostat™ button cell test apparatus is modified and integrated with Sagnac interferometric optical setup and infrared sensor for anode surface deformation and temperature measurement respectively. The cell surface transient temperature is monitored as a function of applied current densities in hydrogen and simulated syngas environment. The surface deformation is also measured over time to estimate the anode material degradation to predict its structural life. The experimental results are useful to validate the SOFC structural and electrochemical models.
Study of Proton Exchange Membrane Fuel Cells (PZT-PEMFCs) With Nozzle and Diffuser
Authors: Ma Hsiao-Kang , Huang Shih-Han , Cheng Ya-Ting , Yu Chen-Chiang , Hou Chrung Guang , Su Ay
Published: February 2010

Previous theoretical studies have shown that piezoelectric proton exchange membrane fuel cells (PZT-PEMFCs) might solve water flooding problems and increase cell performance. The innovative design of PZT-PEMFCs results in more oxygen being compressed into the catalyst layer. This enhances the electrochemical reaction and the current density, especially at a high PZT vibration frequency (64 Hz). In this investigation, a single, valveless PZT-PEMFC experimental fuel cell is built. The results are then compared with those of previous theoretical studies. This study includes an analysis of PZT vibration frequencies, and cell operation temperatures. A Nafion 212 membrane with a reaction area of 2 cm × 2 cm is used to measure the voltage and average current density under different temperatures and vibration frequencies. When the PZT device moves upward and increases the chamber volume, a diffuser directs most of the air to the outlet. In the valveless PZT-PEMFC, both a nozzle and diffuser are used. This innovative design may direct air flow into the cathode channel through the diffuser and prevent air backflow. The nozzle/diffuser design in this study can direct a single directional air flow without valves. The experimental results indicate that the direction in which the cell is mounted have a negligible effect on cell performance due to air flow through the nozzle. The diffuser is not influenced by gravity. The optimal operating temperature for the PZT-PEMFC of this study is 50°C, as higher temperatures dry out the membrane electrode assembly (MEA). The optimal vibration frequency of the PZT-PEMFC is 180Hz, as higher frequencies cause more air intake and solve the problem of water flooding in the cathode channel. This study also concludes that the innovative design of PZT-PEMFCs may equal the performance of an open cathode stack configuration and can be applied in a fuel cell stack without an external air supply device.
Exploration and Prioritization of Fuel Cell Commercialization Barriers for Use in the Development of a Fuel Cell Roadmap and Action Plan for California
Authors: Eichman Josh , Brouwer Jack , Samuelsen Scott
Published: February 2010

Barriers to fuel cell commercialization are often introduced as general challenges, such as cost and durability, without definition of the terms and usually without prioritizing the degree to which each of these barriers hinders the development of fuel cell technology. This work acts to objectively determine the importance of technology barriers to fuel cell commercialization and to develop a list of appropriate actions to overcome these barriers especially as they relate to the California market. Using previous fuel cell roadmaps and action plans along with feedback from the fuel cell community, benchmarks (i.e., the current technology status) and milestones (i.e., the desired technology status) for fuel cell technology are explored. Understanding the benchmarks and milestones enables the development of a list of fuel cell commercialization barriers. These barriers or gaps represent issues, which if addressed, will enhance the market feasibility and acceptance of fuel cell technologies. The research process determined that the best technique to address these barriers and bridge the gaps between fuel cell benchmarks and milestones is to develop specific research projects to address individual commercialization barriers or collections of barriers. This technique allows for a high resolution of issues while presenting the material in a form that is conducive to planning for organizations like industry, regulatory bodies, universities, and government entities that desire to pursue the most promising projects. The current analyses resulted in three distinct research and development areas that are considered most important based upon the results. The first and most important research and development area is associated with technologies that address the connection and interaction of fuel cells with the electric grid. This R&D area is followed in importance by the production, use and availability of opportunity fuels in fuel cell systems. The third most important category concerned the development and infrastructure required for transportation related fuel cell systems. In each of these areas the fuel cell community identified demonstration and deployment projects as the most important types of projects to pursue since they tend to address multiple barriers in many different types of markets for fuel cell technology. Other high priority types of projects are those that addresses environmental and grid related barriers. The analyses found that Cost / Value to Customer, System Integration and Customer Requirements were the most important barriers that affect the development and market acceptance of fuel cell technology.
Study of the Change of Electrical Conductivity of Ni-YSZ Cermet Caused by Carbon Formation
Authors: Kim Yongmin , Kim Jung Hyun , Bae Gyujong , Bae Joongmyeon
Published: February 2010

The change in electrical conductivity of Ni-YSZ cermet used for the solid oxide fuel cell (SOFC) anode was investigated under methane, ethylene and n-butane feeding. Rectangular-shaped pellets have been prepared and exposed to gaseous mixtures including hydrocarbons and nitrogen with or without steam at 750 °C. In the absence of steam, carbon deposition occurred in the Ni-YSZ pellets and consequently the electrical conductivity decreased exponentially. When steam was fed (steam to carbon ratio = 2) to make carbon formation thermodynamically unfavorable, the electrical conductivity was kept constant, except for supplying butane. The effect of the mole fraction of methane on the change of electrical conductivity was also investigated in the CH4-N2 system. When the molar concentration was increased, the rate of decrease of electrical conductivity was increased. The present study helps to understand the effect of carbon deposition on the electrical conductivity of Ni-YSZ anode.
Dilute Ethanol Fueled SOFCs: A Symbiotic Solution Strategy
Authors: Nelson George J. , Haynes Comas , Miller Cameron
Published: February 2010

Biofuels have been broached as being more commercially viable under the biorefinery concept wherein multiple product streams enhance overall economics. An example of this concept is presented in which solid oxide fuel cells (SOFCs) and bioethanol are explored in light of the symbiotic pairing of these two promising alternative energy thrusts. Mutual benefit is manifested via dilute ethanol fueled SOFCs. Due to high operating temperatures SOFCs reject high quality heat that can be used for indirect internal reformation of ethanol in the presence of sufficient water. Additionally, carbon monoxide present in hydrogen-rich reformate can be used as a fuel through shift to hydrogen. Thus, the optimization of an additional dilute product stream for SOFC applications can contribute to the realization of the bioethanol infrastructure. This contribution is two-fold. First, across a domain of viable operating conditions the use of dilute ethanol can potentially improve SOFC system performance above cell stack performance and enable distributed SOFC generation fueled by dilute ethanol, providing a secondary market for higher energy ratio bioethanol. Second, integrating in-plant SOFC co-generation fueled by a fraction of dilute ethanol diverted from the process stream is shown to improve the energy ratio of bioethanol. This improvement results from the electricity and by-product heat from the SOFC partially offsetting the energy intensive anhydrous ethanol production.
Characterization of the Micro Porous Layer - Catalyst Layer Interface Structure in Polymer Electrolyte Fuel Cells
Authors: Hizir F. E. , Swamy T. , Ural S. O. , Kumbur E. C. , Mench M. M.
Published: February 2010

The impact of interfacial contact between the catalyst layer (CL) and diffusion medium (DM) has important implications for the transport of heat, current, and multi-phase flow in the membrane electrode assembly. This work is motivated by the need to obtain a greater understanding of the impact of the micro porous layer (MPL) and CL morphologies and resulting interfacial contact on the fuel cell performance. In this effort, an array of different virgin mating CL and MPL surface structures was investigated. The mating surface morphology of the tested MPL and CL samples were digitally characterized by optical profilometry, a non-destructive imaging technique which measures the 3-D surface topology of samples with a resolution on the order of several nanometers in the vertical direction, and 0.5 to 5 microns in the lateral direction. The benchmark data were utilized in an analytical model developed for virtual reconstruction of a compressed interfacial structure. The results show that for the materials tested, the MPL surface exhibits a higher degree of roughness compared to catalyst surfaces. The MPL surface was observed to have relatively higher roughness than the CL, indicating the potential dominance of the MPL surface morphology on the local transport and interfacial contact across the MPL|CL interface. Elasticity, compression, and uncompressed surface morphology are the key factors controlling the ultimate interfacial contact.
In-Situ Optical Measurements of Water Vapor Concentration and Temperature in a Proton Exchange Membrane Fuel Cell at Steady State and Under Dynamic Cycling
Authors: Sur Ritobrata , Boucher Thomas J. , Renfro Michael W. , Cetegen Baki M.
Published: February 2010

A robust, accurate and fast in-situ sensor was developed for detection of water vapor partial pressure and temperature simultaneously at the anode and cathode channels of a PEM fuel cell. Tunable diode laser absorption spectroscopy (TDLAS) utilizing wavelength modulation (WMS) was employed for these measurements. This method determines the ratio of the second and first harmonics (2f/1f) of the spectroscopic absorption profile of water vapor by the aid of a software lock-in amplifier. Measurements were taken using a diode laser emitting around a wavelength of 1471 nm where the water vapor absorption exhibits significant sensitivity to partial pressure and temperature. Measurements of water vapor concentration and temperature in were taken at steady and dynamic operating conditions in the anode and cathode gas channels near the inlet and outlet ports of a serpentine channel PEM fuel cell with Nafion membrane of active area 50 cm2. Different load and inlet humidity conditions were tested to characterize the operation at different conditions. The partial pressure of water vapor increases towards the exit of both the gas channels, but the increase is found to be more significant on the cathode side. The dynamic operation of the fuel cell was also examined in this study as well as the simultaneous measurements at the anode and cathode gas channels.
Dynamic Model for Understanding Spatial Temperature and Species Distributions in Internal-Reforming Solid Oxide Fuel Cells
Authors: Shaffer Brendan , Brouwer Jacob
Published: February 2010

Direct internal reformation of methane in solid oxide fuel cells (SOFCs) leads to two major performance and longevity challenges: thermal stresses in the cell due to large temperature gradients and coke formation on the anode. A simplified quasi-two-dimensional direct internal reformation SOFC (DIR-SOFC) dynamic model was developed for investigation of the effects of various parameters and assumptions on the temperature gradients across the cell. The model consists of 64 nodes each of which contains four control volumes: the positive electrode, electrolyte, negative electrode (PEN); interconnect; anode gas; and cathode gas. Within each node the corresponding conservation, chemical, and electrochemical reaction equations are solved. The model simulates the counterflow configuration since previous research [8] has shown this configuration to yield the smallest temperature differentials. Steady state simulations revealed several results where the temperature difference across the cell was considerably affected by operating and cell design parameters. Increasing the performance of the cell through modifications to the electrochemical model to simulate modern cell performance produced significant changes in the cell temperature differential. Improved cell performance led to a maximum increase in the temperature differential across the cell of 31 K. An increase in the interconnect thickness also exhibits a considerable reduction in the temperature difference across the PEN. In particular, increasing the interconnect thickness from 3.5 to 4.5 mm can achieve about a 50 K reduction in the cross cell temperature difference. Variation of other physical parameters such as the thermal conductivity of the interconnect and the rib width also showed an effect on the temperature distribution. The sensitivity of temperature distribution to the adiabatic assumption was also performed and results showed a considerable effect near the fuel and air inlets. This resulted in severe temperature gradients approaching 160 K/cm.
Effect of Temperature Gradient on End Cell Water Transport in a Polymer Electrolyte Fuel Cell After Shutdown
Authors: Khandelwal Manish , Mench Matthew M.
Published: February 2010

In a PEFC stack, end cells are subjected to severe conditions compared to the center cell, sometimes resulting in poor end cell performance and early freeze out. In this work, the concept of using controlled temperature gradients to non-parasitically remove excess water from end cell during PEFC stack shutdown has been numerically investigated. To investigate the end cell water transport, an integrated modeling approach focusing both at stack and single cell level is presented. The stack thermal model is developed to obtain detailed temperature distribution across the PEFC stack. Extending the results of the stack thermal model into a single cell level, a two-phase unit fuel cell model is developed to investigate the water and thermal transport in the PEFC components after shutdown, which for the first time includes thermo-osmotic flow in the membrane. The model accounts for capillary and phase-change induced flow in the porous media, and thermo-osmotic and diffusive flow in the polymer membrane. The single cell model is used to estimate the local water distribution with land/channel boundary condition, and the experimentally validated stack thermal model provided the transient temperature boundary condition to simulate the end cells. Model results indicate that a favorable temperature gradient can be formed in the stack to enhance the water drainage rate, thus enhancing the anode end cell cold start performance.
High Temperature Stationary Solid Oxide Fuel Cell Systems in the Renewable Future
Authors: Mueller Fabian , Tarroja Brian
Published: February 2010

Solid oxide fuel cells (SOFCs) are attractive emerging energy conversion devices. Particularly, SOFC electrochemically react fuel and oxygen to generate electricity efficiently with ultra low pollutant emissions. For SOFC systems to be widely utilized in the future, SOFC will have to be effectively integrated with a wide array of energy resources and conversion devices including base-loaded nuclear and coal as well as renewables. Load following generators and/or energy storage will be required to manage intermittent renewables. Base-loaded fuel cell systems (i.e., present day SOFCs) that use potentially dispatchable fuel resources will be increasingly difficult to market. Fuel generators such as SOFCs that can load follow with ultra-low emissions will become increasingly attractive, particularly in future grid scenarios with increased renewables. Simulations results are shown in the paper that demonstrates the intermittent challenge of renewables and the potential for SOFC systems to provide load following capability. SOFCs have the potential to be very attractive load following generators which achieve high efficiencies at part load with low emissions. Research and development is needed to understand solid oxide fuel cell system and control development to minimize dynamics that can degrade the fuel cell during load following. Understanding of degradation of optimally controlled fuel cell is needed to fully understand the true potential of SOFC systems in future grids with increased intermittent renewable penetration.
Designing Micro Fuel Cells for Portable Products
Authors: Flipsen Bas
Published: February 2010

The main driving force in developments of consumer electronics, such as cell phones and laptop computers, is longer run times and more functionality. In this quest for higher energy densities battery characteristics improve at a constant pace. Fuel cells seem to be the next big technology breakthrough improving energy density with a factor 3 to 10 compared to current lithium ion batteries. In particular the Direct Methanol Fuel Cell (DMFC) is an interesting opportunity because of the high energy capacity of methanol and the handling of the fuel making ‘charging’ easy, safe and fast. To get information on the different aspects that determine the boundaries of the DMFC power source, a power source for a MP3 player, the Samsung YP-Z5F, is designed. This design is based on a DMFC plus battery (DMFC hybrid) and utilizes standard available components [1]. Design of a DMFC hybrid power source in a conventional way (standard practice engineering) will not result in a smaller power source for this particular application. The design has a power and energy density lower than the currently available lithium polymer battery, mainly because of the low fuel efficiency of the cell at low temperatures, the use of commercially available but still too bulky components, and a large amount of dead space (≈34%). There are three ways to increase power and energy density of the system. First by increasing the fuel-efficiency of the cells membrane. Second by scaling down the system components to the right proportions and third by improving the systems architecture diminishing empty space. This paper presents the design of a DMFC hybrid with scaled-down components. A literature study is done on the efficiency improvements of DMFC cells. The results are presented in a CAD model and evaluated, comparing the ‘optimized design’ with ‘standard practice’ design and the current lithium-polymer battery. The energy density of the redesigned fuel-cell system is still low compared to the used lithium-polymer battery, but an improvement to the preliminary design.
3D Modeling of Four-Points Probe Method: Application to Characterisation of Fuel Cell’s Bipolar Plates
Authors: Madier Ludovic , Martemianov Serguei , Bigarre´ Janick , Galiano Herve´
Published: February 2010

For reducing fuel cell stack cost new materials for bipolar plates (BPs) are under development. Quality of this new BPs has to be checked. The four-points probe method is commonly used for measuring of electrical conductivity; however sample’s geometry has a strong impact on the accuracy of this method. For this reason, it is not possible to use the classical assumptions for the interpretation of the electrical conductivity measurements with respect to the case of BPs. In this paper, a finite elements numerical model is developed for the calculation of the influence of the sample’s geometry on the accuracy of four-points probe method. The approach presented here allows obtaining the corrective factor for the four-points method taking into account the real geometry of BPs and evaluating the border effects related with the position of the probe. The obtained results can be used for diagnostic of the quality of BPs.
A 3D Single-Phase Numerical Model for a PEMFC Stack
Authors: Le Anh Dinh , Zhou Biao
Published: February 2010

A single-phase, three-dimensional mathematical model has been constructed and implemented to simulate the fluid flow, heat transfer, species transport, electrochemical reaction, and current density distributions in a Proton Exchange Membrane Fuel Cell (PEMFC) stack with parallel-shaped channels. In this study, a complete PEMFC stack with 3 parallel single-cells including the membrane, gas diffusion layers (GDLs), catalyst layers, flow channels, and current collectors was taken into account. The reasonable numerical results show the detailed distributions of fluid flow and species concentrations in the channel and porous media, heat and current transports through the single cells in the stack. Furthermore, this successful modeling of a single-phase PEMFC stack would be a critical step to further develop a general two-phase PEMFC model that could investigate the water management and effects of liquid water on the performance of a fuel cell stack.
Liquid Water Removal Process in PEM Fuel Cell Cathode
Authors: Liu Xiaojing , Zhou Biao
Published: February 2010

In this paper, water behaviors in both the gas flow channels and porous layer at the cathode side of a PEMFC are studied. VOF (Volume of Fluid) method and UDF (User-Defined-Function) are applied. A multi-phase three-dimensional fuel cell cathode side model is constructed with a serpentine shape channel and a porous layer attached to it. Fluid will be allowed to pass through both domains as it can in the real fuel cell system. Parameters of the porous layer will be introduced to the model by the user defined function. The detailed liquid water removal process in this model will be presented, and since the water behaviors in a fuel cell is proved to be one of the most important factors influencing the fuel cell performance, this model can be a good starting point to make more advanced models to investigate fuel cell performance.
Life Prediction of a Solid Oxide Fuel Cell Under Thermal Cycling Conditions
Authors: Liu Lin , Kim Gap-Yong , Chandra Abhijit
Published: February 2010

Typical operating temperature of a solid oxide fuel cell (SOFC) is above 600°C, which leads to severe thermal stresses caused by the difference in coefficients of thermal expansion (CTE) during thermal cycling. Interfacial and peeling stresses are two types of thermal stress that cause the mechanical failure of the SOFC. The paper develops a mathematical model to estimate thermal stresses in a typical electrolyte-supported SOFC (NiO/8YSZ-YSZ-LSM). The proposed model is then utilized to obtain analytical expressions for interfacial and peeling stresses. This model provides insight into the distribution of interfacial and peeling stresses of SOFCs and the cause of catastrophic failure. A model for crack nucleation in multi-layered structures under thermal cycling is generalized and utilized in this study for the life prediction. It is found that the peeling and interfacial shear stresses are more concentrated near free edge areas. The relevant damage evolution rate accelerates as the crack propagates. The work also provides a foundation for future SOFC reliability and life prediction research.
Electrical and Thermal Conductivities of Novel Metal Mesh Hybrid Polymer Composite Bipolar Plates for Proton Exchange Membrane Fuel Cells
Authors: Hsiao Min-Chien , Liao Shu-Hang , Yen Ming-Yu , Ma Chen-Chi M. , Lee Shuo-Jen , Lin Yu-Feng , Hung Chih-Hung
Published: February 2010

Novel metal mesh hybrid polymer composite bipolar plates for proton exchange membrane fuel cells (PEMFCs) have been prepared via inserting a copper or alumina mesh in polymer composites. The composition of polymer composites consisted of 70 wt% graphite powder and 0–2 wt% modified multi-walled carbon nanotubes (m-MWCNTs). Results indicated that the inplane electrical conductivity of m-MWCNTs/polymer composite bipolar plates increased from 156 S cm−1 (0 wt% MWCNT) to 643 Scm−1 (with 1 wt% MWCNT) (D.O.E target > 100 S cm−1). The bulk thermal conductivities of the copper and aluminum mesh hybrid polymer composite bipolar plates increased from 27.2 W m−1 K−1 to 30.0 W m−1 K−1 and 30.4 W m−1 K−1, respectively. Furthermore, the current and power densities of a single fuel cell using copper or alumina mesh hybrid polymer composite bipolar plates are more stable than that of using neat polymer composite bipolar plates, especially in the ohmic overpotential region of the polarization curves of single fuel cell tests. The overall performance confirms that the metal mesh hybrid polymer composite bipolar plates prepared in this study are promising for PEMFC application.
Liquid Water Visualization in an Operating PEMFC by Soft X-Ray Radiography
Authors: Sasabe Takashi , Tsushima Shohji , Hirai Shuichiro , Minami Katsunori , Yada Keiji
Published: February 2010

Liquid water distribution in the operational PEMFC was visualized by soft X-ray radiography. To achieve a maximum sensitivity towards liquid water, low electron beam energy in soft X-ray range was chosen. As a result, the spatial resolution of 1 μm and the temporal resolution of 0.5 sec were obtained. In addition, a correlation between the accumulation of water and the reduction of performance in higher current density range was observed. At low current densities, liquid water accumulation was observed under the ribs, because the longer diffusion length for the rib area conduced to a less effective water removal into the vapor phase. With increase of current density, liquid water was also observed under the channels and the reduction of performance with time was observed. These results suggested that the current density under the rib is much less than under the channel and the anisotropy of GDL and the geometry of flow field had a heavy impact on the liquid water distribution and the cell performance.
Suppression of Methanol Crossover for Direct Methanol Fuel Cells Using a Layer of Nanometer-Sized Pt50-Sn50 Catalyst Particles Deposited on Proton Exchange Membrane Surface
Authors: Wan Chieh-Hao , Wei Jyun-Ming
Published: February 2010

This paper deposited a layer of nanometer-sized Pt50-Sn50 catalyst particles in the proton exchange membrane (PEM) surface by impregnation-reduction (IR) method to suppress the methanol crossover and improve the utilization efficiency of methanol fuel. The suppression of methanol crossover and the performance of membrane electrode assembly (MEA) for the proposed structure were compared to those with normal-PEM structure. The possible mechanism of the suppression of methanol crossover was investigated. SEM, X-ray, EDS and EPMA analysis were used to characterize microstructures, phases, chemical composition and distributions for the obtained electro-catalyst layers. Methanol crossover rate in a DMFC was determined by measuring the CO2 concentration at the cathode exit in real time. Experimental results demonstrate that the PEM with a layer of nanometer-sized Pt50-Sn50 catalyst particles suppresses methanol crossover by up to 18% more than the normal-PEM, and yield a 8% better MEA performance than the normal-MEA. The combination effects of Pt50-Sn50 in PEM as a methanol filter and electrode contribute to the suppression of methanol crossover and improvement of cell performance as well as the utilization efficiency of methanol fuel.
Distribution Study of Species and Current Density During Oxygen Starvation
Authors: Gerard Mathias , Poirot-Crouvezier Jean-Philippe , Hissel Daniel , Pe´ra Marie-Cecile , Memponteil Alain , Bador Bruno
Published: February 2010

In a fuel cell system the stack is strongly coupled with the main system components, among which the compressor is one of the most important. Malfunction of this auxiliary device (delay during peak power, low stoichiometry operation, emergency stop, etc) is directly responsible for bad oxygen distribution in the cathode (substoichiometry reactants feeding). This phenomenon is usually called oxygen starvation. In this study we want to identify the consequences of oxygen starvation on performance and durability of PEMFC stacks and more particularly on the current distribution along the cell. The oxygen concentration decreases along the channel and induces a change of the local electrochemical response; it means that the local current density on the cell is redistributed on the surface. This bad distribution of reactive gas (in a transient time or long time) decreases performance but may also have an effect on cathode degradation like carbon corrosion and platinum dissolution/oxidation. The current distribution along the cell is studied by two approaches (modeling and experiments). The local current is computed by measuring the local induced magnetic field and using Maxwell equations. The experiments are performed with a bi-cell stack developed by CEA with specific design for the magnetic sensors. The 3D model using serpentine bipolar plate meshing is adapted to dynamically compute the catalyst layer local conditions (local current, temperature, gases partial pressure, water activity). It is able to reproduce the conditions of low or high oxygen concentration in the cathode side.
In-Situ Estimation of Faradaic Efficiency for a Direct Methanol Fuel Cell Stack by a Carbon Dioxide Saturated Solution Method
Authors: Qi Liang , Xie Xiaofeng , Alaefour Ibrahim , Pereira Aaron , Li Xianguo
Published: February 2010

A direct methanol fuel cell (DMFC) system consisting of 40 single cells was assembled to study the influence of the transport phenomena at the anode and stack faradaic efficiencies by a CO2 saturated solution method. This method corrected the common experimental error in measuring methanol crossover caused by the simultaneous CO2 permeation from the anode to cathode. Both anode and stack faradaic efficiencies were estimated using this method. An equivalent “carbon-flow current” has been defined and a relationship between the transport phenomena and efficiencies was developed. Also the effect of methanol concentration, methanol flow rate and air flow rate on stack efficiency was studied. The results show that lower methanol flow rate, lower methanol concentration and higher air flow rate are all helpful in decreasing the methanol crossover and increase the stack faradaic efficiency.
Extracting Model Parameters and Paradigms From Neutron Imaging of Dead-Ended Anode Operation
Authors: Siegel Jason B. , Stefanopoulou Anna G. , Yesilyurt Serhat
Published: February 2010

In a PEMFC, feeding dry hydrogen into a dead-ended anode (DEA), reduces the overall system cost, weight and volume due to reduced need for a hydrogen-grade humidification and recirculation subsystems, but requires purging to remove the accumulated water and inert gas. Although the DEA method of operation might be undesirable due to its associated high spatial variability it provides a unique perspective on the evolution of the water accumulation in the anode. Sections of the channel nearest the inlets are significantly drier than those nearest the outlet as shown in the neutron imaging of a 53 cm2 PEMFC. This method allows in-situ visualization of distinct patterns, including water front propagation along the channels. In this paper we utilize neutron imaging of the liquid water distributions and a previously developed PDE model of liquid water flow in the GDL to (a) identify a range of numerical values for the immobile saturation limit, (b) propose a gravity-driven liquid flow in the channels, and (c) derive the two-phase GDL boundary conditions associated with the presence of liquid water in the channel.
Analysis of Pressure Drop and Water Flooding of Two-Phase Flow Along Channels for a PEM Fuel Cell System
Authors: He Jinglin , Choe Song-Yul , Hong Chang-Ouk
Published: February 2010

The flow in gas flow channels of an operating polymer electrolyte membrane (PEM) fuel cell has a two-phase characteristic that includes air, water vapor and liquid water and significantly affects the water flooding, pressure distribution along the channels, and subsequently the performance of the cell and system. Presence of liquid water in channels prevents transport of the reactants to the catalysts and increases the pressure difference between the inlet and outlet of channels, which leads to high parasitic power of pumps used in air and fuel supply systems. We propose a model that enables prediction of pressure drop and liquid water distribution along channels and analysis of water flooding in an operating fuel cell. The model was developed based on a gas-liquid two-phase separated flow that considers the variations of gas pressure, mass flow rate, relative humidity, viscosity, void fraction, and density along the channels on both sides. Effects of operating parameters that include stoichoimetric ratio, relative humidity, and inlet pressure on the pressure drop and water flooding along the channels were analyzed.
In-Situ Neutron Radiography of Water Freezing in a GDL
Authors: Gilbert Andrew , Hidrovo Carlos , Biegalski Steven , Kapsimalis Roger , Deinert Mark
Published: February 2010

Research in fuel cells has increased considerably in the past 10 years largely due to the desire to develop dependable, low emission, and inexpensive alternatives to internal combustion engines. Already deployed in industrial applications as backup power, the widespread utilization of polymer electrolyte membrane fuel cells (PEMFCs) in automobiles has long been a promising prospect. In order to make fuel cells a viable option for such an application, issues of water management must be addressed. Of particular importance in automobile applications is the problem of water within the cathode gas diffusion layer (GDL) and cathode channels freezing in cold temperatures while the car is off. Freezing not only damages the membrane electrode assembly (MEA), it also hinders normal startup and operation of the fuel cell. Neutron radiography has proven effective for imaging the water transport in a fuel cell. Neutrons can easily move through many metals but are heavily attenuated by the hydrogen content of water. This has been shown to be a valuable tool for detecting water in fuel cells. By using neutron radiography the dynamics of water production and movement within a fuel cell can be imaged during normal operation. In this research we take this approach one step further by taking advantage of the water phase density changes to image the freezing dynamics within a fuel cell using neutron radiography. This imaging will aid in the design of MEAs and bipolar plates to be used in cold temperatures. The nuclear research reactor at the University of Texas at Austin includes a neutron radiography system that uses a scintillator screen and a CCD camera. This system has been shown to have sufficient mass resolution to image the small density changes observed in water freezing within a thin GDL. A fuel cell mock-up capable of replicating water production and air usage in a fully functional PEMFC was developed and employed to observe the effects of freezing temperatures on the GDL and bipolar plate channels. Preliminary results show that neutron radiography is well suited for imaging freezing within a fuel cell. This methodology will be further employed in the study of water freezing within the GDL and cathode channels in a fully working PEMFC.
A Phenomenological Model for Degradation of Solid Oxide Fuel Cell Anodes Due to Impurities in Coal Syngas
Authors: Cayan Fatma N. , Pakalapati Suryanarayana R. , Elizalde-Blancas Francisco , Celik Ismail
Published: February 2010

A new phenomenological one-dimensional model is formulated to simulate the typical degradation patterns observed in Solid Oxide Fuel Cell (SOFC) anodes due to coal syngas contaminants such as arsenic (As) and phosphorous (P). The model includes ordinary gas phase diffusion including Knudsen diffusion and surface diffusion within the anode and the adsorption reactions on the surface of the Ni-YSZ based anode. Model parameters such as reaction rate constants for the adsorption reactions are calibrated to match the degradation rates reported in the literature. Preliminary results from implementation of the model demonstrated that the deposition of the impurity on the Ni catalyst starts near the fuel channel/anode interface and slowly moves toward the active anode/electrolyte interface which is in good agreement with the experimental data. Parametric studies performed at different impurity concentrations, operating temperatures and current densities showed that the coverage rate increases with increasing temperature, impurity concentration and current density, as expected.
Modeling of Liquid Water Transport in Cathode Catalyst Layer of PEM Fuel Cells
Authors: Das Prodip K. , Li Xianguo , Liu Zhong-Sheng
Published: February 2010

The performance of a polymer electrolyte membrane (PEM) fuel cell is significantly affected by liquid water generated at the cathode catalyst layer (CCL). Conversely, the ionic conductivity of PEM is directly proportional to its water content; it must have sufficient water. Therefore, it is essential to maintain a delicate water balance, which seems difficult without properly understanding liquid water transport from the CCL. In the present study, a one-dimensional analytical solution of liquid water transport across the CCL is derived from the fundamental transport equations. The effect of CCL wettability on liquid water transport and the effect of liquid water “flooding” on reactant transport have been investigated. It has been observed that hydrophilic characteristic of a CCL plays significant role on the liquid water transport. The liquid water saturation in a hydrophilic CCL can be reduced by increasing the surface wettability or lowering contact angle. Based on a dimensionless time constants analysis, it has been shown that liquid water production from the phase change process is negligible compared to water production from the electrochemical process.
Monte Carlo Investigation of Particle Properties Affecting TPB Formation and Conductivity in Composite Solid Oxide Fuel Cell Electrode-Electrolyte Interfaces
Authors: Martinez Andrew , Brouwer Jacob
Published: February 2010

A previously-developed microstructure model of the Solid Oxide Fuel Cell (SOFC) electrode-electrolyte interface has been applied to the study of particle properties in these devices through the use of the Monte Carlo simulation method. Previous findings that have demonstrated the necessity of accounting for the gaseous phase percolation have been re-emphasized through the current investigation. In particular, the effects of three-phase percolation critically affect the dependence of TPB formation and electrode conductivity on: 1) conducting phase particle size distributions, 2) electronic:ionic conduction phase contrast, and 3) the amount of Mixed Electronic-Ionic Conductor (MEIC) included in the electrode. In particular, the role of differing percolation effectiveness between electronic and ionic phases has been shown to counteract and influence the role of the phase contrast. Porosity, however, has been found to not be a significant factor for the range studied, but does not obviate the necessity of modeling the gas phase. In addition, the current work has investigated the inconsistencies in experimental literature results concerning the optimal particle size distribution. It has been found that utilizing smaller particles with a narrow size distribution is the preferable situation for electrode manufacturing. These findings stress the property-function relationships of fuel cell electrode materials.