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A unique vessel system is being developed to facilitate proton imaging of small-scale shock physics experiments at Los Alamos National Laboratory (LANL). The main components of the system are the Inner Pressure Confinement Vessel (IPCV, which hosts the physics experiment), the Outer Pressure Containment Vessel (OPCV) and Beam Pipes and Auxiliary Hardware (BPAH). The IPCV is an explosively loaded high-pressure vessel which is mounted inside the statically loaded OPCV. The OPCV is attached to a proton beamline. The OPCV and beam pipes form a containment pressure barrier. The detonation of high explosive (HE) inside the IPCV drives material to extreme loading conditions, which are imaged using a proton beam and an imaging system. The IPCV needs to satisfy the ASME Boiler and Pressure Vessel Code, Section VIII, Division 3, Code Case 2564, while allowing for maximum resolution of proton radiography across a sufficiently large field of view. The main components of the IPCV are the vessel body, top and bottom covers, side covers, and radiographic window assemblies. The covers have different feedthroughs mounted on them, such as feedthrough devices for sending or receiving electrical and optical signals across the pressure boundary. The covers also have gas lines with valves for venting the vessel volume. The sealing strategy incorporates a tortuous path and a minimum of 3 O-rings at each vessel cover. The O-ring grooves are designed and tested to seal the vessel before and after the explosive experiment, and to minimize the burp (limited release) during the explosion. While the IPCV is designed for a relatively small HE amount of 30 g TNT equivalent, the radiographic window is located only a few cm away from the HE, which is unique to this specialized high-pressure vessel. To achieve the optimal imaging resolution across the required field of view of 2cm by 2cm, the radiographic windows need to be very thin, located extremely close to the HE, and made of low-attenuating material such as Beryllium. This paper provides an overview of the baseline design, analysis, and testing of the IPCV and its subassemblies.

The results of development testing of the Inner Pressure Confinement Vessel (IPCV) are presented here. The IPCV is part of a two-vessel confinement and containment system designed to support proton radiography of shock physics experiments at Los Alamos National Laboratory (LANL). The IPCV is explosively loaded and designed to confine the high-pressure detonation products, material fragments and any other hazardous materials created by explosively driving materials to extreme loading conditions. The IPCV is designed to meet the requirements of ASME Boiler and Pressure Vessel Code Section VIII, Division 3, Code case 2564. The unique and challenging design consideration for the IPCV is the proximity of the High Explosive (HE) charge to the radiographic imaging windows. The radiographic windows are fabricated from low-attenuating Beryllium which aids in obtaining high resolution radiographic images but is also brittle and susceptible to damage. Several rounds of developmental tests were conducted over the course of a few years. The IPCV is designed to withstand a maximum HE charge size of 30g TNT equivalent. The main components of the IPCV are the Experimental Physics Package (EPP), fragment mitigation assembly, radiographic windows, and gas handling equipment. The radiographic windows are located only a few cm away from the EPP (which houses the HE) and are protected by the fragment mitigation assembly. The gas handling equipment allows for post-shot pressure monitoring and eventual venting of the vessel. The development tests were used to advise the design of the fragment mitigation strategy, develop testing processes and procedures, and collect data for comparison with the engineering models.

Platinum group metal-free (PGM-free) catalysts are a low-cost alternative to expensive PGM catalysts for polymer electrolyte fuel cells. However, due to the low volumetric activity of PGM-free catalysts, the catalyst layer thickness of the PGM-free catalyst electrode is an order of magnitude higher than PGM based electrodes. The thick PGM-free electrodes suffer from increased transport resistance and poor water management, which ultimately limits the fuel cell performance. This manuscript presents the study of water management in the PGM-free electrodes to understand the transport limitations and improve fuel cell performance. In-operando neutron imaging is performed to estimate the water content in different components across the fuel cell thickness. Water saturation in thick PGM electrodes, with similar catalyst layer thickness to PGM-free electrodes, is lower than in the PGM-free electrodes irrespective of the operating conditions, due to high water retention by PGM-free catalysts. Improvements in fuel cell performance are accomplished by enhancing water removal from the flooded PGM-free electrode in three ways: (i) enhanced water removal with a novel microporous layer with hydrophilic pathways incorporated through hydrophilic additives, (ii) water removal through anode via novel GDL in the anode, and (iii) lower water saturation in PGM-free electrode structures with increased catalyst porosity.

We present the results of explosive testing of an Inner Pressure Confinement Vessel (IPCV). The IPCV is explosively-loaded high-pressure vessel which is a part of the containment system to facilitate proton imaging of small-scale shock physics experiments at Los Alamos National Laboratory (LANL). The detonation of high explosive (HE) drives material to extreme loading conditions, which are imaged using a proton beam and an imaging system. The IPCV needs to satisfy the ASME Boiler and Pressure Vessel Code, Section VIII, Division 3, Code Case 2564, while allowing for maximum resolution of proton radiography. The IPCV contains an Experimental Physics Package (EPP), fragment mitigation assembly, and radiographic windows. To achieve the optimal imaging resolution, the radiographic windows need to be very thin, located extremely close to the HE, and made of low-attenuating material such as Beryllium. While the IPCV is designed to a relatively small HE amount of 30 g TNT equivalent, the radiographic window is located only a few cm away from the HE, which is unique to this specialized high-pressure vessel. Fragment mitigation is critical to protecting radiographic windows from any fragments to allow the IPCV to maintain the pressure boundary before and after the explosive experiment. This shielding contains two layers: Boron carbide (B4C) facing the HE and Dyneema (cross plied composite layers made of ultra-high molecular weight polyethylene) facing the window. The B4C plate serves to break up and dull fragments while Dyneema catches fragments and prevents them from contacting the radiographic windows. Design development of the fragment mitigation assembly and attachment was informed by several series of explosive tests at LANL. The tests also addressed the sealing function of vessel covers, gas lines, and isolation valves before and after an explosive experiment.

Los Alamos National Laboratory (LANL) designs and utilizes impulsively loaded pressure vessels for the confinement of experimental configurations involving explosives. For physics experiments with hazardous materials, a two-barrier containment system is needed, where an impulsively (or, explosively) loaded pressure vessel is assembled as an inner confinement vessel, inside an outer containment vessel (subject to quasi-static load in the event of confinement vessel breach). Design of the inner and outer vessels and support structure must account for any directional loads imparted by the blast loading on the inner vessel. Typically there is a shock-attenuating assembly between the inner confinement and outer containment pressure barriers, which serves to mitigate any dynamic load transfer from inner to outer vessel. Depending on the shock-attenuating approach, numerical predictions of these reaction loads can come with high levels of uncertainty due to model sensitivities. Present work here focuses on the numerical predictions and measurements of the reaction loads due to detonating 30 g of TNT equivalent in the Inner Pressure Confinement Vessel (IPCV) for proton imaging of small-scale shock physics experiments at LANL. Direct reaction load measurements from IPCV testing is presented alongside numerical predictions. Using the experimental measurements from the firing site, we refine the tools and methodology utilized for reaction load predictions and explore the primary model sensitivities which contribute to uncertainties. The numerical tools, modeling methodology, and primary drivers of model uncertainty identified here will improve the capability to model detonation experiments and enable design load calculations of other impulsively loaded pressure vessels with higher accuracy.

A containment and confinement pressure vessel system is under development to expand the capability to perform small explosively driven physics experiments at the Proton Radiography facility at Los Alamos National Laboratory (LANL). Two barriers of this vessel system are the Inner Pressure Confinement Vessel (IPCV) and the Outer Pressure Containment Vessel (OPCV). To achieve high spatial resolution of proton images, radiographic windows (covers) of the Inner Vessel are located extremely close to the experiment containing high explosive (HE). While the Inner Vessel is designed to meet the ASME Boiler and Pressure Vessel Code, Section VIII, Division 3, Code Case 2564 criteria, the small separation between the explosive and the pressure-retaining boundary presents a unique requirement for designing dynamically loaded vessels. We present numerical simulations of HE detonation in the Inner Vessel for several HE configurations. Eularian hydrodynamic code is used to calculate pressure-time history on the inner vessel surface. The pressure-time loading is then imported into a Langrangian structural model, and high-fidelity structural dynamic simulations are performed to obtain stress and strain as functions of time. Simulations are compared against experimental measurements from dynamic testing. Dynamic experiments are conducted in a low-fidelity (LoFi) vessel prototype, to measure the pressure and strain in regions of interest in different vessel locations (body, radiographic windows, covers).

A containment system is being developed to expand the capability of proton radiography of small-scale shock physics experiments at Los Alamos National Laboratory (LANL). The detonation of high explosives (HE) drives materials to extreme loading conditions, which are imaged using a proton beam and an imaging system. A qualified confinement and containment boundary needs to exist between a high-explosive experiment and the environment, and is comprised of the Inner Pressure Confinement Vessel (IPCV) and the Outer Pressure Containment Vessel (OPCV). The Inner Vessel is designed to the criteria of the ASME Boiler and Pressure Vessel Code, Section VIII, Division 3, Code Case 2564. The vessel contains an Experimental Physics Package, fragment mitigation structure, and radiographic windows. The windows need to minimize radiographic blur contribution (thin, radiographically transparent material such as Beryllium) over the field of view for imaging, but also need to maintain the pressure boundary during and after the dynamic event. Further, the vessel covers need to seal before, during, and after the experiment . In addition, the covers have miscellaneous feedthroughs, to enable high-voltage signal (for HE detonator), instrumentation and control signals (e.g. valves, pressure and vacuum gauge, optical fibers). We present the preliminary design, analyses, and testing of the Inner Vessel components.

This work describes the performance improvement of a polymer electrolyte fuel cell with a novel class of microporous layers (MPLs) that incorporates hydrophilic additives: one with 30 μm aluminosilicate fibers and another with multiwalled carbon nanotubes with a domain size of 5 μm. Higher current densities at low potentials were observed for cells with the additive-containing MPLs compared to a baseline cell with a conventional MPL, which correlate with improvements in water management. This is also observed for helium and oxygen experiments and by the lower amount of liquid water in the cell, as determined by neutron radiography. Furthermore, carbon-nanotube-containing MPLs demonstrates improved durability compared to the baseline MPL. Microstructural analyses including nanotomography demonstrate that the filler material in both the additive-containing MPLs provide preferential transport pathways for liquid water, which correlate with ex situ measurements. The main advantage provided by these MPLs is im...

Summary We report results of systematic, holistic, diagnostic, and cell studies to elucidate the mechanistic role of the experimentally determined influence of the anode gas-diffusion layer (GDL) on the performance of ultra-thin electrode polymer-electrolyte fuel cells, which can further enable fuel-cell market penetration. Measurements of product water balance and in situ neutron imaging of operational membrane-electrode-assembly water profiles demonstrate how improved performance is due to a novel anode GDL fiber-density modulated structure at the micrometer scale that removes water preferentially out of the anode, a key strategy to manage water in these cells. The banded structure results in low transport-resistance pathways, which affect water-droplet removal from the GDL surface. This interfacial effect is unexpectedly shown to be critical for decreasing overall water holdup throughout the cell. These studies demonstrate a new material paradigm for understanding and controlling fuel-cell water management and related high-power technologies or electrodes where multiphase flow occurs.

Understanding and predicting vortex-induced motion (VIM) of offshore systems for deep seawater applications is crucial to improve the system safety and integrity. We report on experimental tow-tank measurements and numerical simulations of VIM of a deep-draft offshore platform, specifically Paired-Column Semisubmersible (PC-Semi). The study is carried out in model scale (1:54), at subcritical flow regime with Re∼104. Motion of the floating structure has three degrees of freedom: in-line, cross-flow, and yaw. Large periodic cross-flow motion is measured for headings 0°, 11.25°, and 22.5°, for reduced velocities ( U r ) between 5 and 10. Considerably smaller cross-flow amplitude is recorded at 45° heading across the Ur range considered. An extensive sensitivity study is performed using computational fluid dynamics (CFD) to capture the transient displacement history of VIM (in-line, cross-flow, and yaw motion components). Amplitude and period of cross-flow (transverse) motion are obtained from statistical analysis of VIM time history and subsequently used as the validation criterion between the CFD simulation and the model tests. Satisfactory agreement between the CFD results and tow-tank measurements is achieved with a Delayed Detached Eddy Simulation – Shear Stress Transport (DDES-SST) formulation. This work provides experimental results and serves as a practical starting point to set up a CFD problem to estimate amplitude and period of cross-flow VIM motion for offshore engineering applications.

Neutron imaging is a completely non-destructive probe of the water content in operating fuel cells. However, the spatial and temporal resolution of the method limits the application range. We report on our continued imaging detector resolution improvements, where we have achieved a spatial resolution of 2 µm. We applied this new detector capability to measure the water content of two different membrane electrode assemblies where the ionomer to carbon mass ratio (I/C) was the only variable. A 1 cm2 active area test section with parallel flow channels was operated as a differential cell at 80 °C at constant voltage.

In-situ neutron diffraction measurements were performed during heat-treating and uniaxial loading of additively manufactured (AM) GP1 material. Although the measured chemical composition of the GP1 powder falls within the composition specifications of 17-4 PH steel, a fully martensitic alloy in the wrought condition, the crystal structure of the as-built GP1 material is fully austenitic. Chemical analysis of the as-built material shows high oxygen and nitrogen content, which then significantly decreased after heat-treating in a vacuum furnace at 650 °C for one hour. Significant austenite-to-martensite phase transformation is observed during compressive and tensile loading of the as-built and heat-treated material with accompanied strengthening as martensite volume fraction increases. During loading, the initial average phase stress state in the martensite is hydrostatic compression independent of the loading direction. Preferred orientation transformation in austenite and applied load accommodation by variant selection in martensite are observed via measurements of the texture development.

Doped ceria compounds demonstrate excellent radical scavenging abilities and are promising additives to improve the chemical durability of polymer electrolyte membrane (PEM) fuel cells. In this work, Ce0.85Zr0.15O2 (CZO) nanoparticles were incorporated into the cathode catalyst layers (CLs) of PEM fuel cells (based on Nafion XL membranes containing 6.0 μg cm−2 ion-exchanged Ce) at loadings of 10 and 55 μg cm−2. When compared to a CZO-free baseline, CZO-containing membrane electrode assemblies (MEAs) demonstrated extended lifetimes during PEM chemical stability accelerated stress tests (ASTs), exhibiting reduced electrochemical gas crossover, open circuit voltage decay, and fluoride emission rates. The MEA with high CZO loading (55 μg cm−2) demonstrated performance losses, which are attributed to Ce poisoning of the PEM and CL ionomer regions, which is supported by X-ray fluorescence (XRF) analysis. In the MEA with the low CZO loading (10 μg cm−2), both the beginning of life (BOL) performance and the performance after 500 hours of ASTs were nearly identical to the BOL performance of the CZO-free baseline MEA. XRF analysis of the MEA with low CZO loading reveals that the BOL PEM Ce concentrations are preserved after 1408 hours of ASTs and that Ce contents in the cathode CL are not significant enough to reduce performance. Therefore, employing a highly effective radical scavenger such as CZO, at a loading of 10 μg cm−2 in the cathode CL, dramatically mitigates degradation effects, which improves MEA chemical durability and minimizes performance losses.

A nested confinement (inner) and containment (outer) vessel system is under development to conduct small shock-physics experiments in a high-speed proton imaging facility at Los Alamos National Laboratory. The dual vessel system is necessary to serve as a qualified confinement system and containment buffer boundary between a high explosives experiment and the environment. The paper describes the preliminary engineering design and analyses that have been performed on the outer containment pressure vessel, following ASME BPVC Sect. VIII Div. 1, for both pressure and vacuum conditions. Other engineering attributes which will be presented include an internal support structure for a nested inner vessel, an external integrated support and alignment structure for the complete vessel system, and the vacuum and gas handling equipment.

A unique containment and confinement system is under development to conduct small explosively driven physics experiments containing hazardous materials at the Proton Radiography facility at Los Alamos National Laboratory (LANL). In these experiments, the detonation of high explosives (HE) is used to drive materials to extreme loading conditions, where some of the materials tested can be extremely hazardous (e.g. nuclear materials). The main components of the system are the Inner Pressure Confinement Vessel (IPCV, which hosts the physics experiment), the Outer Pressure Containment Vessel (OPCV) and Beam Pipes and Auxiliary Hardware (BPAH). This paper describes the design and preliminary analyses of the IPCV. The body of the IPCV, also referred to as the Inner Vessel, is being designed to the criteria of the ASME Boiler and Pressure Vessel Code, Section VIII, Division 3, Code Case 2564, with the exception of the materials of construction. The closure covers have different devices mounted on them, such as feedthrough devices for sending or receiving electrical and optical signals across the pressure boundary, and valves for venting the vessel interior. The unique feature in the vessel design are the radiographic windows, tentatively made of Beryllium, which need to be strong enough to maintain the pressure boundary during dynamic events, while being radiographically low-attenuating for the purpose of proton imaging.

Charpy test specimens were additively manufactured (AM) on a single stainless steel plate from a 17–4 class stainless steel using a powder-bed, laser melting technique on an EOS M280 direct metal laser sintering (DMLS) machine. Cross-hatched mesh support structures for the Charpy test specimens were varied in strut width and density to parametrically study their influence on the build stability and accuracy as the DMLS process has been known to generate parts with large amounts of residual stress. Neutron diffraction was used to profile the residual stresses in several of the AM samples before and after the samples were removed from the support structure for the purpose of determining residual stresses. The residual stresses were found to depend very little on the properties of the support structure over the limited range studied here. The largest stress component was in the long direction of each of the samples studied and was roughly 2/3 of the yield stress of the material. The stress field was altered considerably when the specimen was removed from the support structure. It was noted in this study that a single Charpy specimen developed a significant tear between the growth plate and support structure. The presence of the tear in the support structure strongly affected the observed stress field: the asymmetric tear resulted in a significantly asymmetric stress field that propagated through removal of the sample from the base plate. The altered final residual stress state of the sample as well as its observed final shape indicates that the tear initiated during the build and developed without disrupting the fabrication process, suggesting a need for in-situ monitoring.

Cerium is a radical scavenger which improves polymer electrolyte membrane (PEM) fuel cell durability. During operation, however, cerium rapidly migrates in the PEM and into the catalyst layers (CLs). In this work, membrane electrode assemblies (MEAs) were subjected to accelerated stress tests (ASTs) under different humidity conditions. Cerium migration was characterized in the MEAs after ASTs using X-ray fluorescence. During fully humidified operation, water flux from cell inlet to outlet generated in-plane cerium gradients. Conversely, cerium profiles were flat during low humidity operation, where in-plane water flux was negligible, however, migration from the PEM into the CLs was enhanced. Humidity cycling resulted in both in-plane cerium gradients due to water flux during the hydration component of the cycle, and significant migration into the CLs. Fluoride and cerium emissions into effluent cell waters were measured during ASTs and correlated, which signifies that ionomer degradation products serve as possible counter-ions for cerium emissions. Fluoride emission rates were also correlated to final PEM cerium contents, which indicates that PEM degradation and cerium migration are coupled. Lastly, it is proposed that cerium migrates from the PEM due to humidification conditions and degradation, and is subsequently stabilized in the CLs by carbon catalyst supports.

Additive manufacturing (AM) processes allow for complex geometries to be developed in a cost- and time-efficient manner in small-scale productions. The unique functionality of AM offers an ideal collaboration between specific applications of human variability and thermal management. This research investigates the intersection of AM, human variability and thermal management in the development of a military helmet heat exchanger. A primary aim of this research was to establish the effectiveness of AM components in thermal applications based on material composition. Using additively manufactured heat pipe holders, the thermal properties of a passive evaporative cooler are tested for performance capability with various heat pipes over two environmental conditions. This study conducted a proof-of-concept design for a passive helmet heat exchanger, incorporating AM components as both the heat pipe holders and the cushioning material targeting internal head temperatures of ≤ 35°C. Copper heat pipes from 3 manufactures with three lengths were analytically simulated and experimentally tested for their effectiveness in the helmet design. A total of 12 heat pipes were tested with 2 heat pipes per holder in a lateral configuration inside a thermal environmental chamber. Two 25-hour tests in an environmental chamber were conducted evaluating temperature (25°C, 45°C) and relative humidity (25%, 50%) for the six types of heat pipes and compared against the analytical models of the helmet heat exchangers. Many of the heat pipes tested were good conduits for moving the heat from the head to the evaporative wicking material. All heat pipes had Coefficients of Performance under 3.5 when tested with the lateral system. Comparisons of the analytical and experimental models show the need for the design to incorporate a re-wetting reservoir. This work on a 2-dimensional system establishes the basis for design improvements and integration of the heat pipes and additively manufactured parts with a 3-dimensional helmet.

Cerium ions enhance the chemical stability and lifetime of polymer electrolyte membrane (PEM) fuel cell components by rapidly and reversibly scavenging degrading radical species which are generated during operation [1]. However, during cell fabrication and discharge, these ions readily migrate between the membrane and catalyst layers (CLs) of the membrane electrode assembly (MEA) [2]. Complete washout of cerium from the MEA has also been observed [3]. It is necessary to understand the mechanisms and magnitude of cerium migration during fuel cell operation, since cerium ions are ineffective outside of the catalyzed area of the MEA. These results can be applied to improve cerium stability in the active area of the MEA and localize it to areas of high radical generation, which can further extend the lifetime of PEM fuel cells. Table 1: AST conditions and flow fields Test Time (h) RH (%) Flow field I 500 30 50 cm2 single serpentine II 2,000 100 25 cm2 tri-serpentine III 2,000 100 25 cm2 single serpentine Membrane chemical stability accelerated stress tests (ASTs) [4] were performed on cerium-containing MEAs at 90°C in single-cell hardware (Fuel Cell Technologies), compressed with 8 x bolts at 50 in-lb of torque, using the conditions and hardware shown in Table 1. Nafion XL (DuPont) membranes were used, which contain a nominal cerium loading of 6 μg Ce/cm2. Carbon-supported platinum electrodes (TKK, 48% Pt, 0.1-0.2 mg Pt/cm2loading) and Sigracet 25BC GDLs (SGL) were also used. X-ray fluorescence (XRF) was performed on MEA components before and after the ASTs in order to measure in-plane cerium content in the membrane and CLs. After 500 hours of OCV operation at 30% RH, cerium moved uniformly from the membrane of MEA I into the CLs (not shown). Here, migration is attributed cell component hot pressing and interactions with the CLs [2]. Membrane cerium was reduced to 3.7± 0.68 μg Ce/cm2, while anode and cathode CL concentrations were increased to 2.3 ± 0.06 and 3.4 ± 0.22 μg Ce/cm2, respectively. At 100% RH, cerium also remained in the active area, however, membrane concentration increased from inlet to outlet (Figure 1a). This gradient may arise due to the increased presence and flow of liquid water during 100% RH operation. Only trace amounts of cerium remained in the CLs, except near the outlet, which suggests that under humidified conditions, the effects of CL interactions on cerium migration are reduced. After 2,000 hours of operation at 100% RH, flow field compression was observed to have implications on cerium migration out of the active area. The cerium profile of MEA III (Figure 1b) shows that it migrated from areas of high compression in the active area (shown in red) into low compression regions outside of the active area. In contrast, compression was higher around the active area of MEA II (Figure 1a), which prevents cerium from leaching from it. However, concentration was non-uniform, as discussed above. These preliminary results indicate that cerium migration and leaching out of the active area are affected by membrane water content and cell clamping pressure. It is believed that other factors such as electrical potential and temperature influence migration, as well. The authors wish to acknowledge the financial support of the Fuel Cell Technologies Program and the Technology Development Manager Nancy Garland. References [1] F. D. Coms, H. Liu, J. E. Owejan, ECS Trans., 16, 1735-1747 (2008). [2] S. M. Stewart, D. Spernjak, R. L Borup, A. Datye, F. Garzon, ECS Lett., 3, F19-F22 (2014). [3] M. V. Lauritzen, S. Knights, T. Cheng, D. W. Banham, E. Kjeang, A. Sadeghi Alavijeh, Fuel Cells 2014 Science & Technology, April 2014, Amsterdam, The Netherlands [4] U.S. DOE, Cell Component Accelerated Stress Test Protocols for PEM Fuel Cells, 2010. Figure 1

PEM fuel cells (PEMFCs) show great promise to increase the fuel efficiency for transportation applications; however, for this application, they must show performance and durability with the requirements for transportation. For transportation applications, the fuel cell will be subjected to frequent power cycling. For example, the DOE/Fuel Cell Tech Team (FCTT) protocol for durability includes load cycling from 0.02 A/cm2 to 1.2 A/cm2 every 0.5 min. The cathode catalyst and catalyst layer have been shown as susceptible to degradation causing loss of performance due to both loss of kinetics for the oxygen reduction reaction and loss of mass transport. Catalyst support-carbon corrosion can result in thinning of the catalyst layer contributing to degradation in performance. To examine the effect of power cycling in situ on carbon corrosion and electrode degradation, we directly measured the catalyst support degradation by measuring CO2 in the cathode outlet by NDIR (Non-Dispersive Infra-Red) while operating a single-cell fuel cell. CO2 present in air was removed by a lime bed prior to introduction to the fuel cell. We operated with a modified DOE/FCTT durability protocol using controlled voltage, and varied the potential limits to explore the effects of the upper potential limit, lower potential limit, the potential step size and time at potential. The upper potential limit was varied from 0.95 to 0.55V; the lower potential limit from 0.40V to 0.80V, with times ranging from 0.5 min to 5 min. The corrosion of three different types of carbon were explored, high surface area (E), vulcan (V), and graphitized (EA). The catalyst support carbon corrosion occurs under normal fuel cell operating conditions and is exacerbated by the voltage cycling inherent in these steps in potential. A series of carbon corrosion spikes during potential cycling is shown in Figure 1 for E-type carbon, varying the upper potential from 0.95 V to 0.60V while keeping the lower potential constant at 0.40V. Sharp spikes in the carbon corrosion rate are observed during a step increase in cell potential with the magnitude of the spikes decreasing as the high cell potential is reduced from 0.95 V to 0.6 V. The carbon corrosion rate at high cell potential (0.95V) decreases with time at potential, indicating formation of passivating carbon surface oxides. Carbon corrosion was measured during the drive cycle measurements for all three types of carbon, with the relative carbon corrosion rates of E>V>EA. The series of step potential carbon corrosion spikes where the potential was varied for the lower potential from 0.40 V to 0.60V while keeping the upper potential constant at 0.95V shows similar results in terms of the carbon corrosion. The magnitude of the spikes decrease as the lower cell potential is raised. These results indicate that the size of the step in potential has a more significant impact on the carbon corrosion rate than does the absolute value of the potential for normal cathode operating potentials. The peak in CO2 evolution occurs when the cell potential increases from high power operation to low power near open circuit. This correlates with when CO2 evolution is observed during cyclic voltammograms, which occurs during the positive sweep at ~ 0.55 to 0.60 V. The evolution of this CO2 peak suggests that oxygen is adsorbed onto the carbon and/or CO is formed on the Pt surface. During long-term operation, a reduction in catalyst layer thickness is observed during drive cycle operation, which can be due to the loss of carbon through carbon corrosion or possibly due to compaction; both effects likely lead to a loss of void volume. This reduction in thickness includes a sharp decrease in catalyst layer thickness within the first 100 hours of operation (30%), eventually reaching ~50% of its thickness after 1000 hours. Most of this reduction in electrode thickness does not appear to be directly due to carbon corrosion as there is little evidence for carbon corrosion from microscopic analysis, especially during the early stages of operation, where the thickness reduction is substantially more than what should be due to carbon corrosion. These results show that carbon corrosion occurs during normal potential operation, and is exacerbated by potential variations during operation. To minimize the carbon corrosion, the size of the steps in potential should be minimized. For a more stable catalyst, another requirement is for the Pt particles to be stabilized on graphitized carbon supports. Acknowledgments Funding for this work is from DOE EERE FCTO, Technology Development Manager Nancy Garland Figure 1

The main impediment to the wide-range spread of proton exchange membrane fuel cells is most probably their low durability – at a reasonable production cost: highly dispersed and carbon supported catalysts developed to lower the cost of PEM systems suffer from a lack of stability due to carbon corrosion and catalyst dissolution. Several specific working conditions have been identified as responsible for accelerated catalyst degradation [1, 2]. Among them, the harshest one may be the fuel cell startup or shutdown -without any particular mitigation. During fuel cell startup and shutdown, a part of the anode compartment is filled with hydrogen while the complementary part remains occupied with oxygen-rich gases. In this case, the electric potential of the cathode facing the oxygen-rich portion can reach values as high as 1.6 V [3, 4] which entails accelerated carbon corrosion and catalyst degradation in the local regions exposed to air in the anode compartment the longest: degradations will be more severe near the anode outlet (inlet) in the case of startup (shutdown) [5] and heterogeneities were also observed recently between the channel itself and the rib [6, 7]. This corrosion phenomenon is now relatively well characterized thanks to segmented cells [8]. In the experimental part of this work, separate testing protocols for fuel cell startup and shutdown were developed to distinguish between their effects on performance degradation. The internal currents and the local potentials (Figure 1) were measured with different membrane-electrode assemblies (MEAs): we examined the influence of the cathode and anode Pt loading, the type of carbon for cathode catalyst support and monitored the time evolution of spatially-resolved performance decrease and electrochemical active surface area (ECSA). Both the CO2 emissions and the charge exchanged –between the passive and the active regions of the cell- increased with the common residence time of air and hydrogen in the anode compartment. However, the evolved CO2 accounted for less than 25 % of the total exchanged charge indicating the predominance of other reactions: water electrolysis, Pt oxidation... Startups were also consistently more damaging than the shutdowns, evidenced by more evolved CO2, severe ECSA decrease, and higher performance losses. The objectives of the modelling part of the work were to quantify mathematically the redox reactions occurring during startups and shutdowns in order to understand in detail the influence of the experimental parameters varied above. The numerical approach is based on a model that takes account variations in gas concentration and platinum oxide coverage between the cell inlet and outlet. Mass transport in the direction perpendicular to the membrane and electrochemical phenomena are modeled locally (along parallel hydrogen and air channels) while the concentration of gases in the channels are imposed as boundary conditions, as functions of space and time, so that this model can be considered as "pseudo2D" [9]. [1] R. Borup et al., Chem. Rev. 2007, 107, 3904-3951. [2] L. Dubau et al., Wiley interdisciplinary reviews-energy and environment, Vol 3, Issue 6, pp 540-560, 2014. [3] Q. Shen et al., J. Power Sources, 189, (2009). [4] C.A. Reiser et al., Solid-State Lett., 8, A273 (2005). [5] A. Lamibrac et al., J. Power Sources, 196, (2011). [6] J. Durst et al., App. Catalysis B. : env. , 138-139, (2013). [7] I.A. Schneider and S. von Dahlen, Electrochem. Solid-State Lett., 14, B30 (2011). [8] J. Dillet et al., J. Power Sources 250C (2014). [9] G Maranzana et al., J. Electrochem. Soc., 162 (7), F694-F706 (2015). Figure 1

International audience; Operating a proton exchange membrane (PEM) fuel cell with a dead-ended anode may lead to local fuel starvation due to the excessive accumulation of liquid water and possibly nitrogen (because of membrane crossover) in the anode compartment. In this paper, we present experimental results obtained with a segmented, linear cell with reference electrodes along the gas channels, used to record local anode and cathode potentials. By simultaneously monitoring the local potentials and current densities during operation, we assessed the impact of fuel starvation on local fuel cell performance during aging protocols consisting of repeated dead-ended anode operation sequences (with anode outlet closed longer than in real use conditions). During the aging protocols, we observed strong local cathode potential excursions close to the anode outlet. The cathode showed non-uniform ElectroChemical Surface Area (ECSA) losses and performance degradation along the cell area. The damage was more severe in the regions suffering the longest from fuel starvation. Similar experiments performed in different operating conditions and with different membrane thickness showed that water management impacts significantly the cathode potential variations and thus the MEA degradation. Most of the MEA degradation is attributed to local cathode potential excursions above 1.2 V although potential cycling between 0.5 V and 0.7 V also had an impact in the regions well supplied with hydrogen (hydrogen purges were triggered when the fuel cell voltage dropped from about 0.7 V to 0.5 V). According to our results, localized and transient hydrogen starvation events may be difficult to detect by considering only the overall fuel cell performance.

Fused deposition modelling (FDM) is an additive manufacturing (AM) technique which involves melting a thermoplastic filament material and subsequently extruding it, layer by layer, to create three-dimensional objects. The nature of this build process yields parts with inhomogeneous compositions, which may result in anisotropic thermal and mechanical properties. In this work, such anisotropies were investigated for different commercially-available FDM materials such as polylactic acid, acrylonitrile butadiene styrene, and polyurethane. Due to the biaxial symmetry of some properties of resulting FDM parts, a transversely isotropic material model was developed for simulating the FDM part response to thermal and mechanical loads. Such a model is more robust than an isotropic model and, when compared to a full orthotropic model, requires fewer elastic constants to be experimentally determined. Ultimately, the development of FDM-specific thermomechanical property data and models for AM parts will provide more accurate parameters for part designs, leading to higher confidence in part qualification.

Experiments and simulations were performed to assess the performance of an HVAC system for the cooling of a Leitz Infinity coordinate measurement machine (CMM) enclosed within a glove box (GB). Manufacturer specifications require maintaining very uniform temperatures with spatial and temporal variations not to exceed 0.3 °C/hr, 0.4 °C/day, and 0.1°C/m. Data were collected at 0.17 Hz by 2 thermocouples located outside the glovebox, 10 static thermocouples located inside the glovebox, and up to 28 thermocouples attached to the moving granite table of the CMM. The latter thermocouples are arrayed in a grid in the volume of interest (VOI) which envelopes the motion of the CMM measuring head above the granite table. Data were collected for periods ranging from 1 to 5 days to observe the effects of temperature variations within the enclosing facility. Simulations were then performed on the enclosed volume of the GB using ANSYS-CFX to better understand the heat loads, and test temperature variation mitigation strategies. These simulations consisted of 18 runs which varied heat input from the CMM motors, inflow gas temperature from the HVAC system into the GB, and non-uniform GB wall temperature boundary conditions. Heat loads from the motors were found to be insignificant influences on the temperature distribution, while fluid entrainment inside the diffuser was discovered to lead to an adverse temperature distribution, and insufficient cooling in the VOI. Velocity distributions were examined by using a TSI VelociCalc 8345 to verify the presence of stagnant regions in the GB. Finally, modifications to the diffuser design were proposed to eliminate entrainment, improve the flow distribution, and enhance temperature uniformity.

This study focuses on the micro-crack formation of Nafion ® -based membrane electrode assemblies (MEAs) after extended direct methanol fuel cell (DMFC) operation. All electrodes, both with metal-black and carbon-supported catalysts, contain some micro-cracks initially; the area covered by these cracks increases both in the anode and cathode after 100-hours of DMFC test. X-ray tomography shows an increase in the crack area in both anode and cathode that correlates with methanol feed concentration and methanol crossover. The MEAs with carbon-supported catalysts and thicker membrane are more resistant to the formation of micro-cracks compared to those with metal-black catalysts and thinner membrane, respectively. The impact of the micro-crack formation on cell performance and durability is limited over the 100-hour DMFC operation, with the long-term impact remaining unknown.

Lean-burn operation of both diesel and gasoline direct injection engines has the potential to increase fuel economy by consuming almost all of the fuel in the presence of excess air. However, the oxygen rich exhaust prevents reduction of NO via three-way catalytic converters commonly used to reduce emissions in stoichiometric engines1. Vehicle manufactures are pursuing selective catalyst reduction (SCR) and exhaust gas recirculation (EGR) systems to meet EPA emissions regulations for lean-burn engines. SCR systems typically use a zeolite catalyst that selectively adsorbs NOx molecules and converts them to N2 and H2O with the injection of a urea water solution. In EGR, exhaust gas is mixed into the intake manifold air in order to lower the temperature of combustion, reducing NOx emissions. EGR systems have been used in gasoline engines for several decades, however the higher oxygen content of diesel exhaust requires that it be pre-cooled before mixing with the air intake. The percentage of recirculation and temperature of the exhaust gas need to be carefully controlled in order to reduce emissions without sacrificing engine efficiency and fuel economy. Exhaust gas sensors are needed to monitor emissions and control and maintain efficient operation of SCR and EGR systems. Oxygen λ-sensors are an integral part of onboard diagnostics (OBD) of today’s stoichiometric vehicle emission systems, but a suitable analog is not available for NOx and NH3 detection and discrimination in lean-burn engines. Mixed-potential sensors fabricated via well-established commercial manufacturing methods present a promising avenue to enable the widespread utilization of NOx and NH3 sensing technology. These electrochemical devices develop a non-Nernstian potential due to differences in the redox kinetics of various gas species at each electrode/ electrolyte/ gas interface. The electrode potential is not fixed by equilibrium thermodynamics as it is in the O2 λ-sensor, but rather by the rates of different electrochemical reactions occurring simultaneously at each electrode/ electrolyte interface. Therefore, device-to-device reproducibility of the triple phase interface, as well as long term morphological stability are required. The patented LANL sensor design incorporates dense electrodes and a porous electrolyte 2. The use of dense electrodes minimizes deleterious heterogeneous catalysis, and the increased morphological stability of dense electrodes yields a robust electrochemical interface, increasing lifetime durability 3. The Materials Synthesis and Integrated Devices group at LANL has worked in collaboration with Electro-Science Laboratories (ESL, King of Prussia, PA) to fabricate planar mixed-potential sensors via the readily scalable, cost-effective high temperature co-fired ceramic (HTCC) technology already employed in the manufacturing of planar O2 λ-sensors. The two sides of a planar, self-heated, tape cast sensor prototype are shown in Figure 1. A Pt-heater with independent leads is printed on the backside of the ceramic substrate. A heater is employed to operate the devices between 400-600˚C where the YSZ possesses sufficient ionic conductivity and the rates of the electrochemical reactions do not impede the response time of the device. The sensing element is printed on the opposite side of the ceramic substrate and consists of dense, screen-printed electrodes coated with a porous YSZ electrolyte layer. We have developed two distinct sensing platforms for NOx and NH3 detection. The NOx sensor consists of La1-xSrxCrO3 and Pt electrodes. Under open circuit conditions, this sensor responds to many of the constituents of concern in emissions monitoring, however, the selectivity may be tuned by operating conditions 4. By operating at a small positive current bias, this sensor becomes preferentially selective to NOx species. Selectivity towards non-methane hydrocarbons (NMHC) is improved by operation at elevated temperatures, however sensitivity is reduced. The NH3 sensor consists of Au/Pd and Pt electrodes and is minimally sensitive to NOx species with varying sensitivity to NMHCs depending on operating temperature. The voltage response of both sensor constructs to various gases is shown in Figure 2. References 1. J. E. Parks, 2nd, Science 327 (5973), 1584-1585 (2010). 2. R. Mukundan, E. L. Brosha and F. H. Garzon, US Patent no.s. 6,605,202 and 6,656,336 (2003). 3. R. Mukundan, E. L. Brosha and F. H. Garzon, Journal of The Electrochemical Society 150 (12), H279 (2003). 4. C. R. Kreller, P. K. Sekhar, W. Li, P. Palanisamy, E. L. Brosha, R. Mukundan and F. H. Garzon, ECS Transactions 50 (12), 307-314 (2012). Acknowledgments The research was funded by the US DOE, EERE, Vehicle Technology Programs. The authors wish to thank Technology Development Manager Roland Gravel.

International audience; Separate testing protocols for fuel cell startup (SU) and shutdown (SD) are developed to distinguish between the effects of \SU\ and \SD\ on performance degradation. The internal currents during \SU\ and \SD\ operation are measured in a segmented cell to evaluate the charge exchanged between the active (H2/Air) and passive (Air/Air) portions of the cell. Cells with different membrane-electrode assemblies (MEAs) are subjected to \SU\ or \SD\ sequences to evaluate the time evolution of spatially resolved decrease of performance and electrochemical active surface area (ECSA). We examine the influence of the cathode and anode Pt loading, and the type of carbon for cathode catalyst support. Both the \CO2\ emissions and the charges exchanged increase with the common residence time of air and hydrogen in the anode compartment. However, the evolved \CO2\ accounts for less than 25% of the total exchanged charge. Startups are consistently more damaging than the shutdowns, evidenced by more evolved \CO2\ and charge exchanged, severe \ECSA\ decrease, and higher performance losses.

Commercial tape casting and screen-printing methods have been used to fabricate a planar, self-heated, mixed-potential NOx sensor for application in vehicle on-board emission control systems. The device consists of dense La0.8Sr0.2CrO3 (LSCrO) and Pt electrodes and a porous YSZ electrolyte on one side of a dense ceramic substrate and a Pt-heater with independent leads on the backside of the substrate. While these sensors have demonstrated high sensitivity and selectivity to NOx when operated at a positive bias, optimization of the sensor device geometry remains an open question. In this work, we used circular cells of dense YSZ with LSCrO working and Pt counter electrodes in order to identify the impedance response of each individual sensor component. The impedance response measured on the cell was then used to identify the rate-limiting processes underlying the response of the planar sensor device.

Component durability of polymer-electrolyte membrane (PEM) fuel cells can be improved by adding cerium cations, which serve to scavenge harmful free radicals and selectively decompose hydrogen peroxide, which are formed during the oxidation reduction reaction (ORR). We have investigated the change in distribution of cerium cations in a hydrogen fuel cell as a function of operating time, considering both cerium containing membranes (commercial XL by DuPont) as well as fuel cells with CeO2 in the cathode catalyst layer. Our results show cerium cations are very mobile in Nafion, and migrate into both the anode and cathode catalyst

International audience; • Degradation due air/air operation due to startup and shutdown in fuel cell studied. • The effect of platinum loading, and carbon support material is studied. • A segmented cathode hardware is utilized to study the effect along the flow field. • In-situ and ex-situ characterization were correlated to elucidate the degradation. • Limiting the anode's ability to reduce oxygen to water is key to mitigating loss. A B S T R A C T Polymer electrolyte membrane fuel cells have durability limitations associated with the startup and shutdown of the fuel cell, which is critical for real-world vehicle commercialization. During startup or shutdown, there exists an active region (hydrogen/air) and a passive region (air/air) between the cell inlet and outlet. Internal currents are generated in the passive region causing high-potential excursion in the cathode leading to accelerated carbon corrosion. In this study, a segmented cathode hardware is used to evaluate the effect of platinum loading on both cathode and anode, and carbon support material on degradation due to repeated series of startups or shutdowns. In situ losses in the performance and electrochemical surface area were measured spatially, and ex situ analysis of the catalyst layer thickness and platinum particle size was performed to understand the effect of startup or shutdown on different membrane electrode assembly materials. Startup degrades the region near anode outlet more, while shutdown degrades the region near anode inlet more compared to the rest of the electrode. While various system mitigation strategies have been reported in the literature to limit this degradation, one materials mitigation strategy is to limit the anode's ability to reduce oxygen to water through increasing the ratio of platinum loading in the cathode to the anode, or by using a bi-functional catalyst.

Neutron imaging was been widely used to study the water distribution in proton exchange membrane fuel cell flow fields and gas diffusion layer. However, due to the limitation of spatial resolution, there has been little focus on the water transport process in the membrane and catalyst layer. Here we report on measurements made on thick membranes under saturation gradients which show no “jump condition” and on thick cathode catalyst layers to understand the water transport issues in a non-precious metal catalyst. Finally, we speculate on the possibility of obtaining neutron images with ~1 µm spatial resolution.

Validation of computational models for polymer electrolyte membrane fuel cell (PEMFC) performance is crucial for understanding the limits of the model predictions. We compare predictions from a multiphase PEMFC computational model with experimental data collected under various current density, temperature and humidification conditions from a single 50 cm 2 PEMFC with a 10 × 10 segmented current collector. Both cell voltage and current distribution measurements are used to quantify the predictive capability of the computational model. Several quantitative measures are used to quantify the error in the model predictions for current distribution, including root mean square error, maximum/minimum local error, and local error averaged from inlet to outlet. The cell voltage predictions were within 15 mV of the experimental data in the current range from 0.1 to 1.2 A cm −2 , and the current distributions were acceptable (less than 30% local error) except for the low temperature case, where the model overpredicted the current distribution. Particular attention was paid to incorporating experimental variability into the model validation process.

Understanding and optimizing water and thermal management in the catalyst layer of proton-exchange-membrane fuel cells is crucial for performance and durability improvements. This is especially the case at low temperatures, where liquid water and even ice may exist. In this article, the durability of a traditional Pt/C dispersed and a nanostructure thin film (NSTF) membrane-electrode assembly (MEA) are examined under wet/dry and freeze/thaw cycles using both in situ and ex situ experiments. Multiple isothermal cold starts result in a performance degradation for the dispersed MEA, while no such a degradation is found in the NSTF. The results are consistent with stand-alone MEA tests, wherein the dispersed catalyst layer results in an exponential increase in the number and size of cracks until it delaminates from the membrane due to the impact of the freeze/thaw process within the catalyst-layer pores. The NSTF catalyst layer shows minimal crack generation without delamination since the ice forms on top of the layer. The results are useful for understanding degradation due to phase-change containing cycles.

Accelerated Stress Tests (ASTs) to characterize carbon corrosion were performed on MEAs based on 3 different carbon supports. High surface area carbon exhibited the best initial performance but the fastest degradation rate. On the other hand, highly graphitized carbon exhibiting the slowest degradation rate but had the lowest initial performance. TEM analysis of the MEAs after corrosion indicated Pt particle size growth in all the catalyst layers in addition to significant thinning of the high surface area carbon-based catalyst layers. Voltage loss breakdown identified mass transport losses resulting from a compaction of the catalyst layer porosity as the greatest contributor to performance loss. Three different membrane ASTs were performed on 2 distinct MEAs (designated P5 and HD6) from Ballard Power Systems and the degradation compared to that observed in the field. The membrane chemical degradation AST resulted in significant membrane thinning not observed in the field. The membrane mechanical degradation AST was able to reproduce the degradation phenomenon observed in the field but had little ability to distinguish between various membranes. A combined mechanical/chemical AST was examined to better simulate the degradation rates observed in the field.

Semisubmersible platforms are good candidates for hydrocarbon exploration and production in the Gulf of Mexico (GoM) and elsewhere. These platforms are a preferred choice for deep-water applications involving higher design throughput. A dry tree application of semisubmersible provides the benefit of having improved well control, direct vertical well access, easy access to production equipment, reduced capital expenditures and other dry tree benefits. These are in addition to the benefits that a semisubmersible platform has over spars and tension leg platforms (TLPs). A public-private partnership has sponsored multiple projects since 2009 with the aim of maturing a dry tree semisubmersible design that is cost-effective and safe like spars and TLPs. Due to the deeper draft of the semisubmersibles proposed for the dry tree applications, vortex induced motion (VIM) is an area of concern that needs to be addressed during the design stage. The research work presented here is a part of the "Vortex Induced Motion Study for Deep Draft Column Stabilized Floaters." The effect of additional damping due to mooring lines and risers on the VIM response of a deep draft semisubmersible is an area of focus of the ongoing project. CFD-based predictions show a significant reduction in the VIM response when additional damping is considered. With the intent of validating the CFD models and to further understand the effect of additional damping on the VIM response, a model test campaign was conducted with different levels of applied additional damping. The paired column semisubmersible and conventional semisubmersible platforms were tested, with and without damping and the results are discussed here. In addition, during model testing, the conventional semisubmersible hull was equipped with a column force measurement system to measure the hydrodynamic forces on the individual columns. The results of the study show that damping plays a significant role in reducing the platform VIM which directly impacts the estimated fatigue damage of mooring lines and risers, and which in turn can reduce the overall cost of the system.

Corrosion of the catalyst support reduces PEM fuel cell performance via catalyst layer (CL) degradation (loss of porosity, catalyst connectivity, and active catalyst surface area). Carbon corrosion was investigated in a segmented cell for cathode gas diffusion layers (GDLs) with and without a microporous layer (MPL) to investigate the spatial aspects of GDL effect on corrosion. The cells were aged in situ using an accelerated stress test (AST) for carbon-support corrosion consisting of consecutive holds at 1.3 V. Carbon corrosion was quantified by measuring CO2 evolution during the AST. Performance degradation was substantial both with and without cathode MPL, but the degradation of the CL after prolonged corrosion was lower in the presence of an MPL. This was corroborated by better cell performance, higher remaining Pt active area, lower kinetic losses and smaller Pt particle size. The cell with an MPL showed increasingly nonuniform current distribution with corrosion time, which is correlated to the distribution of the Pt particle growth across the active area. This cell also showed an increase in mass-transport resistance due to MPL degradation. Without an MPL, GDL carbon fibers caused localized thinning in the cathode CL, originating from the combined effects of compression and corrosion.

Carbon corrosion in PEM fuel cells was induced by a potential-hold accelerated stress test in 2.5 cm2 active area cells. Polarization curves and electrochemical impedance spectra quantified performance degradation from a combination of kinetic and mass transport losses. High-resolution neutron imaging of the operating cells showed a dramatic decrease in water retention at the same current density in the corroded cells, attributed to increasing internal heat generation. Omission of a cathode-side microporous layer resulted in significant differences in both water profiles and performance degradation. Post-mortem characterization of cell components was carried out by x-ray photoelectron spectroscopy to quantify changes in surface functionality.

11th Polymer Electrolyte Fuel Cell Symposium (PEFC) Under the Auspices of the 220th Meeting of the ECS -- OCT, 2011 -- Boston, MA, WOS: 000309598800030, In proton exchange membrane (PEM) electrolyzers, oxygen evolution in the anode and flooding due to water cross-over results in two distinct two-phase transport conditions, and these two phenomena were found to strongly affect the performance. A comprehensive understanding of two-phase flow in PEM electrolyzer is required to increase efficiency and aid in material selection and flow field design. In this study, two-phase transport in an electrolyzer cell is visualized by simultaneous neutron radiography and optical imaging. Optical and neutron data were used in a complementary manner to aid in understanding the two-phase flow behavior. The behavior of the gas bubbles was investigated and two different gas bubble evolution and departure mechanisms are found. It was also found that there is a strong non-uniformity in the gas bubble distribution across the active area, due to buoyancy and proximity to the water and purge gas inlet., ECS, Energy Technol (ETD), Phys & Analyt Electrochem (PAED), Battery (BATT), Ind Electrochem & Electrochem Engn (IEEE), Corros (CORR), Scientific and Research Council of Turkey (TUBITAK); National Science Foundation [CBET-0748063]; U.S. Department of Commerce; NIST Ionizing Radiation Division; Director's Office of NIST; NIST Center for Neutron Research; Department of Energy [DEAI01-01EE50660], Omer F. Selamet would like to thank the Scientific and Research Council of Turkey (TUBITAK) for financial support for this research. Financial support for this work from the National Science Foundation (CBET-0748063) is gratefully acknowledged. We thank professors Ajay K. Prasad and Suresh G. Advani of the University of Delaware for their assistance with the experimental setup. The authors thank Elias Baltic of the NIST for his technical help during the experiments in the NIST. This work was supported by the U.S. Department of Commerce, the NIST Ionizing Radiation Division, the Director's Office of NIST, the NIST Center for Neutron Research, and the Department of Energy through Interagency Agreement No. DEAI01-01EE50660. We also thank to Richard S. Fu for his help during the data analysis.

We present recent work on quantifying the impact of local temperature data in polymer electrolyte membrane fuel cells (PEMFC) model predictions. In particular, we compare differences in prediction of cell voltage and local current distribution when using either i) uniform cathode collector temperature and ii) spatially variable cathode collector temperature obtained from experimental data. We find that changes in temperature of +5/-2 C can result in +/-3 percent change in local current distribution. We conclude that local temperature data should be incorporated into PEMFC models in validation procedures when available.

The gas diffusion layer in a polymer electrolyte fuel cell is the component primarily responsible for effective water management under a wide variety of conditions. The incorporation of hydrophilic alumosilicate fibers in the microporous layer leads to an improvement in the fuel cell performance associated with a decrease in the mass transport resistance especially under high RH operation. This improvement in performance is obtained without sacrificing performance under low RH conditions. The alumosilicate fibers create domains that wick liquid water away from the catalyst layer. The improved mass transport performance is corroborated by AC impedance and neutron radiography analysis and is consistent with an increase in the average pore diameter inside the microporous layer.

Sufficient water content within a polymer electrolyte membrane (PEM) is necessary for adequate ionic conductivity. Membrane hydration is therefore a fundamental requirement for fuel cell operation. The hydration state of the membrane affects the water transport within, as both the diffusion coefficient and electro-osmotic drag depend on the water content. Membrane's water uptake is conventionally measured ex situ by weighing free-swelling samples equilibrated at controlled water activity. In the present study, water profiles in Nafion® membranes were measured using high-resolution neutron imaging. The state-of-the-art, 13 μm resolution neutron detector is capable of resolving water distributions across N1120, N1110 and N117 membranes. It provides a means to measure the water uptake and transport properties of fuel cell membranes in situ.

Water content and dynamics were characterized and compared in situ by simultaneous neutron and optical imaging for three PEM fuel cell flow fields: parallel, serpentine, and interdigitated. Two independent sets of images were obtained simultaneously: liquid water dynamics in the flow field (channels and manifolds) were recorded by a digital camera through an optical window, while the through-thickness integrated water content was measured across the cell area by neutron imaging. Complementary data from the concurrent images allowed distinguishing between the water dynamics on the cathode and the anode side. The transient water content within the cell measured using neutron imaging is correlated with optical data as well as with temporal variations in the cell output and pressure differentials across the flow fields. Water dynamics on both the cathode and anode side were visualized and discussed. The serpentine cell showed stable output across the current range and the highest limiting current. Parallel and interdigitated cells exhibited substantially higher water contents and lower pressure differentials than the serpentine. Anode flooding significantly impeded their performance at high current. At moderate current, cell output correlated with the changes in water distribution in the cathode flow field rather than with the variations in the overall water content. Performance of the interdigitated cell was similar to the serpentine one in spite of the vastly different water contents. The cell's water-content response to a step-change in current revealed three distinct stages of water accumulation. Flow field configuration greatly affected both the amount of water accumulated in the cell and the duration of each stage.

Liquid water formation and transport were investigated by direct experimental visualization in an operational transparent single-serpentine PEM fuel cell. We examined the effectiveness of various gas diffusion layer (GDL) materials in removing water away from the cathode and through the flow field over a range of operating conditions. Complete polarization curves as well as time evolution studies after step changes in current draw were obtained with simultaneous liquid water visualization within the transparent cell. The level of cathode flow field flooding, under the same operating conditions and cell current, was recognized as a criterion for the water removal capacity of the GDL materials. When compared at the same current density (i.e. water production rate), higher amount of liquid water in the cathode channel indicated that water had been efficiently removed from the catalyst layer. Visualization of the anode channel was used to investigate the influence of the microporous layer (MPL) on water transport. No liquid water was observed in the anode flow field unless cathode GDLs had an MPL. MPL on the cathode side creates a pressure barrier for water produced at the catalyst layer. Water is pushed across the membrane to the anode side, resulting in anode flow field flooding close to the H2 exit.