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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.

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.

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.

Previous reports have documented the results of applying dynamic system simulation (DSS) techniques to model a variety of fissile solution systems. The SUPO (Super Power) aqueous homogeneous reactor (AHR) was chosen as the benchmark for comparison of model results to experimental data for steadystate operation.1 Subsequently, DSS was applied to additional AHR to verify results obtained for SUPO and extend modeling to prompt critical excursions, ramp reactivity insertions of various magnitudes and rate, and boiling operations in SILENE and KEWB (Kinetic Experiment Water Boiler).2 Additional models for pressurized cores (HRE: Homogeneous Reactor Experiment), annular core geometries, and accelerator-driven subcritical systems (ADAHR) were developed and results reported.3 The focus of each of these models is core dynamics; neutron kinetics, thermal hydraulics, radiolytic gas generation and transport are coupled to examine the time-based evolution of these systems from start-up through transition to steady-state. A common characteristic of these models is the assumption that (a) core cooling system inlet temperature and flow and (b) plenum gas inlet pressure and flow are held constant; no external (to core) component operations that may result in dynamic change to these parameters are considered. This report discusses extension of models to include explicit reference to cooling structuresmore » and radiolytic gas handling. The accelerator-driven subcritical generic system model described in References 3 and 4 is used as a basis for this extension.« less

The durability of polymer electrolyte membrane (PEM) fuel cells is a major barrier to the commercialization of these systems for stationary and transportation power applications. By investigating cell component degradation modes and defining the fundamental degradation mechanisms of components and component interactions, new materials can be designed to improve durability. To achieve a deeper understanding of PEM fuel cell durability and component degradation mechanisms, we utilize a multi-institutional and multi-disciplinary team with significant experience investigating these phenomena.

Deep Draft Column Stabilized Floaters (DDCSFs) provide a viable dry-tree platform for ultra-deep water applications. A particular area of concern for this concept is its susceptibility to Vortex-Induced-Motions (VIM) due to its deeper draft that results in higher column slenderness ratios than conventional semisubmersibles. The VIM characteristics of platforms have traditionally been assessed through experimental measurements but in recent years Computational Fluid Dynamics (CFD) has been used alongside experiments to predict VIM. Besides solving problems that cannot be addressed by other analysis methods, CFD can be seamlessly integrated into the overall concept design cycle where different geometric parameters can be varied to optimize the performance of the platform. While tank experiments give little fluid flow information without extensive instrumentation, CFD solutions provide flow details that can be used to improve design. CFD also provides a cost-effective solution given the high cost of experiments and facility scheduling constraints. One such DDCSF concept is the Paired-Column semi-submersible (PC-Semi) and the design has a pair of columns instead of one at each of the four sides of the platform. Several design parameters of the columns, like the inter-column spacing, the cross-sectional areas, and the shape of the respective columns can be tuned to minimize the heave response. This study takes a detailed look at the effect of the change in the column-dependent parameters on the overall VIM characteristics of the PC semisubmersible. A two-step approach is presented in which the first step is to identify aspects of the physics that are unique to VIM and then formulate a methodology to predict it within the framework of a set of commercially available CFD tools. The methodology is verified further through a comparison with available experimental data. The second step is to apply the verified methodology and CFD tools to predict the VIM performance of a range of PC-Semi concepts obtained through a systematic variation of the column-dependent design parameters in model scale. Through this exercise the critical design parameters that can either improve or prove detrimental to the VIM performance of the PC-Semi design are identified.Copyright © 2015 by ASME

Vortex-Induced-Motion (VIM) is an important issue in offshore engineering as it impacts the integrity of the mooring system for floating structures such as oil platforms and wind turbine platforms. Understanding and predicting VIM is a challenging task because of the inherent complexity of vortex structure shedding and fluid-structure interaction (FSI) in high Reynolds number flows. Computational Fluid Dynamics (CFD) is one of the key tools in VIM studies and optimization of the offshore systems design. We report a CFD sensitivity study with focus on turbulence model, mesh refinement, and time-step selection. Experimental measurements in a tow-tank facility are used to validate the CFD results. Three types of tank tests are modeled numerically: current drag, oscillating free decay, and VIM. The effect of turbulence model is evaluated by comparing Delayed Detached Eddy Simulation (DDES) and Unsteady Reynolds-Averaged Navier-Stokes (URANS) models. The influence of mesh refinement and time step is investigated by using the grid convergence index (GCI). For present geometry and flow conditions (Re∼105), the DDES turbulence model demonstrates better agreement with experimental measurement in model scale VIM compared to the URANS model. In addition, DDES simulation captures the vortex structure more realistically, as evidenced by Q-criteria and turbulent eddy viscosity distribution. Finally, we show how the mesh refinement and time step selection affect simulation accuracy. Two viscous-flow commercial solvers are tested: the finite-volume solver ANSYS-Fluent™, and the finite-element solver Altair AcuSolve™. The results of this CFD Sensitivity study provide useful guidelines for CFD simulation of FSI and VIM problems for offshore engineering applications.Copyright © 2015 by ASME

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 cm2active area test section with parallel flow channels was operated as a differential cell at 80 °C at constant voltage. Shown in the figure are the imaged water content at 0.4 V, 100% inlet relative humidity for the I/C = 0.9 and I/C = 1.1. Figure Caption: Neutron images of the water content of an operating fuel cell at 0.4 V; a) I/C of 1.1, b) 2I/C of 0.9. In both cases, the cathode is toward the top of the image and the anode toward the bottom. The authors from LANL are supported by DOE Fuel Cell Technologies Office, through the Fuel Cell Performance and Durability (FC-PAD) Consortium; Fuel Cells program manager: Dimitrios Papageorgopoulos. The authors from NIST are supported by the U.S. Department of Commerce, the NIST Radiation and Physics Division, the Director's office of NIST, the NIST Center for Neutron Research, and the Department of Energy interagency agreement No. DE_AI01-01EE50660, program manager: Nancy Garland. Figure 1

Stratified catalyst layers can increase fuel cell performance compared to standard flat catalyst layers1,2 due to improved mass transport because of their irregular thickness and porosity. Stratified catalyst layer structures are expected to have enhanced performance in mass transport region due to improvements in water removal from the catalyst layer. Custom electrodes are fabricated with a custom designed spray coating procedure and catalyst ink recipe. Results from multiple fabrication approaches will be discussed to achieve the electrode structure with the highest performance. One approach used involves a topographical patterning of the catalyst layer, and is based on Ion Power proprietary manufacturing techniques. The second approach uses glass epoxy masks during the spray coating process to create a patterned electrode on the GDL with varying thickness. The mask dimensions mirror the bipolar plate flow-field at the cathode, exposing either the land or channel regions. This way the two extreme cases were investigated and the resulting performance compared. The application of more catalyst material in the channels was found to be more beneficial than when applied in the land regions, likely due to rapid reaction times via better access of the incoming reactant gases through the channels. However, the presence of a thicker catalyst layer under the channel may be an issue for product water removal and oxygen diffusion at high current densities. The best performance was observed when in addition to applying the catalyst preferentially in the channels, a carbon-ionomer filler was used in the land region. The filler is used for both mechanical stabilization of the catalyst layer and to improve ionic and electronic conductivity within the catalyst layer. Two ionomer to carbon (I/C) ratios were tested in the carbon-ionomer filler. The high I/C ratio matches the I/C ratio of the Pt/C ink and is equal to 0.9. This case achieved a significant improvement in the kinetics of the catalyst, but had no effect on mass transport. When the I/C ratio was reduced to 0.6 in the carbon-ionomer filler, there was a visible improvement in mass transport. This is likely due to lower ionomer content which results in less water retention in the catalyst later. In order to better understand the performance mechanisms taking place, the effect of adding hydrophobic agents in the carbon filler is examined. This includes the addition of Polytetrafluoroethylene (PTFE) and Fluorinated ethylene propylene (FEP) to further support water removal at high current densities. Finally, the effect of using thin membranes will be discussed, as by stratifying the catalyst layer, much of the proton conduction occurs in a limited portion of the membrane. Membrane electrode assemblies (MEAs) using 25 micron thick membranes will be compared to significantly thinner membranes of 5 and 10 microns. Acknowledgments This research is supported by DOE Fuel Cell Technologies Office, through the Fuel Cell Performance and Durability (FC-PAD) Consortium; Fuel Cells program manager: Dimitrios Papageorgopoulos. References 1. T. E. Springer, M. S. Wilson, and S. Gottesfeld, J. Electrochem. Soc., 140 (12), 3513–3526 (1993). 2. R. Borup and T. Rockward, US Dep. Energy Annu. Merrit Rev., Project ID: FC052 (2015). https://www.hydrogen.energy.gov/pdfs/review15/fc052_rockward_2015_p.pdf

Cerium ions are additives which enhance the durability of polymer electrolyte fuel cells (PEFCs) by scavenging reactive radical species which are generated during operation.1 These radicals attack vulnerable carboxyl, sulfonic acid, and ether groups in the ion conducting polymer, or ionomer, phases in the polymer electrolyte membrane (PEM) and catalyst layers (CLs) within the membrane electrode assembly (MEA) of the cell.2 During cell fabrication, conditioning, and discharge, Ce rapidly migrates between the PEM and CLs, which reduces efficiency of the Pt catalyst and may negate the addition of the Ce additive.3,4 Therefore, in order to stabilize Ce and localize it to areas of highest radical generation, it is critical to understand the relative influences of different migration mechanisms. In this work, we identified the fundamental Ce migration mechanisms during PEM fuel cell operation. Using a novel elemental analysis technique based on X-ray fluorescence (XRF), we characterized Ce migration due to potential and concentration gradients, water flux, and degradation of Ce-exchanged sulfonic acid groups within the PEM. During accelerated stress tests (ASTs), which are designed to reproduce and enhance stressors the cell will experience in the field, we have observed that < 1% of Ce exits the cell during open circuit voltage (OCV) between 30% and 100% RH. It does, however, irreversibly migrate from the PEM into the CLs at a rate correlated to AST aggressiveness. As a result, Ce accumulation in the CL reduces performance, while Ce migration out of the PEM lowers the PEM’s ability to resist the harmful effects of reactive radicals. In order to determine the relevant Ce transport coefficients during cell operation, Ce migration was induced in PEMs using H2 pump experiments under a range of applied potentials at 50% and 100% RH. Ce profiles were intermittently measured using XRF and fit with a 1-D Nernst-Planck equation. Using this model, diffusion and ion mobility coefficients were extracted, which arise due to concentration and potential gradients during the H2 pump experiments, respectively. The results show the rapid and significant effects of such gradients on Ce movement. Using this knowledge, we propose system control and additional material engineering strategies to mitigate Ce migration due to potential and concentration gradients, in order to reduce performance losses and improve cell durability. We have also determined that the degradation of Ce-exchanged ionomer side chains during aggressive ASTs results in significant Ce migration from the PEM into the CLs. In order to counter undesirable migration due to ionomer degradation, we propose the use of more efficient radical scavengers or improved localization of scavengers near areas of highest radical generation. Zirconia-doped ceria (CZO) has previously been shown to enhance peroxide reactivity while minimizing byproduct radical generation.5 We have demonstrated that adding CZO to the cathode CL, where significant peroxide and radical generation occurs6,7, results in identical initial cell polarization performance compared to CZO-free cells. During PEM chemical stability ASTs, however, CZO reduces gas crossover (Figure 1a) and fluoride emission (Figure 1b) rates, which implies that it is a more effective radical scavenger. Less ionomer degradation results in stable performance of the CZO-enhanced MEA after 500 hours of ASTs. Furthermore, after >1400 hours of ASTs, the Ce concentration in the PEM was maintained at its initial value due to the protective capabilities of CZO. Acknowledgements This research is supported by the U.S. DOE Fuel Cell Technologies Office, through the Fuel Cell Performance and Durability (FC-PAD) Consortium; Fuel Cells Program manager: Dimitrios Papageorgopoulos. Prof. Ajay Prasad and Prof. Suresh Advani also acknowledge support from the University of Delaware’s Fuel Cell Bus Program. This program is funded by the Federal Transit Administration at the Center for Fuel Cell Research at the University of Delaware. References Coms, F. D.; Liu, H.; Owejan, J. E. ECS Trans. 16, 1735–1747 (2008). Rodgers, M. P.; Bonville, L. J.; Kunz, H. R.; Slattery, D. K.; Fenton, J. M. Chem. Rev. 112, 6075–6103 (2012). Stewart, S. M.; Spernjak, D.; Borup, R.; Datye, A.; Garzon, F. ECS Electrochem. Lett. 3, F19–F22 (2014). Baker, A. M.; Mukundan, R.; Spernjak, D.; Judge, E. J.; Advani, S. G.; Prasad, A. K.; Borup, R. L. J. Electrochem. Soc. 163, F1023–F1031 (2016). Stewart, S. M.; Borup, R. L.; Wilson, M. S.; Datye, A.; Garzon, F. H. ECS Trans. 64, 403–411 (2014). Sethuraman, V. A; Weidner, J. W.; Haug, A. T.; Motupally, S.; Protsailo, L. V. J. Electrochem. Soc. 155, B50-B57 (2008). Gubler, L.; Dockheer, S. M.; Koppenol, W. H. J. Electrochem. Soc. 158, B755-B769 (2011). Figure 1

Cerium enhances the durability of polymer electrolyte membrane (PEM) fuel cells by scavenging reactive radical species which are generated during operation. However, during cell fabrication, conditioning, and discharge, Ce migrates through-plane between the PEM and catalyst layers (CLs) due to concentration and potential gradients.1,2 In addition, we have observed in-plane Ce migration due to water gradients and also identified degradation of Ce-exchanged PEM side chains as another possible mechanism for Ce migration.3 Ce migration is detrimental because (1) its accumulation in the CL ionomer can diminish the electrode’s proton conductivity, which generates performance losses4; and (2) its depletion may leave an ionomer region more susceptible to radical attack. Therefore, it is critical to understand the relative influence of different migration mechanisms under a range of operating conditions in order to stabilize Ce in the PEM and localize it to areas of highest radical generation. To understand the effects of potential gradients and relative humidity (RH) on Ce migration, ex situ experiments were performed using uncatalyzed Nafion® XL PEMs (DuPont) which contain ~6 μg/cm2 ion-exchanged Ce. PEM specimens were operated in H2 pump mode in a standard conductivity cell (BekkTech) at 80°C with 50% and 100% RH H2. The evolution of Ce profiles was quantified using X-ray fluorescence (XRF). By comparing the resulting profiles at 4 C of charge transfer for the different potential and RH conditions (Figure 1a), we observe decreased Ce transference at low RH. Under these conditions, decreased PEM water content causes a disproportionate reduction in Ce conductivity relative to proton conductivity. In these experiments, Ce ion mobility induced by a potential gradient leads to a concentration gradient, which, in turn, induces Ce diffusion in the opposite direction. Therefore, the profiles shown in Figure 1a arise from a combination of ion mobility and back-diffusion due to the resulting concentration gradient. In order to decouple these effects, a transient, 1-D model was developed based on Nernst-Einstein ion mobility and Fickian diffusion, in order to solve for the ion mobility and diffusion coefficients. Experimental and model results for the 2 V, 100% RH case are shown in Figure 1b. At 100% RH, both diffusion and ion mobility coefficients were determined to be an order of magnitude higher than at 50% RH. In addition to migration within the PEM, Ce is stabilized in the cathode CL, likely in the CL ionomer and/or carbon CL supports.3 Ce accumulation in the cathode catalyst layer was measured to degrade MEA performance. Different mechanisms have been proposed for the performance loss, including increased proton resistance within the CL, and a reduction of oxygen reduction reaction kinetics. However, the effect of each mechanism on performance loss is difficult to quantify. The relative influence of Ce poisoning on the cathode CL performance will also be discussed. Quantifying the different Ce migration mechanisms has provided a better understanding of the effects of Ce migration in the PEM and CL and the associated losses in operating performance. These results demonstrate that stable Ce compounds, which can be localized to areas of highest reactive radical generation, need to be developed to enhance PEFC durability without compromising performance. Acknowledgements This research is supported by the U.S. DOE Fuel Cell Technologies Office, through the Fuel Cell Performance and Durability (FC-PAD) Consortium; Fuel Cells Program manager: Dimitrios Papageorgopoulos. Prof. Ajay Prasad and Prof. Suresh Advani also acknowledge support from the University of Delaware’s Fuel Cell Bus Program. This program is funded by the Federal Transit Administration at the Center for Fuel Cell Research at the University of Delaware. References Stewart, S. M.; Spernjak, D.; Borup, R.; Datye, A.; Garzon, F. ECS Electrochem. Lett. 3, F19–F22 (2014). Baker, A. M.; Mukundan, R.; Spernjak, D.; Advani, S. G.; Prasad, A. K.; Borup, R. L. ECS Trans. 75, 707–714 (2016) Baker, A. M.; Mukundan, R.; Spernjak, D.; Judge, E. J.; Advani, S. G.; Prasad, A. K.; Borup, R. L. J. Electrochem. Soc. 163, F1023–F1031 (2016). Banham, D.; Ye, S. Y.; Cheng, T.; Knights, S.; Stewart, S. M.; Wilson, M.; Garzon, F. J. Electrochem. Soc. 161, F1075–F1080 (2014). Figure 1

FC-PAD (Fuel Cell – Performance and Durability) consortium coordinates U.S. national laboratory activities related to PEM fuel cell performance and durability. The cost and durability of Polymer Electrolyte Membrane Fuel Cells (PEMFCs) are two major barriers to the commercialization of these systems for transportation power applications. Transportation conditions include operation in the presence of fuel and air impurities, start/stop, freeze/thaw, and humidity and load cycling that result in mechanical and chemical stresses. MEA durability decreases with decreasing catalyst loading, making cost reduction even more difficult. While there has been significant progress in improving PEM Fuel Cell durability with lower cost materials, further improvements are needed to meet the commercialization targets. Degradation in the cathode catalyst layer is the primary cause of performance loss, with both recoverable and irreversible losses occurring. One recoverable loss for PEMFCs includes catalyst poisoning by membrane degradation products [1] which requires removal from the catalyst surface and then from the electrode layer for the performance recovery[2]. Irreversible degradation modes include catalyst dissolution and ripening, loss of alloying agents from Pt-X catalysts, plus the effects of the various forms of carbon used in PEMFC components which include changing hydrophobicity, carbon corrosion and loss of porosity of electrode layers and Gas Diffusion Layers (GDLs). These increasing losses are primarily in the cathode catalyst layer and are attributed to both increasing transport and kinetic losses. The FC-PAD consortium is examining these degradation mechanisms to help develop improved materials and operating strategies. Corrosion of the carbon electrocatalyst support has been measured during drive-cycle operating conditions and increases with increase potential cycling from 0.4 to 0.9 V. Carbon corrosion is one of the major contributors to degradation which leads to changes in the catalyst layer structure and reduces its activity. Reduction in catalyst layer thickness is observed during operation, exacerbated during drive cycles. This reduction can be due to the loss of carbon through carbon corrosion or due to compaction; both effects likely lead to a loss of void volume. Membrane additives which increase membrane life-times, have been measured to migrate into the catalyst layer and appear to be associated with the carbon in the catalyst layers. Low potentials (0.2V) appear to be required to remove membrane fragment adsorbates which decrease catalyst activity. Pt alloy catalysts lose most of their alloying agents during operation; the alloying agents migrated throughout the ionomer. Durability implications of using Pt-X alloy catalysts will be discussed. Results related to the mentioned degradation mechanisms will be presented including characterization from TEM, SEM, XRF, XRD and electrochemical testing. Acknowledgments This work was funded through the DOE FC-PAD Consortium with thanks to DOE EERE FCTO, Fuel Cell Team Leader: Dimitrios Papageoropoulos References: [1] Yu Seung Kim, Melinda Einsla, James E. McGrath, and Bryan S. Pivovar, The Membrane–Electrode Interface in PEFCs: II. Impact on Fuel Cell Durability Journal of The Electrochemical Society, 157 11 B1602-B1607 2010. [2] Jingxin Zhang, Brian A. Litteer, Frank D. Coms, and Rohit Makharia, Recoverable Performance Loss Due to Membrane Chemical Degradation in PEM Fuel Cells, Journal of The Electrochemical Society, 159 (7) F287-F293 (2012)

Commercially viable polymer electrolyte membrane fuel cell (PEMFC) systems for transportation and stationary power applications require optimal combination of performance and durability at a competitive cost. Optimized water management across a fuel cell is key to improving performance and maintaining high conductivity of the ionomer in PEM and catalyst layers (CLs), while preventing excessive flooding in CLs, gas diffusion layers (GDLs), channels, and manifolds. Durability is also greatly affected by water transport as certain degradation mechanisms are accelerated depending on the humidity levels. As well, catalyst layers that suffered carbon corrosion due to long operation and/or high number of starts/stops are more susceptible to flooding. Imaging in situ and ex situ enables crucial insight into the microstructure of the fuel cell components and materials, associated water transport, and their effect on cell performance, dynamic response, and degradation mechanisms. Several case studies are presented to highlight the use of optical imaging, neutron imaging, and micro X-ray computed tomography (microXCT) to investigate fuel cell components across a range of length scales. Laboratory-scale microXCT instruments are used ex situ to resolve the three-dimensional structure of the GDL and electrodes, before and after various accelerated stress tests. Optical imaging in operating fuel cells offers a cost-effective combination of high temporal resolution (tens of frames per second) and high spatial resolution (several µm). The imaging requires special cell design with visual access to the imaged area (GDL surface and channels [1] or catalyst layer surface [2]). We employed optical visualization in a variety of studies, such as to visualize liquid water dynamics in the cathode and anode flow fields, or at the interface between the CL and the GDL. Besides water transport studies, optical imaging can quantify in-plane gas distribution in the flow fields during startups and shutdowns [3]. Neutron imaging is a powerful tool to visualize and quantify water transport in operating fuel cells. It has a high sensitivity to small amounts of water inside a cell, while having high transmission through the common fuel cell hardware. At a lower spatial resolution of 250 µm, neutron imaging is used to measure in-plane water distribution with high temporal resolution and large field of view of up to 20 cm by 20 cm. We studied the performance with novel NSTF (nano structured thin film) electrodes and GDL materials [4], and also visualized water and ice formation when cell was subjected to repeated starts at sub-freezing temperatures [5]. Since neutron imaging provides water content integrated along the neutron beam, we combined neutron imaging with the optical visualization to study water transport in different flow field geometries [6] as well as in an operating electrolyzer [7]. Such approach with simultaneous imaging provides additional information about the water location, and in certain situations allows distinguishing between channel and GDL water, or between anode and cathode channels and manifolds. High-resolution neutron imaging is able to measure water distribution across the cell thickness at spatial resolution as high as 10 µm. Image processing procedure is developed to measure the water uptake and observe Schroeder’s paradox in situ in PEMs [8,9]. Further, properties of the microporous layer (MPL) of the GDL were manipulated to prevent excessive flooding in cathode catalyst layers. Anode flooding is evidenced by optical visualization, simultaneous imaging, and high-resolution neutron imaging. Increased water transport across the membrane, from cathode to anode, can be detrimental as liquid water on the anode side may cause localized fuel starvation and cause irrecoverable degradation, similar to accelerated carbon corrosion during unassisted startups and shutdowns. However, for certain novel cathodes, which are prone to flooding, e.g. nano-structured thin film (NSTF) electrodes and non-precious group metal (non-PGM) catalysts, water removal through the anode has proven to be a viable option to improve the performance by reducing cathode water content. The authors acknowledge support from US Department of Energy EERE FCTO, Los Alamos National Laboratory LDRD (Laboratory Directed Research and Development) program, Federal Transit Administration, US Department of Commerce, and NIST Center for Neutron Research. References: D. Spernjak et al., J. Power Sources 170 pp334 (2007) F.-Y. Zhang et al., J. Electrochem. Soc. 154 (11) B1152 (2007) Y. Ishigami et al., J. Power Sources 269 pp556 (2014) A. J. Steinbach et al., ECS Trans. 33 (1) 1179 (2010) N.Macauley et al, J. Electrochem. Soc. submitted (2016) D. Spernjak et al., J. Power Sources 195 pp3553 (2010) O.F. Selamet et al., Int. J. Hydrogen Energy 38 pp5823 (2013) D. Hussey et al., J. Appl. Phys. 112 (10), 104906 (2012) D. Spernjak et al., ECS Trans. 33 1451 (2010)

The development of stratified catalyst layers promises an increase in catalyst layer performance compared to conventional flat catalyst layers.1,2 Irregular catalyst layer thickness and porosity can lead to enhanced gas and water transport in and out of the catalyst layer, respectively. The stratified structure is expected to have the same performance in the kinetic region where the performance is controlled by the overall Pt loading. However at high current densities, the thinner sections of the stratified catalyst layers should allow for better mass transport properties. Electrode fabrication is done in house with a custom designed spray coating procedure and catalyst ink recipe. Various approaches are being explored to achieve the desired electrode structure. One approach is to densify the catalyst layer in localized regions, and is based on Ion Power proprietary manufacturing techniques. This can involve the use of glass epoxy masks during the spray coating process to directly create a patterned electrode on the membrane that has thicker and thinner regions. Figure 1 illustrates the effect of stratification on the fuel cell performance of a MEA. The second approach is to pattern thicker and thinner sections of an electrode using masks. During spray coating, one mask exposes the lands and the other mask exposes the channels. This way the two extreme cases can be investigated and the resulting performance compared. The concept of having more catalyst material in the channels may be beneficial due to rapid reaction times during influx of reactant gases through the channels. However, the presence of a thicker catalyst layer under the channel may later become an issue for product water removal and oxygen diffusion. On the other hand, spray coating more of the electrode in the land areas may support better product water removal and oxygen diffusion by leaving a thinner catalyst layer under the channels. The best performance is expected from a combination of ordered thin and thick regions in the catalyst layer. Results from various MEAs tested in a single cell on a fully automated test station to evaluate catalyst performance will be presented. The evaluation of new catalyst ink recipes with reduced ionomer to carbon ratios is included in this study to further increase mass transport by reducing ionomer swelling in the catalyst layer at high relative humidity. Acknowledgments This research is supported by DOE Fuel Cell Technologies Office, through the Fuel Cell Performance and Durability (FC-PAD) Consortium; Fuel Cells program manager: Dimitrios Papageorgopoulos. References 1. T. E. Springer, M. S. Wilson, and S. Gottesfeld, J. Electrochem. Soc., 140 (12), 3513–3526 (1993). 2. R. Borup and T. Rockward, US Dep. Energy Annu. Merrit Rev., Project ID: FC052(2015). https://www.hydrogen.energy.gov/pdfs/review15/fc052_rockward_2015_p.pdf Figure 1. Performance of textured and baseline MEA at 275kPa in H2/Air @ 80oC and 100%RH. Figure 1

A major problem with performance for transportation applications is associated with degradation. The performance degradation losses can be separated into two categories: irreversible and recoverable losses. Significantly more work has examined the irreversible degradation losses compared with the recoverable losses because the recoverable losses often can be simply recovered by performing polarization curves[1]. Delineating the component contributions of recoverable performance losses and mitigation of these losses appears to be a growing concern for fuel cell developers. While these degradation mechanisms are recoverable, recovery processes are not necessarily trivial to accomplish in operando in a vehicle. MEA performance degradation is sensitive to operating conditions and both irreversible and reversible materials changes. Some known recoverable losses for PEMFCs include the loss of cathode activity due to surface oxide (hydroxide) formation at high potentials,[2] catalyst poisoning by membrane degradation products [3] and recoverable transport losses due to water transport [1,4]. Platinum oxidation at the cathode results in decreased ORR (Oxygen Reduction Reaction) and with Pt often considered to have two different Tafel slopes; one for a metallic Pt surface, and one for an oxidized Pt surface. In other cases, indications are that membrane degradation products, such as (bi)sulfate are readily adsorbed onto the catalyst and are a large source of recoverable degradation. These require removal from the catalyst surface and then from the electrode layer for the performance recovery.[5] To examine the relative contributions to recoverable degradation, we have conducted in situ and ex situ experiments to separate the effects of Pt oxidation, (bi)sulphate adsorption, water management plus other operational effects including spatial degradation mapping, potential cycling and different levels of RH. One method to identify the effect of membrane degradation products on the performance decay is to compare the effect of non-chemically-stabilized membranes versus the effect of chemically-stabilized membranes. This is shown in Figure 1, where the OCV is shown during two-24-hr periods (with recovery in-between the 24-hr periods) for MEAs with (a) a non-chemically stabilized membrane and (b) a chemically stabilized membrane. Past results have shown significantly more degradation products of fluoride and sulphate anions without the chemical stabilization. Both MEAs show decreasing OCVs, however the decay is significantly more for MEA using the non-chemically stabilized membrane. Segmented cell measurements, made in a 10x10 co-flow cell, do not indicate a substantial spatial difference in the rate of decay during these types of tests. Various methods of recovering the losses were examined to separate the Pt oxidation from the other effects, e.g. decreasing the potential to 0.6 V to reduce the Pt oxide films compared with much lower recovery voltages to desorb adsorbed anions. Recovery protocols in fuel cell mode (H2/air) are also compared without current generation (H2/N2) at potentials of 0.6 V, 0.4 V, 0.3V and 0.2 V including with and without liquid water injection. Liquid water injection was found to be detrimental to the recovery of the fuel cell performance and a voltage of 0.3 V required to good recovery with little difference below that potential. Acknowledgments This research is supported by DOE Fuel Cell Technologies Office, through the Fuel Cell Performance and Durability (FC-PAD) Consortium; Fuel Cells program manager: Dimitrios Papageorgopoulos. References: [1] Cleghorn, S. J. C.; Mayfield, D. K.; Moore, D. A.; Moore, J. C.; Rusch, G.; Sherman, T. W.; Sisofo, N. T.; Beuscher, U., A polymer electrolyte fuel cell life test: 3 years of continuous operation, Journal of Power Sources (2006), 158 (1), 446-454. [2] Yu Seung Kim and Piotr Zelenay in Polymer Electrolyte Fuel Cell Durability, eds. Felix N. Büchi, Minoru Inaba, Thomas J. Schmidt, Springer New York (2009) pp 223-240 [3] Yu Seung Kim, Melinda Einsla, James E. McGrath, and Bryan S. Pivovar, The Membrane–Electrode Interface in PEFCs: II. Impact on Fuel Cell Durability Journal of The Electrochemical Society, 157 11 B1602-B1607 2010. [4] Qing Li, Dusan Spernjak, Piotr Zelenay, Yu Seung Kim, Micro-crack formation in direct methanol fuel cell electrodes, Journal of Power Sources 271 (2014) 561e569 [5] Jingxin Zhang, Brian A. Litteer, Frank D. Coms, and Rohit Makharia, Recoverable Performance Loss Due to Membrane Chemical Degradation in PEM Fuel Cells, Journal of The Electrochemical Society, 159 (7) F287-F293 (2012) Figure 1

The durability of Polymer Electrolyte Membrane Fuel Cells (PEMFCs) is one of the key barriers to their commercialization in automotive applications.1 The development of accelerated stress tests (ASTs) is a critical step in the evaluation and improvement of the durability of PEMFCs. The availability of validated component ASTs can lead to the accelerated development of materials with improved durability. However the ASTs need to faithfully reproduce the degradation mechanisms of interest and not introduce new degradation modes while attempting to accelerate the degradation rates. Furthermore, the acceleration factors with respect to the intended application need to be determined accurately to provide manufacturers with a valuable tool that enables informed decision making regarding cost, performance and durability. For example the DOE – Fuel Cell Technologies program has set lifetime durability targets of 5500 hours for automotive fuel cells and the U.S. Drive Fuel Cell Technical Team (FCTT) has developed AST protocols for the various components.2,3 In this paper we will compare the degradation observed during the ASTs to the degradation observed during simulated drive cycle operation and report on the acceleration factors observed for various materials. We will also propose new ASTs that have greater acceleration factors than the reported FCTT ASTs while retaining the degradation mechanisms of interest. The FCTT has recommended potential cycling from 0.6 V to 1.0 V at 50 mV/sec as the electrocatalyst AST. This potential cycling is performed at 80 °C in 100% RH with H2 (anode) and N2 (cathode) and results in Pt particle growth and loss in electrochemical surface area (ECSA). This loss in ECSA and associated performance loss correlates well to the degradation observed in the field and during simulated drive cycle operation. Figure 1 compares the degradation observed during this AST (Old AST) to that observed during the wet portion of the FCTT: Drive cycle (current cycling from 0.02 to 1.2 A/cm2 at 80 °C and saturated conditions). It is seen that this AST has a 5X acceleration factor for 3 different MEAs that were evaluated. In order to further accelerate this AST, we modified the triangle wave to a square wave and lowered the upper potential to 0.95 V. This new AST consisted of a square wave with upper and lower potentials of 0.95 V and 0.6 V with 3 seconds duration, and was based on literature reports.4 This New AST (Fig. 1) demonstrated a 100X acceleration factor compared to the drive cycle which is a 20 times improvement over the old AST. Moreover, this new AST retained the Pt growth mechanism while minimizing carbon corrosion. The FCTT has 2 recommended ASTs for membranes with one focusing on chemical degradation (OCV hold at 90 °C and 30% RH) and another focusing on mechanical degradation (RH cycling in Air). Our previous results have indicated that both these ASTs are not representative of membrane failure in the field.5 In this paper we will present a new AST that combines the mechanical and chemical degradation and is representative of field failure. This AST consists of cycling the RH between saturated and dry conditions in a H2/Air atmosphere at 90 °C. The duration of the wet and dry cycles are 30s and 45s respectively. The fluoride release rate observed during this AST using DuPont XL® membranes, illustrated in Figure 2, is comparable to the chemical AST and higher than in the mechanical AST. References 1. R. Borup, et al., Chemical Reviews, V. 107, No. 10, 3904-3951 (2007). 2. DOE Cell Component AST and polarization curve Protocols for PEM Fuel Cells (Electrocatalysts, Supports, Membranes and MEAs), Revised December 16, 2010. 3. N. L. Garland, T.G. Benjamin, J. P. Kopasz, ECS Trans., V. 11 No. 1, 923 (2007). 4. A. Ohma, K. Whinohara, A. Liyama, T. Yoshida, and A. Daimaru, ECS Trans., V. 41 No. 1 , 775 (2011). 5. R. Mukundan et al., ECS Trans., V. 50 No. 2, 1003 (2013). Acknowledgements The authors wish to acknowledge the financial support of the Fuel Cell Technologies Program and the Technology Development Manager: Nancy Garland. The authors also wish to acknowledge Ion Power, Inc. for supplying the MEAs and SGL Carbon for the GDLs used in this study. Figure 1

Corrosion of catalyst support-carbon during unprotected start-up and shut-down of H2-air polymer electrolyte fuel cells (PEFC) is a serious concern. Many studies have investigated the underlying mechanisms of current reversals that occur as H2 sweeps the air in the anode compartment during start-up and, conversely, as air sweeps the H2 in anode during shut-down. Carbon corrodes during these transients as high interfacial potentials exceeding 1.5 V develop in the air/air portion of the cell. Protective measures have been proposed to limit these high interfacial potentials and to limit the exposure time to high potentials. Material-based mitigation strategies have also been proposed that include suppressing the oxygen reduction reaction (ORR) on anode and promoting the oxygen evolution reaction (OER) on cathode. Catalyst support-carbon can also corrode below the open-circuit potentials typical of normal operation of automotive fuel cells. Figure 1a is a typical trace of the measured carbon corrosion rate during a simulated drive cycle in which the potential was varied between 0.4 V and 0.95 V. The hold times were 0.5 min at the low potential and 5 min at the high potential. The data are for an Ion Power supplied MEA with 0.15 mg/cm2 Pt loading in cathode, 0.23 Pt/C ratio, and high surface area Ketzenblack (E-type) carbon support. The data are for H2-air at fixed flow rates, 80oC cell temperature, and 100% relative humidity (RH). The corrosion trace in Fig. 1a has a few particularly interesting features. The larger spike in corrosion rate occurs as the cell potential is raised from 0.4 V to 0.95 V. However, the corrosion rate declines rapidly and approaches zero as the cell is held at the high potential for a long time. The smaller spike in corrosion rate occurs as the cell potential is reduced from 0.95 V to 0.4 V. The corrosion rate also declines as the cell is held at the low potential but does not approach zero. Similar features in carbon corrosion rates were observed in other MEAs with Vulcan XC-72 (V-type) and graphitized Ketzenblack (EA-type) carbons as catalyst supports. We have formulated a simple carbon corrosion model to quantitatively capture the essential features of the data. As indicated in Fig. 1a, the surface carbon sites are represented as vacant (C#), active carbon oxides (C#OH), and passive carbon oxides (C#Ox). The active carbon oxides are formed at low potentials. The spike in carbon corrosion after a step increase in potential is associated with the direct oxidation of C#OH to CO2 through its reaction with H2O. Holding the cell at a high potential leads to the formation of C#Ox and displacement of C#OH. The corrosion rate goes down as C#Ox passivates the carbon surface. Also, at potentials higher than 0.4 V, Pt sites begin to convert to PtOH, and at still higher potentials, PtOH converts to PtO. In the model shown in Fig. 1a, a step decrease in potential results in the sites occupied by passive C#Ox being vacated. The vacated sites are available to be occupied by active C#OH which reacts with the OH-like species on adjacent catalyst surface to produce a spike in carbon corrosion. With time, PtOH gradually reduces to Pt, and the carbon corrosion rate decreases to a lower, non-zero value. As partial validation, Fig. 1b shows the modeled steady-state corrosion rate and compares it with the asymptotic rates measured in the experiments when the cell with E-type carbon is held at high potentials for 5 min. Both the model and the data exhibit peak steady-state corrosion rates for E-type carbon at about 0.6 V. As seen in Fig. 1a, the transient corrosion rates can be much higher under potential cycling. Acknowledgments: This work is supported by DOE-EERE-FCTO with Dr. Nancy Garland as the Technology Development Manager. Figure 1

Vehicle manufactures are pursuing selective catalyst reduction (SCR) and exhaust gas recirculation (EGR) systems to meet EPA emissions regulations for lean-burn engines. Exhaust gas sensors are needed to monitor emissions and control and maintain efficient operation of these systems. Oxygen λ-sensors are in integral part of OBD of today’s spark-ignition vehicle emission systems, but a suitable analog is not available for NOx, NH3 and NMHC 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, NH3 and NMHC 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 l-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 (1). 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 (2). 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. 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 (3). 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 NOxspecies with varying sensitivity to NMHCs depending on operating temperature. While these devices exhibit preferential selectivity to target analytes, a single device with absolute selectivity remains elusive. The work herein presents an alternative strategy to absolute selectivity in which the cross-sensitivity of varying sensor elements at varying operating conditions is exploited through the use of Bayesian inference predictive algorithms based on physical models of sensor-analyte interactions. We have previously shown that the individual concentration of individual analyte species can be uniquely determined from a mixtures of two gases based on the sensor voltage response of a single sensor operating at four different current bias points by employing a Bayesian interference model(4). In this work, we seek to expand our proof-of-concept demonstration to uniquely identifying the concentration of NO, NO2, NH3, and C3H6 in complex mixtures relevant to diesel exhaust. In this study we are employing two different sensors, the aforementioned LSCr|YSZ|Pt and Au|YSZ|Pt planar sensor devices, operating at multiple temperatures. Sensor voltage response data collected on a wide range of mixing ratios are used to train the Bayesian model. Complex mixtures not used in the training set will be used to test the accuracy of the model. This research represents the first stages of developing miniaturized mixed-potential electrochemical sensor (MPES) arrays as an all-in-one NOx/NH3/NMHC sensor package that will provide quantitative emissions monitoring and onboard diagnostics (OBD) for all lean-burn engine applications. The MPES arrays have potential to be used in Environmental monitoring of complex gas mixtures. The characteristics of micro-systems that are required to realize the potential of MPES arrays will be discussed and preliminary results to realize these concepts will be presented. Acknowledgements The research was funded by the Los Alamos National Laboratory Directed Research Development Exploratory Research program. References 1. R. Mukundan, E. L. Brosha, F. H. Garzon, US 7,575,709 (2009). 2. R. Mukundan, E. L. Brosha, F. H. Garzon, Journal of The Electrochemical Society 150, H279 (2003). 3. C. R. Kreller, P. K. Sekhar, W. Li, P. Palanisamy, E. L. Brosha, R. Mukundan, F. H. Garzon, ECS Transactions 50, 307-314 (2012). 4. J. Tsitron, C. R. Kreller, P. K. Sekhar, R. Mukundan, F. H. Garzon, E. L. Brosha, A. V. Morozov, Sensors and Actuators B: Chemical 192, 283-293 (2014).

Nitrogen oxide (NOx) and ammonia (NH3) sensing technology is being developed for application in vehicle emissions monitoring of diesel and lean-burn gasoline engines. In lean-burn engines, Selective Catalyst Reduction (SCR) utilizes NH3 to reduce NOx. The purpose of the NH3 sensor is to ensure that the NH3 is completely consumed in the NOx reduction reaction, and it is not emitted from the tailpipe. Both the NOx and NH3 sensors described herein are mixed-potential sensors that measure the non-nernstian potential created at an electrode/electrolyte interface exposed to a mixture of reducing/oxidizing gases. The NOx sensor is composed of La0.8Sr0.2CrO3 (lanthanum strontium chromite) and Pt electrodes with a YSZ electrolyte and a protective porous ceramic overcoat. The NH3 sensor is composed of Au/Pd and Pt electrodes with a YSZ electrolyte and protective overcoat. The purpose of the overcoat is to protect the electrodes from contaminants such as heavy metals and water present in harsh exhaust environments. We examine the effect of the protective overcoat on the sensor performance. Preliminary data on the NOx sensors indicates that the overcoat serves as a thermal blanket; decreasing the temperature difference observed between the heater and the sensing element in bare sensors. Sensing characteristics of response time, selectivity and sensitivity will be compared and discussed.