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Despite the advantages of CO2 electrolyzers, efficiency losses due to mass and ionic transport across the membrane electrode assembly (MEA) are critical bottlenecks for commercial-scale implementation. In this study, more efficient electrolysis of CO2 was achieved by increasing cation exchange membrane (CEM) hydration via the humidification of the CO2 reactant inlet stream. A high current density of 755 mA/cm2 was reached by humidifying the reactant CO2 in a MEA electrolyzer cell featuring a CEM. The power density was reduced by up to 30% when the fully humidified reactant CO2 was introduced while operating at a current density of 575 mA/cm2. We reduced the ohmic losses of the electrolyzer by fourfold at 575 mA/cm2 by fully humidifying the reactant CO2. A semiempirical CEM water uptake model was developed and used to attribute the improved performance to 11% increases in membrane water uptake and ionic conductivity. Our CEM water uptake model showed that the increase in ohmic losses and the limitation of ionic transport were the result of significant dehydration at the central region of the CEM and the anode gas diffusion electrode-CEM interface region, which exhibited a 2.5% drop in water uptake.

Proton exchange membrane fuel cells (PEMFCs), fed with oxygen from ambient air and hydrogen, spontaneously produce power. However, key materials, gas diffusion layers, catalysts, ionomers, and membranes, are sensitive to a multitude of other species that may be inadvertently introduced into PEMFCs even at parts per million (ppm) or less levels. For a commercially relevant catalyst loading of 0.1 mg Pt cm–2 and a severe case such as 100 ppm acetylene in air, the temporary cell voltage loss exceeds 80 % [1]. Exposure to species such as bromomethane is even more problematic because the cell voltage cannot be restored without a specific recovery procedure [2]. The development of preventive and recovery procedures to improve durability is facilitated by contamination mechanisms. Although mass transfer losses were often observed in the presence of organic species [1], the commonest contaminants either in air or released by system components [3], the origin of this effect is currently unclear. It may stem from adsorption on carbon [4] or platinum [5] respectively modifying surface hydrophobicity and liquid water management or reducing the catalyst active surface, which increases the real current density. Several diagnostic techniques able to either characterize water management or oxygen mass transfer were used to evaluate PEMFCs contaminated with propene or methyl methacrylate. Neutron radiographs were obtained for both in-plane and through-plane cell configurations to record changes in liquid water distribution [6,7]. Polarization curves were measured with O2, 21 % O2 in He, and air to derive concentration, molecular diffusion and ionomer permeability mass transfer overpotentials [8]. Overall oxygen mass transfer coefficients were acquired by first operating PEMFCs fed with 1 % to 7 % O2 in He, N2 or CO2 streams under limiting current conditions. Subsequently, a mathematical model was used to fit average limiting current data as a function of oxidant flow rate to extract the overall oxygen mass transfer coefficient. Separation of the overall mass transfer coefficient into molecular and Knudsen diffusion, and ionomer permeability contributions was conducted by plotting mass transfer resistance data (inverse of the mass transfer coefficient) either as a function of the O2 diluent molecular weight or O2 concentration and extrapolating linear correlations to the origin. The original separation method [9] was improved by taking advantage of the insensitivity of Knudsen diffusion to variations in O2 concentration. Propene and methyl methacrylate were selected because they have largely different solubility in water (surrogate hydrophobicity measure), minimize test durations and simplify test procedures (fast contamination and recovery kinetics), and facilitate data interpretation (minimal current distribution and effective concentration changes). Experimental data demonstrated that changes in liquid water distributions were subtle and cannot explain mass transfer losses induced by contaminants. In contrast, ionomer permeability overpotentials and mass transfer coefficients revealed major increases in the presence of both contaminants. These observations support the notion that propene and methyl methacrylate contaminants adsorb on the Pt catalyst, decrease the active area, and bring the real current density closer to the limiting value. Experimental data analyses based on a contamination model that include catalyst surface processes for both O2 and contaminant (adsorption, reaction, desorption) [10] confirmed this hypothesis. The proposed contamination mechanism is similar to the increase in oxygen mass transfer overpotential induced by a decrease in Pt catalyst loading [11]. [1] J. St-Pierre, Y. Zhai, Molecules, 25 (2020) article 1060. [2] Y. Zhai, O. Baturina, D. Ramaker, E. Farquhar, J. St-Pierre, K. Swider-Lyons, Electrochim. Acta, 213 (2016) 482. [3] J. St-Pierre, M. Angelo, K. Bethune, J. Ge, S. Higgins, T. Reshetenko, M. Virji, Y. Zhai, Electrochem. Soc. Trans., 61(23) (2014) 1. [4] X. Zhang, B. Gao, A. E. Creamer, C. Cao, Y. Li, J. Hazard. Mater., 338 (2017) 102. [5] Y. Itagaki, M. Mori, Y. Sadaoka, Curr. Opin. Electrochem., 11 (2018) 72. [6] D. S. Hussey, D. L. Jacobson, M. Arif, K. J. Coakley, D. F. Vecchia, J. Fuel Cell Sci. Technol., 7 (2010) article 021024. [7] D. S. Hussey, D. Spernjak, A. Z. Weber, R. Mukundan, J. Fairweather, E. L. Brosha, J. Davey, J. S. Spendelow, D. L. Jacobson, R. L. Borup, J. Appl. Phys., 112 (2012) article 104906. [8] J. St-Pierre, M. Angelo, K. Bethune, J. Huizingh, T. Reshetenko, M. Virji, Y. Zhai, Modern Fuel Cell Testing Laboratory, in Springer Handbook of Electrochemical Energy, Part D, Chapter 19, Edited by C. Breitkopf, K. Swider-Lyons, Springer, 2017, p. 611. [9] T. Reshetenko, J. St-Pierre, J. Electrochem. Soc., 161 (2014) F1089. [10] J. St-Pierre, J. Electrochem. Soc., 156 (2009) B291. [11] A. Z. Weber, A. Kusoglu, J. Mater. Chem. A, 2 (2014) 17207.

Lithium-ion batteries have received significant research interest due to their advantages in energy and power density, which are important to enabling many devices. One route to further increase energy density is to fabricate thicker electrodes in the battery cell; however, careful consideration must be taken when designing electrodes as to how increasing the thickness impacts the multiscale and multiphase molecular transport processes, which can limit the overall battery operating power. Design of these electrodes necessitates probing the molecular processes when the battery cell undergoes electrochemical charge/discharge. One tool for in situ insights into the cell is neutron imaging, because neutron imaging can provide information of where electrochemical processes occur within the electrodes. In this manuscript, neutron imaging is applied to track the lithiation/delithiation processes within electrodes at different current densities for a full cell with a thick sintered Li(4)Ti(5)O(12) anode and LiCoO(2) cathode. The neutron imaging reveals that the molecular distribution of Li(+) during discharge within the electrode is sensitive to the current density, or equivalently discharge rate. An electrochemical model provides additional insights into the limiting processes occurring within the electrodes. In particular, the impact of tortuosity and molecular transport in the liquid phase within the interstitial regions in the electrodes are considered, and the influence of tortuosity was shown to be highly sensitive to the current density. Qualitatively, the experimental results suggest that the electrodes behave consistent with the packed hard sphere approximation of Bruggeman tortuosity scaling, which indicates that the electrodes are largely mechanically intact but also that a design that incorporates tunable tortuosity could improve the performance of these types of electrodes.

Water retention curves are essential for understanding the hydrologic behavior of partially saturated porous media and modeling flow and transport processes within the vadose zone. We directly measured the main drying and wetting branches of the average water retention function obtained using two-dimensional neutron radiography. Flint sand columns were saturated with water and then drained and rewetted under quasi-equilibrium conditions using a hanging water column setup. Digital images (2048 by 2048 pixels) of the transmitted flux of neutrons were acquired at each imposed matric potential (∼10–15 matric potential values per experiment) at the National Institute of Standards and Technology Center for Neutron Research BT-2 neutron imaging beam line. Volumetric water contents were calculated on a pixel-by-pixel basis using Beer-Lambert’s law after taking into account beam hardening and geometric corrections. To account for silica attenuation and remove scattering effects at high water contents, the volumetric water contents were normalized (to give relative saturations) by dividing the drying and wetting sequences of images by the images obtained at saturation and satiation, respectively. The resulting pixel values were then averaged and combined with information on the imposed basal matric potentials to give average water retention curves. The average relative saturations obtained by neutron radiography showed an approximate one-to-one relationship with the average values measured volumetrically using the hanging water column setup. There were no significant differences (P < 0.05) between the parameters of the van Genuchten equation fitted to the average neutron radiography data and those estimated from replicated hanging water column data. Our results indicate that neutron imaging is a very effective tool for quantifying the average water retention curve.

We report on efforts to improve the achievable spatial resolution in neutron imaging by centroiding the scintillation light from gadolinium oxysulfide scintillators. The current state-of-the-art neutron imaging spatial resolution is about 10 μm, and many applications of neutron imaging would benefit from at least an order of magnitude improvement in the spatial resolution. The detector scheme that we have developed magnifies the scintillation light from a gadolinium oxysulfide scintillator, calculates the center of mass of the scintillation event, resulting in an event-based imaging detector with spatial resolution of about 2 μm.

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.

Models of fluid flow are used to improve the efficiency of oil and gas extraction and to estimate the storage and leakage of carbon dioxide in geologic reservoirs. Therefore, a quantitative understanding of key parameters of rock–fluid interactions, such as contact angles, wetting, and the rate of spontaneous imbibition, is necessary if these models are to predict reservoir behavior accurately. In this study, aqueous fluid imbibition rates were measured in fractures in samples of the Eagle Ford Shale using neutron imaging. Several liquids, including pure water and aqueous solutions containing sodium bicarbonate and sodium chloride, were used to determine the impact of solution chemistry on uptake rates. Uptake rate analysis provided dynamic contact angles for the Eagle Ford Shale that ranged from 51 to 90° using the Schwiebert–Leong equation, suggesting moderately hydrophilic mineralogy. When corrected for hydrostatic pressure, the average contact angle was calculated as 76 ± 7°, with higher values at the fracture inlet. Differences in imbibition arising from differing fracture widths, physical liquid properties, and wetting front height were investigated. For example, bicarbonate-contacted samples had average contact angles that varied between 62 ± 10° and ∼84 ± 6° as the fluid rose in the column, likely reflecting a convergence–divergence structure within the fracture. Secondary imbibitions into the same samples showed a much more rapid uptake for water and sodium chloride solutions that suggested alteration of the clay in contact with the solution producing a water-wet environment. The same effect was not observed for sodium bicarbonate, which suggested that the bicarbonate ion prevented shale hydration. This study demonstrates how the imbibition rate measured by neutron imaging can be used to determine contact angles for solutions in contact with shale or other materials and that wetting properties can vary on a relatively fine scale during imbibition, requiring detailed descriptions of wetting for accurate reservoir modeling.

Abstract Fuel cells have potential for small power applications that require long periods, as much as several decades, of uninterrupted power. Small microwatt fuel cell stacks of varying membrane thickness were designed to allow for passive water management. The passive water management system was incorporated into the stack design by use of exposed Nafion membrane for lateral water transport. 1 An experimental and theoretical study of water management across the operating temperature range was conduction, including operando neutron imaging and the development of a multiphysics two-dimensional model. Stack testing was done between -55 °C to 85 °C using a baseline output current of 10 µA and periodic pulses of 4.5 mA lasting 100 ms, every 100 s, or an average current of 16 µA. The stack spatial water concentration was monitored during various operating conditions through in situ neutron imaging. Figure 1 shows the main points of interest on the stack. Operating temperatures ranged from -55 °C to 85 °C to analyze water formation and subsequent water removal. Freezing and below-freezing temperatures were controlled by use of a liquid nitrogen cooled series of insulated boxes to prevent external water condensation during imaging. These measurements suggest successful water transport laterally, enabling operation from the sub-freezing environment to temperatures near the boiling point of water. Figure 2a depicts a constant operation for two hours at 16 µA and -20 ºC and the subsequent drying process in Figure 2b. Water production due to gas crossover in the stacks was quantified at below-freezing temperatures. At temperatures at and below -20 °C, the water production due to gas crossover was greater than the amount of water removal via the passive water management strategy with the current turned off, rather than a drying of the cell as expected. With the gases turned off as well, drying occurs. This indicates that the cell design must allow for water (ice) build-up during operation at these sub-freezing temperatures for successful operation. To avoid large amounts of hydrogen loss due to membrane crossover, we employ multiple layers of membrane to make a thick, mechanically reinforced and chemically stabilized membrane. A two-dimensional multiphysics model for evaluation of water management was also developed to further describe and explore the passive water management system under different conditions. The fundamental model in its present form shows a similar shape but indicates faster water build-up than experimental data. This discrepancy is currently being resolved to allow for a more accurate representation. Acknowledgments: This work was supported by the U.S. Department of Energy. References: Chintam, M. S. Wilson, T. Rockward, S. Stariha, A. M. Baker, E. L. Brosha, D. S. Hussey, J. M. LaManna, D. L. Jacobson, J. Rau, R. L. Borup, ECS Transactions 86 (13), 233-244. Figure 1

Despite the advantages of CO

Neutrons and X-rays provide excellent complementary, nondestructive probes to understand internal structure of systems across engineering and material science. With its sensitivity to hydrogen, neutrons excel at separating fluids, such as water or oil, from solid and gas phases in three-phase systems whereas X-rays excel at identifying the solid phase. To fully leverage the complementarity of the two methods, the National Institute of Standards and Technology (NIST) has developed the Neutron and X-ray Tomography (NeXT) system which orients a microfocus X-ray generator orthogonally to a reactor sourced thermal neutron beam. This orientation allows for truly simultaneous acquisition of both modalities so that the multi-modal data sets of samples that are evolving with time or undergoing stochastic processes can be directly correlated. The NeXT system has been available for external researchers since 2015 through the NIST Center for Neutron Research user facility program. Significant efforts have resulted in distributable software packages to facilitate image denoising, tomography reconstruction, volume registration, and bivariate histogram segmentation. This paper will give an overview of the NeXT system, explain the process for bivariate histogram segmentation, and provide several examples of use cases for the system.

In this work, we investigated the impact of temperature on two-phase transport in low temperature (LT)-polymer electrolyte membrane (PEM) electrolyzer anode flow channels via in operando neutron imaging and observed a decrease in mass transport overpotential with increasing temperature. We observed an increase in anode oxygen gas content with increasing temperature, which was counterintuitive to the trends in mass transport overpotential. We attributed this counterintuitive decrease in mass transport overpotential to the enhanced reactant distribution in the flow channels as a result of the temperature increase, determined via a one-dimensional analytical model. We further determined that gas accumulation and fluid property changes are competing temperature-dependent contributors to mass transport overpotential; however, liquid water viscosity changes led to the dominant enhancement of reactant water distributions in the anode. We present this temperature-dependent mass transport overpotential as a great opportunity for further increasing the voltage efficiency of PEM electrolyzers.

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...

Micro- to milli-watt fuel cell stacks and systems are designed and tested for applications that require continuous operation for multiple decades and at very wide ambient temperature ranges. Development presumes generic load profiles consisting of small baseline currents with frequent, periodic excursions to higher currents, both delivered at the 3.3 V standard integrated circuit voltage level. Stack design and testing was done between -55˚C to 80˚C using a baseline output current of 10 uA and periodic pulses of 4.5 mA lasting 100 ms, every 100 s. To simplify testing, the stacks are paired with 3.3V output miniature dc-dc converters to supply the respective 33 uW and 15 mW power requirements, with the stack then initially providing about 40 uW and 17 mW reflecting the converter efficiencies at the two sets of output currents/input voltages. Since maximum cell power tends to occur at >0.5 V/cell, 8-cell stacks were chosen so that, if called upon, the maximum power point still exceeds the ~4 V minimum buck converter input. Concerns about air availability and purity impel the use of H2 & O2 reactants; accommodating the fittings and other design constraints lead to a 13x19mm footprint (active area = 0.2 cm2), as shown in Figure 1. Since the stack reactant supplies are “dead-ended”, provision needs to be made to remove product water. In contrast to typical (higher power) fuel cell applications, the primary system challenges with the microwatt system are reactant cross-over and leakage current. As an example of the severity of the issue, a 2020 technical target1 for membrane leakage current is an equivalent DC resistance of 1000 Wcm2. A similar value in this case would yield a cumulative leakage current of about 1.4 mA, causing an otherwise 30 y reactant supply for this system to last less than a year. Moreover, cross-over losses are generally hand-in-hand with leakage. The simple straightforward solution is to use thick membranes, done here primarily by stacking multiple layers of DuPont XL, which provides the benefits of radical scavengers and PTFE reinforcement scrims to enhance long-term chemical and dimensional stability. Other loss mitigation configurations and strategies are being pursued in tandem. The need to provide 17 mW pulses not only limits the maximum membrane thicknesses (and stack resistances), but subjects the individual cells to voltage cycles between about 0.6V and OCV, similar to a catalyst durability AST (accelerated stress test). These issues can be alleviated by pairing the stack with a supercapacitor to minimize the voltage swings during the 17 mW pulses, thus allowing the stack to supply a fairly constant 65 uW (accounting for converter efficiency). Consequently, life and environmental testing are underway to demonstrate the utility of the configuration, but the preferred goal is to achieve the desired results without introducing additional components with potential lifetime and temperature range issues. Long-term room temperature life-testing is underway on stacks with and without supercaps. As of March 2018, a stack subjected to the full 17 mW pulses is approaching 5000 h of operation and has shown little if any drop in performance beyond an initial modest decline. A second stack paired with supercaps has been in operation nearly as long and likewise shows minimal sustained losses, even though it is being subjected to 10x the standard pulse rate. Environmental chamber testing has demonstrated prolonged operation at temperatures ranging from -40˚ to 80˚C. Stack resistance decreases between room and high (80˚C) temperatures, overtly because the proton transport kinetics are enhanced in the already extremely dry membranes. While such dry operation was believed warranted to avoid product water freezing in the catalyst layer causing transport issues, the ability of the stacks to provide 17 mW pulses is challenged by the high stack resistance. Consequently, we have begun investigating the degree to which we can allow water content to increase while maintaining reactant access. Additional future efforts include environmental testing down to -55˚C and further stack design evolution to improve performance and functionality. Figure 1. An 8-cell stack with nitrided titanium bipolar and endplates. Reference 1 DOE Technical Targets for Polymer Electrolyte Membrane Fuel Cell Components. https://www.energy.gov/eere/fuelcells/doe-technical-targets-polymer-electrolyte-membrane-fuel-cell-components Figure 1

Through-plane in operando neutron radiography was used to differentiate between anode feed water and cathode water (transported through the membrane) to resolve oxygen gas bubbles in the anode. In this work, we investigated the impact of cathode purging on the electrochemical performance of polymer electrolyte membrane (PEM) electrolyzers as well as its impact on neutron radiographic imaging to inform our recommendations on how to prescribe an optimal cathode purging rate. We found that excessive dry nitrogen purging in the cathode led to an increase in ohmic resistance and a decrease in the open circuit voltage. Therefore, an optimal nitrogen purging rate must be prescribed during visualization studies to maximize the precision of oxygen gas quantification without unintentionally altering the electrochemical performance. We demonstrated that the optimal cathode nitrogen purge rate can be determined by monitoring the spatial distribution of the anode oxygen gas and that this optimal purge rate is a function of the operating current density.

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.

Enhancement of plant drought stress tolerance by plant growth‐promoting rhizobacteria (PGPR) has been increasingly documented in the literature. However, most studies to date have focused on PGPR‐root/plant interactions; very little is known about PGPR's role in mediating physiochemical and hydrological changes in the rhizospheric soil that may impact plant drought stress tolerance. Our study aimed to advance mechanistic understanding of PGPR‐mediated biophysical changes in the rhizospheric soil that may contribute to plant drought stress tolerance in addition to plant responses. We measured soil water retention characteristics, hydraulic conductivity, and water evaporation in soils with various textures (i.e., pure sand, sandy soil, and clay) as influenced by a representative PGPR (Bacillus subtilis strain UD1022) using the HYPROP system. We found that all PGPR‐treated soils held more water and had reduced hydraulic conductivity and accumulative evaporation, compared to their corresponding controls. We discuss three mechanisms, due to B. subtilis incubation or production of extracellular polymeric substances (EPS), that are potentially responsible for the changes in hydraulic properties and soil evaporation: (i) EPS have a large water holding capacity; (ii) EPS alter soil matrix structure and connectivity of pore space; (iii) EPS modify the physicochemical properties of water (surface tension and viscosity). These results clearly demonstrate PGPR's ability to increase water availability to plants by slowing down evaporation and by increasing the time available for plants to make metabolic adjustments to drought stress.

There are conflicting views in the literature concerning the optimum moisture state for an existing substrate prior to the application of a repair material. Both saturated-surface-dry (SSD) and dry substrates have been found to be preferable in a variety of studies. One confounding factor is that some studies evaluate bonding of the repair material to the substrate via pull-off (direct tension) testing, while others have employed some form of shear specimens as their preferred testing configuration. Available evidence suggests that dry substrate specimens usually perform equivalently or better in shear testing, while SSD ones generally exhibit higher bond strengths when a pull-off test is performed, although exceptions to these trends have been observed. This paper applies a variety of microstructural characterization tools to investigate the interfacial microstructure that develops when a fresh repair material is applied to either a dry or SSD substrate. Simultaneous neutron and X-ray radiography are employed to observe the dynamic microstructural rearrangements that occur at this interface during the first 4 h of curing. Based on the differences in water movement and densification (particle compaction) that occur for the dry and SSD specimens, respectively, a hypothesis is formulated as to why different bond tests may favor one moisture state over the other, also dependent on their surface roughness. It is suggested that the compaction of particles at a dry substrate surface may increase the frictional resistance when tested under slant shear loading, but contribute relatively little to the bonding when the interface is submitted to pull-off forces. For maximizing bond performance, the fluidity of the repair material and the roughness and moisture state of the substrate must all be given adequate consideration.

Fatigue in selective laser melted (SLM) and conventionally manufactured stainless steel (SS) 316 dogbones was studied with neutron imaging methods for attenuation, diffraction, and scattering. Neutron attenuation tomography and Bragg edge imaging did not reveal crack formation. Conversely, to efficiently detect the microcrack evolution, two methods of grating-based neutron interferometry, Talbot-Lau and far-field, were employed. Both interferometry methods detect early crack formation via the dark-field (scattering) images, especially for porous microstructures in the range of 0.6 μm to 2 μm. The dark-field image combines sensitivity to micrometer-sized scattering centers at crack formation with sub-millimeter imaging resolution. The crack formation observed with neutron interferometry dark-field was validated post-imaging with additional fatigue cycles to fracture. Further inspection was performed by scanning electron microscopy (SEM) and optical photography. In the two fatigued dogbones, SLM and conventional crack formation was identified to within 1 mm. Keywords: Additive manufacturing, Neutron interferometry, Bragg edge spectroscopy, Fatigue

Neutron radiography, a non-destructive imaging technique, is applied to study water and solute transport through desalination membranes. Specifically, we use neutron radiography to quantify lithium chloride draw solute concentrations across a thin-film composite membrane during forward osmosis permeation. This measurement provides direct visual confirmation of incomplete support layer wetting and reveals significant dilutive external concentration polarization of the draw solution outside of the membrane support layer. These transport-limiting phenomena have been hypothesized in previous work and are not accounted for in the standard thin-film model of forward osmosis permeation, resulting in inaccurate estimations of membrane transport properties. Our work demonstrates neutron radiography as a powerful measurement tool for studying membrane transport and emphasizes the need for direct experimental measurements to refine the forward osmosis transport model.

Summary A prominent technology for green hydrogen generation is the polymer electrolyte membrane (PEM) electrolyzer. However, the energy efficiency of PEM electrolyzers must improve dramatically to become economically competitive. Here, we engineer the wettability of commercial porous transport layers (PTLs) to make them superhydrophilic. We find that the superhydrophilic PTLs increase the efficiency of PEM electrolyzers by >11% at high current operation (up to 20%). We show via electrochemical analyses and in operando neutron imaging that the improved efficiency stems from reduced gas saturation in the anode PTL, which significantly decreases the mass transport overpotential. We conduct ex situ microfluidic experiments and demonstrate that capillary-driven corner flow is a key physical mechanism responsible for the reduced oxygen gas saturation and enhanced liquid water transport. Our findings illustrate the importance of PTL wettability on mass transport in PEM electrolyzers and enable the design of next-generation electrolyzers with much greater efficiency.

Understanding of fundamental processes and prediction of optimal parameters during the horizontal drilling and hydraulic fracturing process results in economically effective improvement of oil and natural gas extraction. Although modern analytical and computational models can capture fracture growth, there is a lack of experimental data on spontaneous imbibition and wettability in oil and gas reservoirs for the validation of further model development. In this work, we used neutron imaging to measure the spontaneous imbibition of water into fractures of Eagle Ford shale with known geometries and fracture orientations. An analytical solution for a set of nonlinear second-order differential equations was applied to the measured imbibition data to determine effective contact angles. The analytical solution fit the measured imbibition data reasonably well and determined effective contact angles that were slightly higher than static contact angles due to effects of in-situ changes in velocity, surface roughness, and heterogeneity of mineral surfaces on the fracture surface. Additionally, small fracture widths may have retarded imbibition and affected model fits, which suggests that average fracture widths are not satisfactory for modeling imbibition in natural systems.

We report on efforts to improve the achievable spatial resolution in neutron imaging by centroiding the scintillation light from gadolinium oxysulfide scintillators. The current state-of-the-art neutron imaging spatial resolution is about 10 μm, and many applications of neutron imaging would benefit from at least an order of magnitude improvement in the spatial resolution. The detector scheme that we have developed magnifies the scintillation light from a gadolinium oxysulfide scintillator, calculates the center of mass of the scintillation event, resulting in an event-based imaging detector with spatial resolution of about 2 μm.

Proton Exchange Membrane Fuel Cells (PEMFCs) known for their use as portable energy-conversion devices have become increasingly more popular as backup power sources. Some backup power source applications require PEMFCs to startup within seconds after being stored for many years without any external aid. The ability to reach maximum power with a quick startup time in our design comes from the PEMFC being stored with water saturated membranes and catalyst layers. For the lowest overall resistance needed for an immediate startup, the water content of the membrane and catalyst layer ionomer should be maximized, which requires direct contact with liquid water i.e. Schroeder’s paradox. [1] However, any water collecting in the flow-fields or gas diffusion layers (GDLs) is detrimental, as it can obstruct reactant access. Consequently, the water distribution throughout the PEMFC during long-term storage is of critical interest for quick startup times. Previous studies have shown that water content and its distribution within a PEMFC is a good indicator of performance during both normal operation and accelerated stress testing. These studies have been mainly done using neutron imaging. [2,3] The measurements and analysis can be applied to PEMFCs hermetically sealed in storage. Before a PEMFC is stored, neutron imaging can be used to measure the water content and its distribution within the cell. It can then be measured and mapped for a series of identical PEMFCs stored for different lengths of time. Storage experiments have been performed by humidifying and hermetically sealing two identical single cells. Both cells were humidified by controlling the current using dry H2/O2 until a power of 2 W was reached and maintained for 60 seconds. It should be noted that for future experiments the cells will be stored in N2 to prevent any membrane permeation. Cell 1 was stored over night while cell 2 was stored for 14 days. For these cells, the goal was instant startup of at least 1.25 W at 0.5 V with 50 psig H2/O2 and maintaining that power output for at least 60 seconds. Figure 1 shows the results of both startups after storage. Cell 1 (solid grey line) after being stored overnight reached a peak power of 2 W in 1 second and maintained a power of 1.75 W for 120 seconds. Cell 2 (solid blue line) after being stored for 14 days reached a peak power of 3.25 W in 1 second however its power dropped rapidly and after 120 seconds reached the minimum power output of 1.25 W. Water movement within the cell during storage contributed to the different startup behaviors observed. With the use of high resolution neutron imaging in the z-direction (See Figure 2) the water content and distribution can be imaged through all components. The goal of this work is to store identical cells for varying lengths of time, compare the water content distribution in each cell and how it relates to the startup performance, and extrapolate how a cell will startup after multiple years in storage. Acknowledgments: This work is supported by the NNSA Engineering Campaign with long term funding from the Fuel Cell Technologies Office References T.A. Zawodzinski et al. Journal of The Electrochemical Society, 1993. 140 (4) 1041-1047. Spernjak, D. et al. Journal of The Electrochemical Society, 2009. 156 (1) B109-B117. Macauley, N. et al. Journal of The Electrochemical Society, 2016. 163 (13) F1317-F1329. Figure 1

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.

Muhammad Arif died after an extended illness on November 27, 2018 at the age of 64. Arif, as he was always called, was born in Madaripur, Bangladesh, as the first of four children. He attended Dacc...

As a neutron scatters from a target nucleus, there is a small but measurable effect caused by the interaction of the neutron’s magnetic dipole moment with that of the partially screened electric field of the nucleus. This spin-orbit interaction is typically referred to as Schwinger scattering and induces a small rotation of the neutron’s spin on the order of 10(−4) rad for Bragg diffraction from silicon. In our experiment, neutrons undergo greater than 100 successive Bragg reflections from the walls of a slotted, perfect-silicon crystal to amplify the total spin rotation. A magnetic field is employed to insure constructive addition as the neutron undergoes this series of reflections. The strength of the spin-orbit interaction, which is directly proportional to the electric field, was determined by measuring the rotation of the neutron’s spin-polarization vector. Our measurements show good agreement with the expected variation of this rotation with the applied magnetic field, while the magnitude of the rotation is ≈40 % larger than expected.

The NIST Neutron and X-ray Tomography (NeXT) system provides simultaneous, complementary, multimodal information for the characterization of materials. Neutrons and X-rays yield complementary, non-destructive views of a complex system due to the contrast differences that arise from the differences in interaction with matter for the two modes. The NeXT system orients a 90 keV microfocus X-ray tube orthogonally to the thermal neutron beam. This enables truly simultaneous capture of the two modalities, thus it is possible to perform multimodal tomography of dynamic or stochastic samples while penetrating through sample environment equipment such as pressure and flow vessels. NIST has developed and continues to update distributable software for reconstructing and analyzing the multimodal datasets. Tomographic reconstruction offers multiple CPU and GPU based algorithms for parallel and cone beam geometries. Additional analysis routines perform volume registration and region of interest segmentation of the bivariate histogram that leverages the contrast differences between the two modalities. This talk will give an overview of the NeXT system, the algorithm developments underway and provide multiple research applications using the system.

Increasing electrode thickness is one route to improve the energy density of lithium-ion battery cells. However, restricted Li+ transport in the electrolyte phase through the porous microstructure of thick electrodes limits the ability to achieve high current densities and rates of charge/discharge with these high energy cells. In this work, processing routes to mitigate transport restrictions were pursued. The electrodes used were comprised of only active material sintered together into a porous pellet. For one of the electrodes, comparisons were done between using ice-templating to provide directional porosity and using sacrificial particles during processing to match the geometric density without pore alignment. The ice-templated electrodes retained much greater discharge capacity at higher rates of cycling, which was attributed to improved transport properties provided by the processing. The electrodes were further characterized using an electrochemical model of the cells evaluated and neutron imaging of a cell containing the ice-templated pellet. The results indicate that significant improvements can be made to electrochemical cell properties via templating the electrode microstructure for situations where the rate limiting step includes ion transport limitations in the cell.

The flow channel design in a proton exchange membrane fuel cell is critical for transporting reactant gases and removing product water efficiently. Herein, we proposed and performed a comprehensive study of four flow channel designs: straight, wavy, 2D-Nozzle, and the novel 3D-Nozzle. Using the limiting current method, we discovered that the oxygen transport resistance of straight, wavy, and 2D-Nozzle designs are similar confirming the diffusive transport mechanism. In contrast, the oxygen transport resistance of the 3D-Nozzle design is significantly less than that of the other three designs due to the channel-induced convective flux in the gas diffusion layer. As a result, the peak power density of the 3D-Nozzle design is 25% higher than all other designs. The in situ neutron images confirm that the 3D-Nozzle design has less and more evenly distributed water than the straight channel design. Lastly, the simulation results using a three-dimensional finite element COMSOL model show notable in-plane and through-plane convective flux in the gas diffusion layer promoting oxygen and liquid water transfer. The combined experimental and simulation results validate that the novel three-dimensional converging–diverging channel design provides superior water management capability, which in turn improve the performance and robustness of a fuel cell.

With recent developments in direct imaging techniques using x-ray and neutron imaging, there is an increasing need for efficient test setups to study the mechanical and/or transport behavior of porous rocks. Bespoke designs from commercial suppliers are expensive and often difficult to modify. This paper presents a novel design of a portable triaxial cell for imaging deformation (and a suggested adaptation to introduce fluid transport) through rocks/sand/soil under the triaxial states of stress representative of those encountered in the case of groundwater aquifers or subsurface hydrocarbon reservoirs. The design philosophy and the parameters are detailed so that interested researchers can use this experimental setup as a template to design and modify triaxial cells to suit their own experimental requirements. The design has been used in two imaging beamlines: Imaging and Material Science & Engineering (IMAT), ISIS facility, Harwell, Oxfordshire, UK, and BT2 of the National Institute of Standards and Technology Center for Neutron Research, Gaithersburg, MD, USA. The mass attenuation coefficients extracted from the 2D radiograms of the triaxial cell were compared with those reported in the literature. Further suggestions for the adaptation of the triaxial cells for studying the mechanics of deformation and fracture in rocks are included.

Summary: Reaching high current densities is absolutely imperative for electrochemical energy conversion, from fuel cells to CO2 reduction. Here, we identify the existence of a performance indicator for gas-evolving electrochemical energy conversion devices: the critical current density. The critical current density pinpoints a performance inflection point whereby both the gas saturations and mass transport overpotentials suddenly dominate cell performance and exacerbate failure. We elucidate the mass transport behavior of a polymer electrolyte membrane (PEM) water electrolyzer using in operando neutron imaging at operating current densities as high as 9 A ⋅ cm−2. Product gases become heterogeneously distributed in the porous transport layer adjacent to the catalyst layer and promote disastrous local hotspots. Turbulent reactant flows are prerequisites for mitigating these undesired mass transport overpotentials. Optimizing new materials and cell architectures with this performance indicator may unlock higher than previously reported performances for electrochemical energy conversion.

The predominant geometry for a neutron imaging experiment is that of a pinhole camera. This is primarily due to the difficulty in focusing neutrons due to the weak refractive index, which is also strongly chromatic. Proof of concept experiments demonstrated that neutron image forming lenses based on reflective Wolter mirrors can produce quantitative, high spatial resolution neutron images while also increasing the time resolution compared to the conventional pinhole camera geometry. Motivated by these results, we report the design of a neutron microscope where two Wolter mirrors replace condensing and objective lenses, in direct analogy with typical visible light microscopes. Ray tracing results indicate that this system will yield 3 μm spatial resolution images with an acquisition time of order