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In this feasibility study, high-resolution neutron tomography is used to investigate the water distribution in polymer electrolyte membrane fuel cells (PEMFCs). Two PEMFCs were built up with two different gas diffusion layers (GDLs) namely Sigracet® SGL-25BC and Freudenberg H14C10, respectively. High-resolution neutron tomography has the ability to display the water distribution in the flow field channels and the GDLs, with very high accuracy. Here, we found that the water distribution in the cell equipped with the Freudenberg H14C10 material was much more homogenous compared to the cell with the SGL-25BC material.

We present a study on a new type of flow field channel design for polymer electrolyte membrane fuel cells (PEMFCs). Small barriers have been implemented into the flow field channels that force the gas flow to move through the gas diffusion layers in order to improve the supply of the catalyst with reactant gases. We investigated the water distribution in the PEMFC with neutron imaging during operation and compared the results with a comparable reference cell without barriers. We found strong hints for an increased mechanical gas flow resistance by the barriers caused by additional liquid water agglomerations. Furthermore water distribution in the barrier flow field is much more homogenous compared to the reference cell. We assume that both effects, namely the gas flow through the GDL and the homogenous water distribution are responsible for the found performance increase of up to 10%.

Understanding the function of nanoporous materials employed in polymer electrolyte membrane fuel cells (PEMFCs) is crucial to improve their performance, durability, and cost efficiency. Up to now, the water distribution in the nm-sized pore structures was hardly accessible during operation of the cells. Here we demonstrate that phase contrast synchrotron X-ray tomography allows for an in operando quantification of the three-dimensional water distribution within the nm-sized pores of carbon-based microporous layers (MPLs). For this purpose, a fuel cell design optimized for tomographic phase contrast measurements was realized. Water in the pores of the entire MPL was detected and quantified. We found an inhomogeneous distribution of the local water saturation and a sharp boundary between mostly filled MPL and almost empty areas. We attribute the latter observation to the two-phase boundary created because condensation takes place predominantly on one side of the boundary. Furthermore, high water saturation in large areas hints at gas diffusion or transport along preferred three-dimensional paths through the material, therefore bypassing most of the MPL volume. Our approach may contribute significantly to future investigations of nanoporous fuel cell materials under realistic operating conditions.

Complementary in operando X-ray radiography and neutron radiography measurements were conducted to investigate and visualize the initial lithiation in silicon-electrode lithium-ion batteries. By means of X-ray radiography, a significant volume expansion of Si particles and the Si electrode during the first discharge was observed. In addition, many Si particles were found that never undergo electrochemical reactions. These findings were confirmed by neutron radiography, which, for the first time, showed the process of Li alloying with the Si electrode during initial lithiation. These results demonstrate that complementary X-ray and neutron radiography is a powerful tool to investigate the lithiation mechanisms inside Si-electrode based lithium-ion batteries.

Knowledge about degradation and failure of Li ion batteries LIBs is of paramount importance, especially because failure can be accompanied by severe hazards. To contribute to the understanding of such phenomena, synchrotron in line phase contrast X ray tomography was employed to investigate internal cell deformation and degradation caused by an internal short circuit ISC . The tomographic images taken from an uncycled Li Li cell and a short circuited Li Li cell reveal how lithium microstructures LmSs develop during electrochemical stripping and plating during discharge and charge and how the three layer separator used is damaged by growing LmSs and delaminates and melts as a consequence of an ISC. Previously unknown insights into the internal cell degradation and deformation mechanisms caused by an ISC are obtained and provide hints of how the properties of the separator could be modified to improve the reliability and safety of current and next generation LIBs

Carbonate and sandstone reservoirs play an important role in oil industry as they host over 50% of the world’s hydrocarbon reserves. For an accurately assessment of porosity and pore size distribution of such complex pore-network, which affect directly the macroscopic characteristics of multiphase fluid flow, X-ray computed microtomography (micro-CT) emerges as a powerful tool. In contrast to lab-based X-ray micro-CT (XCT), synchrotron X-ray micro-CT (SXCT) images are commonly free of artefacts (i.e. beam hardening) and the unique properties of synchrotron sources enable the X-ray imaging of complex and heterogeneous materials in greater detail, with higher quality, and short acquisition time. This work reports results of cone beam computed microtomography (XCT) in comparison with synchrotron computed microtomography (SXCT) applied to very heterogeneous carbonate and sandstone reservoir rocks. We analyze the quality of the image generated in terms of detection of details and artefacts, the advantages and limitation of each technique, as well as features like contrast, sharpness, and signal-to-noise ratio (SNR). Although SXCT offers significant advantages over XCT, the latter gains in cost of operation, accessibility and user-friendliness.

Materials today 27, 21 - 32 (2018). doi:10.1016/j.mattod.2018.11.003, An intrinsic knowledge gap between current understandings obtained experimentally and the underlying working or degradation mechanisms of rechargeable lithium batteries still remains, giving direct rise to application challenges, e.g., safety issues, predicaments in identifying performance-aging factors and dilemmas in guiding further research directions. Against this background, non-destructive and three-dimensional (synchrotron) X-ray tomography that guarantees a direct visual access to inner electrodes has been employed herein to: in-situ record the evolution of internal short circuits; characterize the behaviors of widely employed separators; investigate the morphological evolution of Li electrodes under different cycling conditions; and study the degradation mechanisms of Li/carbon cells. By incorporating the currently presented results with the previously published studies on those topics, a complete picture of the degradation mechanism of rechargeable lithium batteries has been painted. This advancement of mechanistic understanding supplies the missing pieces of information to bridge fundamental R&D research activities and practical applications., Published by Elsevier Science, Amsterdam [u.a.]

Innovative solutions have been designed to meet the global demand for energy and environmental sustainability, such as enhanced hydrocarbon recovery and geo-sequestration of CO2. These processes involve the movement of immiscible fluids through permeable rocks, which is affected by the interfacial properties of rocks at the pore scale. Overcoming major challenges in these processes relies on a deeper understanding about the fundamental factors that control the rock wettability. In particular, the efficiency of oil recovery strategies depends largely on the 3D wetting pattern of reservoir rocks, which is in turn affected by the adsorption and deposition of ‘contaminant’ molecules on the pores’ surface. Here, we combined high-resolution neutron tomography (NT) and synchrotron X-ray tomography (XRT) to probe the previously unobserved 3D distribution of molecular and mineralogical heterogeneity of oil reservoir rocks at the pore scale. Retrieving the distribution of neutron attenuation coefficients by Monte Carlo simulations, 3D molecular chemical mappings with micrometer dimensions could be provided. This approach allows us to identify co-localization of mineral phases with chemically distinct hydrogen-containing molecules, providing a solid foundation for the understanding of the interfacial phenomena involved in multiphase fluid flow in permeable media.

Phosphoric acid as the electrolyte in high-temperature polymer electrolyte membrane fuel cell plays an essential role in its performance and lifetime. Maldistribution of phosphoric acid in the catalyst layer (CL) may result in performance degradation. In the present study, pore-scale simulations were carried out to investigate phosphoric acid’s multiphase flow in a cathode CL. A reconstructed CL model was built using focused ion beam-SEM images, where distributions of pore, carbon support, binder, and catalyst particles can be identified. The multi-relaxation time lattice Boltzmann method was employed to simulate phosphoric acid invading and leaching from the membrane into the CL during the membrane electrode assembly fabrication process. The predicted redistribution of phosphoric acid indicates that phosphoric acid of low viscosity or low wettability is prone to leaching into the CL. The effective transport properties and the active electrochemical active surface area (ECSA) were computed using a pore-scale model. They were subsequently used in a macroscopic model to evaluate the cell performance. A parametric study shows that cell performance first increases with increasing phosphoric acid content due to the increase of ECSA. However, further increasing phosphoric acid content results in performance degradation due to mass transfer limitation caused by acid flooding.

Combined three-dimensional (3D) mapping of (micro-)structures with elemental and crystalline phase variations is of significant importance for the characterization of materials. Neutron wavelength selective imaging is a spectral imaging technique that exploits unique contrast differences e.g. for mapping dissimilar elemental, isotope, or phase compositions, and has the particular advantage of being applicable to sample volumes on the meso- and macroscale. While being mostly applied as radiography (2D) so far, we herein report that the extension to tomography allows for the display of the full spectral information for every voxel and in 3D. The development is supported by example data from a continuous as well as a pulsed neutron source. As a practical example, we collected 4D data sets (3D + spectral) of plastically deformed metastable stainless steel and herein demonstrate an improved quantification strategy for crystalline phase fractions. These exemplary results illustrate that localized phase transformations can be quantified even in complex geometries within centimeter-sized samples, and we will discuss the limits and future prospects of the technique that is not limited to crystalline materials.

This work describes the effects of catalyst layers (CLs) consisting of hydrophobic PTFE on the performance and water management of PEM fuel cells. Catalyst inks with various PTFE contents were coated on Nafion membranes and characterized using contact angle measurements, SEX-EDX, and mercury porosimetry. Fuel cell tests and electrochemical impedance spectroscopy (EIS) were conducted under varying operating conditions for the prepared materials. At dry conditions, CLs with 5 wt.% PTFE were advantageous for cell performance due to improved membrane hydration, whereas under humid conditions and high air flow rates CLs with 10 wt.% PTFE improved the performance in high current density region. Higher PTFE contents (≥20 wt.%) increased the mass transport resistance due to reduced porosity of the CLs structure. Operando neutron radiography was utilized to study the effects of hydrophobicity gradients within CLs and cathode microporous layer (MPLC) on liquid water distribution. More hydrophobic CLs increased the water content in adjacent layers and improved performance, especially at dry conditions. MPLC with higher PTFE contents increased the overall liquid water within the CLs and GDLs and escalated the water transfer to the anode side. Furthermore, the role of back-diffusion transport mechanism on water distribution was identified for the investigated cells.

The mechanical compression of metal foam flow-field based polymer electrolyte fuel cells (PEFCs) is critical in determining the interfacial contact resistance with gas diffusion layers (GDLs), reactant flow and water management. The distinct scale between the pore structure of metal foams and the entire flow-field warrant a multi-length scale characterization that combines ex-situ tests of compressed metal foam samples and in-operando analysis of operating PEFCs using X-ray computed tomography (CT) and neutron radiography. An optimal ‘medium’ compression was found to deliver a peak power density of 853 mW cm−2. The X-ray CT data indicates that the compression process significantly decreases the mean pore size and narrows the pore size distribution of metal foams. Simulation results suggest compressing metal foam increases the pressure drop and gas velocity, improving the convective liquid water removal. This is in agreement with the neutron imaging results that demonstrates an increase in the mass of accumulated liquid water with minimum compression compared to the medium and maximum compression cases. The results show that a balance between Ohmic resistance, water removal capacity and parasitic power is imperative for the optimal performance of metal foam based PEFCs.

Water transport in polymer electrolyte membrane fuel cells (PEMFCs) is investigated by synchrotron X-ray imaging as a non-destructive testing method. Two different measurement techniques, namely in-situ radiography and quasi-in-situ tomography were combined to reveal the relationship between the structure of the microporous layer (MPL) and the water flow at different operation temperatures. It is shown how water distribution and transport are quantified and visualized. With this method, investigations of different cell materials at different operating parameters such as cell current, gas humidity, gas flow, etc. are conducted.

Two kinds of prototypes of lithium ion batteries (LIBs) for in situ X-ray tomography and radiography are reported. Tomography cells (tomo-cells) and radiography cells (radio-cells) both assembled with silicon/carbon (Si/C) or tin/carbon (Sn/C) working electrodes and Li counter electrodes were investigated with synchrotron X-ray and laboratory X-ray imaging. The structural changes of individual electrode particles during charging and discharging were analyzed. Here we are providing a brief overview to our recent results on this field.

In this study, synchrotron X-ray imaging is used to investigate the water transport inside newly developed GDM (gas diffusion medium) in polymer electrolyte membrane fuel cells. Two different measurement techniques, namely in-situ radiography and quasi-in-situ tomography were combined to reveal the relationship between the structure of the MPL (microporous layer), the operation temperature and the water flow. The newly developed MPL is equipped with randomly arranged holes. It was found that these holes strongly influence the overall water transport in the whole adjacent GDM. The holes act as nuclei for water transport paths through the GDM. In the future, such tailored GDMs could be used to optimize the efficiency and operating conditions of polymer electrolyte membrane fuel cells.

Gas generation within lithium ion batteries (LIBs) gives rise to safety concerns that question their applicability. By employing synchrotron X-ray imaging, the gas and channel evolution occurring in an operating LIB have been directly visualized in their inherent 3D state as a function of discharge and charge. Using the spatial 3D distribution of gas bubbles and channels, the active particles that dictate the performance of a functional LIB were identified and visualized in 3D. Delithiation and lithiation are interpreted as the process of activating particles continuously in a step-by-step way. The present work not only demonstrates the generation and evolution of gas within LIB in 3D, but also reveals the distribution of active particles for the first time. These fundamentally findings presented here shed light on a range of processes that could not previously be characterized in 3D and can provide practical guidance for the design of next-generation LIBs with improved safety.

We present a quantitative analysis of the influence of artificial ageing of gas diffusion layers (GDL) on the water distribution and transport in polymer electrolyte membrane fuel cells (PEMFCs) during cell operation. Water droplet size distributions are measured by means of in-operando neutron radiography. We find a strong correlation between droplet size distribution and GDL ageing time: With increasing GDL ageing, water droplet sizes in the flow field channels strongly decrease, indicating an ineffective water transport that leads to a reduced cell performance. This effect can be assigned to water accumulations on the GDL surface that block the gas supply towards the catalyst layer.

The understanding of dynamic processes in Li-metal batteries is an important consideration to enable the full capacity of cells to be utilised. These processes, however, are generally not directly observable using X-ray techniques due to the low attenuation of Li; and are challenging to visualise using neutron imaging due to the low temporal resolution of the technique. In this work, complementary X-ray and neutron imaging are combined to track the dynamics of Li within a primary Li/SOCl2 cell. The temporal challenges posed by neutron imaging are overcome using the golden ratio imaging method which enables the identification of Li diffusion in operando. This combination of techniques has enabled an improved understanding of the processes which limit rate performance in Li/SOCl2 cells and may be applied beyond this chemistry to other Li-metal cells.

The increasing use of intermittent renewable energy sources calls for novel approaches to large-scale energy conversion and storage. Hydrogen can be readily stored and produced from renewable sources using polymer electrolyte membrane water electrolysers (PEMWEs). Mass transport of water and product gas in the liquid-gas diffusion layer (LGDL) is critical for PEMWE performance, particularly at high current densities. In this work, neutron radiography is deployed to measure the spatial distribution of water within three different LGDLs, while X-ray micro-computed tomography (XCT) is used to characterize the microstructure of the LGDL materials. The combination of these two techniques yields valuable insight into water transport within the LGDL. Significant local water heterogeneity is observed and a link between flow-field geometry/location and LGDL mass transport is identified. It is further shown that the pore volume in these LGDLs is significantly under-utilized, pointing the way towards design optimisation of LGDL materials and architectures.

Dynamic and direct visualization of interfacial evolution is helpful in gaining fundamental knowledge of all solid state lithium battery working degradation mechanisms and clarifying future research directions for constructing next generation batteries. Herein, in situ and in operando synchrotron X ray tomography and energy dispersive diffraction were simultaneously employed to record the morphological and compositional evolution of the interface of InLi anode sulfidesolid electrolyte during battery cycling. Compelling morphological evidence of interfacial degradation during all solid state lithium battery operation has been directly visualized by tomographic measurement. The accompanying energy dispersive diffraction results agree well with the observed morphological deterioration and the recorded electrochemical performance. It is concluded from the current investigation that a fundamental understanding of the phenomena occurring at the solid solid electrode electrolyte interface during all solid state lithium battery cycling is critical for future progress in cell performance improvement andmay determine its final commercial viability

The internal microstructure of a silicon electrode in a lithium ion battery was visualized by operando synchrotron X-ray radioscopy during battery cycling. The silicon particles were found to change their sizes upon lithiation and delithiation and the changes could be quantified. It was found that volume change of a particle is related to its initial size and is also largely determined by the changing surrounding electron-conductive network and internal interface chemical environment (e.g., electrolyte migration, solid-electrolyte interphase propagation) within fractured particles. Moreover, an expansion prolongation phenomenon was discovered whereby some particles continue expanding even after switching the battery current direction and shrinkage would be expected, which is explained by assuming different expansion characteristics of particle cores and outer regions. The study provides new basic insights into processes inside Si particles during lithiation and delithiation and also demonstrates the unique possibilities of operando synchrotron X-ray imaging for studying degradation mechanisms in battery materials.

We present a methodology to characterize the 3D microstructure of Ibuprofen tablets, which strongly influences the strength and solubility kinetics of the final formulation, by means of synchrotron tomography. A physically coherent trinarization for greyscale images of Ibuprofen tablets consisting of the three phases microcrystalline cellulose, Ibuprofen and pores is presented. For this purpose, a hybrid approach is developed combining a trinarization by means of statistical learning with a trinarization based on a watershed algorithm. This hybrid approach allows us to compute microstructure characteristics of tablets using image analysis. A comparison with experimental results shows that there is a significant amount of pores which is below the resolution limit and results from image analysis let us conjecture that these pores make up the major part of the surface between pores and solid. Furthermore, we compute microstructure characteristics, which are experimentally not accessible such as local percolation probabilities and chord length distribution functions. Both characteristics are meaningful in order to quantify the influence of tablet compaction on its microstructure. The presented approach can be used to get better insights into the relationship between production parameters and microstructure characteristics based on 3D image data of Ibuprofen tablets manufactured under different conditions., Comment: 20 pages, 8 figures

The potentials of incoherent X-ray scattering (Compton) computed tomography (CT) are investigated. The imaging of materials of very different atomic number or density at once is generally a perpetual challenge for X-ray tomography or radiography. In a basic laboratory set-up for simultaneous perpendicular Compton scattering and direct beam attenuation tomography are conducted by single channel photon counting line scans. This results in asymmetric distortions of the projection profiles of the scattering CT data set. In a first approach, corrections of Compton scattering data by taking advantage of rotational symmetry yield tomograms without major geometric artefacts. A cylindrical sample composed of PE, PA, PVC, glass and wood demonstrates similar Compton contrast for all the substances, while the conventional absorption tomogram only reveals the two high order materials. Comparison to neutron tomography reveals astonishing similarities except for the glass component (without hydrogen). Therefore, Compton CT offers the potential to replace neutron tomography, which requires much more efforts.

International audience; During solidification of Fe-containing Al-Si casting alloys, various different intermetallic phases are formed. As this can have an impact on mechanical properties we investigated phase formation by applying in-situ time resolved synchrotron X-ray tomography. For a commercially pure Al-10Si-0.3 wt.% Fe alloy, phase morphology and spatial arrangement during solidification was revealed with 1 degrees C temperature resolution. We find that many of the intermetallic phases that have been identified as beta-AlFeSi phases in previous studies are in fact delta phases. It was found that delta plates mainly nucleate on the eutectic Al and Si. delta phase formation terminates almost simultaneously with the final solidification of eutectic phases. Features of delta phase growth such as impingement, branching and deformation of plates are discussed. The phase separation sequence during solidification is described and explained by the existence of ``cells'' of residual liquid in which delta phase formation progresses rapidly due to the supersaturation of solute atoms. (C) 2017 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

We present a synchrotron X-ray tomographic study on the morphology of carbon fiber-based gas diffusion layer (GDL) material under compression. A dedicated compression device is used to provide well-defined compression conditions. A flat compression punch is employed to study the fiber geometry at different degrees of compression. Transport relevant geometrical parameters such as porosity, pore size and tortuosity distributions are calculated. The geometric properties notably change upon compression which has direct impact on transport conditions for gas and fluid flow. The availability of broad 3D paths, which are most important for the transport of liquid water from the catalyst layer through the GDL, is markedly reduced after compression. In a second experiment, we study the influence of the channel-land-pattern of the flow-field on shape and microstructure of the GDL. A flow-field compression punch is employed to reproduce the inhomogeneous compression conditions found during fuel cell assembly. While homogenously compressed underneath the land the GDL is much less and inhomogeneously compressed under the channel. The GDL material extends far into the channel volume where it can considerably influence gas and fluid flow. Loose fiber endings penetrate deeply into the channel and form obstacles for the discharge of liquid water droplets.

We present the first high-resolution synchrotron X-ray study on high temperature polymer electrolyte fuel cells (HT-PEFCs) in through-plane mode. Distribution and evolution of the phosphoric acid in the membrane electrode assembly was monitored in situ/in-operando under different operating conditions at steady states. At current densities of 350 mA cm −2 and 600 mA cm −2 significant changes in local media distribution were detected mainly beneath the channels of the flow field. We assign this effect to a varying degree of acid doping over the active region. Furthermore it was found that parts of the electrode structure were moved and partly irreversibly displaced after cell operation at load conditions. This effect might contribute to structural aging of the electrodes.

The microstructure of electrodes significantly influences the electrochemical performance of lithium-ion batteries. Thus, a deeper understanding of the electrode microstructure provides valuable information for the design of optimized electrode morphologies. One promising approach is called virtual materials testing, where stochastic microstructure models are used for generating a large number of virtual, but realistic morphologies, which serve as input for numerical transport simulations. Doing so, relationships between the microstructure and functional properties of the electrodes can be investigated. In the present paper, we utilize a parametric stochastic microstructure model, which is calibrated to tomographic image data of eight differently manufactured cathodes, where the compaction load has been varied. Since one and the same model type is used for all compaction loads, we can predict the model parameters for an arbitrary compaction load. This allows us to perform predictive simulations, i.e., we are able to generate virtual microstructures that correspond to a compaction load which has not been observed experimentally. The goodness of fit of the microstructure model is validated by comparing phase-based as well as particle-based characteristics of model realizations and tomographic image data. In addition, the suitability of the stochastic microstructure model for predictive simulations is pointed out by cross-validation.

The potential of wavelength resolved neutron transmission experiments is well known. This paper is focused on the performance of the double crystal monochromator which is widely used at steady state neutron sources and compares simulation results based on neutron ray tracing with experimental results in order to provide a better understanding of the device. The influences of crystal mosacities on the neutron beam is reported for the utilised setup and the resulting wavelength gradients along one direction are determined. For the neutron imaging geometry applied, a wavelength gradient of about 0.005 A/cm at the sample position was found. Moreover, a new neutron radiography technique for Bragg edge mapping in imaging experiments utilising a neutron wavelength gradient at the sample position was developed and is reported. Experiments and simulations are found to be in good agreement.

Water dynamics in the membrane electrode assembly (MEA) and flow channels of polymer electrolyte fuel cells (PEFCs) is governed by the complex interplay of many physical and operational factors. The chemical nature and structure of the gas diffusion layer (GDL) plays a large part in this and is affected by the extent to which is mechanically compressed. Here, X-ray computed tomography shows the effect of cell compression on the MEA, and how it differs under the land and channel regions. Multi-orientation neutron radiography reveals the effect of compression on the way in which water accumulates and is transported between land and channel and between cathode and anode. By performing neutron imaging in both the in-plane and through-plane directions, it is possible to determine what constitutes a given ‘thickness’ of water mapped across the extent of an MEA. Changing MEA compression from 25% to 35% has a significant effect on water distribution and dynamics in operational cells. The effect of compression on performance is most marked in the mass transport region, and there are consequences for liquid accumulation in channels and back-diffusion of water from the cathode to the anode.

In this study, the effect of the catalyst layer (CL) and the cathode microporous layer (MPL-C) porosity on the performance and water content of the polymer electrolyte membrane (PEMFC) is investigated. Monodispersed polystyrene (PS) particles are used as pore formers to fabricate CL and MPL with different degrees of porosities. Specially developed miniature Single single cells for neutron imaging experiments with an active area of 8 cm² are implemented with the in-house made catalyst coated membranes and cathode MPLs. The cathode MPLs were coated on a 29 BA substrate, and commercial 29 BC SGL were used on the anode side as the gas diffusion layers (GDLs). Neutron tomography is used to investigate the water content and distribution inside the operating fuel cells. The quantitative analysis of the water thickness under the channels of the cells at RH=120% and I=1 A.cm-2 is shown in figure 1.a. It is observed that the water content has decreased for cell 3 ( CL: 10 wt.% PS, MPL: 10 Vol.% PS) compared to cell 2 ( CL: 5wt.% PS, MPL:10 Vol.% PS) and cell 1 (Reference CL and MPL (without PS)). Polarisation curves for these three cells are provided in Figure 1.b. At current densities higher than I=1 A.cm-2 performance has increased for cell 2 and 3 where the components had higher porosity degree compared to the cell 1 which has a reference CL and reference cathode MPL. The results show that addition of macropores could facilitate the transport of gases and the removal of water at the mass transport regions. This method is currently under investigation by the authors. Further results will be provided in the presentation. Figure 1

The influence of porosity modifications of either microporous layer (MPL) or the catalyst layer of a low temperature Polymer Electrolyte Membrane Fuel Cell (PEMFC) on the performance of the cell has been investigated in many studies. However, we have been focusing in our study on the interactive impact of this morphological modification of both cathode MPL and catalyst layer in the water distribution and transport inside the cell and consequently also on the performance of the cell. In this study, three types of cathode MPLs with different porosity distributions and three type of membrane electrode assemblies (MEA) with different porosity of the catalyst layer are manufactured. Five one-cell fuel cells are investigated using combinations of those in-house made layers. To modify the porosity of the cathode MPL polymer particle in two sizes of 30 µm and 1.5 µm from company Chemisnow® are introduced into the carbon slurry, which are then evaporated out of the MPL during the sintering process. For the catalyst layers, 0.5 µm Polystyrene particles are mixed in the electrode slurry with variable weight fractions and then washed out of the dried MEA. A more detailed description of the production procedure of these layers will be demonstrated in the presentation. Performance of fuel cells using these five combination of cathode MPL and electrode are measured using a Fuel Cell with 25 cm² active area and 3 serpentine channel flow fields. Neutron Radiography is used to investigate the water distribution dynamics and water content of the fuel cell. For this purpose, a specially designed single cell with 8 cm² active area and 3 serpentine channel flow fields is used. The measurements are obtained at two different humidity values on both cathode and anode inlet gases of RH=70% and RH=120% and different current densities. With an exposure time of 10 s, images with 12.4 µm pixel size are achieved. The quantitative analysis of the water thickness in the cathode GDL and cathode electrode of 5 cells at RH=120% for an increasing current density is shown at an exemplary radiogram for water distribution inside the cell with our reference cathode MPL and electrode (without porosity modification). Also the Current (ICell) and Voltage (VCell) evolution of the same cell is shown in figure 1. All cells have reached a water content plateau even before reaching the highest current density of 1A/cm². The cell with a porous catalyst layer and a double layer cathode MPL shows the highest water content on the cathode side of the cell, whereas the cell with a highly porous catalyst layer and a porous MPL shows the lowest water content of all cells, comparable to the water thickness observed for the cell without modified layers. Further results of both water content and performance dependent on porosity changes will be provided in the presentation. Figure 1

We herein report on using a compact and low cost scintillator-camera based neutron detection system for quantitative time-of-flight imaging applications. While powerful pulsed neutron sources emerge and enable unprecedented scientific achievements, one bottleneck is the availability of suitable detectors that provide high count- and high frame- rate capabilities. For imaging applications the achievable spatial resolution/pixel size is obviously another key characteristic. While major effort was so far directed towards the development of neutron counting type imaging detectors, this work demonstrates that a camera based detector system as commonly employed at steady state sources can also be used if a suitable camera is utilized. This is demonstrated at the ESS test beamline (V20) at Helmholtz-Zentrum Berlin by recording the time-of-flight transmission spectrum of steel samples using a CMOS camera at 1 kHz frame rate, revealing the characteristic Bragg edge pattern. This 'simple' setup in the current state presents a useful option of neutron detection and has the potential to overcome many of the existing limitations and could provide a reliable alternative for neutron detector technology in general, given that the camera and scintillator technology keep up the current development speed.

Advanced lithium batteries are crucial for next-generation energy storage systems such as portable electronics and electric vehicles. Due to a low specific capacity (372 mAh/g) batteries using traditional graphite-based anode materials can hardly meet the increasing demands of light weight and high energy density. With theoretical specific capacities that are approximately 10 times higher than that of conventional graphite anodes metallic lithium (3860 mAh/g) and silicon (4200 mAh/g) anodes are regarded as two excellent candidates. Specifically, the energy density of batteries could be dramatically increased approximately from 250 Wh/kg to 440 Wh/kg by replacing graphite with lithium anodes.[1] However these two promising anodes suffer from the dramatic volume change due to microstructure degradation and low coulombic efficiency derived from unstable solid electrolyte interphase (SEI) during cycling. In addition, the dendritic and/or fiber-like deposition on lithium anode could initiate an internal short circuit and thus leads to a fast battery failure. Despite great progress recently achieved on improving electrochemical performance, a clear picture of the microstructure degradation of electrode and battery failure mechanisms is still elusive, largely due to a lack of feasible in situ characterization techniques. Synchrotron X-ray imaging has evolved into a tool of choice allowing in situ or operando analysis of the internal morphology/structure evolution non-invasively.[2, 3] With this technique the internal microstructure of a silicon electrode in a lithium ion battery was visualized during battery operation.[4] The silicon particles were found to change their sizes upon lithiation/delithiation and the changes could be quantified. An expansion prolongation phenomenon of Si particle was discovered involving that some particles continue expanding even after switching the battery current direction and shrinkage would be expected. In addition, Li deposition at the Li/separator interface as well as within a porous carbon matrix was visualized and quantified three dimensionally. The study provides new basic insights into expansion/shrinkage processes inside Si particles and uncovers the Li deposition behavior that can hardly be probed by surface imaging techniques. Finally investigations on Li anode degradation mechanisms are presented.[5] References [1] D. Lin, Y. Liu, Y. Cui, Reviving the lithium metal anode for high-energy batteries. Nature Nanotechnology, 2017, 12, 194. [2] M. Ebner, F. Marone, M. Stampanoni, V. Wood, Visualization and quantification of electrochemical and mechanical degradation in Li ion batteries. Science, 2013, 342, 716-720. [3] K. J. Harry, D. T. Hallinan, D. Y. Parkinson, A. A. MacDowell, N. P. Balsara, Detection of subsurface structures underneath dendrites formed on cycled lithium metal electrodes. Nat Mater, 2014, 13, 69-73. [4] K. Dong, H. Markötter, F. Sun et al., In situ and Operando Tracking of Microstructure and Volume Evolution of Silicon Electrodes by using Synchrotron X-ray Imaging. ChemSusChem, doi:10.1002/cssc.201801969. [5] Fu Sun, Xin He, Xiaoyu Jiang, et al., Advancing Knowledge of Electrochemically Generated Lithium Microstructure and Performance Decay of Lithium Ion Battery by Synchrotron X-ray Tomography, Materials Today, accepted.

Lifetime and durability is one of the most relevant topics regarding PEMFCs. Especially, investigations on water agglomerations within the porous structure of the GDL have become more and more important to completely understand the effects on the fuel cell performance. Besides experimental visualization techniques which help to gain deeper insight the water distribution within the GDL, also different modelling approaches have been developed. We are using and further developing a Monte Carlo model for simulating and analysing the water inventory within PEMFC GDLs taking into account both movements and phase transitions. This model can be employed to identify preferred regions for water agglomerations within the porous structure of the GDL dependent on the surface properties, i.e. the PTFE coverage and the individual structure of the respective material. Our model can use real GDL structures as model input which allows for a direct comparison of the resulting mean water distribution obtained by simulations to experimental results from synchrotron tomography measurements on the same GDL structures. A very good qualitative agreement of the amount of water is found and also preferred regions for water agglomerations are identified consistently for both experiment and simulation.

Handling of water accumulation is still a key issue in fuel cell research. The presented study evaluates the condensate removal capability of three different flow field designs. The designs are compared regarding cell voltage at different current densities using the same operating conditions. The investigated type of meander-shaped channels with a high degree of parallelization shows the best performance with stationary water thickness inside channels throughout the analyzed current densities. To develop and evaluate a condensate removal criterion for fuel cell construction, the pressure drop of each flow field is correlated to the water appearance visualized with neutron radiography. For a further insight, computational fluid dynamics simulation is used to calculate pressure drops present inside the characteristic regions of each flow field. Thus, a characteristic design limit of 20 mbar m −1 for meander-shaped channels is proven to ensure the absence of channel blockage. The meander-shaped channels show specific pressure drops around this limit depending on water production and gas supply. The two other analyzed flow fields suffer from higher channel filling rates: the investigated straight channels with less parallelization fill up with time, while the pattern-structured flow field demonstrates gravity as an additional influence on condensate removal.

In this study, the influence of modifications of the porous transport layer (PTL) on the water management in polymer electrolyte membrane fuel cells is investigated. Laser perforation and milling are used to locally remove either the whole PTL or only the micro porous layer in the PTL. The changed liquid water distribution is visualized using synchrotron radiography and ESEM liquid water imaging. The observed effects are correlated with in-situ performance characteristics, pre-assembly and post-mortem analysis to examine on the beneficial and obstructive effects of the modifications. The analysis reveals that laser-perforation results in PTFE loss and hydrophilic regions which results in a good performance at dry conditions but serious flooding at high humidification conditions. Machined perforations show beneficial effects in the in-situ performance characteristics at high humidification conditions. A draining of the vicinity of the perforations is observed which results in an improved oxygen diffusivity. Water balancing and synchrotron visualization indicate that under the analyzed operating conditions the MPL increases the humidification of the ionomer on the one hand but creates an additional diffusion barrier by liquid water in the interface between the cathode catalyst layer and the MPL on the other hand.

The dynamic water transport of a polymer electrolyte fuel cell was investigated using neutron radiography and CFD simulation. Different operating conditions with dry and humidified gas supply were analyzed at 0.2 and 0.8 A cm-2. The regions with liquid water occurrence were identified using CFD simulations and were verified by neutron radiography. Water accumulation at anode was identified, while at cathode side condensate was sufficiently removed. Thus, the experimentally determined specific pressure drop criterion of 20 mbar m-1 was confirmed for the investigated type of meander-shaped flow field. The observed maximum water thickness was 0.4 mm inside the channel and served as an input for a closer investigation of dynamic water transport using the Volume of Fluid (VOF) model. The water removal capability at anode and cathode were verified with the VOF simulations. The droplet velocity was approximately 44 % / 50 % of gas velocity at anode / cathode for the investigated droplet shapes.

Die dreidimensionalen Wasserverteilungen und -transportwege in den Gasdiffusionslagen (GDL) einer Polymer-Elektrolyt-Membran(PEM)-Brennstoffzelle werden bei verschiedenen Betriebsbedingungen analysiert. Die Methode der quasi In-situ-Röntgen-Tomographie wird für eine dreidimensionale Visualisierung der Wasserverteilung sowie der Struktur der GDL genutzt. Basierend auf Ergebnissen dynamischer radiographischer Messungen werden Wassertransportwege aufgefunden und anschließend mithilfe der Tomographie im Detail untersucht. Die Kombination dieser 2D- und 3D-Techniken erlaubt eine Identifizierung der Transportwege durch die GDL. zwei Mechanismen werden auf der Kathodenseite beobachtet: zum einen Wasser, das aus der GDL in den Kanal transportiert wird, und zum anderen Wasser, das in die GDL zurückgedrückt wird.

Water evolution, distribution, and removal in the cathodes of a running direct methanol fuel cell were investigated by means of synchrotron X-ray radiography. Radiographs with a spatial resolution of around 5 μm were taken every 5 s. Special cell designs allowing for through-plane and in-plane viewing were developed, featuring two mirror-symmetrical flow field structures consisting of one channel with the through-plane design. Evolution and discharge of water droplets and the occurrence of water accumulations in selected regions of the channels were investigated. These measurements revealed a nonuniform distribution of water in the channels. Both irregular and periodic formation of water droplets were observed. In-plane measurements revealed, that the droplets evolve between adjacent carbon fiber bundles of the gas diffusion layer. The water distribution within the channel cross-section fits very well to the pressure difference between cathode channel inlet and outlet. The quick discharge of water droplets causes sudden decreases of the pressure difference up to 4.5 mbar.

Synchrotron imaging allows addressing various important issues in fuel cell research, for example water distribution and transport. The water distribution in polymer electrolyte membrane fuel cells (PEMFCs) was observed quasi in-situ directly after operation by means of synchrotron tomography. The 3D data set was compared with the tomogram of a dry cell in order to separate the water distribution from cell materials. Engineered transport pathways realized by perforating holes through the gas diffusion layer (GDL) are a recent approach to optimize water transport and cell performance. For some parameter sets a cell performance increase and an improvement of stabilization have already been proven. We present high resolution investigations of the water distribution in perforated GDLs of operating PEMFCs by means of in-situ synchrotron radiography. The surrounding areas of the holes exhibited a distinct hydrophilic character.

Kurzfassung Aufgrund des hohen Wirkungsgrades und der vielfältigen Einsatzmöglichkeiten können Brennstoffzellen einen wichtigen Beitrag zur zukünftigen Energieversorgung leisten. Für die Optimierung der Brennstoffzellentechnik ist es erforderlich, die während des Zellbetriebs ablaufenden Prozesse zu verstehen und exakt zu charakterisieren. Ein ausbalanciertes Wassermanagement ist die Grundlage für die optimale Leistungs­fähigkeit einer wasserstoffbetriebenen Zelle. Das während des Betriebs entstehende Wasser muss die Membran ausreichend befeuchten, um deren Protonenleitfähigkeit aufrechtzuerhalten. Andererseits behindern zu große Wasseransammlungen in der Zelle die Gaszufuhr durch die porösen Materialien sowie in den Kanälen der Gasverteilerstrukturen. Alterungsphänomene einzelner Zellkomponenten können die Verteilung der Wasseransammlungen und somit das Wassermanagement empfindlich stören und so die Leistungsfähigkeit der Brennstoffzelle herabsetzen. Zur Analyse der Wasserverteilung werden zerstörungsfreie, bildgebende Methoden, wie die Ex-situ-Neutronentomografie und die In-situ-Synchrotronradiografie, eingesetzt. Diese Methoden können während des Brennstoffzellenbetriebs mit weiteren Messverfahren, beispielsweise der ortsaufgelösten Stromdichtemessung, kombiniert werden. Auf diese Weise werden einzelne Komponenten, wie zum Beispiel die Gasdiffusionsschichten, charakterisiert.

Neutron tomography was applied to study the 3D water distribution in full polymer electrolyte membrane fuel cell (PEMFC) stacks. The water distribution after switch-off of the fuel cell was analyzed over a period of 36 h. We found a slowly increasing water amount in the fuel cell, but only few changes within a time period of 5 h, which is about the time necessary for neutron tomography. In this way, the requirement for successful tomography was obtained. It is demonstrated how the quasi in-situ tomography technique enables us to study the water content in individual flow field channels of three-fold stacks. Flow field as well as stack design issues are addressed by this visualization method showing weak points due to a non-uniform water distribution that can be identified by means of neutron imaging.

Due to its low redox potential and high theoretical specific capacity, Li metal has drawn worldwide research attention because of its potential use in next-generation battery technologies such as Li-S and Li-O2. Unfortunately, uncontrollable growth of Li microstructures (LmSs, e.g., dendrites, fibers) during electrochemical Li stripping/plating has prevented their practical commercialization. Despite various strategies proposed to mitigate LmS nucleation and/or block its growth, a fundamental understanding of the underlying evolution mechanisms remains elusive. Herein, synchrotron in-line phase contrast X-ray tomography was employed to investigate the morphological evolution of electrochemically deposited/dissolved LmSs nondestructively. We present a 3D characterization of electrochemically stripped Li electrodes with regard to electrochemically plated LmSs. We clarify fundamentally the origin of the porous lithium interface growing into Li electrodes. Moreover, cleavage of the separator caused by growing LmS was experimentally observed and visualized in 3D. Our systematic investigation provides fundamental insights into LmS evolution and enables us to understand the evolution mechanisms in Li electrodes more profoundly.

The lithiation and delithiation mechanisms of multiple Sn particles in a customized flat radiography cell were investigated by in amp; 8197;situ synchrotron radiography. For the first time, four de lithiation phenomena in a Sn electrode battery system are highlighted 1 amp; 8197;the de lithiation behavior varies between different Sn particles, 2 amp; 8197;the time required to lithiate individual Sn particles is markedly different from the time needed to discharge the complete battery, 3 amp; 8197;electrochemical deactivation of originally electrochemically active particles is reported, and 4 amp; 8197;a change of electrochemical behavior of individual particles during cycling is found and explained by dynamic changes of de lithiation pathways amongst particles within the electrode. These unexpected findings fundamentaly expand the understanding of the underlying de lithiation mechanisms inside commercial lithium ion batteries LIBs and would open new design principles for high performance next generation LIBs

Silicon Si is proposed to be one of the most promising anode materials for next generation lithium ion batteries, but unsatisfactory discharge capacity and inevitable performance deterioration prevent their commercialisation. In amp; 8197;situ synchrotron X ray tomography is applied to a Si composite electrode based battery in its pristine and first discharged state and the degradation of the electron and or ion conducting network, as well as degradation of Si particles, is quantitatively investigated. Thus, this study is complementary to previous X ray tomographic studies focusing on Si particles only. On the electrode level, the Si particles located in the central part of the electrode primarily experience crack formation; on the particle level, lithiation behaviour is heterogeneous and cavities are formed during electrode preparation and battery operation. The correlation between the electrochemical activities of Si particles and their individual contact with the conducting network is investigated and quantified Si particles will experience lithiation only under the condition that at least 40 amp; 8201; of their surface is electrically and ionically connected

Carbon fiber based felt materials are widely used as gas diffusion layer (GDL) in fuel cells. Their transport properties can be adjusted by adding hydrophobic agents such as polytetrafluoroethylene (PTFE). We present a synchrotron X-ray tomographic study on the felt material Freudenberg H2315 with different PIPE finishing. In this study, we analyze changes in microstructure and shape of GDLs at increasing degree of compression which are related to their specific PTFE load. A dedicated compression device mimicking the channel-land pattern of the flowfield is used to reproduce the inhomogeneous compression found in a fuel cell. Transport relevant geometrical parameters such as porosity, pore size distribution and geometric tortuosity are calculated and consequences for media transport discussed. PTFE finishing results in a marked change of shape of compressed GDLs: surface is smoothed and the invasion of GDL fibers into the flow field channel strongly mitigated. Furthermore, the PTFE impacts the microstructure of the compressed GDL. The number of available wide transport paths is significantly increased as compared to the untreated material. These changes improve the transport capacity liquid water through the GDL and promote the discharge of liquid water droplets from the cell. (C) 2016 Elsevier B.V. All rights reserved.

Two proof-of-concept batteries were designed and prepared for X-ray microtomography and radiography characterizations to investigate the degradation mechanisms of silicon (Si) based half cells during the first cycle. It is highlighted here for the first time that, apart from the significant volume expansion-induced pulverization, the electrochemical “deactivation” mechanism contributes significantly to the capacity loss during the first charge process. In addition, the unexpected electrochemically inactive Si particles are also believed to substantially decrease the energy density due to the inefficient utilization of loaded active material. These unexpected findings, which cannot be deduced from macroscopic electrochemical characterizations, expand the inherent explanations for performance deterioration of Si-anode material based lithium ion batteries (LIBs) and emphasize the vital value of microscopic techniques in revealing the correlation between macroscopic electrode structure and the overall electrochemical performance.

Perforated gas diffusion layers (GDLs) of polymer electrolyte membrane fuel cells (PEMFCs) were investigated by means of in-situ synchrotron X-ray radiography during operation. We found a strong influence of perforations on the water distribution and transport in the investigated Toray TGP-H-090 GDL. The water occurs mainly around the perforations, while the holes themselves show varying water distributions. Some remain dry, while most of them fill up with liquid water after a certain period or might serve as drainage volume for effective water transport.

We present two different methods to increase the size of available neutron beams in order to allow for the investigation of large objects. Application of these methods is demonstrated for radiographic imaging of fuel cells. The first approach is a scanning procedure based on the coordinated translation of detector and sample through the beam. Further advancement was achieved by installing a focusing neutron guide, which offers an expanded neutron beam size after diverging from a focused point source.