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The BAMline at the BESSY II synchrotron X-ray source has enabled research for more than 20 years in widely spread research fields such as materials science, biology, cultural heritage and medicine. As a nondestructive characterization method, synchrotron X-ray imaging, especially tomography, plays a particularly important role in structural characterization. A recent upgrade of key equipment of the BAMline widens its imaging capabilities: shorter scan acquisition times are now possible, in situ and operando studies can now be routinely performed, and different energy spectra can easily be set up. In fact, the upgraded double-multilayer monochromator brings full flexibility by yielding different energy spectra to optimize flux and energy resolution as desired. The upgraded detector (based on an sCMOS camera) also allows exploiting the higher flux with reduced readout times. Furthermore, an installed slip ring allows the sample stage to continuously rotate. The latter feature enables tomographic observation of processes occurring in the time scale of a few seconds. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

The influence of hydrophobicity and porosity of the catalyst layer (CL) and cathode microporous layer (MPLC) on water distribution and performance of polymer electrolyte membrane fuel cell (PEMFC) is investigated. Hydrophobicity of the layers is altered with the addition of PTFE (polytetrafluoroethylene) and mono‐dispersed polymer particles are utilized to introduce the macro‐pores with a diameter of 0.5 µm and 30 µm within the CL and MPLC, respectively. The treated materials are implemented in a specially designed fuel cell with an active area of 8 cm2 to perform operando high‐resolution neutron tomography measurements. At high current density and humid operating conditions, MPLs with higher PTFE content increase the overall water content of the cell. The more hydrophobic MPL (40 wt.% PTFE) performs below the corresponding reference MPL (20 wt.% PTFE), whereas the performance result of double layer MPLC gives hint for further potential improvements of such design. The local water saturation beneath the land regions with the presence of perforated CL and MPLC is increased which is explained by lower capillary pressure barriers of bigger pores. Despite a higher water content, the perforated layers enhance the performance of the cell at both dry (RH 70%) and humid conditions (RH 120%), indicating that the parallel two‐phase flow is facilitated where the oxygen is transported through small pores and the water is preferentially transported through the bigger pores. [ABSTRACT FROM AUTHOR]

Summary: It is well known that the microstructure of electrodes in lithium‐ion batteries has an immense impact on their overall performance. The compaction load during the calendering process mainly determines the resulting morphology of the electrode. Therefore, NCM‐based cathode films from uncompacted (0 MPa) to most highly compacted (1000 MPa) were manufactured, which corresponds to global porosities ranging from about 50% to 18%. All samples have been imaged using synchrotron tomography. These image data allow an extensive analysis of the 3D cathode microstructure with respect to increasing compaction. In addition, the numerous microstructural changes can be quantified using several characteristics describing the morphology of cathode samples. Three characteristics, namely global porosity, global volume fraction of active material and mean cathode thickness, are compared to experimental results. In addition, the microstructural analysis by means of 3D image data and image processing techniques allows the investigation of characteristics which are hard or impossible to ascertain by experiments, for example the continuous pore size distribution and the sphericity distribution of NCM‐particles. Finally, the dependency of microstructural characteristics on compaction load is described by the help of parametric probability distributions. This approach can be used, for example, to predict the distribution of a certain characteristic for an 'unknown' compaction load, which is a valuable information with regard to the optimization and development process of NCM‐cathodes in lithium‐ion batteries. Lay Description: It is well known that the microstructure of electrodes in lithium‐ion batteries has an immense impact on their overall performance. The manufacturing of the batteries includes the so‐called calendering, where the electrodes are compressed with a certain pressure, which is called compaction load. This process step mainly determines the resulting morphology of the electrode and thus the properties of the battery. Therefore, eight cathodes with different compaction loads were manufactured and imaged by synchrotron tomography, which leads to 3D images containing detailed information about the inner structure of the cathode. This image data allows an extensive analysis of the 3D cathode microstructure with respect to increasing compaction. In order to quantify the microstructural changes we use several characteristics describing diverse properties of the morphology. Furthermore, the 3D image data can be used for the computation of characteristics which can not be determined by experiments. Therefore, 3D image data allows us to understand how the microstructure of cathodes is influenced by the compaction load. Finally, we are able to predict the distribution of a certain characteristic for arbitrary compaction loads. This information is valuable with regard to the development of improved lithium‐ion batteries. [ABSTRACT FROM AUTHOR]

Liquid water saturation of pores in the gas diffusion layer (GDL) and the catalyst layer (CL) of polymer electrolyte membrane fuel cells (PEMFC) hinders the transport of the reactant gases, leading to inhomogeneous current density distribution, reduced overall cell performance, and accelerated material degradation. This effect restricts the PEMFC operation, specifically at high current densities. In this study, the simulation results illustrate how the interplay of wettability and pore sizes influences the water distribution within the CL, microporous layer (MPL) of the GDL, and specifically their interface. The liquid water distribution within the porous material is studied employing a voxel-based Monte Carlo (MC) model reflecting the effect of local thermodynamic boundary conditions and inner surface characteristics. Real material structures obtained with a focused ion beam - scanning electron microscope (FIB-SEM) are employed. Local temperature and relative humidity values required as inputs are obtained from sophisticated computational fluid dynamics (CFD) simulations comprising all relevant effects, including the electrochemical reactions. The results show that avoiding drastic changes in wettability at the CL-MPL interface can help to mitigate the possible detrimental water accumulations. Further developing and exploiting this study will contribute to facilitate the systematic material development for better PEMFC performance and durability. [ABSTRACT FROM AUTHOR]

Abstract: Water transport in gas diffusion media (GDM) is investigated by synchrotron radiography and tomography. It is demonstrated that micro porous layer (MPL) cracks improve the water management in polymer electrolyte membrane (PEM) fuel cells. A further treatment by means of laser perforation is expected to enhance this effect. The radiography analysis reveals that water transport is practically not influenced by perforations applied only to the MPL. In contrast, perforations through the whole GDM (including the MPL) have a strong influence on the overall water transport behavior and are therefore considered for a deeper analysis. Performance measurements show a correlation between the perforation size and the fuel cell power density. An optimum is found for a perforation diameter of 60 μm. Synchrotron tomography analysis reveals that this optimum is due to an improved draining effect on the area around the perforation. Moreover, SEM and EDX analysis show a loss of PTFE on the GDM surface in the vicinity of the perforation due to the laser processing. The tomography images reveal water accumulations in this area that can be explained by the hydrophilic surface. [Copyright &y& Elsevier]

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. [ABSTRACT FROM AUTHOR]

Synchrotron X-ray radiography and tomography investigations of a custom-made polymer electrolyte membrane fuel cell optimised for visualisation purposes are presented. The 3D water distribution and transport pathways in the porous carbon fibre gas diffusion layers (GDLs) were investigated. The authors found that water is not only moving from the GDL into the channel, but can also take the opposite way, that is, from the channel into free pore space of the GDL. Such movement of water into the opposite direction has been subject of speculations but has so far not yet been reported and might bring new insights into the general water transport behaviour, which might give new aspects to the general description of water transport processes and influence modelling assumptions to describe the process taking place in the GDL. [ABSTRACT FROM AUTHOR]

Abstract: 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. [Copyright &y& Elsevier]

Abstract: The three-dimensional water distribution and water transport paths in the gas diffusion layer (GDL) and the adjacent micro-porous layer (MPL) of a polymer electrolyte membrane fuel cell (PEMFC) were analyzed during cell operation. The technique of quasi in-situ X-ray tomography was used for a 3D visualization of the water distribution and the structure of the GDL at different operating conditions. Based on findings from in-situ radiographic measurements water transport paths were detected and subsequently examined by tomography. The combination of these 2D and 3D techniques allows for a fully three-dimensionally resolved visualization of transport paths through the GDL. [Copyright &y& Elsevier]

Abstract: Synchrotron X-ray tomography is used to visualize the water distribution in gas diffusion layers (GDL) and flow field channels of a polymer electrolyte membrane fuel cell (PEMFC) subsequent to operation. An experimental setup with a high spatial resolution of down to 10μm is applied to investigate fundamental aspects of liquid water formations in the GDL substrate as well as the formation of water agglomerates in the flow field channels. Detailed analyses of water distribution regarding the GDL depth profile and the dependence of current density on the water amount in the GDL substrate are addressed. Visualizations of water droplets and wetting layer formations in the flow field channels are shown. The three-dimensional insight by means of this quasi in situ tomography allows for a better understanding of PEMFC water management at steady state operation conditions. The effect of membrane swelling as function of current density is pointed out. Results can serve as an essential input to create and verify flow field simulation outputs and single-phase models. [Copyright &y& Elsevier]

Abstract: Water evolution in the gas diffusion layer of a polymer electrolyte membrane fuel cell was visualized in situ by means of synchrotron X-ray radiography. Cracks in the microporous layer were identified as start points of efficient liquid water transfer paths through the gas diffusion layer. Quantitative analysis of the water flow rate through those arbitrarily distributed cracks into the gas channel revealed that they have a strong influence on the overall liquid water transport. This could find entry into future material design and simulation. [Copyright &y& Elsevier]

Abstract: 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. [Copyright &y& Elsevier]

Abstract: 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. [Copyright &y& Elsevier]

Metal foam flow-fields have shown great potential in improving the uniformity of reactant distribution in polymer electrolyte fuel cells (PEFCs) by eliminating the 'land/channel' geometry of conventional designs. However, a detailed understanding of the water management in operational metal foam flow-field based PEFCs is limited. This study aims to provide the first clear evidence of how and where water is generated, accumulated and removed in the metal foam flow-field based PEFCs using in-operando neutron radiography, and correlate the water 'maps' with electrochemical performance and durability. Results show that the metal foam flow-field based PEFC has greater tolerance to dehydration at 1000 mA cm−2, exhibiting a ~50% increase in voltage, ~127% increase in total water mass and ~38% decrease in high frequency resistance (HFR) than serpentine flow-field design. Additionally, the metal foam flow-field promotes more uniform water distribution where the standard deviation of the liquid water thickness distribution across the entire cell active area is almost half that of the serpentine. These superior characteristics of metal foam flow-field result in greater than twice the maximum power density over serpentine flow-field. Results suggest that optimizing fuel cell operating condition and foam microstructure would partly mitigate flooding in the metal foam flow-field based PEFC. Image 1 • Neutron imaging applied to water management in metal foam flow-field based PEFC. • The metal foam flow-field enhances the uniformity of water distribution. • The metal foam flow-field improves the tolerance to dehydration. • The metal foam flow-field is susceptible to flooding at low current density. [ABSTRACT FROM AUTHOR]

Abstract In-depth understanding of water management is essential for the optimization of the performance and durability of polymer electrolyte fuel cells (PEFCs). Neutron imaging of liquid water has proven to be a powerful diagnostic technique, but it cannot distinguish between 'legacy' water that has accumulated in the system over time and 'nascent' water recently generated by reaction. Here, a novel technique is introduced to investigate the spatially resolved water exchange characteristics inside PEFCs. Hydro-electrochemical impedance imaging (HECII) involves making a small AC-sinusoidal perturbation to a cell and measuring the consequential water generated, using neutron radiographs, associated with the stimulus frequency. Subsequently, a least-squares estimation (LSE) analysis is applied to derive the spatial amplitude ratio and phase shift. This technique provides a complementary view to conventional neutron imaging and provides information on the source and 'history' of water in the system. By selecting a suitable perturbation frequency, HECII can be used to achieve an alternative image 'contrast' and identify different features involved in the water dynamics of operational fuel cells. Highlights • Hydro-electrochemical impedance imaging applied to water management of PEFC. • HECII distinguish between 'legacy' and 'nascent' water in the PEFC. • The perturbation frequency of HECII affects water dynamics features. [ABSTRACT FROM AUTHOR]

Abstract In-depth understanding of the effect of compression on the water management in polymer electrolyte fuel cells (PEFCs) is indispensable for optimisation of performance and durability. Here, in-operando neutron radiography is utilised to evaluate the liquid water distribution and transport within a PEFC under different levels of compression. A quantitative analysis is presented with the influence of compression on the water droplet number and median droplet surface area across the entire electrode area. Water management and performance of PEFCs is strongly affected by the compression: the cell compressed at 1.0 MPa demonstrates ∼3.2% and ∼7.8% increase in the maximum power density over 1.8 MPa and 2.3 MPa, respectively. Correlation of performance to neutron radiography reveals that the performance deviation in the mass transport region is likely due to flooding issues. This could be ascribed to the loss of the porosity and increased tortuosity factor of the gas diffusion layer under the land at higher compression pressure. The size and number of droplets formed as a function of cell compression was examined: with higher compression pressure, water droplet number and median droplet surface area rapidly increase, showing the ineffective water removal, which leads to fuel starvation and the consequent performance decay. Highlights • Mechanical compression affects water management in PEFCs. • Minimise compression for improved water management. • Increased compression leads to larger water droplet build-up. [ABSTRACT FROM AUTHOR]

Abstract The water management in proton exchange membrane fuel cells (PEMFC) is strongly influenced by the design of the gas diffusion layers (GDL). Limiting current measurements in small-scale cells operating at high stoichiometries are useful to determine the oxygen transport resistance. The oxygen transport resistance increases, once water condenses inside the GDL. In this study a new electrochemical method for voltage loss estimation of GDL induced oxygen transport losses are presented. This new method, referred to as "transient limiting current" (TLC), is compared with the literature method. TLC allows a direct estimation of oxygen transport resistance at an arbitrarily conditioned state. This study also presents a case study of liquid water visualization of a PEM fuel cell with varying GDLs types. With the help of quasi in-situ synchrotron X-ray computed tomography and time resolved radiography measurements we investigate appearance and distribution of liquid water inside the GDLs under limiting current conditions. Highlights • In-situ characterization of GDLs. • Transient limiting current measurement for prediction of oxygen transport resistance. • Quasi in-situ synchrotron X-ray computed tomography and radiography measurements on GDLs. • Visualization of liquid water inside GDL microstructure. [ABSTRACT FROM AUTHOR]

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 PTFE 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. [ABSTRACT FROM AUTHOR]

Abstract: 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. [Copyright &y& Elsevier]

Abstract: 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. [Copyright &y& Elsevier]

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. • Neutron imaging combined with X-ray computed tomography. • Water distribution operando across active area using neutron radiography. • Decreased water thickness with increasing current density mapped. • Highly heterogeneous distribution of water across active area. [ABSTRACT FROM AUTHOR]