Editors’ Choice—4D Neutron and X-ray Tomography Studies of High Energy Density Primary Batteries: Part I. Dynamic Studies of LiSOCl2 during Discharge

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

Unveiling the Electro‐Chemo‐Mechanical Failure Mechanism of Sodium Metal Anodes in Sodium–Oxygen Batteries by Synchrotron X‐Ray Computed Tomography

Rechargeable sodium–oxygen batteries (NaOBs) are receiving extensive research interests because of their advantages such as ultrahigh energy density and cost efficiency. However, the severe failure of Na metal anodes has impeded the commercial development of NaOBs. Herein, combining in situ synchrotron X-ray computed tomography (SXCT) and other complementary characterizations, a novel electro-chemo-mechanical failure mechanism of sodium metal anode in NaOBs is elucidated. It is visually showcased that the Na metal anodes involve a three-stage decay evolution of a porous Na reactive interphase layer (NRIL): from the initially dot-shaped voids evolved into the spindle-shaped voids and the eventually-developed ruptured cracks. The initiation of this three-stage evolution begins with chemical-resting and is exacerbated by further electrochemical cycling. From corrosion science and fracture mechanics, theoretical simulations suggest that the evolution of porous NRIL is driven by the concentrated stress at crack tips. The findings illustrate the importance of preventing electro-chemo-mechanical degradation of Na anodes in practically rechargeable NaOBs

Entwicklung und Optimierung von radiographischen und tomographischen Verfahren zur Charakterisierung von Wassertransportprozessen in PEM Brennstoffzellenmaterialien

Water transport in polymer electrolyte membrane fuel cells PEMFC was non destructively studied during operation with synchrotron X ray radiography and tomography. The focus was set on the influence of the three dimensional morphology of the cell materials on the water distribution and transport. Water management is still one of the mayor issues in PEMFC research. If the fuel cell is too dry, the proton conductivity of the membrane decreases leading to a performance loss and, in the worst case, to an irreversible damage of the membrane. On the other hand, the presence of water hinders the gas supply and causes a decrease in the cell performance. For this reason, effective water transport is a prerequisite for successful fuel cell operation. In this work the three dimensional water transport through the gas diffusion layer GDL and its correlated with the 3D morphology of the cell materials has been revealed for the first time. It was shown that water is transported preferably through only a few larger pores which form transport paths of low resistance. This effect is pronounced because of the hydrophobic properties of the employed materials. In addition, water transport was found to be bidirectional, i. e. at appropriate locations a back and forth transport between GDL and flow field channels was observed. Furthermore, liquid water in the GDL was found to agglomerate preferably at the ribs of the flow field. This can be explained by condensation due to a temperature gradient in the cell and by the position, which is sheltered from the gas flow. Larger water accumulations in the gas supply channels were mainly attached to the channel wall opposing the GDL. The gas flow can bypass these agglomerations allowing a continuous gas supply. Moreover, it was shown that randomly distributed cracks in the micro porous layers MPL play an important role for the agglomeration of liquid water as they form preferred low resistance transport paths. In this work also perforated MPL GDL materials were investigated. It had been shown in complementary measurements that depending on process parameters perforated MPL GDL materials can have either a positive or in other cases a negative impact on the cell performance gains of up to 20 vs. losses of same magnitude . The water transport was found to be responsible for the different behavior. At its best, the perforations have a drainage effect which facilitates effective water removal. In other cases a flooding of the whole local pore area around the perforation was observed. This area was obviously heat affected by laser perforation procedure and showed a hydrophilic behavior. The transport through the perforations was also found to be bidirectional. In this work, specially adapted measuring techniques were applied to analyze various aspects of water management. For example the combination of dynamic radiographic and three dimensional tomographic measurements has been proven as valuable method to interpret transport phenomena in terms of the underlying cell structure. On top of that a method is applied, which allows for an increased spatial resolution in tomography and the easy switch between radiographic and tomographic measure mode. By comparing the tomographic data of the cell measured subsequent to operation with the dry reference state it was possible to extract the three dimensional quasi in situ water distribution. This allows for more detailed analyses, for example, statistical water cluster size distributions. The extracted water distribution was also used by a group at the ZSW Ulm for the model validation of a grand canonical Monte Carlo simulation

Development and optimization of radiographic and tomographic methods for characterization of water transport processes in PEM fuel cell materials

Wasserverteilungen und -transportphänomene in Polymer-Elektrolyt-Membran-Brennstoffzellen (PEM-BZ) wurden zerstörungsfrei während des Betriebes mit Synchrotronröntgenradiographie und -tomographie untersucht. Der Schwerpunkt der Arbeit befasst sich mit den Struktur-Eigenschafts-Beziehungen der eingesetzten Zellmaterialien. Die Optimierung von Wasserverteilung und -transport, dem sogenannten Wassermanagement, stellt eine der größten Herausforderungen in der Forschung an PEM-BZs dar. Ist die Brennstoffzelle zu trocken, so verringert sich die Protonenleitfähigkeit der Membran und die Leistung fällt ab. Im schlimmsten Fall wird die Membran dadurch irreversibel geschädigt. Ist allerdings zu viel Wasser in der Zelle, werden die Gasströme stark behindert, was ebenfalls einen Leistungsabfall zur Folge hat. Aus diesen Gründen ist ein effektiver Abtransport des in der Zelle erzeugten flüssigen Produktwassers von entscheidender Bedeutung. Es wurden radiographische und tomographische Messverfahren mit Röntgenstrahlung eingesetzt, um das Innere der Zellen zwei- und dreidimensional mit hoher Ortsauflösung während des Betriebes zu untersuchen. In dieser Arbeit wurden erstmals dreidimensionale Wassertransportpfade durch die Gasdiffusionsschicht (GDL) und deren Zusammenhang mit der Struktur der Zellmaterialien aufgedeckt. Es konnte gezeigt werden, dass der Wassertransport über wenige bevorzugte Pfade mit geringem Transportwiderstand erfolgt. Der Effekt wird vermutlich durch die Hydrophobizität der eingesetzten Materialien stark begünstigt. Zudem konnte gezeigt werden, dass der Wassertransport bidirektional ist, dass heißt sowohl durch die GDL in den Kanal aber auch in umgekehrter Richtung stattfindet. Außerdem konnte innerhalb der GDL eine vermehrte Wasseransammlung an den Stegen der Gasversorgungskanäle beobachtet werden, was mit Kondensation aufgrund geringerer Temperatur und der vor dem Gasstrom geschützten Position erklärt wird. Wasser im Kanal befindet sich hauptsächlich an der GDL abgewandten Seite, so dass eine weitere Gasversorgung der Zelle möglich ist. Darüber hinaus konnte gezeigt werden, dass zufällig verteilte Rissstrukturen in den MPLs eine wichtige Rolle für die Agglomerationen von Wasser spielen, da sie häufig den Startpunkt von bevorzugten Transportpfaden bilden. Zusätzlich wurden perforierte GDL / MPL-Materialien untersucht, für die in begleitenden Messungen sowohl Leistungssteigerungen von bis zu 20 % als auch Leistungsabfälle in derselben Größenordnung festgestellt worden sind. Als Ursache wurde der Wassertransport identifiziert, bei dem im optimalen Fall eine Art Drainage-Effekt zu einem sehr effektiven Wasseraustrag führt. In anderen Fällen wiederum wurde ein Auffüllen der Perforationsumgebung in der GDL festgestellt. Dies wird auf eine hydrophile Eigenschaft der GDL an diesen Stellen zurückgeführt, welche durch die Perforationstechnik verursacht wurde. Auch an den Perforationen konnte bidirektionaler Transport festgestellt werden. Die Untersuchungen an den Brennstoffzellen erforderten die Adaption spezieller Messtechniken. So ist z. B. die Kombination von dynamischen radiographischen Messungen mit dreidimensionalen tomographischen Messungen für das Verständnis von Wassertransportprozessen von unschätzbarem Wert. Darüber hinaus wird eine Methode genutzt die erreichbare Auflösung einer Tomographie zu erhöhen und einen einfachen Wechsel zwischen Radiographie und Tomographie von Brennstoffzellen zu ermöglichen. Durch den Vergleich direkt nach dem Betrieb aufgenommener tomographischer Daten mit dem trockenen Referenzzustand der Zelle, ist es gelungen die Wasserverteilung in den Zellmaterialien zu extrahieren. Dies ermöglicht tiefer gehende Analysen, wie z. B. eine statistische Tropfengrößenverteilung. Darüber hinaus wurde die Wasserverteilung von einer Arbeitsgruppe am ZSW Ulm benutzt, um das Modell einer großkanonischen Monte Carlo Simulation zu validieren.Water transport in polymer electrolyte membrane fuel cells (PEMFC) was non-destructively studied during operation with synchrotron X-ray radiography and tomography. The focus was set on the influence of the three-dimensional morphology of the cell materials on the water distribution and transport. Water management is still one of the mayor issues in PEMFC research. If the fuel cell is too dry, the proton conductivity (of the membrane) decreases leading to a performance loss and, in the worst case, to an irreversible damage of the membrane. On the other hand, the presence of water hinders the gas supply and causes a decrease in the cell performance. For this reason, effective water transport is a prerequisite for successful fuel cell operation. In this work the three-dimensional water transport through the gas diffusion layer (GDL) and its correlated with the 3D morphology of the cell materials has been revealed for the first time. It was shown that water is transported preferably through only a few larger pores which form transport paths of low resistance. This effect is pronounced because of the hydrophobic properties of the employed materials. In addition, water transport was found to be bidirectional, i. e. at appropriate locations a back and forth transport between GDL and flow field channels was observed. Furthermore, liquid water in the GDL was found to agglomerate preferably at the ribs of the flow field. This can be explained by condensation due to a temperature gradient in the cell and by the position, which is sheltered from the gas flow. Larger water accumulations in the gas supply channels were mainly attached to the channel wall opposing the GDL. The gas flow can bypass these agglomerations allowing a continuous gas supply. Moreover, it was shown that randomly distributed cracks in the micro porous layers (MPL) play an important role for the agglomeration of liquid water as they form preferred low resistance transport paths. In this work also perforated MPL/GDL-materials were investigated. It had been shown in complementary measurements that depending on process parameters perforated MPL/GDL materials can have either a positive or in other cases a negative impact on the cell performance (gains of up to 20 % vs. losses of same magnitude). The water transport was found to be responsible for the different behavior. At its best, the perforations have a drainage effect which facilitates effective water removal. In other cases a flooding of the whole local pore area around the perforation was observed. This area was obviously heat affected by laser perforation procedure and showed a hydrophilic behavior. The transport through the perforations was also found to be bidirectional. In this work, specially adapted measuring techniques were applied to analyze various aspects of water management. For example the combination of dynamic radiographic and three-dimensional tomographic measurements has been proven as valuable method to interpret transport phenomena in terms of the underlying cell structure. On top of that a method is applied, which allows for an increased spatial resolution in tomography and the easy switch between radiographic and tomographic measure mode. By comparing the tomographic data of the cell measured subsequent to operation with the dry reference state it was possible to extract the three-dimensional quasi in situ water distribution. This allows for more detailed analyses, for example, statistical water cluster size distributions. The extracted water distribution was also used by a group at the ZSW Ulm for the model validation of a grand canonical Monte Carlo simulation

Development of self-supporting MPLs for investigations of water transport in PEM fuel cells

The performance of a polymer electrolyte membrane (PEM) fuel cell has a strong dependence of its water management. The membrane needs humidity to have sufficient ion conductivity. But at high humidity, especially at high current densities, flooding of the electrodes can occur and consequently the available active area begins to decrease. The primary purpose of a micro porous layer (MPL) on a gas diffusion layer (GDL) is the effective wicking of liquid water from the catalyst layer into the diffusion media as well as reducing electrical contact resistance with the adjacent layers. In synchrotron radiography studies the importance of liquid water pathways through the porous structure for the water management is proven. These pathways can be formed by natural cracks in the MPL and the texture of the carbon fibre substrate or by artificial pore paths through the GDL. With artificial paths in a carbon fibre GDL produced by laser perforation an overall performance gain has been obtained. To get additional information about the function of the MPL as an interconnection between the reaction layer and the macro porous carbon fibre substrate a self-supporting MPL was developed. This allows the manufacturing and the following treatments of the MPL independent from the GDL substrate. This MPL consists of a thin nonwoven of synthetics coated on one side with a mixture of carbon and PTFE produced by the dry spraying technology. It is possible to perforate this layer alone and press it with the non-coated side on a commercial GDL without MPL (Sigracet® GDL25BA from SGL). Thus it was feasible to perform experiments for investigation of the influence of artificial pores in the MPL on the water management. As a consequence, the liquid water transport of non-perforated GDL/MPLs is compared to the perforation of both layers as well as to the exclusive perforation of MPL and the GDL, by means of in-situ synchrotron imaging. Further measurements, in particular Ucell(i)-curves up to limiting current densities and electrochemical impedance spectra were done in a 5 cm² fuel cell setup, to obtain a correlation of the global intrinsic properties of the MPL, like through-plane permeability, electrical conductivity or hydrophobicity, with fuel cell performance

Influence of the MPL on PEM fuel cell performance

The gas diffusion layer (GDL) plays a crucial role for PEM fuel cell performance. The main requirements of a GDL are the provision of a gas and water transport as well a significant electrical and thermal conductivity. For the development of PEMFC diffusion media the still unknown influence of the micro porous layer (MPL) on fuel cell performance is a major obstacle. Therefore a self-supporting MPL was developed because it allows basically to measure the properties of the MPL separately from the substrate. This MPL consists of a thin nonwoven of synthetics coated on one side with a mixture of carbon and PTFE produced by the dry spraying technology. For in-situ experiments and some ex-situ measurements these layers are pressed with the non coated side on a commercial GDL without MPL (Sigracet® GDL25BA from SGL). To get a correlation of the global intrinsic properties of the MPL, like through-plane permeability, electrical conductivity or hydrophobicity, to fuel cell performance, Ucell(i)-curves up to limiting current densities and electrochemical impedance spectra are measured in a 5 cm² fuel cell setup. Additionally the function of the MPL structure on water distribution is investigated. Synchrotron radiography studies proved the importance of liquid water pathways through the porous structure for the water management. These pathways can be formed by natural cracks in the MPL and the texture of the carbon fibre substrate or by artificial pore paths through the GDL. With artificial paths in a carbon fiber GDL produced by laser perforation an overall performance gain has been obtained. In the presented work, we performed further experiments to investigate the influence of artificial pores in the MPL on the water management. Therefore the liquid water transport of nonperforated GDL/MPLs is compared to the perforation of both layers as well as to the exclusive perforation of MPL and the GDL, by means of in-situ synchrotron imaging

Synchrotron radiography and tomography of water transport in perforated gas diffusion media

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

Self-Supporting Microporous Layers (MPLs) for PEM Fuel Cells

The gas diffusion layer (GDL) plays a crucial role for PEM fuel cell performance. The main requirements of a GDL are the provision of gas and water transport as well as significant electrical and thermal conductivity. For the development of PEMFC diffusion media the still unknown influence of the micro porous layer (MPL) on fuel cell performance is a major obstacle. Therefore a self-supporting MPL was developed because it allows basically the measurement of MPL properties separately from the substrate. To get a correlation of the global intrinsic properties of the MPL, like electrical conductivity or hydrophobicity to fuel cell performance, I-V curves up to limiting current densities and electrochemical impedance spectra (EIS) are measured in a 5 cm² fuel cell setup. Additionally synchrotron X-ray radiography studies were performed to compare the influence of different MPLs on liquid water distribution during fuel cell operation

Improved water transport in natural and artificial pore paths of gas diffusion layer in PEM fuel cells

Gas diffusion layer in PEM fuel cells have to fulfil many tasks: high electrical and thermal conductivity, gas and water transport. Different pore sizes and pore geometries in gas diffusion layer influence these parameters. For high current densities, water agglomerations in the porous medium limit the fuel cell performance. The geometry of the carbon fibre substrate and natural cracks in the micro porous layer form preferred water pathways through the porous structure. Synchrotron radiography and tomography studies prove the importance of these pore paths for the overall water distribution. Regarding the flow field design it is found that these water paths cause a draining effect, which transports water from under the land to the gas channel. For further improvement a higher and more regular distribution of cracks is necessary. To achieve this, laser perforation based on and mechanical milling techniques were applied. With this treatment a higher overall performance has been obtained due to the improved water transport. This has been proven by synchrotron imaging