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This article reports a comprehensive investigation of the average and local structure of La5.6WO12-δ, which has excellent mixed proton, electron and oxide ion conduction suitable for device applications. Synchrotron X-ray and neutron powder diffraction show that a cubic fluorite supercell describes the average structure, with highly disordered lanthanum and oxide positions. On average, the tungsten sites are sixfold coordinated and a trace [3.7 (1.3)%] of anti-site disorder is detected. In addition to sharp Bragg reflections, strong diffuse neutron scattering is observed, which hints at short-range order. Plausible local configurations are considered and it is shown that the defect chemistry implies a simple 'chemical exchange' interaction that favours ordered WO6 octahedra. The local model is confirmed by synchrotron X-ray pair distribution function analysis and EXAFS experiments performed at the La K and W L-3 edges. It is shown that ordered domains of similar to 3.5 nm are found, implying that mixed conduction in La5.6WO12-δ is associated with a defective glassy-like anion sublattice. The origins of this ground state are proposed to lie in the nonbipartite nature of the face-centred cubic lattice and the pairwise interactions which link the orientation of neighbouring octahedral WO6 sites. This 'function through frustration' could provide a means of designing new mixed conductors.
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.
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.
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.
Investigation of fuel cell materials and liquid water transport by means of synchrotron imaging
(2013)
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.
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.
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.
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.
Synchrotron X-ray absorption edge imaging was used to investigate the ruthenium distribution in both fresh and aged Pt/Ru-based membrane electrode assemblies (MEA) of direct methanol fuel cells. MEAs aged in different ways were analyzed: artificially aged by MeOH depletion and aged for 1700 h in an operating fuel cell stack. An element sensitive tomographic technique – differential X-ray absorption edge tomography – was applied allowing for a 3D-visualization of the ruthenium distribution within the MEA. We found a markedly changed Ru distribution after aging which is correlated to the GDL structure, the flow field geometry, and CO2 transport in the methanol solution.