Water management in proton-exchange membrane fuel cells (PEMFCs) is critical for efficient operation. However, in-situ measurements of the water distribution in an operating PEMFC are challenging. In this study, we investigate key aspects of ultrasonic guided wave (UGW) propagation through the bipolar plate of a PEMFC using 3D scanning laser Doppler vibrometry (3D LDV). Our results provide insights for the design and optimization of UGW-based sensor systems for water detection in PEMFCs.
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.
CT (computed tomography) reconstructions of fuel cell components of a yet unrivaled spatial resolution and quality are presented. This is achieved by application of the novel DIRECTT (Direct Iterative Reconstruction of Computed Tomography Trajectories) algorithm. We focus on two different key issues which essentially rule the fuel cell's durability on different length scales and physical interactions. On the resolution scale of some 100 µm agglomerations of condensed water in flow-field channels are detected by means of quasi-in situ neutron CT (after operation). Five orders of magnitude below nanometer sized Ru catalyst particles on carbon black support are visualized by electron tomography. Both types of experiments are especially adapted to the type of material involved but they are accompanied by severe deviations from ideal CT measuring conditions, as well. In order to overcome the tremendous reconstruction artifacts of standard algorithms, we employ DIRECTT which is described in detail. Comparisons of DIRECTT reconstructions to the conventional filtered back projection, prove the significant improvements in both experimental methods.
Kunststoffe sind wichtige Werkstoffe und heutzutage aus Haushalt und Wirtschaft nicht mehr wegzudenken. Seit Anfang der 1970er-Jahre wird das Vorkommen von Kunststoffpartikeln in Küstengewässern beschrieben. Inzwischen ist Mikroplastik nicht nur im marinen Bereich, sondern auch in weiteren Umweltkompartimenten, Biota und technischen Einrichtungen der Wasserwirtschaft nachgewiesen worden, in dieser Arbeit wird ein Überblick über das Vorkommen von Mikroplastik in der aquatischen Umwelt und ersten bekannten Auswirkungen gegeben. Darüber hinaus werden die aktuellen Untersuchungsverfahren von Probenahme, Probenaufbereitung und Analytik betrachtet und offene fragen diskutiert. Zentral erscheint in diesem Zusammenhang die Entwicklung einer harmonisierten medienübergreifenden Untersuchungsmethodik, die eine Auswertung relevanter Stoffströme ermöglicht und auf notwendige Bewertungskonzepte abgestimmt ist.
The complex nature of liquid water saturation of polymer electrolyte fuel cell (PEFC) catalyst layers (CLs) greatly affects the device performance. To investigate this problem, we present a method to quantify the presence of liquid water in a PEFC CL using small-angle X-ray scattering (SAXS). This method leverages the differences in electron densities between the solid catalyst matrix and the liquid water filled pores of the CL under both dry and wet conditions. This approach is validated using ex situ wetting experiments, which aid the study of the transient saturation of a CL in a flow cell configuration in situ. The azimuthally integrated scattering data are fitted using 3D morphology models of the CL under dry conditions. Different wetting scenarios are realized in silico, and the corresponding SAXS data are numerically simulated by a direct 3D Fourier transformation. The simulated SAXS profiles of the different wetting scenarios are used to interpret the measured SAXS data which allows the derivation of the most probable wetting mechanism within a flow cell electrode.