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Abstract The influence of artificial ageing of gas diffusion layers (GDLs) on the cell performance was investigated using high resolution synchrotron radiography. State-of-the-art GDLs of the type SIGRACET® SGL 25BC were aged for 0 h, 16 h and 24 h in a hydrogen peroxide solution before they were assembled in the fuel cells. In-operando radiographic measurements were combined with voltage and contact angle measurements. Correlations between applied ageing conditions, GDL water saturation and cell performance were revealed. Hereby, all cell operating conditions were tested several times to estimate the reproducibility of in-operando radiographic fuel cell measurements. Water films at the GDL-membrane and at the GDL-flow field interfaces were found and attributed to MPL cracks and large pores in the GDL structure. The combination of these cracks and pores are assumed to play a crucial role for blocked gas paths, leading to an undersupply with reactants and an increased humidification of the membrane. It is shown that water agglomerations directly impact the membrane resistance. We assume that the hydrophobicity of the fibers inside the GDL is more important for the cell performance than water agglomerations at the membrane-GDL interface.

Understanding how gas diffusion electrodes are working is crucial to improve their performance and cost efficiency. One key issue is the electrolyte distribution during operation. Here, operando synchrotron imaging of the electrolyte distribution in silver-based gas diffusion electrodes is presented. For this purpose, a half-cell compartment was designed for operando synchrotron imaging of chronoamperometric measurements. For the first time, the electrolyte distribution could be analyzed in real time (1 s time resolution) even in individual pores as small as a few micrometers. The detailed analyses of dynamic filling processes are an important step for understanding and improving electrodes.

Abstract A novel, realistic 3D model is developed describing the microstructure of non-woven GDL in PEMFC which consists of strongly curved and non-overlapping fibers. The model is constructed by a two-stage procedure. First we introduce a system of random fibers, where the locations of their midpoints are modeled by a 3D Poisson point process and the fibers themselves by random 3D polygonal tracks which represent single fibers in terms of multivariate time series. Secondly, we transform the random fiber system into a system of non-overlapping fibers using an iterative method leaned on the so-called force-biased algorithm. The model is validated by comparing transport-relevant characteristics computed for experimental 3D synchrotron data, and for realizations sampled from the stochastic microstructure model. Finally, we suggest a model for the spatial distribution of PTFE, a wet-proofing agent often used in non-woven GDL, and combine this PTFE model with our new microstructure model for non-woven GDL.

Abstract 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.

Our strategy of polymerizing lithium salt as a polymer electrolyte (3D-SIPE-LiFPA) simultaneously enhances the cycle life and safety characteristics of ultrahigh-energy-density lithium metal batteries (437 W h kg−1).

Thermal neutron imaging is a powerful non-invasive diagnostic technique to study water management within a proton exchange membrane (PEM) fuel cell. To isolate the hydration behavior of membrane, significant increase in detecting thin water films is needed. A humidity cell is developed to study hydration of a PEM using cold neutron imaging. Spatial and temporal changes of PEM water uptake are quantified. Water film as small as 1 μm thick and corresponding to a volume containing 10 ng of liquid water at high spatial resolution is detected. This represents an order of magnitude improvement in water detection efficiency achievable at thermal neutron imaging facilities.

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.

AbstractThe 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 A stochastic multi-layer model is developed describing the microstructure of materials which are built up of strongly curved, but almost horizontally oriented fibers. This fully parametrized model is based on ideas from stochastic geometry and multivariate time series analysis. It consists of independent layers which are stacked together, where each single layer is described by a 2D germ-grain model dilated in 3D. The germs form a Poisson point process and the grains are given by random polygonal tracks describing single fibers in terms of multivariate time series. Exemplarily, on the basis of 2D data from SEM images, the parameters of the multi-layer model are fitted to the microstructure of a non-woven material which is used for gas-diffusion layers in PEM fuel cells. Therefore, an algorithm is presented which automatically extracts typical fiber courses from SEM images. Finally, the multi-layer model is validated by comparing structural characteristics computed for 3D data gained by synchrotron tomography from the same material, and for realizations drawn from the multi-layer model.