Lung-inspired, fractal flow-fields hold great potential in improving the performance of polymer electrolyte membrane fuel cells (PEMFCs) by providing uniform gas distribution across the electrodes ...and ensuring minimum entropy production in the whole system. However, the inherent susceptibility of the fractal flow-fields to flooding renders their use inadequate at high humidity conditions. In-depth understanding of water management in lung-inspired flow-fields is indispensable for the implementation of alternative outlet channel geometries or engineered water removal strategies to alleviate flooding. Here, liquid water formation and transport across the lung-inspired and serpentine flow-field based PEMFCs are evaluated using neutron radiography. The results reveal a propensity to flooding in the interdigitated outlet channels of the fractal flow-field with N = 4 generations as a result of slow gas velocity and narrow channel dimensions, which leads to significant performance deterioration. Neutron images also elucidate the importance of ensuring a well-defined internal channel structure of the fractal flow-fields to prevent backflow of liquid water via wicking and capillary pressure build-up arising from the narrow inlet gas channels and hydrophobic gas diffusion layer.
•Neutron radiographs are presented for the lung-inspired and serpentine flow-fields.•A well-defined channel structure of the fractal flow-field is indispensable.•Water removal strategies required to alleviate flooding in the fractal flow-field.
The spatial resolution aspects of the local modification of porous materials by electron induced graft-polymerization were studied by a combination of experiments and numerical simulations. Using ...blocking masks, only selected regions of the material were exposed to radiation and subsequently grafted. The main focus of this study is the application to gas diffusion layers, a carbonaceous ~200µm thick porous substrate widely used in fuel cells, with the goal of improving water management by locally tuning the wettability. The comparison of experiments performed with different electron energies and corresponding simulations shows good agreement, identifying the energy threshold necessary to graft through the material to be approximately 150keV. The impact of electron energy on spatial resolution was studied, showing that the blurring effects due to electron scattering reach a maximum at around 200keV and are reduced at higher electron energies. Finally, the numerical simulations were used to define the conditions necessary to selectively graft only parts of bi-layer fuel cell materials.
•A method based on mask-assisted electron radiation grafting is used.•Hydrophilic patterns are created through the volume of porous substrates.•The influence of electron energy on the pattern resolution is elucidated.•Results of experiments and Monte Carlo simulations are compared.
Following our two previous publications on material synthesis and on ex situ characterization, we present an experimental in situ study to evaluate the effects of using gas diffusion layers with ...patterned wettability at the cathode side of polymer electrolyte fuel cells. The operando performance was assessed using traditional electrochemical diagnostics (such as polarization curves) combined with the pulsed gas analysis (PGA) method, which allows measuring the mass transport losses. Neutron radiography was performed simultaneously in order to image the water distribution during operation. Using this methodology, the effects of changing the pattern, including a microporous layer (MPL), and varying the operation conditions (temperature and relative humidity of the cathode gas) have been systematically evaluated. It has been confirmed that water redistributes according to the engineered pattern and that the power density is significantly increased thanks to reduced mass transport losses under various conditions.
Despite being a promising technology for automotive applications, polymer electrolyte fuel cells still face challenges to reduce their complexity and cost. One challenge is to achieve good ...humidification, which is essential for a fuel cell membrane, without expensive external humidifiers. Here we present an evaporative cooling concept that manages humidification and cooling simultaneously, and does not require any additional layer to the structure of the cell. To this aim, water flows in the fuel cell itself through a small number of the flowfields' channels. Modified gas diffusion layers, with separate parallel hydrophilic regions, are capable of wicking the water from these supply channels and bring it in contact to the gas flow to evaporate, thus providing cooling and humidification. Our results show that this concept can provide the necessary cooling power and humidification for a cell with completely dry inlet gases at 80 °C, and has the potential for working at higher temperatures.
Engineering the wettability and microstructure of gas diffusion layers offers a powerful strategy to optimize water management in polymer electrolyte fuel cells. To this goal, we recently developed a ...radiation grafting technique to synthesize GDLs with patterned wettability. Despite the promise of this approach, current designs are empirically-driven which hampers the development of advanced wettability patterns. To design materials with improved transport characteristics over a range of water saturations, physically representative models can be employed for the bottom-up design of gas diffusion layers with local variations in hydrophilicity. In this paper, pore network models using topology and size information extracted from high resolution tomographic images of three common gas diffusion layer materials are presented with patterned wettability. We study the influence of the substrate microstructure, the hydrophobic coating load, and the hydrophilic pattern width. It is shown that tuning the wettability leads to enhanced phase separation and increased diffusive transport which is attributed to decreased gas phase tortuosity. The network model elaborates on previous experimental studies, shedding light on the effectiveness of the radiation pattern transference and the importance of matching the masking pattern with the substrate microstructure. The work opens the door for exploration of advanced patterns, coupled with flow from gas flow field designs.
We demonstrated the use of a neutron grating interferometer setup (nGI) with a significantly improved contrast-to-noise ratio of the operando dark-field (DF) contrast visualization of water in gas ...diffusion media (GDM). The nGI parameters were optimized in such a way that we could perform DF imaging of a fully operational fuel cell including two GDM layers (anode and cathode side). The DF contrast is sensitive to the size and shape of microstructures and is in principle not influenced by large water clusters present in flow field channels. Thus, DF imaging can be applied to analyze water present in GDM overlapping with channels, which is not possible by attenuation contrast imaging when the cell is placed perpendicular to the beam direction. In GDM regions overlapping with ribs the distinction of hydrophilic and hydrophobic areas is facilitated as well compared to attenuation contrast imaging. Finally, we show that disturbing artefacts introduced by moving water clusters in the channels are considerably reduced by applying a golden ratio phase stepping scan strategy.
In this paper, we present an experimental study on the development of gas diffusion layer (GDL) materials for fuel cells with dedicated water removal pathways generated using radiation induced ...grafting of hydrophilic compounds onto the hydrophobic polymer coating. The impact of several material parameters was studied: the carbon substrate type, the coating load, the grafted chemical compound and the pattern design (width and separation of the hydrophilic pathways). The corresponding materials were characterized for their capillary pressure characteristic during water imbibition experiments, in which we also evidenced the differences between injection from a narrow distribution channel in the center of the material (and thus strongly relying on lateral transport) and homogeneous injection from one face of the material. All materials parameters were observed to have a significant influence on the water distribution. In particular, the type of substrate has a dramatic impact, with results ranging from a nearly perfect separation of water between hydrophilic and hydrophobic domains for substrates having a narrow pore size distribution to a fully random imbibition of the material for substrates having a broad pore size distribution.
Visualizing the water distribution in porous gas diffusion layers (GDLs) of operating polymer electrolyte fuel cells (PEFCs) is indispensable to understand the impact of water management on ...performance. For this purpose, neutron and X-ray transmission imaging have been used for nearly two decades. Certain limitations inherent to attenuation based imaging methods can be overcome by applying neutron dark-field imaging, which has the ability to selectively visualize structures in the micrometer size range. In this study, we compare dark-field images and transmission images of GDLs filled with water through an injection channel. The high contrast of the dark-field value between a heavy water filled and a dry GDL is suitable to reveal water distribution patterns in the GDL. The water present in the 1 mm wide water injection channel of the test device does not alter the dark-field signal, as this technique is selectively sensitive to microstructures. Therefore, neutron dark-field imaging can be applied for the selective analysis of the water distribution in the GDL overlapping with channel water. In addition to the selective visualization of water distributed in a GDL, we show that neutron dark-field imaging can also be used to visualize GDL damages.
High intensity ultrasound can be used for the production of novel nanomaterials, including metal oxides. According to previous works in this field, the most notable effects are consequence of ...acoustic cavitation. In this context, we have studied the preparation of different materials in the presence of ultrasound, including N-doped TiO2 nanopowder, NiTiO3 nanorods and MnOx thin films. Ultrasound did not show a significant effect in all the cases. Exclusively for NiTiO3 nanorods a reduction of the final particle size occurs upon ultrasonic irradiation. From these results, it can be concluded that the ultrasound irradiation does not always play a key role during the synthesis of metal oxides. The effects seem to be particularly relevant in those cases where mass transport is highly hindered and in those procedures that require the rupture of nanoparticle aggregates to obtain a homogenous dispersion.
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