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A novel method to produce gas diffusion layers with patterned wettability for fuel cells is presented. The local irradiation and subsequent grafting permits full design flexibility and wettability tuning, while modifying throughout the whole material thickness. These water highways have improved operando performance due to an optimized water management inside the cells.

Cold-starts of a polymer electrolyte fuel cell (PEFC), isothermally maintained at various subfreezing temperatures, were visualized using high resolution dynamic in-plane neutron imaging. The results obtained aim to bring new knowledge about the water accumulation mechanisms leading to the voltage drop usually observed in isothermal mode after a given working time, also called voltage failure. In particular, the data presented should be useful for comparison to simulation predictions provided by modeling studies. As main result, water in a condensed phase was observed to accumulate not only in the membrane-electrode-assembly (MEA), but also in the cathode gas diffusion layer (GDL) at −15 • C and even in the cathode gas channels at −10 • C. Moreover, approximately 400 cold-starts were realized without neutron imaging and revealed stochastic distributions of working times. The presence of water in super-cooled state is discussed and finally retained as only valid explanation of the results obtained. Additionally, the sudden freezing of super-cooled water is thought to cause the rapid water accumulation observed in the MEA during the voltage failure.

Systematic variations of current density and asymmetric relative humidity are realized on differential Polymer Electrolyte Fuel Cells (PEFCs). The water distribution is visualized and quantified by high resolution in-plane imaging, and its effect on performance is discussed. Two cells are investigated: one using paper type gas diffusion layers (GDLs) and one with cloth type GDLs. The novel output of this work is an extensive database of local measurements obtained by the differential cell approach and an asymmetric variation of relative humidity. This should be particularly valuable for the validation of modeling studies. The negative impact of water accumulation on the performance is clearly stronger for the paper type GDLs than for the cloth type GDLs. On the contrary, the performance of the cell with cloth GDLs is low in dry conditions. The accumulation of water in the channel region of the cathode GDL has a crucial impact on performance. The anode side is observed to play an important role for water removal. The presence of a maximal liquid water saturation in the GDL for a wide range of current densities is observed and discussed. In-plane water distribution profiles are presented for the channel and rib regions for all conditions.

Improvements in the spatial resolution of neutron imaging were specifically developed for application in fuel cell imaging, where the resolution requirement in different directions may vary by 1 order of magnitude, thus making anisotropic setups attractive. A maximal spatial resolution of 8.7 m could be reached. For the transient studies, the combination of a high resolution of 20 m with exposure times of 10 s proved to resolve the water evolution, both temporally and spatially. Neutron imaging has been applied for several years 1-7 in the determination of liquid water in operating polymer electrolyte fuel cells ͑PEFCs͒, where the usual configuration is through-plane imaging ͑the cell membrane is perpendicular to the beam͒. Recently, in-plane imaging ͑the cell membrane is parallel to the beam͒ has been investigated as well 8,9 to resolve the water distribution in different layers of the fuel cell structure. Such a configuration sets high demands on the spatial resolution in the direction across the membrane. Recently, the possibility of observing liquid water with high resolution using synchrotron radiation was reported as well. 10 However, the lower requirements set by neutron imaging on the fuel cell structural materials, corresponding to lower invasiveness, the possibility of imaging regions, such as the membrane, which is apparently completely opaque to soft X-rays, as well as the possibilities opened by the use of isotope labeling, 11 motivate the further development of high resolution in-plane neutron imaging of fuel cells.In this work, the latest developments in terms of neutron imaging realized at Paul Scherrer Institut ͑PSI͒ are presented. These developments were targeted on the application to fuel cell imaging or any other structures with a high aspect ratio, which set very different requirements on the spatial resolution in different directions.One of the major limitations for the spatial resolution of neutron imaging is set by the inherent blurring of the scintillator screen used as a part of the detector. This unsharpness can originate from the secondary radiation involved in the two-step conversion of neutrons into visible light, as well as from the scattering of light inside the scintillator screen. Using an optimized optical setup and a thin scintillator screen, a resolution of approximately 50 m was demonstrated at the imaging with cold neutrons beam line at PSI 12 using a stationary detector. This detector was based on a charge-coupled device ͑CCD͒ of 2048 ϫ 2048 pixels with a field of view of 27 ϫ 27 mm, resulting in a pixel pitch of 13.5 m.By using the detector in a tilted position, as illustrated in Fig. 1, the projected image of the object is magnified in the direction perpendicular to the tilting axis. The ratio between the observed and real dimensions is calculated by a simple trigonometric relationAs all limitations inherent to the detector apply to the magnified image, the inherent resolution of the detector is effectively increased. The effective resolution r eff can be calculated as r ...

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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.

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