In-situ X-ray tomography on PEFC: The on-stage humidification concept J. Roth, M. Citerne1, R. Gaudenzi, J. Eller, F.N. Büchi phone: +41 56 310 53 96, e-mail: joerg.roth@psi.ch
The investigation and optimization of the water management in PEFC has been in the focus for many years, since it strongly affects performance and durability. The progress in imaging methods based on neutrons and Xrays makes it possible to study the behaviour of liquid water and ice inside the porous opaque structures of gas diffusion layers (GDL) and flow fields. The state of the art temporal and spatial resolutions allow for 3D imaging in the GDL with time constants in the order of seconds for the X-ray tomography with synchrotron radiation (SR). The visualization of the water distribution can be used to test the hypothesis of gas/water interaction in porous structures where capillary forces dominate the physics and capillary fingering takes place [1]. One example of such a test can be found in Eller et al. [2]. The PSI has gathered experience in the field of fuel cell related ex- and in-situ synchrotron imaging experiments over the past 5 years. Mainly the high brilliance of SR at the SLS and the fast setup at the TOMCAT beamline made it possible that tomographic images of fuel cells during operation can be taken. The experimental boundary conditions are quite stringent to get the image quality that allows for quantitative segmentation. In this field PSI is among the most successful and experienced groups.
Figure 1. XTM fuel cell. 1. Anode flow field, 2. Cathode flow field, 3. GDL, 4. MEA, 5. Gaskets, 6. Outer shell, 7. Basement.
The main challenge is the absolute prevention of heat loss in the humidified gas before the active area is reached. Otherwise the dew point is incorrect, and even more severe, liquid water may enter the cell. For this reason, the gas humidification is located as close to the active area as possible.
Several fuel cell setups had been evaluated in the past and are constantly under development. Recently the problem of radiation damage has been encountered and explored [3]. Based on the cognisance, the concept used in [2] is significantly modified.
Concept All the parameters described previously evoke spatial constraints on the concept since the humidification shall be located on the rotation stage (see Figure 2).
Specification The proof of concept of in-situ SR experiments was produced and so far, the experiments have been performed mainly at room temperature and ambient pressures. This is due to the fact that the existing vertical cell design gives high quality images at an increased active area, but is delicate to handle and difficult to make gas tight at elevated pressures. Furthermore, the orientation has the significant drawback of a maximum radiation damage. The new concept is a step back to the horizontal orientation with circular active area of 0.25 mm2 to mitigate the radiation damage on the one hand and easier sealing on the other hand. The design is modular, meaning that flow field modifications can be implemented easily, it is shown in Figure 1. The cell design, introduced here, aims at the extension of the list of controlled variables towards pressure, humidification and sub-zero temperature operation. Here the humidification concept will be further elucidated. For tomographic imaging, the pictures have to be taken from various angles. At synchrotron sources this is done by rotating the sample by at least 180° but up to 540° or more is possible under certain conditions. This means that the feeding tubes will wind up during the scan process and need to be sufficiently flexible (thin) and long enough (50 cm - 100 cm). The requirement of the humidification system is that hydrogen and air/oxygen will reach the active are in a condition of 100% RH at 80°C at a flow rate of 100 ml/min H2 and 50 ml/min O2 at ambient pressure. 1
Figure 2. Fluid process flows sheet of XTM fuel cell. Red: Gas pipe. Blue: Water.
This way, the water and the gas are transported separately and condensation due to heat loss is no problem. Commercial humidification systems fail to meet these constraints and specifications, and a new design was developed. The concept of membrane humidification
Università di Corsica Pasquale Paoli, Corte, France
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