Annual Report 2015 – Electrochemistry Laboratory
Fuel Cells & Electrolysis – Systems & Diagnostics
Combined neutron imaging and small angle scattering study of water management in fuel cells P. Boillat, A. Morin¹, A. Forner-Cuenca, S. Lyonnard¹, G. Gebel¹ phone: +41 56 310 2743, e-mail: pierre.boillat@psi.ch
Water management in polymer electrolyte fuel cells (PEFCs) plays a crucial role due to the dual role of water being on one side necessary in the membrane to ensure proton conduction and on the other side detrimental in the gas flow channels and porous media. Neutron based methods (imaging and scattering) are of high interest for in situ, operando measurements because of the high penetration of neutrons through fuel cell construction materials and the high sensitivity to hydrogen containing compounds such as water. Imaging and small angle scattering can provide complementary results, the first one being able to measure the water distribution with high spatial resolution [1], and the latter giving information about the membrane hydration state [2]. Here, we present a combined imaging and small angle scattering study on the water distribution in a small scale fuel cell, giving an unprecedented insight in the water management of fuel cells.
Experimental A small scale fuel cell (active area 13 x 17 mm2) with a single serpentine channel having a developed length of 10 mm was used. The width of the channel was 1 mm and the width of the ribs 0.8 mm, while the channel depth was 0.5 mm. The polymer electrolyte membrane having a thickness of 50 μm (Nafion 212, Dupont) was hot-pressed between two gas diffusion electrodes based on a carbon paper with a microporous layer (GDL24BC, SGL Carbon Group) on which CEA in-house developed catalyst layer were applied (0.5 mgPt/cm2. The flow fields were machined directly into the aluminum compression plates having a thickness of 30 mm, which were subsequently coated with gold. The cell was designed so that no disturbing elements (e.g. gas ports, heaters) were placed in the path of the beam, allowing three different measuring configurations (through plane and in plane imaging plus small angle scattering) as illustrated in Figure 1.
ity of the gas flows set to 50 %. The gases were fed in counterflow configuration with the hydrogen flowing from top to bottom and the air flowing for bottom to top. The SANS experiments were conducted at the PAXY instrument of the Laboratoire Léon Brillouin (LLB, CEA Saclay, France). In order to measure small angle scattering patterns corresponding to different regions of the cells, a mask consisting of 3 slits having a width of 0.5 mm and a center-to-center distance matched to the cell channel pitch of 1.8 mm was placed in the beam. Using this mask, the ribs and channel regions of the cell were successively measured at three different positions (top, middle and bottom sections of the cell), resulting in a total of 6 measurement regions for one experiment. The measurement time for each region was set to 30 minutes, resulting in a total measurement duration of 3 hours per experiment. Additionally to the operando measurements, the scattering pattern of the dry cell as well as reference scattering patterns with nitrogen at different relative humidities between 50 % and 100 % were recorded. The imaging (through plane and in plane) experiments were conducted at the ICON instrument [3] of the SINQ neutron source (Paul Scherrer Institute, Switzerland). For the in plane high resolution measurements, the specific anisotropic enhancement methods previously developed at PSI [1] were applied. For referencing purposes, images of the dry cells were recorded besides the operando measurements. To ensure to best possible reproducibility, the same fuel cell test infrastructure was used for the measurements at PAXY and at ICON. After the experiments at PAXY, the cell was dried and purged with nitrogen and sealed until the experiments at ICON.
Results An example of SANS results is given in Figure 2, corresponding to the reference measurement at a relative humidity of 100 %. As can be seen in the top part of the figure, the amplitude of the useful signal is relatively small compared to the scattering intensity from the cell body. However, after adequate intensity correction and subtraction of the dry cell pattern, a signal with sufficient quality was obtained (bottom part of Figure 2). Two features were mostly used for extracting useful information from this pattern: The position of the ionomer peak is related to the swelling state of the membrane, and the constant background at high q values (> 0.3 Å-1) corresponds to the incoherent scattering from liquid water. The obtained pattern was fitted using the following function:
Figure 1. Illustration of the three different measurement configurations: Through plane imaging (a), in plane imaging (b) and small angle scattering (c). The cell was operated on technically relevant stoichiometries (1.2 on the anode side, 2.0 on the cathode side), at a temperature of 70 °C, a pressure of 1.5 barabs and with a relative humid-
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(1)
Where the first term corresponds to the ionomer peak represented as a Gaussian function (amplitude Aion, central position mion and standard deviation sion), the second term (Iincoh) represents the incoherent scattering from liquid water and the last term is an arbitrary exponential function used to account for the additional scattering at low q values. The fitting val¹ Commissariat à l’Energie Atomique et aux Energies Alternatives (CEA), Grenoble, France