Imaging and Quantification of Water inside Membrane-Electrode Assembly of Polymer Electrolyte Fuel Cell during Power Generation By Use of Neutron Beam

The polymer electrolyte fuel cells (PEFCs) have attracted attentions as energy-conversion devices of high power density with no emission of carbon dioxide. For the efficient and stable power generation of a PEFC, the water management inside the membrane-electrode assembly (MEA) is very important. A proper water management can adequately moisten the electrolyte membrane and improve the performance of a PEFC, while an excessive water can block the flow channels and the pores in gas diffusion layers (GDLs), as well as the catalyst layers; the mass transports are inhibited from the flow channels to the Pt catalysts resulting in a poor power generation. In case of large liquid water accumulations, the performance of the power generation is temporary decreased. 1 Neutron imaging techniques have been used for understanding the distribution of liquid water inside PEFCs, and water droplets within the pores of the GDL and/or on the flow-channel walls have been mainly reported. 2–4 However, the distribution of smaller amounts of water contained in catalyst layers or an electrolyte membrane has not been reported, which is very important because of the large effect on the mass transport inside the MEA. The visualization of water in an MEA was carried out using a pulse neutron beam at BL22 (RADEN) of MLF, J-PARC, Japan 5 . The neutrons with the wavelength of 5-13.5 Å was separated and used to increase the attenuation of the neutron beam by water. A PEFC with 10 straight channels, 30 mm in length and 1 mm in width, was used. The endplates and current collectors were made of aluminum alloy (A7075) for the neutron penetration. The transmitted neutrons were converted to photons by a scintillator (ZnS(Li)) for imaging with a CCD camera. The neutron beam size was 102.4 mm ×102.4 mm (2048×2048 pixels), while a plate containing boron carbide (B 4 C) powders was used for a window of 50 mm × 50 mm to reduce the unpreferable radio activation of the cell. The cell temperature was kept at 80 o C. To obtain the background image, a dry N 2 gas was supplied both to the anode and the cathode. Figures 1(a) and 1(b) show the background image under the dry condition and the image of water in the MEA at 100% RH (obtained by the division by the background image using an “ImageJ” software), respectively. The values at pixels aligned in the perpendicular direction in the area surrounded by yellow solid lines in Fig. 1(b) were integrated to be shown in Fig. 1(c). The neutron transmittance at th