Effect of Properties of Hydrophilic Microporous Layer (MPL) on PEFC Performance

Abstract

The effect of the microporous layer (MPL) on membrane electrode assemblies (MEAs) has been investigated. The performance of an MEA employing an MPL with a larger pore volume was better under 100%RH condition, which we reported in 2015 (1). Under 30%RH, no distinct relationship was found between the pore volume of the MPL and the MEA performance. However, the MEA employing an MPL with a larger median pore diameter showed higher cell voltage. We reported that the MEA using a hydrophilic MPL showed much better performance in a wide range of pressure and humidity conditions than that using a conventional hydrophobic MPL. We applied our technique of preparing a carbon fiber dispersion to the fabrication of MPLs using carbon particles and carbon fibers with various fiber diameters. Properties of carbon materials and MPLs are shown in Table 1. The pore size distribution (Fig. 1) was measured using mercury intrusion porosimetry. Figure 2 shows the surface SEM images of the MPLs: The MPL made of carbon black (Vulcan XC) has the smallest pore diameter (0.068 μm) and that made of carbon fiber (VGCF-H) has the largest pore diameter (0.77 μm). Although carbon fiber CF-X has the largest fiber diameter of 250 nm, the MPL made of it has the smallest pore volume. This is probably because fibers of CF-X are straight and longer than other carbon fibers, tending not to become tangled and form larger pores. MEAs having an MPL made of four different carbon materials (Vulcan, 24PS, CF-X, and VGCF-H) were prepared, and polarization curves were measured under the conditions of 80 ºC, 30%RH. The IR free cell voltage (@0.1, 0.5, 1.0 and 2.0 A/cm2) for the four MEAs are plotted against the median pore diameter of the MPL in Fig. 3. The larger the pore diameter of the MPL, the higher the cell voltage becomes. The MEA employing an MPL made of VGCF-H, which has the largest median pore diameter, shows the best performance under the dry condition of 30%RH. This is probably because there is little liquid water in the MPL under the dry condition, and the cell voltage is largely dependent on the oxygen diffusivity of the MPL: The larger median pore diameter of the MPL, the better oxygen diffusivity and the higher the cell voltage.