Effect of Pore Volume of Hydrophilic Microporous Layer (MPL) on PEFC Performance

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 both 100%RH and 30%RH conditions.

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 hydrophobic MPL (1, 2). The hydrophilic MPL consists of vapor-grown carbon fiber with a fiber diameter of 150 nm and ionomer. 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 (Auto Pore IV 9500, Shimadzu Co.).

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 a smaller 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, VGCF-H, and CF-X) were prepared, and polarization curves were measured under the conditions of 80 ºC, 100%RH & 30%RH. The estimated mass transfer loss based on the polarization curves (80 ºC, 100%RH) for the four MEAs are plotted against the pore volume of MPL in Fig. 3. The larger the pore volume of MPL, the smaller the mass transfer loss becomes. The MEA employing a hydrophilic MPL with a larger pore volume also showed higher cell voltage under the dry condition of 80 ºC, 30%RH.

It was found that the pores of the hydrophilic MPL play a crucial role in achieving excellent MEA performance, such as mitigating cathode flooding under the wet condition and high cell voltage under the dry condition by preventing the membrane from drying.

References

1. T. Tanuma and S. Kinoshita, Energy Procedia, 28, 12 (2012).

2. T. Tanuma and S. Kinoshita, J Electrochem Soc, 159, B150 (2012).