Gas Diffusion Layer Fabrication with Coating of Carbon Fiber Dispersion
The GDL is a critical component for water management in PEFCs (Polymer Electrolyte Fuel Cells). In the case of when identical catalyst-coated membranes are used, the performance of MEAs is largely dependent on the properties of the GDL, such as reactant permeability, product permeability, electronic conductivity, heat conductivity, and mechanical strength. However, according to the cost analysis by B. D. James (1), the GDL, as well as membranes, and catalyst ink application, is one of the largest cost contributors among the components to the 2012 cost of passenger bus fuel cell stacks. Because hydrophobicity is believed to be key to water management, both the cathode and anode GDL substrates are treated with a hydrophobic material such as PTFE, in order to improve water transport. In addition, a micro-porous layer (MPL), consisting of carbon black mixed with a PTFE dispersion, is typically applied to the substrates, followed by heat treatment at over 300 ºC to remove the dispersant used in the PTFE dispersion. These complicated processes result in increasing the production cost of the GDL.
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 (2, 3). The hydrophilic MPL consists of carbon fiber and ionomer, and thus we applied our technique of preparing a carbon fiber dispersion to the GDL fabrication so that we could reduce manufacturing cost of GDLs.
At first, we used carbon fiber MLD1000 (Toray) to find the best mixing ratio with ionomer and dispersing conditions, and then employed a different carbon fiber K223QG (Mitsubishi Plastics) to improve the performance when used in an MEA. Table 1 compares the properties of the carbon fibers and dispersion-cast GDL1, GDL2. Carbon fiber K223QG has a larger fiber diameter and lower electrical conductivity than MLD1000, which results in lower electric resistivity and better air permeability of the dispersion-cast GDL. Carbon fiber dispersions were coated onto a substrate film, and then dried in a convection oven at 80 ºC. Figure 1 and 2 show the surface photos of commercially available GDLs and the dispersion-cast GDLs, respectively.
Figure 3 shows the polarization curves of MEAs using dispersion-cast GDL1, GDL2, and a reference GDL (SGL 25BC). The cell voltage of the MEA using dispersion-cast GDL2 is lower than those of the other two MEAs due to large high frequency resistance (HFR), which is attributed to the high electrical resistivity of carbon fiber MLD1000. On the other hand, the performance of the MEA using dispersion-cast GDL1 is on a par with that using a reference GDL. Although tensile strength of the dispersion-cast GDLs falls short of the reference GDL, it would be fair to say that dispersion-cast GDLs could work well if they are properly used, because they can be manufactured at less than 1/10 the cost of conventional GDLs.
1. B. D. James, J. M. Molton and W. G. Gollera, Fuel Cell Transportation Cost Analysis, in 2013 Annual Progress Report, p. V, U. S. Department of Energy (DOE) (2013).
2. T. Tanuma and S. Kinoshita, J Electrochem Soc, 159, B150 (2012).