Characterization Studies of a New MEA Structure for Polymer Electrolyte Fuel Cells

A conventional gas diffusion layer (GDL) of a polymer electrolyte fuel cell (PEFC) is typically comprised of a highly porous carbon paper or cloth substrate, which is coated with a thin micro-porous layer (MPL) on the catalyst layer (CL) side. A small amount of hydrophobic material is applied to the GDL to enhance its water removal capabilities. At high current densities, mass transport limitations of fuel or oxidizer in the PEFC occur in porous structures of the GDL, particularly at the cathode, which result in a sharp drop in the output voltage.

A new MEA concept is recently introduced, where the carbon paper substrate is eliminated and the entire gas diffusion layer consists of only the MPL. The MPL is directly deposited onto both sides of the CCM to provide an improvement in the interfacial contact between the MPL and the catalyst layer, and to allow a functional gradation of the MPL for improved mechanical strength as well as optimized gas transport and water removal. A spray deposition method was used for depositing this MPL onto a commercial CCM [1]. This simple MEA structure is easy to handle during fabrication and assembly. Here, we present further characterization studies of the new MEA structure.

Fig. 1(a) and 1(b) show the elemental mapping of the MPL (obtained by EDX) deposited on both sides of the CCM. The hydrophobic material, Poly(vinylidene fluoride) (PVDF), is seen to be uniformly distributed throughout the MPL cross-section. The EDX mapping result indicate that the sprayed MPL homogeneously covers the CCM without damaging the membrane or the catalyst layer. Imperfect contact between the rough surfaces of the MPL/CL, which would cause interfacial gaps and uneven compression leading to disruption of heat transfer and interfacial contact delamination, are eliminated [2]. As shown in Fig. 1(c) and 1(d), the sprayed MPL on the CCM produces a robust interface of the MPL/CL, which results in reduced electronic contact resistance. Fig. 2. shows a comparison of morphological data of pore structures in this new MPL with commercially available GDLs (Freudenberg C4 and SGL 25BC). The micro-pore distribution of the deposited MPL (30 wt% PVDF) indicate similar pore structure when compared to commercial MPLs. Mercury intrusion porosimetry (MIP) is used to quantify porosity and pore size distribution.

The polarization curves for two different MPL thicknesses are shown in Fig. 3. A micro-channel flow field was used to provide the desired pathway for the reactant gases throughout the cell. The cell with the thinner (40µm) MPL exhibits better performance than the thicker (100µm) MPL, especially in the mass transport region. The result shows that this new method of MEA fabrication can provide high power density operation, and that mass transport limitations can be controlled. The ideal MPL pore structure and thickness needs to be optimized to have both sufficient reactant supply, and enhanced liquid water removal from the MEA, while maintaining mechanical stability. Thermal and electrical resistance, permeability, electrochemical impedance spectroscopy, and mechanical properties are used to further characterize the new MEA structure.