A New CCM/MPL Structure for High Power Density Polymer Electrolyte Fuel Cells

Abstract

High power density operation is one path of cost reduction and commercialization of polymer electrolyte fuel cells (PEFCs), and mass transport limitations, particularly of oxygen hinders the operation at required high current densities. Most of the mass transport limitation in a PEFC occur in the gas diffusion layers (GDL), particularly of cathode. Conventional GDLs are typically comprised of highly porous carbon paper or cloth substrate; a small amount of hydrophobic material (usually PTFE) is applied to the GDL to enhance its water removal capabilities. In the last decade, micro-porous coatings (e.g. micro-porous layer, MPL) have been developed to both improve the electrical/thermal contact between the GDL and the catalyst layers, and water management.Kotaka et. al. recently introduced a GDL, where the carbon paper substrate is removed and the entire gas diffusion layer consists of the MPL only [1]. The MPL in their concept is manufactured separately and assembled similar to a conventional GDL. Here, we present a similar concept where the MPL is directly fabricated on the catalyst coated membrane (CCM), forming the GDL. A schematic of the assembly is shown in Fig. 1(a). Due to much smaller thickness and heat/electron conduction cross-section of this GDL, a micro-channel flow field is utilized.The MPL is directly deposited onto both sides of the CCM via spray deposition with a hot plate maintained at 100°C as shown in Fig. 1(b). Poly(vinylidene fluoride) (PVDF)-based copolymer is used as the wet proofing agent because of its lower sintering temperature and high solubility in acetone compared to conventional PTFE and PVDF homopolymer. In order to prepare the MPL solution, carbon particles are added to a PVDF solution obtained by dissolving in dimethylformamide (DMF), and then mixed in an ultrasonic water bath with a homogenizer for 6 hours and stirring for 24 hours. The MPL solution can be applied to the CCM by spraying, screen printing, or doctor blade methods with a thin layer; however spray deposition with a hot plate found to be the most suitable method to avoid damage to the Nafion membrane in the CCM by DMF. Acetone is also added to the MPL solution to accelerate evaporation rate before spraying onto the CCM. A spray station with controlled X-Y table is used to make a uniform MPL surface onto both sides of the CCM. The desired thicknesses of the MPL are between 50 and 100µm to provide enough mechanical stability while maintaining minimal mass transport resistance. After spraying the carbon solution to either side of the CCM, the MPL/CCM/MPL (MCM) is dried at 100°C to evaporate all remaining solvents for 30 min, and then heated between two hot flat plates for 3 hours with a temperature of 170°C to distribute homogeneously PVDF throughout the MPL.Figure 2 shows the SEM image and EDX maps of the CCM with the MPL deposited on one side. A highly porous structure is observed in the deposited MPL layer, which results in open pathways for fuel and oxygen to the CL. Pore size distribution, permeability (with Gurley method), cell performance and electrochemical impedance spectroscopy (EIS) are used to characterize the new diffusion layer structure. Acknowledg e ments Financial support from National Science Foundation (CBET-0748063) is greatly acknowledged.