Investigation of Novel Gas Diffusion Media for Application in Pem Fuel Cell Ribbon Assemblies
Sole, Joshua David
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ABSTRACT A new type of fuel cell architecture, the fuel cell ribbon, is presented. The fuel cell ribbon architecture relies on the gas diffusion layer (GDL) to conduct electrical current in-plane to adjacent cells or collector terminals. The potential advantages of the fuel cell ribbon architecture with respect to conventional fuel cell stacks include reduced manufacturing costs, reduced weight, reduced volume, and reduced component cost. The critical component of fuel cell ribbon assemblies, the gas diffusion media, is investigated herein. Analytical models which focus on the electrical loses within the gas diffusion media of the novel architecture are developed. The materials and treatments necessary to fabricate novel gas diffusion media for fuel cell ribbon assemblies are presented. Experimental results for the novel gas diffusion media of are also presented. One dimensional and two dimensional analytical models were developed for the fuel cell ribbon. The models presented in this work focus on the losses associated with the transport of the electrons in fuel cell ribbon assemblies, rather than the complex system of equations that governs the rate of electron production. The 1-D model indicated that the GDL used in ribbon cells must exhibit an in-plane resistance which is approximately an order of magnitude lower than the resistance of gas diffusion media typically used in conventional fuel cells. A 2-D model was developed with which a parametric study of GDL properties and ribbon cell dimensions was performed. The parametric study indicated that ribbon cells of useful size can be constructed using novel diffusion media that offer reduced resistivity, and that the ribbon cells can produce as much as 80-85% of the power density produced in a conventional fuel cell. Novel gas diffusion media for fuel cell ribbons that have the necessary characteristics suggested by the analytical study were developed.. Properties and performance for a commercially available gas diffusion media, ELAT, were measured as a reference for the novel media developed. The increased thickness PAN (ITPN) series diffusion media was constructed of PAN based fibers exhibiting similar resistive properties to the fibers used in ELAT. The ITPN series of materials were woven in a manner which made them approximately twice the thickness of ELAT, effectively reducing their in-plane resistance to half the resistance exhibited by ELAT. The coarsely woven pitch (CWPT) series of materials were constructed in a manner which yielded a similar number of fibers in the plane of the material to ELAT and a similar material thickness to ELAT, but the fibers used were mesophase pitch based fibers which exhibit a resistivity of approximately one-tenth the resistivity of the fibers used to make the ELAT and ITPN materials. The reduction in fiber resistivity led to the CWPT material having an in-plane resistance an order of magnitude lower than ELAT. The widely used ELAT material exhibited an in-plane resistance of 0.39 Ω/sq., a through-plane area specific resistance of 0.007 Ω-cm2, and a Darcy permeability coefficient of 8.1 Darcys. The novel diffusion materials exhibited in-plane resistances in the range of 0.18-0.036 Ω/sq., through-plane area specific resistances in the range of 0.017-0.013 Ω-cm2, and Darcy permeability coefficients in the range of 30-150 Darcys. Experiments were performed to validate the analytical model and to prove the feasibility of fuel cell ribbon concept. When the novel gas diffusers were adhered to a catalyzed membrane and tested in a ribbon test assembly utilizing serpentine flow channels and in-plane current collection, a range of performance was achieved between 0.28-0.4 A/cm2 at a cell output potential of 0.5 V. In contrast, when ELAT was adhered to a catalyzed membrane and tested in the fixture requiring in-plane conduction, a current density of 0.21 A/cm2 was achieved at 0.5 V. Additionally, the 2-D finite element model was used to predict the performance of a ribbon cell based on the cells performance when a conventional method of through-plane conduction was utilized. The agreement between the experimental data and the model predictions was very good for the ELAT and ITPN materials, whereas the predictions for the CWPT materials showed more significant deviation which was likely due to mass transport and contact resistance effects.
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