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This paper describes a method of measuring the relationship between capillary pressure and porous media saturation in the gas diffusion layer (GDL) of a polymer electrolyte membrane fuel cell (PEMFC). Such a relationship is commonly used to model the liquid water flow in the GDL. The method utilized to characterize the GDL behavior mimics the actual transport of liquid water within the GDL by utilizing the actual fluids of interest in a PEMFC cathode (water and air), and by introducing all water from a single face to simulate the water production at the catalyst layer. Other porosimetry methods rely on totally non-wetting or totally wetting fluids to achieve saturation and consequently the resulting capillary pressure measurements must be scaled to the emulate the situation in the PEMFC GDL. Capillary pressure versus saturation curves for two different GDL materials (one paper, one cloth), each with four different bulk loadings of PTFE (0, 10, 20 and 30 wt%), were measured. Results show that the PTFE loading has a relatively small effect on the capillary pressure within the pressure range normally associated with PEMFC water transport. The results also show that carbon cloth based GDL materials require greater capillary pressures than paper materials to achieve significant saturation and that compression has a homogenizing effect on the pore structure and the slope of the capillary pressure – saturation Pc(S) behavior of both materials. Representative curves for the derivative of the Pc(S) function are developed for each type of diffusion media within the appropriate saturation range.

Electronics of all types, particularly those in the military aviation arena, are decreasing in size while at the same time increasing in power. As a result, newer high-heat-flux electronic components are exceeding the cooling capabilities of conventional single-phase military aviation coldplates and coolants. It is for this reason that we have been investigating new methods to cool the next generation of high-heat-flux military aviation electronics. In this work, a novel method of inducing two-phase conditions within a microchannel heat exchanger has been developed and demonstrated. Micro-orifices placed upstream of each microchannel in a microchannel heat exchanger not only cause an improvement in flow distribution, but can also induce cavitation in the incoming subcooled refrigerant and result in favorable two-phase flow regimes for enhanced heat transfer. In this study, R-134a is used as the coolant in the cavitation enhanced microchannel heat exchanger (CEMC-HX) which has been integrated into a vapor compression refrigeration system. Multiple micro-orifice geometries combined with a fixed microchannel geometry (nominally 250 μm × 250 μm) were investigated over a range of applied base heat fluxes (10–100 W/cm2) and mass fluxes (500–1000 kg/m2-s). Two-phase heat transfer coefficients exceeding 100,000 W/m2-K at refrigerant qualities of less than 5% have been demonstrated due to the achievement of preferential, cavitation-induced, flow regimes such as annular flow. To the author’s knowledge, this is the highest heat transfer coefficient ever reported in the literature for R-134a. Additionally, a four term two-phase heat transfer correlation was developed that achieved a mean absolute error (MAE) of 25.5%.

A promising type of proton exchange membrane fuel cell (PEMFC) architecture, the ribbon fuel cell, 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. This work addresses the critical component of fuel cell ribbon assemblies, which is the GDL. The materials and treatments necessary to fabricate GDLs for fuel cell ribbon assemblies are presented along with experimental results for various candidate gas diffusion materials. An experimentally validated analytical model, which focuses on the electrical losses within the GDL of the ribbon fuel cell, was developed and used to guide design and testing. Low in-plane resistance is extremely important for the ribbon architecture because high in-plane GDL resistance can cause significant variation in current density over the catalyzed area. To reduce the current variation the new GDLs are fabricated with materials that have reduced in-plane resistance. Properties and performance for a common gas diffusion media, ELAT® LT-1200W (BASF Fuel Cell), were measured as a reference for the new gas diffusion layers. The widely used ELAT material exhibited an in-plane resistance of 0.39 Ω/sq, whereas the new diffusion materials exhibited in-plane resistances in the range of 0.18−0.06 Ω/sq. The performance of a ribbon fuel cell was predicted using a two-dimensional model that combines the polarization curve for a conventional bipolar plate type PEMFC and the resistive properties of the GDL material of interest. Experiments were performed to validate the analytical model and to prove the feasibility of the ribbon fuel cell concept. Results show that when the novel GDLs were adhered to a catalyzed membrane and tested in a ribbon fuel cell test assembly utilizing serpentine flow channels and in-plane current collection, a range of performance was achieved between 0.28 A/cm2 and 0.48 A/cm2 at a cell potential of 0.5 V. The agreement between the experimental data and the model predictions was very good for the ELAT and the B1/B polyacrylonitrile (PAN)-based carbon cloth. Differences between predicted and measured performance for a pitch-based GDL material were more significant and likely due to mass transport limitations.

A new method is demonstrated for the simultaneous determination of both the liquid phase relative permeability and the gas phase relative permeability as a function of compression in thin porous materials such as those used as gas diffusion layers (GDLs) in proton exchange membrane fuel cells (PEMFCs). In this method, multiple layers of the material of interest are inserted into the test section and the desired compression is achieved via pneumatic cylinders. The compression of the sample is maintained while both liquid and gas are forced through the medium at a known rate until a steady pressure differential across the compressed medium is achieved. Upon achieving a steady pressure differential, the pneumatic cylinders are retracted and the center layer of the sample material is released and suspended from an analytical balance. The mass measurement yields the liquid saturation of the material, while the flow rate of each component and the common pressure drop are used to determine the relative permeability of each phase. The process is repeated at different flow rates until the dependence of the relative permeability on saturation is established. The relative permeability of liquid water in GDL materials has long been assumed to follow a cubic relationship with saturation similar to what has been observed in packed sand. However, it is shown in this work for a variety of macroporous GDL materials including both carbon fiber paper and carbon fiber cloth, that the relative permeability function is actually a linear function of liquid water saturation. The slope of the linear function is highly dependent on the substrate type, the level of wetproofing that has been applied to the substrate, and the compression of the material. Results are presented for carbon paper and carbon cloth materials that are untreated (no wetproofing) and that have been treated with a wetproofing agent to a level of 20 wt%.