Mass transport studies in conventional and microfabricated free convection proton exchange membrane fuel cells

This thesis presents the design, modeling and testing of both conventional and non-conventional free convection proton exchange membrane fuel cells (PEMFC) which are particularly attractive for low power (<100W), man portable applications. As part of this investigation, experimental data coupled with computational simulations has provided a deeper understanding into the mechanisms that limit operability. In particular, the low temperatures of operation (<50°C) and lack of forced air convection generate high levels of saturation within the cell thereby reducing the available catalytic sites.;A 2-D, single- and two-phase computational model for an open-cathode, free convection PEMFC was developed. The two-phase modeling has provided significant insight into the mass transport limitations caused primarily by liquid water flooding while the single phase simulations present an upper limit in performance assuming product water exists only in vapor form. A set of parametric experiments were performed using conventional gas diffusion media (GDM) along with numerous 1cm2 stainless steel grid-based current collectors having open area ratios of 10%, 25% and 50%. These experiments provided a data set that was used to "tune" and validate the model. Single cell polarization data experienced limiting currents ranging from 190mA/cm2 to 600mA/cm 2 at operating cell temperatures of 38°C-45°C, depending on the open area ratio. The two-phase model captured the effect of current collector porosity as well as the increase in limiting current associated with rising cell temperature.;Once validated, the model was used with confidence as a design tool for MEMS-based tailored mass transfer media (TMTM) that provide more advanced functionality than customary GDM. They were based on a microfabricated hydrophobic silicon mesh comprised of square pores with discrete zones made to be hydrophilic using a carbon-polyethylene oxide treatment. The TMTM were engineered to localize the removal of liquid water and allow saturation free transport of the gas phases to and from the catalyst layer. The hydrophilic zone spacing distance was varied (1250mum and 2500mum) while keeping the overall wetting to non-wetting area ratio at 12.5%.;The TMTM were tested using conventional catalyst on membrane assemblies at low temperatures (ambient and 32.5°C) and were shown to efficiently segregate water and provide unobstructed reactant gas paths to the cell. The ability to control the wetting characteristics of these structures and provided enhanced evaporation to the surroundings has resulted in significant increases in steady state limiting current densities compared to untreated silicon structures (the latter of which delivered 100mA/cm2 and 133mA/cm 2 at 26.6°C and 33.1°C, respectively). Liquid water transport within the catalyst layer has been shown to be a function of the hydrophilic spacing where the TMTM having a hydrophilic separation distance of 1250mum delivered higher limiting current densities (148.9mA/cm2 and 250mA/cm2 for ambient and 32.5°C) than the larger separation distance of 2500mum (134mA/cm2 and 175mA/cm2 for ambient and 32.5°C). Limiting current performance improvements between the treated and untreated substrates ranged from 34-50% at ambient temperature to 32-88% at the 32.5°C operating temperature.;In summary, the work presented herein will not only advance the state of the art in miniaturizing PEMFC's using MEMS technology but it will also open up a path for a more scientific approach to understanding multi-phase transport within porous fuel cell layers and interfaces.