Development of experimental techniques for studying gas diffusion and water transport in the porous media of fuel cells

Abstract

Polymer Electrolyte Membrane (PEM) fuel cells, which produces electricity by oxidation of hydrogen and reduction of oxygen with water as the only waste product, have shown great potential as a zero-emission and high-efficiency source of energy. Despite the numerous advantages of the fuel cell, its performance must still be improved significantly. Numerous studies have identified water management as the major problem hindering commercialization of fuel cells. One of the components playing an important role in water management is the porous gas diffusion layer (GDL) across which reactant transport and water removal occur. Due to a long operation or large output current, the water production rate exceeds the water removal rate. As a result, water accumulates in the pores of the GDL and blocks convective and diffusive transport of reactants. This is the origin of the limiting current. To enhance water management, the reactant transport and water removal mechanisms inside the GDL needs to be thoroughly studied. Despite various numerical models developed to illustrate the multiphase flow in the cell, these mechanisms are not well understood due to the structural complexity of the GDL. In this thesis, experimental techniques are developed for the measurement of gas diffusion and water content inside the GDL. For the latter, the fluorescence microscopy technique is used to measure the pressure and time required for water to penetrate and break through the surface of the GDL. The results show that the breakthrough time and pressure are larger for hydrophobic GDLs. However, the effect of the sample thickness is found to be negligible. An innovative experimental technique is developed for the determination of the effective diffusion co- iii efficient as a function of the water content in the GDL. The results demonstrate that diffusivity is greater for samples with higher porosity. Also, it is shown that the presence of cracks in the micro-porous layer of the GDL increases water accumulation within the pores, reducing significantly diffusive transport within the GDL. These experimental techniques provide basic insights into the transport properties of the GDL which can lead to the design of new materials that enhance transport inside the porous media.