Hydrophobicity, hydrophilicity and two-phase flow in the gas diffusion layer of a polymer electrolyte fuel cell

Polymer electrolyte fuel cells (PEFCs) are versatile electrochemical devices that convert the chemical energy of a fuel, such as hydrogen, and an oxidant, such as air-derived oxygen, directly into electricity, both cleanly and efficiently. In spite of their promise as alternative energy sources, design issues remain when the cell operates at high current density: condensing water vapour at the reacting catalyst layer, on the cathode side of the cell, blocks the pores in the gas diffusion layer (GDL), which is detrimental to cell performance. Substantial experimental evidence indicates that using a hydrophobic, rather than hydrophilic, GDL on the cathode alleviates the problem. However, whilst existing theory confirms the advantages of using a hydrophobic GDL, it does not simultaneously confirm the disadvantage of using a hydrophilic GDL. This thesis uses a combination of asymptotic and numerical methods to investigate this apparent anomaly by considering an isothermal, steady state, generalized Darcy model for two-phase flow in a porous medium; mathematically, this leads to a free-boundary problem to determine the location of the interface between one-phase and two-phase flow. After extensive analysis, it is found that the model predicts significant differences between the flow regimes found in hydrophobic and hydrophilic GDLs. Furthermore, the model results show that hydrophobic GDLs are found to lead to higher current density, and hence better cell performance, than hydrophilic GDLs, as is the case in experiment. The importance of temperature differences across the GDL is also analysed by means of a non-isothermal model. It is found that temperature gradients in the GDL significantly affect the cathode overpotential and outlet temperature at which the onset of two-phase flow occurs; high enough overpotential can lead to the return of gas-phase only flow, which is not the case in the isothermal model