Novel Gas Diffusion Layers for Proton Exchange Membrane Fuel Cells

Polymer electrolyte membrane fuel cells (PEMFCs) have a great potential in becoming a major alternative to fossil fuel combustion technology. One of its major components is the gas diffusion layer (GDL). It is a porous carbon fibre-based material (which is mostly coated with a microporous layer (MPL)) that provides pathways for the transport of reactant gases, excess water, heat, and electrons during PEMFC operation. Hence, there is a need for an effective design of the GDL to optimize the PEMFC performance. One of the efficient and cost-effective tools to optimise fuel cell performance is numerical modelling using for example computational fluid dynamics (CFD) software. However, the anisotropy of the GDL and its anisotropic transport properties are not fully captured in the PEMFC numerical models reported in the literature, leading to unrealistic predictions of the PEMFC performance. In this thesis, a comprehensive 3-D numerical model of a PEMFC, incorporating realistic experimentally measured multidimensional values of the GDL transport properties (gas permeability, diffusivity, thermal conductivity, and electrical conductivity) has been developed to investigate the sensitivity of the fuel cell performance to these anisotropic transport properties. Also, a 2-D model of the transport of reactant species in cathode GDL of the PEMFC was developed. Further, numerical investigations were conducted to study the sensitivity of the PEMFC performance to (i) the interfacial contact resistances between the PEMFC components and (ii) that of a double side MPL-coated GDL. Finally, the impact of a linear porosity gradient distribution in the cathode GDL on the PEMFC global performance and the local distribution of the current density and oxygen concentration in the cathode side of the membrane electrode assembly (MEA) has been investigated. Results from the research show that at low fluid velocities and gas permeability, the transport of the gas reactant species from the channel to the catalyst reaction sites is dominated and controlled by the diffusion mechanism in the GDL. GDL anisotropy has significant impact on the PEMFC performance, overlooking it results in either significant overestimation (if the in-plane values of the transport properties are only considered) or underestimation (if the through-plane values of the transport properties are only considered) of the modelled fuel cell. Also, when designing the carbon fibre paper GDLs, it is recommended that the carbon fibres are more oriented in the through-plane direction than in the through-plane direction to maximise traverse transport properties, in particular electrical conductivity, thermal conductivity, and gas diffusivity. Furthermore, the fuel cell performance is more sensitive to the porosity of the MPL facing the bipolar plate than the MPL facing the catalyst layer. As the porosity of the MPL facing the bipolar plate is predominantly the limiting factor for the distribution of oxygen concentration within the cathode GDL. Also, the grading of the cathode porous transport medium of the PEMFC shows that at optimum design with increasing gradient, the PEMFC global performance as well as the local distribution of the key parameters is improved.