Efficient PEM fuel cells for portable applications

Fuel cells (FCs) have attracted considerable attention for replacing batteries in many electronic devices. Among different types of fuel cells, the proton exchange membrane fuel cell (PEMFC) is most widely investigated. Air-breathing proton exchange membrane fuel cells (AB-PEMFCs) have received more attention in recent years because of the simplifications in the fuel cell system, which makes it a good choice for portable applications. However the simplifications also causes a rather low performance and this is attributed to the low mass and heat transfer coefficients. In this thesis, a mathematical model has been developed in order to investigate the overall performance of the fuel cell system, and the local performances of two important components, i.e. the gas channel (GC) and gas diffusion layer (GDL) have been studied by a CFD model and a gas permeability experiment, respectively.
The mathematical model presented in this thesis is based on the conservation of the mass and heat transfer in order to investigate the effects of the different parameters on the fuel cell overall performance. A new revised water transport relation is applied in this model, which makes it possible to study the effect of the hydrogen relative humidity (RH). The results show that, among all the different operating parameters, the hydrogen RH can significantly improve the performance of AB-PEMFCs and the GDL is an important component in improving the transport and water management issues.
In addition, a computational fluid dynamics (CFD) model for the anode channels in AB-PEMFCs is developed by employing the Volume of Fluid (VOF) method. The dynamics of the liquid water are studied under different flooding conditions. The modelling results show that the initial position of the accumulated droplet and the hydrogen velocity have little effect while the droplet size and the channel wettability can largely influence the local performance in the channels, e.g. the water removal time and the pressure drop. Also it is found that the trade-off between the pressure drop and the removal time should be considered when designing practical products.
Further, the GDL thickness is found to be important in determining the performance of the AB-PEMFCs in the modelling work. In order to produce the GDLs with different thicknesses, an experimental investigation has been conducted to study the effect of the stacking of single GDL layers, and the through-plane gas permeability is investigated, which is one of the most important properties of GDLs. Compared with previous studies, the gas permeability of the GDL stacks is investigated instead of a single GDL layer. The calculation results show that the stacking of layers has only a small influence on the overall gas permeability of the GDL stack. In addition, a tighter contact between each layer in the GDL stacks is found to increase the overall gas permeability of the GDL stacks.