Efficient Polymer Electrolyte Fuel Cells for Portable Applications

Fuel cells offer a promising future for the production of clean energy by efficiently converting chemical energy into electricity without combustion, thereby minimising the environmental impact. Air-breathing polymer electrolyte fuel cells (PEFCs) distinguish themselves from other types of fuel cell due to their simplified design, which utilises ambient air, supplied by natural convection, as an oxidant. This novel approach reduces the system complexity and has the potential to improve the scalability of the fuel cell technology for portable applications. The main motivation of this thesis is to numerically investigate how to improve the output power and the operation stability of air-breathing PEFCs used for portable applications. It consists of a collection of a review paper and three published research papers. There has been a knowledge gap in the literature on how the air-breathing PEFC responds to sudden and large load alterations under different ambient conditions, design parameters and operating conditions. In this context, a dynamic model for an air-breathing PEFC has been developed and then the sensitivity of the transient response and the steady state performance of the fuel cell to the ambient temperature and relative humidity, the thickness and the thermal conductivity of the cathode GDL, and the fuel utilisation have been studied. Further, there has been a lack of knowledge on how natural convection limits the steady-state performance and the transient response of the air-breathing PEFC. To this end, two steady-state, non-isothermal mathematical models have been developed for both air-breathing and conventional polymer electrolyte fuel cells. Namely, with these models, a comparative parametric study has been performed to understand how each type of fuel cell responds to changes in some key parameters (i.e. the porosity and the thickness of the GDL, the membrane thickness and the overall electrical resistance) and subsequently obtain some insights on how to design an efficient air-breathing PEFC. Finally, two dynamic models have been developed for air-breathing and conventional PEFCs to conduct a parametric study on the impact of natural convection and some key parameters (i.e. the GDL porosity, the membrane thickness and the electrical resistance) on the transient response to load alterations and the steady-state performance of the air-breathing PEFC. This has been achieved by comparing the outcomes of the dynamic model with those of the dynamic model of the higher-in-performance and more responsive conventional PEFC. Overall, the results show that not only the performance of the air-breathing PEFC but also its dynamic response to rapid and large load alterations are significantly sensitive to the ambient conditions due to reliance on natural convection. The results also show that it is possible to enhance the transient response and the steady-state performance of the air-breathing PEFCs by optimising some key design parameters.