Structural Modification of Polymer Electrolyte Membrane Fuel Cells Components

Abstract

Polymer electorolyte fuel cells (PEFCs) are being investigated as applications of hydrogen, one of the candidates to replace fossil fuels due to near zero emissions However, since the price merit and the performance are still far from the internal combustion engine due to expensive components, much researcher are still required for commercialization. These components are mainly membrane, electrode, gas diffusion layer, and flow field. Therefore, the main theme of this thesis is to analyze the improved performance and durability by structurally changing the components of the fuel cell.<br/> Chapter 1 briefly introduces the polymer electrolyte membrane fuel cell. Then the features, roles, and challenges of fuel cell components are addressed.<br/> In chapter 2, a metal foam was characterized and applied as a flow field in a PEFC unit cell. In addition, the electrochemical performance of the metal foam was compared with that of the commonly used serpentine flow field. At a relative humidity (RH) of 100%, no significant difference in performance was observed between the metal foam and serpentine flow field. However, the performance of a single cell with the metal foam was superior to that of the common flow field under an RH of 20% under pressurized conditions. Furthermore, the factors affecting fuel cell performance by application of the flow field were discussed.<br/> In chapter 3, we have achieved performance enhancement of PEFC though crack generation on its electrodes. It is the first attempt to enhance the performance of PEFC by using cracks which are generally considered as defects. The pre-defined, cracked electrode was generated by stretching a catalyst-coated Nafion membrane. With the strain-stress property of the membrane that is unique in the aspect of plastic deformation, membrane electrolyte assembly (MEA) was successfully incorporated into the fuel cell. Cracked electrodes with the variation of strain were investigated and electrochemically evaluated. Remarkably, mechanical stretching of catalyst-coated Nafion membrane led to a decrease in membrane resistance and an improvement in mass transport, which resulted in enhanced device performance.<br/> In chapter 4, Guided cracks were successfully generated in an electrode using the concentrated surface stress of a prism-patterned Nafion membrane. An electrode with guided cracks was formed by stretching the catalyst-coated Nafion membrane. The morphological features of the stretched membrane electrode assembly (MEA) were investigated with respect to variation in the prism pattern dimension (prism pitches of 20 μm and 50 μm) and applied strain (S ≈ 0.5 and 1.0). The behaviour of water on the surface of the cracked electrode was examined using environmental scanning electron microscopy. Guided cracks in the electrode layer were shown to be efficient water reservoirs and liquid water passages. The MEAs with and without guided cracks were incorporated into fuel cells, and electrochemical measurements were conducted. As expected, all MEAs with guided cracks exhibited better performance than conventional MEAs, mainly because of the improved water transport. <br/> In chapter 5, to increase the durability of highly loaded platinum- and platinum-nickel alloy catalysts possessing different types of carbon supports, a nitrogen-doped carbon shell was introduced on the catalyst surface through dopamine coating. As the catalyst surfaces were altered following shell formation, the ionomer contents of the catalyst inks were adjusted to optimise the three-phase boundary formation. Single cell tests were then conducted on these inks by applying them in a membrane electrolyte assembly. Furthermore, to confirm the durability of the catalysts under accelerated conditions, the operation was continued for 200 h at 70 °C and at a relative humidity of 100%. Transmission electron microscopy and electrochemical analysis were conducted before and after the durability tests, and the observed phenomena were discussed for catalysts bearing different types of carbon supports.<br/> In appendix A, development of a single cell for X-ray absorption fine structure (XAFS) analysis is described. Also, we developed a new type of single cell system to enable XAFS analysis under real operating condition. In this system, the electrode can be analyzed at practical fuel cell operation conditions, and signals will be obtained in fluorescence mode and transmission mode. This study will play a major role in analyzing fuel cell components in actual driving conditions in the future.<br/>