PEMFC 적용을 위한 CNT-Metal 분리막의 전기적 특성에 관한 연구

Abstract

The polymer electrolyte membrane fuel cells (PEMFCs) show the relatively low operation temperature, low current density, high efficiency, high power density and rather rapid response to the load variation. Thus, it can be applied to fuel cell electric vehicles (FCEVs) and distributed power generation systems. The bipolar plates perform as the current conductors between cells, provide conduits for reactant gases flow, and constitute the backbone of a power stack. They are commonly made of graphite composite for high corrosion resistance and good surface contact resistance. However, the graphite and their composites are unfavorable to manufacturability, permeability of gas, and durability for shock and vibration in comparison to metals. Metal is considered to be a good candidate material for the bipolar plates because of its high bulk electrical conductivity, high heat conductivity, high strength, low gas permeability, and ease of manufacture. However, the corrosion resistance and interfacial contact resistance of metal material should be considered. On the other hand, various methods and techniques must be developed to prevent the metallic corrosion and eliminate the passive layer that causes unacceptable reduction in contact resistance and possible fouling of the catalyst and the ionomer. Thus recently metallic bipolar plates have received considerable attention in the research and industry. So the materials in the bipolar plate of PEMFC should have the characteristics as follows; high corrosion resistance, low interfacial contact resistance, high surface tension with water, light-weight, high mechanical strength, and high volume cost-effective manufacturability. In this thesis, the carbon nanotube (CNT) coated metallic bipolar plates and their effective manufacturing process methods were proposed to secure the mechanical, chemical, electrical properties for the PEMFC in comparison to the conventional bipolar plates with graphite and metals. The optimum conditions by relationship between the material properties and the electrical characteristics were also considered for the PEMFC cells/stacks. Spin-coating and hot-press methods were proposed to prepare the CNT coated metals for the bipolar plates. The contents of CNT were varied by 1.0, 1.5, 2.0, and 2.5 g to the mixed and stirred solution with 30 g of poly tetrafluoro ethylene (PTFE) and 500 ml of ethanol. The metal substrates of copper (Cu) and aluminum (Al) were 20×20 mm of area and 0.5 mm of thickness. Atomic percentage of CNT on the surface of the metals was proportional to the contents in the PTFE solution regardless of metal substrates, by analyses of energy-dispersive X-ray spectroscopy (EDX) and X-ray diffractometer (XRD). Spin-coating method could cover 93.14 and 93.45 at% of CNT on copper and aluminum substrates, respectively. Hot-press method showed the 95.25 and 89.32 at% of CNT on those substrates. Sheet resistance of the CNT-coated copper showed the decreased value of 3.67×10-4 Ω/□ under that of the non-coated metallic copper; however, that of the CNT-coated aluminum showed the somewhat higher value than that of the non-coated metallic aluminum. This thesis successfully demonstrated the methodologies for preparing the CNT-coated metal bipolar plates with the mechanical strength, chemical stability, and electrical conductivity. This study will contribute to the improvement in the efficiency and economics in the bipolar plates for the PEMFC.