The fabrication art of the membrane electrode assembly (MEA) in a proton-exchange membrane (PEM) fuel cell strongly correlates to the cell performance. It has been recognized that defects, for example, high interfacial resistance between the catalyst layers (CLs) and the membrane or cracks in the CLs, may occur during the MEA manufacturing process. These defects could greatly influence the electrochemical performance of the fuel cell. To eliminate those defects and improve the cell performance, in this study, a novel fabrication approach of the MEA for PEM fuel cells is developed. With this method, the Nafion ionomer, employed as a PEM, is directly coated onto both the cathode and anode CLs. As a result, not only an excellent interfacial connection between the PEM and CLs is achieved with a low interfacial resistance, but also cracks are eliminated due to Nafion ionomer penetration into the cracks, forming hydrophilic channels with ionic conduction. Those ionic conduction channels improve the water management, lower the mass transport loss, and facilitate the proton transfer, thus maximizing the three-phase boundary and enhancing the utilization of Pt/C catalysts. By adding an expanded polytetrafluoroethylene film, a favorable mechanical property of the MEA is also achieved. This novel MEA exhibits excellent cell performance under low humidity conditions. Under the H 2 /air operation, the cell performance reaches a high maximum power density of 1.35 W cm −2 .

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Although the microporous membrane prepared based on the melt stretching mechanism has been commercialized for more than 20 years, the formation process from the initial lamellae structure to final fiber connecting bridges and pores is still unclear. In this work, to clarify the transformation mechanism, in situ SAXS and WAXS were carried out during hot stretching at 130 °C to 100%. The scattering patterns from the annealed film, cold stretched film to 20% (stretched at room temperature), and heating to 130 °C were also collected. The preparation technology was similar to that during the commercial fabrication. It was found that during cold stretching to 20% many long and narrow crazes are formed between separated lamellae clusters, and a part of destroyed crystals appeared. After heating to 130 °C, oriented structure and needlelike voids appeared, which was related to the shrinkage of oriented amorphous chains along the transverse direction, due to the tension stress effect. Also some oriented crystal structure was formed. During hot stretching to 20%, the lamellae which are close to the craze wall are rotated as the fibril crystal as the axle and the connecting bridges were formed among the separated lamellae cluster. Further stretching to 100%, these connecting bridges transformed to fiber bridges, contributed by strain-induced crystallization. During the whole hot stretching, the amorphous chains oriented along the machine direction and also crystallized into fiber bridges. This is the first time to clearly describe the lamellae to fiber bridges transformation during the preparation of microporous membrane.

SYNOPSISA series of block copolymers from D,L-lactide and poly(ethy1ene glycol) (PEG) with different molecular weights were synthesized by using Al(i-Bu)3 -H3P04 -H20 as the initiator and identified by 'H-NMR, GPC, and DSC. The elongation at break of the material increased when PEG was introduced, but the strength of the material decreased. The copolymers degrade more smoothly in uitro than did the PLA homopolymer. 0 1995

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