연료전지용 술폰화 고분자 전해질막 개발 및 특성 분석

Abstract

This dissertation is concerned with the development of sulfonated polymer electrolyte membranes (PEMs) with the relation between membrane structure, morphology, and properties for high proton conductivity and electrochemical performances at low humidity conditions. Prior to development of sulfonated polymer electrolyte membranes, a fundamental understanding of polymer structure, proton and water molecule transport, membrane morphology, and mechanical properties was summarized in chapter 1. Current polymer electrolyte membrane researches are focused on developing membranes with synthetic methodologies for tailoring polymer structures that promote nanophase separation of hydrophilic and hydrophobic domains, and modifications for enhancement of the PEM properties such as proton transport and durability. A promising strategy to improve proton conductivity, especially under low-humidity conditions, is the formation of well-connected ion nanochannel. This may be achieved by high concentration of the acid groups to specific chain segments in the polymer and hence promoting the nanophase separation. Sulfonic acid groups have been highly concentrated to specific chain segments in the polymer backbone (block and densely sulfonated copolymer) or separated from the polymer backbone on pendant side chains (graft copolymer). Furthermore, other approaches such as cross-linking, acid-base complexation, and reinforced composite have been suggested to promote well-defined microstructure and PEM properties. In Chapter 2, we present a new approach of morphological transformation for effective proton transport within ionomers, even at partially hydrated states. Highly sulfonated poly(phenylene sulfide nitrile) (XESPSN) random network copolymers were synthesized as alternatives to state-of-the-art perfluorinated polymers such as Nafion. A combination of thermal annealing and cross-linking, which was conducted at 250 °C by simple trimerisation of ethynyl groups at the chain termini, results in a morphological transformation. The resulting nanophase separation between the hydrophilic and hydrophobic domains forms well-connected hydrophilic nanochannels for dramatically enhanced proton conduction, even at partially hydrated conditions. For instance, the proton conductivity of XESPSN60 was 160% higher than that of Nafion 212 at 80 °C and 50% relative humidity. The water uptake and dimensional swelling were also reduced and mechanical properties and oxidative stability were improved after three-dimensional network formation. The fuel cell performance of XESPSN membranes exhibited a significantly higher maximum power density than that of Nafion 212 at elevated temperature conditions. Furthermore, the XESPSN membrane exhibits a much longer duration than the ESPSN membrane during fuel cell operation under a constant current load as a result of its improved mechanical and thermal stabilities. In Chapter 3, a series of end-group cross-linked membranes (Az-XESPSN) were prepared by click reaction to investigate the effects of cross-linking on the morphology and proton transport properties of proton exchange membranes. The morphological transformations resulting from thermal annealing and cross-linking were observed by means of atomic force microscopy (AFM) and transmission electron microscopy (TEM). Compared to the non-cross-linked ESPSN membranes, the Az-XESPSN membranes exhibited lower water uptake and improved mechanical and chemical stabilities. In addition, the Az-XESPSN membranes exhibited higher proton conductivities (0.018-0.028 S cm-1) compared to those of the ESPSN membranes (0.0044-0.0053 S cm-1) and Nafion 212 (0.0061 S cm-1), particularly in conditions of elevated temperature (120 °C) and low relative humidity (35%). Such enhancements can be attributed to a synergistic effect of well-defined hydrophilic ionic clusters and triazole groups that function as proton carriers under anhydrous conditions. Furthermore, the Az-XESPSN membranes exhibited significantly enhanced single cell performance and long-term stability compared to those of ESPSN membranes. In Chapter 4, a new approach to fabricate reinforced composite membranes with improved proton conductivity is designed and prepared using thermally rearranged polybenzoxazole-co-imide (TR-PBOI) nanofibrous membrane incorporated with cross-linked sulfonated poly(phenylene sulfide sulfone) (XESPSN) electrolytes for proton exchange membrane fuel cell (PEMFC) application. It was determined that doping the TR-PBOI membranes with acids plays an important role in improving the wetting properties and interfacial compatibility with electrolytes, as well as proton conductivity with acid-base proton transfer pathways between the sulfonic acid in electrolyte and the benzoxazole group in nanofibrous substrate. Particularly, at low relative humidity conditions, the TR-RCM composite membranes exhibited steep decline in proton conductivity and higher proton conductivity than those of the XESPSN casting membranes and Nafion 212 membrane at low RH conditions. Compared with the XESPSN casting membrane, the TR-RCM composite membranes exhibited much improved water uptake and mechanical properties while effectively suppressing the dimensional water swelling. These excellent membrane properties and enhanced proton conductivity contributed to significantly higher maximum power density and long-term stability than XESPSN casting membrane. In Chapter 5, the conclusions and directions for further studies regarding advanced sulfonated polymer membranes with well-ordered morphology and durability as polymer electrolyte membranes are presented.