NOVEL SULFONATED HYDROCARBON POLYMER ELECTROLYTE MEMBRANES FOR FUEL CELL

Abstract

This dissertation is related with development of sulfonated hydrocarbon polymer electrolyte membranes (PEMs) for high proton conductivity at low humidity condition and electrochemical performance. This dissertation is organized into ten chapters, including introduction chapter, and the main topic of research can be classified as three primary area: (a) cross-liked PEMs, (b) branch and block PEMs (c) solid state modified PEM.

In Chapter 2: Hear we report on the relationship between morphology and fuel cell performance of three types of sulfonated poly(arylene ether sulfone)s (SPAES) prepared from random, block, grafting method under low relative humidity (RH) condition at high temperature (> 100oC). A random copolymer (R-SPAES) with randomly distributed of sulfonic acid groups in polymer matrix had featureless morphology, whereas multiblock copolymer (B-SPAES) shows highly hydrophilic and hydrophobic domains phase-separated morphology with well-connected hydrophilic channels. The grafting polymer (G-SPAES) with sulfonic acid group chains grafted evenly along the polymer chain showed small and narrow phase-separated hydrophilic capillary, as evidenced by TEM. The ion conductivity, water sorption kinetics, and fuel cell performance, especially at low relative humidity, were found to be highly dependent upon the morphology of the membranes. Hydrophilic domain size seems to be important to retain water molecules for high temperature fuel cell applications. G-SPAES outperformed both the R-SPAES and B-SPAES at 120 °C and 35% RH fuel cell operating conditions.

In Chapter 3: Novel sulfonated poly(phenylene sulfide sulfone nitrile) (SPSN) was prepared and subsequently cross-linked via a Friedel-Craft reaction using 4,4’-oxybis(benzoic acid) as a cross-linker to achieve lower water swelling and lower methanol permeability. The dimensional change of SPSN40 was 43.7% in 90oC liquid water but that of the cross-linked membrane, XSPSN40, was 23.3% while maintaining proton conductivity at 0.22 Scm-1. These results show that the Friedel-Craft crosslinking of novel SPSN membrane effectively reduced water uptake and the degree of swelling while improving the dimensional stability and maintaining high proton conductivity.

In Chapter 4: A new approach of morphological transformation was investigated 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 oC by simple trimerisation of ethynyl groups at the chain termini, result in a morphological transformation. The resulting nanophase separation between the hydrophilic and hydrophobic domains form well-connected hydrophilic nanochannels for dramatically enhanced proton conduction, even at partially hydrated conditions. For instance, the proton conductivity of XEPSN60 was 160% higher than that of Nafion® 212 at 80 oC 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 Nafion® 212 under partially hydrated environments. In Chapter 5: Highly sulfonated poly(ether sulfone)s with densely populated flexible acid side chains were prepared for fuel cell applications by polycondensation of 3,3'-dihydroxybenzidine with bis(4-fluorophenyl)sulfone and 4,4'-biphenol, followed by postsulfonation using 1,4-butanesultone at room temperature. The sulfonated polymers gave tough, flexible, and transparent membranes by solvent casting. The membranes had high ion exchange capacity (IEC) values (2.47-2.95 meq. g-1) and displayed good proton conductivities in the range of 13.90-20.90 × 10-2 and 1.08-2.21 × 10-2 S cm-1 at 95% and 35% relative humidity (RH) (80 °C), respectively. In particular, the S-PES-55 membrane with the highest IEC value showed higher or comparable proton conductivity than that of Nafion® 212 in the range of 35-95% RH. The morphologies of these membranes were investigated by STEM analysis, which exhibited well-connected hydrophilic channels due to their high IEC values and densely populated flexible acid side chains. In sharp contrast with many reported highly sulfonated polymers, the membranes showed good dimensional stability regardless of their high IEC values.

In Chapter 6: A novel class of the highly sulfonated fully aromatic comb-shaped copolymers were synthesized by the combination of an OH functionalized polymer main chain with fully aromatic graft (phenylene oxide)(PPO) oligomer, followed by post sulfonation. The sulfonated fully aromatic comb-shaped copolymers gave tough, flexible, and transparent membranes by solvent casting. Although the membranes had lower ion exchange capacity (IEC) values (0.92-1.72 meq. g-1) compare with relative other sulfonated hydrocarbon PEMs, they displayed high proton conductivities in the range of 10.60-20.70 × 10-2 and 1.08-1.25 × 10-2 S cm-1 at 95% and 35% relative humidity (RH) (90 °C), respectively. In particular, the 4(X5-Y14) membrane with the highest IEC value showed higher proton conductivity than that of Nafion 212 in the range of 35-95% RH. Morphological observation by transmission electron microscopy (TEM) exhibited organized well-connected hydrophilic channels and this unique morphology is consistent with phase morphology by mesoscale simulation. This comb-shaped copolymer approach could lead to new PEM materials that meet the demanding requirements for automotive fuel cells.

In Chapter 7: Highly proton-conducting polymer electrolyte membrane (PEMs) materials are presented as alternatives to state-of-the-art perfluorinated polymers such as Nafion®. To achieve stable PEMs with efficient ionic nanochannels, novel fully aromatic ABA triblock copolymers (SP3O-b-PAES-b-SP3O) based on sulfonated poly(2,6-diphenyl-1,4-phenylene oxide)s (A, SP3O) and poly(arylene ether sulfone)s (B, PAES) were synthesized. This molecular design for a PEM was implemented to promote the nanophase separation between the hydrophobic polymer chain and hydrophilic ionic groups, and thus to form well-connected hydrophilic nanochannels that are responsible for the water uptake and proton conduction. Relative to other hydrocarbon-based PEMs, the triblock copolymer membranes showed a dramatic enhancement in proton conductivity under partially hydrated conditions, and superior thermal, oxidative and hydrolytic stabilities, suggesting that they have the potential to be utilized as alternative materials in applications operating under partly hydrated environments.

In Chapter 8: Random di-sulfonated poly(arylene ether sulfone)-silica nanocomposite (FSPAES-SiO2) membranes were physico-chemically tuned via surface fluorination. Surface fluorination after 30 minutes converted about 20% of the C–H bonds on the membrane surface into C–F bonds showing hydrophobicity and electro-negativity at the same time. The membranes with hydrophobic surface properties showed high dimensional stability and low methanol permeability when hydrated for direct methanol fuel cell (DMFC) applications. In particular, the surface enrichment of fluorine atoms led to anisotropic swelling behavior, associated with a stable electrode interface formation. Interestingly, in spite of the use of a random copolymer as a polymer matrix, the low surface free energy of the C–F bonds induced a well-defined continuous ionic channel structure, similar to those of multi-block copolymers. In addition to the morphological transition, fluorine atoms with high electro-withdrawing capability promoted the dissociation of sulfonic acid (–SO3H) groups. Consequently, FSPAES-SiO2 membranes exhibited improved proton conductivity. Thus, FSPAES-SiO2 membranes exhibited significantly improved single cell performances (about 200%) at a constant voltage of 0.4 V in comparison with those of Nafion® 117 and non-fluorinated membranes. Surprisingly, their good electrochemical performances were maintained with very low non-recovery loss over the time period of 1,400 hours and interfacial resistances 380% times lower than those of conventional membrane-electrode assemblies (MEAs) comprising the control hydrocarbon membrane and Nafion® binder for the electrodes.

In Chapter 9: Disulfonated poly(arylene ether sulfone) random and multiblock copolymers with the same chemical architecture state were physicochemically tuned in solid membrane via direct fluorination under a dilute fluorine (F2)-nitrogen (N2) mixture, which was expected to minimize chemical degradation by a highly reactive F2 gas. The direct fluorination and subsequent acidification enabled C-H bonds of aromatic rings in hydrophobic domains or blocks of the copolymers to be selectively converted into C-F bonds. Interestingly, the direct fluorination on the copolymer membranes influenced their morphologies and water/ion transport properties in different ways, even though both sulfonic acid level and swelling ratio of the precursor copolymers were similar. In case of random copolymers, hydrophobic fluorine atoms were mainly enriched on their membrane surface, which resulted in improved dimensional stability even in high sulfonation levels, showing anisotropic swelling behavior. Low surface free energy and electron-withdrawing capability of the C-F bonds and the fluorine atoms induced a well-defined continuous ionic channel structure on their membrane surfaces and enhanced proton dissociation of their sulfonic acid groups, respectively. Consequently, their proton conductivities were improved in spite of reduced water diffusion. On the other hand, the direct fluorination on the multiblock copolymers made their lamellar morphology more developed, particularly in through-plane direction. The peculiar morphological transformation contributed to significantly improved water/ion transport even in partially hydrated states.