EFFECT OF MORPHOLOGY ON THE PROPERTY OF SULFONATED POLYMER MEMBRANE

Abstract

This dissertation is concerned with the relation between polymer morphology and membrane properties for proton exchange membrane fuel cell (PEMFC) application. In particular, the effect of chemical and physical structures of sulfonated aromtatic polymers on membrane properties such as stability and electrochemical performance. This dissertation is composed of seven chapters, including an introductory chapter, which summarizes recent publications on developments of sulfonated aromatic PEMs with various chemical structures and polymer microstructures. A fundamental understanding of chemical structure, polymer microstructure, and electrochemical performance of sulfonated aromactic polymers is important in order to design novel polymer electrolyte membranes (PEMs) with excellent fuel cell performance at medium temperature and low relative humidity (RH). Sulfonated aromatic polymers have distinct microstructure according to their chemical structure. The ionic clusters and/or channels play a critical role in PEMs affecting proton conductivity and water transport, especially at medium temperature and low RH. In addition, physical properties such as water uptake and dimensional swelling behavior depend on polymer morphology. Over the past few decades, many researches have focused on synthesis of sulfonated aromatic polymers as an alternative material to perfluorosulfonic acid polymers (PFSA, e.g., Nafion®, Flemion®, Aciplex®, and so on) and characterization of microstructure. Furthermore, some approaches such as blend, composite, and crosslinking have been suggested to obtain well-defined microstructure of PEMs. In Chapter 2, sulfonated poly(arylene sulfide sulfone nitrile)s (SN) were synthesized to investigate the effects of naphthalene units in the polymer backbone on membrane properties. The small and planar naphthalene in the main chain reduced inter-domain distance, as confirmed by molecular simulations and small angle X-ray scattering patterns. The SN polymer membranes exhibited excellent chemical and mechanical properties, better than those of their phenylene counterpart (SP). In particular, the water uptake and swelling ratio of the SN membranes were much lower than those of the SP membranes. Furthermore, the SN membranes exhibited a greatly reduced methanol permeability (9-17 × 10-8 cm2 s-1) compared to Nafion® 212 (330 × 10-8 cm2 s-1) at 30 oC in 10 M methanol. Moreover, sulfide- and naphthalene-based chemical structure of the SN membranes enhanced their DMFC single cell performance and long-term stability during fuel cell operation. In Chapter 3, ordered morphologies in disulfonated poly(arylene sulfide sulfone nitrile) (SPSN) copolymers were generated via thermal annealing followed by multi-block copolymer synthesis. While SPSN random copolymers (R-SPSN) showed featureless morphologies, the SPSN multi-block copolymers (B-SPSN) exhibited co-continuous lamellar morphologies with a center-to-center inter-domain size of up to 40 nm. In spite of the well-ordered, interconnected hydrophilic domains, the water self-diffusion coefficient (e.g., D = (0.7–2.0) × 10-10 m2 sec-1) and proton conductivity (e.g., σ = 0.16–0.20 S cm-1 in deionized water at 30 oC) through B-SPSN were lower than those of the corresponding R-SPSN (e.g., D = (3.5–3.9) × 10-10 m2 sec-1 and σ = 0.21 S cm-1) due to the relatively lower water uptake of the B-SPSN after thermal annealing. The reduced water uptake of B-SPSN was beneficial to reduction of peroxide degradation rate. Thermal annealing produced significant gains in morphological ordering, and finer control over desired membrane properties for proton conduction applications. In Chapter 4, a series of poly(arylene ether sulfone)s (SPAEs) with densely sulfonated pendant group (4P-XX, XX is molar ratio of hydrophilic repeating unit) or side chain (4S-XX) was prepared for fuel cell applications by polycondensation of the new monomer. The sulfonated polymers gave tough, flexible, and transparent membranes by solvent casting. The ionic exchange capacity (IEC), water uptake, swelling ratio, mechanical properties, thermal stability as well as proton conductivities and single fuel cell performance of the membranes were investigated. The membranes with densely sulfonated pendant group show high proton transport properties, and their proton conductivities exhibit lower dependence on temperature compared with membranes containing densely sulfonated side chain. 4P-38 with high IEC value (2.23 meq. g-1) displays comparable fuel cell performance with Nafion® 212 under low humidity conditions. In Chapter 5, crosslinked sulfonated poly(arylene sulfide nitrile) (XESPSN) membranes showed outstanding thermal and mechanical properties up to 200 oC compared with the pristine and non-crosslinked ESPSN and Nafion®. The XESPSN membranes exhibited a higher proton conductivity (0.011-0.023 S cm-1) than that of as-prepared ESPSN (0.004 S cm-1), particularly under elevated temperature (120 oC) and low relative humidity (35%) conditions due to well-ordered hydrophilic morphology of XESPSN. Therefore, the XESPSN membranes showed significantly outperformed maximum power density (415-485 mW cm-2) than the ESPSN (281 mW cm-2) and Nafion® (314 mW cm-2) membranes in single cell performance tests at 120 oC and 35% relative humidity. Furthermore, the XESPSN membrane exhibited much longer duration time than ESPSN during fuel cell operation with constant current load. In Chapter 6, Inorganic-organic composite membranes were fabricated using zirconium acetylacetonate and biphenol based sulfonated poly(arylene ether sulfone) as a proton conducting inorganic material and polymer matrix, respectively. Amphiphilic surfactant was induced to distribute inorganic material over the polymer membrane. The composite membranes were thermally stable up to 200 oC. Inorganic material improved inter-chain interaction and robustness of polymer membranes resulting excellent mechanical properties. In addition, composite membranes showed an outstanding proton conductivity compared to that of pristine membrane at medium temperature (> 80 oC) and low relative humidity (RH, < 50%) conditions. This is due to the acetylacetonate anions, which bind water molecules and act as an additional proton conducting site and/or medium. Therefore, the composite membranes significantly outperformed comparing to pristine membrane in fuel cell performance tests at medium temperature and low RH. In Chapter 7, the conclusions and directions for further studies regarding advanced sulfonated aromatic polymer membranes with well-ordered morphology as polymer electrolyte membranes are presented.