Effect of Modification of Crosslinking, Composite, Cationic Polymeric Ion Exchange Membranes for Fuel Cell and Reverse Electrodialysis

Abstract

Abstract

An ion exchange polymer membrane (IEM) has been studied as a key part for power generation and energy saving application such as polymer electrolyte membrane fuel cell (PEMFC), reverse electrodialysis (RED) and various desalination technologies. Fundamental understanding of IEM is important in order to design the improved polymer electrolyte membranes (PEMs) with excellent electrochemical performance at low relative humidity (RH), which is caused from hydrophilic and hydrophobic nano-phase separation polymer morphology and several membrane modification methods. In addition, physical properties such as water uptake and dimensional swelling behavior depend strongly on membrane morphology and polymer structure (i.e. main chain structure, side chain and functional group). Over the past few decades, much research has focused in the synthetic development and micro structural characterization of hydrocarbon based PEM materials besides perfluorinated polymer. However various modification approaches based on highly ion conductive polymer have also been kind of strategy to achieve high electrochemical performances in practical condition at medium to high temperature (about 80 ~ 120 oC) and low relative humidity, compensating poor mechanical and chemical property of novel polymer membranes to satisfy the requirements of fuel cell system for practical applications, fuel cell electric vehicles (FCEV) and housing stationary systems. On the other hands, the RED system, which generate power from salinity gradient between see water and river water, operate at room temperature and fully hydrated condition. This operation condition difference leads research strategy to highly ion conductive and permselective membrane with good chemical and mechanical stability. This thesis is organized into six chapters, including introduction chapter, and main research topics can be classified as two primary area, modification of membrane and membrane electrode assembly (MEA) fabricate technique for PEMFC application and modifications of membrane for RED application. In chapter 2, end-group crosslinked sulfonated poly(arylene sulfide nitrile) (XESPSN) membranes are prepared to investigate the effect of crosslinking on the properties of sulfonated aromatic polymer membranes at elevated temperatures (> 100 oC). The morphological transformation during annealing and crosslinking is confirmed by atomic force microscopy. The XESPSN membranes show outstanding thermal and mechanical properties compared to pristine and Nafion®. In addition, the XESPSN membranes exhibit higher proton conductivities (0.011 ~ 0.023 S cm-1) than the pristine ESPSN (0.004 S cm-1), particularly at elevated temperature (120 oC) and low relative humidity (35%) conditions due to its well-ordered hydrophilic morphology after crosslinking. Therefore, the XESPSN membranes demonstrate significantly improved maximum power densities (415 ~ 485 mW cm-2) compared to the pristine ESPSN (281 mW cm-2) and Nafion® (314 mW cm-2) membranes in single cell performance tests conducted at 120 oC and 35% relative humidity. 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, inorganic-organic composite membranes are fabricated using zirconium acetylacetonate nanoparticles and biphenol-based sulfonated poly(arylene ether sulfone) as an inorganic, proton conducting nanomaterial and a polymer matrix, respectively. An amphiphilic surfactant (Pluronic®) induces distribution of the inorganic nanoparticles over the entire polymer membrane. The composite membranes are thermally stable up to 200 oC. Zirconium acetylacetonate improves inter-chain interactions and the robustness of polymer membranes resulting in excellent membrane mechanical properties. In addition, composite membranes show outstanding proton conductivity compared to that of the pristine membrane at medium temperatures (80 ~ 120 oC) and low relative humidity (<50%) conditions. This improvement is due to the presence of acetylacetonate anions, which bind water molecules and act as an additional proton conducting site and/or medium. Therefore, the composite membranes significantly outperform the pristine membrane in fuel cell performance tests at medium temperatures and low relative humidity. In chapter 4, the decal transfer method is regarded as an effective membrane electrode assembly (MEA) fabrication method for industrial processes due to the improved adhesion between the catalyst layers and the hydrocarbon membrane. In this study, three swelling agents (ethanol, 1,5-pentanediol and glycerol) are introduced to the conventional decal methods in order to improve both the transfer ratio of electrodes on the membrane surface and the electrochemical properties. These swelling agents affect the surface energy differences between the swollen catalyst layer and the membrane substrate. Swelling agents also change the structure of the catalyst layer during the preparation (hot pressing) of the MEA. Changing the catalyst layer structure by introducing swelling agents diminishes the charge transfer resistance and internal resistances of MEAs. These improved electrochemical properties lead to the remarkably enhanced single cell performance of a SPAES MEA of 1380 mA cm-2 at 0.6 V, compared to a SPAES MEA fabricated by the conventional decal method (500 mA cm-2). In chapter 5, In this work, three functional groups were introduced in the poly(arylene ether sulfone) (PAES) to investigate the effects of cationic functional groups in PAES on reverse electrodialysis (RED) performances. Basic tetramethyl ammonium (TMA), 1-methyl-imidazolium (IMD) and salt form of 1-azabicyclo[2,2,2]octane (ABCO) as conducting cationic functional groups were employed in PAES, respectively. Controlling the swelling behavior of the membranes was one of important factors increasing the permselectivity while maintaining the high ion conductivity. Among three cationic groups in PAES, imidazolium group substituted PAES (IMD-y-PAES) shows the highest permselectivity but have similar conductivity with other membranes at the same degree of functionalization. The gross power density of IMD-30-PAES samples shows about 1.2 W m-2 which is higher than that of commercial cation exchange membrane.