Byungchan Bae

Anion Conductive Block Poly(arylene ether)s: Synthesis, Properties, and Application in Alkaline Fuel Cells

Anion conductive aromatic multiblock copolymers, poly(arylene ether)s containing quaternized ammonio-substituted fluorene groups, were synthesized via block copolycondensation of fluorene-containing (later hydrophilic) oligomers and linear hydrophobic oligomers, chloromethylation, quaternization, and ion-exchange reactions. The ammonio groups were selectively introduced onto the fluorene-containing units. The quaternized multiblock copolymers (QPEs) produced ductile, transparent membranes. A well-controlled multiblock structure was responsible for the developed hydrophobic/hydrophilic phase separation and interconnected ion transporting pathway, as confirmed by scanning transmission electron microscopic (STEM) observation. The ionomer membranes showed considerably higher hydroxide ion conductivities, up to 144 mS/cm at 80 °C, than those of existing anion conductive ionomer membranes. The durabilities of the QPE membranes were evaluated under severe, accelerated-aging conditions, and minor degradation was recognized by (1)H NMR spectra. The QPE membrane retained high conductivity in hot water at 80 °C for 5000 h. A noble metal-free direct hydrazine fuel cell was operated with the QPE membrane at 80 °C. The maximum power density, 297 mW/cm(2), was achieved at a current density of 826 mA/cm(2).

Sulfonated polystyrene grafted polypropylene composite electrolyte membranes for direct methanol fuel cells

Abstract Polypropylene (PP)-g-sulfonated polystyrene (SPS) composite membranes were prepared by grafting polystyrene (PS) on microporous polypropylene membranes via plasma-induced polymerization. Grafting of polystyrene was established not only inside the pores but also on the surface of PP membranes, followed by the sulfonation reaction. The chemical and physical structure of PS-g-PP membranes was investigated using FTIR and SEM. The thickness and weight of the composite membrane increased with increasing grafting time. Ion exchange capacity (IEC), ion conductivity, and methanol permeability coefficient were measured and analyzed according to grafting reaction and sulfonation time. While both the ion conductivity and methanol permeability coefficient increased with grafting amount, the characteristic factor was comparable to that of Nafion®.

Synthesis and Properties of Sulfonated Block Copolymers Having Fluorenyl Groups for Fuel-Cell Applications

A series of sulfonated poly(arylene ether sulfone)s (SPEs) block copolymers containing fluorenyl groups were synthesized. Bis(4-fluorophenyl)sulfone (FPS) and 2,2-bis(4-hydroxy-3,5-dimethylpheny)propane were used as comonomers for hydrophobic blocks, whereas FPS and 9,9-bis(4-hydroxyphenyl)fluorene were used as hydrophilic blocks. Sulfonation with chlorosulfonic acid gave sulfonated block copolymers with molecular weight (M(w)) higher than 150 kDa. Proton conductivity of the SPE block copolymer with the ion exchange capacity (IEC) = 2.20 mequiv/g was 0.14 S/cm [80% relative humidity (RH)] and 0.02 S/cm (40% RH) at 80 degrees C, which is higher or comparable to that of a perfluorinated ionomer (Nafion) membrane. The longer hydrophilic and hydrophobic blocks resulted in higher water uptake and higher proton conductivity. Scanning transmission electron microscopy observation revealed that phase separation of the SPE block copolymers was more pronounced than that of the SPE random copolymers. The SPE block copolymer membranes showed higher mechanical properties than those of the random ones. With these properties, the SPE block copolymer membranes seem promising for fuel-cell applications.

Surface characterization of microporous polypropylene membranes modified by plasma treatment

Scanning electron microscope and goniometer were used to investigate morphology and wetting property of polypropylene membrane surfaces modified by plasma treatment using different reagents. Surface morphology was significantly affected by the types of reagents. X-ray photoelectron spectroscopy and attenuated total refection-Fourier transform infrared spectroscopy were used to characterize the chemical structure of polypropylene membrane surfaces modified by Freon-116 gas plasma treatment. Many fluorine atoms were observed on the polypropylene surface, and its concentration increased to saturation with increasing plasma treatment time. The wetting behavior of plasma treated polypropylene membrane was well explained in relation with morphology and chemical structure.

ATR-FTIR Study of Water in Nafion Membrane Combined with Proton Conductivity Measurements during Hydration/Dehydration Cycle

We have conducted combined time-resolved attenuated total reflection Fourier transform infrared (ATR-FTIR) and proton conductivity measurements of Nafion NRE211 membrane during hydration/dehydration cycles at room temperature. Conductivity change was interpreted in terms of different states of water in the membrane based on its δ(HOH) vibrational spectra. It was found that hydration of a dry membrane leads first to complete dissociation of the sulfonic acid groups to liberate hydrated protons, which are isolated from each other and have δ(HOH) vibrational frequency around 1740 cm(-1). The initial hydration is not accompanied by a significant increase of the proton conductivity. Further hydration gives rise to a rapid increase of the conductivity in proportion to intensity of a new δ(HOH) band around 1630 cm(-1). This was interpreted in terms of formation of channels of weakly hydrogen-bonded water to combine the isolated hydrophilic domains containing hydrated protons and hydrated sulfonate ions produced during the initial stage of hydration. Upon dehydration, proton conductivity drops first very rapidly due to loss of the weakly hydrogen bonded water from the channels to leave hydrophilic domains isolated in the membrane. Dehydration of the protons proceeds very slowly after significant loss of the proton conductivity.

Fluorene-containing cardo polymers as ion conductive membranes for fuel cells

For polymer electrolyte fuel cells, proton conducting electrolyte membranes are key materials. This review highlights aromatic polymers containing cardo groups and ionic functions as emerging electrolyte materials. First, the role of biphenyl fluorene groups on sulfonated polyimides is discussed. Discussion then focuses on how to balance proton conductivity and membrane stability with sulfonated poly(arylene ether)s making the most of the bulky cardo groups. Block copolymer structure containing fully sulfonated biphenyl fluorene groups in its hydrophilic component and compact hydrophobic component is proposed. Effect of superacid groups onto the properties of fluorene-containing polymers is discussed. Replacing sulfonic acid groups with ammonio groups afforded the cardo polymers highly anion conducting properties, which make them potentially applicable to alkaline fuel cells.

Sulfonated Poly(arylene ether sulfone ketone) Multiblock Copolymers with Highly Sulfonated Blocks. Long-Term Fuel Cell Operation and Post-Test Analyses

The stability of poly(arylene ether sulfone ketone) (SPESK) multiblock copolymer membranes having highly sulfonated hydrophilic blocks was tested in an operating fuel cell. The electrochemical properties and drain water were monitored during the test, followed by post-test analyses of the membrane. During a 2000-h fuel cell operation test at 80 °C and 53% RH (relative humidity) and with a constant current density (0.2 A cm(-2)), the cell voltage showed minor losses, with slight increases in the resistance. In the drain water, anions such as formate, acetate, and sulfate were observed. Post-test analyses of the chemical structure by NMR and IR spectra revealed that the sulfonated fluorenyl group with ether linkage was the most likely to have degraded during the long-term operation, producing these small molecules. The minor oxidative degradation only slightly affected the proton conductivity, water uptake, and phase-separated morphology.

Sulfonated Poly(arylene ether sulfone ketone) Multiblock Copolymers with Highly Sulfonated Block. Fuel Cell Performance

Poly(arylene ether sulfone ketone) (SPESK) multiblock copolymers having highly sulfonated hydrophilic blocks were synthesized and the fuel cell performance with the copolymers was investigated. A membrane electrode assembly (MEA) using an SPESK ionomer with an ion exchange capacity of 1.8 mequiv g(-1) as membrane and Nafion as the electrode binder showed comparable fuel cell performance and ohmic resistance to that using a Nafion NRE 211 membrane at 80 degrees C and 30% relative humidity (RH). A Nafion-free, all-SPESK MEA using SPESK as both the membrane and the binder was operable at 100 degrees C and 50% RH. The fuel cell performance was limited not only by the proton conductivity of the SPESK membrane but also by the low water flux through the membrane and specific adsorption of the ionomer on the platinum catalyst.

Preparation and Characterization of Nafion/Poly(1-vinylimidazole) Composite Membrane for Direct Methanol Fuel Cell Application

The base monomer, 1-vinyl imidazole was impregnated in a Nation 112 membrane and polymerized to poly(l-vinylimidazole) (PVI) by UV irradiation in order to reduce the methanol permeability of the Nation membrane when used in a direct methanol fuel cell. As the PVI content in the composite membrane was increased, the equilibrium water uptake decreased and the size of the hydrated ion clusters was reduced, as confirmed by small angle X-ray scattering analysis. The electrochemical properties of the membrane, such as its proton conductivity, methanol permeability, and electro-osmotic drag, were also affected by the equilibrium water uptake and hydrated pore size. Even a small amount of the base polymer incorporated into the membrane resulted in a significant effect on its proton conductivity and methanol permeability. The methanol transport induced by the electro-osmotic drag was evaluated and the methanol permeability and limiting current density data obtained in this study. Although the absolute number of the electro-osmotic drag was not determined, the trend of change is discussed in relation to the bulk-like water in the composite membranes. The novel composite membrane exhibited improved cell performance compared with a plain Nafion membrane due to its reduced methanol crossover rate.

Block poly(arylene ether sulfone ketone)s containing densely sulfonated linear hydrophilic segments as proton conductive membranes

Synthesis and properties of aromatic block copolymers composed of highly sulfonated phenylene ether sulfone ketone units as hydrophilic blocks and phenylene ether biphenyene sulfone units as hydrophobic blocks are reported. High molecular weight block copolymers 4 (Mw = 275–362 kDa and Mn = 76–144 kDa) with different compositions (number of repeat unit in the hydrophobic blocks (X) = 15, 30, or 60, and that of hydrophilic blocks (Y) = 4, 8, or 12) were synthesized. Transparent and bendable membranes were obtained by casting from the solution of 4. Due to the rigid rod-like structure of the hydrophilic blocks, the nanophase-separated morphology was not as distinct as that of the conventional sulfonated aromatic block copolymer membranes. Highly sulfonated hydrophilic blocks, which contained phenylene rings with sulfonic acid groups and electron-withdrawing sulfone or ketone groups, contributed to the high proton conductivity and improved oxidative stability of 4 membranes. The 4 (X60Y12) membrane with low IEC (1.18 mequiv g−1) showed comparable or higher conductivity than that of Nafion at >80% relative humidity (RH).