Zhaoxia Hu

Cross-Linked Sulfonated Polyimide Membranes for Polymer Electrolyte Fuel Cells

The SO 2 cross-linked membranes of sulfonated polyimides (SPIs) bearing sulfophenoxy side groups were prepared and evaluated as polymer electrolyte membranes for polymer electrolyte fuel cells (PEFCs). They maintained a high mechanical strength and proton conductivity after aging in water at 130°C for 500 h, indicating their high water stability. PEFCs with the SPI membranes showed high performances at 90°C under a high feed-gas humidity of 84% relative humidity (RH), and also fairly high performances even at a low feed-gas humidity of 30% RH due to the back-diffusion of water formed at the cathode. The PEFCs were operated under a constant current density of 0.5 A/cm 2 at 90°C and 84% RH for 1600 h without any reduction in cell performance. There was no change in the Fourier transform infrared spectra for the SPI membranes before and after the durability test. These results indicate that they have a high durability in PEFC operation. The cross-linked SPI membranes have a high potential for PEFCs at higher temperatures above 80°C.

Graft-crosslinked copolymers based on poly(arylene ether ketone)-gc-sulfonated poly(arylene ether sulfone) for PEMFC applications.

Novel poly(arylene ether ketone) polymers with fluorophenyl pendants and phenoxide-terminated wholly sulfonated poly(arylene ether sulfone) oligomers are prepared via Ni(0)-catalyzed and nucleophilic polymerization, respectively, and subsequently used as starting materials to obtain graft-crosslinked membranes as polymer electrolyte membranes. The phenoxide-terminated sulfonated moieties are introduced as hydrophilic parts as well as crosslinking units. The chemical structure and morphology of the obtained membranes are confirmed by (1) H NMR and tapping-mode AFM. The properties required for fuel cell applications, including water uptake and dimensional change, as well as proton conductivity, are investigated. AFM results show a clear nanoscale phase-separation microstructure of the obtained membranes. The membranes show good dimensional stability and reasonably high proton conductivities under 30-90% relative humidity. The anisotropic proton conductivity ratios (σ(formula see text) ) of the membranes in water are in the range 0.65-0.92, and increase with an increase in hydrophilic block length. The results indicate that the graft-crosslinked membranes are promising candidates for applications as polymer electrolyte membranes.

Alkaline stable anion exchange membranes based on poly(phenylene-co-arylene ether ketone) backbones

To improve the alkaline stability of anion exchange membranes (AEMs) based on conventional poly(ether ketone/sulfone) backbones, two novel poly(phenylene-co-arylene ether ketone) (PPAEK) ionomers with a quaternary ammonium (QA-) or imidazolium (IM-) group were prepared by zero-valent nickel-catalyzed coupling polymerization, benzylic bromination and in situ amination, successively. In particular, the cationic head-groups were incorporated as pendants into the polyphenylene moieties. Due to its higher water uptake and well-developed microphase separations, IM-PPAEK exhibited a better ion conduction ability than QA-PPAEK over the entire temperature range. However, QA-PPAEK displayed significantly higher alkaline stability in a 1 M NaOH aqueous solution at 80 °C, i.e., over 80% of its initial ion conductivity was still maintained after 1000 h of testing. The present work provides a promising new synthetic approach to obtain high performance AEMs based on chemically stable polyphenylene units.

Multiblock sulfonated poly(arylene ether sulfone)s with fluorenyl hydrophilic moieties for PEMFC applications

Multiblock sulfonated poly(arylene ether sulfone) (MB-SPAES) ionomers containing fluorenyl hydrophilic moieties were synthesized and converted into proton exchange membranes (PEMs) through solution casting, as well as other two types of MB-SPAES membranes containing hydrophilic biphenyl and hexafluoroisopropyl diphenyl moieties for the comparison. Chemical structures of the MB-SPAES ionomers were confirmed by 1H NMR spectrometer. Fundamental physical properties were characterized based on the ionic group content, the hydrophilic/hydrophobic block structure and length, including ion exchange capacity (IEC), water uptake, size change, mechanical property, proton conductivity, hydrolytic stability and fuel cell performance. All the obtained MB-SPAES membranes were transparent and mechanical ductile, suitable for PEM applications. Water uptake and size change results showed that the MB-SPAES membranes containing fluorenyl hydrophilic moieties absorbed less water and swelled smaller in water than the other two types at similar IECs, indicating their better dimensional stability. Proton conductivity and hydrolytic stability results indicated that the fluorenyl hydrophilic moieties were also favorable to gain better proton conductivity and hydrolytic stability.

Fluorinated poly(ether sulfone) ionomers with disulfonated naphthyl pendants for proton exchange membrane applications

Proton exchange membranes based on fluorinated poly(ether sulfone)s with disulfonated naphthyl pendants (sSPFES) have been successfully prepared by post functionalization through polymeric SNAr reaction. Copolymer structure was confirmed by H-nuclear magnetic resonance spectroscopy and Fourier transform infrared spectroscopy, the physico-chemical properties of the sSPFES membranes were evaluated by thermogravimetric analysis, gel permeation chromatography, electro-chemical impedance spectroscopy, atomic force microscopy, Fenton, water-swelling and fuel cell test. The pendant grafting degree was controlled by varying the feeding amount of the disulfonaphthols, resulting in the ion exchange capacity about 1.28-1.73 mmol/g. The obtained sSPFES membranes were thermal stable, mechanical ductile, and exhibited dimensional change less than 17%, water uptake below 70%, and proton conductivity as high as 0.17-0.28 S/cm at 90°C in water. In a single H2/O2 fuel cell test at 80°C, the sSPFES-B-3.2 membrane (1.61 mmol/g) showed the maximum power output of 593–658 mW/cm2 at 60%-80% relative humidity, indicating their rather promising potential for fuel cell applications.

Synthesis and Properties of Novel Post Cross-Linked Sulfonated Poly (arylene Ether sulfone)s for Proton Exchange Membrane Fuel Cell Applications

A series of novel cross-linked sulfonated poly(arylene ether sulfone)s were prepared. The membrane performance were significantly improved after cross-linking treatment, for example, the cross-linked membranes showed lower water uptake and lower swelling ratio than the corresponding original ones. With the increase of cross-linking extent, the water uptake and dimensional change decreased, while reasonably high proton conductivity was maintained.