Partially sulfonated Poly(arylene ether sulfone)/organically modified metal oxide nanoparticle composite membranes for proton exchange membrane for direct methanol fuel cell

Organic/inorganic composite proton exchange membranes (PEMs) of sulfonated poly(arylene ether sulfone) (sPSF) and organically modified metal oxide nanoparticles are demonstrated. The sPSF ionomers with three different degree of sulfonations (DS) (39, 42, 48%) were synthesized by condensation polymerization, and nanocrystalline titania and zirconia particles were respectively prepared by sol–gel reactions catalyzed by p-toluene sulfonic acid (PTSA) in the presence of acetylacetone (AcAc) as an organic surface modifier. Through structural analyses, non-aggregated anatase titania with ∼7 nm average size and tetragonal zirconia with ∼4 nm size were confirmed. The transparent composite membranes with 1 wt% nanoparticle contents were obtained by simply mixing sPSF and the respective nanoparticles in dimethylsulfoxide (DMSO) followed by membrane casting. Post-treatment in aqueous sulfuric acid resulted in the composite membranes exhibiting reduced proton conductivities and highly improved methanol permeabilities and water uptakes compared to the prestine sPSF membranes. Overall, the zirconia-containing sPSF (with 48% DS) composite membrane exhibited the best PEM property and an active mode DMFC performance tests on the membrane-electrode assemblies (MEA) with various PEMs also revealed that the zirconia nanocomposite membrane exhibits the best power density. The maximum power density from zirconia/sPSF48 composite MEA was measured to be 73 mW/cm2 at 60 °C with 1 M methanol fuel which is 7% higher than that of Nafion-115 MEA. The improved PEM properties of the composite membranes developed in this study can be attributed to the effective barrier effect of both titania and zirconia nanoparticles provided by their small particle sizes (<10 nm) without significant aggregation within the sPSF matrix, and also to the hydrophilic nanoparticle surfaces to enable the improved water retaining properties for the composited PEM.

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