Cross-Linking Density Effect of Fluorinated Aromatic Polyethers on Transport Properties

Abstract

Novel sulfonated poly(arylene ether) copolymers containing cross-linking moiety and fluorine atoms were prepared for proton exchange membranes of fuel cells. The copolymers were synthesized using potassium 2,5-dihydroxybenzenesulfonate (SHQ), 4,4′-(hexafluoroisopropylidene)diphenol (6F-BPA), decafluorobiphenyl (DFBP), and 1-ethynyl-2,4-difluorobenzeneand (CM) as a cross-linking moiety through polycondensation. The chemical structures of the cross-linkable copolymers were analyzed by 1 H NMR, 19 F NMR, and FTIR-ATR spectra. Cross-linked membranes are obtained by thermal curing of the cross-linkable copolymers. The thermal properties of the cross-linked membranes were characterized by TGA and DSC. Intrinsic viscosity and gel fraction were also evaluated in order to measure the molecular weight and cross-linking density, respectively. The cross-linked network structure greatly suppressed the water uptake and swelling of the cross-linked membrane. The proton conductivity and methanol permeability of the membranes decreased with increasing cross-linking moiety. The decreases in the proton conductivity and methanol permeability are mainly caused by the enhanced barrier properties of the membranes due to the introduction of CM. In other words, the membranes blocked the channels for the passage of water and methanol. The proposed membranes showed moderate proton conductivity and significantly lower methanol permeability when compared to Nafion 212. The SFPE90-CM20 membrane in particular showed low water uptake (37.5%), high proton conductivity (0.091 S/cm), and low methanol permeability (37 × 10 -8 cm 2 /s). The selectivities of all the cross-linked membranes, that is, the ratio of proton conductivity to methanol permeability, fell in the range of 122 × 10 3 -587 × 10 3 S • s/cm 3 , and they were thus also much higher than the selectivity of Nafion 212 (56 × 10 3 S • s/cm 3 ).

Sulfonated hydrocarbon membranes for medium-temperature and low-humidity proton exchange membrane fuel cells (PEMFCs)

“…In addition to the curing time, the curing temperature in self-crosslinkable systems is a critical factor in determining their crosslinking degree. Recently, self-crosslinkable polymers have been reported in the literature [166][167][168][169]. Representative examples are SPEEK and SPPSf [166,167] where a portion of the sulfonic acid groups (crosslinking sites) present in the aromatic backbone is sacrificed for thermal conversion into sulfone bridge groups to cure the polymer matrices.…”

“…g −1 [167]) can be a solution to compensate this unavoidable proton conductivity loss. The self-crosslinkable polymer systems also include SPAEs with ethynyl crosslinkable groups randomly located within the polymer backbone [168] or positioned at their terminal ends [169]. The ethynyl groups in SPAEs were effective in removing the thermal bottleneck observed in the aforementioned BPS-FPEB curing system.…”

Acid–base blend membranes consisting of sulfonated poly(ether ether ketone) and 5-amino-benzotriazole tethered polysulfone for DMFC

“…However, covalently cross-linked polymers have a tendency to become brittle in the dry state, which is a critical problem for fuel cell application. The brittleness is possibly caused by the inflexibility of the covalent networks [12][13][14][15]. Because of this reason, more attention is being directed towards the development of ionomer networks containing ionically cross-linked blend systems such as SPPO (poly(2,6-dimethyl-1,4-phenylene oxide))/PBI (polybenzimidazole respectively), SPEEK/PBI, and SPSf/aminated PSf, which are more flexible [16][17][18][19].…”

Poly[2,2′-(m-phenylene)-5,5′-bibenzimidazole] and poly[6-fluoro-3-(pyridin-2-yl)-3,4-dihydro-2H-benzoxazine] based polymer electrolyte membranes for fuel cells at elevated temperature

“…Gel fraction was measured by solvent extraction. 20 The films, 2 cm×2 cm in size, were dried at 70 o C under vacuum for 2 days and weighed (W 1 ), and then the dried films were soaked in an excess of 85 wt% PA solution at 160 o C for 12 h. The films after extraction were washed with distilled water several times. The films were dried for 2 days in a vacuum oven until constant weight and reweighed (W 2 ).…”