Study On Carbonyl And Sulfone Groups Co-crosslinked Sulfonated Polyimide Proton Exchange Membranes

Proton exchange membrance fuel cell (PEMFC) is a promising energy device thatcontinuously converts chemical energy into electric energy. The proton exchangemembrance (PEM) is one of the key components in PEMFC. The state-of-the-artperfuorosulfonic polymer membranes such as Nafion have excellent properties of highproton conductivity and chemical stability. However, there are some disadvantages such aslow operation temperature and high fuel permeability limited their further application.Studies foucused on developing alternative PEMs based on sulfonated aromatic polymershave attracted much interesting.Sulfonated polyimides(SPIs) with six-membered imide rings have been reported aspromising candidates with good film forming and high mechanical properties. However,poor water stability of SPIs limited their further applications. The proton conductivity ofSPI-based PEMs was rather low due to their low ion exchange capacity (IEC) value, whichwas essential for maintaining membrane durability toward water. The water stability ofSPIs were largely improved by structure-property studies, and chemical cross-linkingfurther improved the performance of SPI-based PEMs. In the previous study, the sulfonegroup crosslinking SPI PEMs were developed by a simple process of immersingmembranes in PPMA(phosphorus pentoxide solution in methanesulfonic acid in the ratio of1/10by weight) solution. However, the process exist the problems of low crosslinkingdegree and proton conductivity clearly decreased due to the consumption of sulfonic acidgroups in PEMs.In this study, diamine monomer containing carboxylic acid groups,3,5-bis(4-aminophenoxy) benzoic acid (BAPBa) and sulfonated side-chain type diamine,2,2’-bis(4-sulfophenoxy)benzidine (BSPOB) were prepared. A series of SPIs have beensynthesized by condensation polymerization of1,4,5,8-naphthalenetetracarboxylicdianhydride (NTDA) with BSPOB, BAPBa and3,3’-bis(4-aminophenoxy)benzene(BAPBz)) in m-cresol. SPI-based PEMs were obtained by casting method. The carbonyland sulfone groups co-crosslinked PEMs were prepared by immersing proton form SPImembranes into PPMA.Their basic properties including crosslinking degree, mechanical properties, wateruptake, size-change, proton conductivity and water stability were investigated. The relativeviscosity of synthesized SPIs were higher than7.4dL/g indicating high molecular weight of the polymers. The crosslinking degree of crosslinked SPI PEMs was increased with lowerconsumption of sulfonated groups. The co-crosslinked SPI PEMs showed excellentmechanical properties. Water uptake was obviously reduced after crosslinkingprocess.These SPI PEMs showed low in-plane dimensional change in dry/wet environment,which is beneficial for membrane electrode assembly forming. The carbonyl and sulfonegroups co-crosslinked SPI PEMs showed higher proton conductivity when compared to thesulfone groups crosslinked ones. For example, the value of co-crosslinked SPI PEM was7.8mS cm-1at60oC and50%RH, which was28%higher than that of sulfone crosslinkedSPI PEM. The crosslinking process in this study further improved the water stability of SPIPEMs. After aging in water at130°C for500h, the crosslinked PEMs showed goodmechanical properties and the proton conductivity barely decreased.Fuel cell performance of carbonyl and sulfone groups co-crosslinked SPI PEM wasinvestigated comparing to the sulfone groups crosslinked SPI PEM as well as NR212. Thecell voltage of co-crosslinked SPI PEM was0.62V at1.0A cm-2, while sulfone groupscrosslinked SPI PEM and NR212were0.57V and0.61V, respectively. The maximumpower output of o-crosslinked SPI PEM was0.85W/cm2, which was comparable to that ofNR212, and1.3times higher than that of sulfone groups crosslinked SPI PEM due to thehigher proton conductivity. PEMFC durability performance at110°C showed that the cellvoltage was maintained in the range of0.62V to0.66V for300h.