Synthesis and Characterization of Poly(arylene ether sulfone) Membranes Having Cross-linked Structures and Poly(arylene ether sulfone) Composite Membranes Having Silica and Graphene Oxide Fillers for Fuel Cell Applications

Abstract

Novel sulfonated poly(arylene ether sulfone) (SPAES) based materials were prepared and characterized for their application in proton exchange membranes fuel cells (PEMFCs) in this study. Firstly, Sulfonated poly(arylene ether sulfone) membranes with cross-linked structures (C-SPAES) were simply prepared by simultaneously casting and heating the polymer solutions composed of sulfonated poly(arylene ether sulfone) with azidomethyl side groups (SPAES-N3), cross-linkers such as 1,4-diethynylbenzene and 4,4′-diazido-2,2′-stilbenedisulfonic acid disodium salt tetrahydrate, and a click reaction catalyst such as CuBr and N,N,N′,N″,N″-pentamethyldiethylenetriamine in N,N-dimethylacetamide, where SPAES-N3 were prepared by the substitution of SPAES through chloromethylation followed by azidation reaction. C-SPAES membranes obtained using the optimum amount of the cross-linkers showed much improved chemical and physical stabilities and mechanical strength compared with linear SPAES membrane. Since the cross-linked structures were formed by the cross-linker having sulfonic acid groups, C-SPAES membranes showed higher ion exchange capacity and proton conductivity than the linear SPAES membrane. Although the C-SPAES membrane can absorb more water than the linear SPAES membrane, less volume expansion was observed due to their physically stable cross-linked structures. Secondly, sulfonated poly(arylene ether sulfone) (SPAES) composite membranes were prepared with core-shell silica particles having poly(4-styrenesulfonic acid) (PSSA) and poly(4-vinylpyridine) (P4VP) in shell layers named S-Si and P-Si, respectively, to investigate the effect of acidic and basic silica fillers on membrane properties. The core-shell silica particles were obtained by hydrolysis of vinyltrimethoxysilane followed by radical polymerization of vinyl monomers (4-styrenesulfonic acid sodium salt and 4-vinylpyridine) on the silica particle with vinyl groups. Incorporation of S-Si and P-Si into SPAES increased dimensional stability, mechanical strength, and proton conductivity of the membranes. In particular, P-Si was found to be more effective filler materials to improve these properties by additional well-connected hydrophilic channels having sulfonate/pyridinium structures formed around the silica particles through the acid-base interaction between the pyridine groups of the P4VP shell and the sulfonic acid groups of SPAES. Finally, sulfonated poly(arylene ether sulfone) (SPAES) composite membranes were prepared using thermally treated graphene oxide (GO) and poly(2,5-benzimidazole)-grafted graphene oxide (ABPBI-GO) as fillers for proton exchange membrane fuel cell (PEMFC) applications. Pristine graphene oxide was obtained from graphite by chemical oxidation, and then 3,4-diaminobenzoic acid was reacted with pristine graphene oxide to obtain ABPBI-GO. When and ABPBI-GO were incorporated into the SPAES matrix, the dimensional stability and mechanical strength of the membrane were improved. In particular, SPAES/ABPBI-GO composite membranes exhibited an improved dimensional stability, larger Youngs modulus value, and larger elongation at break value than SPAES/GO composite membranes due to the acid-base interaction between the sulfonic acid group of the SPAES matrix and the basic imidazole unit of ABPBI-GO. In addition SPAES/ABPBI-GO composite membranes possessed a higher proton conductivity than pristine SPAES and SPAES/ABPBI-GO composite membranes because the acid-base interactions can generate additional proton conduction pathways in the membrane structures.