Synthesis and Characterization of Sulfonated Poly(arlyene ether sulfone) based Polymeric Materials and Their Application in Fuel Cells Operating at High-Temperature and Low Humidity Conditions

Abstract

This study presents synthesis and characterization of sulfonated poly(arylene ether sulfone) (SPAES) based polymeric materials for application in polymer electrolyte membrane fuel cells operating at high-temperature and low humidity conditions. Especially, SPAES-based polymer electrolyte membranes having various structures have been described in detail. Firstly, semi-interpenetrating polymer network (semi-IPN) membranes based on SPAES are developed for application in polymer electrolyte membrane fuel cells operating at high temperature (> 80 °C) and low relative humidity (< 50% RH) conditions. Two types of semi-IPN membranes using different cross-linkers are simply prepared by in-situ casting and thermal-initiated radical polymerization of vinyl phosphonic acid (VPA) and two kinds of cross-linkers such as diethylene glycol dimethacrylate (DEGDMA) and bis(2-(methacryloyloxy)ethyl) phosphate (BMAEP), respectively, in N,N-dimethylacetamide solutions of SPAES. The incorporation of VPA units into the SPAES based membrane system improves the proton conductivity especially at high temperature and low humidity conditions. In addition, all the cross-linkers such as DEGDMA and BMAEP, prevent the decrease in the mechanical and chemical stabilities by the presence of aliphatic linear poly(vinyl phosphonic acid) chains in the semi-IPN membranes. Furthermore, the semi-IPN membrane using BMAEP as the cross-linker can prevent the decrease of the proton conductivity by the formation of cross-linked structures because the additional phosphonic acid group in BMAEP can make the additional proton conducting pathways in the semi-IPN membrane. The fuel cell performances of membrane-electrode assemblies (MEAs) prepared with the semi-IPN membranes using DEGDMA (180 mW cm-2 at 120 °C and 40% RH ) and BMAEP (187 mW cm-2 at 120 °C and 40% RH) are found to be superior to that of the MEA from the SPAES membrane (145 mW cm-2 at 120 °C and 40% RH). Durability test results at the operating conditions indicate that all the semi-IPN membranes are electrochemically very stable maintaining the low hydrogen crossover and high power densities. Secondly, a series of pore-filling membranes are prepared by impregnating porous cross-linked benzoxazine-benzimidazole copolymer P(pBUa-co-BI) substrates with SPAESs having different degree of sulfonation for polymer electrolyte membrane fuel cells operating at high-temperatures (> 80 °C) and low-humidity (< 50% RH) conditions. The SPAESs are synthesized by reacting 4,4-dihydroxybiphenyl with the mixtures of disulfonate-4,4-difluorodiphenylsulfone and 4,4-difluorodiphenylsulfone in different ratios. The porous P(pBUa-co-BI) substrates are prepared by extracting dibutyl phthalate (DBP) included in P(pBUa-co-BI) films using methanol. The P(pBUa-co-BI) films are prepared by stepwise heating the casted N,N-dimethylacetamide solution containing the mixtures of poly[2,2′-(m-phenylene)-5,5′-bibenzimidazole] (PBI), 3-phenyl-3,4- dihydro-6-tert-butyl-2H-1,3-benzoxazine (pBUa), and DBP to 220 °C. The pore-filling membranes are found to have much improved dimensional stability and mechanical strength compared with the SPAES membranes. Although the proton conductivity values of the pore-filling membranes are slightly smaller than those of the SPAES membrane, their cell performance is superior to that of the SPAES membrane at 120 °C and 40% RH conditions because ultrathin pore-filling membranes (15-20 µm) having high mechanical strength can be prepared and they can contain a larger content of chemically-bound water Thirdly, proton conductive porous substrates consisting of cross-linked benzoxazine-benzimidazole copolymers are developed for practical application of reinforced pore-filling membranes in polymer electrolyte membrane fuel cells operating at high-temperatures (> 80 °C) and low relative humidity (< 50% RH) conditions. The porous proton conductive substrates are prepared by casting solution mixtures of sodium 3-(4-sulfonatophenyl)-3,4-dihydro-2H-1,3 benzoxazine-6-sulfonate (pS) and poly[2,2′-(m-phenylene)-5,5′-bibenzimidazole] (PBI) with dibutyl phthalate (DBP) as a porogen in N,N-dimethylacetamide, followed by subsequent stepwise heating to 220 °C and extraction of DBP from the P(pS-co-BI) films using methanol. The resulting porous substrates are found to have mechanically robust cross-linked structures, tunable hydrophilicity, and reasonably high proton conductivity. A pore-filling membrane is prepared by impregnating the porous substrate with SPAES having the degree of sulfonation of 70 mol%. The pore-filling membrane exhibits much improved dimensional stability and mechanical strength compared to the linear SPAES membrane and its proton conductivity and cell performance are found to be superior to the pore-filling membrane prepared using the porous substrate based on cross-linked benzoxazine-benzimidazole copolymers without any proton conductive acid groups. Finally, we propose a simple and effective cross-linking technology for the design of a high performance cross-linked SPAES (C-SPAES) membrane using perfluoropolyether (PFPE) as a novel cross-linker for fuel cell applications. The C-SPAES membrane is prepared by in-situ casting and heating the polymer mixture solution of SPAES with chloromethyl side groups and PFPE. The C-SPAES membrane shows much improved physicochemical stability and comparable proton conductivity compared with the SPAES membrane due to the finely phase-separated morphology induced from the cross-linked polymer network structure using PFPE. Under practical operating conditions of automotive fuel cells (90 °C, 50% RH, and 150 kPa), membrane electrode assembly from the C-SPAES membrane shows an outstanding cell performance (1.17 W cm-2 at 0.65 V) compared with that from the SPAES membrane (0.85 W cm-2 at 0.65 V) mainly due to the enhanced interfacial compatibility between the C-SPAES membrane and electrode surfaces.