Multiblock copolymers containing hydrophobic and hydrophilic segments for anion exchange membranes

Abstract

Alkaline fuel cells have the potential to provide a cost effective conversion of energy compared to proton exchange membrane fuel cells that require more expensive noble metal catalysts. The anion exchange membrane (AEM) is an important component that separates electrodes and provides a conducting medium for anions. Proper design of hydrophobic-hydrophilic copolymer membranes that are highly hydroxide conductive yet remain chemically and physically stable under the highly basic conditions is required. Polysulfone (PSf) is an attractive material owing to it excellent mechanical properties and film forming ability, thermal and chemical stability that it is readily modified to bear a variety of functional groups. Multiblock PSf copolymers have been investigated for several decades and for many applications. More recently, hydrophobic-hydrophilic PSf multiblock copolymers have been studied as AEMs and for application in electrochemical applications including alkaline fuel cells. The research in this thesis was focused on the development of new multiblock PSf copolymer materials for AEMs that address current design challenges, but also provide additional insight to the structure-property relationships that will lead to improved AEM performance and base stability. Two main PSf multiblock copolymer systems were investigated. Multiblock copolymers composed of bisphenol A PSf segments and tetramethylbisphenol A (TMBPA) polyformal (PF) segments were studied. The TMBPA PF segments provided compact repeat units of TMBPA with methylene linkages and the ability to bear up to four benzyltrimethylammonium cations. The solution processible PSf quaternary ammonium functionalized PF (PSf-QAPF) multiblock copolymers were shown to form hydroxide conductive, well-connected phase separated membranes with excellent mechanical properties. An alternative polymer system was investigated to prepare more alkaline stable materials. Cylcopolymerization of N,N-diallylpiperidinium chloride was developed to prepare spirocylcic ammonium telechelic oligomers of poly(diallylpiperidinium) (PDApip). Copolymerization of the oligomers with PSf monomers was done followed by ion exchange to provide multiblock PSf-PDApip copolymers that were thermally stable up to 360 °C and highly conductive in the hydroxide form (up to 102 mS·cm-1 at 80 °C). The PDApip homopolymer was exceptionally base stable over 1000 hrs. A series of PSf-PDApipOH multiblock copolymers with similar composition and ion exchange capacity with variation in PDApip block length were prepared and investigated to elucidate the impact of hydrophilic block length on membrane performance. The PSf-PDApip copolymer membranes displayed high hydroxide conductivity under humidified gas conditions, and the conductivity increased with increasing PDApip segment length, which was related to the morphology studied by tapping mode-atomic force microscopy.