Preparation Of Novel Polymer Electrolyte Membranes Based On Poly(Arylene Ether Ketone) For Fuel Cells

The fuel cell is receiving tremendous attention as an alternative to existing powerconversion systems because of its high efficiency and environmental friendly nature.Among several types of fuel cells, proton exchange membrane fuel cells (PEMFCs)have attracted major research activities in the past decade due to their promise fortransportation, residential appliance and portable power applications. Superior protonconductivity, thermal and chemical durability, and oxidative stability have madeperfluorinated sulfonic acid containing ionomers (Nafion) as commonly usedelectrolytes in PEMFCs. However, their high cost, loss in proton conductivity atelevated temperatures or low relative humidity (RH) and high methanol permeabilityhinder their applications in fuel cells. Hence, great efforts have been devoted to thedevelopment of alternative proton exchange membranes. Among these potentialalternatives, sulfonated poly(arylene ether ketone)s (SPAEKs) have beenextensively studied. SPAEKs have also been explored as one of alternative PEMmaterials due to their relatively low production costs, excellent chemical and thermalproperties, as well as low methanol crossover. However, the membranes require ahigh sulfonation level to obtain high proton conductivity. Too high loading of acidicgroups could lead to undesirable large swelling and thus result in a dramatic loss ofmechanical properties. Cross-linking is a powerful and simple method to preparesemi-interpenetrating polymer network membranes by mixing two polymers. In thisway, after a cross-linking treatment the polymer matrix forms a network in which themacromolecular chains of the polyelectrolyte are immobilized and compacted. The polymer electrolyte material is divided into two categories: proton exchangemembranes (PEMs) and anion exchange membranes (AEMs). The proton exchangemembrane is usually a sulfonic acid type, with a fixed group and detachable ions-protons. The dissociation of sulfonic acid in cation exchange membrane requireswater, then the protons travels with water molecules as carrier. Therefore the sulfonicacid type membranes can only be used under low temperature conditions. Unlike theacid PEMs, benefiting from the alkaline media, the electrocatalysis properties ofAEMs were greatly improved. The opposite direction of electro-osmosis reduces themethanol crossover efficiently. These advantages of AEMs can result in higherefficiency and enable the usage of non-precious metal catalysts, greatly reducing thecost of the device.In the second chapter, we chose poly (arylene ether ketone)s bearing carboxylicacid groups (SPAEK-C) with high DS as polymer backbones. Then the SPAEK-C canbe cross-linked at the-COOH site with PVA. The advantage of this system is that thesulfonic acid groups are not involved in the cross-linking reaction. Thus, thecross-linked membranes not only exhibit low water swelling and low methanolcrossover, but also achieve reasonable proton conductivity. Fourier transform infraredspectroscopy is used to characterize and confirm the structure of SPAEK-C and thecross-linked membranes. The proton conductivity of the cross-linked membrane with15%PVA in weight reaches up to0.18S cm-1at80oC (100%relative humidity),which is higher than that of Nafion membrane, while the methanol permeability isnearly five times lower than Nafion. The ion-exchange capacity, water uptake andthermal stability are investigated to confirm their applicability in fuel cells.In the third chapter, we report a new approach for obtaining covalentlycross-linked PEM materials based on a coupling reaction between highly reactiveo-diamino functional PBI oligomers and SPAEK-C. Then the SPAEK-C/PBI blendmembranes were dried at110C for3h to induce cross-links between amino groupsof PBI oligomer and carboxylic acid groups of SPAEK-C.1H NMR measurementsand Fourier transform infrared spectroscopy were used to characterize and confirm the structures of SPAEK-C, PBI oligomers and the cross-linked membranes. Thecross-linked membranes were immersed in various concentrations of phosphoric acidsolution to enhance proton conductivity. These H3PO4doped SPAEK-C/PBImembranes displayed comparable or even higher proton conductivity than that ofNafion117, especially under low humidity conditions. Moreover, the water uptake,acid uptake, mechanical and thermal stability were also investigated in detail.In the fourth chapter, a poly(arylene ether ketone) with3,3′,5,5′-tetramethyl-4,4′-dihydroxybipheny moiety (PAEK-TM) was firstly synthesized. Asa class of engineering thermoplastic materials, PAEK-TM displays high thermalstability, advanced mechanical properties, and excellent resistance to hydrolysis andoxidation. The preparation of anion exchange membranes comprised convertingbenzylic methyl groups to bromomethyl groups by a radical reaction, followed byfunctionalization of the bromomethylated PAEK with alkyl imidazoles, i.e. methyl,butyl and vinyl imidazoliums. The structure of imidazolium functionalized PAEK wasproved by1H NMR spectra. A class of flexible and tough membranes was thenachieved by subsequent film-forming and anion exchange processes. The membranesshowed water uptake and hydroxide conductivity that are comparable or superior tothose of quaternary ammonium (QA) anion exchange membranes. This workdemonstrated a new route for non-QA anion exchange membrane design, avoiding thechloromethylation reagent and precisely controling over the degree and location ofimidazolium groups.