Synthesis And Properties Studies Of Novel Phosphoric Acid-doped Quaternary Ammonium Containing Poly(Arylene Ether Ketone) For High-temperature Proton Exchange Membranes

Fuel cell is an electrochemical energy conversion device, with fast starting rate at lowtemperature, high energy conversion efficiency, low pollutant emission and other advancedfeatures, which is currently considered the most likely choice for green alternative to coal,oil, natural gas and other non-renewable energy resources. The fuel cell conducts ionsthrough the internally ion exchange membrane, to form a complete closed loop with anexternal circuit. The ion exchange membranes as the electrolyte material, determining therate of energy conversion, are the core portion of the fuel cell. The industrialization processwas affected mainly by the lack of commercial electrolyte membranes with comprehensiveproperties. Therefore, the development of high-performance electrolyte membrane materialis the primary task for the development of high-performance fuel cell.The polymer electrolyte material could be divided into two categories: protonexchange membranes and anion exchange membranes. The proton exchange membranes areusually sulfonic acid type, with a fixed group and detachable ions-protons. The sulfonicacid was detached with the help of water, then the protons transfers with water molecules asthe carrier. Therefore the sulfonic acid type membranes can only be used under lowtemperature conditions. Cationic exchange membranes using phosphoric acid as conductionmedia received extensive attention recently since at high temperature under anhydrousconditions, the fuel cells were found with a higher catalytic efficiency, CO tolerance, energyconversion efficiency and a simplified water treatment and cooling systems. However, suchproton exchange membranes also have a prominent drawback: the phosphoric acid is easyto drain from composite membranes, resulting in unstable fuel cell performance. Therefore, anchoring of phosphate to the polymer matrix becomes one of the hot spots of today’shigh-temperature fuel cells researchs.Quaternary ammonium type poly(arylene ether ketone)(QPAEK) can absorbphosphoric acid based on the acid-base and covalent interactions to conduct protons throughthe membranes. With higher phosphoric acid adsorption capacity, more simple preparationprocess and more variable structures, QPAEK as high temperature proton exchangemembrane matrix has being recognized gradually. The brominated polyarylene ether ketonepolymer (BrPAEK) with benzyl bromide has significant reaction activity: on the one hand, anew quaternary ammonium derivative can be obtained from the nucleophilic reaction ofbenzyl bromide with a tertiary amine; on the other hand, it can act as cross-linkablefunctional groups to prepare cross-linked membranes with active diamine monomer. In thisthesis, BrPAEK was synthesized. Then, a series of target BrPAEK with differentbrominated ratio were obtained by controlling the reaction conditions. Thus QPAEKmembranes were either prepared from homogeneous amination method or from soakingamination method. The homogeneous amination method can effectively control the extentand location of the quaternization. And the chemical stability, mechanical properties,thermal decomposition temperature and the micro-phase structure of the resulting QPAEKmembranes were analyzed in detail.The third chapter discusses nature of phosphoric acid doped poly (aryl ether ketone)membrane (PA/QPAEK), with QPAEK as the matrix and with PA as the conductionmedium. First, this chapter discusses the PA content, water content, and PA doping abilityof the composite membranes. The doping level of the resulting PA/QPAEK membranesincreases with both the concentration level of soaking PA solution and the ion exchangecapacity (IEC) of QPAEK. For example, the highest doping level of composite membranesis28.6, derived from QPAEK-5with an ion exchange capacity of2.02mmol g-1, which isreally higher than that of polybenzimidazole (PBI). Secondly, this chapter investigates themechanical properties and the proton conductivity of the composite membranes.Experimental results revealed that the proton conductivity of the composite membranesincreased with the improvement of PA doping content, but its mechanical properties are on the decline. As a result, the PA/QPAEK membranes showed the highest conductivity of59mS cm1at200oC under anhydrous condition. This result is comparable to that of thePA/PBI composite membranes prepared from the Gel process. All the results demonstratedPA/QPAEK composite membranes with a high application prospects in high temperaturefuel cellHowever, there is still obvious dimension swelling and dramatically decline ofmechanical properties of PA/QPAEK composite membranes with adequate PA uptake. Inorder to solve the above problems, the chapters IV and V introduced two methods toprepare PA doped cross-linked poly(aryl ether ketone) membranes (PA/cQPAEK).In the fourth chapter, BrPAEK containing two benzyl bromides for each repeating unitis synthesized, then, a dilute solution of QPAEK is prepared through a further homogeneousquaternary ammonium reaction. Active diamine monomer is introduced into the dilutesolutions, then, a cross-linked QPAEK membrane is prepared. We have discussed about twokinds of cross-linked QPAEK membranes with different corss-linkers, respectively. Theresults indicate that the diamine-cross-linked membranes using the rigid cross-linker showmuch improved properties than the other kind. The PA doping level of thediamine-cross-linked membranes is significantly improved (increased from17.9to31.2).Furthermore, the chemical and mechanical stability and the high-temperature protonconductivity of the diamine-cross-linked membrane are also improved significantly. Fuelcell performance was conducted, and the maximum performance of diamine-cross-linkedmembrane is observed at180oC with a current density of1.06A cm-2and the peak powerdensity of323mW cm-2, proving PA/QPAEK composite membrane the most potentialproton exchange membrane for high temperature fuel cells. In the fifth chapter, BrPAEKwith1.7benzyl bromide for each repeating unit was prepared and the Gel of BrPAEK wasobtained using the highly reactive tertiary amine, resulting in a free-volume enhancedmembranes. Then the Gel-QPAEK membranes with enhanced free-volume are obtainedusing conventional soaking amination method. The infrared spectrum, density testing anddynamic mechanical thermal analysis proved its structure. The Gel method, on the one hand,improved the PA adsorption capacity by the increased free-volume, thereby increasing the proton conductivity of the membrane; on the other hand, the introduction of cross-linkingagent induces a three dimensional network structure in the membrane, improving themechanical stability and the chemical stability of the membrane qualitatively. Thefeasibility of free-volume enhanced cross-linked membranes in high temperature fuel cellswas also discussed, indicating it having a protruding comprehensive performance in thehigh-temperature fuel cell field.