Synthesis of Novel Alkaline Polymer Electrolyte for Alkaline Fuel Cell Applicaitons
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Development of the intrinsically OH- conductive polymeric electrolyte (alkaline polymer electrolyte, APE) is the critical component to enable the wide application of alkaline fuel cell (AFC) technology. Alkaline polymer electrolyte fuel cell (APEFC) based on AFC technology has been revived recently for applications in transportation and portable electronic devices due to its advantages of using non-noble metal catalysts, faster oxygen reduction in alkaline medium, and compact design. The research described in this dissertation aims to synthesize a novel APE, with controlled ionic conductivity and mechanical strength to achieve high fuel cell power density and long durability. Most APEs synthesized up to now use a modification of existing engineering polymer backbones, which are very difficult to balance its mechanical properties with its ionic conductivities. In this research, we copolymerized APE precursor polymers, namely poly (methyl methacrylate-co-butyl acrylate-co vinylbenzyl chloride) (PMBV) from three functional monomers, methyl methacrylate (MMA), butyl acrylate (BA) and vinylbenzyl chloride (VBC), where VBC was the functional group that was attached with trimethylamine (TMA) and was the OH- carrier after ion-exchanging. MMA was used for mechanical support and BA was used to alleviate the brittleness coming from MMA and VBC. We synthesized alkaline polymer electrolytes from bottom-up polymerization of these selected functional monomers using free radical solution and miniemulsion copolymerization techniques. By miniemulsion copolymerization, the properties of the obtained APEs could be precisely controlled by tuning the (1) monomer ratio, (2) glass transition temperature (Tg), (3) molecular weight (MW), and (4) crosslinking the copolymer. The increase in Tg was realized by eliminating BA from monomers, which was a low Tg component. MW was optimized through investigating binary copolymerization kinetics factors (initiator and surfactant). For crosslinking, the newly obtained poly (methyl methacrylate-co-vinylbenzyl chloride) (PMV) was crosslinked as a semi-interpenetrating network (s-IPN) to reduce water uptake and thus enhanced the mechanical strength in a humidified environment for APEFCs. After the optimization, our best quaternized PMBV (QPMBV) series APE membranes could reach a maximum power density of 180 mW/cm2 and the crosslinked QPMV APE could last 420 hours on APEFCs, which was among the best overall performance in APE technologies. In the future, we propose to use fluorinated polymer monomers to redesign the polymer backbone. Another direction in the design of APEs is to reselect the possible functional OH- carrier groups to make APEs more chemically and mechanically stable in a high pH environment. And last but not least, atomic force spectroscopy (AFM) is proposed to observe the APE nanostructure, the ionic conductive path, and the local mechanical strength by applying a small voltage between the tip and stage.