Syntheses of Block Polyelectrolyte and Their Applications for Ion Conducting Membranes in Fuel cells

• Main Authors: Hsing-Chieh Lee, 李興傑
• Other Authors: Chi-Yang Chao
• Format: Others
• Language: en_US
• Published: 2013
• Online Access: http://ndltd.ncl.edu.tw/handle/38034954161636813003

博士 === 國立臺灣大學 === 材料科學與工程學研究所 === 101 === Block polyelectrolytes are block copolymers containing one of the building segments bearing electrolyte groups. Because microphase separation could facilitate microstructure formations into well-defined polyelectrolyte domains to produce continuous ion conducting channels, the use of block polyelectrolytes for the ion conducting membranes in fuel cell applications has been demonstrated to effectively promote the ion conductivity and suppress the methanol permeability. In this dissertation, block polyelectrolytes bearing pendant sulfonic acids or quaternary ammonium groups are designed and synthesized to prepare the corresponding proton exchange membranes (PEM) or anion exchange membranes (AEM). The interplays between the polymer architecture, the morphology of the membrane and the transport properties are systematically investigated. In chapter 2, a series of novel block polyelectrolytes, poly (styrene-block- sulfonated hydroxystyrene) (PS-b-sPHS), containing pendant sulfonic acid groups attached to the backbone via propyl spacers in the sPHS domain were synthesized and characterized. Some unique morphologies, such as hallow channels and lamellar arrangement of strings of beads, were observed as a consequence of equilibrium between microphase separation and coulomb interactions between polyelectrolytes were observed. The combination of microphase separation of block polyelectrolytes and freedom of movement of pendent alkylsulfonic acids was demonstrated to effectively enhance the proton transport and suppress the methanol crossover for the PEMs, leading to the selectivity higher than Nafion® 117 by 5 times at most. In chapter 3, two different side mesogenic moieties were delicatedly incorporated in the hydrophobic segment to change the crystallinity of the hydrophobic domains, which were found to significantly affect the microstructure in nanoscale and microscale and thus to alter the transport properties. The block polyelectrolytes with the long-tailed –C6 side groups exhibit highly crysallinity. In contrast, the block polyelectrolytes with the help of -CN side groups alignment, parallel patterning of nanostructures is fabricated with excellent well-organized microphase separation at microscale (> 15 μm). For this ion conducting membrane enhances the ion conductivity higher than that membrane preparing from –C6 groups by 2 times. In chapter 4, anion exchange membranes were prepared from quaternized block copolymers PS-b-PIN, composed of a hydrophobic polystyrene segment and a quaternized polyisoprene segment bearing pendant quaternary ammonium groups. A new approach is developed to prepare the AEMs without the use of ill-defined and toxic chloromethylation to introduce quaternary ammomium groups. The resulting AEMs exhibit microphase separated morphologies, primarily determined by the compositions of PS-b-PIN. The morphology-transport properties interplays are systematically investigated, showing the interconnectivity of the hydroxide conducting domains is critical to hydroxide conduction but less important for methanol permeation. Through this method, benzylmethylammonium groups, usually seen in the widely studied quaternized polysulfones, are absent to enable the resulting AEMs exhibit high alkaline stability in strong basic environment for long term.