Dissertation Defense – Lisha Liu
Prof. Paul A. Kohl, Advisor, CHBE
Prof. Seung Soon Jang, Co-advisor, MSE
Prof. Meilin Liu, MSE
Prof. William J. Koros, CHBE
Prof. Vladimir Tsukruk, MSE
"Multiblock Copolymer Based Anion Exchange Membranes and Ionomers"
Anion exchange membranes (AEMs) have been a subject of research as hydroxide conducting polymer electrolytes in electrochemical devices like fuel cells and electrolyzers in recent years due to the alkaline conditions facilitating reaction kinetics and allowing for non-precious metal catalysts, which greatly reduces the cost. However, the wide-scale commercialization of the AEMs is impeded by the low ionic conductivity, high water uptake, poor alkaline stability, and poor mechanical properties of current materials. The primary objective of this study is to create anion exchange membranes and ionomers with improved properties, including high ionic conductivity (i.e. greater than 100 mS/cm), excellent chemical stability under alkaline conditions, and low water uptake for better dimensional stability, compared to existing materials in the applications of fuel cells and electrolyzers. To achieve this objective, several new structures for polymeric anion exchange membranes were designed, and the corresponding structure-morphology-property relationship was investigated.
First, multiblock copolymers with partial fluorination and long head-group tethers was designed to improve the ionic conductivity, the alkaline stability, and lower water uptake of AEMs. A systematic study of the effect of the hydrophilic and hydrophobic block lengths and ion exchange capacity of partially fluorinated multiblock copolymer mPEs with long head-group tethers was undertaken to explore the relationship of the chemical structure, morphology and properties of the AEMs. The formation of ion conductive nano-channels for hydroxide ion transport due to nanophase separation of the multiblock copolymers greatly improved the ionic conductivity and reduced the water uptake.
Second, the effect of the number of ionic groups in the hydrophilic segment on the morphology and properties of AEMs was studied. Multiblock copoly(arylene ether)s (mPEs) were synthesized with different ion exchange capacity (IEC) by attaching a different number of cationic head-groups via long alkyl chain tethers. The multiblock copolymer mPEs with 1, 2, 3 and 4 long alkyl chain tethered ionic groups on each repeat unit in hydrophilic block, resulting in different IECs, were compared. As the ionic concentration increased, the ionic conductivity and water uptake of the membranes increased. This was due to the increase of the size of ion conductive channels. 3-tether membrane showed the highest ionic conductivity/IEC, which means that its ionic groups were most efficient for contributing to the ionic conductivity.
Third, the effect of the cationic groups (i.e. size, central atom, etc.) was investigated. Multiblock copoly(arylene ether)s (mPEs) with partial fluorination and two long alkyl head-group tethers on each hydrophilic repeat unit with quaternary trimethyl mmonium (TMHA), quinuclidium (ABCO), and tris(2,4,6-trimethoxyphenyl)phosphonium (TTMPP) were synthesized. Their morphology and properties (i.e. ionic conductivity, water uptake, alkaline stability, mechanical and thermal properties) were compared in order to understand the effects of the size and type of cations on the morphology and physical properties. Alkyl trimethylammonium appears to be the best cation head group among the three cations studied for this backbone. In addition, the membrane preparation method impacts the morphology and properties of the AEMs. Quaternization before membrane casting resulted in better properties than quaternization after membrane casting due to greater phase segregation.
Lastly, a series of anionic ionomers was synthesized and tested in fuel cells and electrolyzers. These ionomers are based on a series of materials which include block copolymer AEMs with alkyl tethers that have been modified to be used as anion conductors. The ionomers were tested for their viability as anion conductors in the cathodic electrode for the cathode hybrid fuel cell and as the anodic electrode of an alkaline electrolyzerThe two sets of results show similar trends: for a homopolymer, lower molecular weight materials provide superior performance compared to their higher molecular weight counterparts. Additionally, the introduction of phase segregation via block copolymer further increased performance in both types of devices.