Synthesis And Characterization Of Novel Alkaline Anion Exchange Membranes

Developing clean, high efficiency and alternative energies technologies becomes more and more important, due to the fast economic development and limited availability of fossil fuels. Fuel cells have been regarded as one of the most promising power generation technologies because of high conversion efficiency and environmental friendliness, and this technology can convert chemical energy to electrical energy. The electrolyte membrane is the heart of fuel cell. Proton exchange membrane has attracted considerable interest due to their excellent properties during the past few decades. Nowadays, the representative membrane material is Nafion which is a poly(perfluorosulfonic acid) membrane with high ionic conductivity, excellent chemical, mechanical and thermal stability. Unfortunately, the use of platinum catalysts limits the practical application of proton exchange membrane fuel cells (PEMFCs) due to the cost and its instability under the fuel cell operating conditions. Recently, there has been a growing interest in alkaline anion exchange membranes (AEMs), because non-precious-metals can be used as stable catalysts for the fuel-cell reactions, making the fuel cell more cost effective. However, the ionic conductivity and alkaline stability of AEMs reported in literatures were generally lower than that of acidic proton exchange membranes (PEMs). Improving the ionic conductivity and alkaline stability remains a challenge for the development of AEMs. Therefore, many research groups are currently focusing their work on the development of high hydroxide-conducting membranes with good stability. We believe that membrane materials are composed of polymer chains and ionic groups. Therefore, we chosed PPO as a precursor of membrane materials to design and prepare the polymers based on the relationship between the structure and the properties of polymers. We prepared high performance anion exchange membranes by designing morphological structure of polymer and introducing ionic groups with good stability. The main results are listed as follows:1. We designed and synthesized a new type of aromatic ABA triblock polymers (QPPO-PAES-QPPO) with two hydrophilic end blocks (A) and a hydrophobic middle block (B). The structures of polymers were characterized by1H NMR, TGA and GPC. In order to unravel the hydrophilic-hydrophobic phase separation, the morphologies of the membranes were examined by small-angle X-ray scattering (SAXS) and AFM. In addition, water uptake, swelling ratio, conductivity and mechanical property of the membranes were examined. The highest conductivity of triblock polymer membrane is129mS/cm. The alkaline stability of the QPPO-PAES-QPPO and QPPO membranes was also preliminarily investigated by immersing membranes in1M NaOH at80℃.2. Novel trifunctional moieties,2,4,6-tri(dimethylaminomethyl)-phenol (TDAP), were synthesized and incorporated onto PPO to obtain a novel ammonium functionalized polymer. As a control, the polymers with one (PPO-MQA) and two (PPO-DQA) ammonium groups on the same ring were also synthesized. Water uptake, swelling ratio, conductivity and mechanical property of the membranes were examined. The results confirmed that PPO-TQA membranes exhibited not only high ionic conductivity and mechanical properties, but also low water swelling. In addition, the alkaline stability of PPO-TQA and QPPO was investigated in1M NaOH at80℃.3. Multiwalled carbon nanotubes functionalized with imidazolium-type ionic liquid polymer, PIL(BF4)-MWCNTs, have been successfully prepared via in situ free radical polymerization of1-vinyl-3-methylimidazolium iodide ([VMIm][I]) and then blended with poly(2,6-dimethyl-1,4-phenylene oxide) containing imidazolium (PPO-MIm) in solution to fabricate composite membranes. The composite membranes were characterized by scanning electron microscopy (SEM) and the SEM images of the membranes show that the PIL(BF4)-MWCNTs can be homogeneously dispersed in the PPO-MIm matrix. Conductivity and mechanical property of the composite membranes were examined. It was demonstrated that the incorporation of the PIL(BF4)-MWCNTs into the membranes of PPO-MIm can increase both conductivity and mechanical property. The composite membrane containing0.3wt%of PIL(BF4)-MWCNTs (P(0.3)) exhibited dramatic enhancements in ionic conductivity (95.3%) and tensile strength (82.9%) in comparison with the membrane without PIL(BF4)-MWCNTs. Therefore, this research demonstrates that incorporation of the functionalized carbon nanotube is a facile and potential strategy for improvements of both ionic conductivity and mechanical properties of alkaline polymer electrolyte membranes.4. We designed and synthesized a novel poly(2,6-dimethyl-1,4-phenylene oxide) with imidazolium-functionalized aromatic side chains by the reaction of imidazole-functionalized PPO and methyl iodide, and then prepared the acid doped PPO membrane by immersing the polymer membrane in phosphoric acid (PA). To achieve a high acid doping level and a high dimensional stability, the cross-linked membranes were prepared by the reaction of the imidazole groups of the polymer and1,4-bis(bromomethyl) benzene. The PA content, area swelling, and tensile strength of the non-crosslinked and the crosslinked membranes doped with a H3PO4solution were measured. The results showed that the crosslinked membrane with a PA content of438%possessed the highest conductivity of54mS/cm at140℃with high dimensional stability and high tensile strength. Moreover, no shape change of the membranes was observed in15days in a Fenton solution containing3%H2O2and4ppm Fe2+at80℃, indicating a high oxidative stability.