The efforts to reduce dependence on fossil fuels due to the rapid increase in environmental problems and fuel depletion are focused on renewable energy throughout the world. Of the various renewable energies, fuel cells are growing rapidly in recent years due to their high energy efficiency, use of various fuels, and low pollutant emissions. Among them, an alkaline fuel cell (AFC) is a promising resource to replace proton exchange membrane fuel cells (PEMFCs), which is the most commercially available, due to its fast oxygen reduction reaction, low fuel permeability, and use in non-precious metal catalysts. Anion exchange membranes (AEMs), a key component that needs to be developed for rapid commercialization of AFC, must have good physical and chemical properties as well as improved ionic conductivity and outstanding alkaline stability. There are two main problems to be solved for the development of AEM. First, the transfer rate of hydroxide ion is slow due to large volume compared to proton and does not provide sufficient phase separation to connect ion channel between hydrophilic and hydrophobic blocks. Secondly, functional groups on AEM such as quaternary ammonium group can easily degrade by OH- attack and show very low durability under alkaline conditions. Therefore, in order to solve the above two problems, various researches have been carried out in this study.
Chapter 1 briefly introduces renewable energy and fuel cells, and shows the principles, advantages, disadvantages, and components of AFC.
Chapter 2 presents AEMs, which are a key material of AFC components, with various structures(main chain and funtional gorups) that can have excellent alkaline stability and electrochemical performance.
Chapter 3 describes the synthesis of partially fluorinated multi-block copolymers containing quaternary ammonium groups for improved ionic conductivity. In this study, the ionic conductivity, physical properties, and the alkaline stability test according to length of hydrophilic precursors and hydrophobic oligomers were investigated in details. The prepared AEM showed a high ionic conductivity of 154 mS cm-1 but presented an unstable dimensional change with a very high water content of over 300%. As mentioned above, high IEC value caused excessive swelling in the membrane, which resulted in poor alkaline stability against OH-.
Chapter 4 investigates a series of heterocyclic-quaternary ammonium type poly(arylene ether) (PAE) random copolymers with moieties of sulfone, ketone, hexafluoroisopropyl, isopropyl, phenolphthalein, or phenylene to identify differences in physicochemical properties of AEM due to polymer backbone structure with electron-withdrawing groups (EWGs) or electron-donating groups (EDGs). The 1-methylpyrrolidine (PYR)-PAE membranes containing EDG exhibited excellent electrochemical performance as well as alkaline stability, compared with PYR-PAE membranes with EWG. Especially, PYR-PAE-phenolphthalien (PhPh) membrane exhibited excellent alkaline stability due to a steric hindrance effect of phenolphthalein, which is a cardo compound having a complete aromatic structure and a large free volume.
Chapter 5 reports organic/inorganic composite membrane using PAE random copolymer and GO-triethoxysilylpropylamine (APTS)-co-(3-bromopropyl)trimethyl ammonium bromide (PTMA) (0.1, 0.3, 0.5, 0.7, and 0.9 wt%) to improve ionic conductivity and chemical stability. The ionic conductivity of the composite membrane showed a tendency to improve up to 0.7 wt%, but it decreased again from 0.9 wt%, because obstruct the ion cluster due to agglomeration by the amphipathic property of the GO-APTS-co-PTMA. Prepared composite membranes have proven to be very promising materials for the development of AEM by demonstrating the improvement of ionic conductivity as well as proper dimensional stability and alkaline stability through the introduction of functionalized GO.
As a result, various chemical structure designs and introduction of inorganic in AEM were carried out to develop AEM with improved electrochemical performance and excellent alkaline stability. The polymer structure with proper physical properties and ionic conductivity was confirmed by proceeding the synthesis of block copolymer according to repeat units of hydrophilic/hydrophobic oligomers. The difference in electrochemical performance and alkaline stability was proved according to the structure of the main chain. It was also concluded that the introduction of functionalized inorganic materials with appropriate proportions lead to improved electrochemical performance and chemical stability for rapid commercialization of AEM.