As the environmental contamination and the depletion of fossil fuels become severe problems, demand for renewable energy is increasing. Fuel cells are efficient energy conversion devices that converts chemical energy pertaining in an fuel and oxidant directly into electrical energy.
Among of them, proton exchange membrane fuel cell(PEMFC) has received a lot of attention owing to its low operating temperature and high energy density. Despite the merits of PEMFCs, commercialization is hampered by the weakness using very expensive materials such as perfluorinated sulfonic acid as electrolytes and noble metals for electrocatalyst. As an alternative, alkaline fuel cell(AFC) which have a faster oxigen reduction reaction and allows the use of non noble metal catalysts is being developed to replace PEMFC with these faults. Anion exchange membrane(AEM) is an essential component of AFC, which acts as an ion transport medium from the cathode to the anode.
In this study, poly(arylene ether sulfone)(PAES) membranes were prepared for 5 species with various degree of bromination(DB) to investigate the electrochemical performance and chemical stability of anion exchange membranes. First, the PAES block copolymer was successfully synthesized through nucleophilic aromatic substitution reaction, followed by the bromination reaction proceeded to have various DB values (10% to 50%) through N-bromosuccinimide(NBS) equivalent control. After brominated PAESs(Br-PAESs) were dissolved in N-methyl-2-pyrrolidone(NMP), 1,4-diazabicyclo[2.2.2]octane (DABCO) was added for the quaternization reaction.
The structures of prepared polymers were confirmed by proton nuclear magnetic resonance(1H-NMR) and Fourier transform infrared spectroscopy(FT-IR). The thermogravimetric analysis(TGA) and differential scanning calorimetry(DSC) analyzes were performed to confirm the thermal properties of synthesized polymers. Ion exchange capacity(IEC), water uptake, hydration number, and swelling ratio were investigated to characterize membranes. Q-PAES with high degree of bromination(DB) exhibited poor physical properties, high ionic conductivity(under 100% relative humidity) in range of 30-51 mS·cm-1 and distinct microphase separation. The alkaline stability of fabricated membranes was carried out 1M KOH solution for 1,000 hours. AEM with high DB showed higher decomposition rates due to the decrease of electron density of the main chain by more functional groups. On the basis of research, higher DB values can raise efficiency of fuel cell but diminish the chemical stability. So, the regulation of DB values may be an important factor in determining the properties of the anion exchange membrane.