Preparation of poly(arylene ether ketone) based anion exchange membrane with pendant pyrimidinium and pyridazinium cation derivatives for alkaline fuel cell

Abstract

As a key constituent of anion exchange membrane fuel cells, anion exchange membrane (AEM) faces a tough challenge to withstand the nucleophilic (OH−) attack on central cation in alkaline atmosphere which causes a constant degradation and thus affects the long term performance of the AEM. Aromatic cations unlike aliphatic amine cations are resistant to Hofmann β-elimination, however due to planar geometry they are more vulnerable to nucleophilic substitution reaction. To understand the problem and to formulate a mechanism to protect the AEM, a series of cationic membranes was fabricated based on poly(arylene ether ketone) having pendant quaternized derivatives of pyrimidinium and pyridazinium cations employing DCC-coupling reaction. The chemical structure characterization of the target products was conducted by 1H NMR and FT-IR spectroscopy. Various physicochemical properties of the membranes were investigated including thermomechanical behavior, ion exchange capacity, water uptake, swelling ratio, ionic conductivity, fuel cell performance and oxidative and chemical stabilities. No serious swelling was observed with adequate water uptake at temperature between 20 °C and 80 °C. The membranes with both pyrimidinium and pyridazinium cations exhibited sufficient ionic conductivity. The highest recorded conductivity was 45.7 mS cm−1 and peak power density was 194 mW cm−2. The oxidative stability of all membranes was high (more than 90%) however, alkaline stability results suggest a different behavior. Pyridazinium cations with substituents simultaneously at C-3 and C-6 positions is thoroughly protected from OH− attack while pyrimidinium (simultaneously protected at C-4 and C-6 positions) degraded at accelerated rate with raising temperature. This work not only explores the possibility of utilizing aromatic cations that weren't used before but also provides a wider understanding of the relationship of aromatic cation structure and membrane stability and performance thus helping in designing stable anion exchange membranes for long-lasting fuel cells.