(35g) Mechanism of Imidazolium Groups Degradation in Alkaline Anion Exchange Membrane

Recently, alkaline electrolyte membrane fuel cells (AEMFCs) have been attracting growing interest. By changing acidic medium into a basic one, the cathode oxygen reduction over-potential can be greatly reduced, resulting in higher efficiency, and the catalysts are more stable in alkaline environment. Moreover, the costs of the fuel cells can be dramatically reduced because AEMFCs can work with non-noble metal catalysts such as nickel and silver.

Since imidazolium cations tend to be attacked by hydroxide, the membrane degradation occurs over time. Those greatly limit the development of alkaline electrolyte membrane fuel cells. Imidazolium groups as cationic head groups are prone to degradation in alkaline conditions through a ring-opening mechanism. In the mechanism, a hydroxide ion performs a nucleophilic attack on the imidazolium ring at α carbon (α carbon with reference to both nitrogen atoms). In this study, density functional theory calculations were used to research the degradation pathways of imidazolium cations to discuss the relative durability of cation bounded by alkyl and aromatic groups. Detailed structure changes of imidazolium ring were explored by means of quantum chemical calculations (QCCs) using a model side chain terminal of dimethylimidazolium (DMIM) cation. The reaction energy barrier of the degradation was obtained from our reaction path calculations. The effect of the medium and the water content was also discussed on the calculated reaction barriers. Good solvation of the head-groups and hydroxide ions is helpful to the chemical stability of alkaline membranes.