The Alkaline Stability of Anion Exchange Membrane for Fuel Cell Applications: The Effects of Alkaline Media

Alkaline alcohols (methanol, ethanol, propanol, and ethylene glycol) have been applied as fuels for alkaline anion exchange membrane fuel cells. However, the effects of alkaline media on the stability of anion exchange membranes (AEMs) are still elusive. Here, a series of organic cations including quaternary ammonium, imidazolium, benzimidazolium, pyridinium, phosphonium, pyrrolidinium cations, and their corresponding cationic polymers are synthesized and systematically investigated with respect to their chemical stability in various alkaline media (water, methanol, ethanol, and dimethyl sulfoxide) by quantitative 1H nuclear magnetic resonance spectroscopy and density functional theory calculations. In the case of protic solvents (water, methanol, and ethanol), the lower dielectric constant of the alkaline media, the lower is the lowest unoccupied molecular orbital (LUMO) energy of the organic cation, which leads to the lower alkaline stability of cations. However, the hydrogen bonds between the anions and protic solvents weaken the effects of low dielectric constant of the alkaline media. The aprotic solvent accelerated the SN2 degradation reaction of "naked" organic cations. The results of this study suggest that both the chemical structure of organic cations and alkaline media (fuels) applied affect the alkaline stability of AEMs.