Density functional theory study on the degradation of fuel cell anion exchange membranes via removal of vinylbenzyl quaternary ammonium head group

The alkaline stability of different tethered amine functional groups of fuel cell anion exchange membranes (AEMs), namely, trimethyl amine (TMA), 1-azabicyclo[2.2.2]octane (ABCO), 1,4-diazabicyclo[2.2.2]octane (DABCO), and N-methylpiperidine (NMP), is investigated using density functional theory (DFT). Among the amine functional groups investigated, ABCO emerged as the most stable exhibiting the highest energy of barrier (EOB) of 33.5 kcal/mol, while DABCO has the lowest EOB of 30.0 kcal/mol due to the presence of an additional electron-withdrawing nitrogen. The calculated lowest unoccupied molecular orbital (LUMO) energy revealed the trend of increasing alkaline stability against nucleophilic attack, consistent with their measured barrier energies: DABCO < TMA < NMP < ABCO. Most importantly, the DFT calculations confirmed the proposed multistep AEM degradation mechanism via the detachment of the whole vinylbenzyl quaternary ammonium group through the following steps: (1) nucleophilic attack leading to the loss of aromaticity with subsequent transformation to a quinodimethane moiety, (2) detachment of the quinodimethane-like intermediate from the polymer backbone by the attack of superoxide and/or peroxy radicals via oxidative cleavage, and (3) the rearomatisation of the reaction intermediate.