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

Abstract

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.

Chemically stable piperidinium cations for anion exchange membranes

“…DFT results have shown that the elimination reaction free energies of both [Bzpyr] + or [Bzpip] + are the lowest among all the reaction pathways, which is in accord with the theoretical predictions shown by other reports. 18,46 Surprisingly, the product of the elimination reaction was not found in the NMR analysis (Fig. 2 in the manuscript and Fig.…”

Molecular Modeling in Anion Exchange Membrane Research: A Brief Review of Recent Applications

“…The degradation mechanism of the vinyl benzyl head group. in the presence of implicit solvation and ions, were studied via the GGA-BLYP COSMO model and DMol [ 117 ]. As a result, it was found that the stability order of head groups is as follows: DABCO < TMA < NMP < ABCO.…”

“…The second step is the detachment of the quinodimethane-like intermediate from the polymer backbone, by attacking superoxide or peroxy radicals via oxidative cleavage. The final step is the rearomatization of the reaction intermediate, as shown in Figure 5 [ 117 ].…”

“… Degradation mechanism of N-methylpiperidine (NMP)-based AEM: ( i ) NMP head group detachment, ( ii ) ring-opening with subsequent formation of alkene, and ( iii ) ring-opening via direct hydroxylation. Reprinted with permission from [ 117 ]. Copyright 2020, for John Wiley and Sons.…”

The alkali degradation of LDPE-based radiation-grafted anion-exchange membranes studied using different ex situ methods

“…1 Alkali stable types are required for many applications, including alkaline membrane fuel cells (AEMFC) and AEM-based electrolysers. [2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19] A high performance class of AEM is made via radiation-graing (RG-AEM), 3,[20][21][22][23][24][25][26][27][28] with examples being published as far back as 1996. 29 Poly(ethylene-co-tetrauoroethylene)-(ETFE)-based RG-AEMs have been the most commonly investigated, with those made using trimethylamine (TMA), N-methylpyrrolidine (MPY), and N-methylpiperidine (MPIP) amination agents having good stabilities in aqueous alkali.…”