Binding and Degradation Reaction of Hydroxide Ions with Several Quaternary Ammonium Head Groups of Anion Exchange Membranes Investigated by the DFT Method

Abstract

Commercialization of anion exchange membrane fuel cells (AEMFCs) has been limited due to the chemical degradation of various quaternary ammonium (QA) head groups, which affects the transportation of hydroxide (OH−) ions in AEMs. Understanding how various QA head groups bind and interact with hydroxide ions at the molecular level is of fundamental importance to developing high-performance AEMs. In this work, the binding and degradation reaction of hydroxide ions with several QA head groups—(a) pyridinium, (b) 1,4-diazabicyclo [2.2.2] octane (DABCO), (c) benzyltrimethylammonium (BTMA), (d) n-methyl piperidinium, (e) guanidium, and (f) trimethylhexylammonium (TMHA)—are investigated using the density functional theory (DFT) method. Results of binding energies (“∆” EBinding) show the following order of the binding strength of hydroxide ions with the six QA head groups: (a) > (c) > (f) > (d) > (e) > (b), suggesting that the group (b) has a high transportation rate of hydroxide ions via QA head groups of the AEM. This trend is in good agreement with the trend of ion exchange capacity from experimental data. Further analysis of the absolute values of the LUMO energies for the six QA head groups suggests the following order for chemical stability: (a) < (b)~(c) < (d) < (e) < (f). Considering the comprehensive studies of the nucleophilic substitution (SN2) degradation reactions for QA head groups (c) and (f), the chemical stability of QA (f) is found to be higher than that of QA (c), because the activation energy (“∆” EA) of QA (c) is lower than that of QA (f), while the reaction energies (“∆” ER) for QA (c) and QA (f) are similar at the different hydration levels (HLs). These results are also in line with the trends of LUMO energies and available chemical stability data found through experiments.

Water Uptake in an Anion Exchange Membrane Based on Polyamine: A First-Principles Study

“…The computed energies are low compared to other studies, 47 − 51 sign of lower chemical stability of the monomer. However, it should be noted that the implicit solvation model significantly underestimates the obtained values which would be higher, considering explicit water molecules.…”

“…The TMA functional groups are subjected to different degradation mechanisms at high temperatures and alkaline environments, mainly to nucleophilic substitution (S N 2), Ylide formation, and Hofmann elimination. ,,− The degradation process is closely related to the level of hydration. Therefore, the interaction of the TMA head group with OH – and different hydration levels will be analyzed with structures 4, 7, and 8, with λ equal to 0, 2, and 4 respectively, simulating the hydrated condition by explicitly adding water molecules to the system.…”

“…To investigate the degradation reaction of the TMA head group at different levels of hydration, the reaction energy (Δ E R ) and the activation energy (Δ E A ) were estimated via DFT calculations of the energies of transition state geometries. ,, …”

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

“…In ab initio calculations, the ground state for a set of atoms is obtained by solving the Schrodinger equation. The time-independent Schrodinger equation for the collection of many atoms is the basis of quantum mechanics (QM) [ 90 , 91 , 92 , 93 , 94 , 95 , 96 , 97 , 98 , 99 , 100 , 101 , 102 , 103 , 104 , 105 , 106 , 107 ]. Most of the computational studies on the chemical stability of AEM study via DFT calculations.…”

“…More recently, Karibayev et al studied the chemical stability and transportation of ion trends, for the various QA head groups using quantum chemical properties, such as binding energies, LUMO energies, nucleophilic substitution reaction, and activation energies [ 94 ]. The results suggested that the trimethylhexylammonium-based QA head group is the most stable QA head group, while the pyridinium-based QA head group is the least stable QA head group.…”