Department of Chemical and Materials Engineering, School of Engineering and Digital Sciences, Nazarbayev University, Nur-Sultan 010000, Kazakhstan.
Laboratory of Advanced Materials and Systems for Energy Storage, Center for Energy and Advanced Materials Science, National Laboratory Astana, Nazarbayev University, Nur-Sultan 010000, Kazakhstan.
Molecules. 2022 Apr 21;27(9):2686. doi: 10.3390/molecules27092686.
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.
阴离子交换膜燃料电池（AEMFCs）的商业化受到各种季铵（QA）头基化学降解的限制，这会影响 AEM 中氢氧根（OH−）离子的传输。了解各种 QA 头基在分子水平上如何与氢氧根离子结合和相互作用对于开发高性能 AEM 至关重要。在这项工作中，使用密度泛函理论（DFT）方法研究了氢氧根离子与几种 QA 头基（a）吡啶鎓、（b）1,4-二氮杂双环[2.2.2]辛烷（DABCO）、（c）苄基三甲基铵（BTMA）、（d）N-甲基哌啶鎓、（e）胍鎓和（f）三甲基己基铵（TMHA）的结合和降解反应。结合能（“∆”EBinding）的结果表明，这六种 QA 头基与氢氧根离子的结合强度顺序为：（a）＞（c）＞（f）＞（d）＞（e）＞（b），这表明基团（b）具有通过 AEM 的 QA 头基快速传输氢氧根离子的能力。这一趋势与实验数据得出的离子交换容量趋势非常吻合。进一步分析这六种 QA 头基的最低未占分子轨道（LUMO）能量的绝对值表明，化学稳定性顺序为：（a）＜（b）～（c）＜（d）＜（e）＜（f）。考虑到 QA 头基（c）和（f）的亲核取代（SN2）降解反应的综合研究，发现 QA（f）的化学稳定性高于 QA（c），因为 QA（c）的活化能（“∆”EA）低于 QA（f），而 QA（c）和 QA（f）在不同水合水平（HL）下的反应能（“∆”ER）相似。这些结果也与 LUMO 能量和实验得出的可用化学稳定性数据的趋势一致。
