Theoretical study of the degradation mechanisms of substituted phenyltrimethylammonium cations

Xiang, Tiancheng; Si, Hongyan

doi:10.1016/j.comptc.2015.04.022

Cited by 8 publications

(9 citation statements)

References 22 publications

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“…One such cation is benzyltrimethylammonium, which has been shown to degrade primarily via S N 2 attack at the benzylic carbon. , Here, the introduction of electron-donating groups can stabilize the cation, although the associated ΔΔ G ⧧ value is a modest 1.6 kcal/mol, even in the most dramatic case studied (i.e., substitution of −N(CH 3 ) 2 at the 3- and 5-positions of the aromatic ring). , A somewhat similar case is the phenyltrimethylammonium cation, which degrades via nucleophilic attack on the methyl groups. In this case, electron-donating −N(CH 3 ) 2 moieties can improve the stability considerably, i.e., by up to 3.7 kcal/mol for triply −N(CH 3 ) 2 -substituted phenyltrimethylammonium . Another set of cations that have been computationally investigated are the substituted guanidinium cations, which primarily degrade via direct attack by OH – on the central carbon, which is followed by an intramolecular deprotonation of the attached −OH to form tetramethylurea.…”

Section: Theorymentioning

confidence: 99%

Electrocatalysis in Alkaline Media and Alkaline Membrane-Based Energy Technologies

et al. 2022

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Hydrogen energy-based electrochemical energy conversion technologies offer the promise of enabling a transition of the global energy landscape from fossil fuels to renewable energy. Here, we present a comprehensive review of the fundamentals of electrocatalysis in alkaline media and applications in alkaline-based energy technologies, particularly alkaline fuel cells and water electrolyzers. Anion exchange (alkaline) membrane fuel cells (AEMFCs) enable the use of nonprecious electrocatalysts for the sluggish oxygen reduction reaction (ORR), relative to proton exchange membrane fuel cells (PEMFCs), which require Pt-based electrocatalysts. However, the hydrogen oxidation reaction (HOR) kinetics is significantly slower in alkaline media than in acidic media. Understanding these phenomena requires applying theoretical and experimental methods to unravel molecularlevel thermodynamics and kinetics of hydrogen and oxygen electrocatalysis and, particularly, the proton-coupled electron transfer (PCET) process that takes place in a proton-deficient alkaline media. Extensive electrochemical and spectroscopic studies, on single-crystal Pt and metal oxides, have contributed to the development of activity descriptors, as well as the identification of the nature of active sites, and the rate-determining steps of the HOR and ORR. Among these, the structure and reactivity of interfacial water serve as key potential and pH-dependent kinetic factors that are helping elucidate the origins of the HOR and ORR activity differences in acids and bases. Additionally, deliberately modulating and controlling catalyst−support interactions have provided valuable insights for enhancing catalyst accessibility and durability during operation. The design and synthesis of highly conductive and durable alkaline membranes/ionomers have enabled AEMFCs to reach initial performance metrics equal to or higher than those of PEMFCs. We emphasize the importance of using membrane electrode assemblies (MEAs) to integrate the often separately pursued/optimized electrocatalyst/support and membranes/ionomer components. Operando/in situ methods, at multiscales, and ab initio simulations provide a mechanistic understanding of electron, ion, and mass transport at catalyst/ionomer/membrane interfaces and the necessary guidance to achieve fuel cell operation in air over thousands of hours. We hope that this Review will serve as a roadmap for advancing the scientific understanding of the fundamental factors governing electrochemical energy conversion in alkaline media with the ultimate goal of achieving ultralow Pt or precious-metal-free highperformance and durable alkaline fuel cells and related technologies.

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Section: Theorymentioning

confidence: 99%

Electrocatalysis in Alkaline Media and Alkaline Membrane-Based Energy Technologies

et al. 2022

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“…To increase understanding of the stability (and degradation) of different cationic groups, theoretical studies were carried out using DFT , or reactive force-field molecular dynamics (ReaxFF MD) simulations . Focusing on electron-donating and electron-withdrawing groups, DFT calculations reveal limited increase of stability by substitution with electron-donating groups for meta positions for substituted benzyltrimethyl ammonium (BTMA) cations and for ortho and para positions for substituted phenyltrimetylammonium cations. , Positively charged groups and electron-withdrawing groups were shown to decrease the stability of substituted BTMA cations . The effect of polymer backbone chemistry on BTMA cation degradation was also explored for fluorinated poly(arylene) ether and for poly(phenylene) .…”

Section: Introductionmentioning

confidence: 99%

Molecular Simulation of Quaternary Ammonium Solutions at Low Hydration Levels

2018

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Much research has focused on the stability of substituted ammonium salts in anion-exchange membranes (AEMs). While cation chemistry dictates AEM stability, chemical degradation has been recently shown to be significantly influenced by the hydration level at which the AEM operates. At low hydration, it is now known that almost every quaternary ammonium may suffer significant decomposition. In this work, we use molecular dynamics simulations to explore the behavior of three common quaternary ammonium cations with stoichiometric hydroxide concentration and at very low hydration. We find that water preferentially solvates hydroxide anions and hence when water is present in sufficient amount (more than four water molecules per ion pair), stability of the cations is expected to significantly improve. However, lower amounts of water result in the formation of isolated molecular clusters and ammonium hydroxide pairing that lead to degradation of the cation. The composition and size of the water–hydroxide–cation clusters that form is shown to be significantly affected by the cation chemistry. Implications of the observed behavior are discussed in view of recent experimental results on the differences in stability of these cations. This study highlights, for the first time, the crucial importance of studying hydroxide–water–cation interactions at low hydration levels.

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“…Specifically, the calculations showed that the double-

N-substituted phenylTMA+ is more stable than the double-

N-substituted benzylTMA+. These results elucidate the effects of substituents on the degradation of model cations and provide a reference for their potential use in anion-exchange membranes [ 112 ].…”

Section: Electronic Structure Calculations Based On the Density Funct...mentioning

confidence: 95%

“…The degradation mechanism of substituted phenyltrimethylammonium head group in alkaline conditions was studied at the B3LYP 6-311G (2d,p) level with the polarizable continuum solvation model (PCM) in water [ 112 ]. Several substituents and their positions on the benzene ring were changed, in order to explore the relation between the orientation effect and the stability of the substituted phenyltrimethylammonium cations.…”

Section: Electronic Structure Calculations Based On the Density Funct...mentioning

confidence: 99%

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

et al. 2022

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Anion Exchange Membrane (AEM) fuel cells have attracted growing interest, due to their encouraging advantages, including high power density and relatively low cost. AEM is a polymer matrix, which conducts hydroxide (OH−) ions, prevents physical contact of electrodes, and has positively charged head groups (mainly quaternary ammonium (QA) groups), covalently bound to the polymer backbone. The chemical instability of the quaternary ammonium (QA)-based head groups, at alkaline pH and elevated temperature, is a significant threshold in AEMFC technology. This review work aims to introduce recent studies on the chemical stability of various QA-based head groups and transportation of OH− ions in AEMFC, via modeling and simulation techniques, at different scales. It starts by introducing the fundamental theories behind AEM-based fuel-cell technology. In the main body of this review, we present selected computational studies that deal with the effects of various parameters on AEMs, via a variety of multi-length and multi-time-scale modeling and simulation methods. Such methods include electronic structure calculations via the quantum Density Functional Theory (DFT), ab initio, classical all-atom Molecular Dynamics (MD) simulations, and coarse-grained MD simulations. The explored processing and structural parameters include temperature, hydration levels, several QA-based head groups, various types of QA-based head groups and backbones, etc. Nowadays, many methods and software packages for molecular and materials modeling are available. Applications of such methods may help to understand the transportation mechanisms of OH− ions, the chemical stability of functional head groups, and many other relevant properties, leading to a performance-based molecular and structure design as well as, ultimately, improved AEM-based fuel cell performances. This contribution aims to introduce those molecular modeling methods and their recent applications to the AEM-based fuel cells research community.

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Theoretical study of the degradation mechanisms of substituted phenyltrimethylammonium cations

Cited by 8 publications

References 22 publications

Electrocatalysis in Alkaline Media and Alkaline Membrane-Based Energy Technologies

Electrocatalysis in Alkaline Media and Alkaline Membrane-Based Energy Technologies

Molecular Simulation of Quaternary Ammonium Solutions at Low Hydration Levels

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

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