Benzyltrimethylammonium Cations Research Articles

Surface Termination on Unstable Methylammonium-based Perovskite Using a Steric Barrier for Improved Perovskite Solar Cells.

Compared to widely adopted low-dimensional/three-dimensional (LD/3D) heterostructure, functional organic cation based surface termination on perovskite can not only realize advantage of defect passivation but also prevent potential disadvantage of the heterostructure induced intercalation into 3D perovskite. However, it is still very challenging to controllably construct surface termination on organic-inorganic hybrid perovskite because the functional organic cations' substitution reaction is easy to form LD/3D heterostructure. Here, we report using a novel benzyltrimethylammonium (BTA) functional cation with rational designed steric hindrance to effectively surface terminate onto methylammonium lead triiodide (MAPbI3) perovskite, which is composed of the most unstable MA cations. The BTA cation is difficult to form a specific 1.5-dimensional perovskite of BTA4Pb3I10 by cation substitution with MA cation, which then provides a wide processing window (around 10 minutes) for surface terminating on MAPbI3 films. Moreover, the BTAI surface terminated BTAI-MAPbI3 shows better passivation effect than BTA4Pb3I10-MAPbI3 heterojunction. Finally, BTAI surface terminated solar cell (0.085 cm2) and mini-module (11.52 cm2) obtained the efficiencies of 22.03% and 18.57%, which are among the highest efficiencies for MAPbI3 based ones.

Surface Termination on Unstable Methylammonium‐based Perovskite Using a Steric Barrier for Improved Perovskite Solar Cells

AbstractCompared to widely adopted low‐dimensional/three‐dimensional (LD/3D) heterostructure, functional organic cation based surface termination on perovskite can not only realize advantage of defect passivation but also prevent potential disadvantage of the heterostructure induced intercalation into 3D perovskite. However, it is still very challenging to controllably construct surface termination on organic–inorganic hybrid perovskite because the functional organic cations’ substitution reaction is easy to form LD/3D heterostructure. Here, we report using a novel benzyltrimethylammonium (BTA) functional cation with rational designed steric hindrance to effectively surface terminate onto methylammonium lead triiodide (MAPbI3) perovskite, which is composed of the most unstable MA cations. The BTA cation is difficult to form a specific 1.5‐dimensional perovskite of BTA4Pb3I10 by cation substitution with MA cation, which then provides a wide processing window (around 10 minutes) for surface terminating on MAPbI3 films. Moreover, the BTAI surface terminated BTAI‐MAPbI3 shows better passivation effect than BTA4Pb3I10‐MAPbI3 heterojunction. Finally, BTAI surface terminated solar cell (0.085 cm2) and mini‐module (11.52 cm2) obtained the efficiencies of 22.03 % and 18.57 %, which are among the highest efficiencies for MAPbI3 based ones.

Micro-block poly(arylene ether sulfone)s with densely quaternized units for anion exchange membranes: Effects of benzyl N-methylpiperidinium and benzyl trimethyl ammonium cations

The piperidinium cation exhibited a substantial impact on the improvement of alkaline stability of anion exchange membranes (AEMs), yet the research regarding the micro-block polymer AEMs modified with it has not been conducted. Herein, we report two series of micro-block poly(arylene ether sulfone)s containing densely benzyl-N-methylpiperidinium (TTP-n-PIPPES) and benzyl-trimethyl-ammonium (TTP-n-QAPES) functionalized units for AMEs, respectively, where the n represents the molar ratio of monomer containing tri-tetraphenyl to the employed bisphenol monomers. The content and arrangement of the two types of cations in both polymer backbones were the same. The hydrophilic phases of TTP-n-PIPPES membranes exhibited isolated island states, wrapped with coherent hydrophobic phases. While the TTP-n-QAPES membranes formed continuous hydrophilic channels but fragmented hydrophobic phases. The discrepancy in the microscopic morphology of membranes and the intrinsic nature of cations thus rendered two types of AEMs with different advantages in macroscopic properties. The TTP-n-PIPPES membranes performed better dimensional stability and alkaline stability (low temperature). In contrast, the preferable capabilities of TTP-n-QAPES membranes were water affinity, hydroxide conductivity, and alkaline stability (high temperature). For instance, the TTP-18.2-PIPPES membrane was 11.7%, 11.4%, and 27.4% lower than the TTP-18.2-QAPES membrane at 80 °C in terms of dimensional change, water uptake, and hydroxide conductivity, respectively.

Alkaline Stability Evaluation of Polymerizable Hexyl-Tethered Ammonium Cations.

One of the important challenges in designing robust alkaline anion exchange membranes is the difficulty associated with the chemical stability of covalently bound cationic units. Here, a systematic study exploring alkaline stabilities of polymerizable hexyltrimethylammonium cations is presented, where the hexyl chain is linked to a phenyl ring through a direct carbon-carbon, phenyl ether, or benzyl ether functionality. For this work, small molecule model compounds, styrenic monomer analogs, and their homopolymers are synthesized. Alkaline stabilities of the small molecule cations and their homopolymers are compared to alkaline stability of benzyltrimethylammonium (BTMA) cation and its homopolymer poly(BTMA), respectively. All the hexyl-tethered cations and their homopolymers are significantly more stable under strongly alkaline conditions (2 m KOD at 80 °C). Moreover, ether-linked cations show comparable stability to the direct carbon-carbon linked cation. Via 1 H NMR analyses, possible degradation mechanisms are investigated for each small molecule cation. Findings of this study strongly suggest that the alkaline stability is dictated by the steric hindrance around the β-hydrogen. This study expands beyond the limits of general knowledge on alkaline stability of alkyl-tethered ammonium cations via the Hofmann elimination route, highlights important design parameters for stable ammonium cations, and demonstrates accessible directly polymerizable alkaline stable ammonium cations.

THz, Raman, IR and DFT studies of noncentrosymmetric metal dicyanamide frameworks comprising benzyltrimethylammonium cations

We report density functional theory (DFT) studies of vibrational modes for benzyltrimethylammonium cations (BeTriMe+) as well as THz, IR and Raman studies of [BeTriMe][M(dca)3(H2O)] (dca = N(CN)2−, dicyanamide; M = Mn2+, Co2+, Ni2+) and their anhydrous analogues. These studies show that the anhydrous BeTriMeMn and BeTriMeNi have the same or very similar structures and loss of water molecules leads to significant changes in the metal-dicyanamide frameworks. In particular, the number of dca modes decreases, suggesting increase of crystal symmetry, probablly related with decrease in the number of non-equivalent dca bridges from two to one. Although it is possible that dehydration leads to a replacement of the coordinate Mn-O (Ni-O) bonds by Mn-N (Ni-N) bonds, wherein N atoms come from the C≡N groups of previously non-bridged dca units, reversibility of the dehydration process indicates that such new bonds are either not formed or are very weak. The anhydrous Mn and Ni compounds undergo similar reversible phase transitions to lower symmetry phases. The driving force for these transitions is most likely ordering of dca linkers but this process is accompanied by weak distortion of the metal-dicyanamide frameworks. In the case of BeTriMeCo, the loss of water molecules also leads to significant changes in the cobalt-dicyanamide framework. However, the structure of this analogue is different from the structures of the Mn and Ni counterparts, the number of unique dca linkers is preserved and the dehydration process is irreversible, suggesting more drastic rearrangement of the metal-dicynamide framework.

Benzyltrimethylammonium cadmium dicyanamide with polar order in multiple phases and prospects for linear and nonlinear optical temperature sensing.

Coordination polymers with multiple non-centrosymmetric phases have sparked substantial research efforts in the materials community. We report the synthesis and properties of a hitherto unknown cadmium dicyanamide coordination polymer comprising benzyltrimethylammonium cations (BeTriMe+). The room-temperature (RT) crystal structure of [BeTriMe][Cd(N(CN)2)3] (BeTriMeCd) is composed of Cd centers linked together by triple dca-bridges to form one-dimensional chains with BeTriMe+ cations located in void spaces between the chains. The structure is polar, the space group is Cmc21, and the spontaneous polarization in the c-direction is induced by an arrangement of BeTriMe+ dipoles. BeTriMeCd undergoes a second-order phase transition (PT) at T1 = 268 K to a monoclinic polar phase P21. Much more drastic structural changes occur at the first-order PT observed in DSC at T2 = 391 K. Raman data prove that the PT at T2 leads to extensive rearrangement of the Cd-dca coordination sphere and pronounced disorder of both dca and BeTriMe+. On cooling, the HT polymorph transforms at T3 = 266 K to another phase of unknown symmetry. Temperature-resolved second harmonic generation (TR-SHG) studies at 800 nm confirm the structural non-centrosymmetry of all the phases detected. Optical studies indicate that BeTriMeCd exhibits at low temperatures an intense emission under 266 nm excitation. Strong temperature dependence of both one-photon excited luminescence and SHG response allowed for the demonstration of two disparate modes of optical thermometry for a single material. One is the classic ratiometric luminescence thermometry employing linear excitation in the ultraviolet region while the other is single-band SHG thermometry, a thus far unprecedented subtype of nonlinear optical thermometry. Thus, BeTriMeCd is a rare example of a dicyanamide framework exhibiting polar order, SHG activity, photoluminescence properties and linear and nonlinear optical temperature sensing capability.

Functionalizing Polystyrene with N-Alicyclic Piperidine-Based Cations via Friedel-Crafts Alkylation for Highly Alkali-Stable Anion-Exchange Membranes.

Different anion-exchange membranes (AEMs) based on polystyrene (PS)-carrying benzyltrimethyl ammonium cations are currently being developed for use in alkaline fuel cells and water electrolyzers. However, the stability in relation to these state-of-the-art cations needs to be further improved. Here, we introduce highly alkali-stable mono- and spirocyclic piperidine-based cations onto PS by first performing a superacid-mediated Friedel–Crafts alkylation using 2-(piperidine-4-yl)propane-2-ol. This is followed by quaternization of the piperidine rings either using iodomethane to produce N,N-dimethyl piperidinium cations or by cyclo-quaternizations using 1,5-dibromopentane and 1,4-dibromobutane, respectively, to obtain N-spirocyclic quaternary ammonium cations. Thus, it is possible to functionalize up to 27% of the styrene units with piperidine rings and subsequently achieve complete quaternization. The synthetic approach ensures that all of the sensitive β-hydrogens of the cations are present in ring structures to provide high stability. AEMs based on these polymers show high alkaline stability and less than 5% ionic loss was observed by 1H NMR spectroscopy after 30 days in 2 M aq NaOH at 90 °C. AEMs functionalized with N,N-dimethyl piperidinium cations show higher stability than the ones carrying N-spirocyclic quaternary ammonium. Careful analysis of the latter revealed that the rings formed in the cyclo-quaternization are more prone to degrade via Hofmann elimination than the rings introduced in the Friedel–Crafts reaction. AEMs with an ion-exchange capacity of 1.5 mequiv g–1 reach a hydroxide conductivity of 106 mS cm–1 at 80 °C under fully hydrated conditions. The AEMs are further tuned and improved by blending with polybenzimidazole (PBI). For example, an AEM containing 2 wt % PBI shows reduced water uptake and much improved robustness during handling and reaches 71 mS cm–1 at 80 °C. The study demonstrates that the critical alkaline stability of PS-containing AEMs can be significantly enhanced by replacing the benchmark benzyltrimethyl ammonium cations with N-alicyclic piperidine-based cations.

(Invited) Interactions of Block Co-Polymer Ionomers on Metal Surfaces for Tuned Electrocatalysis

In acidic fuel cells, the ionomer and catalyst layer were developed empirically, and only recently studies have begun investigating the success of the heterogeneous microstructure and triple-phase boundary. Currently, the catalyst layer is not well understood due to its complexity; however, model interfaces between ionomers and catalyst particles have been used to provide valuable insight on ionomer-catalyst interfacial interactions. It is known from studies with Nafion® that restructuring of the polymer morphology can occur when it exists as an ionomer thin film at a catalyst interface. The behavior of the thin film ionomer changes compared to its bulk characteristics, which ultimately affects electrode kinetics by altering ionic and water transport networks. In the field of alkaline fuel cells, there is limited research published on ionomer developments in general or on specific interactions between ionomers and catalysts. The few fuel cell studies that compare the effects of ionomer chemistry have conflicting conclusions, which likely arise because the ionomer chemistries interact with Pt in different ways. Overall, this provides the motivation for our work to study idealized structures in the electrode, specifically to elucidate and modify interactions occurring at the interface between ionomers and non-PGM catalysts. This insight will enable the rational and systematic development of ionomer chemistry with the ultimate goal of improving electrode kinetics for a variety of important electrochemical reactions.We have synthesized a large number of model cationic block co-polymers, diblocks, triblocks and pentablocks (AB, ABA, ABABA). Where the hydrophobic block is either polyisoprene or polycyclooctadiene, or their hydrogenated analogues, polymethylbutylene or polyethylene. The other block is polychloromethylstyrene that can be quaternized with a variety of amines to vary the interaction with the surface, inmost of work we use the relatively simple benzyltrimethylammonium (TMA) cation or the reportedly more stable and more bulky benzylmethylpyrolidinium (MPRD) cation. Our initial studies using commercial Ag nano powders showed that the olefinic groups of the polymers interacted wit the Ag and that the interaction of the cations with the Ag surface could completely remove the crystallinity from the entire sample in the case of the polyethylene block. Probing the effects of these interactions on electrocatalysis of the oxen reduction reaction is on going and we will report our progress in this paper.To further constrain the polymers we have also studied them as thin films on flat Si wafers and the same substrate coated with thin metallic films and studied by GISAXS. When block co-polymers are spin coated on plain silicon wafers we almost always see no scattering as there is no alignment of the polymer on the silicon surface. When a thin layer of silver is coated onto the silicon, discrete alignment of features perpendicular to the surface are observed for many of the polymers we have synthesized on the nanoscale. The size of these features in the thin films is not always correlated with the block sizes observed in the bulk films by SAXS. AFM imaging of the polymer air interface suggests that these vertically aligned features persist throughout the film, a property presumably needed for transport, but that the size can vary. The data shown below is typical of aligned vertical triblock ABA polymers on Ag. We have also investigated these thin films as a function of temperature and hummidity and observed transients of the changes in the morphology of these materials. In this paper we will show how these interactions can be utilized for the beneficial development of electrocatalysts. Figure 1

Quaternized poly (2, 6-dimethyl-1, 4-phenylene oxide) anion exchange membranes based on isomeric benzyltrimethylammonium cations for alkaline fuel cells

Benzyltrimethylammonium (BTMA) is most frequently-used organic cations in anion exchange membrane (AEM) materials. However, BTMA-based AEMs always suffer from low ionic conductivity and insufficient alkaline stability for practical alkaline fuel cells. Here, we present a systematic investigation of a series of side-chain-type poly (2,6-dimethyl-1,4-phenylene oxide) (PPO) AEMs with constitutional isomerism in BTMA cations. Three isomeric BTMA cations, e.g. meta-BTMA, ortho-BTMA, and para-BTMA, were tethered onto PPO backbones via a flexible spacer using CuAAC reaction, producing side-chain-type AEMs, namely m-QPPO, o-QPPO, and p-QPPO membranes, respectively. As expected, side-chain-type PPO AEMs displayed higher hydroxide conductivity as compared to a control PPO-QA membrane where BTMA cations were directly linked on PPO backbones, due to the microphase-separated morphologies as confirmed by small-angle X-ray scattering (SAXS) results. Although these isomeric quaternized PPO copolymers had identical chemical composition and polymer architectures, they did not share similar properties. Specifically, among three side-chain-type AEMs, the highest hydroxide conductivity of 42.8 mS/cm was observed for m-QPPO membrane having meta-BTMA cations with an ion exchange capacity of 1.93 meq./g at 20 °C, as a result of its high water uptake. In addition to high conductivity, m-QPPO membrane showed superior alkaline stability with respect to o-QPPO and p-QPPO membranes. After 200 h of aging in 1 M NaOH at 60 °C, 85% of the hydroxide conductivity was retained for m-QPPO AEMs, while more than 30% conductivity loss was observed for o-QPPO and p-QPPO membranes. NMR analysis of the aged membrane suggested that SN2 nucleophilic substitution at benzyl groups is the dominant degradation mechanisms. Furthermore, the AEM fuel cells using these PPO AEMs with isomeric BTMA cations were investigated, and the cell with highly conductive and durable m-QPPO membranes exhibited the best performance with a peak power density of 333 mW/cm2 at a current density of 700 mA/cm2 at 60 °C, comparable to other AEMFCs with PPO-based AEMs. Consequently, this work not only provides a facile and effective strategy to precisely synthesize isomeric AEMs, but also contributes to fundamental insights into the structure-property relationship as well as alkaline fuel cell performance for these isomeric BTMA-based AEMs, which are not explored before.

Alkaline Stability and Fuel Cell Performance of Piperidinium-Based Anion Exchange Membranes

There is a growing interest in using anion exchange membranes (AEMs) as separators in alkaline membrane fuel cells (AMFCs) and in other energy conversion and storage systems such as redox flow batteries (RFBs), alkaline water electrolyzers (AWEs) and reverse electrodialysis (RED) cells. The most commonly used cation group in AEMs is the benzyl trimethylammonium cation. However, it had been shown that quaternary ammonium-based AEMs are sensitive towards Hofmann elimination [1] and direct nucleophilic elimination reactions [2] that result in loss of ion exchange capacity (IEC) and ionic conductivity. To solve the alkaline stability issue inherent to quaternary-ammonium-group-containing AEMs, alternative cations such as piperidinium-based cations has been proposed and investigated. The piperidinium-based AEM was able to maintain its ca. 90% of its initial IEC after immersion into 1M KOH at 80 ºC for 30 days. Such piperidinium-based AEMs showed higher alkaline stability than benzyl-trimethylammonium-based AEM (ca. 20% degradation in 1M KOH at 60 ºC for 30 days) [3]. The improved alkaline stability was mainly attributed to the avoidance of attaching quaternary ammonium onto benzylic position. The planar structure of piperidinium-based cation slows down the β-elimination phenomenon. Also, there is no ether linkage with the polymer backbone and hence, the stability of polymer backbone is superior. The chloride ion conductivity of the piperidinium-based AEM was 65 mS/cm at 80 ºC under an IEC of 2.26 mmol/g. An AEMFC was assembled using our piperidinium-based AEM with Pt/C catalysts used for both anode and cathode. A peak power density of 700 mW/cm2 with 2 A/cm2 current density was obtained at 70 ºC without any backpressure.

Effect of Carbonate Anions on Quaternary Ammonium-Hydroxide Interaction

Currently, there are two main challenges in state-of-the-art anion-exchange membrane fuel cells (AEMFCs)—first, cation degradation in the presence of hydroxide anions; second, carbonation process during AEMFC operation. Both degradation and carbonation processes lead to a significant decrease in the ionic conductivity of the anion exchange membranes (AEMs), and, in turn, in the AEMFC performance. In this work, we use molecular dynamics simulations to bring first insights into the contributing factors that lead to changes in the degradation of quaternary ammonium cations due to the presence of carbonate anions. Focusing on low hydration levels, we explore the behavior of benzyltrimethylammonium cation (BTMA+) in the presence of a mixture of hydroxide and carbonate anions at different water:cation ratios. Water is shown to have a stronger affinity toward carbonate than hydroxide. Thus, the introduction of carbonate anions effectively lowers the concentration of free hydroxide anions and thereby decreases the conductivity of the AEM. Lower hydration of the hydroxide anion, in turn, leads to higher coordination of hydroxide compared with carbonate around BTMA+, hence increasing the probability of degradation of the cation. Nonetheless, carbonate competes with hydroxide in its interaction with cation, leading to approximately 20% reduction in hydroxide coordination around the BTMA+ when carbonate is present. We examine in detail these two competing factors—steric shielding of BTMA+ by carbonate and effectively lower hydration of the hydroxide—which are critical for understanding the effect of carbonate on the stability of quaternary ammonium cations.

Quaternary Ammonium Cation Specific Adsorption on Platinum Electrodes: A Combined Experimental and Density Functional Theory Study

Quaternary ammonium cations provide anionic conductivity in polymer ionomers and membranes. While study has been devoted to their alkaline stability and to improving hydroxide conductivity, there are limited studies on the electrode/cationic polymer interface. We use density functional theory (DFT) and cyclic voltammetry to examine the adsorption of tetramethyl-, tetraethyl-, tetrapropyl-, and benzyltrimethylammonium cations to platinum electrode surfaces. DFT results demonstrate that adsorption to Pt(111), Pt(100), and Pt(110) is favorable at low potentials in an alkaline electrolyte and that van der Waals interactions contribute significantly. Near-surface solvation weakens the adsorption of the long alkyl chain cations, and promotes the adsorption of the shorter chain cations, suggesting adsorption of shorter-chain cations may be possible at low potentials even in acidic electrolytes. The cations retain most of their charge on adsorption, which contributes to repulsive interactions between adsorbates. Steric hindrance further contributes to coverage dependence, with only low coverage adsorption (∼1/9–1/4 monolayer) being stable within the electrochemical window of an aqueous electrolyte. Our experimental results show that the quaternary ammonium cations blocked surface sites, hindering the adsorption of hydrogen and hydroxide. This observation is qualitatively consistent with DFT results, showing that these organic cations favorably adsorb at low potentials in alkaline electrolytes.

Cationic Side-Chain Attachment to Poly(Phenylene Oxide) Backbones for Chemically Stable and Conductive Anion Exchange Membranes

A series of chemically stable and ionically conductive side-chain anion exchange membranes (AEMs) based on poly(2,6-dimethyl-1,4-phenylene oxide) (PPO) backbones and alkyltrimethylammonium cations are reported in this work. Two alkyltrimethylammonium groups with n-propyl (C3) and n-pentyl (C5) alkyl chains were tethered onto PPO backbones through secondary amine moieties, resulting in two side-chain AEMs, that is, NC3Q-PPO and NC5Q-PPO and NC5 Q-PPO. In comparison to benzylic QA groups (e.g., benzyltrimethylammonium cations in quarternized PPO (QPPO) and benzylalkyldimethylammonium cations in comb-shaped PPOs (i.e., QC3-PPO and QC6-PPO)), the alkyltrimethylammonium cations of the side-chain PPOs, which do not possess highly reactive benzylic protons adjacent to both the aromatic ring and the cation, showed superior alkaline stability. After 30 days of aging in 1 mol/L NaOH solution at 80 °C, the retention of the conductivities of NC3Q-PPO (IEC = 2.17 mmol/g), NC5Q-PPO-40 (IEC = 2.03 mmol/g), and NC5Q-PPO-...

Anion Exchange Membrane Separators Based on Polystyrene-Block-Poly(ethylene-ran-butylene)-Block-Polystyrene Triblock Copolymers

There is a growing interest in using anion exchange membranes (AEMs) as separators in alkaline membrane fuel cells (AMFCs) and in other energy conversion and storage systems such as redox flow batteries (RFBs), alkaline water electrolyzers (AWEs) and reverse electrodialysis (RED) cells. The most commonly used cation in AEMs is the benzyl trimethylammonium cation. It is easy to synthesize due to the basicity of the trimethylamine, results in relatively large ion exchange capacities (IECs), and the resulting AEMs have good ionic conductivities. AEMs based on the benzyl trimethylammonium cation are good candidates for separators in redox flow batteries. However, it had been shown that the quaternary-ammonium-based AEMs are sensitive towards Hofmann elimination and direct nucleophilic elimination reactions under alkaline conditions, which impedes their use in AMFCs and AWEs. To solve the alkaline stability problem encountered by quaternary ammonium cations, researchers have investigated cations without alpha hydrogens to avoid degradation through ylide formation. Gu and Yan (1) first synthesized a polysulfone based ionomer containing the benzyl tris-(2,4,6-trimethoxyphenyl) phosphonium (TTMP+) cation and they found it had an outstanding stability under alkaline conditions. In our lab, we attempted to attach TTMP to polyphenylene oxide (PPO) and other polymer backbones to make alkaline stable AEMs, but the resultant AEMs were extremely brittle. Therefore, we have devoted attention to studying more robust backbones.Polystyrene-block-poly-(ethylene-ran-butylene)-block-polystyrene (SEBS) is a chemically stable and elastomeric triblock copolymer, which deforms elastically to accommodate bulky cations (TTMP+) without becoming too brittle. There have been several attempts to make SEBS-based AEM by chloromethylation, however, the degree of functionalization (DF) obtained were relatively low due to gelation during the reaction (2, 3). In our work, we have optimized the chloromethylation conditions by selecting the appropriate solvent, temperature and reaction time. The resulting polymers have DFs up to 0.3 (mol of chloromethyl groups per polymer repeat unit). We have been able to synthesize highly robust SEBS-based AEMs containing benzyl trimethyl ammonium (TMA+) and benzyl tris- (2,4,6-trimethoxyphenyl) phosphonium (TTMP+) cations. 1H-NMR spectroscopy, ion exchange capacity and tensile tests were employed to characterize the AEMs. We will report the results obtained.

Alkaline Stability of Poly(phenylene oxide) Based Anion Exchange Membranes Containing Imidazolium Cations

There is a growing interest in using anion exchange membranes (AEMs) as separators in alkaline membrane fuel cell (AMFCs) and in other energy conversion and storage systems such as redox flow batteries (RFBs), alkaline water electrolyzers (AWEs) and reverse electrodialysis (RED) cells. The most commonly used cation group in AEMs is the benzyl trimethylammonium cation. However, it had been shown that quaternary ammonium-based AEMs are sensitive towards Hofmann elimination (1) and direct nucleophilic elimination reactions (2) that result in loss of ion exchange capacity (IEC) and ionic conductivity. To solve the alkaline stability issue inherent to quaternary-ammonium-group-containing AEMs, several alternative cations, including imidazolium, benzimidazolium, guanidinium, phosphonium and metal cations have been proposed and investigated. Imidazolium-based AEMs have drawn researchers’ interests mainly because of their high hydroxide ion conductivities. In addition to 1-methylimidazole and 1,2-dimethylimidazole, a series of modified imidazole bases, namely 1-heptyl-2-methyl-1H-imidazole and 1-dodecyl-2-methyl-1H-imidazole, were synthesized, wherein these modified bases contained a long alkyl chain at the N-3 position . Anion exchange membranes (AEMs) were prepared with derived imidazolium cations either grafted onto the benzyl position of poly(phenylene oxide)(PPO) or affixed using a hexyl (6-carbon) spacer chain. AEMs with cations affixed to the benzyl position were synthesized by bromination of PPO followed by reaction with the imidazole bases. AEMs with the hexyl spacer were synthesized by Friedel-Crafts acylation of 6-bromo-1-hexanoyl chloride on PPO, followed by reduction of the ketone group and reaction with the imidazole bases. The alkaline stability of the various imidazolium-cation-containing AEMs was evaluated by: 1) measuring the change in IEC after immersion in 1M KOH at 60°C for up to 14 hours; and 2) using 1-D and 2-D NMR spectroscopy to investigate any changes in chemical structure. Both NMR and IEC experiments showed that PPO functionalized with modified imidazolium cations (with long alkyl chain at the N-3 position) did not exhibit better alkaline stability than benchmark AEMs made with 1-methylimidazole. However, the use of a six-carbon spacer did improve AEM alkaline stability when the AEM was prepared directly using 1-methylimidazole and 1,2-dimethylimidazole, but not when the AEM was prepared with the modified imidazolium cations (with a long alkyl chain at the N-3 position).

Alkaline Chemical Stability and Ion Transport in Polymerized Ionic Liquids with Various Backbones and Cations

The development of anion exchange membranes (AEMs) with high alkaline chemical stability, as well as high ion conductivity, is crucial to the implementation of long-lasting, low-cost (nonplatinum) alkaline fuel cells (AFCs). In this study, 12 polymerized ionic liquids (PILs) were synthesized with various backbones and cations (backbones: ethyl methacrylate, undecyl methacrylate, undecyl acrylate, styrene; covalently attached cations: butylimidazolium, trimethylammonium, butylpyrrolidinium). 1H NMR spectroscopy was employed to determine the alkaline degradation mechanisms and extent of degradation at high pH (in D2O) at 60 °C for 1 week (168 h). For the butylpyrrolidinium cation, ethyl and undecyl methacrylate backbones proved to be more stable than the undecyl acrylate backbone. Styrene (vinylbenzene) functionalized with butylpyrrolidinium cation surpassed the stability of the benchmark benzyltrimethylammonium (BTMA) cation, where the former possessed the highest overall observed chemical stability with 0...

Study of Polyphenylene Oxide Membranes Containing Long and Short Alkyl Side Chains

Investigation on the subject of structure and chemical stability was carried out on two anion exchange membranes that have a great potential for being candidates in alkaline fuel cells. Both of the membranes contain poly(2,6-dimethylphenylene oxide) (PPO) backbone, one has benzyltrimethyl ammonium cation (BTMA) while the other has long (10 carbons) alkyl side chain pendant to the nitrogen-centered cation (C10) as can be seen in Scheme. Membrane with the long alkyl side chain contributes to better hydrophilic–hydrophobic separation which significantly influence on the ionic channels of the membranes.Membrane morphology and in particular hydrophilic-hydrophobic separation was investigated using tapping-mode AFM. Both of the membranes exhibit phase-separated morphology and connectivity among the hydrophilic domains. AFM measurements show that long alkyl side chain contributed to better phase separation in the C10 membrane, where the hydrophilic domain size was ∼8 nm compared to ∼5 nm in BTMA. In addition, chemical stabilities of the membranes were evaluated under accelerated-aging conditions. Degradation process was determined by measuring FTIR, Elemental analysis and EDS during the aging. It was found that long side chain contributes to its chemical stability under basic and high temperature conditions. Figure 1

Effects of ion pairing on the capillary electrophoretic separation and ultraviolet-absorption detection of quaternary ammonium cations using meso-octamethylcalix[4]pyrrole as the background electrolyte additive.

Five quaternary ammonium cations, including tetramethylammonium, tetraethylammonium, hexadecyltrimethylammonium, benzyltrimethylammonium, and 1-butyl-3-methylimidazolium, have been separated by capillary electrophoresis. A direct ultraviolet method has been achieved when tetrabutylammonium fluoride was the background electrolyte and meso-octamethylcalix[4]pyrrole was the background electrolyte additive. The ultraviolet spectra of meso-octamethylcalix[4]pyrrole and cation mixtures showed that redshifts can be attributed to the size of cations, and the maximum absorption wavelength shifted from 218 to 230 nm when tetrabutylammonium cation was substituted with tetramethylammonium cation or tetraethylammonium cation. Conductivity measurements were performed to evaluate the ion-pairing effect of tetrabutylammonium fluoride in a mixture of acetonitrile/ethanol (80:20, v/v), and the ion-pairing formation constant, Kip, was calculated (Kip = 14.8 ± 0.3 L/mol) using the Fuoss extended model. Ion pairing also occurs between cations of the analytes and counterion, a fluoride complex of meso-octamethylcalix[4]pyrrole. The tetramethylammonium cations associate more strongly with this counterion than the tetraethylammonium cation that contributes to the change of selectivity in capillary electrophoresis separation. The effective mobilities of the cations with trimethyl groups, such as tetramethylammonium cation, benzyltrimethylammonium cation, and hexadecyltrimethylammonium cation, decreased faster than others with the increase of meso-octamethylcalix[4]pyrrole concentration, highlighting the fact that the ion-pairing effect played an important role in this method.

Hydroxide Degradation Pathways for Substituted Benzyltrimethyl Ammonium: A DFT Study

The stability of cations used in the alkaline exchange membranes has been a major challenge. In this paper, degradation energy barriers were investigated by density functional theory for substituted benzyltrimethyl ammonium (BTMA+) cations. It was found that electron-donating substituent groups at meta-position(s) of the benzyl ring could result in increased degradation barriers. However, after investigating more than thirty substituted BTMA+ cations, the largest improvement in degradation barrier found was only 6.7 kJ/mol. This suggests a modest (8×) improvement in stability for this type of approach may be possible, but for anything greater other approaches will need to be pursued.

Computational Modeling of Degradation of Substituted Benzyltrimethyl Ammonium

The stability of cations used in the alkaline exchange membranes has been a major challenge for alkaline membrane fuel cells. In this paper, we investigated the degradation barriers by density functional theory for substituted benzyltrimethyl ammonium (BTMA+) cations. BTMA+ is one of the most commonly used cations for alkaline exchange membranes. We found that substituted cations with electron-donating substituent groups at meta-position of the benzyl ring could result in improved degradation barriers. However, after investigating more than thirty substituted BTMA+ cations with ten different substituent groups, the largest improvement of degradation barriers calculated was only 1.6 kcal/mol. While this may be a meaningful increase in stability for some applications, it suggests that only a modest improvement in stability is possible through substitution of BTMA+.