Novel alkaline anion exchange membranes containing pendant benzimidazolium groups for alkaline fuel cells

Anion Exchange Membranes for Alkaline Polymer Electrolyte Fuel Cells-A Concise Review

Solid anion exchange membrane (AEM) electrolytes are an essential commodity considering their importance as separators in alkaline polymer electrolyte fuel cells (APEFC). Mechanical and thermal stability are distinguished by polymer matrix characteristics, whereas anion exchange capacity, transport number, and conductivities are governed by the anionic group. The physico-chemical stability is regulated mostly by the polymer matrix and, to a lesser extent, the cationic head framework. The quaternary ammonium (QA), phosphonium, guanidinium, benzimidazolium, pyrrolidinium, and spirocyclic cation-based AEMs are widely studied in the literature. In addition, ion solvating blends, hybrids, and interpenetrating networks still hold prominence in terms of membrane stability. To realize and enhance the performance of an alkaline polymer electrolyte fuel cell (APEFC), it is also necessary to understand the transport processes for the hydroxyl (OH^-) ion in anion exchange membranes. In the present review, the radiation grafting of the monomer and chemical modification to introduce cationic charges/moiety are emphasized. In follow-up, the recent advances in the synthesis of anion exchange membranes from poly(phenylene oxide) via chloromethylation and quaternization, and from aliphatic polymers such as poly(vinyl alcohol) and chitosan via direct quaternization are highlighted. Overall, this review concisely provides an in-depth analysis of recent advances in anion exchange membrane (AEM) and its viability in APEFC.

Anion-Exchange Membrane Water Electrolyzers

This Review provides an overview of the emerging concepts of catalysts, membranes, and membrane electrode assemblies (MEAs) for water electrolyzers with anion-exchange membranes (AEMs), also known as zero-gap alkaline water electrolyzers. Much of the recent progress is due to improvements in materials chemistry, MEA designs, and optimized operation conditions. Research on anion-exchange polymers (AEPs) has focused on the cationic head/backbone/side-chain structures and key properties such as ionic conductivity and alkaline stability. Several approaches, such as cross-linking, microphase, and organic/inorganic composites, have been proposed to improve the anion-exchange performance and the chemical and mechanical stability of AEMs. Numerous AEMs now exceed values of 0.1 S/cm (at 60-80 °C), although the stability specifically at temperatures exceeding 60 °C needs further enhancement. The oxygen evolution reaction (OER) is still a limiting factor. An analysis of thin-layer OER data suggests that NiFe-type catalysts have the highest activity. There is debate on the active-site mechanism of the NiFe catalysts, and their long-term stability needs to be understood. Addition of Co to NiFe increases the conductivity of these catalysts. The same analysis for the hydrogen evolution reaction (HER) shows carbon-supported Pt to be dominating, although PtNi alloys and clusters of Ni(OH)2 on Pt show competitive activities. Recent advances in forming and embedding well-dispersed Ru nanoparticles on functionalized high-surface-area carbon supports show promising HER activities. However, the stability of these catalysts under actual AEMWE operating conditions needs to be proven. The field is advancing rapidly but could benefit through the adaptation of new in situ techniques, standardized evaluation protocols for AEMWE conditions, and innovative catalyst-structure designs. Nevertheless, single AEM water electrolyzer cells have been operated for several thousand hours at temperatures and current densities as high as 60 °C and 1 A/cm2, respectively.

Performance of Quaternized Polybenzimidazole-Cross-Linked Poly(vinylbenzyl chloride) Membranes in HT-PEMFCs

High-temperature proton-exchange membrane fuel cells (HT-PEMFCs) are mostly based on acid-doped membranes composed of polybenzimidazole (PBI). A severe drawback of acid-doped membranes is the deterioration of mechanical properties upon increasing acid-doping levels. Cross-linking of different polymers is a way to mitigate stability issues. In this study, a new ion-pair-coordinated membrane (IPM) system with quaternary ammonium groups for the application in HT-PEMFCs is introduced. PBI cross-linked with poly(vinylbenzyl chloride) and quaternized with three amines (DABCO, quinuclidine, and quinuclidinol) are manufactured and compared to the state-of-the-art commercial Dapazol PBI membrane ex situ as well as by evaluating their HT-PEMFC performance. The IPMs show reduced swelling and better mechanical properties upon doping, which enables a reduction in membrane thickness while maintaining a comparably low gas crossover and mechanical stability. The HT-PEMFC based on the best-performing IPM reaches up to 530 mW cm^-2 at 180 °C under H₂/air conditions at ambient pressure, while Dapazol is limited to less than 430 mW cm^-2 at equal parameters. This new IPM system requires less acid doping than conventional PBI membranes while outperforming conventional PBI membranes, which renders these new membranes promising candidates for application in HT-PEMFCs.

Progress in neutron techniques: towards improved polymer electrolyte membranes for energy devices

Design and implementation of advanced membrane formulations for selective transport of ions and molecular species are critical for creating the next generations of fuel cells and separation devices. It is necessary to understand the detailed transport mechanisms over time- and length-scales relevant to the device operation, both in laboratory models and in working systems under realistic operational conditions. Neutron scattering techniques including quasi-elastic neutron scattering, reflectivity and imaging are implemented at beamline stations at reactor and spallation source facilities worldwide. With the advent of new and improved instrument design, detector methodology, source characteristics and data analysis protocols, these neutron scattering techniques are emerging as a primary tool for research to design, evaluate and implement advanced membrane technologies for fuel cell and separation devices. Here we describe these techniques and their development and implementation at the ILL reactor source (Institut Laue-Langevin, Grenoble, France) and ISIS Neutron and Muon Spallation source (Harwell Science and Technology Campus, UK) as examples. We also mention similar developments under way at other facilities worldwide, and describe approaches such as combining optical with neutron Raman scattering and x-ray absorption with neutron imaging and tomography, and carrying out such experiments in specialised fuel cells designed to mimic as closely possible actualoperandoconditions. These experiments and research projects will play a key role in enabling and testing new membrane formulations for efficient and sustainable energy production/conversion and separations technologies.

Effect of Different Quaternary Ammonium Groups on the Hydroxide Conductivity and Stability of Anion Exchange Membranes

Anion exchange membrane fuel cells (AEMFCs) are encouraging electrochemical structures for the competent and complaisant conversion of energy. Herein, the development of brominated poly(2,6-dimethyl phenylene oxide) (BPPO)-based anion exchange membranes (AEMs) with different quaternary ammonium groups for AEMFCs was reported. The successful preparation of AEMs was proved by utilizing proton nuclear magnetic resonance and Fourier transform infrared spectroscopy. They were explored in terms of water uptake (W _R), ion exchange capacity (IEC), hydration number (λ), linear swelling ratio (LSR), morphology, tensile strength (TS), and elongation at break (E _b). The alkaline stability of the prepared AEMs was assessed and compared with each other. The experimental outcomes demonstrated that the N-methylpyrrolidinium-based membrane (MPyPPO) exhibited higher alkaline stability, whereas the N-methylimidazolium-based membrane (MImPPO) showed the lowest alkaline stability among the prepared AEMs. Similarly, the hydroxide conductivity of the prepared AEMs was measured and compared with each other. The pyrrolidinium-based membrane (MPyPPO) exhibited higher hydroxide conductivity among the prepared AEMs.

Transport and Electrochemical Characteristics of CJMCED Homogeneous Cation Exchange Membranes in Sodium Chloride, Calcium Chloride, and Sodium Sulfate Solutions

Recently developed and produced by Hefei Chemjoy Polymer Material Co. Ltd., homogeneous CJMC-3 and CJMC-5 cation-exchange membranes (CJMCED) are characterized. The membrane conductivity in NaCl, Na2SO4, and CaCl2 solutions, permeability in respect to the NaCl and CaCl2 diffusion, transport numbers, current-voltage curves (CVC), and the difference in the pH (DpH) of the NaCl solution at the desalination compartment output and input are examined for these membranes in comparison with a well-studied commercial Neosepta CMX cation-exchange membrane produced by Astom Corporation, Japan. It is found that the conductivity, CVC (at relatively low voltages), and water splitting rate (characterized by DpH) for both CJMCED membranes are rather close to these characteristics for the CMX membrane. However, the diffusion permeability of the CJMCED membranes is significantly higher than that of the CMX membrane. This is due to the essentially more porous structure of the CJMCED membranes; the latter reduces the counterion permselectivity of these membranes, while allowing much easier transport of large ions, such as anthocyanins present in natural dyes of fruit and berry juices. The new membranes are promising for use in electrodialysis demineralization of brackish water and natural food solutions.

Anion Exchange Membranes Obtained from Poly(arylene ether sulfone) Block Copolymers Comprising Hydrophilic and Hydrophobic Segments

The anion exchange membrane may have different physical and chemical properties, electrochemical performance and mechanical stability depending upon the monomer structure, hydrophilicity and hydrophobic repeating unit, surface form and degree of substitution of functional groups. In current work, poly(arylene ether sulfone) (PAES) block copolymer was created and used as the main chain. After controlling the amount of NBS, the degree of bromination (DB) was changed in Br-PAES. Following that, quaternized PAES (Q-PAES) was synthesized through quaternization. Q-PAES showed a tendency of enhancing water content, expansion rate, ion exchange capacity (IEC) as the degree of substitution of functional groups increased. However, it was confirmed that tensile strength and dimensional properties of membrane reduced while swelling degree was increased. In addition, phase separation of membrane was identified by atomic force microscope (AFM) image, while ionic conductivity is greatly affected by phase separation. The Q-PAES membrane demonstrated a reasonable power output of around 64 mW/cm² while employed as electrolyte in fuel cell operation.

Effect of Micromorphology on Alkaline Polymer Electrolyte Stability

Recent studies demonstrated that the chemical stability of alkaline polymer electrolytes (APEs) could be improved by reducing the inductive effect between cations and backbones. Therefore, pendent cations were recommended. However, microphase-separated morphologies would be generated by elongating the spacer between cations and backbones, which have a significant influence on the chemical stability of APEs too. In order to analyze how the patterns of micromorphology affect the chemical stability of the materials, in the present work, four APEs ( a₁-QAPS, a₃-QAPS, a₅-QAPS, and a₇-QAPS) with different lengths of side chain between polysulfone and quaternary ammonium are synthesized. The longer the side chain is, the more obvious the microphase separation for the a _x-QAPS membranes is observed. After immersing in a hot alkaline solution (80 °C, 1 M KOH) for 30 days, a₅-QAPS exhibits the highest chemical stability. The ion exchange capacity and ionic conductivity of  a₅-QAPS film are reduced by 10.0 and 10.5%, respectively. The weight loss of  a₅-QAPS membrane is 8.0%, which is similar to the value of the pristine backbone. The increased chemical stability can be ascribed to the suitable micromorphology constructed in  a₅-QAPS sample. Besides,  a₅-QAPS membrane shows a high conductivity of 75.5 mS cm^-1, whereas the swelling ratio is limited to 15.0% in liquid water at 80 °C. In addition, a peak power density of 339.1 mW cm^-2 is obtained by applying a₅-QAPS as the APE to the H₂-O₂ fuel cell at 60 °C.

The Alkaline Stability of Anion Exchange Membrane for Fuel Cell Applications: The Effects of Alkaline Media

Alkaline alcohols (methanol, ethanol, propanol, and ethylene glycol) have been applied as fuels for alkaline anion exchange membrane fuel cells. However, the effects of alkaline media on the stability of anion exchange membranes (AEMs) are still elusive. Here, a series of organic cations including quaternary ammonium, imidazolium, benzimidazolium, pyridinium, phosphonium, pyrrolidinium cations, and their corresponding cationic polymers are synthesized and systematically investigated with respect to their chemical stability in various alkaline media (water, methanol, ethanol, and dimethyl sulfoxide) by quantitative 1H nuclear magnetic resonance spectroscopy and density functional theory calculations. In the case of protic solvents (water, methanol, and ethanol), the lower dielectric constant of the alkaline media, the lower is the lowest unoccupied molecular orbital (LUMO) energy of the organic cation, which leads to the lower alkaline stability of cations. However, the hydrogen bonds between the anions and protic solvents weaken the effects of low dielectric constant of the alkaline media. The aprotic solvent accelerated the SN2 degradation reaction of "naked" organic cations. The results of this study suggest that both the chemical structure of organic cations and alkaline media (fuels) applied affect the alkaline stability of AEMs.

Synthesis of gemini basic ionic liquids and their application in anion exchange membranes

A gemini-type basic morpholine ionic liquid ([Nbmd][OH]) was synthesized via a two-step method with morpholine, bromododecane and 1,4-dibromobutane as raw materials, and its structure was characterized by 1H NMR and FT-IR spectroscopy. Meanwhile, a series of anion exchange membranes ([Nbmd][OH] x -QCS) were prepared with quaternized chitosan (QCS) as the polymer matrix and [Nbmd][OH] as the dopant owing to its strong alkalinity and good solubility. The structures of the [Nbmd][OH] x -QCS composite membranes were characterized in detail by FT-IR spectroscopy, the OH- conductivity by AC impedance spectroscopy, and the morphological features by scanning electron microscopy (SEM), thermal gravity analysis (TGA), etc. The results show that the [Nbmd][OH] x -QCS composite membranes have uniform surfaces and cross-section morphology. Increasing the content of [Nbmd][OH] not only enhances the thermal stability but also increases the OH- conductivity; the thermal decomposition temperature of the [Nbmd][OH]40-QCS membrane is nearly 20 °C higher than that of the pristine QCS membrane, and the maximum OH- conductivity is approximately 1.37 × 10-2 S cm-2 at 70 °C. The methanol permeability of the [Nbmd][OH]40-QCS membrane in 1 M methanol at room temperature is 2.21 × 10-6 cm-2 s-1, which is lower than that of Nafion®115, indicating a promising potential use in alkaline direct methanol fuel cells. Moreover, the [Nbmd][OH]40-QCS membrane exhibits the best alkaline stability of all the membranes prepared in this work, retaining approximately 81% of its initial conductivity after immersion in 3 M KOH solution for 120 h at 70 °C.

Grotthuss versus Vehicular Transport of Hydroxide in Anion-Exchange Membranes: Insight from Combined Reactive and Nonreactive Molecular Simulations

Combined reactive and nonreactive polarizable molecular dynamics simulations were used to probe the transport mechanisms of hydroxide in hydrated anion-exchange membranes (AEMs) composed of poly(p-phenylene oxide) functionalized with the quaternary ammonium cationic groups. The direct mapping of membrane morphologies between two models allowed us to investigate the contributions of vehicular and Grotthuss mechanisms in hydroxide motion and correlate these mechanisms with the details of local structure. In AEMs with nonblocky polymer structure, where anion transport occurs through narrow (subnanometer size) percolating water channels, simulations indicate the importance of the Grotthuss mechanism. In nonreactive simulations, in order to diffuse through bottlenecks in the water channels, the hydroxide anion has to lose part of its hydration structure, therefore creating a large kinetic barrier for such events. However, when the Grotthuss mechanism is involved, the hydroxide transport through these bottlenecks can easily occur without loss of anion hydration structure and with a much lower barrier.

Anion-Exchange Membranes for Alkaline Fuel-Cell Applications: The Effects of Cations

Alkaline anion-exchange membrane fuel cells (AEMFCs) are attracting much attention because of their potential use of nonprecious electrocatalysts. The anion-exchange membrane (AEM) is one of the key components of AEMFCs. An ideal AEM should possess high hydroxide conductivity and sufficient long-term durability at elevated temperatures in high-pH solutions. Herein, recent progress in research into the alkaline stability behavior of cations (including quaternary ammonium, imidazolium, guanidinium, pyridinium, tertiary sulfonium, phosphonium, benzimidazolium, and pyrrolidinium) and their analogous AEMs, which have been investigated by both experimental studies and theoretical calculations, is reviewed. Effects, including conjugation, steric hindrance e, σ-π hyperconjugation, and electrons, on the alkaline stability of cations and their analogous AEMs have been discussed. The aim of this article is to provide an overview of some key factors for the future design of novel cations and their analogous AEMs with high alkaline stability.

Highly fouling-resistant brominated poly(phenylene oxide) membranes using surface grafted diethylenetriamine

Composite BPPO/DETA ultrafiltration membranes show decreased membrane fouling and enhanced protein rejection with very high flux recovery ratios.

Tetrakis(dialkylamino)phosphonium Polyelectrolytes Prepared by Reversible Addition-Fragmentation Chain Transfer Polymerization

A tetrakis(dialkylamino)phosphonium cation ([P(NR₂)₄]⁺) was appended to a styrenic monomer and explored in reversible addition-fragmentation chain transfer polymerization (RAFT) to conduct random copolymerizations of the cationic monomer with styrene. Well-defined polyelectrolytes with molecular weights up to ∼30 100 and dispersities between ∼1.2 and 1.4 were obtained. Up to 18.9 mol % of the ionic monomer could be incorporated into the polymer with hexafluorophosphate or bis(trifluoromethane)sulfonimide acting as the counterion during polymerization. Differential scanning calorimetry of the hexafluorophosphate polymers revealed glass transition temperatures higher than polystyrene likely due to interactions between the anion and the polymer. Thermogravimetric analysis indicated these materials have high thermal stability with decomposition temperatures approaching 400 °C.

Synthesis and investigation of imidazolium functionalized poly(arylene ether sulfone)s as anion exchange membranes for all-vanadium redox flow batteries

A serials of imidazolium functionalized poly(arylene ether sulfone) as anion exchange membranes (AEMs) for all-vanadium redox flow battery (VRB) application are synthesized successfully in this study.