Anion exchange membranes with well-developed conductive channels: Effect of the functional groups

Cycloaliphatic Quaternary Ammonium Functionalized Poly(oxindole biphenyl) Based Anion-Exchange Membranes for Water Electrolysis: Stability and Performance

Mechanically robust anion-exchange membranes (AEMs) with high conductivity and long-term alkali resistance are needed for water electrolysis application. In this work, aryl-ether free polyaromatics containing isatin moieties were prepared via super acid-catalyzed copolymerization, followed by functionalization with alkaline stable cyclic quaternary ammonium (QA) cationic groups, to afford high performance AEMs for application in water electrolysis. The incorporation of side functional cationic groups (pyrrolidinium and piperidinium) onto a polymer backbone via a flexible alkyl spacer aimed at conductivity and alkaline stability improvement. The effect of cation structure on the properties of prepared AEMs was thoroughly studied. Pyrrolidinium- and piperidinium-based AEMs showed similar electrolyte uptakes and no obvious phase separation, as revealed by SAXS and further supported by AFM and TEM data. In addition, these AEMs displayed high conductivity values (81. 5 and 120 mS cm-1 for pyrrolidinium- and piperidinium-based AEM, respectively, at 80 °C) and excellent alkaline stability after 1 month aging in 2M KOH at 80 °C. Especially, a pyrrolidinium-based AEM membrane preserved 87% of its initial conductivity value, while at the same time retaining its flexibility and mechanical robustness after storage in alkaline media (2M KOH) for 1 month at 80 °C. Based on 1H NMR data, the conductivity loss observed after the aging test is mainly related to the piperidinium degradation that took place, probably via ring-opening Hofmann elimination, alkyl spacer scission and nucleophilic substitution reactions as well. The synthesized AEMs were also tested in an alkaline water electrolysis cell. Piperidinium-based AEM showed superior performance compared to its pyrrolidinium analogue, owing to its higher conductivity as revealed by EIS data, further confirming the ex situ conductivity measurements.

Research Progress in Energy Based on Polyphosphazene Materials in the Past Ten Years

With the rapid development of electronic devices, the corresponding energy storage equipment has also been continuously developed. As important components, including electrodes and diaphragms, in energy storage device and energy storage and conversion devices, they all face huge challenges. Polyphosphazene polymers are widely used in various fields, such as biomedicine, energy storage, etc., due to their unique properties. Due to its unique design variability, adjustable characteristics and high chemical stability, they can solve many related problems of energy storage equipment. They are expected to become a new generation of energy materials. This article briefly introduces the research progress in energy based on polyphosphazene materials in the past ten years, on topics such as fuel cells, solar cells, lithium batteries and supercapacitors, etc. The main focus of this work is on the defects of different types of batteries. Scholars have introduced different functional group modification that solves the corresponding problem, thus increasing the battery performance.

Clustered piperidinium-functionalized poly(terphenylene) anion exchange membranes with well-developed conductive nanochannels

Anion exchange membrane fuel cells (AEMFCs) attract considerable attention owing to their high-power density and potential utilization of cheap non-noble metal catalysts. However, anion exchange membranes (AEMs) still face the problems of low conductivity, poor dimensional and chemical stability. To address these issues, AEMs with clustered piperidinium groups and ether-bond-free poly(terphenylene) backbone (3QPAP-x, x = 0.3, 0.4, and 0.5) were designed. Transmission electron microscope results show that the clustered ionic groups are responsible for fabricating well-developed conductive nanochannels and restraining the swelling behavior of the membranes. 3QPAP-0.4 and 3QPAP-0.5 AEMs exhibit higher conductivity (117.5 mS cm^-1, 80 °C) and lower swelling ratio than that of commercial FAA-3-50 (80.4 mS cm^-1, 80 °C). The conductivity of 3QPAP-0.5 only decreased by 10.4% after treating with 1 M NaOH at 80 °C for 720 h. The Hofmann elimination degradation of the cationic groups is restrained by the long flexible alkyl chain between cations. Based on the high performance of 3QPAP-0.5, an H₂-O₂-type AEMFC reaches 291.2 mW cm^-2 (60 °C), which demonstrates that the as-prepared AEMs are promising for application in fuel cells.

Polybenzimidazole Ultrathin Anion Exchange Membrane with Comb-Shape Amphiphilic Microphase Networks for a High-Performance Fuel Cell

A comb-shape amphiphilic cationic side chain is proposed to well-balance the water sorption in anion exchange membranes (AEMs), in which the cationic group is in between of an ether-containing hydrophilic spacer and an alkyl hydrophobic spacer. By fully grafting the amphiphilic side chains onto polybenzimidazole (PBI), comb-shape amphiphilic microphase networks are well-developed in the AEMs, in which the alkyl hydrophobic network greatly restricts water swelling and the ether-containing hydrophilic network keeps the hydration of the cationic groups and enlarges the ion conductive channel. The as-prepared membranes achieve a high conductivity of about 91.2 mS cm^-1, an extremely low swelling ratio of about 8.1% at 80 °C, and good mechanical properties at a hydrated state (tensile strength and elongation at a break of about 14.6 MPa and 77.5%, respectively). Benefits from the balanced water sorption in AEMs, the H₂/O₂ fuel cell with a 10 μm ultrathin membrane could withstand 80 °C and 0.1 MPa back pressure and achieve a high open circuit voltage of about 1.0 V and a high peak power density of about 631.5 mW cm^-2. This work provides a new insight into the design of high-performance AEM.

Characterizing Cation Chemistry for Anion Exchange Membranes-para-Alkyl-Substituted Benzyltetramethylimidazolium and Benzyltrimethylammonium Salts in Base

Base stability and degradation of imidazolium-functional materials have been a focus of research on anion exchange membranes for electrochemical devices. Despite extensive work, mechanistic questions can cloud efforts to correlate results from model compounds and polymeric membranes. Here, we report an NMR study of behavior in aqueous KOH of benzyltetramethylimidazolium and benzyltrimethylammonium salts with and without para-alkyl-substitution to probe whether base attack at the alkyl substituent plays a role in the reaction pathways. In all cases, the major products from the imidazolium salts are consistent with hydrolytic ring fragmentation as the principal mode of decomposition. Also, relative decomposition kinetics for both types of salts provided no conclusive evidence for a change in reaction mechanism upon introduction of the alkyl substituent. While our data do not rule out base attack at the alkyl group in the cases where rate differences were noted, these differences appear to be better attributed to reaction medium effects of submicrometer oil phase droplets arising from salt aggregation.

Fast Bulky Anion Conduction Enabled by Free Shuttling Phosphonium Cations

Highly conductive anion-exchange membranes (AEMs) are desirable for applications in various energy storage and conversion technologies. However, conventional AEMs with bulky HCO₃ ^- or Br^- as counterion generally exhibit low conductivity because the covalent bonding restrains the tethered cationic group's mobility and rotation. Here, we report an alternative polyrotaxane AEM with nontethered and free-shuttling phosphonium cation. As proved by temperature-dependent NMR, solid-state NMR, and molecular dynamics simulation, the phosphonium cation possesses a thermally trigged shuttling behavior, broader extension range, and greater mobility, thus accelerating the diffusion conduction of bulky anions. Owing to this striking feature, high HCO₃ ^- conductivity of 105 mS cm^-1 at 90°C was obtained at a relatively lower ion-exchange capacity of 1.17 mmol g^-1. This study provides a new concept for developing highly conductive anion-exchange membranes and will catalyze the exploration of new applications for polyrotaxanes in ion conduction processes.

Poly(meta/para-Terphenylene-Methyl Piperidinium)-Based Anion Exchange Membranes: The Effect of Backbone Structure in AEMFC Application

A series of poly(meta/para-terphenylene-methyl piperidinium)-based anion exchange membranes devoid of benzylic sites or aryl ether bonds, that are vulnerable to degradation by hydroxide ions, are synthesized and investigated for their application as novel anion exchange membranes. The copolymers are composed of both linear para-terphenyl units and kink-structured meta-terphenyl units. The meta-connectivity in terphenyl units permits the polymer backbones to fold back, maximizing the interactions among the hydrocarbon polymer chains and enhancing the peripheral formation of ion aggregates, due to the free volume generated by the kink structure. The effects of the copolymer composition between para-terphenyl and meta-terphenyl on the morphology and the electrochemical and physicochemical properties of the corresponding polymer membranes are investigated.

Fabrication of Conducting Nanochannels Using Accelerator for Fuel Cell Membrane and Removal of Radionuclides: Role of Nanoparticles

Latent tracks in pure polymer and its nanohybrid are fabricated by irradiating with swift heavy ions (SHI) (Ag⁺) having 140 MeV energy followed by selective chemical etching of the amorphous path, caused by the irradiation of SHI, to generate nanochannels of size ∼80 nm. Grafting is done within the nanochannels utilizing free radicals generated from the interaction of high-energy ions, followed by tagging of ionic species to make the nanochannels highly ion-conducting. The uniform dispersion of two-dimensional nanoparticles better controls the size and number density of the nanochannels and, thereby, converts them into an effective membrane. The nanoparticle and functionalization induce a piezoelectric β-phase in the membrane. The functionalized membrane removes the radioactive nuclide like ²⁴¹Am⁺³ (α-emitting source) efficiently (∼80% or 0.35 μg/cm²) from its solution/waste. This membrane act as a corrosion inhibitor (92% inhibition efficiency) together with its higher proton conduction (0.13 S/m) ability. The higher ion-exchange capacity, water uptake, ion conduction, and high sorption by the nanohybrid membrane are explored with respect to the extent of functionalization and control over nanochannel dimension. A membrane electrode assembly has been fabricated to construct a complete fuel cell, which exhibits superior power generation (power density of 45 mW/cm² at a current density of 298 mA/cm²) much higher than that of the standard Nafion, measured in a similar condition. Further, a piezoelectric matrix along with its anticorrosive property, high sorption characteristics, and greater power generation makes this class of material a smart membrane that can be used for many different applications.

Micro-block versus random quaternized poly(arylene ether sulfones) with highly dense quaternization units for anion exchange membranes

A systematic study was carried out to investigate the effect of different distributions of conducting groups in segments for poly(arylene ether sulfone)s.

Characterizing Cation Chemistry for Anion Exchange Membranes-A Product Study of Benzylimidazolium Salt Decompositions in the Base

Imidazolium functionality has played a prominent role in research on anion exchange membranes for use in alkaline electrochemical devices. Base stability and degradation of these materials has been much studied, but in many instances, product pathways have not been thoroughly delineated. We report an NMR study of base-induced decomposition products from three benzylimidazolium salts bearing varying extents of methyl substitution on the imidazolium ring. The major products are consistent with a hydrolytic ring fragmentation pathway as the principal mode of decomposition. We observe several new products not previously reported in the literature on imidazolium salt degradation, including benzilic acid rearrangement products formally derived from intermediate 1,2-dicarbonyl compounds or their equivalents. However, the overall reactions are complex, the yields of observed products do not account for all consumed starting materials, and mechanistic ambiguities remain.

Poly(2,6-dimethyl-1,4-phenylene oxide)s with Various Head Groups: Effect of Head Groups on the Properties of Anion Exchange Membranes

Poly(2,6-dimethyl-1,4-phenylene oxide)s (PPOs)-based anion exchange membranes (AEMs) with four of the most widely investigated head groups were prepared. Through a combination of experimental and simulation approaches, the effects of the different types of head groups on the properties of the AEMs, including hydroxide conductivity, water content, physicochemical stability, and fuel cell device performance were fully explored. Unlike other studies, in which the conductivity was mostly investigated in liquid water, the conductivity of the PPO-based AEMs in 95% relative humidity (RH) conditions as well as in liquid water was investigated. The conductivity trend in 95% RH condition was different from that in liquid water but corresponded well with the actual cell performance trend observed, suggesting that the AEM fuel cell performance under in situ cell conditions (95% RH, 60 °C, H₂/O₂) is more closely related to the conductivity measured ex situ under 95% RH conditions (60 °C) than in liquid water. On the basis of the conductivity data and molecular simulation results, it was concluded that the predominant hydroxide ion-conducting mechanism in liquid water differs from that in the operating fuel cell environment, where the ionomers become hydrated only as a result of water vapor transported into the cells.

Multiscale Modeling of Structure, Transport and Reactivity in Alkaline Fuel Cell Membranes: Combined Coarse-Grained, Atomistic and Reactive Molecular Dynamics Simulations

In this study, molecular dynamics (MD) simulations of hydrated anion-exchange membranes (AEMs), comprised of poly(p-phenylene oxide) (PPO) polymers functionalized with quaternary ammonium cationic groups, were conducted using multiscale coupling between three different models: a high-resolution coarse-grained (CG) model; Atomistic Polarizable Potential for Liquids, Electrolytes and Polymers (APPLE&P); and ReaxFF. The advantages and disadvantages of each model are summarized and compared. The proposed multiscale coupling utilizes the strength of each model and allows sampling of a broad spectrum of properties, which is not possible to sample using any of the single modeling techniques. Within the proposed combined approach, the equilibrium morphology of hydrated AEM was prepared using the CG model. Then, the morphology was mapped to the APPLE&P model from equilibrated CG configuration of the AEM. Simulations using atomistic non-reactive force field allowed sampling of local hydration structure of ionic groups, vehicular transport mechanism of anion and water, and structure equilibration of water channels in the membrane. Subsequently, atomistic AEM configuration was mapped to ReaxFF reactive model to investigate the Grotthuss mechanism in the hydroxide transport, as well as the AEM chemical stability and degradation mechanisms. The proposed multiscale and multiphysics modeling approach provides valuable input for the materials-by-design of novel polymeric structures for AEMs.