The effect of polymer backbones and cation functional groups on properties of anion exchange membranes for fuel cells

Alkali-Stable Cations and Anion Exchange Membranes

Anion exchange membranes (AEMs) based energy conversion and storage devices have attracted attention as an innovative technology due to their advantageous alkaline catalytic kinetics and cost-effectiveness. AEMs play a crucial role in these devices and have shown significant progress in terms of ionic conductivity, mechanical properties, alkaline stability, and other essential characteristics. Nevertheless, their durability remains a limiting factor preventing the large-scale deployment of AEMs based devices. The attack of hydroxide ions on the cations is an inherent issue that needs to be addressed to enhance the lifetime of the AEMs. Therefore, the design of more stable cationic groups is essential to maintain the initial properties of AEMs and extend the device lifetime. This concept systematically summarizes the development and stability enhancement strategies of the cationic groups for AEMs in recent years, with particular emphasis on the emerging cyclic cationic groups. Furthermore, the stability differences of cations in small molecules versus AEMs are systematically discussed, as well as prospective research toward stable AEMs.

Multiscale Understanding of Anion Exchange Membrane Fuel Cells: Mechanisms, Electrocatalysts, Polymers, and Cell Management

Anion exchange membrane fuel cells (AEMFCs) are among the most promising sustainable electrochemical technologies to help solve energy challenges. Compared to proton exchange membrane fuel cells (PEMFCs), AEMFCs offer a broader choice of catalyst materials and a less corrosive operating environment for the bipolar plates and the membrane. This can lead to potentially lower costs and longer operational life than PEMFCs. These significant advantages have made AEMFCs highly competitive in the future fuel cell market, particularly after advancements in developing non-platinum-group-metal anode electrocatalysts, anion exchange membranes and ionomers, and in understanding the relationships between cell operating conditions and mass transport in AEMFCs. This review aims to compile recent literature to provide a comprehensive understanding of AEMFCs in three key areas: i) the mechanisms of the hydrogen oxidation reaction (HOR) and the oxygen reduction reaction (ORR) in alkaline media; ii) recent advancements in the synthesis routes and structure-property relationships of cutting-edge HOR and ORR electrocatalysts, as well as anion exchange membranes and ionomers; and iii) fuel cell operating conditions, including water management and impact of CO2. Finally, based on these aspects, the future development and perspectives of AEMFCs are proposed.

Diamine Crosslinked Addition-Type Diblock Poly(Norbornene)s-Based Anion Exchange Membranes with High Conductivity and Stability for Fuel Cell Applications

Anion exchange membranes (AEMs) as a kind of important functional material are widely used in fuel cells. However, synthetic AEMs generally suffer from low conductivity, poor alkaline stability, and poor dimensional stability. Constructing efficient ion transport channels is widely regarded as one of the most effective strategies for developing AEMs with high conductivity and low swelling ratio. Herein we demonstrate a versatile strategy to prepare the AEMs with both high conductivity and excellent alkali stability via all-carbon hydrogen block copolymer backbone hydrophilic crosslinking and introducing flexible alkoxy spacer chains. Additionally, we investigated the impact of the crosslinking degree on the AEMs' performances. It was found that the dosage of the hydrophilic crosslinker has a significant impact on the construction of efficient ion transport channels in the AEMs. Amazingly, the CL30-aPNB-TMHDA-TMA exhibited the highest hydroxide conductivity (138.84 mS cm-1), reasonable water uptake (54.96%), and a low swelling ratio (14.07%) at 80 °C. Meanwhile, the membrane showed an excellent alkaline stability in a 1 M NaOH solution at 80 °C for 1008 h (ion exchange capacity (IEC) and OH- conductivity remained at 91.9% and 89.12%, respectively). The single cells assembled with CL30-aPNB-TMHDA-TMA exhibited a peak power density of 266.2 mW cm-2 under a current density of 608 mA cm-2 at 80 °C. The novel developed composite strategy of flexible alkoxy side chains with hydrophilic crosslinking modification is potentially promised to be an effective approach to develop the high-performance AEMs.

Diverse applications of versatile quaternized chitosan salts: A review

In the ever-evolving world of materials science, modifying natural polymers has garnered significant attention across diverse industries, driven by their inherent availability and cost-effectiveness. Among these, chitosan, a pseudo-natural cationic polymer, has emerged as a versatile player, finding applications in medical, pharmaceutical, filtration, and textile sectors, owing to its exceptional biodegradability, non-allergenicity, antimicrobial properties, and eco-friendly nature. However, the limitations of chitosan, such as low surface area, poor solubility at neutral to alkaline pH, and inadequate thermal-mechanical properties, have prompted researchers to explore innovative modification strategies, including graft copolymerization, quaternization, and cross-linking. This review delves into the remarkable potential of a specific chitosan derivative, N-[(2-hydroxy-3-trimethylammonium) propyl] chitosan salts (N-HTCS), a quaternized form of chitosan. This review uniquely examines the properties and multifaceted applications of N-HTCS, spanning biomedical, textile, food packaging, and environmental domains. The outstanding features of N-HTCS, including antioxidant, anticancer, and antimicrobial bioactivity, as well as biocompatibility, biodegradability, hemostatic, piezoelectric, superparamagnetic, water solubility, and permeation-enhancing effects, offer novel solutions to the limitations of unmodified chitosan. Notably, while previous reviews have addressed the significance of chitosan, this work presents a groundbreaking focus on the N-HTCS derivative, providing a fresh perspective and paving the way for the design and engineering of cutting-edge N-HTCS-based devices and applications. The comprehensive coverage of this review aims to inspire researchers and industry professionals to explore the untapped potential of this remarkable chitosan derivative, unlocking new frontiers in material science and technology.

Aryl ether-free polymer electrolytes for electrochemical and energy devices

Anion exchange polymers (AEPs) play a crucial role in green hydrogen production through anion exchange membrane water electrolysis. The chemical stability of AEPs is paramount for stable system operation in electrolysers and other electrochemical devices. Given the instability of aryl ether-containing AEPs under high pH conditions, recent research has focused on quaternized aryl ether-free variants. The primary goal of this review is to provide a greater depth of knowledge on the synthesis of aryl ether-free AEPs targeted for electrochemical devices. Synthetic pathways that yield polyaromatic AEPs include acid-catalysed polyhydroxyalkylation, metal-promoted coupling reactions, ionene synthesis via nucleophilic substitution, alkylation of polybenzimidazole, and Diels-Alder polymerization. Polyolefinic AEPs are prepared through addition polymerization, ring-opening metathesis, radiation grafting reactions, and anionic polymerization. Discussions cover structure-property-performance relationships of AEPs in fuel cells, redox flow batteries, and water and CO2 electrolysers, along with the current status of scale-up synthesis and commercialization.

Tuning Alkaline Anion Exchange Membranes through Crosslinking: A Review of Synthetic Strategies and Property Relationships

Alkaline anion exchange membranes (AAEMs) are an enabling component for next-generation electrochemical devices, including alkaline fuel cells, water and CO2 electrolyzers, and flow batteries. While commercial systems, notably fuel cells, have traditionally relied on proton-exchange membranes, hydroxide-ion conducting AAEMs hold promise as a method to reduce cost-per-device by enabling the use of non-platinum group electrodes and cell components. AAEMs have undergone significant material development over the past two decades; however, challenges remain in the areas of durability, water management, high temperature performance, and selectivity. In this review, we survey crosslinking as a tool capable of tuning AAEM properties. While crosslinking implementations vary, they generally result in reduced water uptake and increased transport selectivity and alkaline stability. We survey synthetic methodologies for incorporating crosslinks during AAEM fabrication and highlight necessary precautions for each approach.

Electrocatalysis in Alkaline Media and Alkaline Membrane-Based Energy Technologies

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 molecular-level 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 high-performance and durable alkaline fuel cells and related technologies.

Semi-Continuous Desalination and Concentration of Small-Volume Samples

Electrodialysis is an electric-field-mediated process separating ions exploiting selective properties of ion-exchange membranes. The ion-exchange membranes create an ion-depleted zone in an electrolyte solution adjacent to the membrane under DC polarization. We constructed a microfluidic system that uses the ion-depleted zone to separate ions from the processed water solution. We tested the separation performance by desalting a model KCl solution spiked with fluorescein for direct observation. We showed both visually and by measuring the conductivity of the output solutions that the system can work in three modes of operation referred to as continuous desalination, desalination by accumulation, and unsuccessful desalination. The mode of operation can easily be set by changing the control parameters. The desalination factors for the model KCl solution reached values from 80 to 100%, depending on the mode of operation. The concentration factor, given as a ratio of concentrate-to-feed concentrations, reached zero for desalination by accumulation when only diluate was produced. The water recovery, therefore, was infinite at these conditions. Independent control of the diluate and concentrate flow rates and the DC voltage turned our system into a versatile platform, enabling us to set proper conditions to process various samples.

Conductivity and Stability Properties of Anion Exchange Membranes: Cation Effect and Backbone Effect

The rise of heterocycle cations, a new class of stable cations, has fueled faster growth of research interest in heterocycle cation-attached anion exchange membranes (AEMs). However, once cations are grafted onto backbones, the effect of backbones on properties of AEMs must also be taken into account. In order to comprehensively study the influence of cations effect and backbones effect on AEMs performance, a series of AEMs were prepared by grafting spacer cations, heterocycles cations, and aromatic cations onto brominated poly(2,6-dimethyl-1,4-phenylene oxide) (BPPO) or poly(vinylbenzyl chloride) (PVB) backbones, respectively. Spacer cation [trimethylamine (TMA), N,N-dimethylethylamine (DMEA)]-attached AEMs showed general ion transportation and stability behaviors, but exhibited high cationic reaction efficiency. Heterocycle cation [1-methylpyrrolidine (MPY), 1-methylpiperidine (MPrD)]-attached AEMs showed excellent chemical stability, but their ion conduction properties were unimpressive. Aromatic cation [1-methylimidazole (MeIm), N,N-dimethylaniline (DMAni)]-attached AEMs exhibited superior ionic conductivity, while their poor cations stabilities hindered the application of the membranes. Besides, it was found that PVB-based AEMs had excellent backbone stability, but BPPO-based AEMs exhibited higher OH^- conductivity and cation stability than those of the same cations grafted PVB-based AEMs due to their higher water uptake (WU). For example, the ionic conductivities (ICs) of BPPO-TMA and PVB-TMA at 80 °C were 53.1 and 38.3 mS cm^-1 , and their WU was 152.3 and 95.1 %, respectively. After the stability test, the IC losses of BPPO-TMA and PVB-TMA were 21.4 and 32.2 %, respectively. The result demonstrated that the conductivity and stability properties of the AEMs could be enhanced by increasing the WU of the membranes. These findings allowed the matching of cations to the appropriate backbones and reasonable modification of the AEM structure. In addition, these results helped to fundamentally understand the influence of cation effect and backbone effect on AEM performance.

Recent Insights on Catalyst Layers for Anion Exchange Membrane Fuel Cells

Anion exchange membrane fuel cells (AEMFCs) performance have significantly improved in the last decade (>1 W cm-2 ), and is now comparable with that of proton exchange membrane fuel cells (PEMFCs). At high current densities, issues in the catalyst layer (CL, composed of catalyst and ionomer), like oxygen transfer, water balance, and microstructural evolution, play important roles in the performance. In addition, CLs for AEMFCs have different requirements than for PEMFCs, such as chemical/physical stability, reaction mechanism, and mass transfer, because of different conductive media and pH environment. The anion exchange ionomer (AEI), which is the soluble or dispersed analogue of the anion exchange membrane (AEM), is required for hydroxide transport in the CL and is normally handled separately with the electrocatalyst during the electrode fabrication process. The importance of the AEI-catalyst interface in maximizing the utilization of electrocatalyst and fuel/oxygen transfer process must be carefully investigated. This review briefly covers new concepts in the complex AEMFC catalyst layer, before a detailed discussion on advances in CLs based on the design of AEIs and electrocatalysts. The importance of the structure-function relationship is highlighted with the aim of directing the further development of CLs for high-performance AEMFC.

Densely Quaternized Fluorinated Poly(fluorenyl ether)s with Excellent Conductivity and Stability for Vanadium Redox Flow Batteries

Cationic group distribution and elemental composition are two key factors determining the conductivity and stability of anion exchange membranes (AEMs) for vanadium redox flow batteries (VRFBs). Herein, fluorinated tetra-dimethylaminomethyl-poly(fluorenyl ether)s (TAPFE)s were designed as the polymer precursors, which were reacted with 6-bromo-N,N,N-trimethylhexan-1-aminium bromide to introduce di-quaternary ammonium (DQA) containing side chains. The resultant DQA-TAPFEs with a rigid fluorinated backbone and flexible multi-cationic side chains exhibited distinct micro-phase separation as probed by small-angle X-ray scattering (SAXS) and atomic force microscopy (AFM). DQA-TAPFE-20 with an ion exchange capacity (IEC) of 1.55 mmol g^-1 exhibited a SO₄^2- conductivity of 10.1 mS cm^-1 at room temperature, much higher than that of a control AEM with an identical backbone but spaced out cationic groups, which had a similar IEC of 1.60 mmol g^-1 but a SO₄^2- conductivity of only 3.2 mS cm^-1. Due to the Donnan repulsion effect, the DQA-TAPFEs exhibited significantly lower VO²⁺ permeability than Nafion 212. The VRFB assembled with DQA-TAPFE-20 achieved an energy efficiency of 80.4% at 80 mA cm^-1 and a capacity retention rate of 82.9% after the 50th cycling test, both higher than those of the VRFB assembled with Nafion 212 and other AEMs in the literature. Therefore, the rationally designed DQA-TAPFEs are promising candidates for VRFB applications.

Application of Crosslinked Polybenzimidazole-Poly(Vinyl Benzyl Chloride) Anion Exchange Membranes in Direct Ethanol Fuel Cells

Crosslinked membranes have been synthesized by a casting process using polybenzimidazole (PBI) and poly(vinyl benzyl chloride) (PVBC). The membranes were quaternized with 1,4-diazabicyclo[2.2.2]octane (DABCO) to obtain fixed positive quaternary ammonium groups. XPS analysis has showed insights into the changes from crosslinked to quaternized membranes, demonstrating that the crosslinking reaction and the incorporation of DABCO have occurred, while the ¹³C-NMR corroborates the reaction of DABCO with PVBC only by one nitrogen atom. Mechanical properties were evaluated, obtaining maximum stress values around 72 MPa and 40 MPa for crosslinked and quaternized membranes, respectively. Resistance to oxidative media was also satisfactory and the membranes were evaluated in single direct ethanol fuel cell. PBI-c-PVBC/OH 1:2 membrane obtained 66 mW cm⁻² peak power density, 25% higher than commercial PBI membranes, using 0.5 bar backpressure of pure O₂ in the cathode and 1 mL min⁻¹ KOH 2M EtOH 2 M aqueous solution in the anode. When the pressure was increased, the best performance was obtained by the same membrane, reaching 70 mW cm⁻² peak power density at 2 bar O₂ backpressure. Based on the characterization and single cell performance, PBI-c-PVBC/OH membranes are considered promising candidates as anion exchange electrolytes for direct ethanol fuel cells.