Alkaline-stable anion exchange membranes: A review of synthetic approaches

Revealing the Crystalline Architecture of Semicrystalline Ion Exchange Membranes for the Design of Conductive and Durable Alkaline Anion Exchange Membranes

Alkaline anion exchange membrane (AAEM) fuel cells offer a cost-effective alternative to proton exchange membrane (PEM) fuel cells by eliminating the need for expensive precious metal catalysts. In both PEMs and AAEMs, semicrystalline polymers are a common choice, as the crystalline domains can act as mechanical reinforcements that limit swelling and promote mechanical durability in the material. However, spatially resolved characterization of crystalline organization in ion exchange membranes beyond ensemble-averaged X-ray scattering is underrepresented, likely in part due to ionization damage limitations in soft materials. Here, we resolve the nanometer-size crystallites in semicrystalline ion exchange membranes by applying cryogenic four-dimensional scanning transmission electron microscopy (cryo-4D-STEM) along with data-processing algorithms designed to optimize signals at a low dose to minimize radiation damage. We investigate the effects of synthesis components, including molecular weight and thermal treatment, on a model system of AAEMs in comparison to Nafion, the most commonly used and commercially successful PEM today. We find that excess water uptake in polymer membranes, a property directly associated with weak mechanical durability and with possible negative impacts on ion conductivity, can be reduced by over 30% by varying the polymer's crystalline morphology through changes in synthesis parameters such as molecular weight and thermal history. Our results indicate that this improvement is correlated with smaller crystalline domains with a more homogeneous distribution. More broadly, these results demonstrate how the crystalline architecture of polymer membranes can be tuned through their chemistry and thermal treatment in order to improve their conductivity and durability for commercial fuel cell performance.

Alkylation as a strategy for optimizing water uptake and enhancing selectivity in polyethyleneimine-based anion-exchange membranes for brine mining via electrodialysis

Brine treatment poses a significant challenge for the growing desalination industry, yet it also holds valuable elements and a substantial amount of water. To efficiently extract these elements and increase water recovery, the development of advanced, highly selective separation technologies is urgently needed. This study addresses this challenge by optimizing polyethyleneimine (PEI)-based anion exchange membranes (AEMs) through an alkylation strategy to enhance water uptake control and ion selectivity. The aim is to achieve the high separation efficiency required for effective reverse osmosis (RO) brine mining via electrodialysis. The careful design of functional amine groups with a mixed composition of alkyl substituents enabled the development of membranes with reduced water uptake and high charge density, providing the best conductivity/selectivity ratio, enhanced ion selectivity, and decreased water-splitting activity. The unmodified PEI-membrane already demonstrated a competitive performance compared to common commercial AEMs membranes used in electrodialysis, such as FujiFilm® AEM Type 1 and 2, Ralex® AM-PP, and Neosepta® AMX. However, the alkylation further improved the performance significantly. Among modified membranes, PEI alkylated with propyl followed by methyl (PEI-Pr-Me) achieved the highest current efficiency of 93 %, while PEI alkylated with a mixture of four (C1C4)n-alkyl groups had the highest Cl⁻/SO42⁻-selectivity coefficients of up to 8.7 and the lowest water transfer across the membrane. This tailored functionalization approach presents a promising pathway for improving AEMs' performance in desalination brine treatment, enabling more efficient water and mineral recovery.

Tetraarylphosphonium Cations with Excellent Alkaline-Resistant Performance for Anion-Exchange Membranes

To realize the robust anion exchange membrane (AEM)-based water splitting modules and fuel cells, the design and synthesis of tetraarylphosphonium (TAP) cations are described as a new class of cationic building blocks that exhibit remarkable alkaline stability under harsh conditions. TAP cations with highly sterically demanding aromatic substituents were efficiently synthesized from triarylphosphine derivatives and highly reactive arynes, whose alkaline degradation proved to be suppressed dramatically by the sterically demanding substituents. In the case of bis(2,5-dimethylphenyl)bis(2,4,6-trimethylphenyl)phosphonium, for example, approximately 60% of the cation survived for 27 d under the forced conditions (i.e., in 4 M KOH/CD3OH at 80 °C), while tetraphenylphosphonium degraded completely within 10 min in 1 M KOH/CD3OH at that temperature. Through the decomposition of the alkaline-stable TAP cations, not only triarylphosphine oxides, which are often reported to form via the nucleophilic attack toward the cationic phosphorus center, but also triarylphosphines were detected, which suggested the presence of other degradation mechanisms due to the sterically demanding aromatic substituents. In kinetic analyses, bis(2,5-dimethylphenyl)bis(2,4,6-trimethylphenyl)phosphonium was found to exhibit 52 times higher stability compared to benzyltrimethylammonium, which is often employed as the cationic building block for AEMs.

Unraveling the Synergistic Role of Sm3+ Doped NiFe-LDH as High-Performance Electrocatalysts for Improved Anion Exchange Membrane and Water Splitting Applications

Effective first-row transition metal-based electrocatalysts are crucial for large-scale hydrogen energy generation and anion exchange membrane (AEM) devices in water splitting. The present work describes that SmNi0.02Fe-LDH nanosheets on nickel foam are used as a bifunctional electrocatalyst for water splitting and AEM water electrolyzer study. Tuning the Ni-to-Fe ratios in NiFe-LDH and doping with Sm ions improves the electrical structure and intrinsic activity. SmNi0.02Fe-LDH has higher oxygen evolution reaction (OER), HER, and TWS activity, achieving 10 mA cm⁻2 current density at lower overpotentials (230 mV, 95 mV, and 1.62 V, respectively). In AEMWE cells, SmNi0.02Fe-LDH as a cathode and anode pair exhibits outstanding activity (0.9 A cm⁻2 at 2 V) and stability over 120 h. Density Functional Theory (DFT) investigations reveal that the Sm doping in NiFe-LDH surface enhances its bifunctional activity toward OER and HER. These findings emphasize the potential of non-noble composites for long-term water electrolysis in total water splitting and AEMWE applications.

Polyarylmethylpiperidinium (PAMP) for Next Generation Anion Exchange Membranes

Terawatt-scale hydrogen production using anion exchange membrane water electrolyzers (AEM-WEs) requires the development of facilely prepared, alkali-stable, and high-performance anion exchange membranes (AEMs). State-of-the-art polyarylpiperidinium AEMs fail to match the alkaline stability of piperidinium due to conformational deformation caused by the stiff cardo structure. Herein, polyarylmethylpiperidinium (PAMP) AEM with pendant structure is constructed by utilizing 4-formylpiperidine as a functional monomer. The inclusion of an aldehyde group in the synthesis enhances polymerization reactivity, reduces the amount of superacid required, and ensures that the piperidinium is suspended from the ether-free polymer backbone. The accelerated aging analysis demonstrates that the pendant structure of piperidinium effectively suppresses the Hofmann elimination, resulting in PAMP AEM with exceptional alkali stability, surpassing that of the commercial PiperION-A40. Most importantly, when assembled with non-noble metal OER/HER catalysts, the related AEM-WE achieves a remarkably high transient current density of 7.35 A cm-2 (@2 V, 80 °C, 1 m KOH). Moreover, the AEM-WE can operate stably at industrial current density over 1500 h (≈1.70 V at 1.0 A cm-2).

Facilitating Rapid OH-/H2O Transport in Anion Exchange Membranes via Ultra-Stable Heteroatom-Free Micropores

Efficient OH- conduction in anion exchange membranes (AEMs) is pivotal for the advancement and industrialization of sustainable electrochemical technologies in alkaline environments, including water electrolysis, fuel cells, and CO2 electroreduction. We here designed AEMs with a novel class of rigid heteroatom-free micropores (HFMs), engineered at the molecular level to facilitate rapid ionic transport in an ultra-stable manner. By manipulating monomers, our design strategically controls the torsional angles and energy barriers within the polymeric backbones, creating sub-nanometer ionic channels that precisely regulate porosity. These hydrophilic micropores significantly enhance the mobility of OH-/H2O, achieving over a 150 % increase in self-diffusion coefficient compared to commercial AEMs and elevating OH- conductivity to a leading 215 mS cm-1 at 80 °C. Moreover, the robust carbon-carbon bond construction in HFMs offers the stability that is four orders of magnitude higher under severe alkaline conditions compared to existing wisdoms, with a demonstrated operational lifespan of over 4000 hours. The integration of HFM-AEMs into water electrolyzers not only supports the use of platinum group metal-free catalysts but also exhibits exceptional energy efficiency and extended durability, highlighting their substantial potential for wide-ranging applications in emerging electrochemical technologies.

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.

Composite Anion Exchange Membranes Containing a Long-Side Chain Ionomer and Exfoliated Lamellar Double Hydroxides

Anion Exchange Membranes (AEMs) are promising materials for electrochemical devices, such as fuel cells and electrolyzers. However, the main drawback of AEMs is their low durability in alkaline operating conditions. A possible solution is the use of composite ionomers containing inorganic fillers stable in a basic environment. In this work, composite anion exchange membranes are prepared from poly (2,6-dimethyl-1,4-phenylene oxide) with quaternary ammonium groups on long-side chains (PPO-LC) and exfoliated Mg/Al lamellar double hydroxide (LDH) as inorganic filler added in different percentages (2, 5, and 10%). The mechanical stiffness of the membranes increases significantly by the addition of exfoliated LDH up to 5%. The ionic conductivity is measured as a function of the temperature in fully humidified conditions and as a function of relative humidity (RH). The maximum conductivity is observed for 5% LDH. The average activation energy for conductivity amounts to 0.20 ± 0.01 eV in fully humidified conditions and >50% RH. Thermogravimetric analysis of membranes before and after alkaline degradation tests (2 M KOH @ 80 °C, 48 h) reveals that the sample with 5% LDH has improved stability (19% vs. 36% of degradation). The stability tests are also investigated, measuring the ionic conductivity and the water uptake. A protective effect of LDH on the alkaline degradation of quaternary ammonium groups is clearly evidenced and opens the way to the use of different compounds and exfoliation methods in the LDH family.

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.

Synthesis and characterization of soluble pyridinium-containing copolyimides

Novel ionene-type cationic copolyimides based on 4,4'-oxydiphthalic anhydride (ODPA), 4,4'-(1,4-phenylenediisopropylidene)bisaniline (BIS P), and 2,6-diaminopyridine (DAP) were synthesized. The copolyimides were obtained in two stages: first, the copolyimides with the 0/1, 0.2/0.8, 0.3/0.7, 0.5/0.5, 0.6/0.4 and 1/0 DAP/Bis P ratios were obtained through thermal imidization, and then quaternization of soluble copolyimides with methyl iodide was conducted for 24 or 48 h. The samples were characterized via FTIR, NMR and EDX methods to confirm their structure and composition. The cationic copolyimides with a DAP content of less than 0.3 showed initial weight loss (onset) at about 250 °C, according to TGA results and demonstrated solubility in chloroform. The highest ionic conductivity value of 0.234 S cm-1 was showed by the sample with 0.3 DAP content and 0.15 degree of quaternization. The stability of the membranes in alkaline media was evaluated using FTIR and TGA. It was shown that samples with a DAP content of more than 0.3 lost their integrity probably owing to partial hydrolysis of imide rings, while copolyimides with a DAP content of 0.2 and 0.3 remained stable.

Protocol for Evaluating Anion Exchange Membranes for Nonaqueous Redox Flow Batteries

Nonaqueous redox flow batteries often suffer from reduced battery lifetime and decreased coulombic efficiency due to crossover of the redox-active species through the membrane. One method to mitigate this undesired crossover is to judiciously choose a membrane based on several criteria: swelling and structural integrity, size and charge of redox active species, and ionic conductivity. Most research to date has focused on reducing crossover by synthesizing modified redox-active molecules and/or new membranes. However, no standard protocol exists to compare membranes and a comprehensive study comparing membranes has yet to be done. To address both these limitations, we evaluate herein 26 commercial anion exchange membranes (AEMs) to assess their compatibility with common nonaqueous solvents and their resistance to crossover by using neutral and cationic redox-active molecules. Ultimately, we found that all the evaluated AEMs perform poorly in organic solvents due to uncontrolled swelling, low ionic conductivity, and/or high crossover rates. We believe that this method, and the generated data, will be useful to evaluate and compare the performance of all AEMs─commercial and newly synthesized─and should be implemented as a standard protocol for future research.

Machine Learning-Aided Design of Highly Conductive Anion Exchange Membranes for Fuel Cells and Water Electrolyzers

Alkaline anion exchange membrane (AEM)-based fuel cells (AEMFCs) and water electrolyzers (AEMWEs) are vital for enabling the efficient and large-scale utilization of hydrogen energy. However, the performance of such energy devices is impeded by the relatively low conductivity of AEMs. The conventional trial-and-error approach to designing membrane structures has proven to be both inefficient and costly. To address this challenge, a fully connected neural network (FCNN) model is developed based on acid-catalyzed AEMs to analyze the relationship between structure and conductivity among 180,000 AEM variations. Under machine learning guidance, anilinium cation-type membranes are designed and synthesized. Molecular dynamics simulations and Mulliken charge population analysis validated that the presence of a large anilinium cation domain is a result of the inductive effect of N+ and benzene rings. The interconnected anilinium cation domains facilitated the formation of a continuous ion transport channel within the AEMs. Additionally, the incorporation of the benzyl electron-withdrawing group heightened the inductive effect, leading to high conductivity AEM variant as screened by the machine learning model. Furthermore, based on the highly active and low-cost monomers given by machine learning, the large-scale synthesis of anilinium-based AEMs confirms the potential for commercial applications.

Ionic nanoporous membranes from self-assembled liquid crystalline brush-like imidazolium triblock copolymers

There is a need to generate mechanically and thermally robust ionic nanoporous membranes for separation and fuel cell applications. Herein, we report a general approach to the preparation of ionic nanoporous membranes through custom synthesis, self-assembly, and subsequent chemical manipulations of ionic brush block copolymers. We synthesized polynorbornene-based triblock copolymers containing imidazolium cations balanced by counter anions in the central block, side-chain liquid crystalline units, and sidechain polylactide end blocks. This unique platform comprises: (1) imidazolium/bis(trifluoromethanesulfonyl)imide (TFSI) as the middle block, which has an excellent ion-exchange ability, (2) cyanobiphenyl liquid crystalline end block, a sterically hindered hydrophobic segment, which is chemically stable and immune to hydroxide attack, (3) polylactide brush-like units on the other end block that is easily etched under mild alkaline conditions and (4) a polynorbornene backbone, a lightly crosslinked system that offers mechanical robustness. These membranes retain their morphology before and after backbone crosslinking as well as etching of polylactide sidechains. The ion exchange performance and dimensional stability of these membranes were investigated by water uptake capability and swelling ratio. Moreover, the length of the carbon spacer in the imidazolium/TFSI central block moiety endowed the membrane with improved ionic conductivity. The ionic nanoporous materials are unusual due to their singular thermal, mechanical, alkaline stability and ion transport properties. Applications of these materials include electrochemical actuators, solid-state ionic nanochannel biosensors, and ion-conducting membranes.

Functionalized Triblock Copolymers with Tapered Design for Anion Exchange Membrane Fuel Cells

Triblock copolymers such as styrene-b-(ethylene-co-butylene)-b-styrene (SEBS) have been widely used as an anion exchange membrane for fuel cells due to their phase separation properties. However, modifying the polymer architecture for optimized membrane properties is still challenging. This research develops a strategy to control the membrane morphology based on quaternized SEBS (SEBS-Q) by dual-tapering the interfacial block sequences. The structural and transport properties of SEBS-Q with various tapering styles at different hydration levels are systematically investigated by coarse-grained molecular simulations. The results show that the introduction of the tapered regions induces the formation of a bicontinuous water domain and promotes the diffusivity of the mobile components. The interplay between the solvation of the quaternary groups and the tapered fraction determines the conformation of polymer chains among the hydrophobic-hydrophilic subdomains. The strategy presented here provides a new path to fabricating fuel cell membranes with controlled microstructures.

Bioadhesive-Inspired Ionomer for Membrane Electrode Assembly Interface Reinforcement in Fuel Cells

Anion exchange membrane fuel cells promise a sustainable and ecofriendly energy conversion pathway yet suffer from insufficient performance and durability. Drawing inspiration from mussel foot adhesion proteins for the first time, we herein demonstrate catechol-modified ionomers that synergistically reinforce the membrane electrode assembly interface and triple-phase boundary inside catalyst layers. The resulting ionomers present exceptional alkaline stability with only slight ionic conductivity declines after treatment in 2 M NaOH aqueous solution at 80 °C for 2500 h. Adopting catechol-modified ionomer as both anion exchange membrane and binder achieves a single-cell performance increase of 34%, and more importantly, endows fuel cell operation at a current density of 0.4 A cm-2 for over 300 h with negligible performance degradation (with a cell voltage decay rate of 0.03 mV h-1). Combining theoretical and experimental investigations, we reveal the molecular adhesion mechanism between the catechol-modified ionomer and Pt catalyst and illuminate the effect on the catalyst layer microstructure. Of fundamental interest, this bioadhesive-inspired strategy is critical to enabling knowledge-driven ionomer design and is promising for diverse membrane electrode assembly configurational applications.

Polyarylene-Based Anion Exchange Membranes for Fuel Cells

Anion exchange membrane fuel cell (AEMFC) is an emerging and promising technology that can help realize a carbon-neutral, sustainable economy. Also, compared to the proton exchange membrane counterpart, AEMFC can achieve comparable cell outputs with lower costs due to the applicability of non-platinum group metal electrocatalysts for the reaction on the electrodes' surfaces. However, the wide application of the AEMFCs has been impeded by the unsatisfactory stability and performance of the hydroxide-conductive membranes in the past. Recently researchers have made breakthroughs using polyarylene (PA)-based AEMs. This article summarizes the recent advances of a class of AEMs with aromatic backbone without ether bonds, mainly synthesized by Friedel-Crafts polycondensation. Such PA-based AEMs showed high chemical/mechanical stabilities and ionic conductivity, and even the fuel cell with those AEMs showed impressive peak power density of up to 2.58 W cm-2. In this concept article, we classify major strategies for making PA-based AEMs to show the recent trends, highlight synthesis, characterization, and properties, and provide a brief outlook.

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.

Separators and Membranes for Advanced Alkaline Water Electrolysis

Traditionally, alkaline water electrolysis (AWE) uses diaphragms to separate anode and cathode and is operated with 5-7 M KOH feed solutions. The ban of asbestos diaphragms led to the development of polymeric diaphragms, which are now the state of the art material. A promising alternative is the ion solvating membrane. Recent developments show that high conductivities can also be obtained in 1 M KOH. A third technology is based on anion exchange membranes (AEM); because these systems use 0-1 M KOH feed solutions to balance the trade-off between conductivity and the AEM's lifetime in alkaline environment, it makes sense to treat them separately as AEM WE. However, the lifetime of AEM increased strongly over the last 10 years, and some electrode-related issues like oxidation of the ionomer binder at the anode can be mitigated by using KOH feed solutions. Therefore, AWE and AEM WE may get more similar in the future, and this review focuses on the developments in polymeric diaphragms, ion solvating membranes, and AEM.

Stable Anion Exchange Membrane Bearing Quinuclidinium for High-performance Water Electrolysis

Anion exchange membranes (AEMs) are core components in anion exchange membrane water electrolyzers (AEM-WEs). However, the stability of functional quaternary ammonium cations, especially under high temperatures and harsh alkaline conditions, seriously affects their performance and durability. Herein, we synthesized a 1-methyl-3,3-diphenylquinuclidinium molecular building unit. Density functional theory (DFT) calculations and accelerated aging analysis indicated that the quinine ring structure was exceedingly stable, and the SN2 degradation mechanism dominated. Through acid-catalyzed Friedel-Crafts polymerization, a series of branched poly(aryl-quinuclidinium) (PAQ-x) AEMs with controllable molecular weight and adjustable ion exchange capacity (IEC) were prepared. The stable quinine structure in PAQ-x was verified and retained in the ex situ alkaline stability. Furthermore, the branched polymer structure reduces the swelling rate and water uptake to achieve a tradeoff between dimensional stability and ionic conductivity, significantly improving the membrane's overall performance. Importantly, PAQ-5 was used in non-noble metal-based AEM-WE, achieving a high current density of 8 A cm-2 at 2 V and excellent stability over 2446 h in a gradient constant current test. Based on the excellent alkaline stability of this diaryl-quinuclidinium group, it can be further considered as a multifunctional building unit to create multi-topological polymers for energy conversion devices used in alkaline environments.

CO2 Electrolyzers

CO2 electrolyzers have progressed rapidly in energy efficiency and catalyst selectivity toward valuable chemical feedstocks and fuels, such as syngas, ethylene, ethanol, and methane. However, each component within these complex systems influences the overall performance, and the further advances needed to realize commercialization will require an approach that considers the whole process, with the electrochemical cell at the center. Beyond the cell boundaries, the electrolyzer must integrate with upstream CO2 feeds and downstream separation processes in a way that minimizes overall product energy intensity and presents viable use cases. Here we begin by describing upstream CO2 sources, their energy intensities, and impurities. We then focus on the cell, the most common CO2 electrolyzer system architectures, and each component within these systems. We evaluate the energy savings and the feasibility of alternative approaches including integration with CO2 capture, direct conversion of flue gas and two-step conversion via carbon monoxide. We evaluate pathways that minimize downstream separations and produce concentrated streams compatible with existing sectors. Applying this comprehensive upstream-to-downstream approach, we highlight the most promising routes, and outlook, for electrochemical CO2 reduction.

Highly Hydrophilic Zirconia Composite Anion Exchange Membrane for Water Electrolysis and Fuel Cells

To prepare anion exchange membranes with high water electrolysis and single fuel cell performance, an inorganic-organic composite (IOC) strategy with click cross-linked membranes coated with different contents of hydrophilic polar nanozirconia is proposed to fabricate composite membranes (CM) PBP-SH-Zrx. The performance test results showed that the CM PBP-SH-Zr4 not only has good through-plane ionic conductivity (167.7 mS cm-1, 80 °C), but also exhibits satisfactory dimensional stability (SR 16.5%, WU 206.4%, 80 °C), especially demonstrating excellent alkaline stability with only 16% degradation (2 M NaOH for 2200 h). In water electrolysis, the "microgap" between the membrane and catalyst layer (solid-solid interface) is alleviated, and the membrane electrode assembly (MEA) interfacial compatibility (liquid-solid-solid interface) is enhanced. The CM PBP-SH-Zr4 showed the lowest charge transfer resistance (Rct, 0.037 Ω cm2) and a high current density of 2.5 A cm-2 at 2.2 V, while the voltage drop was 0.361 mV h-1 after 360 h of endurance (six start-stop cycles) at 60 °C and 500 mA cm-2, proving a good water electrolysis durability. Moreover, an acceptable peak power density of 0.464 W cm-2 at 80 °C is achieved in a H2/O2 fuel cell with a PBP-SH-Zr4-AEM. Therefore, the IOC strategy can enhance the membrane's comprehensive performance and interface compatibility of MEA and may promote the development of anion exchange membranes (AEMs) for water electrolysis and fuel cells.

Anion Exchange Ionomers: Design Considerations and Recent Advances - An Electrochemical Perspective

Alkaline-based electrochemical devices, such as anion exchange membrane (AEM) fuel cells and electrolyzers, are receiving increasing attention. However, while the catalysts and membrane are methodically studied, the ionomer is largely overlooked. In fact, most of the studies in alkaline electrolytes are conducted using the commercial proton exchange ionomer Nafion. The ionomer provides ionic conductivity; it is also essential for gas transport and water management, as well as for controlling the mechanical stability and the morphology of the catalyst layer. Moreover, the ionomer has distinct requirements that differ from those of anion-exchange membranes, such as a high gas permeability, and that depend on the specific electrode, such as water management. As a result, it is necessary to tailor the ionomer structure to the specific application in isolation and as part of the catalyst layer. In this review, an overview of the current state of the art for anion exchange ionomers is provided, summarizing their specific requirements and limitations in the context of AEM electrolyzers and fuel cells.

Advanced membrane-based electrode engineering toward efficient and durable water electrolysis and cost-effective seawater electrolysis in membrane electrolyzers

Researchers have been seeking for the most technically-economical water electrolysis technology for entering the next-stage of industrial amplification for large-scale green hydrogen production. Various membrane-based electrolyzers have been developed to improve electric-efficiency, reduce the use of precious metals, enhance stability, and possibly realize direct seawater electrolysis. While electrode engineering is the key to approaching these goals by bridging the gap between catalysts design and electrolyzers development, nevertheless, as an emerging field, has not yet been systematically analyzed. Herein, this review is organized to comprehensively discuss the recent progresses of electrode engineering that have been made toward advanced membrane-based electrolyzers. For the commercialized or near-commercialized membrane electrolyzer technologies, the electrode material design principles are interpreted and the interface engineering that have been put forward to improve catalytic sites utilization and reduce precious metal loading is summarized. Given the pressing issues of electrolyzer cost reduction and efficiency improvement, the electrode structure engineering toward applying precious metal free electrocatalysts is highlighted and sufficient accessible sites within the thick catalyst layers with rational electrode architectures and effective ions/mass transport interfaces are enabled. In addition, this review also discusses the innovative ways as proposed to break the barriers of current membrane electrolyzers, including the adjustments of electrode reaction environment, and the feasible cell-voltage-breakdown strategies for durable direct seawater electrolysis. Hopefully, this review may provide insightful information of membrane-based electrode engineering and inspire the future development of advanced membrane electrolyzer technologies for cost-effective green hydrogen production.

Fabrication of Dual-Functional Bacterial-Cellulose-Based Composite Anion Exchange Membranes with High Dimensional Stability and Ionic Conductivity

Anion exchange membranes (AEMs) are increasingly becoming a popular research area due to their ability to function with nonprecious metals in electrochemical devices. Nevertheless, there is a challenge to simultaneously optimize the dimensional stability and ionic conductivity of AEMs due to the "trade-off" effect. Herein, we adopted a novel strategy of combining filling and cross-linking using functionalized bacterial cellulose (PBC) as a dual-functional porous support and brominated poly(phenylene oxide) (Br-PPO) as the cross-linking agent and filler. The PBC nanofiber framework together with cross-linking can provide a reliable mechanical support for the subsequent filled polymer, thus improving the mechanical properties and effectively limiting the size change of the final quaternized-PPO (QPPO)-filled PBC composite membrane. The composite membrane showed a very low swelling ratio of only 10.35%, even at a high water uptake (81.83% at 20 °C). Moreover, the existence of multiple -NR3+ groups in the cross-link bonds between BC and Br-PPO can provide extra OH- ion transport sites, contributing to the increase in ionic conductivity. The final membrane demonstrated a hydroxide ion conductivity of 62.58 mS cm-1, which was remarkably higher than that of the pure QPPO membrane by up to 235.93% (80 °C). The successful preparation of the PBC3/QPPO membrane provides an effective avenue to tackle the trade-off effect through a dual-functional strategy.

Dimensionally Stable Anion Exchange Membranes Based on Macromolecular-Cross-Linked Poly(arylene piperidinium) for Water Electrolysis

The advancement of anion exchange membranes (AEMs) with superior ionic conductivity has been greatly hindered due to the inherent "trade-off" between membrane swelling and ionic conductivity. To resolve this dilemma, macromolecular covalently cross-linked C-FPVBC-x AEMs were fabricated by combining partially functionalized ether-bond-free polystyrene (FPVBC) with poly(arylene piperidinium). The results from atomic force microscopy reveal that an increase in the ratio of FPVBC promotes the fabrication of microphase separation morphology, resulting in a high ionic conductivity of 40.15 mS cm-1 (30 °C) for the C-FPVBC-1.7 membrane. Molecular dynamics simulations further examine the ionic conduction effect of cross-linked AEMs. Besides, the unique cross-linking structure significantly improves mechanical and alkaline stability. After treatment in 1 M KOH at 50 °C for 1200 h, the C-FPVBC-1.7 membrane shows only a 6.9% decrease in conductivity. The C-FPVBC-1.7 AEM-based water electrolyzer achieves a high current density of 890 mA cm-2 at 2.4 V (80 °C) and maintains good stability, enduring over 100 h at 100 mA cm-2 (50 °C). These results demonstrate the significant potential of macromolecularly cross-linked AEMs for practical applications in water electrolysis.

Ring-opening metathesis polymerization of N-methylpyridinium-fused norbornenes to access antibacterial main-chain cationic polymers

Cationic polymers have been identified as a promising type of antibacterial molecules, whose bioactivity can be tuned through structural modulation. Recent studies suggest that the placement of the cationic groups close to the core of the polymeric architecture rather than on appended side chains might improve both their bioactivity and selectivity for bacterial cells over mammalian cells. However, antibacterial main-chain cationic polymers are typically synthesized via polycondensations, which do not afford precise and uniform molecular design. Therefore, accessing main-chain cationic polymers with high degrees of molecular tunability hinges upon the development of controlled polymerizations tolerating cationic motifs (or cation progenitors) near the propagating species. Herein, we report the synthesis and ring-opening metathesis polymerization (ROMP) of N-methylpyridinium-fused norbornene monomers. The identification of reaction conditions leading to a well-controlled ROMP enabled structural diversification of the main-chain cationic polymers and a study of their bioactivity. This family of polyelectrolytes was found to be active against both Gram-negative (Escherichia coli) and Gram-positive (Methicillin-resistant Staphylococcus aureus) bacteria with minimal inhibitory concentrations as low as 25 µg/mL. Additionally, the molar mass of the polymers was found to impact their hemolytic activity with cationic polymers of smaller degrees of polymerization showing increased selectivity for bacteria over human red blood cells.

Dual reinforced composite membranes from in-situ ionic crosslinked quaternized chitosan filled quaternized polyvinylidene fluoride nanofiber for alkaline direct methanol fuel cell

The main obstacle of high-performance cationic functionalization chitosan (CS) as anion exchange membranes (AEMs) is the trade-off between mechanical stability and ionic conductivity. Here, in-situ ionic crosslinking between the deprotonated hydroxyl group and quaternary ammonium group under alkaline conditions was ingeniously applied to improve the mechanical stability of highly quaternized CS (HQCS) with high IEC (>2 mmol g-1). Meanwhile, to further reduce the swelling and enhance the hydroxide conductivity, a mechanically robust hydroxide ion conduction network, quaternized electrospun poly(vinylidene fluoride) (QPVDF) nanofiber, was subsequently used as the filling substrate of in-situ crosslinked HQCS to prepare dual reinforced thin AEMs. The introduction of a robust QPVDF nanofiber mat can not only greatly improve the mechanical properties and limit swelling, but also create facile ion transport channels. Notably, the HQCS/QPVDF-74.0 composite membrane demonstrates perfect dimensional stability, high mechanical performance and excellent alkaline stability, as well as superior ionic conductivity of 66.2 mS cm-1 at 80 °C. The thus assembled alkaline direct methanol fuel cell displays a maximum power density of 132.30 mW cm-2 using 5 M KOH and 3 M methanol as fuels at 80 °C with satisfactory durability.

Predicting the anion conductivities and alkaline stabilities of anion conducting membrane polymeric materials: development of explainable machine learning models

Anion exchange membranes (AEMs) are core components in fuel cells and water electrolyzers, which are crucial to realize a sustainable hydrogen society. The low anion conductivity and durability of AEMs have hindered the commercialization of AEM-based devices, and research and development (R&D) to improve AEM materials is often resource-intensive. Although machine learning (ML) is commonly used in many fields to accelerate R&D while reducing resource consumption, it is rarely used in the AEM field. Three problems hinder the adoption of ML models, namely, the low explainability of ML models; complication with expressing both homopolymers and copolymers in unity to train a single ML model; and difficulty in building a single ML model that comprehends various polymer types. This study presents the first ML models that solve all three problems. Our models predicted the anion conductivity for a diverse set of unseen AEM materials with high accuracy (root mean squared error = 0.014 S cm-1), regardless of their state (freshly synthesized or degraded). This enables virtual pre-synthesis screening of novel AEM materials, reducing resource consumption. Moreover, human-comprehensible prediction logic revealed new factors affecting the anion conductivity of AEM materials. Such capability to reveal new important variables for AEM materials design could shift the paradigm of AEM R&D. This proposed method is not limited to AEM materials, instead it presents a technology that is applicable to the diverse set of polymers currently available.

Durable Multiblock Poly(biphenyl alkylene) Anion Exchange Membranes with Microphase Separation for Hydrogen Energy Conversion

Anion exchange membrane fuel cells (AEMFCs) and water electrolysis (AEMWE) show great application potential in the field of hydrogen energy conversion technology. However, scalable anion exchange membranes (AEMs) with desirable properties are still lacking, which greatly hampers the commercialization of this technology. Herein, we propose a series of novel multiblock AEMs based on ether-free poly(biphenyl ammonium-b-biphenyl phenyl)s (PBPA-b-BPPs) that are suitable for use in high performance AEMFC and AEMWE systems because of their well-formed microphase separation structures. The developed AEMs achieved outstanding OH- conductivity (162.2 mS cm-1 at 80 °C) with a low swelling ratio, good alkaline stability, and excellent mechanical durability (tensile strength >31 MPa and elongation at break >147 % after treatment in 2 M NaOH at 80 °C for 3750 h). A PBPA-b-BPP-based AEMFC demonstrated a remarkable peak power density of 2.41 W cm-2 and in situ durability for 330 h under 0.6 A cm-2 at 70 °C. An AEMWE device showed a promising performance (6.25 A cm-2 at 2 V, 80 °C) and outstanding in situ durability for 3250 h with a low voltage decay rate (<28 μV h-1 ). The newly developed PBPA-b-BPP AEMs thus show great application prospects for energy conversion devices.

Poly(Ethylene Piperidinium)s for Anion Exchange Membranes

The lack of anion exchange membranes (AEMs) that possess both high hydroxide conductivity and stable mechanical and chemical properties poses a major challenge to the development of high-performance fuel cells. Improving one side of the balance between conductivity and stability usually means sacrificing the other. Herein, we used facile, high-yield chemical reactions to design and synthesize a piperidinium polymer with a polyethylene backbone for AEM fuel cell applications. To improve the performance, we introduced ionic crosslinking into high-cationic-ratio AEMs to suppress high water uptake and swelling while further improving the hydroxide conductivity. Remarkably, PEP80-20PS achieved a hydroxide conductivity of 354.3 mS cm-1 at 80 °C while remaining mechanically stable. Compared with the base polymer PEP80, the water uptake of PEP80-20PS decreased by 69 % from 813 % to 350 %, and the swelling decreased substantially by 85 % from 350.0 % to 50.2 % at 80 °C. PEP80-20PS also showed excellent alkaline stability, 84.7 % remained after 35 days of treatment with an aqueous KOH solution. The chemical design in this study represents a significant advancement toward the development of simultaneously highly stable and conductive AEMs for fuel cell applications.

Semi-Interpenetrating Network Anion Exchange Membranes by Thiol-Ene Coupling Reaction for Alkaline Fuel Cells and Water Electrolyzers

In this work, a thiol-ene coupling reaction was employed to prepare the semi-interpenetrating polymer network AEMs. The obtained QP-1/2 membrane exhibits high hydroxide conductivity (162.5 mS cm^-1 at 80 °C) with a relatively lower swelling ratio, demonstrating its mechanical strength of 42 MPa. This membrane is noteworthy for its improved alkaline stability, as the semi-interpenetrating network effectively limits the attack of hydroxide. Even after being treated in 2 M NaOH at 80 °C for 600 h, 82.5% of the hydroxide conductivity is maintained. The H₂/O₂ fuel cell with QP-1/2 membrane displays a peak power density of 521 mW cm^-2. Alkaline water electrolyzers based on QP-1/2 membrane demonstrated a current density of 1460 mA cm^-2 at a cell voltage of 2.00 V using NiCoFe catalysts in the anode. All the results demonstrate that a semi-interpenetrating structure is a promising way to enhance the mechanical property, ionic conductivity, and alkaline stability of AEMs for the application of alkaline fuel cells and water electrolyzers.

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.

Crosslinked Polynorbornene-Based Anion Exchange Membranes with Perfluorinated Branch Chains

To investigate the effect of perfluorinated substituent on the properties of anion exchange membranes (AEMs), cross-linked polynorbornene-based AEMs with perfluorinated branch chains were prepared via ring opening metathesis polymerization, subsequent crosslinking reaction, and quaternization. The crosslinking structure enables the resultant AEMs (CFnB) to exhibit a low swelling ratio, high toughness, and high water uptake, simultaneously. In addition, benefiting from the ion gathering and side chain microphase separation caused by their flexible backbone and perfluorinated branch chain, these AEMs had high hydroxide conductivity up to 106.9 mS cm^-1 at 80 °C even at low ion content (IEC < 1.6 meq g^-1). This work provides a new approach to achieve improved ion conductivity at low ion content by introducing the perfluorinated branch chains and puts forward a referable way to prepare AEMs with high performance.

Pyrrolidinium-Based Hyperbranched Anion Exchange Membranes with Controllable Microphase Separated Morphology for Alkaline Fuel Cells

It is well acknowledged that the microphase-separated morphology of anion exchange membranes (AEMs) is of vital importance for membrane properties utilized in alkaline fuel cells. Herein, a rigid macromolecule poly(methyldiallylamine) (PMDA) is incorporated to regulate the microphase morphology of hyperbranched AEMs. As expected, the hyperbranched poly(vinylbenzyl chloride) (HB-PVBC) is guided to distribute along PMDA chains, and longer PMDA cha leads to a more distinct microphase morphology with interconnected ionic channels. Consequently, high chloride conductivity of 10.49 mS cm^-1 at 30 °C and suppressed water swelling ratio lower than 30% at 80 °C are obtained. Furthermore, the β-H of pyrrolidinium cations in the non-antiperiplanar position increases the energy barrier of β-H elimination, leading to conformationally disfavored Hofmann elimination and increased alkaline stability. This strategy is anticipated to provide a feasible way for preparing hyperbranched AEMs with clear microphase morphology and good overall properties for alkaline fuel cells.

Alkaline Stability of Anion-Exchange Membranes

Recently, the development of durable anion-exchange membrane fuel cells (AEMFCs) has increased in intensity due to their potential to use low-cost, sustainable components. However, the decomposition of the quaternary ammonium (QA) cationic groups in the anion-exchange membranes (AEMs) during cell operation is still a major challenge. Many different QA types and functionalized polymers have been proposed that achieve high AEM stabilities in strongly alkaline aqueous solutions. We previously developed an ex situ technique to measure AEM alkaline stabilities in an environment that simulates the low-hydration conditions in an operating AEMFC. However, this method required the AEMs to be soluble in DMSO solvent, so decomposition could be monitored using ¹H nuclear magnetic resonance (NMR). We now report the extension of this ex situ protocol to spectroscopically measure the alkaline stability of insoluble AEMs. The stability ofradiation-grafted (RG) poly(ethylene-co-tetrafluoroethylene)-(ETFE)-based poly(vinylbenzyltrimethylammonium) (ETFE-TMA) and poly(vinylbenzyltriethylammonium) (ETFE-TEA) AEMs were studied using Raman spectroscopy alongside changes in their true OH^- conductivities and ion-exchange capacities (IEC). A crosslinked polymer made from poly(styrene-co-vinylbenzyl chloride) random copolymer and N,N,N',N'-tetraethyl-1,3-propanediamine (TEPDA) was also studied. The results are consistent with our previous studies based on QA-type model small molecules and soluble poly(2,6-dimethylphenylene oxide) (PPO) polymers. Our work presents a reliable ex situ technique to measure the true alkaline stability of AEMs for fuel cells and water electrolyzers.

Alkali-Stable Anion Exchange Membranes Based on Poly(xanthene)

Poly(xanthene)s (PXs) carrying trimethylammonium, methylpiperidinium, and quinuclidinium cations were synthesized and studied as a new class of anion exchange membranes (AEMs). The polymers were prepared in a superacid-mediated polyhydroxyalkylation involving 4,4'-biphenol and 1-bromo-3-(trifluoroacetylphenyl)-propane, followed by quaternization reactions with the corresponding amines. The architecture with a rigid PX backbone decorated with cations via flexible alkyl spacer chains resulted in AEMs with high ionic conductivity, thermal stability and alkali-resistance. For example, hydroxide conductivities up to 129 mS cm^-1 were reached at 80 °C, and all the AEMs showed excellent alkaline stability with less than 4% ionic loss after treatment in 2 M aq. NaOH at 90 °C during 720 h. Critically, the diaryl ether links of the PX backbone remained intact after the harsh alkaline treatment, as evidenced by both ¹H NMR spectroscopy and thermogravimetry. Our combined findings suggest that PX AEMs are viable materials for application in alkaline fuel cells and electrolyzers.

Examining the Alkaline Stability of Tris(dialkylamino)sulfoniums and Sulfoxoniums

Herein, a synthetic method was developed to prepare a series of tris(dialkylamino)sulfonium and sulfoxonium cations from sulfur monochloride. Alkaline stability studies of these two cation families in 2 M KOH/CD₃OH solution at 80 °C revealed how degradation pathways change as a function of the oxidation state of the S center, as determined by ¹H NMR spectroscopy. The sulfonium cations (⁺S(NR₂)₃) typically degrade by nucleophilic attack at the sulfur atom with loss of an amino group and a proton transfer reaction to produce sulfoxides, while the sulfoxoniums (⁺O═S(NR₂)₃) tend to degrade by loss of an R group to form sulfoximines. From the group of sulfoniums and sulfoxoniums explored in this work, the tris(piperidino)sulfoxonium cation was noted to have excellent alkaline stability. This sulfoxonium should be suitable for future examination as a tethered cation in anion-exchange membranes (AEMs), or as a phase-transfer catalyst in biphasic reactions.

Physically and Chemically Stable Anion Exchange Membranes with Hydrogen-Bond Induced Ion Conducting Channels

Anion exchange membranes (AEMs) with desirable properties are the crucial components for numerous energy devices such as AEM fuel cells (AEMFCs), AEM water electrolyzers (AEMWEs), etc. However, the lack of suitable AEMs severely limits the performance of devices. Here, a series of physically and chemically stable AEMs have been prepared by the reaction between the alkyl bromine terminal ether-bond-free aryl backbone and the urea group-containing crosslinker. Morphology analyses confirm that the hydrogen bonding interaction between urea groups is capable of driving the ammonium cations to aggregate and further form continuous ion-conducting channels. Therefore, the resultant AEM demonstrates remarkable OH− conductivity (59.1 mS cm−1 at 30 °C and 122.9 mS cm−1 at 90 °C) despite a moderate IEC (1.77 mmol g−1). Simultaneously, due to the adoption of ether-bond-free aryl backbone and alkylene chain-modified trimethylammonium cation, the AEM possesses excellent alkaline stability (87.3% IEC retention after soaking in 1 M NaOH for 1080 h). Moreover, the prepared AEM shows desirable mechanical properties (tensile stress > 25 MPa) and dimensional stability (SR = 20.3% at 90 °C) contributed by the covalent-bond and hydrogen-bond crosslinking network structures. Moreover, the resulting AEM reaches a peak power density of 555 mW cm−2 in an alkaline H2/O2 single fuel cell at 70 °C without back pressure. This rational structural design presented here provides inspiration for the development of high-performance AEMs, which are crucial for membrane technologies.

Well-Defined Nanostructures by Block Copolymers and Mass Transport Applications in Energy Conversion

With the speedy progress in the research of nanomaterials, self-assembly technology has captured the high-profile interest of researchers because of its simplicity and ease of spontaneous formation of a stable ordered aggregation system. The self-assembly of block copolymers can be precisely regulated at the nanoscale to overcome the physical limits of conventional processing techniques. This bottom-up assembly strategy is simple, easy to control, and associated with high density and high order, which is of great significance for mass transportation through membrane materials. In this review, to investigate the regulation of block copolymer self-assembly structures, we systematically explored the factors that affect the self-assembly nanostructure. After discussing the formation of nanostructures of diverse block copolymers, this review highlights block copolymer-based mass transport membranes, which play the role of “energy enhancers” in concentration cells, fuel cells, and rechargeable batteries. We firmly believe that the introduction of block copolymers can facilitate the novel energy conversion to an entirely new plateau, and the research can inform a new generation of block copolymers for more promotion and improvement in new energy applications.