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

A twisted imidazole-tethered aromatic polymer for high-performance membranes in vanadium-based redox flow batteries

We report on a twisted aromatic polymer, the first xanthene-based example bearing protonated imidazole groups. Its special architecture enables superior ion selectivity, conductivity, and chemical stability, providing excellent performance in both all-vanadium and iron-vanadium redox flow batteries.

Constructing Hydrophilic Polymer Membranes with Microporosity for Aqueous Redox Flow Batteries

Ion selective membranes (ISMs) are key components of aqueous redox flow batteries (ARFBs), and their property in selective ion transport largely determines the energy storage efficiency of ARFBs. Traditional ISMs are based on microphase-separated structures and have been advanced for many years, but most of them show poor performance as membrane separators in ARFBs due to their conductivity-selectivity. In recent years, using confined micropores instead of dense hydrophilic regions as ion channels has been demonstrated to effectively break this tradeoff. We here summarize the synthetic strategies for constructing hydrophilic polymer membranes with microporosity and highlight the performance of some typical microporous ISMs in ARFBs. We also propose fundamental issues that remain to be addressed for the further development of ISMs.

Ether-Free Alkaline Polyelectrolytes for Water Electrolyzers: Recent Advances and Perspectives

Anion exchange membrane (AEM) water electrolyzers (AEMWEs) have attracted great interest for their potential as sustainable, environmentally friendly, low-cost sources of renewable energy. Alkaline polyelectrolytes play a crucial role in AEMWEs, determining their performance and longevity. Because heteroatom-containing polymers have been shown to have poor durability in alkaline conditions, this review focuses on ether-free alkaline polyelectrolytes, which are more chemically stable. The merits, weaknesses, and challenges in preparing ether-free AEMs are summarized and highlighted. The evaluation of synthesis methods for polymers, modification strategies, and cationic stability will provide insights valuable for the structural design of future alkaline polyelectrolytes. Moreover, the in situ degradation mechanisms of AEMs and ionomers during AEMWE operation are revealed. This review provides insights into the design of alkaline polyelectrolytes for AEMWEs to accelerate their widespread commercialization.

Cationic groups in polystyrene/O-PBI blends influence performance and hydrogen crossover in AEMWE

This study examines the effect of various quaternary ammonium groups on AEMWE performance and hydrogen crossover in blends of quaternized polystyrenes with O-PBI. Due to their higher hydroxide conductivity (69 mS cm-1 at 80 °C, 90% RH), trimethylammonium groups enable AEMWE to reach 1.0 A cm-2 at 2.0 V. The trimethylammonium groups exhibit low hydrogen crossover, ranging from 1.5% to 0.3%, across current densities of 50 to 1000 mA cm-2. Low hydrogen crossover is essential for AEMWE in terms of safety and efficiency.

Ultramicroporous crosslinked polyxanthene-poly(biphenyl piperidinium)-based anion exchange membranes for water electrolyzers operating under highly alkaline conditions

Anion exchange membrane water electrolyzers (AEMWEs) suffer from low efficiencies and durability, due to the unavailability of appropriate anion exchange membranes (AEM). Herein, a rigid ladder-like polyxanthene crosslinker was developed for the preparation of ultramicroporous crosslinked polyxanthene-poly(biphenyl piperidinium)-based AEMs. Due to the synergetic effects of their ultramicroporous structure and microphase-separation morphology, the crosslinked membranes showed high OH- conductivity (up to 163 mS cm-1 at 80 °C). Furthermore, these AEMs also exhibited moderate water uptake, excellent dimensional stability, and remarkable alkaline stability. The single-cell AEMWE based on QPBP-PX-15% and equipped with non-noble catalysts achieved a current density of 3000 mA cm-2 at 2.03 V (compared to PiperION's 2.26 V) in 6 M KOH solution at 80 °C, which outperformed many AEMWEs that used platinum-group-metal catalysts. Thus, the crosslinked AEMs developed in this study showed significant potential for application in AEMWEs fed with concentrated alkaline solutions.

Poly(Dibenzothiophene-Terphenyl Piperidinium) for High-Performance Anion Exchange Membrane Water Electrolysis

The anion exchange membrane water electrolysis is widely regarded as the next-generation technology for producing green hydrogen. The OH- conductivity of the anion exchange membrane plays a key role in the practical implementation of this device. Here, we present a series of Z-S-x membranes with dibenzothiophene groups. These membranes contain sulfur-enhanced hydrogen bond networks that link surrounding surface site hopping regions, forming continuous OH- conducting highways. Z-S-20 has a high through-plane OH- conductivity of 182±28 mS cm-1 and ultralong stability of 2650 h in KOH solution at 80 °C. Based on rational design, we achieved a high PGM-free alkaline water electrolysis performance of 7.12 A cm-2 at 2.0 V in a flow cell and demonstrated durability of 650 h at 2 A cm-2 at 40 °C with a cell voltage increase of 0.65 mV/h.

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.

Ultramicroporous Tröger's Base Framework Membranes for pH-Neutral Aqueous Organic Redox Flow Batteries

Processable polymers of intrinsic microporosity (PIMs) are emerging as promising candidates for next-generation ion exchange membranes (IEMs). However, especially with high ion exchange capacity (IEC), IEMs derived from PIMs suffer from severe swelling, thus, resulting in decreased selectivity. To solve this problem, we report ultramicroporous polymer framework membranes constructed with rigid Tröger's Base network chains, which are fabricated via an organic sol-gel process. These membranes demonstrate excellent antiswelling, with swelling ratios below 4.5% at a high IEC of 2.09 mmol g-1, outperforming currently reported PIM membranes. The rigid ultramicropore confinement and charged modification of pore channels endow membranes with both very high size-exclusion selectivity and competitive ion conductivity. The membranes thus enable the efficient and stable operation of pH-neutral aqueous organic redox flow batteries (AORFBs). This work presents the advantages of polymer framework materials as IEMs and calls for increasing attention to extending their varieties and utilization in other applications.

High Free Volume Polyelectrolytes for Anion Exchange Membrane Water Electrolyzers with a Current Density of 13.39 A cm-2 and a Durability of 1000 h

The rational design of the current anion exchange polyelectrolytes (AEPs) is challenging to meet the requirements of both high performance and durability in anion exchange membrane water electrolyzers (AEMWEs). Herein, highly-rigid-twisted spirobisindane monomer is incorporated in poly(aryl-co-aryl piperidinium) backbone to construct continuous ionic channels and to maintain dimensional stability as promising materials for AEPs. The morphologies, physical, and electrochemical properties of the AEPs are investigated based on experimental data and molecular dynamics simulations. The present AEPs possess high free volumes, excellent dimensional stability, hydroxide conductivity (208.1 mS cm-1 at 80 °C), and mechanical properties. The AEMWE of the present AEPs achieves a new current density record of 13.39 and 10.7 A cm-2 at 80 °C by applying IrO2 and nonprecious anode catalyst, respectively, along with outstanding in situ durability under 1 A cm-2 for 1000 h with a low voltage decay rate of 53 µV h-1 . Moreover, the AEPs can be applied in fuel cells and reach a power density of 2.02 W cm-2 at 80 °C under fully humidified conditions, and 1.65 W cm-2 at 100 °C, 30% relative humidity. This study provides insights into the design of high-performance AEPs for energy conversion devices.

Rapid Postgrafting Reaction to Prepare Quaternized Polycarbazoles

We report a rapid postgrafting reaction to prepare alkyl ammonium functionalized polycarbazoles from a commercially available monomer. This novel synthetic approach provides benefit to preparing the high molecular weight quaternized polycarbazoles within 1 h of Friedel-Crafts polycondensation, avoiding the synthesis and purification step to prepare a functionalized monomer. The postgrafting reaction produces hexyl alkyl ammonium functionalized polycarbazole with 100% grafting degree. However, the postgrafting reaction produced only 60% grafting with propyl alkyl ammonium due to the competitive elimination reaction because of the higher acidity of β-hydrogen in the propyl alkyl group resulting from the proximity of the bromide and ammonium groups. The hexyl alkyl ammonium functionalized polycarbazole has a high hydroxide conductivity of 103 mS cm-1 at 80 °C and showed excellent alkaline stability with less than 3% loss of ion group after 1 M NaOH treatment at 80 °C for 500 h. This study highlights that the postgrafting reaction provides a pathway for the scale-up synthesis of quaternized aryl ether-free polyaromatics.

Alkaline Membranes toward Electrochemical Energy Devices: Recent Development and Future Perspectives

Anion-exchange membranes (AEMs) that can selectively transport OH-, namely, alkaline membranes, are becoming increasingly crucial in a variety of electrochemical energy devices. Understanding the membrane design approaches can help to break through the constraints of undesired performance and lab-scale production. In this Outlook, the research progress of alkaline membranes in terms of backbone structures, synthesis methods, and related applications is organized and discussed. The evaluation of synthesis methods and description of membrane stability enhancement strategies provide valuable insights for structural design. Finally, to accelerate the deployment of relevant technologies in alkaline media, the future priority of alkaline membranes that needs to be addressed is presented from the perspective of science and engineering.