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The lack of high-performance and substantial supply of anion-exchange membranes is a major obstacle to future deployment of relevant electrochemical energy devices. Here, we select two isomers (m-terphenyl and p-terphenyl) and balance their ratio to prepare anion-exchange membranes with well-connected and uniformly-distributed ultramicropores based on robust chemical structures. The anion-exchange membranes display high ion-conducting, excellent barrier properties, and stability exceeding 8000 h at 80 °C in alkali. The assembled anion-exchange membranes present a desirable combination of performance and durability in several electrochemical energy storage devices: neutral aqueous organic redox flow batteries (energy efficiency of 77.2% at 100 mA cm −2 , with negligible permeation of redox-active molecules over 1100 h), water electrolysis (current density of 5.4 A cm −2 at 1.8 V, 90 °C, with durability over 3000 h), and fuel cells (power density of 1.61 W cm −2 under a catalyst loading of 0.2 mg cm −2 , with open-circuit voltage durability test over 1000 h). As a demonstration of upscaled production, the anion-exchange membranes achieve roll-to-roll manufacturing with a width greater than 1000 mm. The design of highly selective yet robust anion exchange membranes remains a challenge. Here, the authors prepare a stable polymer membrane composed of terphenyl isomers, demonstrate roll-to-roll manufacturing, and assess its properties in redox flow batteries, water electrolyzers and fuel cells.

2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) derivatives are typical catholytes in aqueous organic redox flow batteries (AORFBs), but reported lifetime of them is limited. We find that the increase of Hirshfeld charge decreases the Gibbs free energy change ( ΔG ) values of side reactions of TEMPO, a near-linear relationship, and then exacerbates their degradation. Here we predict and synthesize a TEMPO derivative, namely TPP-TEMPO, by analyzing the Hirshfeld charge. TPP-TEMPO, with the smallest Hirshfeld charge and highest ΔG , is an order of magnitude more stable than TMA-TEMPO (N,N,N-2,2,6,6-heptamethylpiperidinyl oxy-4-ammonium chloride, a control with the largest Hirshfeld charge and lowest ΔG ). We further elaborate on their decomposition pathways, identify byproducts, and mitigate degradation by supporting electrolyte engineering. Finally, a TPP-TEMPO/BTMAP-Vi (1,1′-bis[3-(trimethylammonio)propyl]-4,4′-bipyridinium tetrachloride) cell achieves a capacity density of ~12 Ah L − 1 and a low capacity fade rate of 0.0018% per cycle (or 0.0067% per hour). Organic catholytes for all-organic aqueous redox flow batteries have limited cycling lifetimes. Here, authors adjust the Hirshfeld charge of nitroxide radical derivatives to mitigate degradation, resulting in reduced capacity fade rate and extended battery lifetime.

Studying ion transport in the interaction confinement regime has important implications for membrane design and advanced electrochemical devices. A key example is the rapid-charging capability of aqueous organic redox flow batteries, enabled by near-frictionless Na + /K + transport within triazine framework membranes. However, achieving similar breakthroughs for devices using anions ( e.g. , Cl - ) is challenging due to the suppression of anion transport under confinement, known as the charge asymmetry effect. We present a series of anion-selective covalent triazine framework membranes with comparable densities of subnanometer ion transport channels and identical micropore size distributions, which help to overcome the charge asymmetry effect and promote fast anion conduction. We demonstrate that regulating the charge distribution in the membrane frameworks reduces the energy barrier for anion transport, resulting in nearly doubled Cl - conductivity and adding almost no additional energy barrier for F - transport. This membrane enables an aqueous organic redox flow battery using Cl - ions to operate at high current densities, exceeding battery performance demonstrated by current membranes. These findings could benefit various electrochemical devices and inspire single-species selectivity in separation membranes. The suppressed Cl - transport under micropore confinement is a critical challenge. Here, authors report tailoring the pore chemistry of framework polymer membranes, which alters the interaction between Cl - and membranes and speeds Cl - transport, improving flow battery performance.

Alkaline polyelectrolyte fuel cell now receives growing attention as a promising candidate to serve as the next generation energy-generating device by enabling the use of non-precious metal catalysts (silver, cobalt, nickel et al. ). However, the development and application of alkaline polyelectrolyte fuel cell is still blocked by the poor hydroxide conductivity of anion exchange membranes. In order to solve this problem, we demonstrate a methodology for the preparation of highly OH − conductive anion exchange polyelectrolytes with good alkaline tolerance and excellent dimensional stability. Polymer backbones were grafted with flexible aliphatic chains containing two or three quaternized ammonium groups. The highly flexible and hydrophilic multi-functionalized side chains prefer to aggregate together to facilitate the formation of well-defined hydrophilic-hydrophobic microphase separation, which is crucial for the superior OH − conductivity of 69 mS/cm at room temperature. Besides, the as-prepared AEMs also exhibit excellent alkaline tolerance as well as improved dimensional stability due to their carefully designed polymer architecture, which provide new directions to pursue high performance AEMs and are promising to serve as a candidate for fuel cell technology.

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.

Synthetic polyelectrolytes, capable of fast transporting protons, represent a challenging target for membrane engineering in so many fields, for example, fuel cells, redox flow batteries, etc. Inspired by the fast advance in molecular machines, here we report a rotaxane based polymer entity assembled via host–guest interaction and prove that by exploiting the thermally triggered translational motion (although not in a controlled manner) of mechanically bonded rotaxane, exceptionally fast proton transfer can be fulfilled at an external thermal input. The relative motion of the sulfonated axle to the ring in rotaxane happens at ~60 °C in our cases and because of that a proton conductivity (indicating proton transfer rate) of 260.2 mS cm −1 , which is much higher than that in the state-of-the-art Nafion, is obtained at a relatively low ion-exchange capacity (representing the amount of proton transfer groups) of 0.73 mmol g −1 . Proton exchange is critical in many applications, such as in conductive proton exchange membranes, but achieving fast proton exchange still remains a challenge. Here the authors report fast proton exchange in a rotaxane based polymer by exploiting thermally triggered translational motion of the mechanically bonded rotaxane.

The enhancement of separation processes and electrochemical technologies such as water electrolysers 1 , 2 , fuel cells 3 , 4 , redox flow batteries 5 , 6 and ion-capture electrodialysis 7 depends on the development of low-resistance and high-selectivity ion-transport membranes. The transport of ions through these membranes depends on the overall energy barriers imposed by the collective interplay of pore architecture and pore–analyte interaction 8 , 9 . However, it remains challenging to design efficient, scaleable and low-cost selective ion-transport membranes that provide ion channels for low-energy-barrier transport. Here we pursue a strategy that allows the diffusion limit of ions in water to be approached for large-area, free-standing, synthetic membranes using covalently bonded polymer frameworks with rigidity-confined ion channels. The near-frictionless ion flow is synergistically fulfilled by robust micropore confinement and multi-interaction between ion and membrane, which afford, for instance, a Na + diffusion coefficient of 1.18 × 10 −9  m 2  s –1 , close to the value in pure water at infinite dilution, and an area-specific membrane resistance as low as 0.17 Ω cm 2 . We demonstrate highly efficient membranes in rapidly charging aqueous organic redox flow batteries that deliver both high energy efficiency and high-capacity utilization at extremely high current densities (up to 500 mA cm –2 ), and also that avoid crossover-induced capacity decay. This membrane design concept may be broadly applicable to membranes for a wide range of electrochemical devices and for precise molecular separation. The authors develop a strategy that allows the diffusion limit of ions in water to be approached for large-area, free-standing, synthetic membranes using covalently bonded polymer frameworks with rigidity-confined ion channels.

To ameliorate the trade-off effect between ionic conductivity and water swelling of anion exchange membranes (AEMs), a crosslinked, hyperbranched membrane (C-HBM) combining the advantages of densely functionalization architecture and crosslinking structure was fabricated by the quaternization of the hyperbranched poly(4-vinylbenzyl chloride) (HB-PVBC) with a multiamine oligomer poly(N,N-Dimethylbenzylamine). The membrane displayed well-developed microphase separation morphology, as confirmed by small angle X-ray scattering (SAXS) and transmission electron microscopy (TEM). Moreover, the corresponding high ionic conductivity, strongly depressed water swelling, high thermal stability, and acceptable alkaline stability were achieved. Of special note is the much higher ratio of hydroxide conductivity to water swelling (33.0) than that of most published side-chain type, block, and densely functionalized AEMs, implying its higher potential for application in fuel cells.

Ion and water transfer in subnanometer‐sized confined channels of hydrophilic microporous polymer membranes show enormous potential in tackling the ubiquitous trade‐off between permeability and selectivity for energy and environment‐related membrane technologies. To this end, a variety of hydrophilic polymers of intrinsic microporosity (HPIMs) have been developed. Herein, the synthetic strategies toward HPIMs are summarized, including post‐synthetic modification of polymers to introduce polar groups (e. g., amines, hydroxy groups, carboxylic acids, tetrazoles) or charged moieties (e. g., quaternary ammonium salts, sulfonic acids), and the polymerization of hydrophilic monomers. The advantages of HPIM membranes over others when employed in energy conversion and storage, acid gas capture and separation, ionic diodes, and ultrafiltration, are highlighted. The Minireview summarizes the typical synthetic routes toward hydrophilic polymers of intrinsic microporosity (HPIMs), including post‐synthetic modification and polymerization of functional monomers. It also highlights the advantages of HPIM membranes over other systems when used in energy conversion and storage, acid gas capture and separation, ionic diodes, and ultrafiltration.