Thermally triggered polyrotaxane translational motion helps proton transfer

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.

Engineering solid nanochannels with macrocyclic host-guest chemistry for stimuli responses and molecular separations

Biological channels in the cell membrane play a critical role in the regulation of signal transduction and transmembrane transport. Researchers have been committed to building biomimetic nanochannels to imitate the above significant biological processes. Unlike the fragile feature of biological channels, numerous solid nanochannels have aroused extensive interests for their controllable chemical properties on the surface and superior mechanical properties. Surface functionalization has been confirmed to be vital to determine the properties of solid nanochannels. Macrocyclic hosts (e.g., the crown ethers, cyclodextrins, calix[n]arenes, cucurbit[n]urils, pillar[n]arenes, and trianglamine) can be tailored to the interior surface of the nanochannels with the performance of stimuli response and separation. Macrocycles have good reversibility and high selectivity toward specific ions or molecules, promoting functionalies of solid nanochannels. Hence, the combination of macrocyclic hosts and solid nanochannels is conducive to taking both advantages and achieving applications in functional nanochannels (e.g., membranes separations, biosensors, and smart devices). In this review, the most recent advances in nanochannel membranes decorated by macrocyclic host-guest chemistry are briefed. A variety of macrocyclic hosts-based responsive nanochannels are organized (e.g., the physical stimuli and specific molecules or ions stimuli) and nanochannels are separated (e.g., water purifications, enantimerseparations, and organic solvent nanofiltration), respectively. Hopefully, this review can enlighten on how to effectively build functional nanochannels and facilitate their practical applications in membrane separations.

Short hydrogen-bond network confined on COF surfaces enables ultrahigh proton conductivity

The idea of spatial confinement has gained widespread interest in myriad applications. Especially, the confined short hydrogen-bond (SHB) network could afford an attractive opportunity to enable proton transfer in a nearly barrierless manner, but its practical implementation has been challenging. Herein, we report a SHB network confined on the surface of ionic covalent organic framework (COF) membranes decorated by densely and uniformly distributed hydrophilic ligands. Combined experimental and theoretical evidences have pointed to the confinement of water molecules allocated to each ligand, achieving the local enrichment of hydronium ions and the concomitant formation of SHBs in water-hydronium domains. These overlapped water-hydronium domains create an interconnected SHB network, which yields an unprecedented ultrahigh proton conductivity of 1389 mS cm⁻¹ at 90 °C, 100% relative humidity.

When hydronium ions are enriched in confined water, short hydrogen bonds (SHBs) form due to the constrained space of excess protons between pairs of water molecules. Here authors demonstrate a SHB network confined on the surface of ionic COF membranes with tunable -SO₃H groups, with proton conductivity of 1389 mS cm^-1 at 90 ^oC.

Fast Bulky Anion Conduction Enabled by Free Shuttling Phosphonium Cations

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

Pillar[5]arene-based ion-pair recognition for constructing a [2]pseudorotaxane with supramolecular interaction induced LCST behavior

Here, we report a novel [2]pseudorotaxane based on perbromoethylated pillar[5]arene/imidazolium iodide ionic liquid ion-pair recognition and this pseudorotaxane shows supramolecular interaction induced LCST behavior in solution.

Weakly Humidity-Dependent Proton-Conducting COF Membranes

State-of-the-art proton exchange membranes (PEMs) often suffer from significantly reduced conductivity under low relative humidity, hampering their efficient application in fuel cells. Covalent organic frameworks (COFs) with pre-designable and well-defined structures hold promise to cope with the above challenge. However, fabricating defect-free, robust COF membranes proves an extremely difficult task due to the poor processability of COF materials. Herein, a bottom-up approach is developed to synthesize intrinsic proton-conducting COF (IPC-COF) nanosheets (NUS-9) in aqueous solutions via diffusion and solvent co-mediated modulation, enabling a controlled nucleation and in-plane-dominated IPC-COF growth. These nanosheets allow the facile fabrication of IPC-COF membranes. IPC-COF membranes with crystalline, rigid ion nanochannels exhibit a weakly humidity-dependent conductivity over a wide range of humidity (30-98%), 1-2 orders of magnitude higher than that of benchmark PEMs, and a prominent fuel cell performance of 0.93 W cm^-2 at 35% RH and 80 °C arising from superior water retention and Grotthuss mechanism-dominated proton conduction.

Modification of Supramolecular Membranes with 3D Hydrophilic Slide-Rings for the Improvement of Antifouling Properties and Effective Separation

A three-dimensional (3D) strategy for the fabrication of ethylene vinyl alcohol (EVAL) membranes with a dynamic surface was developed based on sliding supramolecular polymer brushes (SSPBs). The SSPBs with a 3D hydrophilic structure were introduced into the alkyne-EVAL membrane matrix via an azide-alkyne click coupling reaction. The self-mobile hydrophilic slide-rings in the SSPB provided a proactive exclusion system. This resulted in reduced direct contact of the membrane surface with multiple pollutants such as bovine serum albumin (BSA) and oil droplets. The EVAL-SSPB membrane demonstrated increased surface hydrophilicity, underwater oleophobicity, and antifouling properties. More importantly, the abundant hydrophilic rings in the membrane matrix result in supramolecular assembly and efficient hydrophilic sliding channels. This resulted in a dramatic increase in the water flux [2000 L/(m² h)] while retaining a 96% rejection of BSA and oil/water emulsions. The results of the study indicate that three effects of the cyclodextrins rings, i.e., the hydrophilic effect, the exclusion effect, and the sliding effect, enabled the improved membrane performance. The demonstrated 3D fabrication strategy is versatile, facile, and scalable, which allows for its application to various other membranes. The fabricated materials possess excellent permeability and separation efficiencies, which make them attractive candidates for use as separation membranes with novel functions.