In Situ-Doped Superacid in the Covalent Triazine Framework Membrane for Anhydrous Proton Conduction in a Wide Temperature Range from Subzero to Elevated Temperature

Rapid and Scalable Preparation of High-Crystalline Carboxyl-Functionalized Covalent Triazine Frameworks via Friedel-Crafts Reaction

The Friedel-Crafts reaction has been extensively applied to the preparation of various porous organic polymers because of its simple operation and abundant building blocks. However, due to its poor reversibility and excessive random reactive sites, the synthesis of crystalline organic polymers/frameworks by Friedel-Crafts reaction has never been realized so far. Herein, we develop a molecular confined Friedel-Crafts reaction strategy to achieve rapid preparation (within only 30 minutes) of highly crystalline covalent triazine frameworks (CTFs) with tailorable functionality for the first time. Theoretical calculations and detailed experiments revealed the critical role of carboxyl groups in aromatic benzene monomer during the CTFs crystallization, which can not only effectively cause a suitable activation barrier for the electrophilic attack of 2,4,6-trichloro-1,3,5-triazine through their electron-withdrawing properties, but also introduce anchoring effects (molecular confinement effects) to facilitate the ordered polymerization at specific sites in 2D planar direction. Benefiting from the convenient synthetic route and low-cost raw materials, we could unprecedentedly realize the kilogram-scale fabrication of carboxyl-functionalized high-crystalline CTFs. Moreover, thanks to the hydrophilic and ionizable properties of carboxyl groups, the obtained functionalized CTFs exhibited excellent aqueous dispersibility and superior solution processability.

Anhydrous Solid-State Proton Conduction in Crystalline MOFs, COFs, HOFs, and POMs

Strategic design of solid-state proton-conducting electrolytes for application in anhydrous proton-exchange membrane fuel cells (PEMFCs) has gained burgeoning interest due to a spectrum of advantageous features, including higher CO tolerance and ease in the water management systems. Toward this direction, crystalline materials like metal-organic frameworks (MOFs), covalent organic frameworks (COFs), hydrogen-bonded organic frameworks (HOFs), and polyoxometalates (POMs) are emerging PEM materials, offering strategic structural engineering through crystallography, thus enabling ultrahigh anhydrous proton conductivity up to 10-2-10-1 S/cm. This Perspective highlights significant progress achieved thus far with such crystalline platforms in the domain of anhydrous proton conduction across a wide temperature window (sub-zero to above 100 °C). Based on their structural backgrounds, these platforms are categorized into four classes (viz. MOFs, COFs, HOFs, and POMs) with a detailed evolutionary timeline since their emergence early in 2009. Insightful discussions with a key focus on the strategies undertaken to attain anhydrous proton conductivity along with implementation in fuel cell technology through membrane electrode assembly are presented. A section on "Critical Analysis and Future Prospects" provides decisive key viewpoints on those overlooked issues with future endorsement (e.g., performance assessment with CO tolerance analysis and fuel cell test stand) for further development while comparing them with other anhydrous platforms from both academic and industrial perspectives.

Self-Standing Covalent Organic Polymer Membrane with High Stability and Enhanced Ion-Sieving Effect for Flow Battery

Fabrication of ion-conducting membranes with continuous sub-nanometer channels holds fundamental importance for flow batteries in achieving safe integration of renewable energy into grids. Self-standing covalent organic polymer (COP) membranes provide feasibility due to their rapid and selective ion transport. However, the development of a scale-up possible, mechanically robust and chemically stable membranes remains a significant challenge. Herein, using irreversible strong secondary amine linkage, we propose a self-standing COP membrane with sub-nanometer pores ranging from 4.5 to 6.4 Å, by a simple and efficient in situ polymerization approach. This membrane exhibits enhanced selectivity for proton and vanadium ions, especially excellent electrochemical stability, delivering an energy efficiency of over 80 % at the current density of 200 mA cm-2 over 1000 cycles for an all-vanadium redox flow battery (VFB). This study provides novel insights for COP-based ion-sieving membranes in sustainable energy fields.

Electrospun Sulfonated Poly(ether ether ketone) and Chitosan/Poly(vinyl alcohol) Bifunctional Nanofibers to Accelerate Proton Conduction at Subzero Temperature

Multilayered microstructures can accelerate the proton conduction process in proton exchange membranes (PEMs). Herein, we design and construct PEMs with microstructures based on bifunctional nanofibers and sulfonated poly(ether ether ketone) (SPEEK) nanofibers. Specifically, the bifunctional nanofibers composed of poly(vinyl alcohol) and chitosan are prepared and then combined with the electrospun SPEEK nanofibers. The stable microstructure is derived from the compatible interfacial property of nanofibers and the formed hydrogen bonds. The multilayered microstructure consisting of nanofibers accelerates the proton conduction even at subzero temperature because of regulating the proton conduction pathways. Specifically, the (SKNF/CPNF/SKNF)/PA membrane exhibits the proton conductivities of (0.951 ± 0.138) × 10-2 S/cm at -30 °C and (7.32 ± 0.37) × 10-2 S/cm at 160 °C. Additionally, the fine proton conductivity stability is demonstrated by the proton conductivity in the long-term test and the cooling/heating cycle test, such as 1.67 × 10-2 S/cm at -30 °C (after 1000 h), 4.52 × 10-2 S/cm at 30 °C (after 810 h), 1.12 × 10-2 S/cm at -30 °C, and 1.01 × 10-1 S/cm at 30 °C in the cooling/heating process (5 cycles). The single fuel cell possesses an open-circuit voltage of 0.886 V and a peak power density of 0.508 W/cm2 at 130 °C.

The preparation of hexaphenylsilole-based conjugated microporous polymer for fluorescence sensing o-nitrophenol

A first hexaphenylsilole-based conjugated microporous polymer, termed THPS, was prepared by a Friedel-Crafts arylation reaction of 1,1,2,3,4,5-hexabenylthirole with 2,4,6-trichloro-1,3,5-triazine catalyzed by anhydrous aluminium trichloride (AlCl3) in 1,2-dichloroethane. Its Brunauer-Emmett-Teller and Langmuir surface area reached up to 1579 and 2081 m2 g-1, and total pore volume and micropore pore volume were severally 0.8325 and 0.7322 cm3 g-1. THPS could fluorescence-sense o-nitrophenol with high sensitivity and selectivity. Its fluorescence quenching constant (KSV) and limits of detection (LODs) were severally 8.32 × 103 L mol-1 and 3.61 × 10-10 mol L-1. The fluorescence quenching of THPS by o-nitrophenol was originated from the combined action of both photo-induced electron transfer mechanism and resonance energy transfer mechanism.

A series of polyoxometalate-based COF composites by one-pot mechanosynthesis of thioether to sulfone

An effective combination of polyoxometalates (POMs) and porous materials is a feasible method to solve the homogeneity of POMs and synthesize extremely stable POM-based catalysts. Herein, by using simple mechanochemical synthesis, we fabricated a series of composites constructed by Keggin-POMs, p-phenylenediamine (Pa-1), and 1,3,5-triformylphloroglucinol (Tp), which in situ form a stable covalent organic framework (Keggin-POMs@TpPa-1). Notably, the different Keggin-POMs@TpPa-1 composites showed different catalytic effects on thioether oxidation reaction under mild conditions. From the comparison, the catalytic effect of PW12@TpPa-1 with its added amount of 27% H3PW12O40 is superior to that of other composites, whose catalytic efficiency can reach 99%. This study provides some inspiration for designing diverse POM-modified catalysts with outstanding stability and efficiency using COFs as supports.

Recent Progress of Crystalline Porous Frameworks for Intermediate-Temperature Proton Conduction

Proton exchange membranes (PEMs), especially for work under intermediate temperatures (100-200 °C), have attracted great interest because of the high CO toleration and facial water management of the corresponding proton exchange membrane fuel cells (PEMFCs). Traditional polymer PEMs faced challenges of low stability and proton carrier leaking. Crystalline porous materials, such as metal-organic frameworks (MOFs) and covalent organic frameworks (COFs), are promising to overcome these issues contributed by nanometer-sized channels. Herein we summarized the recent development of MOF/COF-based intermediate-temperature proton conductors. The strategies of framework engineering and pore impregnation were introduced in detail for raising proton conductivity. The proton-conducting mechanism was described as well. This spotlight will provide new insight into the fabrication of MOF/COF proton conductors under intermediate-temperature and anhydrous conditions.

Dynamics and Proton Conduction of Heterogeneously Confined Imidazole in Porous Coordination Polymers

The nanoconfinement of proton carrier molecules may contribute to the lowing of their proton dissociation energy. However, the free proton transportation does not occur as easily as in liquid due to the restricted molecular motion from surface attraction. To resolve the puzzle, herein, imidazole is confined in the channels of porous coordination polymers with tunable geometries, and their electric/structural relaxations are quantified. Imidazole confined in a square-shape channels exhibits dynamics heterogeneity of core-shell-cylinder model. The core and shell layer possess faster and slower structural dynamics, respectively, when compared to the bulk imidazole. The dimensions and geometry of the nanochannels play an important role in both the shielding of the blocking effect from attractive surfaces and the frustration filling of the internal proton carrier molecules, ultimately contributing to the fast dynamics and enhanced proton conductivity.

Crystalline Porphyrazine-Linked Fused Aromatic Networks with High Proton Conductivity

Fused aromatic networks (FANs) have been studied in efforts to overcome the low physicochemical stability of metal-organic frameworks (MOFs) and covalent organic frameworks (COFs), while preserving crystallinity. Herein, we describe the synthesis of a highly stable and crystalline FAN (denoted as Pz-FAN) using pyrazine-based building blocks to form porphyrazine (Pz) linkages via an irreversible reaction. Unlike most COFs and FANs, which are synthesized from two different building blocks, the new Pz-FAN is formed using a single building block by self-cyclotetramerization. Controlled and optimized reaction conditions result in a highly crystalline Pz-FAN with physicochemical stability. The newly prepared Pz-FAN displayed a high magnitude (1.16×10^-2  S cm^-1 ) of proton conductivity compared to other reported FANs and polymers. Finally, the Pz-FAN-based membrane was evaluated for a proton-exchange membrane fuel cell (PEMFC), which showed maximum power and current densities of 192 mW cm^-2 and 481 mA cm^-2 , respectively.

Turning Electronic Waste to Continuous-Flow Reactor Using Porous Aromatic Frameworks

Extraction of valuable metals such as gold from electronic wastes (e-waste) is regarded as a promising way of environmental remediation; however, this process is still confronted with the cost-ineffective product for normal usages like electronic devices or jewelry. Therefore, there would be merits in directly converting gold from e-waste to materials of higher value, for example, catalysts for pollutant treatment. Herein, a porous aromatic framework (PAF) with cationic sites, named iPAF-7, was synthesized and exhibited rapid extraction of gold from e-waste. Au@iPAF-7 completely converted nitroarenes to arylamines within 10 s at a rate constant of 7.8 × 10^-2 s^-1, which is much higher than that of any other gold nanoparticle (AuNP) catalysts with solid supports reported so far. Furthermore, considering the limitations and difficulties of operating powder materials, the aerogel monolith incorporating iPAF-7 was successfully fabricated, which retained the excellent gold extraction ability and catalytic activity of its powder form, thus exhibiting its potential application for continuous-flow catalysis of nitroarene reduction.

Electrostatic attraction induces cationic covalent-organic framework to pack inorganic acid ions for promoting proton conduction

A cationic COF (CCOF) was imported to afford stable proton conductive material while the positively charged CCOF can constrain the inorganic anion groups into the framework through an ion exchange process. The inorganic anions could play the role of proton donors to provide a rich source of protons, as well as the proton acceptors for proton transmission, thus forming an interlinked proton conduction pathway within the framework. The proton conductivities of COF materials loaded with different anions have been improved to varying degrees, and the conductivity of H₂PO₄^-@CCOF is up to 3.86 × 10^-2 S cm^-1 at 100 °C.

Graphitic Aza-Fused π-Conjugated Networks: Construction, Engineering, and Task-Specific Applications

2D π-conjugated networks linked by aza-fused units represent a pivotal category of graphitic materials with stacked nanosheet architectures. Extensive efforts have been directed at their fabrication and application since the discovery of covalent triazine frameworks (CTFs). Besides the triazine cores, tricycloquinazoline and hexaazatriphenylene linkages are further introduced to tailor the structures and properties. Diverse related materials have been developed rapidly, and a thorough outlook is necessitated to unveil the structure-property-application relationships across multiple subcategories, which is pivotal to guide the design and fabrication toward enhanced task-specific performance. Herein, the structure types and development of related materials including CTFs, covalent quinazoline networks, and hexaazatriphenylene networks, are introduced. Advanced synthetic strategies coupled with characterization techniques provide powerful tools to engineer the properties and tune the associated behaviors in corresponding applications. Case studies in the areas of gas adsorption, membrane-based separation, thermo-/electro-/photocatalysis, and energy storage are then addressed, focusing on the correlation between structure/property engineering and optimization of the corresponding performance, particularly the preferred features and strategies in each specific field. In the last section, the underlying challenges and opportunities in construction and application of this emerging and promising material category are discussed.

Zwitterionic Covalent Organic Frameworks: Attractive Porous Host for Gas Separation and Anhydrous Proton Conduction

Ionic covalent organic frameworks (COFs) consisting of an anionic or cationic skeleton and corresponding counterions have demonstrated great potential in many application fields such as ion conduction, molecular separation, and catalysis. However, arranging anionic and cationic groups into the same COF to form zwitterionic materials is still unexplored. Herein we design the synthesis of three zwitterionic COFs as attractive porous hosts for SO₂/CO₂ separation and anhydrous proton conduction. The separated cationic and anionic groups in zwitterionic COFs' channels can act as two different polar sites for SO₂ adsorption, allowing zwitterionic COFs to achieve a high SO₂ adsorption capacity (216 mL/g, 298 K) and impressive SO₂/CO₂ selectivity (118, 298 K). Furthermore, after loading with triazole/imidazole, the zwitterionic groups in COFs' channels can induce complete proton carrier deprotonation, producing more freely migrating protons. The free protons migrate along a continuous hydrogen-bonding network in zwitterionic COFs' channels, leading to outstanding anhydrous proton conductivity up to 4.38 × 10^-2 S/cm, which is much higher than other N-heterocyclic-doped porous materials under anhydrous conditions. Proton dissociation energy calculations combined with frequency-dependent dielectric analysis give insight into the role of zwitterionic COFs for proton conductivity. Our work provides the possibility to design well-defined zwitterionic frameworks for gas separation and ion conduction.

Proton conductors with wide operating temperature domains achieved by applying a dual-modification strategy to MIL-101

Developing an efficient strategy for obtaining proton conductors with wide working temperature domains is of great significance for the wide application of proton conductors. To date, proton conductors that have high proton conductivity from subzero temperatures to high temperatures above 100 °C have been very rare. Herein, we prepared two composites, H₃PO₄@MIL-101-SO₃H(Cr) (1) and H₂SO₄@MIL-101-NH₂(Al) (2) by applying a dual-modification strategy to functionalize MOF MIL-101, that is, incorporating acidic guest molecules into the channels of MIL-101 while modifying the MIL-101 backbone with functional groups. Both composites have high proton conductivity over a broad temperature domain (-40 °C to above 150 °C) due to the complementary conduction or synthetic conduction of the backbone functional group and acidic guest molecules in different temperature ranges. The proton conductivities of 1 are 0.9 × 10^-1 S cm^-1 at 65 °C and 95% RH, 7.5 × 10^-5 S cm^-1 at -40 °C and 1.4 × 10^-2 S cm^-1 at 150 °C. Further, the proton conductivities of 2 are 5.8 × 10^-2 S cm^-1 at 65 °C and 95% RH, 7.1 × 10^-4 S cm^-1 at -40 °C and 2.5 × 10^-4 S cm^-1 at 170 °C. All the proton conductivities of the two composites in three temperature domains (low, moderate and high temperature) are at a high level among those of reported proton conductors. Moreover, their proton conductivities have good stability and durability in the broad temperature region from subzero temperatures to high temperatures above 100 °C.

Ionic Covalent Organic Frameworks for Energy Devices

Covalent organic frameworks (COFs) are a class of porous crystalline materials whose facile preparation, functionality, and modularity have led to their becoming powerful platforms for the development of molecular devices in many fields of (bio)engineering, such as energy storage, environmental remediation, drug delivery, and catalysis. In particular, ionic COFs (iCOFs) are highly useful for constructing energy devices, as their ionic functional groups can transport ions efficiently, and the nonlabile and highly ordered all-covalent pore structures of their backbones provide ideal pathways for long-term ionic transport under harsh electrochemical conditions. Here, current research progress on the use of iCOFs for energy devices, specifically lithium-based batteries and fuel cells, is reviewed in terms of iCOF backbone-design strategies, synthetic approaches, properties, engineering techniques, and applications. iCOFs are categorized as anionic COFs or cationic COFs, and how each of these types of iCOFs transport lithium ions, protons, or hydroxides is illustrated. Finally, the current challenges to and future opportunities for the utilization of iCOFs in energy devices are described. This review will therefore serve as a useful reference on state-of-the-art iCOF design and application strategies focusing on energy devices.

Anhydrous Proton Conduction in Crystalline Porous Materials with a Wide Working Temperature Range

Crystalline porous materials (CPMs), exhibiting high surface areas, versatile structural topologies, and tunable functionality, have attracted much attention in the field of proton exchange membrane fuel cells (PEMFC) for their great potential in solid electrolytes. However, most hydrated CPM proton conductors suffer from the narrow working temperature and the high water/humidity dependence. Considering the practical application in different working environments, CPMs with high anhydrous conductivity from subzero to moderate temperature (>100 °C) are desirable, but it is still a huge challenge. Herein we summarized our recent research work in the anhydrous CPM proton conductors, including to rationally tune the structures of CPMs by using the strategies of pore engineering and protonic species control to achieve wide working temperature conduction, as well as to clarify the conducting mechanism. This spotlight will provide clues to flexibly design and fabricate wide-working-temperature CPM conductors with high protonic conductivity.