Host-Guest Assembly of H-Bonding Networks in Covalent Organic Frameworks for Ultrafast and Anhydrous Proton Transfer

Supramolecular structure@MXenes for photocatalytic applications - a review

Recently, supramolecules have emerged as innovative and eco-friendly options for photocatalytic applications due to their tunable porous structures and photophysical properties. However, their low thermal stability and chemical stability pose a significant challenge. To address this, combining supramolecules with more stable materials like MXenes, which have a low Fermi energy level, is a useful strategy, in which they can form heterostructures that enhance stability and improve photocatalytic activity. The synthesis process, whether through in situ or post-synthesis modifications, plays a crucial role in controlling the formation of both covalent and non-covalent interactions, as well as the morphology of the heterostructures. These interactions and the resulting morphology significantly influence the recombination and separation of charge carriers (electron-hole pairs), ultimately affecting the stability and recyclability of the heterostructures in photocatalytic applications. In this review, we discuss the importance of supramolecule/MXene heterostructures, detailing their synthesis and morphology, as well as the mechanisms involved in various applications.

In-Situ Gelled Covalent Organic Framework Membrane with Vacancies-Enhanced Anhydrous Proton Conductivity

The development of high-performance anhydrous proton-exchange membranes (APEMs) for electrochemical techniques remains a significant challenge. Covalent organic frameworks (COFs) offer a promising solution for APEMs due to their tunable channels and functionalizable skeletons. However, COFs are typically porous powders, which create extreme difficulties in processing them into self-standing APEMs, thereby limiting their practical applications. In this study, we propose a novel strategy for preparing COF-based APEMs for high-temperature proton exchange membrane fuel cell (HT-PEMFC) applications through acidification and gelation. In the gel, COF acts as both a gelling agent and proton trap, inhibits guest acid flow, and captures protons from the acid, leading to the formation of proton vacancies in the COF gel and greatly accelerating proton migration. As a result, COF gel membranes exhibit conductivities that far surpass that of the guest acid itself, exceeding 0.1 S cm-1 at temperatures above 140 °C, outperforming most reported COF materials. Notably, membrane electrode assemblies of HT-PEMFCs fabricated with a COF gel achieve a maximum power density of 150 mW cm-2 at 180 °C and anhydrous conditions. Our approach introduces an innovative strategy for the fabrication of self-standing COF-based APEMs, representing a significant breakthrough in the field of COF-based APEMs for fuel cell technology.

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.

Covalent organic frameworks and their composites as enhanced energy storage materials

The advancement in materials chemistry promoted the growth of energy storage systems such as capacitors, supercapacitors and batteries. Covalent organic frameworks and nanomaterials have significantly improved the performance of various energy storage systems. Because of the unique properties of these materials, like high surface area, tunable architectures, and enhanced conductivity, researchers have developed effective and durable energy storage solutions for multiple applications. These findings are significant for meeting the demand for reliable and sustainable energy storage materials in order to save energy for a better future of mankind. As the demand for reliable and sustainable energy storage materials is increasing, the scientific community is more focussed towards the development of covalent organic frameworks (COFs). The high surface area, thermal and chemical stability, structural tunability, porosity, and low density of COFs make them appropriate for energy storage applications. Their potential to produce advanced energy storage devices with better performance and durability is further reinforced by their ability to be customized for specific applications and amplified for conductive materials. This review covers the designs and synthetic techniques of COFs and their composites specifically suitable for energy storage uses. It further highlights their use as cathode and anode materials in supercapacitors, COF based electrolytes and batteries. The review further includes the flexibility and efficiency of COFs in energy storage applications. Furthermore, it addresses the challenges and their potential solutions regarding the use of COFs in energy storage devices. By providing a comprehensive understanding of the advantages and limitations of COFs, this review aims to inform and inspire future advancements in energy storage technologies.

High Proton Conductivity of Acid Impregnated COFs Stabilized by Post-Oxidation

The investigation of proton conduction processes within artificial nanopores using phosphoric acid (H3PO4) and sulfuric acid (H2SO4) not only sheds light on the mechanisms of proton conduction for these strong acids in confined environments, while also provides critical insights into the proper understanding of biological transmembrane proton transport. However, the synthesis of stable and acid-resistant host frameworks is yet a major challenges. By following that, the present study is conducted with the aim of improving the chemical stability of an imine-based COF (CPOF-10) by converting it into an amide-linked COF (CPOF-11) via a post-oxidative approach. In which, the integration of an appropriate amount of imidazole groups into the framework facilitates the efficient impregnation of liquid proton-conducting acids. The obtained results indicate the ten times greater proton conductivity of H3PO4@CPOF-11 than that of H3PO4@CPOF-10, thereby, successfully achieving 8.63 × 10-2 S cm-1 at 160 °C, under nitrogen (N2) atmosphere. Moreover, the highly stable CPOF-11 tolerated H2SO4 doping, delivering a high proton conductivity of up to 1.70 × 10-1 S cm-1 at 140 °C, with a significantly low activation energy of 0.05 eV. To the best of the knowledge, this activation energy (0.05 eV) of H2SO4@CPOF-11 is found to be one of the lowest value among all the reported proton-conducting materials. Thus, this study will provide new understanding for the fabrication of advanced porous organic materials in fuel cells application.

Perfluoroalkyl Functionalized Superhydrophobic Covalent Organic Frameworks for Excellent Oil-Water Membrane Separation and Anhydrous Proton Conduction

Rapid economic development has led to oil pollution and energy shortage. Membrane separation has attracted much attention due to its simplicity and efficiency in oil-water-separation. The development of membrane materials with enhanced separation properties is essential to improve the separation-efficiency. Proton exchange membrane fuel cells (PEMFCs) are expected to replace conventional engines due to their high-power-conversion rates and other favorable properties. Anhydrous-proton-conducting materials are vital components of PEMFCs. However, developing stable proton-conducting materials that exhibit high conductivity at varying temperatures remains challenging. Herein, two covalent organic frameworks (COFs) with long-side-chains are synthesized, and their corresponding COF@SSN membranes. Both membranes can effectively separate oil-water mixtures and water-in-oil emulsions. The TFPT-AF membrane achieves a maximum oil-flux of 6.05 × 105 g h-1 m-2 with an oil-water separation efficiency of above 99%, which is almost unchanged after 20 consecutive uses. COF@H3PO4 doped with different ratios of H3PO4 is prepared, the results show that the perfluorocarbon-chain system has  excellent anhydrous proton conductivity , achieving an ultra-high proton-conductivity of 3.98 × 10-1 S cm-1 at 125 °C. This study lays the foundation for tailor-made-functionalization of COF through pre-engineering and surface-modification, highlighting the great potential of COFs for oil-water separation and anhydrous-proton-conductivity.

Accelerating Anhydrous Proton Transport in Covalent Organic Frameworks: Pore Chemistry and its Impacts

Proton conduction is important in both fundamental research and technological development. Here we report designed synthesis of crystalline porous covalent organic frameworks as a new platform for high-rate anhydrous proton conduction. By developing nanochannels with different topologies as proton pathways and loading neat phosphoric acid to construct robust proton carrier networks in the pores, we found that pore topology is crucial for proton conduction. Its effect on increasing proton conductivity is in an exponential mode other than linear fashion, endowing the materials with exceptional proton conductivities exceeding 10-2 S cm-1 over a broad range of temperature and a low activation energy barrier down to 0.24 eV. Remarkably, the pore size controls conduction mechanism, where mesopores promote proton conduction via a fast-hopping mechanism, while micropores follow a sluggish vehicle process. Notably, decreasing phosphoric acid loading content drastically reduces proton conductivity and greatly increases activation energy barrier, emphasizing the pivotal role of well-developed proton carrier network in proton transport. These findings and insights unveil a new general and transformative guidance for designing porous framework materials and systems for high-rate ion conduction, energy storage, and energy conversion.

Advances in ionogels for proton-exchange membranes

To ensure the long-term performance of proton-exchange membrane fuel cells (PEMFCs), proton-exchange membranes (PEMs) have stringent requirements at high temperatures and humidities, as they may lose proton carriers. This issue poses a serious challenge to maintaining their proton conductivity and mechanical performance throughout their service life. Ionogels are ionic liquids (ILs) hybridized with another component (such as organic, inorganic, or organic-inorganic hybrid skeleton). This design is used to maintain the desirable properties of ILs (negligible vapor pressure, thermal stability, and non-flammability), as well as a high ionic conductivity and wide electrochemical stability window with low outflow. Ionogels have opened new routes for designing solid-electrolyte membranes, especially PEMs. This paper reviews recent research progress of ionogels in proton-exchange membranes, focusing on their electrochemical properties and proton transport mechanisms.

A Stimuli-Responsive Dual-Emitting Covalent Organic Framework Shows Selective Sensing of Highly Corrosive Acidic Media via Fluorescence Turn-On Signal with White Light Emission

Luminescent covalent organic frameworks (LCOFs) have been employed as platforms for sensing analytes. Judicial incorporation of appropriate functional units inside the framework leads to the different electronic states in the presence of external stimuli, e.g., temperature, pH, etc. We report herein a new COF (TPEPy) as a solid-state acid sensor specific for the highly acidic environments that range from pH ∼0.5 to ∼3.0. This COF shows a protonation-induced reversible color change from bright yellow to deep red upon decreasing the pH from 3 to 0.5 and vice versa. No visual color change was, however, observed above pH 3.0. Photoluminescence (PL) studies show that the intrinsic emission peak of the TPEPy COF at 530 nm is shifted to 420 nm owing to the N-protonation of the imine nitrogen of COF within this pH range. Extensive studies demonstrate that the protonation behavior of the COF is counterion dependent. This was revealed when different acids, e.g., HCl, HNO3, HBr, and HI, were employed. The intensity of the proton-induced emission peak at 420 nm depends significantly upon the counterions with the order of HCl > HNO3 > HBr > HI. These anions interact with the protonated TPEPy COF by cation-anion and H-bonding interactions. Further, the pristine COF showed near white light emission at a particular pH of 2.5 (CIE coordinates 0.27, 0.32). From the PL spectrophotometric titrations, the deprotonation pKa was experimentally found to be 1.8 ± 0.02 for the TPEPy COF. The sensor reported herein is reversible, reusable, and regenerable and is useful for assessing pH fluctuations within a strongly acidic range via digital signaling.

High anhydrous proton conductivity and a smart proton transportation approach of a sulfate coordination polymer

We successfully synthesized a one-dimensional cobalt sulfate coordinating polymer, whose simple hydrogen bond web structure facilitated the analysis of the proton transfer process. At 175 °C, without humidity, the conductivity is 0.0311 S cm-1, which exceeds those of most of the organic inorganic hybrid materials under anhydrous conditions (world record rank 8). Based on its crystal structure and theoretical calculations, the subversive proton conduction pathway was inferred clearly. We, for the first time, found that the proton smartly chose the path with a lower energy barrier but not the one with short distance to transport avoiding short circuit.

Acidified Nitrogen Self-Doped Porous Carbon with Superprotonic Conduction for Applications in Solid-State Proton Battery

Solid proton electrolytes play a crucial role in various electrochemical energy storage and conversion devices. However, the development of fast proton conducting solid proton electrolytes at ambient conditions remains a significant challenge. In this study, a novel acidified nitrogen self-doped porous carbon material is presented that demonstrates exceptional superprotonic conduction for applications in solid-state proton battery. The material, designated as MSA@ZIF-8-C, is synthesized through the acidification of nitrogen-doped porous carbon, specifically by integrating methanesulfonic acid (MSA) into zeolitic imidazolate framework-derived nitrogen self-doped porous carbons (ZIF-8-C). This study reveals that MSA@ZIF-8-C achieves a record-high proton conductivity beyond 10-2  S cm-1 at ambient condition, along with good long-term stability, positioning it as a cutting-edge alternative solid proton electrolyte to the default aqueous H2 SO4 electrolyte in proton batteries.

Recent advancements of covalent organic frameworks (COFs) as proton conductors under anhydrous conditions for fuel cell applications

Recent electrochemical energy conversion devices require more advanced proton conductors for their broad applications, especially, proton exchange membrane fuel cell (PEMFC) construction. Covalent organic frameworks (COFs) are an emerging class of organic porous crystalline materials that are composed of organic linkers and connected by strong covalent bonds. The unique characteristics including well-ordered and tailorable pore channels, permanent porosity, high degree of crystallinity, excellent chemical and thermal stability, enable COFs to be the potential proton conductors in fuel cell devices. Generally, proton conduction of COFs is dependent on the amount of water (extent of humidity). So, the constructed fuel cells accompanied complex water management system which requires large radiators and airflow for their operation at around 80 °C to avoid overheating and efficiency roll-off. To overcome such limitations, heavy-duty fuel cells require robust proton exchange membranes with stable proton conduction at elevated temperatures. Thus, proton conducting COFs under anhydrous conditions are in high demand. This review summarizes the recent progress in emerging COFs that exhibit proton conduction under anhydrous conditions, which may be prospective candidates for solid electrolytes in fuel cells.

Post-synthetic modifications of covalent organic frameworks (COFs) for diverse applications

Covalent organic frameworks (COFs) are porous and crystalline organic polymers, which have found usage in various fields. These frameworks are tailorable through the introduction of diverse functionalities into the platform. Indeed, functionality plays a key role in their different applications. However, sometimes functional groups are not compatible with reaction conditions or can compete and interfere with other groups of monomers in the direct synthetic method. Also, pre-synthesis of bulky moieties in COFs can negatively affect crystal formation. To avoid these problems a post-synthetic modification (PSM) approach is a helpful tactic. Also, with the assistance of this strategy porous size can be tunable and stability can be improved without considerable effect on the crystallite. In addition, conductivity, hydrophobicity/ hydrophilicity, and chirality are among the features that can be reformed with this method. In this review, different types of PSM strategies based on recent articles have been divided into four categories: (i) post-functionalization, (ii) post-metalation, (iii) chemical locking, and (iv) host-guest post-modifications. Post-functionalization and chemical locking methods are based on covalent bond formation while in post-metalation and host-guest post-modifications, non-covalent bonds are formed. Also, the potential of these post-modified COFs in energy storage and conversion (lithium-sulfur batteries, hydrogen storage, proton-exchange membrane fuel cells, and water splitting), heterogeneous catalysts, food safety evaluation, gas separation, environmental domains (greenhouse gas capture, radioactive element uptake, and water remediation), and biological applications (drug delivery, biosensors, biomarker capture, chiral column chromatography, and solid-state smart nanochannels) have been discussed.

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.

Design, Preparation, and High Intrinsic Proton Conductivity of Two Highly Stable Hydrazone-Linked Covalent Organic Frameworks

Assembling crystalline materials with high stability and high proton conductivity as a potential alternative to the Nafion membrane is a challenging topic in the field of energy materials. Herein, we concentrated on the creation and preparation of hydrazone-linked COFs with super-high stability to explore their proton conduction. Fortunately, two hydrazone-linked COFs, TpBth and TaBth, were solvothermally prepared by using benzene-1,3,5-tricarbohydrazide (Bth), 2,4,6-trihydroxy-benzene-1,3,5-tricarbaldehyde (Tp), and 2,4,6-tris(4-formylphenyl)-1,3,5-triazine (Ta) as monomers. Their structures were simulated by Material Studio 8.0 software and confirmed by the PXRD pattern, demonstrating a two-dimensional framework with AA packing. The presence of a large number of carbonyl groups as well as -NH-NH₂- groups on the backbone is responsible for their super-high water stability as well as high water absorption capacity. AC impedance tests demonstrated a positive correlation between the water-assisted proton conductivity (σ) of the two COFs and the temperature and humidity. Under 100 °C/98% RH, the highest σ values of TpBth and TaBth can reach 2.11 × 10^-4 and 0.62 × 10^-5 S·cm^-1, which are among the high σ values of the reported COFs. Their proton-conductive mechanisms were highlighted by structural analyses as well as N₂ and H₂O vapor adsorption data and activation energy values. Our systematic research affords ideas for the synthesis of proton-conducting COFs with high σ values.

High Proton Conductivity of Magnetically Ordered 2D Oxalate-Bridged [MnIICrIII] Coordination Polymers with Irregular Topology

Two heterometallic coordination polymers {[NH(CH₃)₂(C₂H₅)]₈[Mn₄Cl₄Cr₄(C₂O₄)₁₂]}_n (1) and {[NH(CH₃)-(C₂H₅)₂]₈[Mn₄Cl₄Cr₄(C₂O₄)₁₂]}_n (2) were obtained by slow evaporation of an aqueous solution containing the building block [A]₃[Cr(C₂O₄)₃] [A = (CH₃)₂(C₂H₅)NH⁺ or (CH₃)(C₂H₅)₂NH⁺] and MnCl₂·2H₂O. The isostructural compounds comprise irregular two-dimensional (2D) oxalate-bridged anionic layers [Mn₄Cl₄Cr₄(C₂O₄)₁₂]_n^8n- with a Shubnikov plane net fes topology designated as (4·8²), interleaved by the hydrogen-bonded templating cations (CH₃)₂(C₂H₅)NH⁺ (1) or (CH₃)(C₂H₅)₂NH⁺ (2). They exhibit remarkable humidity-sensing properties and very high proton conductivity at room temperature [1.60 × 10^-3 (Ω·cm)^-1 at 90% relative humidity (RH) of 1 and 9.6 × 10^-4 (Ω·cm)^-1 at 94% RH of 2]. The layered structure facilitates the uptake of water molecules, which contributes to the enhancement of proton conductivity at high RH. The better proton transport observed in 1 compared to that in 2 can be tentatively attributed to the higher hydrophilicity of the cations (CH₃)₂(C₂H₅)NH⁺, which is closely related to their affinity for water molecules. The original topology of the anionic networks in both compounds leads to the development of interesting magnetic phases upon cooling. The magnetically ordered ground state can be described as the coupling of ferromagnetic spin chains in which Mn²⁺ and Cr³⁺ ions are bridged by bis(bidentate) oxalate groups into antiferromagnetic planes through monodentate-bidentate oxalate bridges in the layers, which are triggered to long-range order below temperature 4.45 K via weaker interlayer interactions.

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.