Acidic Groups Functionalized Carbon Dots Capping Channels of a Proton Conductive Metal-Organic Framework by Coordination Bonds to Improve the Water-Retention Capacity and Boost Proton Conduction

Proton Conduction in Zirconium-Based Metal-Organic Frameworks for Advanced Applications

Zirconium-based metal-organic frameworks (Zr-MOFs) have emerged as a promising class of crystalline porous materials, attracting significant interest in the field of proton conduction due to their exceptional chemical stability, structural flexibility, and functional tunability. Notably, proton-conducting Zr-MOFs show immense potential for diverse advanced technological applications. In this Spotlight on Applications paper, we provide an overview of proton-conducting Zr-MOFs and spotlight the recent progress of their utilization as proton exchange membranes in proton exchange membrane fuel cells (PEMFCs), light-responsive systems for proton pumps, and chemical sensors for formic acid detection. Furthermore, we also discussed the challenges, future prospects, and opportunities for promoting the application of proton-conducting Zr-MOFs.

Directed Regulation of Proton Transport Pathways in MOF-808

The directed regulation of proton transport pathways in proton conductors, facilitated by the well-defined crystal structures of metal-organic frameworks (MOFs), is important for the development of advanced materials. In this study, MOF-808-2.5SO4-His is synthesized by progressive directed modification on the framework using functional molecules. The incorporation of sulfate and imidazole groups into MOF-808-2.5SO4-His results in a high proton conductivity of 1.37 × 10-2 S cm-1 at 70 °C and 98% relative humidity (RH). The analysis of temperature-dependent proton conductivity indicates that MOF-808-2.5SO4-His facilitates proton transport through the Grotthuss mechanism at 98% RH and the temperature range of 30-70 °C. Additionally, MOF-808-2.5SO4-His exhibits good cycling stability and durability in performance. This feasible approach enhances the comprehension of proton transfer mechanism and promotes the development of viable strategies for controllable construction of proton conductors.

High Proton Conductivity Enhancement Obtained by a Covalent Postsynthesis Modification Approach for Two Metal-Organic Frameworks

This study demonstrates an effective strategy to enhance proton conductivity by synthesizing 2 three-dimensional metal-organic frameworks (MOFs), [Zn(DTD22)]n (MOF 1) and [Cd2(DTD22)2]n (MOF 2), (DTD22 = 4,4″-diamino-[1,1':4',1″-terphenyl]-2,2″-dicarboxylic acid). The DTD22 ligand used formed a continuous hydrogen-bonding network in the structure, constructing excellent hydrophilic channels. MOF 1 and MOF 2 were further postsynthesized and modified (PSM) by Schiff base reaction, and 4-chloro-3-formylbenzenesulfonic acid ligands containing -SO3H and -Cl were successfully introduced into the framework to form PSM-MOF 1 and PSM-MOF 2. Experiments showed that this modification significantly enhanced the proton conductivity of the materials, especially at 90 °C and 98% RH: PSM-MOF 1 (2.38 × 10-1 S·cm-1) and PSM-MOF 2 (3.50 × 10-1 S·cm-1). In comparison, the conductivities of unmodified MOF 1 and MOF 2 were 8.55 × 10-2 S·cm-1 and 9.50 × 10-5 S·cm-1, respectively. The present study demonstrates that the proton conductivity of MOFs can be effectively enhanced by the covalent postmodification method, which provides a new idea for the application of MOFs.

Enhanced Proton Conduction in Metal-Organic Frameworks through Single-Crystal to Single-Crystal Transformation

In this work, an anionic framework Co-MOF (1) was elaborately constructed, which underwent single-crystal-to-single-crystal (SC-SC) transformation to produce 1-Cr and 1-Fe after immersion in a CrCl3 or FeCl3 solution. Despite the similar crystal structure, the significantly enhanced proton conductivities of 1-Cr and 1-Fe far exceed that of 1 at all humidity and temperature conditions. Even at 30 °C and 98% RH, the proton conductivity of 1-Cr and 1-Fe can reach up to high values of 1.49 × 10-2 and 6.39 × 10-3 S cm-1, respectively, surpassing that of 1 by over 5000 times under identical conditions. The partial alteration of the proton-conducting carriers from metal-water cluster [Co(H2O)6]·6H2O] (1) to metal-hydroxyl-water clusters [Cr(OH)4(H2O)2]·6H2O] (1-Cr) and [Fe(OH)4(H2O)2]·6H2O] (1-Fe) can be attributed for the above-mentioned enhanced performance. The introduction of hydroxyl by SC-SC transformation can establish interconnected proton conduction pathways within the proton channels, which greatly facilitate proton conduction, affording much lower activation energies (0.12 eV for 1-Cr, 0.18 eV for 1-Fe, and 0.28 eV for 1). This research demonstrated that SC-SC transformation not only achieved significantly improved proton conduction but also contributed to a deeper understanding of the structure-property relationships, providing new insights into the design of advanced materials with enhanced proton conductivity.

Synergistic effect promoting proton transport in metal-organic framework aerogels

One of the significant challenges encountered by metal-organic frameworks (MOFs) in proton conduction is their limited processability. In this study, we successfully synthesized UiO-66-COOH aerogel and UiO-66-2COOH aerogel, which exhibit high proton conductivities and remarkable temperature cycling stability in terms of performance and structural integrity.

Symmetry related proton conductivity tunability via aliovalent metal substitution in imidazolium templated stable metal-organic framework hybrid membranes

Proton-conducting materials have gained popularity owing to their extensive applications in biologic/chemical sensors, supercapacitors, proton sieving, and proton-exchange-membrane fuel cells. To date, the most commercially used polymer membrane has been the Nafion series that exhibits conductivity exceeding 0.1 S cm-1, however, this series is expensive, has poor dimensional stability, and requires a complex synthesis process. The key criterion for selecting Nafion alternatives is to achieve the systematic integration of high proton conductivity with high stability through a simple and efficient approach. In this study, we used an aliovalent metal substitution strategy to design serial metal-organic frameworks (MOFs), including tetragonal T-Cd-BTC (CH3NH2CH3)2[Cd(BTC)](H2O) and quasi-cubic quasi-C-In-BTC (C4H7N2)[In(BTC)] and Im@quasi-C-In-BTC (C3H5N2)2[In(BTC)] frameworks, with 2-methylimidazolium and imidazolium cations as templates, respectively. Because of the aliovalent substitution of In(III) for Cd(II), both the metal-oxygen bond strength and unit cell symmetry gradually increased, resulting in an increase in the thermal stability of quasi-C-In-BTC and Im@quasi-C-In-BTC at temperatures of up to 700 K. Compared with in situ loaded 2-methylimidazolium quasi-C-In-BTC, Im@quasi-C-In-BTC prepared by incorporating the imidazolium cation into the pores of activated quasi-C-In-BTC exhibited a higher proton conductivity of 7.1 × 10-2 S cm-1 at 338 K and 95 % relative humidity. Thus, Im@quasi-C-In-BTC demonstrated real-life application. This result was confirmed by integrating Im@quasi-C-In-BTC with a poly(vinyl pyrrolidone)-poly(vinylidene fluoride) polymer matrix. Density functional theory simulations indicated that Im@quasi-C-In-BTC was strongly acidic and had high water-adsorption capacities, which contributed to extensive hydrogen-bond networks and strong host-guest interactions, in accordance with the experimental finding.

Enhancing leachate management with antibacterial nanocomposites incorporating plant-based carbon dots and Satureja Khuzestanica essential oils

Landfill leachate, a complex mixture of pollutants, poses a significant environmental hazard. This study reports the synthesis and characterization of superabsorbent nanocomposites (SANs) designed for enhanced performance in waste management applications. SANs were prepared using carboxymethyl cellulose (CMC) and sodium polyacrylate (SPA) as the main components, carbon dots (CDs) to improve absorption, and Satureja Khuzestanica essential oil (SEO) for antibacterial performance. The results demonstrated that the addition of CDs significantly increased the absorption capacity and liquid retention of the samples, with a water absorption capacity reaching up to 8621 %. Furthermore, the samples exhibited high mechanical strength, with tensile strength improving by over 100 % in the presence of CDs. The inclusion of SEO provided strong antibacterial activity against Escherichia coli and Staphylococcus aureus, with inhibition zones measuring up to 26 mm. These SANs, with their high absorption capacity, mechanical robustness, and antibacterial properties, show great potential for improving waste management practices, particularly in leachate absorption strategies.

Metal-organic framework (MOF)/C-dots and covalent organic framework (COF)/C-dots hybrid nanocomposites: Fabrications and applications in sensing, medical, environmental, and energy sectors

Developing new hybrid materials is critical for addressing the current needs of the world in various fields, such as energy, sensing, health, hygiene, and others. C-dots are a member of the carbon nanomaterial family with numerous applications. Aggregation is one of the barriers to the performance of C-dots, which causes luminescence quenching, surface area decreases, etc. To improve the performance of C-dots, numerous matrices including metal-organic frameworks (MOFs), covalent-organic frameworks (COFs), and polymers have been composited with C-dots. The porous crystalline structures, which are constituents of metal nodes and organic linkers (MOFs) or covalently attached organic units (COFs) provide privileged features such as high specific surface area, tunable structures, and pore diameters, modifiable surface, high thermal, mechanical, and chemical stabilities. Also, the MOFs and COFs protect the C-dots from the environment. Therefore, MOF/C-dots and COF/C-dots composites combine their features while retaining topological properties and improving performances. In this review, we first compare MOFs with COFs as matrices for C-dots. Then, the recent progress in developing hybrid MOFs/C-dots and COFs/C-dots composites has been discussed and their applications in various fields have been explained briefly.

Imparting Stable and Ultrahigh Proton Conductivity to a Layered Rare Earth Hydroxide via Ion Exchange

Proton conductors are essential functional materials with a wide variety of potential applications in energy storage and conversion. In order to address the issues of low proton conductivity and poor stability in conventional proton conductors, a simple and valid ion-exchange method was proposed in this study for the introduction of stable and ultrahigh proton conductivity in layered rare earth hydroxides (LRHs). Test analyses by solid-state nuclear magnetic resonance, Fourier transform infrared spectroscopy, and powder X-ray diffraction revealed that the exchange of H2PO4- not only does not disrupt the layered structure of LRHs, but also creates more active proton sites and channels necessary for proton transport, thereby creating a high-performance proton conductor (LRH-H2PO4-). By utilizing this ion-exchange method, the proton conductivity of LRHs can be significantly enhanced from a low level to an ultrahigh level (>10-2 S·cm-1), while maintaining excellent long-term stability. Moreover, through methodically manipulating the guest ions and molecules housed within the interlayers of LRHs, a comprehensive explanation has been presented regarding the proficient mechanism of proton conduction in LRH-H2PO4-. As a result, this investigation presents a feasible and available approach for advancing proton conductor.

Construction of Hydration Layer for Proton Transport by Implanting the Hydrophilic Center Ag0 in Nickel Metal-Organic Frameworks

The directional arrangement of H2O molecules can effectively regulate the ordered protons transfer to improve transport efficiency, which can be controlled by the interaction between materials and H2O. Herein, a strategy to build a stable hydration layer in metal-organic framework (MOF) platforms, in which hydrophilic centers that can manipulate H2O molecules are implanted into MOF cavities is presented. The rigid grid-Ni-MOF is selected as the supporting material due to the uniformly distributed cavities and rigid structures. The Ag0 possesses potential combination ability with the hydrophilic substances, so it is introduced into the MOF as hydration layer centers. Relying on the strong interaction between Ag0 and H2O, the H2O molecules can rearrange around Ag0 in the cavity, which is intuitively verified by DFT calculation and molecular dynamics simulation. The establishment of a hydration layer in Ag@Ni-MOF regulates the chemical properties of the material and gives the material excellent proton conduction performance, with a proton conductivity of 4.86 × 10-2 S cm-1.

Bioadhesive and electroactive hydrogels for flexible bioelectronics and supercapacitors enabled by a redox-active core-shell PEDOT@PZIF-71 system

Stretchable and conductive hydrogels are rapidly emerging as new generation candidates for wearable devices. However, the poor electroactivity and bioadhesiveness of traditional conductive hydrogels has limited their applications. Herein, a mussel-inspired strategy is proposed to prepare a specific core-shell redox-active system, consisting of a polydopamine (PDA) modified zeolitic imidazolate framework 71 (ZIF-71) core, and a poly 3,4-ethylenedioxythiopene (PEDOT) shell. Owing to the abundant catechol groups, PEDOT can be assembled on the surface of ZIF-71 to create a redox-active system. The core-shell nanoparticles could act as a redox-active nanofiller to develop a conductive polyacrylamide (PAM) hydrogel with energy-storage properties. The core-shell PEDOT@PZIF-71 system provides a mussel-inspired environment in the hydrogel matrix and endows the hydrogel with stretchability and adhesiveness. The hydrogel can be applied as a functional electrode for both bioelectronics and supercapacitors. Moreover, this hydrogel exhibits favorable biocompatibility and can be implanted in vivo for biosignal measurement without causing inflammation. This redox-active core-shell PEDOT@PZIF-71 system provides a promising strategy for the design of hydrogel-based wearable electronic devices.

SCSC Transformation and Post-Synthesis Modification of MOFs with Proton Conduction and Ratiometric Fluorescence-Sensing Properties

The modification of metal-organic framework (MOF) materials to facilitate their practical applications is an extremely challenging and meaningful topic. In this work, two stepwise modification strategies for MOFs were conducted. First, we have demonstrated a single-crystal-to-single-crystal (SCSC) transformation from a microporous three-dimensional (3D) MOF to a two-dimensional (2D) coordination polymer (CP). The centrosymmetric [Cd(3-bpdb)(MeO-ip)]_n (1) transforms into a chiral [Cd₂(3-bpdb)(MeO-ip)₂(CH₃OH)₂]_n (2), which is triggered by the reaction time with methanol that acts as a structure-directing agent. The conversion relationship of 1 to 2 at different reaction times was studied in detail. Density functional theory (DFT) calculations clearly state that the irreversible formation of 2 is thermodynamically favorable. Intriguingly, 2 exhibits good proton conduction of 1.34 × 10^-3 S cm^-1 under 363 K and 98% relative humidity (RH) due to unique H-bond network characteristics. To the best of our knowledge, there are very few cases of 3D to 2D SCSC transformation stimulated by reaction time. The results have important implications for understanding the SCSC transformation mechanism and synthetic chemistry. On the other hand, the lanthanide³⁺-functionalized hybrids (Ln³⁺-MOF), Ln³⁺@1, were continuously prepared by incorporating luminescent Ln³⁺ ions into the structure of 1 through encapsulating post-synthesis modification (PSM). Tb³⁺@1 exhibits double emission in water and shows visual ratiometric fluorescence behavior for sensing glutamic acid (Glu), tryptophan (Trp), and Al³⁺, which is more reliable and accurate than single emission. Our work may not only provide new insights into the multiple modification of MOF materials but also promote the practical application of such materials.

A Potential Roadmap to Integrated Metal Organic Framework Artificial Photosynthetic Arrays

Metal organic frameworks (MOFs), a class of coordination polymers, gained popularity in the late 1990s with the efforts of Omar Yaghi, Richard Robson, Susumu Kitagawa, and others. The intrinsic porosity of MOFs made them a clear platform for gas storage and separation. Indeed, these applications have dominated the vast literature in MOF synthesis, characterization, and applications. However, even in those early years, there were hints to more advanced applications in light-MOF interactions and catalysis. This perspective focuses on the combination of both light-MOF interactions and catalysis: MOF artificial photosynthetic assemblies. Light absorption, charge transport, H2O oxidation, and CO2 reduction have all been previously observed in MOFs; however, work toward a fully MOF-based approach to artificial photosynthesis remains out of reach. Discussed here are the current limitations with MOF-based approaches: diffusion through the framework, selectivity toward high value products, lack of integrated studies, and stability. These topics provide a roadmap for the future development of fully integrated MOF-based assemblies for artificial photosynthesis.

Enhancing Proton Conductivity of Nafion Membrane by Incorporating Porous Tb-Metal-Organic Framework Modified with Nitro Groups

A rigid carboxylate ligand with a nitro functional group was selected to coordinate with Tb(III) cation, and Tb-MOF ({[Tb₄(L)₄(OH)₄(H₂O)₃]·8H₂O}_n, H₂L = 2-nitroterephthalic acid) with large porous and excellent hydrophilicity was obtained successfully. The obtained Tb-MOF was filled into the Nafion matrix to improve its proton conduction performance. The Tb-MOF/Nafion composite membrane was characterized by PXRD, IR, and thermogravimetry (TG) and for water uptake, area swelling, and proton conductivity. The activity energy, E_a, value of the composite membrane, which is a very important factor affecting the proton conduction performance of the membrane, was fitted and calculated. It was revealed that Tb-MOF can improve the proton conductivities of composite membranes, and the improvement degree and E_a value were both affected by Tb-MOF content. When Tb-MOF content was 5%, the proton conductivity of the composite membrane was 1.53 × 10^-2 S·cm^-1 at 100% RH and 80 °C, which is 1.81 times that of the pure Nafion membrane. A MOF containing a nitro functional group was first doped into Nafion in this study and exhibited excellent performance for improving composite membrane proton conductivity. This study will provide a valuable reference for designing different functionalized MOFs to promote the proton conductivities of proton exchange membranes.

Ionothermal synthesis of a highly crystalline zirconium phosphate proton conductor

A highly crystalline one-dimensional zirconium phosphate, (NH₄)₂[ZrF(PO₄)(HPO₄)] (ZrP-3), was facilely synthesized by the ionothermal method. The robust structure and rich hydrogen-bonded network make ZrP-3 an excellent proton conductor by having a proton conductivity higher than 10^-2 S cm^-1 at 90 °C and 95% RH. The remarkable stability makes ZrP-3 a promising solid electrolyte material for proton exchange membrane fuel cells.

A multifunctional anionic metal-organic framework for high proton-conductivity and photoreduction of CO2 induced by cation exchange

Metal-Organic Frameworks (MOFs) provide an ideal platform for loading various guests owing to their available spaces, which can be developed as a class of multifunctional materials. Herein, we cover the...