Unique Proton Dynamics in an Efficient MOF-Based Proton Conductor

Breathing Behavior and Superprotonic Conductivity of Two-Dimensional Flexible Metal-Organic Frameworks Tuned with Alkoxy Groups

Flexible metal-organic frameworks (FMOFs) exhibit reversible structural transitions ("breathing" behaviors), which can regulate the proton transport passageway effectively. This property offers remarkable advantages for improving the proton conductivity. Our objective of this work is to design a single-variable flexibility synergistic strategy for the fabrication of FMOFs with high conductivity. Herein, four two-dimensional FMOFs, {[Co(4-bpdb)(R-ip)]·xsolvents}n (x = rich, 1-4), have been successfully designed and assembled (4-bpdb = 1,4-bis(4-pyridyl)-2,3-diaza-1,3-butadiene and R-ip = MeO/EtO/n-PrO/n-BuO-isophthalate). Upon the release and/or absorption of different solvent molecules, they display reversible breathing behaviors, thereby resulting in the formation of the partial and complete solvent-free compounds {[Co(4-bpdb)(R-ip)]·ysolvents}n (y = free or poor, 1A-4A). This breathing behavior involves the synergistic self-adaption of the dynamic torsion of alkoxy groups and reversible structural transformation, leading to remarkable changes in cell parameters and void space, as evidenced by single-crystal X-ray diffraction, powder X-ray diffraction, and N2 and CO2 adsorption analyses. At 363 K and 98% relative humidity, 2A exhibits the best proton conductivity among the FMOFs. Its conductivity reaches 4.08 × 10-2 S cm-1 and is one of the highest conductivities shown by reported unmodified MOF-based proton conductors.

Synthesis and proton-conductive behaviour of two MOFs with covalently bonded imidazoles in the channels

Immobilization of imidazole molecules as proton carriers into MOFs to facilitate proton conduction is a general strategy for developing high proton conductive materials. Herein, we designed two imidazole substituted phthalic acid ligands and constructed two novel MOFs, {[Zr6(OH)16(H3L1)4]Cl8·20H2O}n [Zr-MOF; H3L1 = 2-(1H-imidazol-4-yl) methylaminoterephthalic acid] and {Gd(HCOO)(H2L2)2}n [Gd-MOF; H3L2 = 5-(1H-imidazol-4-yl)methylaminoisophthalic acid] and fully studied their porous nature, stability and water-assisted proton conduction. The resulting Zr-MOF exhibits a high proton conductivity of 1.82 × 10-2 S cm-1 at 98% RH and 80 °C, while Gd-MOF has a proton conductivity of 3.01 × 10-3 S cm-1 at 98% RH and 60 °C.

Tunnel-Structured Phosphate Exhibiting High Proton Conductivity and Thermal Stability over a Wide Intermediate Temperature Range

For the practical application of fuel cells in vehicles, it is a challenge to develop a proton solid electrolyte that coexhibits thermal stability and high proton conductivity at wide intermediate temperatures. Here, we report on the tunnel structured phosphate KNi1-xH2x(PO3)3·yH2O, which exhibits high proton conductivity at room temperature up to 500 °C, with the conductivity value reaching 1.7 × 10-2 S cm-1 at 275 °C for x = 0.18. This material, composed of the smallest cations that form the tunnel framework with face-shared (KO6) and (NiO6) chains and PO4 tetrahedral chains, retained the rigid framework up to 600 °C. Two oxygen sites of water molecules located adjacent to each other along the PO4 tetrahedral chains in the tunnel provided the proton conduction pathway. The sample maintained a conductivity of 5.0 × 10-3 S cm-1 for 10 h at 150 °C while changing the measurement atmosphere to a N2 gas flow, a 4% H2-96% Ar gas flow, and an O2 gas flow. The conductivity value at x = 0.18 obtained from the DC measurement was in the order of 10-6 S cm-1, close to the instrument's measurement limit. These results demonstrate that tunnel phosphate has potential as a proton solid electrolyte for next-generation fuel cells.

Sulfur Migration Enhanced Proton-Coupled Electron Transfer for Efficient CO2 Desorption with Core-Shelled C@Mn3O4

Transforming hazardous species into active sites by ingenious material design was a promising and positive strategy to improve catalytic reactions in industrial applications. To synergistically address the issue of sluggish CO2 desorption kinetics and SO2-poisoning solvent of amine scrubbing, we propose a novel method for preparing a high-performance core-shell C@Mn3O4 catalyst for heterogeneous sulfur migration and in situ reconstruction to active -SO3H groups, and thus inducing an enhanced proton-coupled electron transfer (PCET) effect for CO2 desorption. As anticipated, the rate of CO2 desorption increases significantly, by 255%, when SO2 is introduced. On a bench scale, dynamic CO2 capture experiments reveal that the catalytic regeneration heat duty of SO2-poisoned solvent experiences a 32% reduction compared to the blank case, while the durability of the catalyst is confirmed. Thus, the enhanced PCET of C@Mn3O4, facilitated by sulfur migration and simultaneous transformation, effectively improves the SO2 resistance and regeneration efficiency of amine solvents, providing a novel route for pursuing cost-effective CO2 capture with an amine solvent.

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.

Photomodulation of Proton Conductivity by Nitro-Nitroso Transformation in a Metal-Organic Framework

The design of a highly and photomodulated proton conductor is important for advanced potential applications in chemical sensors and bioionic functions. In this work, a metal-organic framework (MOF; Gd-NO2) with high proton conductivity is synthesized with a photosensitive ligand of 5-nitroisophthalic acid (BDC-NO2), and it provides remote-control photomodulated proton-conducting behavior. The proton conduction of Gd-NO2 reaches 3.66 × 10-2 S cm-1 at 98% relative humidity (RH) and 25 °C, while it decreases by ∼400 times after irradiation with a 355 nm laser. The newly generated and disappearing FT-IR characteristic peaks reveal that this photomodulated process is realized by the photoinduced transformation from BDC-NO2 to 5-nitroso-isophthalic acid (BDC-NO). According to density functional theory, the smaller electronegativity of the -NO group, the longer distance of the hydrogen bond between BDC-NO and H2O molecules, and the lower water adsorption energy of BDC-NO indicate that the irradiated sample possesses a poorer hydrophilicity and has difficulty forming rich hydrogen-bonded networks, which results in the remarkable decrease of proton conductivity.

Ultra-fast Proton Conduction and Photocatalytic Water Splitting in a Pillared Metal-Organic Framework

Proton-exchange membrane fuel cells enable the portable utilization of hydrogen (H2) as an energy resource. Current electrolytic materials have limitation, and there is an urgent need to develop new materials showing especially high proton conductivity. Here, we report the ultra-fast proton conduction in a novel metal-organic framework, MFM-808, which adopts an unprecedented topology and a unique structure consisting of two-dimensional layers of {Zr6}-clusters. By replacing the bridging formate with sulfate ligands within {Zr6}-layers, the modified MFM-808-SO4 exhibits an exceptional proton conductivity of 0.21 S·cm-1 at 85 °C and 99% relative humidity. Modeling by molecular dynamics confirms that proton transfer is promoted by an efficient two-dimensional conducting network assembled by sulfate-{Zr6}-layers. MFM-808-SO4 also possesses excellent photocatalytic activity for water splitting to produce H2, paving a new pathway to achieve a renewable hydrogen-energy cycle.

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.

Enhanced proton conductivity by guest molecule exchange in an acylamide-functionalized metal-organic framework

Metal-organic frameworks (MOFs) as types of proton conductive materials have attracted much attention. Here, an acylamide-functionalized 3D MOF, [Ni₃(TPBTC)₂(stp)₂(H₂O)₄]·2DMA·32H₂O, has been successfully constructed via combining Ni(NO₃)₂, TPBTC (TPBTC = benzene-1,3,5-tricarboxylic acid tris-pyridin-4-ylamide) and 2-H₂stp (2-H₂stp = 2-sulfoterephthalic acid monosodium salt) under solvothermal conditions. Single-crystal X-ray diffraction revealed that there are uncoordinated guest DMA molecules in the pores of the compound. On removal of guest DMA molecules, the proton conductivity of the compound increased to 2.25 × 10^-3 S cm^-1 at 80 °C and 98% RH which is about 110 times that of the original material. It is hoped that this work can provide essential insight for designing and obtaining improved crystalline-state proton conducting materials by considering the influences of guest molecules on proton conduction properties of porous materials.

Bi-MOF Modulating MnO2 Deposition Enables Ultra-Stable Cathode-Free Aqueous Zinc-Ion Batteries

The Mn-based materials are considered as the most promising cathodes for zinc-ion batteries (ZIBs) due to their inherent advantages of safety, sustainability and high energy density, however suffer from poor cyclability caused by gradual Mn²⁺ dissolution and irreversible structural transformation. The mainstream solution is pre-adding Mn²⁺ into the electrolyte, nevertheless faces the challenge of irreversible Mn²⁺ consumption results from the MnO₂ electrodeposition reaction (Mn²⁺  → MnO₂ ). This work proposes a "MOFs as the electrodeposition surface" strategy, rather than blocking it. The bismuth (III) pyridine-3,5-dicarboxylate (Bi-PYDC) is selected as the typical electrodeposition surface to regulate the deposition reaction from Mn²⁺ to MnO₂ . Because of the unique less hydrophilic and manganophilic nature of Bi-PYDC for Mn²⁺ , a moderate MnO₂ deposition rate is achieved, preventing the electrolyte from rapidly exhausting Mn²⁺ . Simultaneously, the intrinsic stability of deposited R-MnO₂ is enhanced by the slowly released Bi³⁺ from Bi-PYDC reservoir. Furthermore, Bi-PYDC shows the ability to accommodate H⁺ insertion/extraction. Benefiting from these merits, the cathode-free ZIB using Bi-PYDC as the electrodeposition surface for MnO₂ shows an outstanding cycle lifespan of more than 10 000 cycles at 1 mA cm^-2 . This electrode design may stimulate a new pathway for developing cathode free long-life rechargeable ZIBs.

Anhydrous Superprotonic Conductivity in the Zirconium Acid Triphosphate ZrH5 (PO4 )3

The development of solid-state proton conductors with high proton conductivity at low temperatures is crucial for the implementation of hydrogen-based technologies for portable and automotive applications. Here, we report on the discovery of a new crystalline metal acid triphosphate, ZrH₅ (PO₄ )₃ (ZP3), which exhibits record-high proton conductivity of 0.5-3.1×10^-2  S cm^-1 in the range 25-110 °C in anhydrous conditions. This is the highest anhydrous proton conductivity ever reported in a crystalline solid proton conductor in the range 25-110 °C. Superprotonic conductivity in ZP3 is enabled by extended defective frustrated hydrogen bond chains, where the protons are dynamically disordered over two oxygen centers. The high proton conductivity and stability in anhydrous conditions make ZP3 an excellent candidate for innovative applications in fuel cells without the need for complex water management systems, and in other energy technologies requiring fast proton transfer.

Effect of Proton Transfer on Electrocatalytic Water Oxidation by Manganese Phosphates

The effect of proton transfer on water oxidation has hardly been measurably established in heterogeneous electrocatalysts. Herein, two isomorphous manganese phosphates (NH₄ MnPO₄  ⋅ H₂ O and KMnPO₄  ⋅ H₂ O) were designed to form an ideal platform to study the effect of proton transfer on water oxidation. The hydrogen-bonding network in NH₄ MnPO₄  ⋅ H₂ O has been proven to be solely responsible for its better activity. The differences of the proton transfer kinetics in the two materials indicate a fast proton hopping transfer process with a low activation energy in NH₄ MnPO₄  ⋅ H₂ O. In addition, the hydrogen-bonding network can effectively promote the proton transfer between adjacent Mn sites and further stabilize the Mn^III -OH intermediates. The faster proton transfer results in a higher proportion of zeroth-order in [H⁺ ] for OER. Thus, proton transfer-affected electrocatalytic water oxidation has been measurably observed to bring detailed insights into the mechanism of water oxidation.

Metal-Organic Framework-Derived N-Doped Porous Carbon for a Superprotonic Conductor at above 100 °C

The development of proton conductors capable of working at above 100 °C is of great significance for proton exchange membrane electrolysis cells (PEMECs) and proton exchange membrane fuel cells (PEMFCs) but remains to be an enormous challenge to date. In this work, we demonstrate for the first time that the N-doped porous carbon derived from metal-organic frameworks (MOFs) with great superiority can be exploited for high-performing proton conductors at above 100 °C. Through the pyrolysis of ZIF-8, the N-doped porous carbon (ZIF-8-C) featuring high chemical resistance to Fenton's reagent was readily prepared and then served as a robust host to accommodate H₃PO₄ molecules for proton transport. Upon impregnation with H₃PO₄, the resulting PA@ZIF-8-C exhibits low water swelling and high proton conduction of over 10^-2 S cm^-1 at a temperature above 100 °C, which is superior to many reported proton conductors. This work provides a new approach for the design of high-performing proton conductors at above 100 °C.

Coordination Polymers Constructed from an Adaptable Pyridine-Dicarboxylic Acid Linker: Assembly, Diversity of Structures, and Catalysis

4,4'-(Pyridine-3,5-diyl)dibenzoic acid (H₂pdba) was explored as an adaptable linker for assembling a diversity of new manganese(II), cobalt(II/III), nickel(II), and copper(II) coordination polymers (CPs): [Mn(μ₄-pdba)(H₂O)]_n (1), {[M(μ₃-pdba)(phen)]·2H₂O}_n (M = Co (2), Ni (3)), {[Cu₂(μ₃-pdba)₂(bipy)]·2H₂O}_n (4), {[Co(μ₃-pdba)(bipy)]·2H₂O}_n (5), [Co₂(μ₃-pdba)(μ-Hbiim)₂(Hbiim)]_n (6), and [M(μ₄-pdba)(py)]_n (M = Co (7), Ni (8)). The CPs were hydrothermally synthesized using metal(II) chloride precursors, H₂pdba, and different coligands functioning as crystallization mediators (phen: 1,10-phenanthroline; bipy: 2,2'-bipyridine, H₂biim: 2,2'-biimidazole; py: pyridine). Structural networks of 1-8 range from two-dimensional (2D) metal-organic layers (1-3, 5-8) to three-dimensional (3D) metal-organic framework (MOF) (4) and disclose several types of topologies: sql (in 1), hcb (in 2, 3, 5), tfk (in 4), 3,5L66 (in 6), and SP 2-periodic net (6,3)Ia (in 7, 8). Apart from the characterization by standard methods, catalytic potential of the obtained CPs was also screened in the Knoevenagel condensation of benzaldehyde with propanedinitrile to give 2-benzylidenemalononitrile (model reaction). Several reaction parameters were optimized, and the substrate scope was explored, revealing the best catalytic performance for a 3D MOF 4. This catalyst is recyclable and can lead to substituted dinitrile products in up to 99% product yields. The present study widens the use of H₂pdba as a still poorly studied linker toward designing novel functional coordination polymers.

Metal-Organic Frameworks and Their Composites for Environmental Applications

From the point of view of the ecological environment, contaminants such as heavy metal ions or toxic gases have caused harmful impacts on the environment and human health, and overcoming these adverse effects remains a serious and important task. Very recent, highly crystalline porous metal-organic frameworks (MOFs), with tailorable chemistry and excellent chemical stability, have shown promising properties in the field of removing various hazardous pollutants. This review concentrates on the recent progress of MOFs and MOF-based materials and their exploit in environmental applications, mainly including water treatment and gas storage and separation. Finally, challenges and trends of MOFs and MOF-based materials for future developments are discussed and explored.

Enhanced Mechanical Stability and Proton Conductivity Performance from the Dense Mn(II)-Metal-Organic Framework to Porous Mn(II)-Fe(III)-Metal-Organic Framework

Postsynthetic modification (PSM) of the metal-organic framework (MOF) has been demonstrated to be an effective strategy to enhance performance. In this particular work, the anion framework Mn-MOF {[Mn₃O(H₂O)₃(HTC)]^2-} (HTC^6- = (5'-(3,5-dicarboxyphenyl)-[1,1':3',1″-terphenyl]-3,3″,5,5″-tetracarboxylate] was obtained, and NH₂(CH₃)₂⁺ ions were filled within the pores to balance the charge. In order to release the internal pores of Mn-MOF, the trivalent Fe(III) was introduced instead of Mn(II) nodes, resulting in the porous Mn_1-xFe_x-MOF, and the NH₂(CH₃)₂⁺ ions were simultaneously deported from the pores. The content of Fe(III) in Mn_1-xFe_x-MOF was highly dependent on the concentration of Fe(III) solution, and the maximum could be up to Mn_0.05Fe_0.95-MOF with a BET surface area of 1209.457 m² g^-1. Compared to the amorphization of dense Mn-MOF at 0.8 GPa in a diamond anvil cell, the mechanical stability of porous Mn_0.05Fe_0.95-MOF has been dramatically enhanced, and the framework integrity could be maintained up to 16.5 GPa. The proton conductivity for the Mn_1-xFe_x-MOF series was also investigated, where Mn_0.93Fe_0.07-MOF showed the best performance of 1.47 × 10^-2 S cm^-1 under 70 °C and 98% RH due to the onset of reversed charge from the anionic framework to cationic framework and the formation of the most compact hydrogen bonding net. This work has not only provided an example for the PSM strategy but also illustrated that the versatile functionalities of MOF materials were mainly ascribed to the tunable porosity.

Three layered cucurbit[6]uril-based metal-organic rotaxane networks functionalized by sulfonic groups for proton conduction

Three new cucurbit[6]uril (CB[6])-based metal-organic rotaxane networks (MORNs) (named CUST-711, CUST-712, and CUST-713) functionalized by a sulfonic group (-SO₃H) have been designed and synthesized via a hydrothermal method. All three compounds exhibited similar two-dimensional (2D) wave layer structures. Their stability under different temperature and relative humidity conditions has been investigated and all the compounds showed excellent stability. Furthermore, their proton conduction properties were also discussed in detail. Due to different structures and sulfonic group sites, the three compounds exhibited different proton conduction abilities of which CUST-712 exhibited an intrinsic relatively high proton conductivity (1.75 × 10^-4 S cm^-1 at 85 °C and 97% relative humidity). These results provide ideas for the design and synthesis of functional CB[6]-based metal-organic rotaxane frameworks (MORFs) as proton conducting materials.

Highly Efficient Proton Conduction in the Metal-Organic Framework Material MFM-300(Cr)·SO4(H3O)2

The development of materials showing rapid proton conduction with a low activation energy and stable performance over a wide temperature range is an important and challenging line of research. Here, we report confinement of sulfuric acid within porous MFM-300(Cr) to give MFM-300(Cr)·SO₄(H₃O)₂, which exhibits a record-low activation energy of 0.04 eV, resulting in stable proton conductivity between 25 and 80 °C of >10^–2 S cm^–1. In situ synchrotron X-ray powder diffraction (SXPD), neutron powder diffraction (NPD), quasielastic neutron scattering (QENS), and molecular dynamics (MD) simulation reveal the pathways of proton transport and the molecular mechanism of proton diffusion within the pores. Confined sulfuric acid species together with adsorbed water molecules play a critical role in promoting the proton transfer through this robust network to afford a material in which proton conductivity is almost temperature-independent.

Comparative Studies on the Proton Conductivities of Hafnium-Based Metal-Organic Frameworks and Related Chitosan or Nafion Composite Membranes

Hafnium (Hf)-based UiO-66 series metal-organic frameworks (MOFs) have been widely studied on gas storage, gas separation, reduction reaction, and other aspects since they were first prepared in 2012, but there are few studies on proton conductivity. In this work, one Hf-based MOF, Hf-UiO-66-fum showing UiO-66 structure, also known as MOF-801-Hf, was synthesized at room temperature using cheap fumaric acid as the bridging ligand, and then imidazole units were successfully introduced into MOF-801-Hf to obatin a doped product, Im@MOF-801-Hf. Note that both MOF-801-Hf and Im@MOF-801-Hf demonstrate excellent thermal, water, and acid-base stabilities. Expectedly, the maximum proton conductivity (σ) of Im@MOF-801-Hf (1.46 × 10^-2 S·cm^-1) is nearly 4 times greater than that of MOF-801-Hf (3.98 × 10^-3 S·cm^-1) under 100 °C and 98% relative humidity (RH). To explore their possible practical application value, we doped them into chitosan (CS) or Nafion membranes as fillers, namely, CS/MOF-801-Hf-X, CS/Im@MOF-801-Hf-Y, and Nafion/MOF-801-Hf-Z (X, Y, and Z are the doping percentages of MOF in the membrane, respectively). Intriguingly, it was found that CS/MOF-801-Hf-6 and CS/Im@MOF-801-Hf-4 indicated the highest σ values of 1.73 × 10^-2 and 2.14 × 10^-2 S·cm^-1, respectively, under 100 °C and 98% RH and Nafion/MOF-801-Hf-9 also revealed a high σ value of 4.87 × 10^-2 S·cm^-1 under 80 °C and 98% RH, which showed varying degrees of enhancement compared to the original MOFs or pure CS and Nafion membranes. Our study illustrates that these Hf-based MOFs and related composite membranes offer great potential in electrochemical fields.

Ultrastable Polyoxometalate-Encapsulated Supramolecular Metal-Organic Nanotubes for Single-Crystal Proton Conduction

Two unique polyoxometalate (POM)-encapsulated tubular materials with the formula K(H₂O)₆[M₆(btp)₆(H₂O)₂₂](P₂W₁₈O₆₂)₃(Hbtp)₅(btp)₃·52H₂O [M = Mn (1) and Co (2); btp = 2,6-bis(1,2,4-triazol-1-yl)pyridine] were designed and synthesized based on the Dawson POM and V-type btp ligand, as confirmed by IR, X-ray diffraction (XRD), and element analysis. Single-crystal XRD analyses of compound 1 show that two kinds of remarkable metal-organic supramolecular nanotubes, including trigonal and hexagonal nanotubes, are constructed along the c-axis direction via π···π-packing interactions between {Mn₃(btp)₃} rings and the btp ligands, of which [α-P₂W₁₈O₆₂]^6- anions are confined in channels, making the entire structure extraordinarily stable. Meanwhile, the coordinated [α-P₂W₁₈O₆₂]^6- anion within the hexagonal channel makes the channel highly hydrophilic and attracts a number of guest water molecules to fill in the free space, conducive to proton transport. Therefore, the single-crystal sample of 1 exhibits a high proton conductivity of 6.39 × 10^-3 S cm^-1 along one-dimensional channels, 30 times higher than that of a pellet sample at 358 K and 98% relative humidity.

Stable Lanthanide Metal-Organic Frameworks with Ratiometric Fluorescence Sensing for Amino Acids and Tunable Proton Conduction and Magnetic Properties

Four new isostructural lanthanide metal-organic frameworks (MOFs), namely {[Ln(DMTP-DC)_1.5(H₂O)₃]·DMF}_n [H₂DMTP-DC = 2',5'-dimethoxytriphenyl-4,4″-dicarboxylic acid; Ln^III = Eu^III (1), Gd^III (2), Tb^III (3), and Dy^III (4)], have been synthesized and characterized. Single-crystal structure analysis reveals that 1-4 are three-dimensional Ln-MOFs with rich H-bonding of coordinated H₂O molecules in the network channels. The X-ray diffraction patterns indicate that Ln-MOF 1 displays good stabilities in organic solvents and aqueous solutions with distinct pH values. Both 1 and 3 show characteristic emission of Ln^III ions. Ln-MOF 1 can be used as a ratiometric fluorescence sensor for arginine and lysine in aqueous solution, and the detection limits are 24.38 μM for arginine and 9.31 μM for lysine. All 1-4 show proton conductivity related to relative humidity (RH) and temperature, and the maximum conductivity values of 1-4 at 55 °C and 100% RH are 9.94 × 10^-5, 1.62 × 10^-4, 1.71 × 10^-4, and 2.67 × 10^-4 S·cm^-1, respectively. The value of σ increases with the decrease in ionic radius, indicating that the radius of the Ln^III ions can regulate the proton conductivity of these MOFs. Additionally, 2 exhibits a significant magnetocaloric effect (MCE) with a magnetic entropy change (-ΔS_m) of 18.86 J kg^-1 K^-1 for ΔH = 7 T at 2 K, and 4 shows weak field-induced slow relaxation of magnetization. The coexistence of good fluorescence sensing capability, attractive proton conductivity, and relatively large MCE in Ln-MOFs is rare, and thus, 1-4 are potentially multifunctional MOF materials.

Phosphinate MOFs Formed from Tetratopic Ligands as Proton-Conductive Materials

Metal-organic frameworks (MOFs) are attracting attention as potential proton conductors. There are two main advantages of MOFs in this application: the possibility of rational design and tuning of the properties and clear conduction pathways given by their crystalline structure. We hereby present two new MOF structures, ICR-10 and ICR-11, based on tetratopic phosphinate ligands. The structures of both MOFs were determined by 3D electron diffraction. They both crystallize in the P3̅ space group and contain arrays of parallel linear pores lined with hydrophilic noncoordinated phosphinate groups. This, together with the adsorbed water molecules, facilitates proton transfer via the Grotthuss mechanism, leading to a proton conductivity of up to 4.26 × 10^-4 S cm^-1 for ICR-11. The presented study demonstrates the high potential of phosphinate MOFs for the fabrication of proton conductors.

Two-dimensional coordination polymers with high proton conductivity and ultrafast highly efficient molecular sieving constructed by the structural inductive effect

A family of unique 2D-layered Cu-based coordination polymers (abbreviated as HNNU-1α, 1β and 1γ) with different halide anions were successfully constructed using a zwitterion pyridiniumolate as the structural inductive agent (SIA). More importantly, we found that the laminates of HNNU-1α exhibit ultrafast highly-efficient molecular sieving in a water system, and HNNU-1α to 1γ display a good proton conductivity of ca. 2.2 × 10^-2, 4.9 × 10^-5, and 3.0 × 10^-4 S cm^-1 at 90 °C and 98% relative humidity (RH), respectively.

High Protonic Conductivity of Three Highly Stable Nanoscale Hafnium(IV) Metal-Organic Frameworks and Their Imidazole-Loaded Products

Attracted by the exceptional structural rigidity and inherent porous structures of the Hf-based metal-organic frameworks (MOFs), we adopted a rapid synthesis approach to preparing three nanoscale MOFs, Hf-UiO-66 (1), Hf-UiO-66-(OH)₂ (2), and Hf-UiO-66-NH₂ (3), and systematically explored the water-assisted proton conductivities of the original ones and the post-modified products. Interestingly, the proton conductivities (σ) of all three MOFs exhibit significant temperature and humidity dependence. At 98% RH and 100 °C, their optimal σ values can reach up to 10^-3 S·cm^-1. Consequently, imidazole units are loaded into 1-3 to obtain related MOFs, Im@1, Im@2, and Im@3, and the σ values of the imidazole-loaded products are boosted to 10^-2 S·cm^-1. Note that these modifications not only do not change the frameworks of the pristine MOFs but also do not affect their high chemical and water stability. The proton-conductive mechanisms of these MOFs before and after modification have been thoroughly discussed based on structural analyses, N₂ and H₂O vapor adsorptions, and activation energy values. The excellent structural stability as well as the durability and stability of their proton conduction ability indicate that these MOFs can be used in the field of fuel cells and so on.

Superprotonic Conductivity of UiO-66 with Missing-Linker Defects in Aqua-Ammonia Vapor

The design and preparation of proton-conducting metal-organic frameworks (MOFs) with superconductivity are of significance for the proton-exchange membrane fuel cell (PEMFC). Introducing functional structural defects to enhance proton conductivity is a good approach. Here, we synthesized a series of UiO-66 (first synthesized in the University of Oslo) with missing-linker defects and investigated the effect of defect numbers on the proton conductivity of the samples. Among them, 60-UiO-66-1.8 (60 represents the synthesis temperature and 1.8 the number of defects) prepared with 3-mercaptopropionic acid as a modulator has the best proton conductivity, which is 3 × 10^-2 S cm^-1 at 100 °C and under 98% relative humidity (RH). The acidic sites induced by missing-linker defects further promote the chemisorption of ammonia molecules, resulting in the formation of a richer hydrogen-bond network and hence boosting the proton conductivity to 1.04 × 10^-1 S cm^-1 at 80 °C, which is one of the highest values among the reported MOF-based proton conductor. Therefore, this work provides a new strategy for enhancing proton conduction in MOF-based materials.

Series of Stable Anionic Lanthanide Metal-Organic Frameworks as a Platform for Pollutant Separation and Efficient Nanoparticle Catalysis

Eight new stable porous lanthanide metal-organic frameworks (Ln-OFs), namely, [Ln₂(BPTC)₂][(CH₃)₂NH₂]₂ [Ln = Ho (1), Eu (2), Gd (3), Dy (4), Er (5), Tm (6), Yb (7), Lu (8)], were prepared by 3,3',5,5'-biphenyltetracarboxylic acid (H₄BPTC) and lanthanide ions by solvothermal reactions. Complexes 1-8 show a three-dimensional (3D) 6,6-connected network {4¹²·6³}·{4⁸·6⁶·8} topology based on binuclear (Ln₂) clusters and feature a one-dimensional curving porous channel occupied by exchangeable dimethylamine cations ([(CH₃)₂NH₂]⁺) in the 3D anionic frameworks. The occupied [(CH₃)₂NH₂]⁺ in the anionic channels exhibited excellent ion-exchange ability, which is favorable to Pd²⁺ and cationic dye adsorption. Consequently, 1-8 were used to load Pd nanoparticles to catalyze the reduction of nitrophenols and adsorb and desorb methyl blue (MB). The catalytic reaction efficiencies of Pd@1-8 were higher than that of Pd/C (5 wt %) in the hydrogenation reaction of p-nitrophenol (p-NP). Moreover, Pd@1 exhibited good cycle stability and achieved nearly 100% p-NP conversion after eight cycles. Meanwhile, compound 1 also exhibited a high adsorption ability of MB, possessing an adsorption capacity of 1.41 g·g^-1 (second only to 1.49 g·g^-1 reported in the literature) selectively over rhodamine B (RhB) and methyl orange (MO) in aqueous solutions. Remarkably, the skeleton of 1 remained stable after four adsorption-desorption cycles of MB in aqueous solution.

Proton Conductive Lanthanide-Based Metal-Organic Frameworks: Synthesis Strategies, Structural Features, and Recent Progress

In the fields of proton exchange membrane fuel cells as well as impedance recognition, molecular sieve, and biochemistry, the development of proton conductive materials is essential. The design and preparation of the next generation of proton conductive materials-crystalline metal-organic framework (MOF) materials with high proton conductivity and excellent water stability-are facing great challenges. Due to the large radius and high positive charge of lanthanides, they often interact with organic ligands to exhibit high coordination numbers and flexible coordination configurations, resulting in the higher stability of lanthanide-based MOFs (Ln-MOFs) than their transition metal analogues, especially regarding water stability. Therefore, Ln-MOFs have attracted considerable attention. This review offers a view of the latest progress of proton conductive Ln-MOFs, including synthesis strategy, structural characteristics, and advantages, proton conductivity, proton conductive mechanism, and applications. More importantly, by discussing structure-property relationships, we searched for and analyzed design techniques and directions of development of Ln-MOFs in the future. The latest progress of synthesis strategy, structural characteristics, proton conductive properties and mechanism and applications on Ln-MOFs. Ln-MOFS Lanthanide-based MOFs, MOF metal-organic framework, PEMFC proton exchange membrane fuel cells.

Process of metal-organic framework (MOF)/covalent-organic framework (COF) hybrids-based derivatives and their applications on energy transfer and storage

The fossil-fuel shortage and severe environmental issues have posed ever-increasing demands on clean and renewable energy sources, for which the exploration of electrocatalysts has been a big challenge toward energy transfer and storage. Some indispensable features of electrocatalysts, such as large surface area, controlled structure, high porosity, and effective functionalization, have been proved to be critical for the improvement of electrocatalytic activities. Recently, the rapid expansion of metal-organic frameworks (MOFs), covalent-organic frameworks (COFs), and porous-organic polymers has provided extensive opportunities for the development of various electrocatalysts. Moreover, combining diverse descriptions of porous-organic frameworks (such as MOFs and COFs) can generate amazing and fantastic properties, affording the formed MOF/COF (including core-shell MOF@MOF and MOF@COF and layer-on-layer MOF-on-MOF or COF-on-MOF) heterostructures wide applications in diverse fields, especially in clean energy and energy transfer. To further boosts electronic conductivity, catalytic performances, and energy storage abilities, these MOF/COF hybrid materials have been widely utilized as versatile precursors for the manufacture of transition metal catalysts embedded within mesoporous carbon nitrides (M@CN_x) and porous carbon nitride frameworks (CN_x) via a facile pyrolysis process. Given that these M@CN_x and CN_x hybrids are composed of abundant catalytic centers, rich functionalities, and large specific surface areas, vast applications in energy transfer and energy storage fields can be realized. In this mini-review, we summarize the preparation strategies of MOF/COF-based hybrids, as well as their derivatives, nanostructure formation mechanism of M@CN_x and CN_x hybrids from MOF/COF-based hybrid materials, and their applications as catalysts for driving diverse reactions and electrode materials for energy storage. Further, current challenges and future prospects of applying these derivatives into energy conversion and storage devices are also discussed.

A Preinstalled Protic Cation as a Switch for Superprotonic Conduction in a Metal-Organic Framework

Metal-organic frameworks (MOFs), made from various metal nodes and organic linkers, provide diverse research platforms for proton conduction. Here, we report on the superprotonic conduction of a Pt dimer based MOF, [Pt₂(MPC)₄Cl₂Co(DMA)(HDMA)·guest] (H₂MPC, 6-mercaptopyridine-3-carboxylic acid; DMA, dimethylamine). In this framework, a protic dimethylammonium cation (HDMA⁺) is trapped inside a pore through hydrogen bonding with an MPC ligand. Proton conductivity and X-ray measurements revealed that trapped HDMA⁺ works as a preinstalled switch, where HDMA⁺ changes its relative position and forms an effective proton-conducting pathway upon hydration, resulting in more than 10⁵ times higher proton conductivity in comparison to that of the dehydrated form. Moreover, the anisotropy of single-crystal proton conductivity reveals the proton-conducting direction within the crystal. The present results offer insights into functional materials having a strong coupling of molecular dynamic motion and transport properties.

High proton conductivity in a charge carrier-induced Ni(ii) metal–organic framework

A new tetradentate phosphonate ligand-based Ni-MOF has been synthesized and employed as an efficient proton-conducting material upon doping with sulphuric acid.

Proton-conducting metal–organic frameworks with linkers containing anthracenyl and sulfonate groups

Co(dia)1.5(Hsip)(H2O)·H2O (1) and Zn2(μ-OH)(dia)2(sip)·2H2O (2) were prepared from the same set of ligand precursors. They exhibited bnn and dia topologies, respectively. Factors that contributed to the higher proton conductivity of 1 were presented.

Stable isomeric layered indium coordination polymers for high proton conduction

Stable isomeric layered indium coordination polymers with different coordinated anionic sites for high proton conduction.

Nanochannel-Based {BaZn}-Organic Framework for Catalytic Activity on Cycloaddition Reaction of Epoxides with CO2 and Deacetalization-Knoevenagel Condensation

Because of the integrated properties from chemically dissimilar metals, microporous heterometallic MOFs have wider potential applicability, which prompts us to explore the tendency collocation of different metal cations in the...

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

Crystalline porous materials, such as metal-organic frameworks (MOFs) and covalent organic frameworks (COFs), have been demonstrated to be versatile material platforms for the development of solid proton conductors. However, most crystalline porous proton conductors suffer from decreasing proton conductivity with increasing temperature due to releasing water molecules, and this disadvantage severely restricts their practical application in electrochemical devices. In this work, for the first time, hydrophilic carbon dots (CDs) were utilized to hybridize with high proton conductivity MOF-802, which is a model of MOF proton conductors, aiming to improve its water-retention capacity and thus enhance proton conduction. The resultant CDs@MOF-802 exhibits impregnable proton conduction with increasing temperature, and the proton conductivity reaches 10^-1 S cm^-1, much superior to that of MOF-802, making CDs@MOF-802 one of the most efficient MOF proton conductors reported so far. This study provides a new strategy to improve the water-retention capacity of porous proton conductors and further realize excellent proton conduction.

Enhancement of Proton Conductivity in Fe-Metal-Organic Frameworks by Postsynthetic Oxidation and High-Performance Hybrid Membranes with Low Acidity

The postsynthetic oxidation (PSO) of metal nodes in metal-organic frameworks (MOFs) has received widespread attention because PSO can significantly improve the performance of materials without changing the framework. This study investigates the influence of PSO on the proton conductivity of MOFs. The PSO product {[Fe^III₃L₂(H₂O)₆]•3(OH)}_n (2) is obtained by oxidizing {[Fe^II₃L₂(H₂O)₆]•3H₂O}_n (1) with Cu(NO₃)₂. At 98% RH and 70 °C, the proton conductivity of 2 is 66 times higher than that of 1, indicating that PSO can promote proton conduction. In the PSO process, metal ions shuttle in the MOF framework to functionalize the pores, and the change in the guest molecule forms more host-guest collaborative hydrogen bonds. All of these have made a significant contribution to proton conduction. Because 2 exhibits high proton conductivity (2.66 × 10^-4 S·cm^-1) at 98% RH and 80 °C, we doped 2 into a highly economical poly(vinylidene fluoride) (PVDF)/polyvinylpyrrolidone (PVP) substrate to make a hybrid membrane. The resulting hybrid membrane exhibits a high proton conductivity of 1.77 × 10^-3 S·cm^-1 at 98% RH and 80 °C, which is 4 times higher than the proton conductivity of the PVDF/PVP membrane and 6.6 times higher than that of 2.

Hourglass-Type Polyoxometalate-Based Crystalline Material as an Efficient Proton-Conducting Solid Electrolyte

Proton exchange membrane fuel cells are limited because they are limited to working temperatures and are susceptible to damage by dramatic electrochemical environments such as hydrogen peroxide/radicals. It is necessary to develop new proton-conducting materials that are water-stable and can operate at high temperatures. The hourglass reduced molybdophosphate-based compound (H₂bimb)₃[Zn₃(H₆P₄Mo₆O₃₁)₂] (bimb = 1,4-bis[(1H-imidazol-1-yl)methyl]benzene) was designed and synthesized under solvothermal conditions. Single-crystal X-ray diffraction analyses demonstrated noticeably that CUST-571 was composed of an hourglass {Zn[P₄Mo₆]₂} structure, which consisted of two fully reduced half-units {P₄Mo₆}. It was found that CUST-571 possessed an excellent proton conductivity of 4.54 × 10^-3 S cm^-1 at 85 °C and 98% RH (relative humidity). In addition, CUST-571 is capable of an excellent catalytic decomposition of H₂O₂, which is beneficial to increase the life of fuel cells. On the basis of the aforementioned results, CUST-571 may be a promising proton-conducting polyoxometalate hybrid material in the future.

Single-crystal superprotonic conductivity in an interpenetrated hydrogen-bonded quadruplex framework

A proton-transporting pathway is crucial to the conduction mechanism in fuel cells and biological systems. Here, we report a novel 5-fold interpenetrated three-dimensional (3D) hydrogen-bonded quadruplex framework, which exhibits an ultrahigh single-crystal proton conductivity of 1.2(1) × 10^-2 S cm^-1 at 95 °C and 98% relative humidity, benefitting from the spiral H₃O⁺/H₂O chains in 1D pore channels studded with COOH/COO^- groups.

Three Robust Blue-Emitting Anionic Metal-Organic Frameworks with High Stability and Good Proton Conductivities

Three anionic luminescent metal-organic frameworks (LMOFs; [M(tcbpe)(CH₃)₂NH₂]·H₂O; M = In³⁺, Eu³⁺, Gd³⁺; tcbpe = 4',4‴,4‴″,4‴‴'-(ethene-1,1,2,2-tetrayl)tetrakis[(1,1'-biphenyl)-4-carboxylic acid]) are synthesized by employing the tetraphenylethene core ligand H₄tcbpe with M³⁺ ions. They stack in the similarly 4-fold-interpenetrated three-dimensional porous structure. All give blue emission when excited at 365 nm, with fluorescence quantum yields of 34.8% (MOF-In), 7.1% (MOF-Eu), and 28.1% (MOF-Gd). Somewhat surprisingly, these three complexes are extremely stable both in various solvents and across a broad pH range: MOF-In is stable between pH = 0 and 14, and MOF-Eu and MOF-Gd are stable between pH = 0 and 13. Additionally, they also show good proton conductivities of 2.29 × 10^-5 S·cm^-1 (MOF-In), 2.02 × 10^-4 S·cm^-1 (MOF-Eu), and 1.24 × 10^-4 S·cm^-1 (MOF-Gd) at high temperature under 98% relative humidity. To the best of our knowledge, this is the first reported LMOF series combining aggregation-induced emission behavior with good proton conductivities.