Sulfonic Groups Lined along Channels of Metal–Organic Frameworks (MOFs) for Super-Proton Conductor

Combined Effect of Hydrophilic Pore and the Type of Protons on Proton Conductivity in Porous Metal-Organic Frameworks: A Feasible Approach to Achieve a Super Proton Conductor under Hydrated Conditions

There has been a steady growth of interest in proton-conductive metal-organic frameworks (MOFs) due to their potential utility in proton-exchange membrane fuel cells. To attain a super proton conductivity (>1 × 10-2 S cm-1) in a MOF-based proton conductor is a key step toward practical application. Currently, most studies are focused on enhancing the proton conductivity of porous MOFs by controlling a single factor, such as the type of protons or hydrophilic pore or hydrogen bond. However, a limited contribution from a single factor cannot afford to remarkably increase the proton conductivity of the MOF and form a super proton conductor. Herein, we constructed two distinct porous MOFs, {(H3O+)4[Cu12(ci)12(OH)4(H2O)12]·3H2O·9DMF} (Cu-ci-3D, H2ci = 1H-indazole-5-carboxylic acid, DMF = N,N'-dimethylformamide) and {[Co(Hppca)2]·2HN(CH3)2·CH3OH·2H2O} (Co-ppca-2D, H2ppca = 5-(pyridin-3-yl)-1H-pyrazole-3-carboxylic acid), to tune their proton conductivities at high relative humidity (RH) using the combined effect of hydrophilic pore and the type of protons, ultimately achieving super proton conduction. Excitingly, Cu-ci-3D indeed harvests a super proton conductivity of 1.37 × 10-2 S cm-1 at 353 K and ∼97% RH, superior to some previously reported MOF-based proton conductors. The results present a unique perspective for developing high-performance MOF-based proton conductors and understanding their structure-performance relationships.

Comparative Study on the Proton Conduction Behaviors of Two Acidic Amphiphilic and Hydrophilic Coordination Compounds in Nafion Composite Membranes

The acidic amphiphilic compound H[Co(H2L1)(HL1)(phen)]·3H2O (H4(Co-L1), H3L1 = 5-(3', 5'-dicarboxylphenyl)-pyridine-2-carboxylic, phen = phenanthroline) and the hydrophilic compound [Ni(HL2)(H2O)5]·H2O (H(Ni-L2), H3L2 = 5-(3',5'-dicarboxylphenyl)-pyridine-3-carboxylic) were synthesized via hydrothermal reactions at acidic conditions. The acidity of H4(Co-L1) is stronger than of H(Ni-L2); while the hydrogen bond continuity in H4(Co-L1) extended monodirectionally, which is smaller compared to the three-directional extension observed in H(Ni-L2). The proton conduction behaviors of these two compounds as fillers of Nafion composite membranes have been investigated. The results indicate that the optimal doping amounts of H4(Co-L1) and H(Ni-L2) are 2 and 1%, respectively; the proton conductivities of H4(Co-L1)/Nafion-2 and H(Ni-L2)/Nafion-1 composite membranes are 0.243 and 0.212 S·cm-1, respectively, which are approximately 50.2 and 30.6% higher than that of pure Nafion membrane, respectively. A higher doping amount of H4(Co-L1) can be attributed to its hydrophobic phen ligand, which promotes compatibility with Nafion membrane and reduces aggregation. Hydrogen bond continuity has a more significant effect on proton conductivity than acidity at relatively low doping amounts; conversely, this relationship reverses at relatively high doping amounts.

Room-Temperature Superprotonic Conductivity beyond 10-1 S cm-1 in a Co(II) Coordination Polymer

An efficient design of crystalline solid-state proton conductors (SSPCs) is crucial for the progress of clean energy applications. Developing such materials to make them work at room temperature with a conductivity of ≥10-1 S cm-1 is of significant interest in terms of technical and commercial aspects. Utilizing the recently highlighted "coordinated-water-driven proton conduction" approach, herein, we have rationally synthesized two highly stable and scalable 1D Co(II) coordination polymers (CPs) as SSPCs, PCM-2 {[Co(bpy)(H2O)2(NO3)2]·H2O}n and PCM-3 {[Co2(bpy)2(SO4)2(H2O)6].4H2O}n, with distinct alignments in coordinated water and coordinated oxo-anions (nitrate and sulfate, respectively). The acidity of the metal-bound water molecules in PCM-2 is further enhanced through cooperative long-range continuous H bonds with coordinated Brønsted basic nitrates (proton acceptors), leading to ultrahigh superprotonic conductivities even at 25 °C (1.03 × 10-1 S cm-1 under 95% RH), and reached a maximum of 2.99 × 10-1 S cm-1 at 85 °C (95% RH). The conductivity at 25 °C is even higher than that of commercial Nafion 117 (6.74 × 10-2 S cm-1 at 100% RH). The absence of such an H-bonding interaction in PCM-3 (closed loops) resulted in a lesser conductivity of 5.87 × 10-5 S cm-1 (95% RH, 85 °C). PCM-2 represents the first example of SSPC exhibiting conductivity in the order 10-1 S cm-1 at ambient temperature (25 °C) with excellent recyclability.

Nanoarchitectonics Design Strategy of Metal-Organic Framework and Bio-Metal-Organic Framework Composites for Advanced Wastewater Treatment through Adsorption

Freshwater depletion is an alarm for finding an eco-friendly solution to treat wastewater for drinking and domestic applications. Though several methods like chlorination, filtration, and coagulation-sedimentation are conventionally employed for water treatment, these methods need to be improved as they are not environmentally friendly, rely on chemicals, and are ineffective for all kinds of pollutants. These problems can be addressed by employing an alternative solution that is effective for efficient water treatment and favors commercial aspects. Metal organic frameworks (MOFs), an emerging porous material, possess high stability, pore size tunability, greater surface area, and active sites. These MOFs can be tailored; thus, they can be customized according to the target pollutant. Hence, MOFs can be employed as adsorbents that effectively target different pollutants. Bio-MOFs are a kind of MOFs that are incorporated with biomolecules, which also possess properties of MOFs and are used as a nontoxic adsorbent. In this review, we elaborate on the interaction between MOFs and target pollutants, the role of linkers in the adsorption of contaminants, tailoring strategy that can be employed on MOFs and Bio-MOFs to target specific pollutants, and we also highlight the effect of environmental matrices on adsorption of pollutants by MOFs.

A multifunctional cobalt-organic framework for proton conduction and selective sensing of Fe3+ ions

Developing multifunctional metal-organic frameworks (MOFs) is a new research trend. MOFs have shown remarkable performances in both proton conduction and fluorescence sensing, but the MOFs integrating the two performances are scarce. Herein, a Co-MOF, [Co₆(oba)₄(Hatz)(atz)(H₂O)₂(μ₃-OH)₂(μ₂-OH)]·H₂O (1, H₂oba = 4,4-oxybis(benzoic acid), Hatz = 5-amino-1H tetrazole), has been assembled by Co²⁺ ions with H₂oba and Hatz ligands, providing a unique example of multifunctional MOFs with both proton conduction and fluorescence sensing performances. The framework of 1 displays a pillar-layer structure built by the oba ligand as a pillar and a layer composed of Co-clusters and atz linkers. Because large-scale single crystals of 1 were successfully synthesized, the proton conduction ability of 1 was investigated using single crystal samples. 1 exhibits highly anisotropic conduction with conductivity values of 1.1 × 10^-3 S cm^-1 along the [001] direction and 9.1 × 10^-6 S cm^-1 along the [010] direction at 55 °C and 95% RH, respectively. Meanwhile, the fluorescence sensing of 1 towards metal ions was studied in aqueous solutions. Attractively, 1 may sensitively and selectively detect Fe³⁺ ions in the presence of other interfering ions by fluorescence quenching.

Visible-light-assisted persulfate activation by SnS2/MIL-88B(Fe) Z-scheme heterojunction for enhanced degradation of ibuprofen

Herein, a highly efficient Z-scheme SnS₂/MIL-88B (Fe) (SnSFe) heterojunction was successfully synthesized to use both as photocatalysts and persulfate (PS) activator for ibuprofen (IBP) degradation. Flower-liked SnS₂ was uniformly loaded on MIL-88B (Fe), and SnSFe retained the original polyhedral morphology of MIL-88B (Fe). The highest removal of IBP was achieved in the presence of SnSFe with 0.5% SnS₂(SnSFe0.5). Characteristic results and density functional theory calculations demonstrated that the enhanced degradation of IBP was due to the difference in Fermi energy levels of SnS₂ and MIL-88B (Fe) leading to electrons transferred from SnS₂ to MIL-88B (Fe), and SnO bond was formed in SnSFe. , OH and O₂^- were the main active species in SnSFe0.5/PS/visible light system. Z-scheme heterojunction of SnSFe was constructed to propose the degradation mechanism. This research revealed that the synergism of photocatalysis and PS activation using SnS₂/Fe-based MOFs composites possessed great potentials in wastewater remediation.

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.

Post-synthesis functionalization of ZIF-90 with sulfonate groups for high proton conduction

Introducing sulfonic acid groups into MOF materials is one of the effective approaches to enhance proton conduction. Here, we attempted to prepare a new post-modified ZIF-90-based material by addition reaction of the aldehyde group with bisulfite to obtain partially functionalized ZIF-90-SO₃Na_(2.3). ZIF-90-SO₃Na_(2.3) exhibits a high proton conductivity of 2.26 × 10^-2 S cm^-1 at 98% RH and 100 °C.

A Porous Sulfonated 2D Zirconium Metal-Organic Framework as a Robust Platform for Proton Conduction

By using the strategy of pre-assembly chlorosulfonation applied to a linker precursor, the first sulfonated zirconium metal-organic framework (JUK-14) with two-dimensional (2D) structure, was synthesized. Single-crystal X-ray diffraction reveals that the material is built of Zr₆ O₄ (OH)₄ (COO)₈ oxoclusters, doubly 4-connected by angular dicarboxylates, and stacked in layers spaced 1.5 nm apart by the presence of sulfonic groups. JUK-14 exhibits excellent hydrothermal stability, permanent porosity confirmed by gas adsorption studies, and shows high (>10^-4  S/cm) and low (<10^-8  S/cm) proton conductivity under humidified and anhydrous conditions, respectively. Post-synthesis inclusion of imidazole improves the overall conductivity increasing it to 1.7×10^-3  S/cm at 60 °C and 90 % relative humidity, and by 3 orders of magnitude at 160 °C. The combination of 2D porous nature with robustness of zirconium MOFs offers new opportunities for exploration of the material towards energy and environmental applications.

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.

Simultaneous Control of Pore-Space Partition and Charge Distribution in Multi-Modular Metal-Organic Frameworks

We report here a strategy for making anionic pacs type porous materials by combining pore space partition with charge reallocation. The method uses the first negatively charged pore partition ligand (2,5,8-tri-(4-pyridyl)-1,3,4,6,7,9-hexaazaphenalene, H-tph) that simultaneously enables pore partition and charge reallocation. Over two dozen anionic pacs materials have been made to demonstrate their excellent chemical stability and a high degree of tunability. Notably, Ni₃ -bdt-tph (bdt=1,4-benzeneditetrazolate) exhibits month-long water stability, while CoV-bdt-tph sets a new benchmark for C₂ H₂ storage capacity under ambient conditions for ionic MOFs. In addition to tunable in-framework modules, we show feasibility to tune the type and concentration of extra-framework counter cations and their influence on both stability and capability to separate industrial C₃ H₈ /C₃ H₆ and C₆ H₆ /C₆ H₁₂ mixtures.

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.

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.

Unique protonconduction 3D ZnII metal organic framework exposure to aquaammonia vapor to enhance conductivity

The ZnII MOF with proton-conductivity obtained an optimal conductivity of 1.38 × 10−3 S cm−1 (100 °C) under 2 M aquaammonia vapor.

Preparation and characterization of a newly constructed multifunctional Co(ii)–organic framework: proton conduction and adsorption of Congo red in aqueous medium

The efficient adsorption of CR over Co-MOF 1 as well as the pH-dependent proton-conducting mechanism of the composite Co-MOF–Nafion membrane.

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.

Hybrid Nafion Membranes of Ionic Hydrogen-Bonded Organic Framework Materials for Proton Conduction and PEMFC Applications

As the high-power density and environmentally friendly energy resources, proton exchange membrane fuel cells (PEMFCs) have a promising future in portable power generation. Herein, the hybrid Nafion membranes of ionic hydrogen-bonded organic frameworks (iHOFs) for PEMFC applications are demonstrated. By adjusting the position of sulfonic groups on naphthalene disulfonic acid compounds, four iHOFs with different types of hydrogen bonds were synthesized successfully based on 1,1'-diamino-4,4'-bipyridylium and naphthalene disulfonic acid. The formation of hydrogen bond interactions between amino and sulfonate groups provides a rich hydrogen bond network, which makes such iHOFs have high conductivity, and the maximum value is 2.76 × 10^-3 S·cm^-1 at 100 °C and 98% RH. Besides, composite membrane materials were obtained by mixing Nafion and iHOFs, and the maximum proton conductivity values can achieve 1.13 × 10^-2 S·cm^-1 for 6%-iHOF-3/Nafion and 2.87 × 10^-3 S·cm^-1 for 6%-iHOF-4/Nafion membranes at 100 °C under 98% RH. Through the H₂/O₂ fuel cell performance test by using iHOF/Nafion as the solid electrolyte, the maximum power and current density values of hybrid membranes are 0.36 W·cm^-2 and 1.10 A·cm^-2 for 6%-iHOF-3/Nafion and 0.42 W·cm^-2 and 1.20 A·cm^-2 for 6%-iHOF-4/Nafion at 80 °C and 100% RH. This work provides a practicable approach for establishing high-performance proton exchange hybrid membranes by doping high proton-conducting iHOFs into the Nafion matrix.

Robust ionic liquid@MOF composite as a versatile superprotonic conductor

A highly performing proton conducting composite was prepared through the impregnation of EMIMCl ionic liquid in the mesoporous MIL-101(Cr)-SO₃H MOF. The resulting EMIMCl@MIL-101(Cr)-SO₃H composite displays high thermal and chemical stability, alongside retention of a high amount of EMIMCl even at temperatures as high as 500 K, as well as under moisture conditions. Remarkably, this composite exhibits outstanding proton conductivity not only at the anhydrous state (σ_{473 K} = 1.5 × 10^-3 S cm^-S) but also under humidity (σ_{(343 K/60%-80%RH)} ≥ 0.10 S cm^-1) conditions. This makes EMIMCl@MIL-101(Cr)-SO₃H a unique candidate to act as a solid state proton conductor for PEMFC applications under versatile conditions.

Tunable Proton Conductivity and Color in a Nonporous Coordination Polymer via Lattice Accommodation to Small Molecules

Nonporous coordination polymers (npCPs) able to accommodate molecules through internal lattice reorganization are uncommon materials with applications in sensing and selective gas adsorption. Proton conduction, extensively studied in the analogue metal-organic frameworks under high-humidity conditions, is however largely unexplored in spite of the opportunities provided by the particular sensitivity of npCPs to lattice perturbations. Here, AC admittance spectroscopy is used to unveil the mechanism behind charge transport in the nonporous 1·2CH3 CN. The conductance in the crystals is found to be of protonic origin. A vehicle mechanism is triggered by the dynamics of the weakly coupled acetonitrile molecules in the lattice that can be maintained by a combination of thermal cycles, even at low humidity levels. An analogue 1·pyrrole npCP is formed by in situ exchange of these weakly bound acetonitrile molecules by pyrrole. The color and conduction properties are determined by the molecules weakly bonded in the lattice. This is the first example of acetonitrile-mediated proton transport in an npCP showing distinct optical response to different molecules. These findings open the door to the design of switchable protonic conductors and capacitive sensors working at low humidity levels and with selectivity to different molecules.

In Situ Aliovalent Nickle Substitution and Acidic Modification of Nanowalls Promoted Proton Conductivity in InOF with 1D Helical Channel

Proton-conductive materials have attracted increasing attention because of their broad explorations in chemical sensors, water electrolysis, fuel cells, and biological systems. Especially, metal-organic frameworks (MOFs) have been demonstrated to be extremely promising candidates as proton-exchange membrane (PEM) fuel cells. Compared with other configurations, MOFs with one-dimensional (1D) channels have the characteristics of enhancing the host-guest interaction and promoting the anisotropic motion of proton carriers in restricted volume, which are beneficial for acquiring rich proton sources and forming successive hydrogen bonds to improve proton conductivity. We are endeavored to screen and find a helical three-dimensional (3D) framework InOF-1, namely, [In₂(OH)₂(BPTC)]·6H₂O (BPTC^4- = 3,3',5,5'-biphenyl tetracarboxylate), as a typical 1D-channel MOF, which is pristinely grafted with spirally distributed -OH groups on the channel surface. Accompanied by an aliovalent substitution Ni(II) for In(III), isostructural NiOF-1 ([Ni₂(BPTC)(HCOOH)₂]·3H₂O) is successfully prepared and massive formic acids are anchored at interior walls, which are interacted with adsorbed water molecules via the formation of stronger O-H···O bonds. This interaction between host-guest molecules and dynamics of lattice water has already led to a remarkable conductivity of InOF-1 (σ = 7.86 × 10^-3 S/cm at 328 K under 95% RH). The synergistic effect of the acidic-modified nanowall, contracted volume, and enhanced adsorption of water molecules in the NiOF-1 channel contributes to a high conductivity value of 3.41 × 10^-2 S/cm (at 328 K under 95% RH). Moreover, the proton conduction mechanism is further visually presented by molecular dynamic (MD) simulation. In contrast to InOF-1, aliovalent-substituted and acidic-modified NiOF-1 has a stronger host-guest interaction and more abundant hydrogen-bond networks, resulting in shorter proton migration distances and more frequent proton hopping, in agreement with the experimental results.

Single-crystal-to-single-crystal transformations among three Mn-MOFs containing different water molecules induced by reaction time: crystal structures and proton conductivities

Three Mn-MOFs {[Mn₃(μ₄-L)₂(H₂O)₇]·4H₂O}_n (1), {[Mn₃(μ₅-L)₂(H₂O)₆]·4H₂O}_n (2) and {[Mn₃(μ₇-L)₂(H₂O)₂]}_n (3) (H₃L = 5-(6-carboxypyridin-3-yl)isophthalic acid) were obtained under different reaction times and temperatures. Interestingly, induced by reaction time, compound 1 can lose one water molecule and SC-SC transform into compound 2. Similarly, compound 2 can also SC-SC transform into 3. Studies on two SC-SC transformation processes were carried out and the transformation mechanisms were deduced, which were verified by TG analyses. Different numbers of water molecules in the three compounds resulted in different coordination environments of the metal cation, coordination modes of the L^3- ligand, continuities of hydrogen bonds, dimensions of framework and porosities. The AC impendence spectra studies revealed that compounds 1-3 can enhance the proton conductivities of the Nafion composite membrane to about 47.77%, 36.88% and 21.28%, respectively. It is speculated that the highest proton conductivity of compound 1 may be due to its continuous hydrogen bond chain and highest water uptake, which were mainly decided by the number of water molecules.

High and Tunable Proton Conduction in Six 3D-Substituted Imidazole Dicarboxylate-Based Lanthanide-Organic Frameworks

Six isostructural three-dimensional (3D) Ln(III)-organic frameworks, {[Ln₂(HMIDC)₂(μ₄-C₂O₄)(H₂O)₃]·4H₂O}_n [Ln^III = Gd^III (1), Eu^III (2), Sm^III (3), Nd^III (4), Pr^III (5), and Ce^III (6)], have been fabricated by using a multifunctional ligand of 2-methyl-1H-imidazole-4,5-dicarboxylic acid (H₃MIDC). Ln-metal-organic frameworks (MOFs) 1-6 present 3D structures and possess abundant H-bonded networks between imidazole-N atoms and coordinated and free water molecules. All the six Ln-MOFs demonstrate humidity- and temperature-dependent proton conductivity (σ) having the optimal values of 2.01 × 10^-3, 1.40 × 10^-3, 0.93 × 10^-3, 2.25 × 10^-4, 1.11 × 10^-4, and 0.96 × 10^-4 S·cm^-1 for 1-6, respectively, at 100 °C/98% relative humidity, in the order of Ce^III (6) < Pr^III (5) < Nd^III (4) < Sm^III (3) < Eu^III (2) < Gd^III (1). In particular, the σ for 1 is 1 order of magnitude higher than that for 6, and it enhances systematically according to the decreasing order of the ionic radius, indicating that the lanthanide-contraction tactics can effectively regulate the proton conductivity while retaining the proton conduction routes. This will offer valuable guidance for the acquisition of new proton-conducting materials. In addition, the outstanding water stability and electrochemical stability of such Ln-MOFs will afford a solid material basis for future applications.

Humidity-sensitive irreversible phase transformation of open-framework zinc phosphate and its water-assisted high proton conduction properties

Open-framework zinc phosphate (NMe4)(ZnP2O8H3) undergoes irreversible phase transformation. Structural transformation with α (NMe4·Zn[HPO4][H2PO4] the low-temperature phase) and β (NMe4·ZnH3[PO4]2 the high-temperature phase) (Tc = 149 °C) and conduction properties were investigated by single-crystal X-ray diffraction, differential scanning calorimetry, and alternating current (ac) impedance. The open-framework material was sensitive to humidity and β proton conductivity was higher than 10-2 S cm-1 at room temperature and 98% relative humidity (RH). Given that the high proton conductivity of the open-framework material can compete with that of many advanced proton conductors based on metal-organic frameworks (MOFs), it has broad application prospects in various electrochemical devices.

Two 6/10-connected Cu12S6 cluster-based organic frameworks: crystal structure and proton conduction

Nowadays, although the exploration of proton conductive materials has ranged from traditional sulfonated polymers to novel crystalline solid materials such as MOFs, COFs, and HOFs, research on crystalline cluster-based organic framework materials is very limited. Here, a pair of homologues Cu(i)-based organic framework containing a Cu12S6 cluster, [Cu12(MES)6(H2O)3]n (1) and {[Cu12(MPS)6(H2O)4]·6H2O}n (2) (H2MES = 2-mercaptoethanesulfonate acid and H2MPS = 2-mercaptoethanesulfonate acid), were hydrothermally synthesized under the same conditions and fully investigated for their proton conduction. Their structures were characterized by means of single-crystal X-ray diffraction, elemental analysis, thermogravimetric analyses, and PXRD measurements. The two MOFs show significant structural differences in the topological fashions. MOF 1 has a three-dimensional network and can be simplified into two topology types: a 10-connected gpu structure with a Schläfli symbol (312·426·57) and a 3,12-connected new topology with a point symbol {3·42}2{310·418·519·614·74·9}. MOF 2 also has a three-dimensional framework and topology as a 6-connected pcu primitive cubic network with a Schläfli symbol {412·63}. The two MOFs show different proton conduction parameters, but both indicate temperature-dependent proton conductive features. Intriguingly, the two MOFs exhibit high water stability and their proton conductivities are 3.63 × 10-5 and 2.75 × 10-5 S cm-1 under 333 K and 98% RH, respectively. The suggested mechanism for the synthesis for 1 and 2, and their proton conductivity performance comparison has been discussed in detail. In addition, Hirshfeld surface and fingerprint analysis on the two MOFs were computed to compare contacts between the molecules, which is essential for analyzing the relationships between their hydrogen bonds and proton conductivity properties.

Multivariate Sulfonic-Based Titanium Metal-Organic Frameworks as Super-protonic Conductors

The proton-conducting performances of a microporous Ti-based metal-organic framework (MOF), MIP-207, were successfully tuned using a multicomponent ligand replacement strategy to gradually introduce a controlled amount of sulfonic acid groups as a source of Brönsted acidic sites while keeping the robustness and ecofriendly synthesis conditions of the starting material. Typically, multivariate sulfonic-based solids MIP-207-(SO₃H-IPA)_x-(BTC)_1-x were prepared by combining various ratios of trimesate 1,3,5-benzenetricarboxylate (BTC) moieties and 5-SO₃H-isophthalate (SO₃H-IPA). The best sulfonic-MOF candidate that combines structural integrity with high proton conductivity values (e.g., σ = 2.6 × 10^-2 S cm^-1 at 363 K/95% relative humidity) was further investigated using ab initio molecular dynamics simulations. These calculations supported that the -SO₃H groups act as proton donors and revealed that the proton transfer mechanism results from the solvation structure of protons through the fast Zundel/hydronium interconversion along the continuous H-bonded network connecting the adsorbed water molecules.

A Rare 2D Framework Cu(atz)2 with Ferrimagnetic and High Proton Conductivity

Materials combining proton conductivity and magnetism have attracted great attention in recent years due to their intriguing application in sensors and fuel cells. Herein a two-dimensional metal-organic framework, [Cu(atz)₂ (H₂ O)₂ ]⋅H₂ O (1) (Hatz=5-aminotetrazole), has been obtained in a green synthesis method. The single-crystal structure revealed that the atz^- ligands as linkers coordinate with copper ions to sql networks, between which water molecules are immobilized through hydrogen bonds. The resulting complex 1 exhibits a high proton conductivity of 1.11×10^-4 and 6.19×10^-4  S cm^-1 at room temperature and 333 K, respectively, under 98% RH with an activation energy of 0.56 eV. Upon dehydration, the proton conductivity of 1_dg drops by an order of magnitude. Furthermore, the magnetic behavior changes from long-range ferrimagnetic ordering of 1 to canted antiferromagnetic behaviour of 1_dg.

Defective Hierarchical Pore Engineering of a Zn-Ni MOF by Labile Coordination Bonding Modulation

The construction and modulation of hierarchical pore structure in metal-organic frameworks (MOFs) has become a hot topic owing to the advantages of hierarchical pore MOFs (HP-MOFs) in matter storage and mass transfer related applications. Herein, we report the engineering of crystalline defect in a bimetallic MOF for the construction and tuning of HP-MOF. A microporous MOF system showing metal-center-dependent water stability, namely, {[M₃F(bdc)₃ tpt] (solvents)}_n (M = Zn²⁺ and Ni²⁺, H₂bdc = 1,4-benzenedicarboxylic acid, tpt = 2,4,6-tris(4-pyridyl)triazine), was utilized as a platform for the construction of HP-MOF. By tuning the Zn²⁺/Ni²⁺ ratio in the reactant, a bimetallic MOF with a highly tunable Zn²⁺/Ni²⁺ ratio could be obtained. The relatively labile Zn²⁺-based coordination bonding in the bimetallic MOF could be readily and targeted broken through water treatment for the engineering of crystalline defects-based hierarchical pore structure. The resultant HP-MOF reveals a dramatically increased pore volume with the presence of mesopore and macropore. In addition, the anionic framework of HP-MOF could be utilized for the selective adsorption of a cationic dye methylene blue, and a relatively high capacity (250 mg·g^-1, five times compared with the pristine microporous MOF) could be achieved.

An ultra-stable hafnium phosphonate MOF platform for comparing the proton conductivity of various guest molecules/ions

A porous hafnium-phosphonate MOF was synthesized using imidazole ionic liquids (ILs), namely PHOS-100(Hf), which has exceptional chemical stability in aqueous environments, even fuming acids. Its rigid framework with permanent porosity makes PHOS-100 an ideal candidate as a platform to fill with different functional guests such as acidic HCl, H2SO4, or H3PO4. The as-synthesized ILs@PHOS-100 exhibits significant humidity-dependent proton conductivities, increasing by four orders of magnitude from 45% RH to 95% RH at 25 °C. After post-treatment with strong acids, the acids@PHOS-100 show enhanced proton conduction at low relative humidities.

Rational strategies for proton-conductive metal-organic frameworks

Since the transition of energy platforms, proton-conducting materials have played a significant role in broad applications for electrochemical devices. In particular, solid-state proton conductors (SSPCs) are emerging as the electrolyte in fuel cells (FC), a promising power generation technology, because of their high performance and safety for operating in a wide range of temperatures. In recent years, proton-conductive porous metal-organic frameworks (MOFs) exhibiting high proton-conducting properties (>10-2 S cm-1) have been extensively investigated due to their potential application in solid-state electrolytes. Their structural designability, crystallinity, and porosity are beneficial to fabricate a new type of proton conductor, providing a comprehensive conduction mechanism. For the proton-conductive MOFs, each component, such as the metal centres, organic linkers, and pore space, is manipulated by a judicious predesign strategy or post-synthetic modification to improve the mobile proton concentration with an efficient conducting pathway. In this review, we highlight rational design strategies for highly proton-conductive MOFs in terms of MOF components, with representative examples from recent years. Subsequently, we discuss the challenges and future directions for the design of proton-conductive MOFs.

Proton conductivities of four low dimensional MOFs: affected by the amount of chelated ligands

The smaller the proportion of the chelated ligand, the more water molecules in the compound, which increases the chance of forming continuous hydrogen bonds, enhances the water absorption ability, and improves the proton conductivity of the compound.

A water-stable open-framework zirconium(iv) phosphate and its water-assisted high proton conductivity

A water stable open-framework zirconium(iv) phosphate (ZrP) has a low anhydrous proton conductivity of 7.73 × 10−7 S cm−1 at 220 °C, and a water assisted high proton conductivity of 4.41 × 10−2 S cm−1 at 60 °C and 98% RH.