Modulating proton diffusion and conductivity in metal-organic frameworks by incorporation of accessible free carboxylic acid groups

Binding of carbon dioxide and acetylene to free carboxylic acid sites in a metal-organic framework

The functionalisation of organic linkers in metal-organic frameworks (MOFs) to improve gas uptake is well-documented. Although the positive role of free carboxylic acid sites in MOFs for binding gas molecules has been proposed in computational studies, relatively little experimental evidence has been reported in support of this. Primarily this is because of the inherent synthetic difficulty to prepare MOF materials bearing free, accessible -COOH moieties which would normally bind to metal ions within the framework structure. Here, we describe the direct binding of CO2 and C2H2 molecules to the free -COOH sites within the pores of MFM-303(Al). MFM-303(Al) exhibits highly selective adsorption of CO2 and C2H2 with a high selectivity for C2H2 over C2H4. In situ synchrotron X-ray diffraction and inelastic neutron scattering, coupled with modelling, highlight the cooperative interactions of adsorbed CO2 and C2H2 molecules with free -COOH and -OH sites within MFM-303(Al), thus rationalising the observed high selectivity for gas separation.

Exploring the Water Oxidation Catalytic Activity of a Mn-Based Magnetic Metal-Organic Framework: The Role of Proton Conductivity and Oxygen Evolution Reaction Overpotential

The present work evaluates the water oxidation catalytic activity of a Mn-based metal-organic framework (MOF), which we envisioned to reduce the oxygen evolution reaction (OER) overpotential because of its high electrical conductivity, facilitated by solvent-encapsulated structural features. The presence of Mn centers induces interesting magnetic features in the MOF, which exhibits impressive cryogenic magnetic refrigeration with a ΔSM value of 29.94 J kg-1 K-1 for a field change of ΔH = 5T at 2.3 K. To the best of our knowledge, the ΔSM value of the current system ranked the highest position among the published examples. The crystal structure aligns perfectly with the thematic expectations and features as many as ten metal-coordinated water molecules, forming an extensive web of a hydrogen-bonded network while facing toward the porous channel filled with another set of much-anticipated entrapped lattice water molecules. Such structural features are expected to manifest high proton conductivity, and detailed investigation indeed yields the best value for the system at 1.57 × 10-4 S/cm at 95% humidity and 85 °C. In order to evaluate the thematic notion of a one-to-one relationship between OER and improved electrical conductivity, extensive electrocatalytic water splitting (WS) investigations were carried out. The final results show highly encouraging WS ability of the Mn-MOF, showing the electrocatalytic surface area value of the active species as 0.0686 F/g with a turnover frequency value of 0.043 [(mol. O2) (mol. Mn-MOF)-1 s-1]. Another fascinating aspect of the current communication is the excellent synergy observed between the experimental WS outcomes and the corresponding theoretical data calculated using density functional theory (DFT). Consequently, a plausible mechanism of the overall OER and the role of the Mn-MOF as a water oxidation catalyst, along with the importance of water molecules, have also been derived from the theoretical calculations using DFT.

Stable Europium(III) Metal-Organic Framework Demonstrating High Proton Conductivity and Fluorescence Detection of Tetracyclines

A europium(III) metal-organic framework (MOF), namely, {[[(CH₃)₂NH₂]₃Eu₂(DTTP-2OH)₂(HCOO)(H₂O)]·4H₂O}_n (Eu-MOF, H₄DTTP-2OH = 2',5'-dihydroxy-[1,1':4',1″-terphenyl]-3,3″,5,5″-tetracarboxylic acid) has been assembled through solvothermal method. The Eu-MOF is a three-dimensional (3D) (4,4,8)-connected topological framework with binuclear Eu(III) clusters as secondary building units, in which a richly ordered hydrogen bonding network formed among the free H₂O molecules, dimethylamine cations, and phenolic hydroxyl groups provides a potential pathway for proton conduction. The proton conductivity reaches the category of superionic conductors (σ > 10^-4 S cm^-1) at room temperature with a maximum conductivity of 1.91 × 10^-3 S cm^-1 at 60 °C and 98% RH. Moreover, it also can be used as a fluorescence sensor in aqueous solution with detection limits of 0.14 μM for tetracycline, 0.13 μM for oxytetracycline and 0.11 μM for doxycycline. These results pave new methods for constructing MOFs with high proton conductivity and responsive fluorescence.

Synthesis and In Situ Monitoring of Mechanochemical Preparation of Highly Proton Conductive Hydrogen-Bonded Metal Phosphonates

Crystalline porous materials are recognized as promising proton conductors for the proton exchange membrane (PEM) in fuel cell technology owing to their tunable framework structure. However, it is still a challenging bulk synthesis for real-world applications of these materials. Herein, we report the mechanochemical gram-scale synthesis of two isostructural metal hydrogen-bonded organic frameworks (MHOFs) of Co(II) and Ni(II) based on 1-hydroxyethylidenediphosphonic acid (HEDPH₄) with 2,2'-bipyridine (2,2'-bipy): Co(HEDPH₃)₂(2,2'-bipy)·H₂O (1) and Ni(HEDPH₃)₂(2,2'-bipy)·H₂O (2). In situ monitoring of the mechanochemical synthesis using different synchrotron-based techniques revealed a one-step mechanism - the starting materials are directly converted to the product. With the existence of extensive hydrogen bonds with amphiprotic uncoordinated phosphonate hydroxyl and oxygen atoms, both frameworks exhibited proton conduction in the range of 10^-4 S cm^-1 at room temperature under humid conditions. This study demonstrates the potential of green mechanosynthesis for bulk material preparation of framework-based solid-state proton conductors.

A Manganese(II) 3D Metal–Organic Framework with Siloxane-Spaced Dicarboxylic Ligand: Synthesis, Structure, and Properties

A new metal–organic framework {[Mn4(Cx)3(etdipy)5]·2ClO4}n (1) was prepared via the complexation of manganese ion from a Mn(ClO4)2 source with 1,3-bis(carboxypropyl)tetramethyldisiloxane (Cx) and 1,2-di(4-pyridyl)ethylene (etdipy) in the presence of 2,4-lutidine as a deprotonating agent. The single-crystal X-ray diffraction analysis revealed a dense 3D framework structure. The presence in the structure of flexible tetramethyldisiloxane moieties, which tend to orient themselves at the interface with the air, gives the compound a highly hydrophobic character, as indicated by the result of the water vapor sorption analysis in the dynamic regime, as well as the shape and stability of the water droplet on the crystalline mass of the compound. The compound is an electrical insulator, and due to its hydrophobicity, this characteristic is unaffected by environmental dampness. The thermal analysis indicated thermal stability up to about 300 °C and an unusual thermal transition for an MOF structure, more precisely a glass transition at 24 °C, the latter also being attributed to the flexible segments in the structure. The magnetic studies showed dominant antiferromagnetic interactions along the metal ion chain in compound 1.

Easy Handling and Cost-Efficient Processing of a Tb3+-MOF: The Emissive Capacity of the Membrane-Immobilized Material, Water Vapour Adsorption and Proton Conductivity

The development of convenient, non-complicated, and cost-efficient processing techniques for packing low-density MOF powders for industry implementation is essential nowadays. To increase MOFs' availability in industrial settings, we propose the synthesis of a novel 3D Tb-MOF (1) and a simple and non-expensive method for its immobilization in the form of pellets and membranes in polymethacrylate (PMMA) and polysulphone (PSF). The photoluminescent properties of the processed materials were investigated. To simulate industrial conditions, stability towards temperature and humidity have been explored in the pelletized material. Water-adsorption studies have been carried out in bulk and processed materials, and because of the considerable capacity to adsorb water, proton-conduction studies have been investigated for 1.

Mechanochemical Synthesis of Phosphonate-Based Proton Conducting Metal-Organic Frameworks

Water-stable metal-organic frameworks (MOFs) with proton-conducting behavior have attracted great attention as promising materials for proton-exchange membrane fuel cells. Herein, we report the mechanochemical gram-scale synthesis of three new mixed-ligand phosphonate-based MOFs, {Co(H₂PhDPA)(4,4'-bipy)(H₂O)·2H₂O}_n (BAM-1), {Fe(H₂PhDPA)(4,4'-bipy) (H₂O)·2H₂O}_n (BAM-2), and {Cu(H₂PhDPA)(dpe)₂(H₂O)₂·2H₂O}_n (BAM-3) [where H₂PhDPA = phenylene diphosphonate, 4,4'-bipy = 4,4'-bipyridine, and dpe = 1,2-di(4-pyridyl)ethylene]. Single-crystal X-ray diffraction measurements revealed that BAM-1 and BAM-2 are isostructural and possess a three-dimensional (3D) network structure comprising one-dimensional (1D) channels filled with guest water molecules. Instead, BAM-3 displays a 1D network structure extended into a 3D supramolecular structure through hydrogen-bonding and π-π interactions. In all three structures, guest water molecules are interconnected with the uncoordinated acidic hydroxyl groups of the phosphonate moieties and coordinated water molecules by means of extended hydrogen-bonding interactions. BAM-1 and BAM-2 showed a gradual increase in proton conductivity with increasing temperature and reached 4.9 × 10^-5 and 4.4 × 10^-5 S cm^-1 at 90 °C and 98% relative humidity (RH). The highest proton conductivity recorded for BAM-3 was 1.4 × 10^-5 S cm^-1 at 50 °C and 98% RH. Upon further heating, BAM-3 undergoes dehydration followed by a phase transition to another crystalline form which largely affects its performance. All compounds exhibited a proton hopping (Grotthuss model) mechanism, as suggested by their low activation energy.

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.

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.

Dysprosium(III) Metal-Organic Framework Demonstrating Ratiometric Luminescent Detection of pH, Magnetism, and Proton Conduction

A multifunctional metal-organic framework, (Hdmbpy)[Dy(H₂dobdc)₂(H₂O)]·3H₂O (Dy-MOF, H₄dobdc = 2,5-dihydroxyterephthalic acid, dmbpy = 4,4'-dimethyl-2,2'-bipyridine), was synthesized and structurally characterized. The metal center Dy^III is connected by four carboxyl groups to form the [Dy₂(CO₂)₄] binuclear nodes, which are further interconnected by eight separate H₂dobdc^2- ligands to form a three-dimensional (3D) framework including hydrophilic triangular channels and abundant hydrogen-bonding networks. Dy-MOF has good stability in aqueous solution as well as in harsh acidic or alkaline solutions (pH range: 2.0-12.0). Furthermore, the luminescence signal of Dy-MOF undergoes a visualized color change as the acidity of the solution alters, which is the typical behavior of pH ratiometric probe. At a 100% relative humidity, Dy-MOF exhibits a high proton conductivity σ (1.70 × 10^-4 S cm^-1 at 303 K; 1.20 × 10^-3 S cm^-1 at 343 K) based on the proton hopping mechanism, which can be classified as a superionic conductor with σ exceeding 10^-4 S cm^-1. Additionally, the ferromagnetic interaction and magnetic relaxation behavior are simultaneously achieved in Dy-MOF. Herein, the combination of luminescence sensing, magnetism, and proton conduction in a single-phase 3D MOF may offer great potential applications in smart multitasking devices.

Angstrom-scale ion channels towards single-ion selectivity

Artificial ion channels with ion permeability and selectivity comparable to their biological counterparts are highly desired for efficient separation, biosensing, and energy conversion technologies. In the past two decades, both nanoscale and sub-nanoscale ion channels have been successfully fabricated to mimic biological ion channels. Although nanoscale ion channels have achieved intelligent gating and rectification properties, they cannot realize high ion selectivity, especially single-ion selectivity. Artificial angstrom-sized ion channels with narrow pore sizes <1 nm and well-defined pore structures mimicking biological channels have accomplished high ion conductivity and single-ion selectivity. This review comprehensively summarizes the research progress in the rational design and synthesis of artificial subnanometer-sized ion channels with zero-dimensional to three-dimensional pore structures. Then we discuss cation/anion, mono-/di-valent cation, mono-/di-valent anion, and single-ion selectivities of the synthetic ion channels and highlight their potential applications in high-efficiency ion separation, energy conversion, and biological therapeutics. The gaps of single-ion selectivity between artificial and natural channels and the connections between ion selectivity and permeability of synthetic ion channels are covered. Finally, the challenges that need to be addressed in this research field and the perspective of angstrom-scale ion channels are discussed.

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.

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.

MOFs in the time domain

Many of the proposed applications of metal-organic framework (MOF) materials may fail to materialize if the community does not fully address the difficult fundamental work needed to map out the 'time gap' in the literature - that is, the lack of investigation into the time-dependent behaviours of MOFs as opposed to equilibrium or steady-state properties. Although there are a range of excellent investigations into MOF dynamics and time-dependent phenomena, these works represent only a tiny fraction of the vast number of MOF studies. This Review provides an overview of current research into the temporal evolution of MOF structures and properties by analysing the time-resolved experimental techniques that can be used to monitor such behaviours. We focus on innovative techniques, while also discussing older methods often used in other chemical systems. Four areas are examined: MOF formation, guest motion, electron motion and framework motion. In each area, we highlight the disparity between the relatively small amount of (published) research on key time-dependent phenomena and the enormous scope for acquiring the wider and deeper understanding that is essential for the future of the field.

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.

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

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

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.

Construction of Highly Proton-Conductive Zr(IV)-Based Metal-Organic Frameworks From Pyrrolo-pyrrole-Based Linkers with a Rhombic Shape

To date, numerous zirconium cluster-based metal-organic frameworks (Zr-MOFs) with attractive physical properties have been achieved thanks to tailorable organic linkers and versatile Zr clusters. Nevertheless, in comparison with the most-used high-symmetry organic linkers, low-symmetry linkers have rarely been exploited in the construction of Zr-MOFs. Despite challenges in predicting the structure and topology of the MOF, linker desymmetrization presents opportunities for the design of Zr-MOFs with unusual topologies and unexpected functionalities. Herein, we report for the first time the construction of two robust Zr-MOFs (IAM-7 and IAM-8) from two pyrrolo-pyrrole-based low-symmetry tetracarboxylate linkers with a rare rhombic shape. The low symmetry of the linkers arises from the asymmetric pyrrolo-pyrrole core and the varying branch lengths, which play a critical role in the structural diversity between IAM-7 and IAM-8 seen from the structural analysis and lead to hydrophilic channels that contain uncoordinated carboxylate groups in the structure of IAM-7. Furthermore, the proton conductivity of IAM-7 displays a high temperature and humidity dependence where the proton conductivity increases from 2.84 × 10^-8 S cm^-1 at 30 °C and 40% relative humidity (RH) to 1.42 × 10^-2 S cm^-1 at 90 °C and 95% RH, making it among one of the most conductive Zr-MOFs. This work not only enriches the library of Zr-MOFs but also offers a platform for the design of low-symmetry linkers toward the structural diversity or irregularity of MOFs as well as their structure-related properties.

High Proton Conductivity in Nafion/Ni-MOF Composite Membranes Promoted by Ligand Exchange under Ambient Conditions

Metal-organic frameworks (MOFs) have appeared to be promising competitive candidates as crystalline porous materials for proton conduction. Explorations of the method of preparation of proton conductive MOFs and the proton transfer mechanism have enabled them to attract widespread attention, and tremendous efforts have been made to improve the proton conductivity of MOFs. On the basis of our previous work, we explicitly propose that ligand exchange can upgrade the proton conduction performance of MOFs. Using MOF-azo as the precursor, the proton conductivities of exchange products MOF-bpy and MOF-bpe increase by 3.5- and 2.8-fold, respectively. After the MOFs had been doped into the Nafion matrix to prepare composite membranes, the proton conduction performance of composite membranes filled with subproducts (2.6 × 10^-2 and 1.95 × 10^-2 S cm^-1) is significantly better than that of a composite membrane filled with a parent product (1.12 × 10^-2 S cm^-1) under ambient conditions (without heating or humidifying). The ligand exchange strategy presented herein demonstrates great promise for the development of high-proton conductivity MOFs and MOF composites with expanded future applications.

Positive Cooperative Protonation of a Metal-Organic Framework: pH-Responsive Fluorescence and Proton Conduction

Positive cooperative binding, a phenomenon prevalent in biological processes, holds great appeal for the design of highly sensitive responsive molecules and materials. It has been demonstrated that metal-organic frameworks (MOFs) can show positive cooperative adsorption to the benefit of gas separation, but potential binding cooperativity is largely ignored in the study of sensory MOFs. Here, we report the first demonstration of positive cooperative protonation of a MOF and the relevant pH response in fluorescence and proton conduction. The MOF is built of Zr-O clusters and bipyridyl-based tetracarboxylate linkers and has excellent hydrolytic stability. It shows a unique pH response that features two synchronous abrupt turn-off and turn-on fluorescent transitions. The abrupt transitions, which afford high sensitivity to small pH fluctuations, are due to cooperative protonation of the pyridyl sites with a Hill coefficient of 1.6. The synchronous dual-emission response, which leads to visual color change, is ascribable to proton-triggered switching between (n, π*) and (π, π*) emissions. The latter emission can be quenched by electron donating anion-dependent through photoinduced electron transfer and ground-state charge transfer. Associated with cooperative protonation, the proton conductivity of the MOF is abruptly enhanced at low pH by two orders, but overhigh acid concentration is adverse because excessive anions can interrupt the conducting networks. Our work shows new perspectives of binding cooperativity in MOFs and should shed new light on the development of responsive fluorescent MOFs and proton conductive materials.

Enhanced proton conductivity in a flexible metal–organic framework promoted by single-crystal-to-single-crystal transformation

Transformation of MFM-722(Pb)-DMA to MFM-722(Pb)-H₂O leads to an increase in proton conductivity linked to a structural transition.

Structural, optical, dielectric and electrical transport properties of a [Mg(H2O)6]2+-templated proton conducting, semiconducting and photoresponsive 3D hydrogen bonded supramolecular framework

{[Co(2,5-Pdc)2,(H2O)2]2−·[Mg(H2O)6]2+·4(H2O)} (where 2,5-pdc = 2,5-pyridinedicarboxylate): a proton conducting semiconducting photoresponsive [Mg(H2O)6]2+ templated 3D hydrogen bonded supramolecular framework (HSF).

Metal-Organic Frameworks as a Versatile Platform for Proton Conductors

Metal-organic frameworks (MOFs) are an intriguing type of crystalline porous materials that can be readily built from metal ions or clusters and organic linkers. Recently, MOF materials, featuring high surface areas, rich structural tunability, and functional pore surfaces, which can accommodate a variety of guest molecules as proton carriers and to systemically regulate the proton concentration and mobility within the available space, have attracted tremendous attention for their roles as solid electrolytes in fuel cells. Recent advances in MOFs as a versatile platform for proton conduction in the field of humidity condition proton-conduction, anhydrous atmosphere proton-conduction, single-crystal proton-conduction, and including MOF-based membranes for fuel cells, are summarized and highlighted. Furthermore, the challenges, future trends, and prospects of MOF materials for solid electrolytes are also discussed.

Proton-conducting layered structures based on transition metal oxo-clusters supported by Sb(iii) tartrate scaffolds

Two transition metal-antimony oxo-cluster based compounds, H₅{MCd(H₂O)₆[M(H₂O)₃Co₃Sb^VSb(μ₃-O)₈(l-tta)₆]}·7H₂O (1) (M = Cd_0.5 + Co_0.5) and H₃K₅(H₂O)₁₁{Cd(H₂O)₄[Cd(H₂O)Fe₄Cd₂Sb₆(μ₄-O)₅(μ₃-O)₃(l-tta)₆][Cd(H₂O)₂Fe₄Cd₂Sb₆(μ₄-O)₄(μ₃-O)₄(l-tta)₆][Cd(H₂O)₂Fe₄Cd₂Sb₆(μ₄-O)₄(μ₃-O)₄(l-tta)₆Cd(H₂O)₅]}·17H₂O (2) (L-H₄tta = l-tartaric acid) were hydrothermally synthesized and characterized. Compound 1 features a [MCo₃Sb^VSb(μ₃-O)₈(l-tta)₆(H₂O)₃]^9- cluster, while compound 2 contains three types of clusters, namely, [Cd(H₂O)Fe₄Cd₂Sb₆(μ₄-O)₅(μ₃-O)₃(l-tta)₆]^4-, [Cd(H₂O)₂Fe₄Cd₂Sb₆(μ₄-O)₄(μ₃-O)₄(l-tta)₆]^4- and [Cd(H₂O)₂Fe₄Cd₂Sb₆(μ₄-O)₄(μ₃-O)₄(l-tta)₆Cd(H₂O)₅]^2-. All the clusters are of sandwich-type with {Sb₃(μ₃-O)(l-tta)} scaffolds on the top and bottom. The Cd (and M in 1) ions interconnect the clusters into layered structures in both compounds. To the best of our knowledge, this is the first report of transition metal-antimony oxo-clusters that simultaneously contain the first-row and second-raw transition metal ions, and compound 1 represents the first example of such type of clusters that contain Sb(v). The two compounds exhibit proton conductivity with the values of 2.43 × 10^-3 and 2.95 × 10^-3 S cm^-1 at 85 °C under 98% relative humidity, respectively.

PbX2(OOCMMIm) (X = Cl, Br): photoluminescent organic-inorganic hybrid lead halide compounds with high proton conductivity

Differences in the electronegativity and hydrophilicity of halogens lead to differences in proton-conducting and photoluminescence properties in hybrid organic-inorganic lead halide compounds of [PbX₂(OOCMMIm)]_n (X = Cl (1), Br (2), HOOCMMIm = 1-carboxymethyl-3-methylimidazolium).