Proton-conductive Metal-organic Frameworks Research Articles

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.

Proton Conducting Metal-Organic Frameworks (MOFs) via Post Synthetic Transmetallation and Water Induced Structural Transformations.

Post Synthetic Modification (PSM) of Metal-Organic Frameworks (MOFs) is a crucial strategy for developing new MOFs with enhanced functional properties compared to their parent one. PSM can be accomplished through various methods:1) modification of organic linkers; 2) exchange of metal ions or nodes; and 3) inclusion or exchange of solvent/guest molecules. Herein, PSM of bimetallic and monometallic MOFs containing biphenyl dinitro-tetra-carboxylates (NCA) are demonstrated. The tetra carboxylate NCA, produces monometallic Cd-MOF-1 and Cu-MOF-1 and bimetallic CoZn-MOF in solvothermal reactions with the corresponding metal salts. The CoZn-MOF undergoes post-synthetic transmetallation with Cd(NO3)2 and Cu(NO3)2 in aqueous solution to yield Cd-MOF-2 and Cu-MOF-2, respectively. Additionally, green crystals of Cu-MOF-1 found to undergo a single-crystal-to-single-crystal (SCSC) transformation to blue crystals of Cu-MOF-3 upon dipped into water at room temperature. These MOFs demonstrate notable proton conductivities ranging from 10-3 to 10-4 S cm-1 under variable temperatures and humidity levels. Among them, Cu-MOF-3 achieves the highest proton conductivity of 1.36×10-3 S cm-1 at 90 °C and 98 % relative humidity, attributed to its continuous and extensive hydrogen bonding network, which provides effective proton conduction pathways within the MOF. This work highlights a convenient strategy for designing proton-conducting MOFs via post-synthetic modification.

Prominent Intrinsic Proton Conduction in Two Robust Zr/Hf Metal-Organic Frameworks Assembled by Bithiophene Dicarboxylate.

To date, developing crystalline proton-conductive metal-organic frameworks (MOFs) with an inherent excellent proton-conducting ability and structural stability has been a critical priority in addressing the technologies required for sustainable development and energy storage. Bearing this in mind, a multifunctional organic ligand, 3,4-dimethylthiophene[2,3-b]thiophene-2,5-dicarboxylic acid (H2DTD), was employed to generate two exceptionally stable three-dimensional porous Zr/Hf MOFs, [Zr6O4(OH)4(DTD)6]·5DMF·H2O (Zr-DTD) and [Hf6O4(OH)4(DTD)6]·4DMF·H2O (Hf-DTD), using solvothermal means. The presence of Zr6 or Hf6 nodes, strong Zr/Hf-O bonds, the electrical influence of the methyl group, and the steric effect of the thiophene unit all contribute to their structural stability throughout a wide pH range as well as in water. Their proton conductivity was fully examined at various relative humidities (RHs) and temperatures. Creating intricate and rich H-bonded networks between the guest water molecules, coordination solvent molecules, thiophene-S, -COOH, and -OH units within the framework assisted proton transfer. As a result, both MOFs manifest the maximum proton conductivity of 0.67 × 10-2 and 4.85 × 10-3 S·cm-1 under 98% RH/100 °C, making them the top-performing proton-conductive Zr/Hf-MOFs. Finally, by combining structural characteristics and activation energies, potential proton conduction pathways for the two MOFs were identified.

Hydrogen-Bonded Metal-Organic Framework Nanosheet as a Proton Conducting Membrane for an H2/O2 Fuel Cell.

Proton-conducting metal-organic frameworks (MOFs) have attracted attention as potential electrolytes for fuel cells. However, research progress in utilizing MOFs as electrolytes for fuel cells has been limited, mainly due to challenges associated with issues such as the fabrication of MOF membranes, and hydrogen crossover through the MOF's pores. Here, proton conductivity and fuel cell performance of a self-standing membrane prepared from of a bismuth subgallateMOF nanosheets with non-porous structure are reported. The fabricated MOF nanosheet membrane with no binding agent exhibits structural anisotropy. The proton conductivity in the membrane thickness direction (4.4×10-3Scm-1) at 90°C and RH 100% is observed to be higher than that in the in-plane direction of the membrane (3.3×10-5Scm-1). The open circuit voltage (OCV) of a fuel cell with ≈120µm proton conducting membrane is 1.0V. The non-porous nature of the MOF nanosheets contributes to the relatively high OCV. A fuel cell using ≈40µm membrane as proton conducting electrolyte records a maximum of 25mWcm-2 power density and a maximum of 109mAcm-2 current density with 0.91V OCV at 80°C in humid conditions.

Improving the proton conductivity of MOF materials by regulating the pore space

The construction of metal-organic frameworks (MOFs) with structural tunability, designability, and high stability is crucial in proton conduction studies. In this paper, MOF-801 and UiO-66 were used as precursors. A solvothermal synthesis method was employed to modulate the pore space of composite MOFs. The hydrogen-bonding network and structure within the pore cavities are adjusted by varying the molar ratios of fumaric acid and terephthalic acid to improve the proton transfer rate and proton conductivity of the materials. A total of 14 materials, MOF/MOG-801&UiO-66-X (X is the molar ratio of fumaric acid and terephthalic acid) were synthesised, the variation of proton conductivity with temperature of which was investigated at different tempertures and humidities. The results shows that MOG-801&UiO-66–1:1 has the best proton conductivity with 0.12 S cm−1 at 353 K and 100 % RH, which is 5 times higher than that of MOF-801&UiO-66–1:3 (2.54 × 10−2 S cm−1), 11 times higher than that of MOF-801 and 22 times higher than that of UiO-66. Based on the calculated activation energy and structural characterisation, the proton conduction mechanism of the composite was investigated. This work opens up novel research fields for the design of new proton-conducting MOF materials.

Incorporating flexible sulfonate-imidazolium ion pair-functionalized MOF in cross-linked polyvinyl alcohol membrane for achieving superprotonic conduction

Designing high-performance metal–organic frameworks (MOFs) based proton exchange membranes for fuel cells simultaneously with a high-density proton carrier and continuous hydrogen-bonded networks remains a challenge. The present study has endeavored to construct a crystalline Cu-MOF with chemically stable based on flexible sulfonate-imidazolium ion pair ligand in which flexible sulfonic acid chains orderly anchoring on the channel wall. Moreover, the cross-linked polyvinyl alcohol (CL-PVA) was choosing as matrix to enhance the practicality of Cu-MOF synergistically in fuel cells due to its hydrophilic and hydrophobic structure distribution. The CL-PVA/Cu-MOF-X (X = 5, 10, 15, 20 wt%) composite membranes exhibit strong water affinity and thus significantly enhanced water uptake which also along with good mechanical properties. The highly hydrophilic hydroxyl groups in CL-PVA and the sulfonic acid groups in Cu-MOF form a continuous hydrogen-bonded pathway, which result to the superprotonic conduction. Among them, CL-PVA/Cu-MOF-15 shows the highest proton conductivity of 2.4 × 10-2 S cm−1 but the smallest activation energy Ea (41 kJ/mol) at 80 °C and 90 % RH, indicating more easily transportation of H+. The single-cell power density of CL-PVA/Cu-MOF-15 is up to 49.3 mW cm−2, which will open a new avenue for the practical application of proton-conducting MOFs in proton exchange membrane fuel cells.

Two proton-conductive metal-organic frameworks constructed by pyridyl imidazole dicarboxylate ligands

To explore the proton conduction in more MOFs bearing N-heterocyclic carboxylate, herein, a 1-D metal-organic framework (MOF), [Mn(3-HPyIDC)(H2O)2]n (1) (3-H3PyIDC = 2-(3-pyridyl)-4,5-imidazole dicarboxylic acid) and a 3-D MOF, [Cd(4-HPyIDC)(H2O)]n (2) (4-H3PyIDC = 2-(4-pyridyl)-4,5-imidazole dicarboxylic acid) were successfully prepared. Their excellent thermal and water stability was verified across thermogravimetric and powder X-ray diffraction determinations. The AC impedance test shows that their proton conductivity boosts with increasing temperatures and humidities, and the optimal value can be above 10−4 S/cm. The frameworks containing a large number of free carboxyl groups or coordination H2O units together with the H-bonded networks are very conducive to proton transfer. Their proton-conducting mechanisms were further investigated in light of the calculated values of activation energies.

Proton conductive thin films of metal-organic framework for impedance detection of formic acid

The development of a highly efficient sensor capable of detecting formic acid is crucial for monitoring air quality and determining the leakage because of the strong corrosivity and pungency. In this work, we present the superiority of a proton conductive metal-organic framework (MOF) thin film as a sensing material for highly efficient detection of formic acid at room temperature. Specifically, MOF-801, a zirconium-based MOF, exhibits exceptional chemical stability and inherent proton conduction, along with a significant surface area and high porosity. Through the solvothermal synthetic method, highly uniform and densely packed thin films of MOF-801 are successfully fabricated on glass substrates. The resulting sensor, based on MOF-801 thin films, displays exceptional sensitivity, selectivity, and durability for detecting formic acid at room temperature, outperforming both other MOFs based sensors and the traditional metal-oxide-based sensors that operate at high temperatures.

Effect of Missing-Linker Defects and Ion Exchange on Stability and Proton Conduction of a Sulfonated Layered Zr-MOF.

Intentionally introduced defects into solid materials create opportunities to control and tune their diverse physicochemical properties. Despite the growing interest in defect-engineered metal-organic frameworks (MOFs), there are still only a handful of studies on defective proton-conducting MOFs, including no reports on two-dimensional ones. Ion-conducting materials are fundamentally of great importance to the development of energy storage and conversion devices, including fuel cells and batteries. In this work, we demonstrate the introduction of missing-linker defects into a sulfonated proton conductive 2D zirconium-based MOF (JUK-14), using a facile post-synthetic approach and compare the stability and performance of the resulting materials, including proton conductivity, as well as adsorption of N2, CO2, and H2O molecules. We also discuss the associated presence of interlayer counterions and their effect on the properties and stability. Our approach to defect engineering can be extended to other layered MOFs and used for tuning their activity.

A superprotonic membrane derived from proton-conducting metal-organic frameworks and cellulose nanocrystals

•A PCMOF10 and CNC composite membrane exhibits unprecedented proton conductivity •The composite membrane shows excellent thermal, water, and mechanical stability •Abundant hydrogen bonds between MOFs and CNCs contribute to the membrane’s performance •High MOF loading in the composite enables retention of the MOF’s superprotonic conduction Metal-organic frameworks (MOFs) have emerged as proton conductors. However, their applications have been limited by their inability to form a self-standing membrane and their significant performance loss when incorporated into composite membranes. Herein, we report a simple strategy for fabricating a structurally and chemically stable proton-conducting MOF/cellulose nanocrystal (PCMOF/CNC) membrane. This PCMOF/CNC composite membrane has an extremely high MOF loading (89%) and low CNC content. With water-stable PCMOF10, this loading enables the retention of MOF properties, and the composite exhibits a proton conductivity of 1.44 × 10−2 S cm−1 at 85°C and 95% relative humidity. The derived composite maintains its free-standing form, superprotonic conductivity during month-long heating/cooling cycles (indicating high mechanical strength), and excellent thermal and water resistance. We attribute the unprecedented proton conductivity and mechanical integrity of the membrane to the abundant hydrogen bonds between PCMOF10 and CNCs, confirmed through Fourier transform infrared spectroscopy and theoretical computations. Metal-organic frameworks (MOFs) have emerged as proton conductors. However, their applications have been limited by their inability to form a self-standing membrane and their significant performance loss when incorporated into composite membranes. Herein, we report a simple strategy for fabricating a structurally and chemically stable proton-conducting MOF/cellulose nanocrystal (PCMOF/CNC) membrane. This PCMOF/CNC composite membrane has an extremely high MOF loading (89%) and low CNC content. With water-stable PCMOF10, this loading enables the retention of MOF properties, and the composite exhibits a proton conductivity of 1.44 × 10−2 S cm−1 at 85°C and 95% relative humidity. The derived composite maintains its free-standing form, superprotonic conductivity during month-long heating/cooling cycles (indicating high mechanical strength), and excellent thermal and water resistance. We attribute the unprecedented proton conductivity and mechanical integrity of the membrane to the abundant hydrogen bonds between PCMOF10 and CNCs, confirmed through Fourier transform infrared spectroscopy and theoretical computations.

Thermal-Response Proton Conduction in Schiff Base-Incorporated Metal-Organic Framework Hybrid Membranes under Low Humidity Based on the Excited-State Intramolecular Proton Transfer Mechanism.

Stimulus-responsive proton conduction materials have attracted enormous interest as a new kind of "smart material". It is desirable to develop the appropriate stimulus signal and high proton-conducting materials with an excellent proton-conducting switch ratio (γ), but it remains a great challenge. Here, it can be found for the first time that 4-((2-hydroxybenzylidene)amino)benzenesulfonic acid (HBABSA) has obvious thermal isomerization when porous solids act as matrixes at the ambient temperatures, which is different from that in the crystalline state at 77 K. Therefore, we proposed a host-guest metal-organic framework (MOF) composite, namely, MOF-808 incorporated with HBABSA (HBABSA@MOF-808), which has a proton-conducting switch ratio (γ) of 16 between 338 and 343 K due to the thermally induced isomerization of HBABSA molecules in the MOF pores. The strong binding between the keto-type HBABSA and MOF at the relatively low temperatures can efficiently suppress the proton conduction, while the enol-type one provides more mobile protons for conduction at the high temperatures due to the excited-state intramolecular proton transfer mechanism. Further, the HBABSA@MOF-808 as a filler is blended into polyvinyl alcohol and poly(2-acrylamide-2-methyl-1-propane sulfonic acid) to form hybrid membranes. The hybrid membrane with the highest content of the MOF composite displays a high proton conductivity of 5.57 × 10-3 S·cm-1 under 353 K and 57% RH along with a good switch ratio of 5.4. The development of thermal-response proton-conducting MOF materials is opening up a unique pathway for remote control, thermal sensing, intelligent batteries, and other fields.

Conductive Metal–Organic Frameworks for Rechargeable Lithium Batteries

Currently, rechargeable lithium batteries are representative of high-energy-density battery systems. Nevertheless, the development of rechargeable lithium batteries is confined by numerous problems, such as anode volume expansion, dendrite growth of lithium metal, separator interface compatibility, and instability of cathode interface, leading to capacity fade and performance degradation of batteries. Since the 21st century, metal–organic frameworks (MOFs) have attracted much attention in energy-related applications owing to their ideal specific surface areas, adjustable pore structures, and targeted design functions. The insulating characteristics of traditional MOFs restrict their application in the field of electrochemistry energy storage. Recently, some teams have broken this bottleneck through the design and synthesis of electron- and proton-conductive MOFs (c-MOFs), indicating excellent charge transport properties, while the chemical and structural advantages of MOFs are still maintained. In this review, we profile the utilization of c-MOFs in several rechargeable lithium batteries such as lithium-ion batteries, Li–S batteries, and Li–air batteries. The preparation methods, conductive mechanisms, experimental and theoretical research of c-MOFs are systematically elucidated and summarized. Finally, in the field of electrochemical energy storage and conversion, challenges and opportunities can coexist.

Proton conducting metal-organic frameworks with light response for multistate logic gates.

The simulation of neurons receiving stimulation and transmitting signals by proton conduction has great potential applications in electrochemistry and biology. In this work, copper tetrakis(4-carboxyphenyl)porphyrin (Cu-TCPP), which is a proton conductive metal organic framework (MOF) with photothermal response, is adopted as the structural framework, with the in situ co-incorporation of polystyrene sulfonate (PSS) and sulfonated spiropyran (SSP) to prepare the composite membranes. The resultant PSS-SSP@Cu-TCPP thin-film membranes were used as the logic gates i.e., NO gate, NOR gate and NAND gate because of the photothermal effect of Cu-TCPP MOFs and the photoinduced conformational changes of SSP. This membrane exhibits the high proton conductivity of 1.37 × 10-4 S cm-1. Under the conditions of 55 °C and 95% relative humidity (RH), using 405 nm laser irradiation with 400 mW cm-2 and 520 nm laser irradiation with 200 mW cm-2 as inputs, the device can be adjusted between various steady states, and the value of the conductivity is regarded as the output with different thresholds in different logic gates. Before and after laser irradiation, the electrical conductivity changes dramatically, and the ON/OFF switching ratio reached 1068. The application of three logic gates is realized by constructing circuits with LED lights. Depending on the convenience of light and the easy measurement of conductivity, this kind of device with light source as input and electrical signal as output provides the possibility to realize the remote control of chemical sensors and complex logic gates devices.

Abundant Hydrophilic Atoms Enhanced the Proton Conductivity of a Stable Metal–Organic Framework in a Wide Relative Humidity Range

The low proton conductivity of proton-conducting solids at reduced relative humidity (RH) hinders the performance of fuel cells. Hence, we propose an assumption that a metal–organic framework (MOF) with a large portion of hydrophilic atoms may hold the adsorbed water molecules at reduced RH and thus maintain a high proton conductivity. As a proof, a Cu-based MOF whose ligand contains a large portion of O and N atoms is investigated and verified to be able to sustain the high water uptake capacity from 100 to 43% RH. Benefiting from the high and steady water uptake, the proton conductivity surpasses 10–3 S cm–1 even at 43% RH, higher than that of most reported proton-conducting MOFs at low RH. Moreover, the conductivity reaches up to 1.80 × 10–2 S cm–1 at 80 °C and 100% RH and is maintained up to 120 h, not only demonstrating an excellent stability but also reaching the highest proton conductivity benchmark (>10 – 2 S cm–1) of MOFs. Also, the synthesis time of this MOF is shortened to 7 h with a high yield of 99% at room temperature, superior to the reported proton-conducting MOFs. Molecular dynamics simulation further proves that the abundant hydrophilic atoms of the MOF keep the high water density in the pores in such a wide RH range, leading to the integrity of the hydrogen bonding networks to smoothly conduct protons. This work not only discovers a highly proton-conducting MOF in a wide RH range, but also provides a strategy to discover conductive MOFs by the portion of hydrophilic atoms in the MOFs, opening a cost-saving avenue to provide highly proton-conductive materials.

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.

Switched Proton Conduction in Metal-Organic Frameworks.

Stimuli-responsive materials can respond to external effects, and proton transport is widespread and plays a key role in living systems, making stimuli-responsive proton transport in artificial materials of particular interest to researchers due to its desirable application prospects. On the basis of the rapid growth of proton-conducting porous metal–organic frameworks (MOFs), switched proton-conducting MOFs have also begun to attract attention. MOFs have advantages in crystallinity, porosity, functionalization, and structural designability, and they can facilitate the fabrication of novel switchable proton conductors and promote an understanding of the comprehensive mechanisms. In this Perspective, we highlight the current progress in the rational design and fabrication of stimuli-responsive proton-conducting MOFs and their applications. The dynamic structural change of proton transfer pathways and the role of trigger molecules are discussed to elucidate the stimuli-responsive mechanisms. Subsequently, we also discuss the challenges and propose new research opportunities for further development.

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.

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.

Water-mediated proton conductive properties of three water-stable metal-organic frameworks constructed by pyromellitic acid

Selecting aromatic polycarboxylic acid ligands to self-assemble metal-organic frameworks (MOFs) with good water stability and proton conductivity is an expected preparation strategy. Herein, three proton conductive MOFs, 3D {[Zn2(1,2,4,5-BTA)](4,4′-tmdp)2]·5H2O}n (1) (1,2,4,5-H4BTA ​= ​1,2,4,5-benzenetetracarboxylic acid; 4,4′-tmdp ​= ​4,4′-trimethylenedipyridine), 2D {[Zn2(1,2,4,5-BTA)(4,4′-tmdp)(H2O)3]·2.25H2O}n (2), 3D {Cd(1,2,4,5-BTA)0.5}n (3) have been synthesized according to the literature method or hydrothermal method and structurally characterized by single-crystal X-ray diffraction and other approaches. All three MOFs have excellent thermal and water stability. EIS determinations show that with the increase of temperature and humidity, their proton conductivities gradually enhanced. Note that at 100 ​°C and 98% RH, their optimal proton conductivity values can attain about 10-4 ​S ​cm-1, which are higher than those of similar MOFs. Consequently, their proton conduction properties and mechanism were compared and speculated on the basis of activation energy calculation and crystal data analyses. We firmly believe that this research will provide new opportunities for the design and preparation of multicarboxylic acid-based crystalline proton conducting materials.

A proton-conductive metal-organic framework based on imidazole and sulphate ligands.

A metal-organic framework (MOF) built from a combination of metal cations, neutral azole ligands and sulphate anions, [Cu2(DHBDI)3(SO4)2]n (1, DHBDI = 1H,5H-benzo[1,2-d:4,5-d']diimidazole), was synthesized. MOF 1 exhibits good chemical stability in acids, bases and boiling water while showing high hydrophilicity. Meanwhile, MOF 1 exhibits a proton conductivity of 1.14 × 10-3 S cm-1 at 90 °C and 98% RH, among the best for MOF materials with uncoordinated N sites. Temperature-dependent conductivity measurements suggest a vehicle mechanism (Ea = 0.64 eV) for proton transport.