Proton-conductive Metal-organic Frameworks Research Articles

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

Designing high-performance proton-conducting metal-organic frameworks simultaneously having highly hydrothermal stability and a high-density proton carrier remains a great challenge. Fe-MIL-88B is a classic metal-organic framework (MOF) with a large-size one-dimensional (1D) channel lined with a high-density uncoordinated metal atom for postfunctionalization; however this MOF cannot act as a proton conductor due to the weak hydrothermal stability. Here, we prepared an ultrastable isostructure Cr-MIL-88B, which is subsequently functionalized by anchoring 3-pyridinesulfonic acid and 2-(4-pyridyl) ethanesulfonic acid on the naked Cr atoms exposed on the surface of the host-framework, producing two new MOFs, i.e. Cr-MIL-88B-pyridine sulfonic acid (abbreviated as Cr-MIL-88B-PSA) and Cr-MIL-88B-pyridine ethanesulfonic acid (abbreviated as Cr-MIL-88B-PESA). Thus, Cr atoms on the host framework were modified by functional sulfonic groups, which stick out toward the center of the channel forming ordered high-density sulfonic groups as proton donors along the open channel and achieving the highest proton conductivity of 4.50 × 10-2 S cm-1 for Cr-MIL-88B-PESA and 1.58 × 10-1 S cm-1 for Cr-MIL-88B-PSA, surpassing that of the Nafion membrane.

Proton-Conductive 3D LnIII Metal-Organic Frameworks for Formic Acid Impedance Sensing.

Metal-organic frameworks (MOFs) as new classes of proton-conducting materials have been highlighted in recent years. Nevertheless, the exploration of proton-conducting MOFs as formic acid sensors is extremely lacking. Herein, we prepared two highly stable 3D isostructural lanthanide(III) MOFs, {(M(μ3 -HPhIDC)(μ2 -C2 O4 )0.5 (H2 O))⋅2 H2 O}n (M=Tb (ZZU-1); Eu (ZZU-2)) (H3 PhIDC=2-phenyl-1H-imidazole-4,5-dicarboxylic acid), in which the coordinated and uncoordinated water molecules and uncoordinated imidazole N atoms play decisive roles for the high-performance proton conduction and recognition ability for formic acid. Both ZZU-1 and ZZU-2 show temperature- and humidity-dependent proton-conducting characteristics with high conductivities of 8.95×10-4 and 4.63×10-4 S cm-1 at 98 % RH and 100 °C, respectively. Importantly, the impedance values of the two MOF-based sensors decrease upon exposure to formic acid vapor generated from formic aqueous solutions at 25 °C with good reproducibility. By comparing the changes of impedance values, we can indirectly determine the concentration of HCOOH in aqueous solution. The results showed that the lowest detectable concentrations of formic acid aqueous solutions are 1.2×10-2 mol L-1 by ZZU-1 and 2.0×10-2 mol L-1 by ZZU-2. Furthermore, the two sensors can distinguish formic acid vapor from interfering vapors including MeOH, N-hexane, benzene, toluene, EtOH, acetone, acetic acid and butane. Our research provides a new platform of proton-conductive MOFs-based sensors for detecting formic acid.

Two Highly Stable Proton Conductive Cobalt(II)-Organic Frameworks as Impedance Sensors for Formic Acid.

Metal-organic frameworks (MOFs) have been extensively explored as advanced chemical sensors in recent years. However, there are few studies on MOFs as acidic gas sensors, especially proton conductive MOFs. In this work, two new proton-conducting 3D MOFs, {[Co3 (p-CPhHIDC)2 (4,4'-bipy)(H2 O)]⋅2 H2 O}n (1) (p-CPhH4 IDC=2-(4-carboxylphenyl)-1 H-imidazole-4,5-dicarboxylic acid; 4,4'-bipy=4,4'-bipyridine) and {[Co3 (p-CPhHIDC)2 (bpe)(H2 O)]⋅3 H2 O}n (2) (bpe=trans-1,2-bis(4-pyridyl)ethylene) have been solvothermally prepared and investigated their formic acid sensing properties. Both MOFs 1 and 2 show temperature- and humidity-dependent proton conductive properties and exhibit optimized proton conductivities of 1.04×10-3 and 7.02×10-4 S cm at 98 % relative humidity (RH) and 100 °C, respectively. The large number of uncoordinated carboxylic acid sites, free and coordination water molecules, and hydrogen-bonding networks inside the frameworks are favorable to the proton transfer. By measuring the impedance values after exposure to formic acid vapor at 98 % or 68 % RH and 25 °C, both MOFs indicate reproducibly high sensitivity to the analyte. The detection limit of formic acid vapor is as low as 35 ppm for 1 and 70 ppm for 2. Meanwhile, both MOFs also show commendable selectivity towards formic acid among interfering solutions. The proton conducting and formic acid sensing mechanisms have been suggested according to the structural analysis, Ea calculations, N2 and water vapor absorptions, PXRD and SEM measurements. This work will open a new avenue for proton-conductive MOF-based impedance sensors and promote the potential application of these MOFs for indirectly monitoring the concentrations of formic acid vapors.

Field effect transistor based on proton conductive metal organic framework (CuBTC)

Metal organic frameworks (MOFs) are crystalline materials with ultrahigh porosity and tunable physico-chemical properties. But most MOFs are electrical insulators which restricts their use in electronic devices. In the present study, a field effect transistor (FET) based on a proton-conductive MOF was investigated. Imidazole was used as a proton conductor to increase the conductivity of the copper benzene tricarboxylate (CuBTC) MOF and, further, it was used to develop the FET. The CuBTC was synthesized using the traditional solvothermal method. The proton-conductive MOF was synthesized by post-modification of the CuBTC via the pore filling method using imidazole molecules, which achieved a proton conductivity of ~1.04 × 10−4 S cm−1 at 70 °C (anhydrous conditions). The proton conductive MOF was characterized by various techniques; spectroscopically by Fourier transform infrared spectroscopy, x-ray photoelectron spectroscopy, x-ray absorption spectroscopy, energy dispersive spectroscopy, alternating current (a.c.) impedance spectroscopy; thermally using thermogravimetric analysis; structurally using powder x-ray diffraction; morphologically using field emission scanning electron microscopy; electrically by FETs.

Impressive Proton Conductivities of Two Highly Stable Metal-Organic Frameworks Constructed by Substituted Imidazoledicarboxylates.

There is great interest in the promising applications of proton-conductive metal-organic frameworks (MOFs) in the field of electrochemistry. Thus, seeking more types of MOFs with high proton conductivity is of great importance. Herein, we designed and prepared two substituted imidazoledicarboxylate-based MOFs, {[Cd( p-TIPhH2IDC)2]·H2O} n [1; p-TIPhH3IDC = 2- p-(1 H-1,2,4-triazolyl)phenyl-1 H-4,5-imidazoledicarboxylic acid] and [Sr(DMPhH2IDC)2] n [2; DMPhH3IDC = 2-(3,4-dimethylphenyl)-1 H-imidazole-4,5-dicarboxylic acid], and fully explored their water-assisted proton conduction. The best conductivity for 1 of 1.24 × 10-4 S·cm-1 is higher than that of most previous conductive Cd-MOFs under similar conditions. 2 has the highest conductivity (0.92 × 10-3 S·cm-1) among the reported conductive Sr-MOFs. Via structural analysis, Ea values, water vapor adsorptions, and powder X-ray diffraction and scanning electron microscopy tests, reasonable proton pathways and conduction mechanisms were highlighted. It should be emphasized that the N-heterocyclic units (imidazole and triazole) and carboxyl and hydrogen-bonding networks in the frameworks all play crucial roles in the transmission of proton conductivity. Our research offers more choice for the preparation of desired proton-conductive materials.

Functionality in metal-organic framework minerals: proton conductivity, stability and potential for polymorphism.

Rare metal-organic framework (MOF) minerals stepanovite and zhemchuzhnikovite can exhibit properties comparable to known oxalate MOF proton conductors, including high proton conductivity over a range of relative humidities at 25 °C, and retention of the framework structure upon thermal dehydration. They also have high thermodynamic stability, with a pronounced stabilizing effect of substituting aluminium for iron, illustrating a simple design to access stable, highly proton-conductive MOFs without using complex organic ligands.

A Highly Proton-Conductive 3D Ionic Cadmium-Organic Framework for Ammonia and Amines Impedance Sensing.

Lately, the progressive study of metal-organic frameworks (MOFs) for the detection of ammonia and amines has made infusive achievements. Nevertheless, the investigation of proton-conductive MOFs used to detect the low concentrations of ammonia and amine gases at different relative humidities (RHs) at room temperature is relatively restricted. Herein, by solvothermal reaction of Cd(NO3)2 with 2-methyl-1 H-imidazole-4,5-dicarboxylic acid (H3MIDC), a three-dimensional ionic MOF {Na[Cd(MIDC)]} n (1) bearing ordered one-dimensional channels was successfully synthesized. Our research indicates that the uncoordination carboxylate sites are beneficial to proton transfer and the recognition of ammonia and amine compounds. The optimized proton conductivity of 1 reaches a high value of 1.04 × 10-3 S·cm-1 (100 °C, 98% RH). The room temperature sensing properties of ammonia and amine gases were explored under 68, 85, and 98% RHs, respectively. Satisfactorily, the detection limits of MOF 1 toward ammonia, methylamine, dimethylamine, trimethylamine, and ethylamine are 0.05, 0.1, 0.5, 1, and 4 ppm, respectively, which is one of the best room-temperature sensors for ammonia among previous sensors based on proton-conductive MOFs. The proton conducting and sensing mechanisms were highlighted as well.

Extra Water- and Acid-Stable MOF-801 with High Proton Conductivity and Its Composite Membrane for Proton-Exchange Membrane.

Proton-exchange membranes (PEMs), characterized by selectively permitting the transfer of protons and acting as a separator in electrochemical devices, have attracted immense attention. The composite membrane, fabricated from organic polymer matrix and high proton-conducting metal-organic framework (MOF), integrates the excellent physical and chemical performances of the polymer and MOF, achieving collective properties for good-performance PEMs. In this study, we demonstrate that MOF-801 shows remarkable proton conductance with σ = 1.88 × 10-3 S cm-1 at 298 K and 98% relative humidity (RH), specifically, together with extra stability to hydrochloric acid or diluting sodium hydroxide aqueous solutions and boiling water. Furthermore, the composite membranes (denoted MOF-801@PP- X, where X represents the mass percentage of MOF-801 in the membrane) have been fabricated using the sub-micrometer-scale crystalline particles of MOF-801 and blending the poly(vinylidene fluoride)-poly(vinylpyrrolidone) matrix, and these PEMs display high proton conductivity, with σ = 1.84 × 10-3 S cm-1 at 325 K 98% RH. A composite membrane as PEM was assembled into H2/O2 fuel cell for tests, indicating that these membrane materials have vast potential for PEM application on electrochemical devices.

A Water-Stable Proton-Conductive Barium(II)-Organic Framework for Ammonia Sensing at High Humidity.

In view of environmental protection and the need for early prediction of major diseases, it is necessary to accurately monitor the change of trace ammonia concentration in air or in exhaled breath. However, the adoption of proton-conductive metal-organic frameworks (MOFs) as smart sensors in this field is limited by a lack of ultrasensitive gas-detecting performance at high relative humidity (RH). Here, the pellet fabrication of a water-stable proton-conductive MOF, Ba( o-CbPhH2IDC)(H2O)4] n (1) ( o-CbPhH4IDC = 2-(2-carboxylphenyl)-1 H-imidazole-4,5-dicarboxylic acid) is reported. The MOF 1 displays enhanced sensitivity and selectivity to NH3 gas at high RHs (>85%) and 30 °C, and the sensing mechanism is suggested. The electrochemical impedance gas sensor fabricated by MOF 1 is a promising sensor for ammonia at mild temperature and high RHs.

Shrink-wrapping water to conduct protons

For proton-conducting metal–organic frameworks (MOFs) to find application as the electrolyte in proton-exchange membrane fuel cells, materials with better stability and conductivity are required. Now, a structurally flexible MOF that is also highly stable is demonstrated to possess high proton conductivity over a range of humidities.

A flexible metal–organic framework with a high density of sulfonic acid sites for proton conduction

The design of stable electrolyte materials with high proton conductivity for use in proton exchange membrane fuel cells remains a challenge. Most of the materials explored have good conductivity at high relative humidity (RH), but significantly decreased conductivity at reduced RH. Here we report a chemically stable and structurally flexible metal–organic framework (MOF), BUT-8(Cr)A, possessing a three-dimensional framework structure with one-dimensional channels, in which high-density sulfonic acid (–SO3H) sites arrange on channel surfaces for proton conduction. We propose that its flexible nature, together with its –SO3H sites, could allow BUT-8(Cr)A to self-adapt its framework under different humid environments to ensure smooth proton conduction pathways mediated by water molecules. Relative to other MOFs, BUT-8(Cr)A not only has a high proton conductivity of 1.27 × 10−1 S cm−1 at 100% RH and 80 °C but also maintains moderately high proton conductivity at a wide range of RH and temperature. Proton-conducting metal-organic frameworks (MOFs) could be used as the electrolytes in proton exchange membrane fuel cells but chemically stable materials that perform well at low humidity are still sought. Here the authors prepare a stable, structurally flexible MOF that maintains high proton conductivity under a wide range of humidity.

Proton Conductance of a Superior Water-Stable Metal-Organic Framework and Its Composite Membrane with Poly(vinylidene fluoride).

Proton-exchange membranes (PEMs) as separators have important technological applications in electrochemical devices, including fuel cells, electrochemical sensors, electrochemical reactors, and electrochromic displays. The composite membrane of a proton-conducting metal-organic framework (MOF) and an organic polymer combines the unique physical and chemical nature of the polymer and the high proton conductivity of the MOF, bringing together the best of both components to potentially fabricate high-performance PEMs. In this study, we have investigated the proton-transport nature of a zirconium(IV) MOF, MOF-808 (1). This superior-water-stability MOF shows striking proton conductivity with σ = 7.58 × 10-3 S·cm-1 at 315 K and 99% relative humidity. The composite membranes of 1 and poly(vinylidene fluoride) (PVDF) have further been fabricated and are labeled as 1@PVDF-X, where X represents the mass percentage of 1 (as X%) in 1@PVDF-X and X = 10-55%. The composite membranes exhibit good mechanical features and durability for practical application and a considerable proton conductivity of 1.56 × 10-4 S·cm-1 in deionized water at 338 K as well. Thus, the composite membranes show promising applications as alternative PEMs in diverse electrochemical devices.

Unique Proton Dynamics in an Efficient MOF-Based Proton Conductor.

Recently, research on metal-organic frameworks (MOFs) serving as a new type of proton conductive material has resulted in many exciting achievements. However, direct observation of a well-established proton-transfer mechanism still remains challenging in MOFs and other crystalline compounds, let alone other conductive materials. Herein we report the solvothermal synthesis of a new proton-conducting MOF, (Me2NH2)[Eu(L)] (H4L = 5-(phosphonomethyl)isophthalic acid). The compound consists of a layered anionic framework [Eu(L)]- and interlayer-embedded counter cations (Me2NH2)+, which interact with adjacent uncoordinated O atoms of phosphonate groups to form strongly (N-H···O) hydrogen-bonded chains aligned parallel to the c-axis. Facile proton transfer along these chains endows the compound with single-crystal anhydrous conductivity of 1.25 × 10-3 S·cm-1 at 150 °C, and water-assisted proton conductivity for a compacted pellet of microcrystalline crystals attains 3.76 × 10-3 S·cm-1 at 100 °C and 98% relative humidity (RH). Proton dynamics (vibrating and transfer) within N-H···O chains of the compound are directly observed using a combination of anisotropic conductivity measurements and control experiments using large single-crystals and pelletized samples, in situ variable-temperature characterization techniques including powder X-ray diffraction (PXRD), single-crystal X-ray diffraction (SCXRD), diffuse reflectance infrared Fourier transform spectrum (DRIFTS), and variable-temperature photoluminescence. In particular, a scarce single-crystal to single-crystal (SCSC) transformation accompanied by proton transfer between an anionic structure (Me2NH2)[Eu(L)] and an identical neutral framework [Eu(HL)] has been identified.

Electrochemical synthesis of metal organic framework films with proton conductive property

It is a big challenge to construct proton-conductive metal organic framework (MOF) films for successful utilization of MOF materials in proton-conducting devices. Herein, we propose an electrochemical strategy for the preparation of highly porous NENU-3 films on the conductive copper substrates. The Keggin-type heteropolyacid of phosphotungstic acid (PTA) as the molecular clusters is integrated into Cu3(BTC)2 cavities by the one-step electrochemical approach. The PTA plays a dual role of guest molecules in the cages as well as a strong electrolyte in the synthesis solution. Further, as-prepared NENU-3 films exhibit a fascinating proton conductivity under ~97% RH and at 24°C, possibly originating from the Keggin PTA clusters, as proton carriers and the hydrophilic framework of Cu2 (BTC)3. This study may offer a new strategy to assemble the proton-conductive MOF films, and the good combination of heteropolyacids and MOF host materials presents a promising platform for expanding the applications of MOFs in electrochemical devices.

Highly conductive and robust composite anion exchange membranes by incorporating quaternized MIL-101(Cr)

With well-defined channels and tunable functionality, metal-organic frameworks (MOFs) have inspired the design of a new class of ion-conductive compounds. In contrast to the extensive studies on proton-conductive MOFs and related membranes attractive for fuel cells, rare reports focus on MOFs in preparation of anion exchange membranes. In this study, chloromethylated MIL-101(Cr) was prepared and incorporated into chloromethylated poly (ether ether ketone) (PEEK) as a multifunctional filler to prepare imidazolium PEEK/imidazolium MIL-101(Cr) (ImPEEK/ImMIL-101(Cr)) anion exchange membrane after synchronous quaternization. The successful synthesis and chloromethylation of MIL-101(Cr) were verified by transmission electron microscopy, X-ray diffraction and Fourier transform infrared spectroscopy while the enhanced performance of composite membranes in hydroxide conductivity, mechanical strength and dimensional stability were evaluated by alternating-current impedance, electronic stretching machine and measurement of swelling ratio. Specifically, incorporating 5.0wt% ImMIL-101(Cr) afforded a 71.4% increase in hydroxide conductivity at 20°C, 100% RH. Besides, the composite membranes exhibited enhanced dimensional stability and mechanical strength due to the rigid framework of ImMIL-101(Cr). At room temperature and the ImMIL-101(Cr) content of 10wt%, the swelling ratio of the ImPEEK/ImMIL-101(Cr) was 70.04% lower while the tensile strength was 47.5% higher than that of the pure membrane.

ChemInform Abstract: Hydrated Proton‐Conductive Metal—Organic Frameworks

AbstractReview: [37 refs.

Hydrated Proton-Conductive Metal-Organic Frameworks.

Recent investigations into proton conduction in metal-organic frameworks (MOFs) indicate that MOFs are promising materials as a new class of proton conductors. Hydrated proton-conductive MOFs show not only high proton conductivity of approximately 10-2 S cm-1 , which is comparable to that of a practical organic polymer, but also structural visibility of proton-conducting pathways inside the materials owing to their high crystallinity. Herein, studies on the design, synthesis, and proton-conductive properties of MOFs with hydrated proton-conductive systems are introduced.

Water sandwiched by a pair of aromatic rings in a proton-conducting metal-organic framework.

The interactions between water molecules and aromatic rings are known to be common and important in physics, chemistry and life sciences. Benzene-water complexes are the main prototype systems for O-Hπ and lone-pair (lp)π interactions in theoretical research, however solid state examples are very rare. Here, the solid state example of water sandwiched by a pair of aromatic rings is observed in a silver-mellitate framework, where lpπ and O-Hπ interactions coexist. The coexistence of these two interactions has been further verified by theoretical calculations. In addition, ammonium ions and water molecules as proton sources, and strongly H-bonded nets as the pathway of proton transport, make the reported MOFs (metal-organic frameworks) exhibit distinct proton conduction.

Proton-Conductive Metal–Organic Frameworks

Abstract Proton-conductive electrolytes are key materials in fuel cells. We introduced acidic functional groups into a porous coordination polymer (PCP), or metal–organic framework (MOF), and constructed proton-conductive PCP/MOFs. To achieve this, a novel synthetic method for introducing acidic groups in PCP/MOF was invented. The proton conductivities of various PCP/MOF materials were investigated by AC impedance spectroscopy, and some of the materials showed high proton conductivity up to 8 × 10−3 S cm−1 at ambient temperature. We also investigated the dependency of proton conductivity on functional groups and found a relationship between proton conductivity, the acidity of the functional groups, and the hydrogen-bond networks formed inside the pores of PCP/MOFs. These PCP/MOF materials have high crystallinity, and the frameworks and arrangement of guests in the inner pore were clearly determined by X-ray crystallographic analysis. The relationship between proton conductivity and hydrogen-bond networks was investigated. This study thus establishes a novel field for investigating highly proton-conductive materials.

A porous proton-relaying metal-organic framework material that accelerates electrochemical hydrogen evolution.

The availability of efficient hydrogen evolution reaction (HER) catalysts is of high importance for solar fuel technologies aimed at reducing future carbon emissions. Even though Pt electrodes are excellent HER electrocatalysts, commercialization of large-scale hydrogen production technology requires finding an equally efficient, low-cost, earth-abundant alternative. Here, high porosity, metal-organic framework (MOF) films have been used as scaffolds for the deposition of a Ni-S electrocatalyst. Compared with an MOF-free Ni-S, the resulting hybrid materials exhibit significantly enhanced performance for HER from aqueous acid, decreasing the kinetic overpotential by more than 200 mV at a benchmark current density of 10 mA cm−2. Although the initial aim was to improve electrocatalytic activity by greatly boosting the active area of the Ni-S catalyst, the performance enhancements instead were found to arise primarily from the ability of the proton-conductive MOF to favourably modify the immediate chemical environment of the sulfide-based catalyst.