Unique Proton Dynamics in an Efficient MOF-Based Proton Conductor

Solvent-Assisted Modification to Enhance Proton Conductivity and Water Stability in Metal Phosphonates

Although proton-conductive metal phosphonates with well-defined structure offer a favorable platform for exploring their structure-property relationship, investigating of the synergic effect of phosphonate groups and functional moieties on proton conduction is rare. In this work, we have synthesized two new copper phosphonates, [Cu(4-cppH)(4,4'-bipy)(H₂O)₃] (FJU-80) and [Cu(4-cppH)(4,4'-bipy)]·H₂O·DMF (FJU-81), by the method of solvent-assisted modification, giving a 1D metal coordination polymer and a 3D metal open framework, respectively. Single-crystal X-ray diffraction shows that FJU-80 is full of hydrogen-bonding sites contributed from the improved synergic effect of phosphonate groups, carboxylate groups, and coordinated water molecules, thereby facilitating continuous hydrogen-bonding networks, whereas FJU-81 only has discrete hydrogen-bonding fragments. Powder X-ray diffraction and impedance analyses confirm that FJU-80 possesses higher water stability as well as improved proton conductivity, indicating that solvent-assisted modification is effective in increasing the hydrogen-bonding sites from phosphonate groups and functional moieties and then realizing facile proton transfer.

In Situ Fluorescence Imaging of the Levels of Glycosylation and Phosphorylation by a MOF-Based Nanoprobe in Depressed Mice

Elucidating the relationship between glycosylation and phosphorylation of protein post-translational modifications is of great significance for understanding most diseases. Mass spectrometry has been widely used in research of protein phosphorylation and glycosylation. However, to realize in situ and dynamic analysis of the levels of phosphorylation and glycosylation in cells and in vivo, mass spectrometry still has certain difficulties. Herein, a nano-MOF-based fluorescent probe with Zr(IV) and boric acid as the active center was designed and prepared. Fluorescence detection and imaging of phosphate is achieved through the specific interaction of Zr(IV) and phosphate. With aim to achieve specific recognition of glycosylation sites, the boronic acid group was modified in the MOF structure, and the fluorescence of the MOFs was regulated by the alizarin red. Thus, the glycosylation sites were recognized by the competition between alizarin red and glycosyl. Finally, the nanoprobe was successfully applied for in situ fluorescence imaging of the levels of glycosylation and phosphorylation in depressed mice.

Proton Conductive Luminescent Thermometer Based on Near-Infrared Emissive {YbCo2} Molecular Nanomagnets

Lanthanide(III)-based coordination complexes have been explored as a source of bifunctional molecular materials combining Single-Molecule Magnet (SMM) behavior with visible-to-near-infrared photoluminescence. In pursuit of more advanced multifunctionality, the next target is to functionalize crystalline solids based on emissive molecular nanomagnets toward high proton conductivity and an efficient luminescent thermometric effect. Here, a unique multifunctional molecule-based material, (H₅O₂)₂(H)[Yb^III(hmpa)₄][Co^III(CN)₆]₂·0.2H₂O (1, hmpa = hexamethylphosphoramide), composed of molecular {YbCo₂}^3- anions noncovalently bonded to acidic H₅O₂⁺ and H⁺ ions, is reported. The resulting Yb^III complexes present a slow magnetic relaxation below 6 K and room temperature NIR 4f-centered photoluminescence sensitized by [Co(CN)₆]^3- ions. The microporous framework, built on these emissive magnetic molecules, exhibits a high proton conductivity of the H-hopping mechanism reaching σ of 1.7 × 10^-4 S·cm^-1 at 97% relative humidity, which classifies 1 as a superionic conductor. Moreover, the emission pattern is strongly temperature-dependent which was utilized in achieving a highly sensitive single-center luminescent thermometer with a relative thermal sensitivity, S_r > 1% K^-1 in the 50-175 K range. This work shows an unprecedented combination of magnetic, optical, and electrical functionalities in a single phase working as a proton conductive NIR-emissive thermometer based on Single-Molecule Magnets.

Particle size dependence of proton conduction in a cationic lanthanum phosphonate MOF

Particle size-dependent proton conduction is studied in a lanthanum(iii) metal–organic framework, PCMOF21-AcO [La₂(H2L)_1.5(AcO)₃·(H₂O)_5.59], with a 3-D network linked by dicationic bis(dimethylphosphonato)bipiperidinium units.

A dual-functional MOF for high proton conduction and sensitive detection of ascorbic acid

A new Eu-MOF detects AA with turn off fluorescence and the proton conductivity of the Im@Eu-MOF is ten times higher than that of the En-MOF.

Application of MOF-based materials in electrochemical sensing

A summary of the most recent advancements of metal–organic frameworks (MOFs) for electrochemical sensing is listed in this frontier article.

Polyoxometalate-based hydrogen-bonded organic frameworks as a new class of proton conducting materials

To develop new types of crystalline proton conducting materials for fuel cells, a polyoxometalate-based hydrogen-bonded organic framework (PHOF) based on Keggin-type [PMo₁₂O₄₀]³⁻ and phenylimidazole (PHOF 1) has been prepared.

Structural features of proton-conducting metal organic and covalent organic frameworks

Proton conductivity in MOFs and COFs have been attracted due to their applicability as electrolytes in proton exchange membrane fuel cells. A short overview with recent updates on the structural features of MOFs and COFs for proton conduction are presented here.

From an organic ligand to a metal–organic coordination polymer, and to a metal–organic coordination polymer–cocrystal composite: a continuous promotion of the proton conductivity of crystalline materials

A new strategy was proposed to increase proton conductivities in metal–organic coordination polymers (MOCPs) commencing from organic ligands, i.e. coordination inducement and MOCP–cocrystal composite formation.

Proton conduction in two Cu/Zn dimer-based hydrogen-bonded supramolecular frameworks from imidazole multi-carboxylate

At 98% RH and 100 °C, two dimer-based HSFs show the best σ values of 3.13 × 10⁻⁴ S cm⁻¹ for 1 and 0.55 × 10⁻⁴ S cm⁻¹ for 2, which can be compared to the values of reported HSFs.

Anisotropic proton conduction realized by a layered vanadium selenite single crystal

A new layered vanadium selenite single crystal exhibits anisotropic proton conductivities along the [100] and [001] directions with the Grotthuss and Vehicle mechanisms, respectively.

The promotion of proton conductivity by immobilizing molybdovanadophosphoric acids in metal–organic frameworks

A novel solid-state electrolyte based on a MOF impregnated with polyoxometalate (PMoV_x@MIL-101) achieves favourable proton conductivity.

A CsCl-type inorganic cluster-based high-symmetry crystal built from {Mo4.55V7.45PO40}10.45- with a high ratio of vanadium to molybdenum and {(H2O)0.3@K6(H2O)12}6+ clusters exhibiting proton conduction below the freezing point of water

As a class of anionic oxoclusters of early transition metals in their highest oxidation states, polyoxometalates (POMs) show considerable structural versatility and unique chemical and physical properties, making them promising multifunctional materials. In this study, a Keggin-type POM has been achieved, with a formula of [(H2O)0.3@K6(H2O)12]H4.45[PV7.45Mo4.55O40]·11H2O (1), and its microcrystals and nanocrystals have been obtained, respectively. This POM was characterized by elemental analysis for C, H and N, ICP-MS, TG, PXRD, SEM, X-band EPR and XPS techniques. Single crystal X-ray diffraction analysis demonstrated that 1 shows a rare extended structure with a high-connected three-dimensional (3D) all inorganic network of a Keggin-type POM, built from {Mo4.55V7.45PO40}10.45- polyoxoanions and {(H2O)0.3@K6(H2O)12}6+ clusters with CsCl-type crystal structure. In addition, to the best of our knowledge, 1 shows the highest ratio of vanadium to molybdenum among Keggin-type POMs reported thus far. Most interestingly, 1 exhibits intrinsic proton conduction below the freezing point of water, with a proton conductivity of 6.90 × 10-7 S cm-1 at 249 K and further reaching 3.36 × 10-6 S cm-1 at 272 K and Ea = 0.44 eV at 249-272 K.

Persistent Superprotonic Conductivity in the Order of 10−1 S·cm−1 Achieved Through Thermally Induced Structural Transformation of a Uranyl Coordination Polymer

Despite tremendous efforts having been made in the exploration of new high-performance proton-conducting materials, systems with superprotonic conductivity higher than 10−1 S·cm−1 are scarcely reported. We show here the utilization of bridging uranyl oxo atoms, traditionally termed cation–cation interaction (CCI), as the hydrogen bond acceptor to build a dense and ordered hydrogen bond network, affording a unique uranyl-based proton-conducting coordination polymer (H3O)4UO2(PO4)2 (HUP-1). This compound contains a densely connected hydronium network that is substantially stabilized by uranyl oxo atoms and exhibits high proton conductivities over a wide temperature range. At 98 °C, 98% relative humidity, a superprotonic conductivity of 1.02 × 10−1 S·cm−1 is observed for the system, one of the highest values reported for a solid-state proton-conducting material. This property originates from the thermally induced phase transformation from HUP-1 to another uranyl compound also with a CCI bond, (H3O)UO2PO4·(H2O)3 (HUP-2), accompanied by the partial generation of phosphorus acid that is further trapped in the structure of HUP-2, demonstrated by solid-state NMR analysis. The superprotonic conductivity of H3PO4@HUP-2 is persistent under the testing condition.

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).

An Alkali Metal Ion-Exchanged Metal-Phosphate (C2H10N2) xNa1- x[Mn2(PO4)2] with High Proton Conductivity of 10-2 S·cm-1

A two-dimensional layered inorganic-organic hybrid metal hydrogenophosphate (1) was treated with 0.1 M NaOH-ethanol solution, which resulted in a Na⁺-ion substitution product that exhibits excellent thermal and aqueous stability with 1, as well as much higher proton conductivity (σ = 10^-2 S·cm^-1) even at low temperature (283 K). This is because Na⁺ ions in aqueous solution make a more dense and extensive H-bonding network of water molecules, which enables protons to more easily transfer along the network.

Enhancement of Intrinsic Proton Conductivity and Aniline Sensitivity by Introducing Dye Molecules into the MOF Channel

The encapsulation of dyes into metal-organic frameworks (MOFs) has generated a variety of platforms for luminescence, but little attention has been paid to their application in proton conduction. Here, a cationic MOF {{[In₃OL_1.5(H₂O)₃](NO₃)}·(DMA)₃·(CH₃CN)₆·(H₂O)₃₀} _n (FJU-10, H₄L = 4,4',4″,4‴-(1,4-phenylenbis(pyridine-4,2,6-triyl))-tetrabenzoic acid, DMA = N, N-dimethylacetamide) was synthesized, and the dye molecule 8-hydroxy-1,3,6-pyrenetrisulfonic acid trisodium salt (HPTS) was further added to the MOF growth solution, but during the reaction, HPTS was nitrated and nitrated HPTS was encapsulated into the FJU-10 to obtain dye@FJU-10. As a result, the intrinsic proton conductivity of dye@FJU-10 is nearly 5 times higher than that of FJU-10 at 90 °C. Dye@FJU-10 exhibits more sensitive fluorescence quenching toward aniline than FJU-10 in DMF solution (the detection limits of FJU-10 and dye@FJU-10 are as low as 0.58 and 0.62 μM, respectively). Here, it is demonstrated for the first time that intrinsic proton conductivity can be effectively improved by encapsulating a nitrated HPTS dye into an MOF.

Metal-Organic Frameworks for Hydrogen Energy Applications: Advances and Challenges

Hydrogen is in limelight as an environmental benign alternative to fossil fuels from few decades. To bring the concept of hydrogen economy from academic labs to real world certain challenges need to be addressed in the areas of hydrogen production, storage, and its use in fuel cells. Crystalline metal-organic frameworks (MOFs) with unprecedented surface areas are considered as potential materials for addressing the challenges in each of these three areas. MOFs combine the diverse chemistry of molecular linkers with their ability to coordinate to metal ions and clusters. The unabated flurry of research using MOFs in the context of hydrogen energy related activities in the past decade demonstrates the versatility of this class of materials. In the present review, we discuss major strategical advances that have taken place in the field of "hydrogen economy and MOFs" and point out issues requiring further attention.

Synthesis of Anionic Metal-Organic Zeolites for Selective Gas Adsorption and Ion Exchange

We report here a new method to construct metal-organic zeolites (MOZs) via the mixing of dicarboxylic acid and tetrazole ligands. Two anionic MOZs, MOZ-200 and MOZ-201 with BCT and ATN topologies, were developed successfully by this approach and exhibit high selective CO₂ uptake and Ln³⁺ ion exchange.

High Proton Conductivity Achieved by Encapsulation of Imidazole Molecules into Proton-Conducting MOF-808

Metal-organic frameworks (MOFs), as newly emerging materials, show compelling intrinsic structural features, e.g., the highly crystalline nature and designable and tunable porosity, as well as tailorable functionality, rendering them suitable for proton-conducting materials. The proton conduction of MOF is significantly improved using the postsynthesis or encapsulation strategy. In this work, the MOF-based proton-conducting material Im@MOF-808 has been prepared by incorporating the imidazole molecules into the pores of proton-conducting MOF-808. Compared with MOF-808, Im@MOF-808 not only possesses higher proton conductivity of 3.45 × 10^-2 S cm^-1 at 338 K and 99% RH, superior to that of any imidazole-encapsulated proton-conducting materials reported to date, but also good durable and stable proton conduction. Moreover, the thermal stability of H-bond networks is much improved owing to the water molecules partially replaced by higher boiling point imidazole molecules. Additionally, it is further discussed for the possible mechanism of imidazole encapsulation into the pores of MOF-808 to enhance proton conduction.

Synthesis, Characterization, and Proton Conduction Behavior of Thorium and Cerium Phosphonates

Thorium (Th⁴⁺) and cerium (Ce⁴⁺) phosphonates have been synthesized by the sol-gel method using various phosphonic acids and analyzed using elemental and CHN analysis, spectral analysis (Fourier transform infrared spectroscopy and X-ray diffraction), thermogravimetric analysis, scanning electron microscopy, and energy-dispersive X-ray spectroscopy. The proton transport study was performed by measuring proton conductance at different temperatures using an impedance analyzer. The Grotthuss mechanism of the proton conduction has been discussed thoroughly on the basis of results obtained from the impedance measurement; the specific proton conduction (σ) and activation energy (E _a) have been evaluated and compared with the reported proton-conducting system having similar phosphonate moieties.

A bismuth oxide/graphene oxide nanocomposite membrane showing super proton conductivity and low methanol permeability

Proton exchange membrane fuel cells are still limited as state-of-art proton exchange membranes perform poorly at high and low temperature and are easily damaged by harsh electrochemical conditions such as reactive peroxide species. One effective solution to this issue is to develop new types of proton conductive materials that are capable of working in a broad temperature range. A simple vacuum-assisted filtration method is employed to obtain a well-ordered new proton-conducting membrane by immobilizing nanosized bismuth oxide clusters [H6Bi12O16] (NO3)10·6(H2O) {H6Bi12O16} onto graphene oxide (GO) supports (named as {H6Bi12O16}/GO). {H6Bi12O16}/GO is stable in acidic media and has high proton conductivity over the temperature range from -40 to 80 °C. The proton conductivity of the {H6Bi12O16}/GO membrane is 0.564 S cm-1 at 80 °C in aqueous solution (in plane), and 0.1 S cm-1 at 80 °C and 97% RH (out of plane), respectively. Without loss of high proton conductivity, the membrane also exhibited 100-fold lower methanol permeability than a Nafion 117 membrane. Moreover, {H6Bi12O16}/GO displayed good catalytic decomposition of hydrogen peroxide and superior humidity response and recovery properties. These advantages mean that {H6Bi12O16}/GO holds great promise as a solid-state electrolyte that can potentially be applied in energy conversion devices in the future.

Proton transfer in hydrogen-bonded degenerate systems of water and ammonia in metal-organic frameworks

Porous crystalline metal-organic frameworks (MOFs) or porous coordination polymers (PCPs) are emerging as a new class of proton conductors with numerous investigations. Some of the MOFs exhibit an excellent proton-conducting performance (higher than 10-2 S cm-1) originating from the interesting hydrogen(H)-bonding networks with guest molecules, where the conducting medium plays a crucial role. In the overwhelming majority of MOFs, the conducting medium is H2O because of its degenerate conjugate acid-base system (H3O+ + H2O ⇔ H2O + H3O+ or OH- + H2O ⇔ H2O + OH-) and the efficient H-bonding ability through two proton donor and two acceptor sites with a tetrahedral geometry. Considering the systematic molecular similarity to water, ammonia (NH3; NH4 + + NH3 ⇔ NH3 + NH4 +) is promising as the next proton-conducting medium. In addition, there are few reports on NH3-mediated proton conductivity in MOFs. In this perspective, we provide overviews of the degenerate water (hydronium or hydroxide)- or ammonia (ammonium)-mediated proton conduction system, the design strategies for proton-conductive MOFs, and the conduction mechanisms.

Metalo Hydrogen-Bonded Organic Frameworks (MHOFs) as New Class of Crystalline Materials for Protonic Conduction

Recently, proton conduction has been a thread of high potential owing to its wide applications in fuel-cell technology. In the search for a new class of crystalline materials for protonic conductors, three metalo hydrogen-bonded organic frameworks (MHOFs) based on [Ni(Imdz)₆ ]²⁺ and arene disulfonates (MHOF1 and MHOF2) or dicarboxylate (MHOF3) have been reported (Imdz=imidazole). The presence of an ionic backbone with charge-assisted H-bonds, coupled with amphiprotic imidazoles made these MHOFs protonic conductors, exhibiting conduction values of 0.75×10^-3 , 3.5×10^-4 and 0.97×10^-3  S cm^-1 , respectively, at 80 °C and 98 % relative humidity, which are comparable to other crystalline metal-organic framework, coordination polymer, polyoxometalate, covalent organic framework, and hydrogen-bonded organic framework materials. This report initiates the usage of MHOF materials as a new class of solid-state proton conductors.

High proton conduction in an excellent water-stable gadolinium metal–organic framework

Proton conductivity of a new gadolinium(iii) metal–organic framework, is among the highest values for proton-conducting MOFs.

High quantum yield pure blue emission and fast proton conduction from an indium–metal–organic framework

A 3D anionic In³⁺–MOF integrates the multifunctionality of pure blue luminescence with a high quantum yield of 61.4% and water-mediated fast proton conduction with σ = 2.90–9.22 × 10⁻³ S cm⁻¹ at 25–60 °C and 99% RH.

Supramolecular hydrogen-bonded organic networks through acid–base pairs as efficient proton-conducting electrolytes

The research of developing new proton-conducting materials via a simple and cost-effective method is vital in fuel cell technology.

Dual-cubic-cage based lanthanide sulfate–carboxylpyrazolate frameworks with high hydrolytic stability and remarkable proton conduction

Lanthanide sulfate–carboxylpyrazolate nets, which have double cuboid cavities, encapsulated single-ion hydrates, tolerance in water for 6 months and even under a 200 °C hydrothermal condition tested for 3 days, and high proton-conductivity of over 10⁻³ S cm⁻¹ at 25 °C and at 97% relative humidity are reported.

6,6′-(Perfluoropropane-2,2-diyl)bis(2,3-dihydrophthalazine-1,4-dione)-based coordination polymers and their sensing properties towards Cr2O72−

The structures of six new diacylhydrazidate-based coordination polymers were reported. Based on their better photoluminescence properties, their sensing abilities towards Cr2O₇²⁻ were investigated.

A water stable layered Tb(iii) polycarboxylate with high proton conductivity over 10−2 S cm−1 in a wide temperature range

An unprecedented Tb(iii) polycarboxylate, {[Tb₄(TTHA)₂(H₂O)₄]·7H₂O}_n (1), has been synthesized.