Characterization of Proton Dynamics for the Understanding of Conduction Mechanism in Proton Conductive Metal‐Organic Frameworks

A multifunctional material of a terbium-based metal-organic framework showing fluorescence detection of Hg2+ and proton conductivity

A novel terbium-based metal-organic framework, namely {[(CH3CH2)2NH2]2[Tb2(TTDI)2(H2O)1.7 (HCOOH)0.3]·DEF·3H2O}n (Tb-MOF, H4TTDI = 5-[5-(3,5-dicarboxyphenyl) thieno [3,2-b] thiophen-2-yl] benzene-1,3-dicarboxylic acid) was synthesized and characterized. The Tb-MOF is a three-dimensional porous framework based on binuclear clusters and crystallizes in the triclinic P1̄ space group. Fluorescence sensing experiments indicated that the Tb-MOF can recognize Hg2+ ions in aqueous solutions by the fluorescence quenching effect with a detection limit of 0.94 μM. The fluorescence quenching effect in the presence of Hg2+ ions primarily arises from the interaction between S in the ligand and Hg2+ ions. Additionally, the Tb-MOF displays proton conduction properties with a maximum proton conductivity of 5.99 × 10-4 S cm-1 at 50 °C and 98% relative humidity, and the proton conduction behavior follows the Grotthuss mechanism. Therefore, the Tb-MOF is a potential bifunctional material with fluorescence sensing of Hg2+ and proton conduction.

Directed Regulation of Proton Transport Pathways in MOF-808

The directed regulation of proton transport pathways in proton conductors, facilitated by the well-defined crystal structures of metal-organic frameworks (MOFs), is important for the development of advanced materials. In this study, MOF-808-2.5SO4-His is synthesized by progressive directed modification on the framework using functional molecules. The incorporation of sulfate and imidazole groups into MOF-808-2.5SO4-His results in a high proton conductivity of 1.37 × 10-2 S cm-1 at 70 °C and 98% relative humidity (RH). The analysis of temperature-dependent proton conductivity indicates that MOF-808-2.5SO4-His facilitates proton transport through the Grotthuss mechanism at 98% RH and the temperature range of 30-70 °C. Additionally, MOF-808-2.5SO4-His exhibits good cycling stability and durability in performance. This feasible approach enhances the comprehension of proton transfer mechanism and promotes the development of viable strategies for controllable construction of proton conductors.

Fabricating MOF-GO Composites by Modulating Graphene Oxide Content to Achieve Superprotonic Conductivity

Metal-organic frameworks (MOFs) have emerged as crucial materials for proton conductivity, especially in the context of the growing need for alternative energy sources. Enhancing the proton conductivity of MOFs has been a major focus with one effective approach involving the integration of MOFs with graphene oxide (GO) to form composite materials. In this study, Cr-MIL-101 MOF is selected, and its growth on GO sheets has been achieved through in situ crystallization, leading to the formation of MOF-GO composites with varying GO content, MIL-101/GO(x%), (x = 1%, 2%, and 5%). The oxygen functional groups on the 2D-GO layer e.g., carboxyl, hydroxyl, and epoxy groups improve both the acidity and hydrophilicity of the composite, which directly contributes to improved proton conductivity. All the composites, fabricated in this work, exhibit higher conductivity than that of the parent MOF due to the additional acidic functional groups introduced by GO. Among the different composites, the MIL-101/GO(2%) composite exhibits the highest proton conductivity, achieving superprotonic conductivity value of 0.105 S cm-1 at 80 °C and 98% relative humidity (RH). These results highlight the potential of MOF-GO composites for their application as nanofillers in proton exchange membranes for proton exchange membrane fuel cells (PEMFCs) and other energy-related technologies.

Heteroatom-embedded Mellitic Triimido COFs for efficient proton conduction

Ionic covalent organic frameworks (iCOFs) are promising materials for energy storage devices due to their ionic functional groups, which facilitate ion transport, and their highly ordered pores of their frameworks, which provide ideal pathways for long-term ion transport under harsh electrochemical conditions. In this study, we attempted for the first time to synthesize an unprecedented iCOF using a heteroatom-embedded mellitic triimido COF framework that enables practical ion channels on the Angstrom scale. This iCOF was subsequently evaluated as an anhydrous proton-conducting material. The heterocyclic pyridine group of the 2,5-diaminopyridine (DAPy) linker plays an important role, not only as an AB stacking-inducing group but also as a proton acceptor that interacts with impregnated H3PO4. The resulting PA@MTI-DAPy-COF exhibited high proton conductivity of 3.68 × 10-2 S cm-1 at 150oC under anhydrous conditions. This work paves the way for constructing efficient proton-conducting channels by leveraging the stacking structure of COF skeletons.

Inherent Ultrahigh Proton Conductivity of Two Highly Stable COOH-Functionalized Hafnium-Based Metal-Organic Frameworks

Although there has been some recent interest in the proton conductivity (σ) of highly stable carboxyl metal-organic frameworks (MOFs) made of tetravalent metal ions, given their potential applications in fuel cells and electrochemical sensing, research on MOFs constructed by hafnium(IV) ions needs to be expanded significantly. Based on this, we used two common and easily prepared phenylpoly(carboxylic acid) ligands, 1,2,4-phenyltricarboxylic acid and 1,2,4,5-phenyltetracarboxylic acid, to react with hafnium tetrachloride, respectively, creating two porous hafnium(IV)-based MOFs, UiO-66-COOH-Hf (1) and UiO-66-(COOH)2-Hf (2), with the same structure as UiO-66-Hf but with different numbers of free carboxylic groups. A series of stability assays revealed that the two MOFs had excellent structural rigidity, including thermal and water stability. More crucially, alternating current impedance experiments demonstrate that the σ of the two MOFs varies positively with humidity and temperature, reaching up to 10-3 S·cm-1 (1: 2.83 × 10-3 S·cm-1 and 2: 4.35 × 10-3 S·cm-1) under the right conditions (98% relative humidity and 100 °C). The latter roughly doubles the proton conductivity of the former, which is due to the difference in the number of free carboxyl groups, as confirmed by the structural analysis and proton conduction mechanism investigation. The high intrinsic σ of the two MOFs lays a solid foundation for their future application and affords new inspiration for developing high-performance proton-conductive materials.

Metal Effect on the Proton Conduction of Three Isostructural Metal-Organic Frameworks and Pseudo-Capacitance Behavior of the Cobalt Analogue

Three isostructural transition metal-organic frameworks, [M(bta)0.5(bpt)(H2O)2]·2H2O (M = Co (1), Ni (2), Zn (3), H4bta = 1,2,4,5-benzenetetracarboxylic acid, bpt = 4-amino-3,5-bis(4-pyridyl)-1,2,4-triazole), were successfully constructed using different metal cations. These frameworks exhibit a three-dimensional network structure with multiple coordinated and lattice water molecules within the framework, contributing to high stability and a rich hydrogen-bond network. Proton conduction studies revealed that, at 333 K and 98% relative humidity, the proton conductivities (σ) of MOFs 1-3 reached 1.42 × 10-2, 1.02 × 10-2, and 6.82 × 10-3 S cm-1, respectively. Compared to the proton conductivity of the initial ligands, the σ values of the complexes increased by 2 orders of magnitude, with the activation energies decreasing from 0.36 to 0.18 eV for 1, 0.09 eV for 2, and 0.12 eV for 3. An in-depth analysis of the correlation between different metal centers and proton conduction performance indicated that the varying coordination abilities of the metal cations and the water absorption capacities of the frameworks might account for the differences in conductivity. Additionally, the potential of 1 as a supercapacitor electrode material was assessed. 1 exhibited a specific capacitance of 61.13 F g-1 at a current density of 0.5 A g-1, with a capacitance retention of 82.4% after 5000 cycles, making it a promising candidate for energy storage applications.

Superprotonic conductivity of ketoenamine covalent-organic frameworks grafted by imidazole-based units

The achievement of covalent organic frameworks (COFs) with high stability and exceptional proton conductivity is of tremendous practical importance and challenge. Given this, we hope to prepare the highly stable COFs carrying CN connectors and enhance their proton conductivity via a post-modification approach. Herein, one COF, TpTta, was successfully synthesized by employing 1,3,5-triformylphloroglucinol (Tp) and 4,4',4″-(1,3,5-triazine-2,4,6-triyl)-trianiline (Tta) as starting materials, which has a β-ketoenamine structure bearing a large amount of -NH groups and intramolecular H-bonds. TpTta was then post-modified by inserting imidazole (Im) and histamine (His) molecules, yielding the corresponding COFs, Im@TpTta and His@TpTta, respectively. As a result, their proton conductivities were surveyed under changeable temperatures (30-100 °C) and relative humidities (68-98 %), revealing a degree of temperature and humidity dependence. Impressively, under identical conditions, the optimum proton conductivities of the two post-modified COFs are 1.14 × 10-2 (Im@TpTta) and 3.45 × 10-3 S/cm (His@TpTta), which are significantly greater than that of the pristine COF, TpTta (2.57 × 10-5 S/cm). Finally, their proton conduction mechanisms were hypothesized based on the computed activation energy values, water vapor adsorption values, and structural properties of these COFs. Additionally, the excellent electrochemical stability of the produced COFs was expressed, as well as the prospective application value.

High H2O-Assisted Proton Conduction in One Highly Stable Sr(II)-Organic Framework Constructed by Tetrazole-Based Imidazole Dicarboxylic Acid

Solid electrolyte materials with high structural stability and excellent proton conductivity (σ) have long been a popular and challenging research topic in the fuel cell field. This problem can be addressed because of the crystalline metal-organic frameworks' (MOFs') high structural stability, adjustable framework composition, and dense H-bonded networks. Herein, one highly stable Sr(II) MOF, {[Sr(H2tmidc)2(H2O)3]·4H2O}n (1) (H3tmidc = 2-(1H-tetrazolium-1-methylene)-1H-imidazole-4,5-dicarboxylic acid) was successfully fabricated, which was structurally characterized by single-crystal X-ray diffraction and electrochemically examined by the AC impedance determination. The results demonstrated that the σ of the compound manifested a positive dependence on temperature and humidity, and the optimal proton conductivity is as high as 1.22 × 10-2 S/cm under 100 °C and 98% relative humidity, which is at the forefront of reported MOFs with ultrahigh σ. The analysis of the proton conduction mechanism reveals that numerous tetrazolium groups, carboxyl groups, coordination, and crystallization water molecules in the framework are responsible for the high efficiency of proton transport. This work offers a fresh perspective on how to create novel crystalline proton conductive materials.

A stable Mn(II) coordination polymer demonstrating proton conductivity and quantitative sensing of oxytetracycline in aquaculture

The construction and investigation of dual-functional coordination polymers (CPs) with proton conduction and luminescence sensing is of great significance in clean energy and agricultural monitoring fields. In this work, an Mn-based coordination polymer (Mn-CP), namely, [Mn0.5(HL)] (H2L = HOOCC6H4C6H4CH2PO(OH)OCH3), was hydrothermally synthesized. Mn-CP has a one-dimensional (1D) chain structure, in which uncoordinated -COOH groups can serve as potential sites for fluorescence sensing. Moreover, Mn-CP shows good water and pH stabilities, offering the feasibility for proton conduction and sensing applications. Mn-CP displays comparatively high proton conductivity of 1.07 × 10-4 S cm-1 at 368 K and 95% relative humidity (RH), which is promising for proton conduction materials. Moreover, it can serve as a repeatable, highly selective, and visualized fluorescence sensor for detecting oxytetracycline (OTC). More importantly, Mn-CP reveals an amazing quantitative sensing of OTC in actual samples such as seawater, aquaculture freshwater, soil infiltration solutions, and tap water. This work proves the excellent application potential of dual-functional CPs in the field of clean energy and environmental protection, especially for the fluorescence detection of antibiotics in aquaculture systems.

Icing on the Cake: Imidazole-Anchored Strategy To Enhance the Proton Conductivity of Two Isostructural Ce(IV)/Hf(IV) Metal-Organic Frameworks

In the field of proton conduction, the acquisition of crystalline metal-organic frameworks (MOFs) with high stability and ultrahigh proton conductivity has been of great research value and is worth continuous exploration. Here, we greenly synthesized a three-dimensional porous MOF (MOF-801-Ce) by using [(NH4)2Ce(NO3)6 and fumaric acid as starting materials and solvothermally synthesized Hf-UiO-66-NO2 by using HfCl4 and 2-nitroterephthalic acid as starting materials. A series of measurements have shown that both MOFs exhibit good water stability, acid-base stability, and thermal stability and demonstrate outstanding proton conductivity. At 100 °C and 98% relative humidity (RH), the proton conductivities (σ) could be 2.59 × 10-3 S·cm-1 for MOF-801-Ce and 0.89 × 10-3 S·cm-1 for Hf-UiO-66-NO2. To pursue higher proton conductivity, we further adopted the evaporation approach to encapsulate imidazole molecules in the pores of the two compounds, achieving the imidazole-encapsulated MOFs, Im@MOF-801-Ce and Im@Hf-UiO-66-NO2. As expected, their σ values were significantly boosted by almost an order of magnitude up to 10-2 S·cm-1. Finally, their proton-conductive mechanisms were explored in light of the structural information, gas adsorption/desorption, and other tests. The outstanding structural stability of these MOFs and their durability of the proton conduction capability manifested that they have great promise in electrochemical fields.

Ultrahigh Proton Conductivities of Postmodified Hf(IV) Metal-Organic Frameworks and Related Chitosan-Based Composite Membranes

Recently, researchers have focused on preparing and studying proton exchange membranes. Metal-organic frameworks (MOFs) are candidates for composite membrane fillers due to their high crystallinity and structural characteristics, and Hf-based MOFs have attracted our attention with their high porosity and high stability. Therefore, in this study, Hf-based MOFs were doped into a cost-effective chitosan matrix as fillers to fabricate composite films having excellent proton conductivity (σ). First, the nanoscale MOFs Hf-UiO-66-(OH)₂ (1) and Hf-UiO-66-NH₂ (2) were chemically modified by a ligand design strategy to obtain SA-1 and CBD-2 bearing free -COOH units. The proton conductivities of SA-1 and CBD-2 under optimal test conditions reached 1.23 × 10^-2 and 0.71 × 10^-2 S cm^-1. After that, we prepared composite membranes CS/SA-1 and CS/CBD-2 by the casting method; tests revealed that the introduction of MOFs improved the stabilities and σ values of the membranes, and their best σ could reach above 10^-2 S cm^-1 under 100 °C/98% RH. Further structural characterization and activation energy calculation revealed the conductive mechanism of the composite films. This investigation not only proposes a novel chemical modification method for optimizing the σ of MOFs but also promotes the development of MOF-doped composite membranes and provides a basis for future applications of MOFs in fuel cells.

Syntheses and High Proton Conductivities of Two 3D Zr(IV)/Hf(IV)-MOFs from Furandicarboxylic Acid

With the gradual progress of research on proton-conducting metal-organic framework (MOFs), it has become a challenging task to find MOF materials that are easy to prepare and have low toxicity, high stability, and splendid proton conductivity. With the abovementioned objectives in mind, we selected the non-toxic organic ligand 2,5-furandicarboxylic acid and the low toxic quadrivalent metals zirconium(IV) or hafnium(IV) as starting materials and successfully obtained 2 three-dimensional porous MOFs, [M6O4(OH)4(FDC)4(OH)4(H2O)4] [M = ZrIV (1) and HfIV (2)], with ultrahigh water stability using a rapid and green synthesis approach. Their proton conductive ability is remarkable, thanks to the large number of Lewis acidic sites contained in their porous frameworks and the abundant H-bonding network, hydroxyl groups, as well as coordination and crystalline water molecules. The positive correlation of their proton conductivity with relative humidity (RH) and the temperature was observed. Notably, their optimized proton conductivities are 2.80 × 10-3 S·cm-1 of 1 and 3.38 × 10-3 S·cm-1 of 2 under 100 °C/98% RH, which are at the forefront of Zr(IV)/Hf(IV) MOFs with prominent proton conductivity. Logically, their framework features, nitrogen/water adsorption/desorption data, and activation energy values are integrated to deduce their proton conductivity and conducting mechanism differences.

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.

Proton Transfer Dynamics-Mediated CO2 Electroreduction

Electrochemical CO₂ reduction reaction (CO₂ RR) is crucial to addressing environmental crises and producing chemicals. Proton activation and transfer are essential in CO₂ RR. To date, few research reviews have focused on this process and its effect on catalytic performance. Recent studies have demonstrated ways to improve CO₂ RR by regulating proton transfer dynamics. This Concept highlights the use of regulating proton transfer dynamics to enhance CO₂ RR for the target product and discusses modulation strategies for proton transfer dynamics and operative mechanisms in typical systems, including single-atom catalysts, molecular catalysts, metal heterointerfaces, and organic-ligand modified metal catalysts. Characterization methods for proton transfer dynamics during CO₂ RR are also discussed, providing powerful tools for the hydrogen-involving electrochemical study. This Concept offers new insights into the CO₂ RR mechanism and guides the design of efficient CO₂ RR systems.

A porous Ti-based metal-organic framework for CO2 photoreduction and imidazole-dependent anhydrous proton conduction

The anhydrous proton conductivity of Im@IEF-11 resulting from the integration of imidazole and porous IEF-11 has been investigated, and the highest proton conductive value can reach up to 7.64 × 10^-2 S cm^-1. Furthermore, IEF-11 is also developed to reduce CO₂ due to its reasonable structure and suitable energy band, and its CO formation rate is 31.86 μmol g^-1 h^-1.

New Type of Nanocomposite CsH2PO4-UiO-66 Electrolyte with High Proton Conductivity

New (1−x)CsH2PO4−xUiO-66 electrolytes with high proton conductivity and thermal stability at 230−250 °C were developed. The phase composition and proton conductivity of nanocomposites (x = 0−0.15) were investigated in detail. As shown, the UiO-66 matrix is thermally and chemically suitable for creating composites based on CsH2PO4. The CsH2PO4 crystal structure remains, but the degree of salt crystallinity changes in nanocomposites. As a result of interface interaction, dispersion, and partial salt amorphization, the proton conductivity of the composite increases by two orders of magnitude in the low-temperature range (up to 200 °C), depending on the UiO-66 fraction, and goes through a maximum. At higher temperatures, up to 250 °C, the conductivity of nanocomposites is close to the superprotonic values of the original salt at low UiO-66 values; then, it decreases linearly within one order of magnitude and drops sharply at x > 0.07. The stability of CsH2PO4-UiO-66 composites with high proton conductivity was shown. This creates prospects for their use as proton membranes in electrochemical devices.

Comparative Studies on the Proton Conductivities of Hafnium-Based Metal-Organic Frameworks and Related Chitosan or Nafion Composite Membranes

Hafnium (Hf)-based UiO-66 series metal-organic frameworks (MOFs) have been widely studied on gas storage, gas separation, reduction reaction, and other aspects since they were first prepared in 2012, but there are few studies on proton conductivity. In this work, one Hf-based MOF, Hf-UiO-66-fum showing UiO-66 structure, also known as MOF-801-Hf, was synthesized at room temperature using cheap fumaric acid as the bridging ligand, and then imidazole units were successfully introduced into MOF-801-Hf to obatin a doped product, Im@MOF-801-Hf. Note that both MOF-801-Hf and Im@MOF-801-Hf demonstrate excellent thermal, water, and acid-base stabilities. Expectedly, the maximum proton conductivity (σ) of Im@MOF-801-Hf (1.46 × 10^-2 S·cm^-1) is nearly 4 times greater than that of MOF-801-Hf (3.98 × 10^-3 S·cm^-1) under 100 °C and 98% relative humidity (RH). To explore their possible practical application value, we doped them into chitosan (CS) or Nafion membranes as fillers, namely, CS/MOF-801-Hf-X, CS/Im@MOF-801-Hf-Y, and Nafion/MOF-801-Hf-Z (X, Y, and Z are the doping percentages of MOF in the membrane, respectively). Intriguingly, it was found that CS/MOF-801-Hf-6 and CS/Im@MOF-801-Hf-4 indicated the highest σ values of 1.73 × 10^-2 and 2.14 × 10^-2 S·cm^-1, respectively, under 100 °C and 98% RH and Nafion/MOF-801-Hf-9 also revealed a high σ value of 4.87 × 10^-2 S·cm^-1 under 80 °C and 98% RH, which showed varying degrees of enhancement compared to the original MOFs or pure CS and Nafion membranes. Our study illustrates that these Hf-based MOFs and related composite membranes offer great potential in electrochemical fields.

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.

The role of sulfonate groups and hydrogen bonding in the proton conductivity of two coordination networks

The proton conductivity of two coordination networks, [Mg(H₂O)₂(H₃L)]·H₂O and [Pb₂(HL)]·H₂O (H₅L = (H₂O₃PCH₂)₂-NCH₂-C₆H₄-SO₃H), is investigated by AC impedance spectroscopy. Both materials contain the same phosphonato-sulfonate linker molecule, but have clearly different crystal structures, which has a strong effect on proton conductivity. In the Mg-based coordination network, dangling sulfonate groups are part of an extended hydrogen bonding network, facilitating a "proton hopping" with low activation energy; the material shows a moderate proton conductivity. In the Pb-based metal-organic framework, in contrast, no extended hydrogen bonding occurs, as the sulfonate groups coordinate to Pb²⁺, without forming hydrogen bonds; the proton conductivity is much lower in this material.

Metal@COFs Possess High Proton Conductivity with Mixed Conducting Mechanisms

The inherent porous structures and aligned functional units inside the skeleton of covalent organic frameworks (COFs) provide an extraordinary promise for post-modification and deservedly expand their application in the field of proton conduction. Herein, we tactfully introduced copper ions into a two-dimensional COF (TpTta) furnished with ample N,O-chelating sites by a post-modification strategy to achieve two copper(II)-modified products, namely, CuCl₂@TpTta-3 and CuCl₂@TpTta-10. Inspiringly, the two modified COFs demonstrated the higher conductivities of 1.77 × 10^-3 and 8.81 × 10^-3 S cm^-1 under 100 °C and 98% relative humidity, respectively, among the previously reported COFs with higher σ values. In comparison to the pristine COFs, the σ values of CuCl₂@TpTta-3 and CuCl₂@TpTta-10 are boosted by 2 orders of magnitude. On the basis of structural analyses, nitrogen and water vapor adsorption tests, and proton conduction mechanism analysis, we deeply analyzed the reason why the conductivity of the modified COFs was significantly increased. To the best of our knowledge, it is the first time to employ the CuCl₂-modified strategy to boost the conductivity of COFs, which offers a wise idea for the fabrication of highly conductive materials in the field of fuel cells.

High Protonic Conductivity of Three Highly Stable Nanoscale Hafnium(IV) Metal-Organic Frameworks and Their Imidazole-Loaded Products

Attracted by the exceptional structural rigidity and inherent porous structures of the Hf-based metal-organic frameworks (MOFs), we adopted a rapid synthesis approach to preparing three nanoscale MOFs, Hf-UiO-66 (1), Hf-UiO-66-(OH)₂ (2), and Hf-UiO-66-NH₂ (3), and systematically explored the water-assisted proton conductivities of the original ones and the post-modified products. Interestingly, the proton conductivities (σ) of all three MOFs exhibit significant temperature and humidity dependence. At 98% RH and 100 °C, their optimal σ values can reach up to 10^-3 S·cm^-1. Consequently, imidazole units are loaded into 1-3 to obtain related MOFs, Im@1, Im@2, and Im@3, and the σ values of the imidazole-loaded products are boosted to 10^-2 S·cm^-1. Note that these modifications not only do not change the frameworks of the pristine MOFs but also do not affect their high chemical and water stability. The proton-conductive mechanisms of these MOFs before and after modification have been thoroughly discussed based on structural analyses, N₂ and H₂O vapor adsorptions, and activation energy values. The excellent structural stability as well as the durability and stability of their proton conduction ability indicate that these MOFs can be used in the field of fuel cells and so on.

Proton Conductive Lanthanide-Based Metal-Organic Frameworks: Synthesis Strategies, Structural Features, and Recent Progress

In the fields of proton exchange membrane fuel cells as well as impedance recognition, molecular sieve, and biochemistry, the development of proton conductive materials is essential. The design and preparation of the next generation of proton conductive materials-crystalline metal-organic framework (MOF) materials with high proton conductivity and excellent water stability-are facing great challenges. Due to the large radius and high positive charge of lanthanides, they often interact with organic ligands to exhibit high coordination numbers and flexible coordination configurations, resulting in the higher stability of lanthanide-based MOFs (Ln-MOFs) than their transition metal analogues, especially regarding water stability. Therefore, Ln-MOFs have attracted considerable attention. This review offers a view of the latest progress of proton conductive Ln-MOFs, including synthesis strategy, structural characteristics, and advantages, proton conductivity, proton conductive mechanism, and applications. More importantly, by discussing structure-property relationships, we searched for and analyzed design techniques and directions of development of Ln-MOFs in the future. The latest progress of synthesis strategy, structural characteristics, proton conductive properties and mechanism and applications on Ln-MOFs. Ln-MOFS Lanthanide-based MOFs, MOF metal-organic framework, PEMFC proton exchange membrane fuel cells.

Hydrated Metal Ions as Weak Bronsted Acids Show the Promoting Effects in Proton Conduction

It is well-known that the hydrated metal ions can act as Bronsted acids, which tend to donate protons increasing the acidic proton concentration in materials, as well as the proton...

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.

Control of Proton-Conductive Behavior with Nanoenvironment within Metal-Organic Materials

Solid-state proton-conductive materials have been of great interest for several decades due to their promising application as electrolytes in fuel cells and electrochemical devices. Metal-organic materials (MOMs) have recently been intensively investigated as a new type of proton-conductive materials. The highly crystalline nature and structural designability of MOMs make them advantageous over conventional noncrystalline proton-conductive materials-the detailed investigation of the structure-property relationship is feasible on MOM-based proton conductors. This review aims to summarize and examine the fundamental principles and various design strategies on proton-conductive MOMs, and shed light on the nanoconfinement effects as well as the importance of hydrophobicity on specific occasions, which have been often disregarded. Besides, challenges and future prospects on this field are presented.

Functionalized Metal-Organic Frameworks for Hg(II) and Cd(II) Capture: Progresses and Challenges

Mercury and cadmium are deemed to be the most harmful heavy metal ions for elimination due to their persistent bio-accumulative and bio-expanding toxic effects. Although many technologies have been developed for capturing Hg(II) and Cd(II) ions from aqueous solution, developing efficient and practical capature technology remains a big challenge. Metal-organic frameworks (MOFs) have been considered as the most promising adsorbents for Hg(II) and Cd(II) removal due to their high porosity and easy functionalization, and various of MOF-based adsorbents based on different synthetic strategies have been prepared and studied. In this article, the progresses of MOF-based absorbents for Hg(II) and Cd(II) capture are described according to the synthetic strategies and the types of functional groups, and the comparison and practical analysis of various adsorbents are also presented.

Dynamics studied by Quasielastic Neutron Scattering (QENS)

Quasielastic neutron scattering (QENS) allows measurement of the molecular displacements in time and space, from pico- to tens of nanoseconds and from Ångstroms to nanometers, respectively. The method probes dynamics from fast vibrational modes down to slow diffusive motion. Every scattering experiment leads to a dynamic structure factor $$S\left( {\vec Q,\omega } \right)$$ S Q → , ω or its spatial and temporal Fourier transform (van Hove correlation function $$G\left( {\vec r,t} \right)$$ G r → , t ). This shows exactly where the atoms are and how they move. In this manuscript the basics of the QENS method are presented and a few examples highlighting the potentials of QENS are given: (i) diffusion of liquids and gases in nano- and mesoporous materials; (ii) hydrogen dynamics in a high temperature polymer electrolyte fuel cell (HT-PEFC) and (iii) influence of the surface interactions on polymer dynamics in nanopores.

Improving proton conduction of the Prussian blue analogue Cu3[Co(CN)6]2·nH2O at low humidity by forming hydrogel composites

Composites of Prussian blue analogue (PBA) adsorbed imidazole-acetic acid with polyvinyl alcohol hydrogel show excellent water-retention capacity and fast proton conduction at 25% RH in 298–353 K, herein X is the mass ratio of PBA to hydrogel.