Designing UiO-66-Based Superprotonic Conductor with the Highest Metal-Organic Framework Based Proton Conductivity

Proton Conduction in Zirconium-Based Metal-Organic Frameworks for Advanced Applications

Zirconium-based metal-organic frameworks (Zr-MOFs) have emerged as a promising class of crystalline porous materials, attracting significant interest in the field of proton conduction due to their exceptional chemical stability, structural flexibility, and functional tunability. Notably, proton-conducting Zr-MOFs show immense potential for diverse advanced technological applications. In this Spotlight on Applications paper, we provide an overview of proton-conducting Zr-MOFs and spotlight the recent progress of their utilization as proton exchange membranes in proton exchange membrane fuel cells (PEMFCs), light-responsive systems for proton pumps, and chemical sensors for formic acid detection. Furthermore, we also discussed the challenges, future prospects, and opportunities for promoting the application of proton-conducting Zr-MOFs.

High Proton Conductivity Enhancement Obtained by a Covalent Postsynthesis Modification Approach for Two Metal-Organic Frameworks

This study demonstrates an effective strategy to enhance proton conductivity by synthesizing 2 three-dimensional metal-organic frameworks (MOFs), [Zn(DTD22)]n (MOF 1) and [Cd2(DTD22)2]n (MOF 2), (DTD22 = 4,4″-diamino-[1,1':4',1″-terphenyl]-2,2″-dicarboxylic acid). The DTD22 ligand used formed a continuous hydrogen-bonding network in the structure, constructing excellent hydrophilic channels. MOF 1 and MOF 2 were further postsynthesized and modified (PSM) by Schiff base reaction, and 4-chloro-3-formylbenzenesulfonic acid ligands containing -SO3H and -Cl were successfully introduced into the framework to form PSM-MOF 1 and PSM-MOF 2. Experiments showed that this modification significantly enhanced the proton conductivity of the materials, especially at 90 °C and 98% RH: PSM-MOF 1 (2.38 × 10-1 S·cm-1) and PSM-MOF 2 (3.50 × 10-1 S·cm-1). In comparison, the conductivities of unmodified MOF 1 and MOF 2 were 8.55 × 10-2 S·cm-1 and 9.50 × 10-5 S·cm-1, respectively. The present study demonstrates that the proton conductivity of MOFs can be effectively enhanced by the covalent postmodification method, which provides a new idea for the application of MOFs.

Magnetic metal-organic frameworks-based ratiometric SERS aptasensor for sensitive detection of patulin in apples

Patulin (PAT), a major contaminant in apples, poses a considerable food safety risk, necessitating a rapid, sensitive and reliable detection method. This study developed a novel magnetic metal-organic frameworks (MOFs)-based ratiometric surface-enhanced Raman scattering (SERS) aptasensor. The sensor consists of magnetic MOFs loaded with 4-Mercaptobenzoic acid (4-MBA) internal labeled AuMBA@Ag as the SERS substrate, and gold nanorods (AuNRs) modified with Rhodamine 6G and aptamers as capture probes. This strategy enhances sensitivity through magnetic separation and abundant SERS hotspots provided by the magnetic MOF nanocomposites. The SERS intensity ratio showed a negative correlation with PAT concentrations from 0.01 to 100 ng/mL, with a LOD of 0.0465 ng/mL. Moreover, the aptasensor achieved 95.90 % ∼ 105.83 % recoveries in apple samples, indicating high accuracy and anti-interference capability. The excellent sensing performance demonstrates great potential of the SERS nanosensor for mycotoxin detection in actual food matrices.

Proton-conducting copper-based MOFs for fuel cells

Metal-organic frameworks (MOFs) are emerging as promising alternatives for proton-conductive materials due to their high porosity, large surface area, stability, and relatively low cost. Among these, copper-based MOFs (Cu-MOFs) stand out with unique advantages, including open metal sites, variable valence states, and strongly electrophilic Cu centers. In this review, we discuss recent advances and developments in the use of Cu-MOFs as proton-conductive materials, with a particular focus on their application as proton exchange membranes (PEMs). We introduce the most common strategies employed to date and review the key features that have contributed to the construction of efficient proton transport pathways in Cu-MOFs. Additionally, we review PEMs fabricated via direct thin-film deposition or as mixed-matrix membranes (MMMs) incorporating Cu-MOF fillers. Finally, we address the challenges that must be overcome in the coming years to develop more robust Cu-MOFs and to create more efficient thin films and Cu-MOF-based MMMs.

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.

A natural pigment-based nanosized colorimetric sensor for freshness evaluation of aquatic products

This work proposes an innovative method for monitoring the freshness of aquatic products using a nanosized colorimetric sensor (NCS). Six porous organic frameworks (POFs) were utilised to modify and sensitize four food-friendly natural pigments. Four optimal nano-pigments were selected based on their RGB-responsive interaction with NH3. Meanwhile, pigments exhibited significant changes in Vis-NIR spectra after nanosizing. Results indicated that the optimal NCS performs well in predicting aquatic products' total volatile alkaline nitrogen (TVB-N) content. While the prediction model based on image data did not benefit from nanosizing, the model constructed from spectral data demonstrated high accuracy and stability. The best PLS model achieved a prediction set correlation coefficient (Rp) of 0.9891 and a residual prediction deviation value of 6.66 by combining with variable combination population analysis-iteratively retaining informative variables (VCPA-IRIV) algorithm. Thus, the nanofabrication of POFs on pigments shows promise for developing high-precision and stable freshness prediction models.

Giant {Mo132} polyoxometalate isolated with diverse organic cations: a systematic proton conductivity study

The development of efficient and stable proton conductors is a pivotal area of research due to their transformative potential in alternative energy technologies. Recently, there has been a surge of interest in synthesizing proton conductors based on polyoxometalate (POM) materials, attributed to their highly negatively charged and oxygen-rich surfaces. In this study, we report on a highly water-soluble giant POM, (NH4)42[Mo132O372(CH3COO)30(H2O)72]·ca.300H2O·ca.10CH3COONH4 (designated as {Mo132}), which was rendered insoluble in water by exchanging its ammonium cations with larger organic cations, specifically histidinium, pyridinium, bipyridinium, and methyl viologen, resulting in His-Mo132, Py-Mo132, Bpy-Mo132 and MV-Mo132, respectively. These ion-exchanged compounds were thoroughly characterized through comprehensive spectral analyses, elemental analyses and microscopic studies. The substitution with organic cations containing nitrogen centres not only rendered {Mo132} insoluble, but also increased the number of proton hopping sites, thereby enhancing proton transport. Consequently, His-Mo132, Py-Mo132, Bpy-Mo132 and MV-Mo132 demonstrated impressive proton conductivity. Among these, Py-Mo132 stood out with a proton conductivity of 1.07 × 10-2 S cm-1 under 98% relative humidity at 80 °C. All four compounds exhibited proton conduction predominantly via the Grotthuss mechanism. Furthermore, stability assessments of these Mo132-based proton conductors were conducted under operational conditions to evaluate their performance in practical applications.

Engineering Metal-MOF Interfaces for Selective CO₂ Hydrogenation to Methanol

The hydrogenation of CO₂ to methanol is a promising pathway toward sustainable fuel production and carbon recycling. A key factor in the efficiency of this process lies in the interaction between the metal catalyst and its support. Metal-Organic Frameworks (MOFs) have emerged as highly effective platforms due to their tunable structures, large surface areas, and ability to form stable interfaces with single-atom metals or metal nanoparticles. These metal-MOF interfaces are crucial for stabilizing active sites, preventing sintering, and enhancing catalytic performance. In this concept paper, we explore the role of these interfaces in promoting CO₂ hydrogenation, focusing on Cu-Zn, Cu-Zr, and Zn-Zr interfaces. The formation of strong interactions between metal sites and MOF nodes enables precise control over the dispersion and electronic environment of the active species, significantly improving methanol selectivity and long-term stability. By analyzing recent advancements in MOF-supported catalysts, this work highlights the concept of engineered metal-MOF interfaces to drive the development of next-generation catalysts for efficient methanol synthesis from CO₂.

Superprotonic Conductivity by Synergistic Blending of Coordination Polymers with Organic Polymers: Fabrication of Durable and Flexible Proton Exchange Membranes

Creation of an efficient and cost-effective proton exchange membrane (PEM) has emerged as a propitious solution to address the challenges of renewable energy development. Coordination polymers (CPs) have garnered significant interest due to their multifunctional applications and moldability, along with long-range order. To leverage the potential of CPs in fuel cells, it is essential to integrate microcrystalline CPs into organic polymers to prepare membranes and avoid grain boundary issues. In this study, we designed and synthesized CPs containing imidazole and sulfonate moieties via gel-to-crystal transformation. The integration of CPs into the PVDF-PVP matrix resulted in superprotonic conductivity in the order of 10-2 S cm-1 at room temperature (30 °C) and 98 % RH. The proton conductivity achieved with CP-integrated composite membrane was 4.69×10-2 S cm-1 at 80 °C and 98 % RH, the highest among all CP/MOF-integrated PVDF-PVP membranes under hydrous conditions. The excellent compatibility of CPs with PVDF-PVP produced highly flexible membranes with superior mechanical, chemical, and thermal stability. About 25 times higher proton conductivity value was achieved with membrane, compared to intrinsic CPs, at RT and 98 % RH. Thus, we present a cost-effective CP-integrated mixed-matrix membrane with superprotonic conductivity and long-term durability for cutting-edge fuel cell development.

Uncoordinated Carboxyl Groups as Proton Sources in Polyoxometalate-Based Metal-Organic Frameworks Enhance Proton Conduction

To select appropriate organic ligands is an effective strategy to enhance the proton conductivities of polyoxometalate-based metal-organic frameworks (POMOFs). Two new Dawson-type POMOFs, named CUST-961 and CUST-962, have been designed and synthesized via combining Htzbc selected by hard and soft acid and base theory and density functional theory calculation, transition metal ions, alkali metal ions (Na+ and K+), and Dawson-type polyoxometalates ([P2W18]6-) under the hydrothermal method. Their stabilities under different temperatures and relative humidities (RHs) have been investigated through powder X-ray diffraction and thermogravimetric analysis. Both CUST-961 and CUST-962 exhibited excellent aqueous and thermal stabilities. The alternating current (AC) impedance spectrum tests revealed that the proton conductivity of CUST-961 could reach 1.4 × 10-4 S cm-1 at 95 °C and 98% RH, which is about 3 times that of CUST-962. The different proton conductivities between the two compounds are due to the fact that CUST-961 possesses more uncoordinated carboxylic acid groups, as confirmed by attenuated total reflection infrared spectroscopy and 1H solid-state nuclear magnetic resonance spectroscopy, which can not only act as the proton source but also establish a richer hydrogen bonding network to enhance proton conduction. This work provides a new strategy and insight for the design and preparation of polyoxometalate-based proton conductive materials.

Close-packing effect of water clusters within metal-organic framework pores on proton conductivity: a dielectric relaxation phenomenon in loose space and colossal dielectric permittivity

Proton-conducting metal-organic frameworks (MOFs) have attracted tremendous attention for their promising application in proton-exchange membrane fuel cells. Water clusters play an extremely important role in the proton-conduction process and affect the proton conductivity of host materials. To date, the close-packing effect of water clusters within pores on proton conductivity due to the amorphous structure of commercial proton-exchange membranes is unclear. Herein, we prepared two crystalline MOFs containing different water clusters, namely, [Sm2(fum)3(H2O)4]·3H2O (Sm-fum-7H2O) and [Er2(fum)3(H2O)4]·8H2O (Er-fum-12H2O) (H2fum = fumaric acid), and regulated their proton conductivities by changing the water clusters. As expected, Sm-fum-7H2O showed a high proton conductivity of 6.89 × 10-4 S cm-1 at 333 K and ∼97% RH because of the close packing of the water clusters within the pores triggered by a lanthanide contraction effect, outperforming that of Er-fum-12H2O and some previously reported MOFs. Additionally, Sm-fum-7H2O and Er-fum-12H2O demonstrated high dielectric functions, reaching 2.22 × 103 and 1.42 × 105 at 102.5 Hz, respectively, making Er-fum-12H2O a highly dielectric material. More importantly, broadband dielectric spectroscopy measurements indicated that there was a dielectric relaxation process in Er-fum-12H2O with an activation energy of 0.59 eV. The present findings provide a better understanding of the crucial role of confined water clusters in proton conductivity and the novel phenomenon of the coexistence of proton conduction and dielectric relaxation in crystalline MOF materials.

Bottom-up synthesis of ion-pair-bridged metal-organic framework for H+ conduction

As a novel conceptual synthesis of a metal-organic framework (MOF)-based proton conductor, UiO-66 based on a pyridinedicarboxylic acid phosphate (PyDC-PA) ion pair linker has been developed, in which the phosphoric acid is fixed to the N donor moiety of pyridine via an ionic bond.

Hexagonal Mo Bronze: Single Crystal Structures, Electrocatalytic Hydrogen Evolution, and Proton Conductivity

Molybdenum trioxide (MoO3) is a well-known transition metal oxide that has drawn much attention as a functional material having numerous applications. However, a vast majority of studies have primarily focused on α-MoO3, the thermodynamically stable polymorph of MoO3. This present work encompasses the synthesis of single crystals of two metastable hexagonal MoO3 described by the formulas {Mn0.03Na0.01}@[Mo0.93VIMo0.07VO3] (1) and {Cu0.01Na0.01}@[Mo0.97VIMo0.03VO3] (2), their comprehensive structural characterization by single-crystal X-ray crystallography, and routine spectral and microscopic studies. Interestingly, compound 1 acts as an efficient electrocatalyst for the hydrogen evolution reaction (HER) as well as an effective proton conductor in comparison to the performance of compound 2. The HER activity of compound 1 is characterized by an overpotential of 340 mV@1 mA cm-2 and a low Tafel slope of 75 mV/decade. The catalyst (compound 1) displays a Faradaic efficiency of 88% with a turnover frequency of 2.9 s-1. The proton conductivity value of this compound (1) is determined to be 4.9 × 10-3 S cm-1 at 55 °C under 98% relative humidity; the relevant proton conduction is operated by the Grotthuss mechanism with an activation energy of 0.17 eV.

Room-Temperature Superprotonic Conductivity beyond 10-1 S cm-1 in a Co(II) Coordination Polymer

An efficient design of crystalline solid-state proton conductors (SSPCs) is crucial for the progress of clean energy applications. Developing such materials to make them work at room temperature with a conductivity of ≥10-1 S cm-1 is of significant interest in terms of technical and commercial aspects. Utilizing the recently highlighted "coordinated-water-driven proton conduction" approach, herein, we have rationally synthesized two highly stable and scalable 1D Co(II) coordination polymers (CPs) as SSPCs, PCM-2 {[Co(bpy)(H2O)2(NO3)2]·H2O}n and PCM-3 {[Co2(bpy)2(SO4)2(H2O)6].4H2O}n, with distinct alignments in coordinated water and coordinated oxo-anions (nitrate and sulfate, respectively). The acidity of the metal-bound water molecules in PCM-2 is further enhanced through cooperative long-range continuous H bonds with coordinated Brønsted basic nitrates (proton acceptors), leading to ultrahigh superprotonic conductivities even at 25 °C (1.03 × 10-1 S cm-1 under 95% RH), and reached a maximum of 2.99 × 10-1 S cm-1 at 85 °C (95% RH). The conductivity at 25 °C is even higher than that of commercial Nafion 117 (6.74 × 10-2 S cm-1 at 100% RH). The absence of such an H-bonding interaction in PCM-3 (closed loops) resulted in a lesser conductivity of 5.87 × 10-5 S cm-1 (95% RH, 85 °C). PCM-2 represents the first example of SSPC exhibiting conductivity in the order 10-1 S cm-1 at ambient temperature (25 °C) with excellent recyclability.

Synthesis and proton-conductive behaviour of two MOFs with covalently bonded imidazoles in the channels

Immobilization of imidazole molecules as proton carriers into MOFs to facilitate proton conduction is a general strategy for developing high proton conductive materials. Herein, we designed two imidazole substituted phthalic acid ligands and constructed two novel MOFs, {[Zr6(OH)16(H3L1)4]Cl8·20H2O}n [Zr-MOF; H3L1 = 2-(1H-imidazol-4-yl) methylaminoterephthalic acid] and {Gd(HCOO)(H2L2)2}n [Gd-MOF; H3L2 = 5-(1H-imidazol-4-yl)methylaminoisophthalic acid] and fully studied their porous nature, stability and water-assisted proton conduction. The resulting Zr-MOF exhibits a high proton conductivity of 1.82 × 10-2 S cm-1 at 98% RH and 80 °C, while Gd-MOF has a proton conductivity of 3.01 × 10-3 S cm-1 at 98% RH and 60 °C.

Thermodynamically Stable Functionalization of Microporous Aromatic Frameworks with Sulfonic Acid Groups by Inserting Methylene Spacers

Porous aromatic frameworks (PAFs) are an auspicious class of materials that allow for the introduction of sulfonic acid groups at the aromatic core units by post-synthetic modification. This makes PAFs promising for proton-exchange materials. However, the limited thermal stability of sulfonic acid groups attached to aromatic cores prevents high-temperature applications. Here, we present a framework based on PAF-303 where the acid groups were added as methylene sulfonic acid side chains in a two-step post-synthetic route (SMPAF-303) via the intermediate chloromethylene PAF (ClMPAF-303). Elemental analysis, NMR spectroscopy, electrochemical impedance spectroscopy and X-ray photoelectron spectroscopy were used to characterize both frameworks and corroborate the successful attachment of the side chains. The resulting framework SMPAF-303 features high thermal stability and an ion-exchange capacity of about 1.7 mequiv g-1. The proton conductivity depends strongly on the adsorbed water level. It reaches from about 10-7 S cm-1 for 33% RH to about 10-1 S cm-1 for 100% RH. We attribute the strong change to a locally alternating polarity of the inner surfaces. The latter introduces bottleneck effects for the water molecule and oxonium ion diffusion at lower relative humidities, due to electrolyte clustering. When the pores are completely filled with water, these bottlenecks vanish, leading to an unhindered electrolyte diffusion through the framework, explaining the conductivity rise.

Construction of Hydration Layer for Proton Transport by Implanting the Hydrophilic Center Ag0 in Nickel Metal-Organic Frameworks

The directional arrangement of H2O molecules can effectively regulate the ordered protons transfer to improve transport efficiency, which can be controlled by the interaction between materials and H2O. Herein, a strategy to build a stable hydration layer in metal-organic framework (MOF) platforms, in which hydrophilic centers that can manipulate H2O molecules are implanted into MOF cavities is presented. The rigid grid-Ni-MOF is selected as the supporting material due to the uniformly distributed cavities and rigid structures. The Ag0 possesses potential combination ability with the hydrophilic substances, so it is introduced into the MOF as hydration layer centers. Relying on the strong interaction between Ag0 and H2O, the H2O molecules can rearrange around Ag0 in the cavity, which is intuitively verified by DFT calculation and molecular dynamics simulation. The establishment of a hydration layer in Ag@Ni-MOF regulates the chemical properties of the material and gives the material excellent proton conduction performance, with a proton conductivity of 4.86 × 10-2 S cm-1.

Post-synthetic modified luminescent metal-organic framework for the detection of berberine hydrochloride in a traditional Chinese herb

In this work, a novel fluorescence sensor UiO-66-PSM based on post-synthetic modified metal-organic frameworks was prepared for the detection of berberine hydrochloride (BBH) in the traditional Chinese herb Coptis. UiO-66-PSM was synthesized by a simple Schiff base reaction with UiO-66-NH2 and phthalaldehyde (PAD). The luminescence quenching can be attributed to the photo-induced electron transfer process from the ligand of UiO-66-PSM to BBH. The UiO-66-PSM sensor exhibited fast response time, low detection limit, and high selectivity to BBH. Moreover, the UiO-66-PSM sensor was successfully applied to the quantitative detection of BBH in the traditional Chinese herb Coptis, and the detection results obtained from the as-fabricated fluorescence sensing assay were consistent with those of high-performance liquid chromatography (HPLC), indicating that this work has potential applicability for the detection of BBH in traditional Chinese herbs.

Insight into the Isoreticularity of Li-MOFs for the Design of Low-Density Solid and Quasi-Solid Electrolytes

Isoreticularity in metal organic frameworks (MOFs) allows the design of the framework structure and tailoring the pore aperture at the molecular level. The optimal pore volume, long-range order of framework expansion, and crystallite size (grain size) could enable improving Li-ion conduction, thereby providing a unique opportunity to design high-performance solid and quasi-solid electrolytes. However, definitive understanding of the pore aperture, framework expansion, and crystallite size on the Li-ion conduction and its mechanism in MOFs remains at the exploratory stage. Among the different MOF subfamilies, Li-MOFs created by the isoreticular framework expansion using dicarboxylates of benzene, naphthalene, and biphenyl building blocks emerge as low-density porous solids with exceptional thermal stability to study the solid-state Li+ transport mechanisms. Herein, we report the subtle effect of the isoreticularity in Li-MOFs on the performance of solid and quasi-solid-state Li+ conduction, providing new insight into Li+ transport mechanisms in MOFs for the first time. Our experimental and computational results show that the reticular design on an isostructural extended framework structure with the optimal pore aperture and crystallite size can influence the Li+ conductivity, exhibiting comparable ionic conductivities to solid polymer electrolytes at room temperature. Aligning with the computational studies, our experimental absorption spectral traces of solid electrolytes prepared by encapsulating lithium salt (LiClO4) and the plasticizer (ethylene carbonate) with Li-MOFs confirm the participation of the free and bound states of Li+ in a pore filling-driven ion conduction mechanism. We postulate that porous channels of Li-MOFs aid free Li+ to move through the pores via a vehicle-type mechanism, in which the pore-filled plasticizer acts as a carrier for mobile Li+ while the framework's functional sites transport the bound state of Li+ via an ion hopping mechanism from one crystallite site to another. Our computational studies performed on the Li+ conduction pathway validated the postulated pore filling mechanism and confirmed the involvement of bridging complexes, formed by binding Li+ onto the framework's functional sites as well as to the pore-filled ethylene carbonates. The Li+ diffusion energy barrier profiles along with the respective conformational changes during the diffusion of Li+ in solid electrolytes prepared from Li-BDC MOF and Li-NDC MOF strongly support the cooperative movement of Li+ ions via ion hopping along the framework's edges and vehicle-type transfer, involving the pore-filled plasticizer. Our findings suggest that cooperative function of the optimal pore volume, framework expansion, and crystallite size play a unique role in Li-ion conduction, thereby providing design guidelines for the low-density solid and quasi-solid electrolytes.

A Multifunctional Co-Based Metal-Organic Framework as a Platform for Proton Conduction and Ni trophenols Reduction

The design and development of proton conduction materials for clean energy-related applications is obviously important and highly desired but challenging. An ultrastable cobalt-based metal-organic framework Co-MOF, formulated as [Co2(btzip)2(μ2-OH2)] (namely, LCUH-103, H2btzip = 4, 6-bis(triazol-1-yl)-isophthalic acid) had been successfully synthesized via the hydrothermal method. LCUH-103 exhibits a three-dimensional framework and a one-dimensional microporous channel structure with scu topology based on the binuclear metallic cluster {Co2}. LCUH-103 indicated excellent chemical and thermal stability; peculiarly, it can retain its entire framework in acid and alkali solutions with different pH values for 24 h. The excellent stability is a prerequisite for studying its proton conductivity, and its proton conductivity σ can reach up to 1.25 × 10-3 S·cm-1 at 80 °C and 100% relative humidity (RH). In order to enhance its proton conductivity, the proton-conducting material Im@LCUH-103 had been prepared by encapsulating imidazole molecules into the channels of LCUH-103. Im@LCUH-103 indicated an excellent proton conductivity of 3.18 × 10-2 S·cm-1 at 80 °C and 100% RH, which is 1 order of magnitude higher than that of original LCUH-103. The proton conduction mechanism was systematically studied by various detection means and theoretical calculations. Meanwhile, LCUH-103 is also an excellent carrier for palladium nanoparticles (Pd NPs) via a wetness impregnation strategy, and the nitrophenols (4/3/2-NP) reduction in aqueous solution by Pd@LCUH-103 indicated an outstanding conversion efficiency, high rate constant (k), and exceptional cycling stability. Specifically, the k value of 4-NP reduction by Pd@LCUH-103 is superior to many other reported catalysts, and its k value is as high as 1.34 min-1 and the cycling stability can reach up to 6 cycles. Notably, its turnover frequency (TOF) value is nearly 196.88 times more than that of Pd/C (wt 5%) in the reaction, indicating its excellent stability and catalytic activity.

A simple MOF constructed using Pb(II) with strong polarizing force: a filler of Nafion membrane to increase proton conductivity

Metal-organic frameworks (MOFs) are promising competitive candidates as fillers for Nafion proton exchange membrane (PEM). Increasing efforts have been made to explore methods for synthesizing MOF fillers and the mechanism by which MOF doping improves the proton conductivity (σH+) values of composite membranes. In this study, a Pb(II) cation with strong polarizing force was selected for the hydrothermal reaction with a simple sulfoterephthalate ligand (H3L). Pb-MOF [Pb2L(OH)]n was obtained, which was constructed using Pb-O layers and deprotonated sulfoterephthalate L3- and exhibited good thermal and water stability. Different amounts of Pb-MOF particles were doped into Nafion to fabricate Pb-MOF/Nafion-x composite membranes, which were characterized using SEM, PXRD, IR spectroscopy, TGA, and other methods. It was found that doping Pb-MOF can apparently improve the water absorbability and thermal stability of the composite membrane. The σH+ of the Pb-MOF/Nafion-7 composite membrane was the highest and 2.14 times that of the pure Nafion membrane at 353 K. The higher proton conduction properties may be explained by the strong polarization force, and Pb(II) cations on the surface of Pb-MOF can decrease the bond energy of the O-H bond of absorbed water molecules and increase the acidity of the composite membrane. The phenomena in this study and our previous study confirm that acidity is the most important factor in favor of proton conductivity.

Molecular Catalysis of Energy Relevance in Metal-Organic Frameworks: From Higher Coordination Sphere to System Effects

The modularity and synthetic flexibility of metal-organic frameworks (MOFs) have provoked analogies with enzymes, and even the term MOFzymes has been coined. In this review, we focus on molecular catalysis of energy relevance in MOFs, more specifically water oxidation, oxygen and carbon dioxide reduction, as well as hydrogen evolution in context of the MOF-enzyme analogy. Similar to enzymes, catalyst encapsulation in MOFs leads to structural stabilization under turnover conditions, while catalyst motifs that are synthetically out of reach in a homogeneous solution phase may be attainable as secondary building units in MOFs. Exploring the unique synthetic possibilities in MOFs, specific groups in the second and third coordination sphere around the catalytic active site have been incorporated to facilitate catalysis. A key difference between enzymes and MOFs is the fact that active site concentrations in the latter are often considerably higher, leading to charge and mass transport limitations in MOFs that are more severe than those in enzymes. High catalyst concentrations also put a limit on the distance between catalysts, and thus the available space for higher coordination sphere engineering. As transport is important for MOF-borne catalysis, a system perspective is chosen to highlight concepts that address the issue. A detailed section on transport and light-driven reactivity sets the stage for a concise review of the currently available literature on utilizing principles from Nature and system design for the preparation of catalytic MOF-based materials.

High-temperature low-humidity proton exchange membrane with "stream-reservoir" ionic channels for high-power-density fuel cells

The perfluorosulfonic acid (PFSA) proton exchange membrane (PEM) is the key component for hydrogen fuel cells (FCs). We used in situ synchrotron scattering to investigate the PEM morphology evolution and found a "stream-reservoir" morphology, which enables efficient proton transport. The short-side-chain (SSC) PFSA PEM is fabricated under the guidance of morphology optimization, which delivered a proton conductivity of 193 milliSiemens per centimeter [95% relativity humidity (RH)] and 40 milliSiemens per centimeter (40% RH) at 80°C. The improved glass transition temperature, water permeability, and mechanical strength enable high-temperature low-humidity FC applications. Performance improvement by 82.3% at 110°C and 25% RH is obtained for SSC-PFSA PEM FCs compared to Nafion polymer PEM devices. The insights in chain conformation, packing mechanism, crystallization, and phase separation of PFSAs build up the structure-property relationship. In addition, SSC-PFSA PEM is ideal for high-temperature low-humidity FCs that are needed urgently for high-power-density and heavy-duty applications.

The proton conduction behavior of two 1D open-framework metal phosphates with similar crystal structures and different hydrogen bond networks

Two open-framework zinc phosphates [C₃N₂H₁₂][Zn(HPO₄)₂] (1) and [C₆N₄H₂₂]_0.5[Zn(HPO₄)₂] (2) were synthesized via hydrothermal reaction and characterized by powder X-ray diffraction, thermogravimetric analysis and scanning electron microscopy. Both compounds have a similar crystal structure and macroscopic morphology. However, the difference in equilibrium cations, in which the propylene diamine is for 1 and the triethylenetetramine is for 2, results in a significant distinction in the dense hydrogen grid. The diprotonated propylene diamine molecule in 1 is more favorable for forming a hydrogen-bond network in three dimensions than in 2, in which the twisted triethylenetetramine forms a hydrogen bond grid with the inorganic framework only in two dimensions owing to its large steric effect. This distinction further leads to a disparity in the proton conductivity of both compounds. The proton conductivity of 1 can reach 1.00 × 10^-3 S cm^-1 under ambient conditions (303 K and 75% RH) and then increase to 1.11 × 10^-2 S cm^-1 at 333 K and 99% RH, which is the highest value among the open-framework metal phosphate proton conductors operated in the same conduction. In contrast, the proton conductivity of 2 is four orders of magnitude smaller than 1 at 303 K and 75% RH and two orders smaller than 1 at 333 K and 99% RH.

A multifunctional cobalt-organic framework for proton conduction and selective sensing of Fe3+ ions

Developing multifunctional metal-organic frameworks (MOFs) is a new research trend. MOFs have shown remarkable performances in both proton conduction and fluorescence sensing, but the MOFs integrating the two performances are scarce. Herein, a Co-MOF, [Co₆(oba)₄(Hatz)(atz)(H₂O)₂(μ₃-OH)₂(μ₂-OH)]·H₂O (1, H₂oba = 4,4-oxybis(benzoic acid), Hatz = 5-amino-1H tetrazole), has been assembled by Co²⁺ ions with H₂oba and Hatz ligands, providing a unique example of multifunctional MOFs with both proton conduction and fluorescence sensing performances. The framework of 1 displays a pillar-layer structure built by the oba ligand as a pillar and a layer composed of Co-clusters and atz linkers. Because large-scale single crystals of 1 were successfully synthesized, the proton conduction ability of 1 was investigated using single crystal samples. 1 exhibits highly anisotropic conduction with conductivity values of 1.1 × 10^-3 S cm^-1 along the [001] direction and 9.1 × 10^-6 S cm^-1 along the [010] direction at 55 °C and 95% RH, respectively. Meanwhile, the fluorescence sensing of 1 towards metal ions was studied in aqueous solutions. Attractively, 1 may sensitively and selectively detect Fe³⁺ ions in the presence of other interfering ions by fluorescence quenching.

Post-synthesis functionalization of ZIF-90 with sulfonate groups for high proton conduction

Introducing sulfonic acid groups into MOF materials is one of the effective approaches to enhance proton conduction. Here, we attempted to prepare a new post-modified ZIF-90-based material by addition reaction of the aldehyde group with bisulfite to obtain partially functionalized ZIF-90-SO₃Na_(2.3). ZIF-90-SO₃Na_(2.3) exhibits a high proton conductivity of 2.26 × 10^-2 S cm^-1 at 98% RH and 100 °C.

Three layered cucurbit[6]uril-based metal-organic rotaxane networks functionalized by sulfonic groups for proton conduction

Three new cucurbit[6]uril (CB[6])-based metal-organic rotaxane networks (MORNs) (named CUST-711, CUST-712, and CUST-713) functionalized by a sulfonic group (-SO₃H) have been designed and synthesized via a hydrothermal method. All three compounds exhibited similar two-dimensional (2D) wave layer structures. Their stability under different temperature and relative humidity conditions has been investigated and all the compounds showed excellent stability. Furthermore, their proton conduction properties were also discussed in detail. Due to different structures and sulfonic group sites, the three compounds exhibited different proton conduction abilities of which CUST-712 exhibited an intrinsic relatively high proton conductivity (1.75 × 10^-4 S cm^-1 at 85 °C and 97% relative humidity). These results provide ideas for the design and synthesis of functional CB[6]-based metal-organic rotaxane frameworks (MORFs) as proton conducting materials.

Insight into the High Proton Conductivity of One-/Two-Dimensional Cadmium Phosphites

The study describes the synthesis and structural attributes of two new cadmium phosphites, [Cd{OP(O)(OH)H}₂(4,4'-bipy)] (1) and [H₂pip][Cd(HPO₃)₂(H₂O)]·H₂O (2). The structure of 1 adopts a two-dimensional motif featuring alternate [Cd-μ₂-O]₂ and [Cd-O-P-O]₂-cyclic rings, while the inorganic chains are held together by 4,4'-bipyridine. The presence of strong hydrogen bonding interactions between the appended H₂PO₃ groups (O---O = 2.55 Å) provides a facile proton conduction pathway and results in a proton conductivity of 3.2 × 10^-3 S cm^-1 at 75 °C under 77% relative humidity (RH). Compound 2 comprises an anionic framework formed by vertex-shared [Cd-O-P-O]₂-cyclic rings, while the [H₂pip] cations between the adjacent chains assist a well-directed O-H---O hydrogen-bonded network between coordinated water, lattice water, and phospite groups. The bulk proton conductivity value under conditions as in 1 reaches 4.3 × 10^-1 S cm^-1. For both 1 and 2, the proton conductivity remains practically unchanged under ambient temperatures (25-35 °C), suggesting their potential in low-temperature fuel cells.

A Porous Sulfonated 2D Zirconium Metal-Organic Framework as a Robust Platform for Proton Conduction

By using the strategy of pre-assembly chlorosulfonation applied to a linker precursor, the first sulfonated zirconium metal-organic framework (JUK-14) with two-dimensional (2D) structure, was synthesized. Single-crystal X-ray diffraction reveals that the material is built of Zr₆ O₄ (OH)₄ (COO)₈ oxoclusters, doubly 4-connected by angular dicarboxylates, and stacked in layers spaced 1.5 nm apart by the presence of sulfonic groups. JUK-14 exhibits excellent hydrothermal stability, permanent porosity confirmed by gas adsorption studies, and shows high (>10^-4  S/cm) and low (<10^-8  S/cm) proton conductivity under humidified and anhydrous conditions, respectively. Post-synthesis inclusion of imidazole improves the overall conductivity increasing it to 1.7×10^-3  S/cm at 60 °C and 90 % relative humidity, and by 3 orders of magnitude at 160 °C. The combination of 2D porous nature with robustness of zirconium MOFs offers new opportunities for exploration of the material towards energy and environmental applications.

Superprotonic conduction of intrinsically zwitterionic microporous polymers based on easy-to-make squaraine, croconaine and rhodizaine dyes

Porous organic polymers (POPs) have been prepared via a novel metal free polycondensation between a tritopic indole-based monomer and squaric, croconic and rhodizonic acids. Each of the three POPs exhibited high BET surface areas (331-667 m² g^-1) and zwitterionic structures. Impedance measurements revealed that the intrinsic POPs were relatively weak proton conductors, with a positive correlation between the density of oxo-groups and the proton conduction. Doping the materials with LiCl vastly improved the proton conductivity up to a value of 0.54 S cm^-1 at 90 °C and 90% relative humidity.

Stable Lanthanide Metal-Organic Frameworks with Ratiometric Fluorescence Sensing for Amino Acids and Tunable Proton Conduction and Magnetic Properties

Four new isostructural lanthanide metal-organic frameworks (MOFs), namely {[Ln(DMTP-DC)_1.5(H₂O)₃]·DMF}_n [H₂DMTP-DC = 2',5'-dimethoxytriphenyl-4,4″-dicarboxylic acid; Ln^III = Eu^III (1), Gd^III (2), Tb^III (3), and Dy^III (4)], have been synthesized and characterized. Single-crystal structure analysis reveals that 1-4 are three-dimensional Ln-MOFs with rich H-bonding of coordinated H₂O molecules in the network channels. The X-ray diffraction patterns indicate that Ln-MOF 1 displays good stabilities in organic solvents and aqueous solutions with distinct pH values. Both 1 and 3 show characteristic emission of Ln^III ions. Ln-MOF 1 can be used as a ratiometric fluorescence sensor for arginine and lysine in aqueous solution, and the detection limits are 24.38 μM for arginine and 9.31 μM for lysine. All 1-4 show proton conductivity related to relative humidity (RH) and temperature, and the maximum conductivity values of 1-4 at 55 °C and 100% RH are 9.94 × 10^-5, 1.62 × 10^-4, 1.71 × 10^-4, and 2.67 × 10^-4 S·cm^-1, respectively. The value of σ increases with the decrease in ionic radius, indicating that the radius of the Ln^III ions can regulate the proton conductivity of these MOFs. Additionally, 2 exhibits a significant magnetocaloric effect (MCE) with a magnetic entropy change (-ΔS_m) of 18.86 J kg^-1 K^-1 for ΔH = 7 T at 2 K, and 4 shows weak field-induced slow relaxation of magnetization. The coexistence of good fluorescence sensing capability, attractive proton conductivity, and relatively large MCE in Ln-MOFs is rare, and thus, 1-4 are potentially multifunctional MOF materials.

New Generation of MOF-Monoliths Based on Metal Foams

Herein, it has been developed a method to prepare metallic foams starting from Zamak5 (ZnAlCu alloy) with different pore sizes. The Zamak5 metallic foam is designed to serve as a support and metallic precursor of ZIF-8. In this way, composite materials MOF-metal can be prepared, these composites have a large number of application in energy exchange processe such as: adsorption or chemical reactions. Additionally, this method of sythesizing MOFs is environmentally friendly thanks to absence of solvents. Hanerssing the low melting point of the linker, the linker is infiltrated into the foam where the foam and the linker react to form the ZIF-8. In this way we have managed to transform part of the foam into ZIF-8 crystals that remain adhered to the foam. The foams have been characterized and modeled studying the mechanical and electrical properties, finding that both can be predected by various models. Among these, Ashby and Mortensen models for mechanical properties and Ashby and Percolation model for electrical properties stand.

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.

Dense Dithiolene Units on Metal-Organic Frameworks for Mercury Removal and Superprotonic Conduction

With 2-COOH and 4-SH donors all packed onto the benzene ring, tetrasulfanyl terephthalic acid (TST) is a simple yet fully equipped ligand to move the field of metal-coordination materials─it is now accomplished. The hard-soft carboxyl-thiol synergy is leveraged here in selectively bonding the carboxyl units to Zr(IV) ions to form the same cubic net of UiO-66 (this being based on the terephthalic linker)─with the free-standing dithiolene units equipping the grid of ZrTST. The 3D network of ZrTST averages about 7.6 connections [as in Zr₆O₄(OH)₄(C₈H₄O₄S₄)_3.8], with the other 4.4 sealed by acetate ions. The ZrTST solid is stable in boiling water (it is formed in water/acetic acid/ethane dithiol) and remains ordered even above 300 °C. The thiol-enabled ZrTST (powder) takes up mercury from water with a high distribution coefficient K_d (e.g., 1.2 × 10⁶ mL·g^-1); it also shows proton conductivity (1.9 × 10^-3 S·cm^-1 at 90 °C and 90% relative humidity), which, most notably, increases to a highest value of 3.7 × 10^-1 S·cm^-1 after oxidizing the -SH into the -SO₃H groups.

High proton conductivity in a charge carrier-induced Ni(ii) metal–organic framework

A new tetradentate phosphonate ligand-based Ni-MOF has been synthesized and employed as an efficient proton-conducting material upon doping with sulphuric acid.

Porosity regulation of metal-organic frameworks for high proton conductivity by rational ligand design: mono- versus disulfonyl-4,4′-biphenyldicarboxylic acid

Porous crystalline metal-organic frameworks (MOFs) bearing sulfonic groups (–SO3H) are receiving increasing attention as solid-state proton-conductors because the –SO3H group can not only enhance the proton concentration but also form...

Acidic Groups Functionalized Carbon Dots Capping Channels of a Proton Conductive Metal-Organic Framework by Coordination Bonds to Improve the Water-Retention Capacity and Boost Proton Conduction

Crystalline porous materials, such as metal-organic frameworks (MOFs) and covalent organic frameworks (COFs), have been demonstrated to be versatile material platforms for the development of solid proton conductors. However, most crystalline porous proton conductors suffer from decreasing proton conductivity with increasing temperature due to releasing water molecules, and this disadvantage severely restricts their practical application in electrochemical devices. In this work, for the first time, hydrophilic carbon dots (CDs) were utilized to hybridize with high proton conductivity MOF-802, which is a model of MOF proton conductors, aiming to improve its water-retention capacity and thus enhance proton conduction. The resultant CDs@MOF-802 exhibits impregnable proton conduction with increasing temperature, and the proton conductivity reaches 10^-1 S cm^-1, much superior to that of MOF-802, making CDs@MOF-802 one of the most efficient MOF proton conductors reported so far. This study provides a new strategy to improve the water-retention capacity of porous proton conductors and further realize excellent proton conduction.