High Proton Conductivity of a Cadmium Metal-Organic Framework Constructed from Pyrazolecarboxylate and Its Hybrid Membrane

Improving Proton-Conducting Stability by Regulating Pore Size of MOF Materials through Mixed Grinding

An effective strategy to improve the proton conductivity of metal-organic frameworks (MOFs) is to regulate the pore size of composite materials. In this work, composite materials of MOF-808@MOG-808-X (X is the mass ratios of MOF-808 to MOG-808) was successfully prepared by grinding and blending. MOF-808@MOG-808-1:2 was optimal for its suitable pore structure, which facilitates the practical construction of hydrogen bonding networks, promotes rapid and stable proton conduction, and enables the proton conductivity, achieving a 1 + 1 > 2 effect. At 353 K and 93% relative humidity (RH), the maximum proton conductivity of MOF-808@MOG-808-1:2 reaches 1.08 × 10-1 S·cm-1. Next, MOF-808@MOG-808-1:2 was blended with chitosan (CS) to obtain composite proton exchange membranes (PEMs), namely, CS@MOF-808@MOG-808-1:2-Y (Y = 5%, 10%, or 15%) with the maximum proton conductivity reaching 1.19 × 10-2 S·cm-1 at 353 K and 93% RH for CS@MOF-808@MOG-808-1:2-10% with additional stability. The conductive mechanisms of the composite materials were revealed by activation energy calculation. This investigation not only proposes a simple grinding-blending method for the development of MOF-doped composite materials for proton conductivity but also provides a producting material basis for future applications of MOFs in proton exchange membrane fuel cells (PEMFCs).

Achieving High Proton Conductivity in MIL-91(Al) Aerogel

The processability and sustainability of proton conductors are two important indicators of their application. Here, MIL-91(Al) with an intrinsic proton conduction framework originating from protonated phosphonate groups was cross-linked with poly(vinyl alcohol) (PVA) to obtain MIL-91(Al) aerogel through freeze-drying. This simple and inexpensive strategy not only facilitated the processing of MIL-91(Al) powder but also resulted in a molded MIL-91(Al) aerogel having a high proton conductivity of 1.02 × 10-2 S cm-1 at 70 °C and 100% relative humidity. Furthermore, MIL-91(Al) aerogel was recyclable and reusable, in line with the principles of environmental protection and sustainability. To the best of our knowledge, this is the first example of using a metal-organic framework aerogel as a proton conductor, which may develop a new model system in this field.

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.

High Proton Conductivity of the UiO-66-NH2-SPES Composite Membrane Prepared by Covalent Cross-Linking

A sulfonated poly(ethersulfone) (SPES)-metal-organic framework (MOF) film with excellent proton conductivity was synthesized by anchoring UiO-66-NH₂ to the main chain of the aromatic polymer through the Hinsberg reaction. The chemical bond was formed between the amino group in MOFs and the -SO₂Cl group in chlorosulfonated poly(ethersulfones) to conduct protons in the proton channel of the membrane, making the membrane have excellent proton conductivity. UiO-66-NH₂ is successfully prepared as a result of the consistency of the experimental and simulated powder X-ray diffraction (PXRD) patterns of MOFs. The existence of absorption peaks of characteristic functional groups in Fourier transform infrared (FTIR) spectra proved the successful preparation of SPES, PES-SO₂Cl, and a composite film. The results of the AC impedance test indicate that the composite film with a 3% mass fraction has the best proton conductivity of 0.215 S·cm^-1, which is 6.2 times higher than that of the blended film without a chemical bond at 98% RH and 353 K. To our knowledge, there are rarely any reports on the preparation of a composite membrane by directly linking MOFs and the membrane matrix with chemical bonds. This work provides a good way to synthesize the highly conductive proton exchange film.

Pyrazine-Functionalized Donor-Acceptor Covalent Organic Frameworks for Enhanced Photocatalytic H2 Evolution with High Proton Transport

The well-defined 2D or 3D structure of covalent organic frameworks (COFs) makes it have great potential in photoelectric conversion and ions conduction fields. Herein, a new donor-accepter (D-A) COF material, named PyPz-COF, constructed from electron donor 4,4',4″,4'″-(pyrene-1,3,6,8-tetrayl)tetraaniline and electron accepter 4,4'-(pyrazine-2,5-diyl)dibenzaldehyde with an ordered and stable π-conjugated structure is reported. Interestingly, the introduction of pyrazine ring endows the PyPz-COF a distinct optical, electrochemical, charge-transfer properties, and also brings plentiful CN groups that enrich the proton by hydrogen bonds to enhance the photocatalysis performance. Thus, PyPz-COF exhibits a significantly improved photocatalytic hydrogen generation performance up to 7542 µmol g^-1 h^-1 with Pt as cocatalyst, also in clear contrast to that of PyTp-COF without pyrazine introduction (1714 µmol g^-1 h^-1 ). Moreover, the abundant nitrogen sites of the pyrazine ring and the well-defined 1D nanochannels enable the as-prepared COFs to immobilize H₃ PO₄ proton carriers in COFs through hydrogen bond confinement. The resulting material has an impressive proton conduction up to 8.10 × 10^-2 S cm^-1 at 353 K, 98% RH. This work will inspire the design and synthesis of COF-based materials with both efficient photocatalysis and proton conduction performance in the future.

Space-Confined Nucleation of Semimetal-Oxo Clusters within a [H7P8W48O184]33- Macrocycle: Synthesis, Structure, and Enhanced Proton Conductivity

Spatially confined assembly of semimetallic oxyanions (AsO₃^3- and SbO₃^3-) within a [H₇P₈W₄₈O₁₈₄]^33- (P₈W₄₈) macrocycle has afforded three nanoscale polyanions, [{As^III₅O₄(OH)₃}₂(P₈W₄₈O₁₈₄)]^32- (As₁₀), [(Sb^IIIOH)₄(P₈W₄₈O₁₈₄)]^32- (Sb₄), and [(Sb^IIIOH)₈(P₈W₄₈O₁₈₄)]^24- (Sb₈), which were crystallized as the hydrated mixed-cation salts (Me₂NH₂)₁₃K₇Na₂Li₁₀[{As^III₅O₄(OH)₃}₂(P₈W₄₈O₁₈₄)]·32H₂O (DMA-KNaLi-As₁₀), K₂₀Li₁₂[(Sb^IIIOH)₄(P₈W₄₈O₁₈₄)]·52H₂O (KLi-Sb₄), and (Me₂NH₂)₈K₆Na₅Li₅[(Sb^IIIOH)₈(P₈W₄₈O₁₈₄)]·65H₂O (DMA-KNaLi-Sb₈), respectively. A multitude of solid- and solution-state physicochemical techniques were employed to systematically characterize the structure and composition of the as-made compounds. The polyanion of As₁₀ represents the first example of a semimetal-oxo cluster-substituted P₈W₄₈ and accommodates the largest As^III-oxo cluster in polyoxometalates (POMs) reported to date. The number of incorporated SbO₃^3- groups in Sb₄ and Sb₈ could be customized by a simple variation of Sb^III-containing precursors. Encapsulation of semimetallic oxyanions inside P₈W₄₈ sets out a valid strategy not only for the development of host-guest assemblies in POM chemistry but also for their function expansion in emerging applications such as proton-conducting materials, for which DMA-KNaLi-As₁₀ showcases an outstanding conductivity of 1.2 × 10^-2 S cm^-1 at 85 °C and 70% RH.

Enhancing Proton Conductivity of Nafion Membrane by Incorporating Porous Tb-Metal-Organic Framework Modified with Nitro Groups

A rigid carboxylate ligand with a nitro functional group was selected to coordinate with Tb(III) cation, and Tb-MOF ({[Tb₄(L)₄(OH)₄(H₂O)₃]·8H₂O}_n, H₂L = 2-nitroterephthalic acid) with large porous and excellent hydrophilicity was obtained successfully. The obtained Tb-MOF was filled into the Nafion matrix to improve its proton conduction performance. The Tb-MOF/Nafion composite membrane was characterized by PXRD, IR, and thermogravimetry (TG) and for water uptake, area swelling, and proton conductivity. The activity energy, E_a, value of the composite membrane, which is a very important factor affecting the proton conduction performance of the membrane, was fitted and calculated. It was revealed that Tb-MOF can improve the proton conductivities of composite membranes, and the improvement degree and E_a value were both affected by Tb-MOF content. When Tb-MOF content was 5%, the proton conductivity of the composite membrane was 1.53 × 10^-2 S·cm^-1 at 100% RH and 80 °C, which is 1.81 times that of the pure Nafion membrane. A MOF containing a nitro functional group was first doped into Nafion in this study and exhibited excellent performance for improving composite membrane proton conductivity. This study will provide a valuable reference for designing different functionalized MOFs to promote the proton conductivities of proton exchange membranes.

Rationalizing Structural Hierarchy in the Design of Fuel Cell Electrode and Electrolyte Materials Derived from Metal-Organic Frameworks

Metal-organic frameworks (MOFs) are arguably a class of highly tuneable polymer-based materials with wide applicability. The arrangement of chemical components and the bonds they form through specific chemical bond associations are critical determining factors in their functionality. In particular, crystalline porous materials continue to inspire their development and advancement towards sustainable and renewable materials for clean energy conversion and storage. An important area of development is the application of MOFs in proton-exchange membrane fuel cells (PEMFCs) and are attractive for efficient low-temperature energy conversion. The practical implementation of fuel cells, however, is faced by performance challenges. To address some of the technical issues, a more critical consideration of key problems is now driving a conceptualised approach to advance the application of PEMFCs. Central to this idea is the emerging field MOF-based systems, which are currently being adopted and proving to be a more efficient and durable means of creating electrodes and electrolytes for proton−exchange membrane fuel cells. This review proposes to discuss some of the key advancements in the modification of PEMs and electrodes, which primarily use functionally important MOFs. Further, we propose to correlate MOF-based PEMFC design and the deeper correlation with performance by comparing proton conductivities and catalytic activities for selected works.

A multifunctional anionic metal-organic framework for high proton-conductivity and photoreduction of CO2 induced by cation exchange

Metal-Organic Frameworks (MOFs) provide an ideal platform for loading various guests owing to their available spaces, which can be developed as a class of multifunctional materials. Herein, we cover the...