A Water Stable Magnesium MOF That Conducts Protons over 10–2 S cm–1

Prominent Intrinsic Proton Conduction in Two Robust Zr/Hf Metal-Organic Frameworks Assembled by Bithiophene Dicarboxylate

To date, developing crystalline proton-conductive metal-organic frameworks (MOFs) with an inherent excellent proton-conducting ability and structural stability has been a critical priority in addressing the technologies required for sustainable development and energy storage. Bearing this in mind, a multifunctional organic ligand, 3,4-dimethylthiophene[2,3-b]thiophene-2,5-dicarboxylic acid (H2DTD), was employed to generate two exceptionally stable three-dimensional porous Zr/Hf MOFs, [Zr6O4(OH)4(DTD)6]·5DMF·H2O (Zr-DTD) and [Hf6O4(OH)4(DTD)6]·4DMF·H2O (Hf-DTD), using solvothermal means. The presence of Zr6 or Hf6 nodes, strong Zr/Hf-O bonds, the electrical influence of the methyl group, and the steric effect of the thiophene unit all contribute to their structural stability throughout a wide pH range as well as in water. Their proton conductivity was fully examined at various relative humidities (RHs) and temperatures. Creating intricate and rich H-bonded networks between the guest water molecules, coordination solvent molecules, thiophene-S, -COOH, and -OH units within the framework assisted proton transfer. As a result, both MOFs manifest the maximum proton conductivity of 0.67 × 10-2 and 4.85 × 10-3 S·cm-1 under 98% RH/100 °C, making them the top-performing proton-conductive Zr/Hf-MOFs. Finally, by combining structural characteristics and activation energies, potential proton conduction pathways for the two MOFs were identified.

Tunnel-Structured Phosphate Exhibiting High Proton Conductivity and Thermal Stability over a Wide Intermediate Temperature Range

For the practical application of fuel cells in vehicles, it is a challenge to develop a proton solid electrolyte that coexhibits thermal stability and high proton conductivity at wide intermediate temperatures. Here, we report on the tunnel structured phosphate KNi1-xH2x(PO3)3·yH2O, which exhibits high proton conductivity at room temperature up to 500 °C, with the conductivity value reaching 1.7 × 10-2 S cm-1 at 275 °C for x = 0.18. This material, composed of the smallest cations that form the tunnel framework with face-shared (KO6) and (NiO6) chains and PO4 tetrahedral chains, retained the rigid framework up to 600 °C. Two oxygen sites of water molecules located adjacent to each other along the PO4 tetrahedral chains in the tunnel provided the proton conduction pathway. The sample maintained a conductivity of 5.0 × 10-3 S cm-1 for 10 h at 150 °C while changing the measurement atmosphere to a N2 gas flow, a 4% H2-96% Ar gas flow, and an O2 gas flow. The conductivity value at x = 0.18 obtained from the DC measurement was in the order of 10-6 S cm-1, close to the instrument's measurement limit. These results demonstrate that tunnel phosphate has potential as a proton solid electrolyte for next-generation fuel cells.

Four Lanthanide(III) Metal-Organic Frameworks Fabricated by Bithiophene Dicarboxylate for High Inherent Proton Conduction

Currently, it is still a challenge to directly achieve highly stable metal-organic frameworks (MOFs) with superior proton conductivity solely through the exquisite design of ligands and the attentive selection of metal nodes. Inspired by this, we are intrigued by a multifunctional dicarboxylate ligand including dithiophene groups, 3,4-dimethylthieno[2,3-b]thiophene-2,5-dicarboxylic acid (H2DTD), and lanthanide ions with distinct coordination topologies. Successfully, four isostructural three-dimensional lanthanide(III)-based MOFs, [Ln2(DTD)3(DEF)4]·DEF·6H2O [LnIII = TbIII (Tb-MOF), EuIII (Eu-MOF), SmIII (Sm-MOF), and DyIII (Dy-MOF)], were solvothermally prepared, in which the effective proton transport will be provided by the coordinated or free solvent molecules, the crystalline water molecules, and the framework components, as well as a large number of highly electronegative S and O atoms. As expected, the four Ln-MOFs demonstrated the highest proton conductivities (σ) being 0.54 × 10-3, 3.75 × 10-3, 1.28 × 10-3, and 1.92 × 10-3 S·cm-1 for the four MOFs, respectively, at 100 °C/98% relative humidity (RH). Excitingly, Dy-MOF demonstrated an extraordinary ultrahigh σ of 1 × 10-3 S·cm-1 at 30 °C/98% RH. Additionally, the plausible proton transport mechanisms were emphasized.

Superprotonic Conductivity in a Metalloporphyrin-Based SMOF (Supramolecular Metal-Organic Framework)

Metal-organic frameworks and supramolecular metal-organic frameworks (SMOFs) exhibit great potential for a broad range of applications taking advantage of the high surface area and pore sizes and tunable chemistry. In particular, metalloporphyrin-based MOFs and SMOFs are becoming of great importance in many fields due to the bioessential functions of these macrocycles that are being mimicked. On the other hand, during the last years, proton-conducting materials have aroused much interest, and those presenting high conductivity values are potential candidates to play a key role in some solid-state electrochemical devices such as batteries and fuel cells. In this way, using metalloporphyrins as building units we have obtained a new crystalline material with formula [H(bipy)]2[(MnTPPS)(H2O)2]·2bipy·14H2O, where bipy is 4,4'-bipyidine and TPPS4- is the meso-tetra(4-sulfonatephenyl) porphyrin. The crystal structure shows a zig-zag water chain along the [100] direction located between the sulfonate groups of the porphyrin. Taking into account those structural features, the compound was tested for proton conduction by complex electrochemical impedance spectroscopy (EIS). The as-obtained conductivity is 1 × 10-2 S·cm-1 at 40 °C and 98% relative humidity, which is a remarkably high value.

MOF-Based Chemiresistive Gas Sensors: Toward New Functionalities

The sensing performances of gas sensors must be improved and diversified to enhance quality of life by ensuring health, safety, and convenience. Metal-organic frameworks (MOFs), which exhibit an extremely high surface area, abundant porosity, and unique surface chemistry, provide a promising framework for facilitating gas-sensor innovations. Enhanced understanding of conduction mechanisms of MOFs has facilitated their use as gas-sensing materials, and various types of MOFs have been developed by examining the compositional and morphological dependences and implementing catalyst incorporation and light activation. Owing to their inherent separation and absorption properties and catalytic activity, MOFs are applied as molecular sieves, absorptive filtering layers, and heterogeneous catalysts. In addition, oxide- or carbon-based sensing materials with complex structures or catalytic composites can be derived by the appropriate post-treatment of MOFs. This review discusses the effective techniques to design optimal MOFs, in terms of computational screening and synthesis methods. Moreover, the mechanisms through which the distinctive functionalities of MOFs as sensing materials, heterostructures, and derivatives can be incorporated in gas-sensor applications are presented.

Anhydrous Superprotonic Conductivity in the Zirconium Acid Triphosphate ZrH5 (PO4 )3

The development of solid-state proton conductors with high proton conductivity at low temperatures is crucial for the implementation of hydrogen-based technologies for portable and automotive applications. Here, we report on the discovery of a new crystalline metal acid triphosphate, ZrH₅ (PO₄ )₃ (ZP3), which exhibits record-high proton conductivity of 0.5-3.1×10^-2  S cm^-1 in the range 25-110 °C in anhydrous conditions. This is the highest anhydrous proton conductivity ever reported in a crystalline solid proton conductor in the range 25-110 °C. Superprotonic conductivity in ZP3 is enabled by extended defective frustrated hydrogen bond chains, where the protons are dynamically disordered over two oxygen centers. The high proton conductivity and stability in anhydrous conditions make ZP3 an excellent candidate for innovative applications in fuel cells without the need for complex water management systems, and in other energy technologies requiring fast proton transfer.

Biobased hydrogels and their composite containing MgMOF74 for the removal of textile dyes and wastewater treatment

In this work, we report the synthesis of a biobased hydrogel comprised of collagen, chitosan, and polyurethane for the removal of textile dyes. The adsorption capacity of this hydrogel was improved by adding a magnesium metal-organic framework to the semi-interpenetrating polymer matrix yielding a composite hydrogel. Removal of Bismarck Brown and Congo red was studied, and the experimental results fit Freundlich's model. Both hydrogel formulations were tested for the removal of textiles dyes from wastewaters. The magnesium-metal organic framework improved the efficiency of the biobased hydrogel for the removal of direct and mordant dyes reaching up to 89 ± 2%. The composite hydrogel was tested for the removal of Congo Red in a fixed bed column observing the breakthrough point after 168 min. Also, a flocculant material was prepared from collagen and chitosan and was tested for the removal of direct red dye from wastewater removing up to 80 ± 1%. The pretreated wastewater by coagulation-flocculation was treated by adsorption yielding a global removal efficiency of 99%. Finally, the studied hydrogels are potentially biodegradable being completely degraded by the proteolytic action after 22 days. PRACTITIONER POINTS: Composite hydrogels of collagen, chitosan, and MgMOF74 removed efficiently textile dyes from wastewater in batch systems and fixed bed columns. A biobased flocculant of collagen and chitosan significantly improved water quality after coagulation flocculation. Hydrogels were reusable for four cycles, and they can be proteolytically degraded after 22 days.

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.

Highly Efficient Proton Conduction in the Metal-Organic Framework Material MFM-300(Cr)·SO4(H3O)2

The development of materials showing rapid proton conduction with a low activation energy and stable performance over a wide temperature range is an important and challenging line of research. Here, we report confinement of sulfuric acid within porous MFM-300(Cr) to give MFM-300(Cr)·SO₄(H₃O)₂, which exhibits a record-low activation energy of 0.04 eV, resulting in stable proton conductivity between 25 and 80 °C of >10^–2 S cm^–1. In situ synchrotron X-ray powder diffraction (SXPD), neutron powder diffraction (NPD), quasielastic neutron scattering (QENS), and molecular dynamics (MD) simulation reveal the pathways of proton transport and the molecular mechanism of proton diffusion within the pores. Confined sulfuric acid species together with adsorbed water molecules play a critical role in promoting the proton transfer through this robust network to afford a material in which proton conductivity is almost temperature-independent.

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.

Supramolecular Proton Conductors Self-Assembled by Organic Cages

Proton conduction is vital for living systems to execute various physiological activities. The understanding of its mechanism is also essential for the development of state-of-the-art applications, including fuel-cell technology. We herein present a bottom-up strategy, that is, the self-assembly of Cage-1 and -2 with an identical chemical composition but distinct structural features to provide two different supramolecular conductors that are ideal for the mechanistic study. Cage-1 with a larger cavity size and more H-bonding anchors self-assembled into a crystalline phase with more proton hopping pathways formed by H-bonding networks, where the proton conduction proceeded via the Grotthuss mechanism. Small cavity-sized Cage-2 with less H-bonding anchors formed the crystalline phase with loose channels filled with discrete H-bonding clusters, therefore allowing for the translational diffusion of protons, that is, vehicle mechanism. As a result, the former exhibited a proton conductivity of 1.59 × 10^-4 S/cm at 303 K under a relative humidity of 48%, approximately 200-fold higher compared to that of the latter. Ab initio molecular dynamics simulations revealed distinct H-bonding dynamics in Cage-1 and -2, which provided further insights into potential proton diffusion mechanisms. This work therefore provides valuable guidelines for the rational design and search of novel proton-conducting materials.

A confinement of N-heterocyclic molecules in a metal–organic framework for enhancing significant proton conductivity

A series of N-heterocyclic⊂VNU-23 materials have been prepared via the impregnation procedure of N-heterocyclic molecules into VNU-23. Their structural characterizations, PXRD, FT-IR, Raman, TGA, ¹H-NMR, SEM-EDX, and EA, confirmed that N-heterocyclic molecules presented within the pores of parent VNU-23, leading to a remarkable enhancement in proton conductivity. Accordingly, the composite with the highest loading of imidazole, Im_13.5⊂VNU-23, displays a maximum proton conductivity value of 1.58 × 10⁻² S cm⁻¹ (85% RH and 70 °C), which is ∼4476-fold higher than H⁺⊂VNU-23 under the same conditions. Remarkably, the proton conductivity of Im_13.5⊂VNU-23 exceeds the values at 85% RH for several of the reported high-performing MOF materials. Furthermore, Im_13.5⊂VNU-23 can retain a stable proton conductivity for more than 96 h, as evidenced by FT-IR and PXRD analyses. These results prove that this hybrid material possesses potential applications as a commercial proton exchange membrane fuel cell.

A series of N-heterocyclic⊂VNU-23 materials have been prepared via the impregnation procedure of N-heterocyclic molecules into VNU-23.

Proton-conducting metal–organic frameworks with linkers containing anthracenyl and sulfonate groups

Co(dia)1.5(Hsip)(H2O)·H2O (1) and Zn2(μ-OH)(dia)2(sip)·2H2O (2) were prepared from the same set of ligand precursors. They exhibited bnn and dia topologies, respectively. Factors that contributed to the higher proton conductivity of 1 were presented.

High Proton Conduction in Three Highly Water-Stable Hydrogen-Bonded Ferrocene-Based Phenyl Carboxylate Frameworks

To acquire more new crystalline proton conductive materials, three ferrocene-based phenyl carboxylate frameworks (FCFs), [FcCO(o-C₆H₄COOH)] (FCF 1) (Fc = (η⁵-C₅H₅)Fe(η⁵-C₅H₄)), [m-FcC₆H₄COOH] (FCF 2), and [p-FcC₆H₄COOH] (FCF 3), supported by hydrogen bonds and π···π interactions were prepared. Their structures and phase purities are clarified by single-crystal X-ray diffraction or powder X-ray diffraction (PXRD). In addition, their high thermal and water stability were confirmed by thermogravimetric analyses, PXRD, and scanning electron microscopy determinations. Proton conductivity (σ) of 1-3 was studied under different relative humidities (RHs) and temperatures, and it was found that their σ boosted with the increase of humidity and temperature. Under 100 °C and 98% RH, their optimal σ values are 0.77 × 10^-3, 1.94 × 10^-4, and 3.46 × 10^-3 S·cm^-1, respectively. Consequently, their proton conductive mechanisms were proposed by means of activation energy calculation and structural analysis. Note that they are good proton conductive materials and are expected to be used in proton exchange membrane fuel cells.

Proton-Conductive Cerium-Based Metal-Organic Frameworks

In this study, proton-conducting behaviors of a cerium-based metal-organic framework (MOF), Ce-MOF-808, its zirconium-based isostructural MOF, and bimetallic MOFs with various Zr-to-Ce ratios are investigated. The significantly increased proton conductivity (σ) and decreased activation energy (E_a) are obtained by substituting Zr with Ce in the nodes of MOF-808. Ce-MOF-808 achieves a σ of 4.4 × 10^-3 S/cm at 25 °C under 99% relative humidity and an E_a of 0.14 eV; this value is among the lowest-reported E_a of proton-conductive MOFs. Density functional theory calculations are utilized to probe the proton affinities of these MOFs. As the first study reporting the proton conduction in cerium-based MOFs, the finding here suggests that cerium-based MOFs should be a better platform for the design of proton conductors compared to the commonly reported zirconium-based MOFs in future studies on energy-related applications.

Dual-Functional Coordination Polymer with High Proton Conductivity and a Low-Detection-Limit Fluorescent Probe

A coordination polymer with dual functions of high proton conductivity and highly sensitive fluorescent sensors demonstrates a great application potential. In this work, a cadmium-based coordination polymer (denoted as CP 1) with hydrothermal stability was synthesized. The abundant coordination water, lattice water, and amino groups make an extended hydrogen-bonding pathway for efficient proton migration, which endows CP 1 with the highest proton conductivity of 2.41 × 10^-3 S·cm^-1 at 353 K and 98% RH. Especially, the proton conductivity of the chitosan (CS) hybrid membrane containing CP 1 reaches a maximum value of 2.62 × 10^-2 S·cm^-1 under 343 K and 98% RH, which increases almost 7 times higher than that of the pure CS membrane due to the host-guest collaboration. Furthermore, luminescence studies revealed that CP 1 is a high-sensitivity and good-selectivity fluorescent probe for the detection of trace amounts of l-histidine with a lowest detection limit of 1.0 × 10^-8 M.

Encapsulating NH4Br in a metal organic framework: achieving remarkable proton conduction in a wide relative humidity range

Proton-conducting materials are key components for constructing high-energy-density electronic devices. In this work, by accumulating NH₄Br into the nanospace of the classical metal organic framework MIL-101-Cr, a proton conductivity as high as 1.53 × 10^-1 S cm^-1 was achieved at 363 K and 100% RH. The proton conduction of NH₄Br@MIL-101-Cr was also high even at lower relative humidity; for instance, it was ∼10^-2 S cm^-1 at 75% RH. The activation energy was calculated to be 0.11 eV for NH₄Br@MIL-101-Cr, indicative of tight H-bond networks and a low barrier to proton transfer, and confirming the occurrence of pure proton conduction as well.

Construction of unimpeded proton-conducting pathways in solution-processed nanoporous polymer membranes

Developing proton-conducting membranes with three-dimensional conductivity and expedited interfacial contact is requested in the field of fuel cells. Here, we present a design strategy by combining solution processing and material flexibility into amorphous and porous polymers. We design a nanoporous polymer whose skeleton contains dihydrophenazine as a proton-accepting site, and subsequently protonate these sites to produce abundant charges on the polymer skeletons, which enables ionic polymers to be well dispersed in organic solvents and guarantees that they can be fabricated into uniform and amorphous membranes in a solution-processed manner. Importantly, after protonation, the dihydrophenazines change to proton-donating sites, which exhibit dynamic local motions that assist proton exchange on the polymer skeletons and thus construct three-dimensional and unimpeded proton-conduction pathways, with a striking proton conductivity of 0.30 S cm^-1 (298 K and 90% relative humidity), a low resistance of 3.02 Ω, and a H⁺ transport number of 0.98 that was very close to the upper limitation of 1.0.

Efficiently Boosting Moisture Retention Capacity of Porous Superprotonic Conducting MOF-802 at Ambient Humidity via Forming a Hydrogel Composite Strategy

Metal-organic frameworks (MOFs) provided a versatile platform for the development of new solid protonic electrolytes but faced great challenges regarding their low chemical stability and poor moisture retention capacity. Herein, we presented the proton-conducting study for zirconium-based MOF-802, revealing that MOF-802 possessed excellent features of extra aqueous and acidic stabilities and room-temperature superprotonic conduction with a proton conductivity of 1.05 × 10^-2 S cm^-1 at 288 K under 98% relative humidity (RH). Unfortunately, due to the liberation of water molecules from pores/channels, the proton conductivity of MOF-802 dropped significantly at the temperature above 318 K. To solve this issue, for the first time, MOF-802 was hybridized with poly(vinyl alcohol) (PVA) to form MOF-802@PVA hydrogel composites, where the moisture retention capacity of MOF-802 was greatly improved, giving the high room-temperature proton conductivity over 10^-3 S cm^-1 under ambient humidity. This work paves a new way to improve the moisture retention capacity and proton-conducting performances of porous proton conductors.

Ultrahigh Proton Conduction via Extended Hydrogen-Bonding Network in a Preyssler-Type Polyoxometalate-Based Framework Functionalized with a Lanthanide Ion

The exploration of composition-structure-function relationship in proton-conducting solids remains a challenge in materials chemistry. Polyoxometalate-based compounds have been long considered as candidates for proton conductors; however, their low structural stability and a large decrease in conductivity under reduced relative humidity (RH) have limited their applications. To overcome such limitations, the hybridization of polyoxometalates with proton-conducting polymers has emerged as a promising method. Besides, 4f lanthanide ions possess a high coordination number, which can be utilized to attract water molecules and to build robust frameworks. Herein, a Preyssler-type polyoxometalate functionalized with a 9-coordinate Eu³⁺ (Eu[P₅W₃₀O₁₁₀K]^11-) is newly synthesized and combined with poly(allylamine) with amine moieties as protonation sites. The resulting robust crystalline composite exhibits an ultrahigh proton conductivity >10^-2 S cm^-1 at 368 K and 90% RH, which is still >10^-3 S cm^-1 at 50% RH, due to the strengthened and extended hydrogen-bonding network.

Phase Transformation Dynamics in Sulfate-Loaded Lanthanide Triphosphonates. Proton Conductivity and Application as Fillers in PEMFCs

Phase transformation dynamics and proton conduction properties are reported for cationic layer-featured coordination polymers derived from the combination of lanthanide ions (Ln3+) with nitrilo-tris(methylenephosphonic acid) (H6NMP) in the presence of sulfate ions. Two families of materials are isolated and structurally characterized, i.e., [Ln2(H4NMP)2(H2O)4](HSO4)2·nH2O (Ln = Pr, Nd, Sm, Eu, Gd, Tb, Er, Yb; n = 4-5, Series I) and [Ln(H5NMP)]SO4·2H2O (Ln = Pr, Nd, Eu, Gd, Tb; Series II). Eu/Tb bimetallic solid solutions are also prepared for photoluminescence studies. Members of families I and II display high proton conductivity (10-3 and 10-2 S·cm-1 at 80 °C and 95% relative humidity) and are studied as fillers for Nafion-based composite membranes in PEMFCs, under operating conditions. Composite membranes exhibit higher power and current densities than the pristine Nafion membrane working in the range of 70-90 °C and 100% relative humidity and with similar proton conductivity.

In Situ-Doped Superacid in the Covalent Triazine Framework Membrane for Anhydrous Proton Conduction in a Wide Temperature Range from Subzero to Elevated Temperature

Synthesis of solid-state proton-conducting membranes with low activation energy and high proton conductivity under anhydrous conditions is a great challenge. Here, we show a simple and convenient way to prepare covalent triazine framework membranes (CTF-Mx) with acid in situ doping for anhydrous proton conduction in a wide temperature range from subzero to elevated temperature (160 °C). The low proton dissociation energy and continuous hydrogen bond network in CTF-Mx make the membrane achieve high proton conductivity from 1.21×10^-3 S cm^-1 (-40 °C) to 2.08×10^-2 S cm^-1 (160 °C) under anhydrous conditions. Molecular dynamics and proton relaxation time analyses reveal proton hopping at low activation energies with greatly enhanced mobility in the CTF membranes.

Photoswitchable Metal-Organic Framework Thin Films: From Spectroscopy to Remote-Controllable Membrane Separation and Switchable Conduction

The preparation of functional materials from photoswitchable molecules where the molecular changes multiply to macroscopic effects presents a great challenge in material science. An attractive approach is the incorporation of the photoswitches in nanoporous, crystalline metal-organic frameworks, MOFs, often showing remote-controllable chemical and physical properties. Because of the short light-penetration depth, thin MOF films are particularly interesting, allowing the entire illumination of the material. In the present progress report, we review and discuss the status of photoswitchable MOF films. These films may serve as model systems for quantifying the isomer switching yield by infrared and UV-vis spectroscopy as well as for uptake experiments exploring the switching effects on the host-guest interaction, especially on guest adsorption and diffusion. In addition, the straightforward device integration facilitates various experiments. In this way, unique features were demonstrated, such as photoswitchable membrane separation with continuously tunable selectivity, light-switchable proton conductivity of the guests in the pores, and remote-controllable electronic conduction.

Rational strategies for proton-conductive metal-organic frameworks

Since the transition of energy platforms, proton-conducting materials have played a significant role in broad applications for electrochemical devices. In particular, solid-state proton conductors (SSPCs) are emerging as the electrolyte in fuel cells (FC), a promising power generation technology, because of their high performance and safety for operating in a wide range of temperatures. In recent years, proton-conductive porous metal-organic frameworks (MOFs) exhibiting high proton-conducting properties (>10-2 S cm-1) have been extensively investigated due to their potential application in solid-state electrolytes. Their structural designability, crystallinity, and porosity are beneficial to fabricate a new type of proton conductor, providing a comprehensive conduction mechanism. For the proton-conductive MOFs, each component, such as the metal centres, organic linkers, and pore space, is manipulated by a judicious predesign strategy or post-synthetic modification to improve the mobile proton concentration with an efficient conducting pathway. In this review, we highlight rational design strategies for highly proton-conductive MOFs in terms of MOF components, with representative examples from recent years. Subsequently, we discuss the challenges and future directions for the design of proton-conductive MOFs.

Excellent humidity sensor based on ultrathin HKUST-1 nanosheets

An excellent humidity sensor based on ultrathin HKUST-1 nanosheets was developed and some insights for the morphology–activity relationship were provided.

A water-stable molecular cadmium phosphonate bearing 2-(2-pyridyl)benzimidazole as a highly sensitive luminescence sensor for the selective detection of bisphenol AF and bisphenol B

A highly water-stable molecular cadmium phosphonate bearing 2-(2-pyridyl)benzimidazole has been used as a sensor platform for the luminescence detection of bisphenol AF (BPAF) and bisphenol B (BPB) in water with good sensitivity and selectivity.

Fabrication of new structures from a 3D cobalt phosphonate network: structural transformation and proton conductivity investigation

Three new compounds with novel structures were prepared from cobalt 318, among which, 0-dimensional structural compound 1 shows a higher proton conductivity.

Proton conductivity in mixed cation phosphate, KMg1-xH2x(PO3)·yH2O, with a layered structure at low-intermediate temperatures

Proton solid electrolytes, which exhibit high proton conductivity at a wide range of low-intermediate temperatures (150-300 °C), are key materials for the development of fuel cells for automobiles and cogeneration systems. In this study, a benitoite-type polyphosphate, KMg1-xH2x(PO3)·yH2O, which has a non-combustible and layered structure, was investigated as a new proton conductor. The benitoite-type KMg1-xH2x(PO3)·yH2O was synthesised by a coprecipitation method. The solid solution formed in the range of x = 0-0.100 in KMg1-xH2x(PO3)3·yH2O. Multi-step weight loss due to dehydration was observed for TG/DTA measurement at 30 °C and 150 °C. We observed enhanced peaks of the vibration bands at around 1117 cm-1 and 1229 cm-1, which were attributed to the symmetric and asymmetric PO2 vibration modes, and at 743 cm-1 and 970 cm-1 due to the ns(P-O-P) and nas(P-O-P) modes as well as broad absorbance peaks at 2300 cm-1 and 2700 cm-1 corresponding to the vibration modes of ns(P-O-H) with increasing x for FTIR spectra, which suggest the introduction of protons to the crystal structure. Proton conductivity increased from x = 0 to 0.10 and then decreased at x = 0.125, where the impurity phase was observed. The sample with x = 0.10 in benitoite-type KMg1-xH2x(PO3)3·yH2O exhibited high proton conductivity of 1.4 × 10-3 S cm-1 at 150 °C and 6.5 × 10-3 S cm-1 at 250 °C under a non-humidified N2 gas flow.

Recent Progress in the Development of Composite Membranes Based on Polybenzimidazole for High Temperature Proton Exchange Membrane (PEM) Fuel Cell Applications

The rapid increasing of the population in combination with the emergence of new energy-consuming technologies has risen worldwide total energy consumption towards unprecedent values. Furthermore, fossil fuel reserves are running out very quickly and the polluting greenhouse gases emitted during their utilization need to be reduced. In this scenario, a few alternative energy sources have been proposed and, among these, proton exchange membrane (PEM) fuel cells are promising. Recently, polybenzimidazole-based polymers, featuring high chemical and thermal stability, in combination with fillers that can regulate the proton mobility, have attracted tremendous attention for their roles as PEMs in fuel cells. Recent advances in composite membranes based on polybenzimidazole (PBI) for high temperature PEM fuel cell applications are summarized and highlighted in this review. In addition, the challenges, future trends, and prospects of composite membranes based on PBI for solid electrolytes are also discussed.

Optimizing the Proton Conductivity of Fe-Diphosphonates by Increasing the Relative Number of Protons and Carrier Densities

Proton conductive materials have attracted extensive interest in recent years due to their fascinating applications in sensors, batteries, and proton exchange membrane fuel cells. Herein, two Fe-diphosphonate chains (H₄-BAPEN)_0.5·[Fe^III(H-HEDP)(HEDP)_0.5(H₂O)] (1) and (H₄-TETA)₂·[Fe^III₂Fe^II(H-HEDP)₂(HEDP)₂(OH)₂]·2H₂O (2) (HEDP = 1-hydroxyethylidenediphosphonate, BAPEN = 1,2-bis(3-aminopropylamino)ethane, and TETA = triethylenetetramine) with different templating agents were prepared by hydrothermal reactions. The valence states of the Fe centers were demonstrated by ⁵⁷Fe Mössbauer spectra at 100 K, with a high-spin Fe^III state for 1 and mixed high-spin Fe^III/Fe^II states for 2. Their magnetic properties were determined, which featured strong antiferromagnetic couplings in the chain. Importantly, the proton conductivity of both compounds at 100% relative humidity was explored at different temperatures, with 2.79 × 10^-4 S cm^-1 at 80 °C for 1 and 7.55 × 10^-4 S cm^-1 at 45 °C for 2, respectively. This work provides an opportunity for improving proton conductive properties by increasing the relative number of protons and the carrier density using protonated flexible aliphatic amines.

Enhanced proton conductivity of metal organic framework at low humidity by improvement in water retention

A series of composites have been fabricated by introducing ionic liquid (IL) (ship) into chromium terephthalate MIL-101 (bottle) by ship-in-bottle method (IL@MIL-101s), the resulting IL@MIL-101s are endowed to high water retention, which is essential to proton conducting on multiple energy-involved applications at the low relative humidity (RH). The humidifying IL can lower water loss and increase water uptake, and thus improves water retention properties of the composites aided by the mesoporous MIL-101 at low RH. The hydropenic proton transfer pathways are modeled inside MOF and between IL-MOF, diminishing energy barrier routes for proton hopping, and thus a promotive proton transfer is rendered via Grotthuss mechanism. Specially, the IL@MIL-101 (SIB-3) unfolds a high proton conductivity (σ = 4.4 × 10^-2 S cm^-1) at RH as low as ~23%, five orders of magnitude increase than that of parent MIL-101 (1.1 × 10^-7 S cm^-1) at 323 K. Besides, IL@MIL-101s as fillers are incorporated into polymer blends to form hybrid membranes, appearing the relatively high proton conductivity (4.3 × 10^-3 S cm^-1) under ~23% RH at 323 K.