Superionic Conduction over a Wide Temperature Range in a Metal–Organic Framework Impregnated with Ionic Liquids

High-Performance Proton Field-Effect Transistor Based on Two-Dimensional Cd Vacancy-Resided Cd0.85PS3Li0.15H0.15

Ion transport is a critical phenomenon underpinning numerous biological, physical, and chemical systems. Proton transistors leveraging proton transport face significant limitations, such as a low on-off ratio and deficient carrier mobility, which restrict their applicability in biological and other scenarios. This study explores the use of two-dimensional (2D) vacancy-residing transition metal phosphorus trichallcogenide-based membranes as the active layer for proton field-effect transistors. The synthesized Cd0.85PS3Li0.15H0.15 membrane exhibits a well-organized layered structure and high hydrophilicity, with nanometer-sized interlayers containing interconnected water networks. These distinct features facilitate proton conduction, leading to a high proton conductivity value of 0.83 S cm-1 at 98% relative humidity and 90 °C, with an activation energy of 0.26 eV. The Cd0.85PS3Li0.15H0.15-based proton transistor demonstrates tunability via gate voltage, thereby enabling effective modulation of proton flow across source and drain electrodes. The transistor notably showcases superior switching characteristics, with an on/off ratio surpassing 5.51 and a carrier mobility of 8.84 × 10-2 cm2 V-1 s-1. The underlying mechanism for this performance enhancement is attributed to electric-field-induced switching in Cd vacancies. This research boosts the development of highly versatile ionotropic devices by introducing advanced 2D ion-conductive membranes.

Determination of standard molar volume of 1-hexyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide on titanium dioxide surface

The fluids near the solid substrate display different properties compared to the bulk fluids owing to the asymmetric interaction between the fluid and substrate; however, to the best of our knowledge, no work has been conducted to determine the interfacial properties of fluids experimentally. In this work, we combined a pycnometer with experimental measurements and data processing to determine the standard thermodynamic properties of interfacial fluids for the first time. In the study, 1-hexyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([Hmim][NTf2]) and titanium dioxide (P25) were chosen as the probes to prove the concept. It was found that, with the combination of the Gay-Lussac pycnometer and the colligative law, together with selecting a suitable solvent, it is possible and reliable to determine the standard molar volume of the immobilized [Hmim][NTf2]. Compared to the bulk phase, the molar volumes of [Hmim][NTf2] on the P25 surface reduce by 20.8%-23.7% at temperatures from 293.15 to 323.15 K, and the reduction degrees decrease with increasing temperatures. The newly determined standard thermodynamic data was used to obtain the model parameters of hybrid electrolyte perturbed-chain statistical associating fluid theory density functional theory (ePC-SAFT-DFT), and further predictions of the density of interfacial ionic liquids with different film thicknesses were proved to be reliable in comparison with the experiment results.

Recent progress and perspectives on metal-organic frameworks as solid-state electrolytes for lithium batteries

Solid-state electrolytes (SSEs) are the key materials in the new generation of all-solid-state lithium ion/metal batteries. Metal-organic frameworks (MOFs) are ideal materials for developing solid electrolytes because of their structural diversity and porous properties. However, there are several significant issues and obstacles involved, such as lower ion conductivity, a smaller ion transport number, a narrower electrochemical stability window and poor interface contact. In this review, a comprehensive analysis and summary of the unique ion-conducting behavior of MOF-based electrolytes in rechargeable batteries are presented, and the different design principles of MOF-based SSEs are classified and emphasized. Accordingly, four design principles for achieving these MOF-based SSEs are presented and the influence of SSEs combined with MOFs on the electrochemical performance of the batteries is described. Finally, the challenges in the application of MOF materials in lithium ion/metal batteries are explored, and directions for future research on MOF-based electrolytes are proposed. This review will deepen the understanding of MOF-based electrolytes and promote the development of high-performance solid-state lithium ion/metal batteries. This review not only provides theoretical guidance for research on new MOF-based SSE systems, but also contributes to further development of MOFs applied to rechargeable batteries.

On the Diffusion of Ionic Liquids in ILs@ZIF-8 Composite Materials: A Density Functional Theory Study

Recently, composite materials consisting of ionic liquids (ILs) and metal-organic frameworks (MOFs) have attracted a great deal of attention due to their fantastic properties. Many theoretical studies have been performed on their special structures and gas separation applications. Yet, the mechanism for the diffusion of ILs inside MOF channels still remains unclear. Here, the DFT calculations (e.g., rigid and relaxed potential energy surface, PES, scan) together with frontier orbital analysis, natural charge analysis, and energy decomposition analysis were performed to investigate the diffusion behavior of a typical IL, [C4mim][PF6], into the ZIF-8 SOD cage. The PES profiles indicate that it is quite difficult for the cation [C4min]+ to diffuse into the cage of ZIF-8 through the pristine pores because of the large imidazole steric hindrance, which results in a large energy barrier of ca. 40 kcal·mol-1 at the least. Interestingly, the PES reveals that a successful diffusion could be obtained by thermal contributions, which enlarge the pore size through swing effects at higher temperatures. For example, both [C4mim]+ and [PF6]- could easily diffuse through the channel of the ZIF-8 SOD cage when the pore size was increased to 6.9 Å. Subsequently, electronic structure analyses reveal that the main interactions between [PF6]- or [C4mim]+ and ZIF-8 are the steric repulsion interactions. Finally, the effects of the amounts of [C4mim][PF6] on the ZIF-8 structures were investigated, and the results show that two pairs of [C4mim][PF6] per SOD cage are the most stable in terms of the interaction between energies and structural changes. With these findings, we propose that the high-temperature technique could be employed during the synthesis of IL@MOF membranes, to enrich their family members and their industrial applications.

Confining Ionic Liquids in Developing Quasi-Solid-State Electrolytes for Lithium Metal Batteries

The concept of confining ionic liquids (ILs) in developing quasi-solid-state electrolytes (QSSEs) has been proposed, where ILs are dispersed in polymer networks/backbones and/or filler/host pores, forming the so-called confinement, and great research progress and promising research results have been achieved. In this review, the progress and achievement in developing QSSEs using IL-confinement for lithium metal batteries (LMBs), together with advanced characterizations and simulations, were surveyed, summarized, and analyzed, where the influence of specific parameters, such as IL (type, content, etc.), substrate (type, structure, surface properties, etc.), confinement methods, and so on, was discussed. The confinement concept was further compared with the conventional one in other research areas. It indicates that the IL-confinement in QSSEs improves the performance of electrolytes, for example, increasing the ionic conductivity, widening the electrochemical window, and enhancing the cycle performance of the assembled cells, and being different from those in other areas, that is, the IL-confinement concept in the battery area is in a broad extent. Finally, insights into developing QSSEs in LMBs with the confinement strategy were provided to promote the development and application of QSSE LMBs.

Enabling High-Performance All-Solid-State Batteries via Guest Wrench in Zeolite Strategy

All-solid-state batteries with a high energy density and safety are desirable candidates for next-generation energy storage applications. However, conventional solid electrolytes for all-solid-state batteries encounter limitations such as poor ionic conduction, interfacial compatibility, instability, and high cost. Herein, taking advantage of the ingenious capability of zeolite to incorporate functional guests in its void space, we present an innovative ionic activation strategy based on the "guest wrench" mechanism, by introducing a pair of cation and anion of LiTFSI-based guest species (GS) into the supercage of the LiX zeolite, to fabricate a zeolite membrane (ZM)-based solid electrolyte (GS-ZM) with high Li ionic conduction and interfacial compatibility. The restriction of zeolite frameworks toward the framework-associated Li ions is significantly reduced through the dynamic coordination of Li ions with the "oxygen wrench" of TFSI- at room temperature as shown by experiments and Car-Parrinello molecular dynamics simulations. Consequently, the GS-ZM shows an ∼100% increase in ionic conductivity compared with ZM and an outstanding Li+ transference number of 0.97. Remarkably, leveraging the superior ionic conduction of GS-ZM with the favorable interface structure between GS-ZM and electrodes, the assembled all-solid-state Li-ion and Li-air batteries based on GS-ZM exhibit the best-level electrochemical performance much superior to batteries based on liquid electrolytes: a capacity retention of 99.3% after 800 cycles at 1 C for all-solid-state Li-ion batteries and a cycle life of 909 cycles at 500 mA g-1 for all-solid-state Li-air batteries. The mechanistic discovery of a "guest wrench" in zeolite will significantly enhance the adaptability of zeolite-based electrolytes in a variety of all-solid-state energy storage systems with high performance, high safety, and low cost.

Liquid-Liquid Phase Separation of Aqueous Ionic Liquids in Covalent Organic Frameworks for Thermal Switchable Proton Conductivity

Covalent organic frameworks (COFs) have regular channels that can accommodate guest molecules to provide highly conductive solid electrolytes. However, designing smart, conductive COFs remains a great challenge. Herein, we report the first example of PEG-functionalized ionic liquids (ILs) anchored on the COF walls by strong hydrogen bonding to fabricate thermally responsive COFs (ILm@COF). We found that similar to the traditional IL/water mixture, the ILs undergo lower critical solution temperature (LCST)-type phase behavior within COF nanopores under high moisture levels. However, the phase separation temperature of aqueous IL decreases in COF channels due to the strong interaction between the IL and COF. Thus, the proton conductivity of ILm@COF can be reversibly switched by phase miscibility and separation in COF nanopores, and there is no obvious decrease even after 20 switching cycles. Our work provides important clues for understanding liquid-liquid phase separation in a confined nanospace and opens a new pathway to switchable proton conductivity.

Ionic Liquids Functionalized MOFs for Adsorption

Metal-organic frameworks (MOFs) and ionic liquids (ILs) represent promising materials for adsorption separation. ILs incorporated into MOF materials (denoted as IL/MOF composites) have been developed, and IL/MOF composites combine the advantages of MOFs and ILs to achieve enhanced performance in the adsorption-based separation of fluid mixtures. The designed different ILs are introduced into the various MOFs to tailor their functional properties, which affect the optimal adsorptive separation performance. In this Perspective, the rational fabrication of IL/MOF composites is presented, and their functional properties are demonstrated. This paper provides a critical overview of an emergent class of materials termed IL/MOF composites as well as the recent advances in the applications of IL/MOF composites as adsorbents or membranes in fluid separation. Furthermore, the applications of IL/MOF in adsorptive gas separations (CO2 capture from flue gas, natural gas purification, separation of acetylene and ethylene, indoor pollutants removal) and liquid separations (separation of bioactive components, organic-contaminant removal, adsorptive desulfurization, radionuclide removal) are discussed. Finally, the existing challenges of IL/MOF are highlighted, and an appropriate design strategy direction for the effective exploration of new IL/MOF adsorptive materials is proposed.

A Metal-Organic Framework Based Quasi-Solid-State Electrolyte Enabling Continuous Ion Transport for High-Safety and High-Energy-Density Lithium Metal Batteries

Solid-state lithium metal batteries are promising next-generation rechargeable energy storage systems. However, the poor compatibility of the electrode/electrolyte interface and the low lithium ion conductivity of solid-state electrolytes are key issues hindering the practicality of solid-state electrolytes. Herein, rational designed metal-organic frameworks (MOFs) with the incorporation of two types of ionic liquids (ILs) are fabricated as quasi-solid electrolytes. The obtained MOF-IL electrolytes offer continuous ion transport channels with the functional sulfonic acid groups serving as lithium ion hopping sites, which accelerate the Li⁺ transport both in the bulk and at the interfaces. The quasi-solid MOF-IL electrolytes exhibit competitive ionic conductivities of over 3.0 × 10^-4 S cm^-1 at room temperature, wide electrochemical windows over 5.2 V, and good interfacial compatibility, together with greatly enhanced Li⁺ transference numbers compared to the bare IL electrolyte. Consequently, the assembled quasi-solid Li metal batteries show either superior stability at low C rates or improved rate performance, related to the species of ILs. Overall, the quasi-solid MOF-IL electrolytes possess great application potential in high-safety and high-energy-density lithium metal batteries.

MOFs Containing Solid-State Electrolytes for Batteries

The use of metal-organic frameworks (MOFs) in solid-state electrolytes (SSEs) has been a very attractive research area that has received widespread attention in the modern world. SSEs can be divided into different types, some of which can be combined with MOFs to improve the electrochemical performance of the batteries by taking advantage of the high surface area and high porosity of MOFs. However, it also faces many serious problems and challenges. In this review, different types of SSEs are classified and the changes in these electrolytes after the addition of MOFs are described. Afterward, these SSEs with MOFs attached are introduced for different types of battery applications and the effects of these SSEs combined with MOFs on the electrochemical performance of the cells are described. Finally, some challenges faced by MOFs materials in batteries applications are presented, then some solutions to the problems and development expectations of MOFs are given.

A porous cobalt(II)-organic framework exhibiting high room temperature proton conductivity and field-induced slow magnetic relaxation

A two-dimensional (2D) cobalt(II) metal-organic framework (MOF) constructed by a ditopic organic ligand, formulated as {[Co(Hbic)(H₂O)]·4H₂O}_n (1) (H₂bic = 1H-benzimidazole-5-carboxylic acid), was hydrothermally synthesized and structurally characterized. Single-crystal X-ray diffraction shows that the distorted octahedral Co²⁺ ions, as coordination nodes, are bridged to form 2D honeycomb networks, which are further organized into a 3D supramolecular porous framework through multiple hydrogen bonds and interlayer π-π interactions. Dynamic crystallography experiments reveal the anisotropic thermal expansion behavior of the lattice, suggesting a flexible hydrogen-bonded 3D framework. Interestingly, hydrogen-bonded (H₂O)₄ tetramers were found to be located in porous channels, yielding 1D proton transport pathways. As a result, the compound exhibited a high room-temperature proton conductivity of 1.6 × 10^-4 S cm^-1 under a relative humidity of 95% through a Grotthuss mechanism. Magnetic investigations combined with theoretical calculations reveal giant easy-plane magnetic anisotropy of the distorted octahedral Co²⁺ ions with the experimental and computed D values being 87.1 and 109.3 cm^-1, respectively. In addition, the compound exhibits field-induced slow magnetic relaxation behavior at low temperatures with an effective energy barrier of U_eff = 45.2 cm^-1. Thus, the observed electrical and magnetic properties indicate a rare proton conducting SIM-MOF. The foregoing results provide a unique bifunctional cobalt(II) framework material and suggest a promising way to achieve magnetic and electrical properties using a supramolecular framework platform.

Exploring ionic liquid-laden metal-organic framework composite materials as hybrid electrolytes in metal (ion) batteries

This review focuses on the combination of metal-organic frameworks (MOFs) and ionic liquids (ILs) to obtain composite materials to be used as solid electrolytes in metal-ion battery applications. Benefiting from the controllable chemical composition, tunable pore structure and surface functionality, MOFs offer great opportunities for synthesizing high-performance electrolytes. Moreover, the encapsulation of ILs into porous materials can provide environmentally benign solid-state electrolytes for electrochemical devices. Due to the versatility of MOF-based materials, in this review we also explore their use as anodes and cathodes in Li- and Na-ion batteries. Finally, solid IL@MOF electrolytes and their implementation into Li and Na batteries have been analyzed, as well as the design and advanced manufacturing of solid IL@MOF electrolytes embedded on polymeric matrices.

Remarkable proton conducting behavior driven by synergistic effects in acid-base conjugated MOFs impregnated with sulphuric acid molecules

The Coulomb force between protons and negative charges usually leads to low proton diffusion and poor proton conductivity. Herein, acid-base conjugated MOFs of UiO-66-SO₃^--NH₃⁺ were constructed by linking sulfonic acid and amine groups to an organic skeleton. According to the XPS and elemental analysis, the ratio of 2-aminoterephthalic acid/2-sulfoterephthalic acid was ∼1.9 in UiO-66-SO₃^--NH₃⁺. After introducing sulphuric acid molecules, the MOF-based electrolyte exhibited a remarkable proton conductivity of 5.40 × 10^-1 S cm^-1 and low activation energy of 0.15 eV at 100% RH and 90 °C. This remarkable proton conducting behavior was generated by the synergistic system in the MOF, which contained certain synergistic effects such as between -SO₃^-⋯H⁺⋯SO₄^2- and NH₃⁺⋯SO₄^2-, therefore possessing a Grotthuss mechanism, which facilitates facile proton transport.

Solitary Medium of a Multifunctional Ionic Liquid for Crystallizing Hierarchically Porous Metal-Organic Frameworks

Hierarchically porous metal-organic frameworks (HP-MOFs), dominating both the micro- and mesoporous regimes, show high potentials in various applications especially those involving bulky biomolecules. The templating method has been proven to be effective in the fabrication of HP-MOFs; however, complicated synthetic systems containing solvents, templates, and additives are frequently employed. Here we report the first example of designing a poly(ethylene glycol)-based alkylammonium and bromide multifunctional ionic liquid (IL) as a solitary medium to construct HP-MOFs, avoiding the involvement of any additional media. Besides the ready solubilization of MOF precursors in the multifunctional IL due to a poly(ethylene glycol) chain as the solubilizer, the ionic moiety facilitates electrostatic interaction to create a templating effect. Hence, UiO-66 with hierarchical porosity has been successfully fabricated, and such a methodology can also be applied to the construction of other HP-MOFs. The resultant HP-UiO-66 is efficient in the encapsulation of bulky biomolecule cytochrome c, and the adsorption capacity is obviously superior to that of the microporous counterpart.

Molecular-Level Understanding of Surface Roughness Boosting Segregation Behavior at the ZIF-8/Ionic Liquid Interfaces

Here, we perform a series of classical molecular dynamics simulations for two different [HEMIM][DCA] and [BMIM][BF₄] ionic liquids (ILs) on the ZIF-8 surface to explore the interfacial properties of metal-organic framework (MOFs)/IL composite materials at the molecular level. Our simulation results reveal that the interfacial structures of anions and cations on the ZIF-8 surface are dominated by the surface roughness due to the steric hindrance, which is extremely different from the driving mechanism based on solid-ion interactions of ILs on flat solid surfaces. At the ZIF-8/IL interfaces, the open sodalite (SOD) cages of the ZIF-8 surface can block most of the large-size cations outside and significantly boost the segregation behavior of anions and cations. In comparison with the [BMIM][BF₄] IL, the [HEMIM][DCA] IL has much more anions entering into the open SOD cages owing to the combination of stronger ZIF-8-[DCA]^- interactions and more ordered arrangement of [DCA]^- anions on the ZIF-8 surface. Furthermore, more and stronger ZIF-8-[BF₄]^- hydrogen bonds (HBs) are found to exist on the cage edges than the ZIF-8-[DCA]^- HBs, further preventing [BF₄]^- anions from entering into SOD cages. By more detailed analyses, we find that the hydrophobic interaction has an important influence on the interfacial structures of the side chains of [HEMIM]⁺ and [BMIM]⁺ cations, while the π-π stacking interaction plays a key role in determining the interfacial structures of the imidazolium rings of both cations. Our simulation results in this work provide a molecular-level understanding of the underlying driving mechanism on segregation behavior at the ZIF-8/IL interfaces.

Ionic Liquid@Metal-Organic Framework as a Solid Electrolyte in a Lithium-Ion Battery: Current Performance and Perspective at Molecular Level

Searching for a suitable electrolyte in a lithium-ion battery is a challenging task. The electrolyte must not only be chemically and mechanically stable, but also be able to transport lithium ions efficiently. Ionic liquid incorporated into a metal-organic framework (IL@MOF) has currently emerged as an interesting class of hybrid material that could offer excellent electrochemical properties. However, the understanding of the mechanism and factors that govern its fast ionic conduction is crucial as well. In this review, the characteristics and potential use of IL@MOF as an electrolyte in a lithium-ion battery are highlighted. The importance of computational methods is emphasized as a comprehensive tool to investigate the atomistic behavior of IL@MOF and its interaction in electrochemical environments.

Superprotonic Conductivity of a Functionalized Metal-Organic Framework at Ambient Conditions

Seeking fast proton transport pathways at ambient conditions is desirable but challenging. Here, we report a strategy to synthesize a composite material with a polyoxometalate (POM) and an ionic liquid (IL) confined in stable metal-organic framework (MOF) channels through electrostatic interaction. The obtained SO₃H-IL-PMo₁₂@MIL-101 possesses fast proton transfer, and its proton conductivity can reach 1.33 × 10^-2 S cm^-1 at ambient conditions (30 °C, 70% relative humidity (RH)), which is the highest value among the MOF-based proton conductors operated in an ambient environment. Therefore, it has the potential of becoming a room-temperature proton conductor without a humidifier. Importantly, the composite material is further fabricated into a composite membrane for proton-exchange membrane fuel cells (PEMFCs), which can deliver a power density of 0.93 mW cm^-2 at 30 °C and 98% RH. This result can lay a fundamental basis for the application of MOF-based proton conductors in the area of electrochemical energy conversion.

Robust ionic liquid@MOF composite as a versatile superprotonic conductor

A highly performing proton conducting composite was prepared through the impregnation of EMIMCl ionic liquid in the mesoporous MIL-101(Cr)-SO₃H MOF. The resulting EMIMCl@MIL-101(Cr)-SO₃H composite displays high thermal and chemical stability, alongside retention of a high amount of EMIMCl even at temperatures as high as 500 K, as well as under moisture conditions. Remarkably, this composite exhibits outstanding proton conductivity not only at the anhydrous state (σ_{473 K} = 1.5 × 10^-3 S cm^-S) but also under humidity (σ_{(343 K/60%-80%RH)} ≥ 0.10 S cm^-1) conditions. This makes EMIMCl@MIL-101(Cr)-SO₃H a unique candidate to act as a solid state proton conductor for PEMFC applications under versatile conditions.

Structural and Dynamic Insights into the Conduction of Lithium-Ionic-Liquid Mixtures in Nanoporous Metal-Organic Frameworks as Solid-State Electrolytes

Metal-organic framework (MOF)-based separators in Li-ion batteries (LIBs) have the potential to improve the battery performance. The mobility and conduction of lithium and organic ionic liquids (ILs) in these materials acting as (quasi) solid-state electrolytes are crucial for the battery power output. Here, we investigate the mobility of a Li-based IL in MOF nanopores and unveil the details of the conduction mechanism by molecular dynamics (MD) simulations. A complex conductivity depending on the Li-IL loading and on the IL composition is observed. Most importantly, the presence of Li prevents the collapse of the conductivity at high IL loadings. The fully atomistic MD simulations including guest-guest and guest-host interactions elucidate the competing mechanisms: Li follows a Grotthuss-like conduction mechanism with large mobility. While at small pore fillings, the Li conduction is limited by the large distance between the anions facilitating the Grotthuss-like conduction; the conduction at high pore fillings is governed by field-induced concentration inhomogeneities. Because of the small MOF pore windows, which hinders the simultaneous passage of the large IL cations and anions in opposite directions, the IL shows field-induced MOF pore blocking and ion bunching. The regions of low anion concentration and high cation concentration represent barriers for Li, decreasing its mobility. In comparison to Li-free IL, the IL bunching effect is attenuated by the formation of charge-neutral Li-anion complexes, resulting in a tremendously increased conductivity at maximum pore filling. The exploitation of this mechanism may enhance the development of advanced batteries based on IL and nanoporous separators.

Recent advances in process engineering and upcoming applications of metal-organic frameworks

Progress in metal-organic frameworks (MOFs) has advanced from fundamental chemistry to engineering processes and applications, resulting in new industrial opportunities. The unique features of MOFs, such as their permanent porosity, high surface area, and structural flexibility, continue to draw industrial interest outside the traditional MOF field, both to solve existing challenges and to create new businesses. In this context, diverse research has been directed toward commercializing MOFs, but such studies have been performed according to a variety of individual goals. Therefore, there have been limited opportunities to share the challenges, goals, and findings with most of the MOF field. In this review, we examine the issues and demands for MOF commercialization and investigate recent advances in MOF process engineering and applications. Specifically, we discuss the criteria for MOF commercialization from the views of stability, producibility, regulations, and production cost. This review covers progress in the mass production and formation of MOFs along with future applications that are not currently well known but have high potential for new areas of MOF commercialization.

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.

Comparison of the ionic conductivity properties of microporous and mesoporous MOFs infiltrated with a Na-ion containing IL mixture

IL@MOF (IL: ionic liquid; MOF: metal-organic framework) materials have been proposed as a candidate for solid-state electrolytes, combining the inherent non-flammability and high thermal and chemical stability of the ionic liquid with the host-guest interactions of the MOF. In this work, we compare the structure and ionic conductivity of a sodium ion containing IL@MOF composite formed from a microcrystalline powder of the zeolitic imidazolate framework (ZIF), ZIF-8 with a hierarchically porous sample of ZIF-8 containing both micro- and mesopores from a sol-gel synthesis. Although the crystallographic structures were shown to be the same by X-ray diffraction, significant differences in particle size, packing and morphology were identified by electron microscopy techniques which highlight the origins of the hierarchical porosity. After incorporation of Na0.1EMIM0.9TFSI (abbreviated to NaIL; EMIM = 1-ethyl-3-methylimidazolium; TFSI = bis(trifluoromethylsulfonyl)imide), the hierarchically porous composite exhibited a 40% greater filling capacity than the purely microporous sample which was confirmed by elemental analysis and digestive proton NMR. Finally, the ionic conductivity properties of the composite materials were probed by electrochemical impedance spectroscopy. The results showed that despite the 40% increased loading of NaIL in the NaIL@ZIF-8micro sample, the ionic conductivities at 25 °C were 8.4 × 10-6 and 1.6 × 10-5 S cm-1 for NaIL@ZIF-8meso and NaIL@ZIF-8micro respectively. These results exemplify the importance of the long range, continuous ion pathways contributed by the microcrystalline pores, as well as the limited contribution from the discontinuous mesopores to the overall ionic conductivity.

Salt nanoconfinement in zirconium-based metal-organic frameworks leads to pore-size and loading-dependent ionic conductivity enhancement

The effect of nanoscale confinement of a salt on its ionic conductivity was studied for [NEt4][TFSI] melt-loaded in three isoreticular zirconium-based MOFs: UiO-66, UiO-67, and PCN-56. Conductivity of the MOF-salt composites was up to a factor of 50 higher than the pure salt, and maximized with slightly less than full loading of the MOFs.

Flexible Porous Organic Polymer Membranes for Protonic Field-Effect Transistors

Artificial transistors represent an ideal means for meeting the requirements in interfacing with biological systems. It is pivotal to develop new proton-conductive materials for the transduction between biochemical events and electronic signals. Herein, the first demonstration of a porous organic polymer membrane (POPM) as a proton-conductive material for protonic field-effect transistors is presented. The POPM is readily prepared through a thiourea-formation condensation reaction. Under hydrated conditions and at room temperature, the POPM delivers a proton mobility of 5.7 × 10^-3 cm² V^-1 s^-1 ; the charge carrier densities are successfully modulated from 4.3 × 10¹⁷ to 14.1 × 10¹⁷ cm^-3 by the gate voltage. This study provides a type of promising modular proton-conductive materials for bioelectronics application.

Metal-Organic Frameworks as a Versatile Platform for Proton Conductors

Metal-organic frameworks (MOFs) are an intriguing type of crystalline porous materials that can be readily built from metal ions or clusters and organic linkers. Recently, MOF materials, featuring high surface areas, rich structural tunability, and functional pore surfaces, which can accommodate a variety of guest molecules as proton carriers and to systemically regulate the proton concentration and mobility within the available space, have attracted tremendous attention for their roles as solid electrolytes in fuel cells. Recent advances in MOFs as a versatile platform for proton conduction in the field of humidity condition proton-conduction, anhydrous atmosphere proton-conduction, single-crystal proton-conduction, and including MOF-based membranes for fuel cells, are summarized and highlighted. Furthermore, the challenges, future trends, and prospects of MOF materials for solid electrolytes are also discussed.

Sodium Ion Conductivity in Superionic IL-Impregnated Metal-Organic Frameworks: Enhancing Stability Through Structural Disorder

Metal-organic frameworks (MOFs) are intriguing host materials in composite electrolytes due to their ability for tailoring host-guest interactions by chemical tuning of the MOF backbone. Here, we introduce particularly high sodium ion conductivity into the zeolitic imidazolate framework ZIF-8 by impregnation with the sodium-salt-containing ionic liquid (IL) (Na0.1EMIM0.9)TFSI. We demonstrate an ionic conductivity exceeding 2 × 10-4 S · cm-1 at room temperature, with an activation energy as low as 0.26 eV, i.e., the highest reported performance for room temperature Na+-related ion conduction in MOF-based composite electrolytes to date. Partial amorphization of the ZIF-backbone by ball-milling results in significant enhancement of the composite stability towards exposure to ambient conditions, up to 20 days. While the introduction of network disorder decelerates IL exudation and interactions with ambient contaminants, the ion conductivity is only marginally affected, decreasing with decreasing crystallinity but still maintaining superionic behavior. This highlights the general importance of 3D networks of interconnected pores for efficient ion conduction in MOF/IL blends, whereas pore symmetry is a less stringent condition.

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

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

Significantly enhancing the lithium-ion conductivity of solid-state electrolytes via a strategy for fabricating hollow metal–organic frameworks

A solid-state electrolyte based on hollow metal organic framework (MOF) materials was prepared. Due to its special hollow structure, the hollow MOF-based solid-state electrolyte exhibits superior ionic conductivity, which can store more lithium ions and transmits lithium ions by the wall of the cavity.