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

High-Concentration Self-Assembly of Zirconium- and Hafnium-Based Metal-Organic Materials

Metal-organic frameworks (MOFs) are crystalline, porous solids constructed from organic linkers and inorganic nodes that are promising for applications in chemical separations, gas storage, and catalysis, among many others. However, a major roadblock to the widespread implementation of MOFs, including highly tunable and hydrolytically stable Zr- and Hf-based frameworks, is their benchtop-scalable synthesis, as MOFs are typically prepared under highly dilute (≤0.01 M) solvothermal conditions. This necessitates the use of liters of organic solvent to prepare only a few grams of MOF. Herein, we demonstrate that Zr- and Hf-based frameworks (eight examples) can self-assemble at much higher reaction concentrations than are typically utilized, up to 1.00 M in many cases. Combining stoichiometric amounts of Zr or Hf precursors with organic linkers at high concentrations yields highly crystalline and porous MOFs, as confirmed by powder X-ray diffraction (PXRD) and 77 K N2 surface area measurements. Furthermore, the use of well-defined pivalate-capped cluster precursors avoids the formation of ordered defects and impurities that arise from standard metal chloride salts. These clusters also introduce pivalate defects that increase the exterior hydrophobicity of several MOFs, as confirmed by water contact angle measurements. Overall, our findings challenge the standard assumption that MOFs must be prepared under highly dilute solvothermal conditions for optimal results, paving the way for their scalable and user-friendly synthesis in the laboratory.

Ethylene oxide functionalization enhances the ionic conductivity of a MOF

Varying the degree of ethylene oxide (EO) functionalization of the zirconium MOF UiO-68 affords two novel MOFs; UiO-68-EO and UiO-68-2EO exhibit solvent-free ionic conductivity upon loading LiTFSI in their pores. Incorporating EO chains provides a pathway for lithium ion migration between the coordinated sites and results in an ionic conductivity of 3.8 × 10^-7 S cm^-1 and 3.9 × 10^-4 S cm^-1 at 90 °C for UiO-68-EO/LiTFSI and UiO-68-2EO/LiTFSI respectively.

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.

Insights in the Ionic Conduction inside Nanoporous Metal-Organic Frameworks by Using an Appropriate Equivalent Circuit

The conduction of protons and other ions in nanoporous materials, such as metal-organic frameworks (MOFs), is intensively explored with the aim of enhancing the performance of energy-related electrochemical systems. The ionic conductivity, as a key property of the material, is typically determined by using electrochemical impedance spectroscopy (EIS) in connection with a suitable equivalent circuit. Often, equivalent circuits are used where the physical meaning of each component is debatable. Here, we present an equivalent circuit for the ionic conduction of electrolytes in nanoporous, nonconducting materials between inert and impermeable electrodes without faradaic electrode reactions. We show the equivalent circuit perfectly describes the impedance spectra measured for the ion conduction in MOFs in the form of powders pressed into pellets as well as for MOF thin films. This is demonstrated for the ionic conduction of an aprotic ionic liquid, and of various protic solvents in different MOF structures. Due to the clear physical meaning of each element of the equivalent circuit, further insights into the electrical double layer forming at the MOF-electrode interface can be obtained. As a result, EIS combined with the appropriate reference circuit allows us to make statements of the quality of the MOF-substrate interface of different MOF-film samples.