Proton conduction and impedance sensing of a highly stable copper–organic framework from imidazole dicarboxylate

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.

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.

A mixed strategy to fabricate two bifunctional ligand-based Ag-complexes with high proton conductivity

High proton conductivity materials BAg-1 and BAg-2 were obtained using a mixed strategy with the same main bifunctional ligand.

Multiple Strategies to Fabricate a Highly Stable 2D CuIICuI-Organic Framework with High Proton Conductivity

Using multifunctional organic ligands with multiple acidic groups (carboxylate and sulfonate groups) to synthesize metal-organic frameworks (MOFs) bearing effective H-bond networks is a promising strategy to obtain highly proton conductive materials. In this work, a highly stable two-dimensional MOF, [Cu^II₅Cu^I₂(μ₃-OH)₄(H₂O)₆(L)₂(H₂L)₂]·3H₂O (denoted as YCu161; H₃L = 6-sulfonaphthalene-1,4-dicarboxylic acid) containing mixed-valence [Cu^II₅Cu^I₂(μ₃-OH)₄]⁸⁺ subunits, was successfully prepared. It exhibited excellent stability and temperature- and humidity-dependent proton conduction properties. Its optimal proton conductivity reached 1.84 × 10^-3 S cm^-1 at 90 °C and 98% relative humidity. On the basis of a crystal structure analysis, water vapor adsorption test results, and activation energy calculations, we deduced the proton conduction pathway and mechanism. Apparently, uncoordinated sulfonic and carboxyl groups and a network of abundant H-bonds inside the framework were responsible for the efficient proton transfer. Therefore, the strategy of selecting suitable bifunctional ligands to construct two-dimensional Cu-cluster-based MOFs with excellent proton conductivity is feasible.

Structural diversity and magnetic properties of six ferrocenyl monocarboxylate Mn(ii), Ni(ii) and Co(ii) complexes with 1D aqua, carboxyl or dinuclear hydroxyl bridges

Six ferrocenyl monocarboxylate Mn(ii), Ni(ii) and Co(ii) complexes with different types of magnetic coupling bridges were synthesized successfully. 1–6 display intriguing structure diversity and magnetic properties.

Proton conductivity studies on five isostructural MOFs with different acidity induced by metal cations

Five isostructural MOFs display very different proton conductivities despite the same proton transfer pathway. This difference is caused by the different coordination ability between the metal cations and the ligand.

High protonic conduction in two metal–organic frameworks containing high-density carboxylic groups

The proton conductivities of two stable 2D MOFs are much higher than those of most non-porous PC-MOFs and comparable to those of porous PC-MOFs.

Proton conduction in two Cu/Zn dimer-based hydrogen-bonded supramolecular frameworks from imidazole multi-carboxylate

At 98% RH and 100 °C, two dimer-based HSFs show the best σ values of 3.13 × 10⁻⁴ S cm⁻¹ for 1 and 0.55 × 10⁻⁴ S cm⁻¹ for 2, which can be compared to the values of reported HSFs.

Water-assisted proton conductivity of two highly stable imidazole multi-carboxylate-based MOFs

The proton conduction and proton mechanisms of two highly stable imidazole multi-carboxylate-based MOFs have been investigated and discussed.