Imidazole molecules were frequently incorporated into porous materials to improve their proton conductivity. To investigate how different arrangements of imidazoles in metal-organic frameworks (MOFs) affect the overall proton conduction, we designed and prepared a MOF-based model system. It includes an Fe-MOF as the blank, an imidazole@Fe-MOF (Im@Fe-MOF) with physically adsorbed imidazole, and an imidazole-Fe-MOF (Im-Fe-MOF), which contains chemically coordinated imidazole molecules. The parent Fe-MOF, synthesized from the exchange of carboxylates in the preformed [Fe(μ-O)](carboxylate) clusters and multitopic carboxylate ligands, serves as a control. The Im@Fe-MOF was prepared by encapsulating free imidazole molecules into the pores of the Fe-MOF, whereas the Im-Fe-MOF was obtained in situ, in which imidazole ligands coordinate to the metal nodes of the framework. Proton-conductivity analyses revealed that the proton conductivity of Im-Fe-MOF was approximately two orders of magnitude greater than those of Fe-MOF and Im@Fe-MOF at room temperature. The high proton conductivity of 1.21 × 10 S cm at 60 °C for Im-Fe-MOF ranks among the highest performing MOFs ever reported. The results of the density functional theory calculations suggest that coordinated imidazole molecules in Im-Fe-MOF provide a greater concentration of protons for proton transportation than do coordinated water molecules in Fe-MOF alone. Besides, Im-Fe-MOF exhibits steadier performance than Im@Fe-MOF does after being washed with water. Our investigation using the above ideal crystalline model system demonstrates that compared to disorderly arranged imidazole molecules in pores, the immobilized imidazole molecules by coordination bonds in the framework are more prone to form proton-conduction pathways and thus perform better and steadier in water-mediated proton conduction.

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A water-stable metal–organic framework (MOF)-based composite, H2SO4@MIL-101-SO3H, configured with functionalized sulfonate groups bound to the polymeric backbone and nonvolatile strong acid H2SO4 encapsulated into pores, is demonstrated as a superior proton-conducting material, owing to the resultant multiple proton-transfer pathways. This solid material exhibits a superprotonic conductivity of 1.82 S cm–1 (70 °C, 90% RH) that is comparable to that of other excellent MOF-based proton conductors reported thus far. More importantly, the splendid conductivity (0.92 × 10–2 S cm–1, −40 °C) of the title material achieves an enormous improvement compared with that of all proton-conducting systems demonstrated at subzero temperatures.

It is a great challenge to obtain oriented design and synthesis of new two-dimensional covalent organic frameworks (2D-COFs) with proper CN stoichiometry, N position, and band structures as specific electrocatalysts. Driven by density functional theory (DFT) calculations, we designed and synthesized a new phenazine-linked 2D-COF (COF-C 4 N) by solvothermal reaction of triphenylenehexamine (TPHA) and hexaketocyclohexane (HKH). Structural analysis confirmed that COF-C 4 N possesses an ordered crystalline structure with a highly conjugated basal plane and better stability. COF-C 4 N exhibited OER performance with a low overpotential of 349 mV at 10 mA cm −2 and a Tafel slope of 64 mV dec −1 . A combination of theoretical and experimental studies revealed that better OER performance is attributed to better crystallinity and stability, an appropriate band gap, and an N position that promotes the formation of C active sites around N atoms. The strategy of theory-driven design and targeting synthesis of 2D-COFs for OER may provide a new way to further develop metal-free materials for clean energy application.