Ligand-Functionalized MIL-68-type Indium(III) Metal–Organic Frameworks with Prominent Intrinsic Proton Conductivity

Indium-based metal–organic frameworks (In-MOFs) have now become an attractive class of porous solids in materials science and electrochemistry due to their diverse structures and promising applications. In the field of proton conduction, to find more crystalline MOFs with splendid proton-conductive properties, herein, five three-dimensional isostructural In-MOFs, MIL-68-In or MIL-68-In-X (X = NH2, OH, Br, or NO2) using terephthalic acid (H2BDC) or functionalized terephthalic acids (H2BDC-X) as multifunctional linkages were efficiently fabricated. First, the outstanding structural stability of the five MOFs, including thermal and water stability, was verified by thermal analysis and powder X-ray diffraction. Subsequently, the H2O-mediated proton conductivities (σ) were fully assessed and compared. Notably, their σ evinced a significant positive correlation between the temperature or relative humidity (RH) and varied with the functional groups on the organic ligands. Impressively, their highest σ values are up to 10–3–10–4 S/cm (100 °C/98% RH) and change in this order: MIL-68-In-OH (1.72 × 10–3 S/cm) > MIL-68-In-NH2 (1.70 × 10–3 S/cm) > MIL-68-In-NO2 (4.47 × 10–4 S/cm) > MIL-68-In-Br (4.11 × 10–4 S/cm) > MIL-68-In (2.37 × 10–4 S/cm). Finally, the computed activation energy values under 98 or 68% RHs are assessed, and the related proton conduction mechanisms are speculated. Moreover, after electrochemical testing, these MOFs illustrate remarkable structural rigidity, laying a meritorious material foundation for future applications.