Exceptional Proton Conduction Performance under Low Humidity of Four Zirconium(IV)/Hafnium(IV) Metal–Organic Frameworks

In this study, isophthalic acid (IPA) and 5-hydroxyisophthalic acid (5-OH-IPA) were employed as bridging ligands to successfully prepare four isostructural three-dimensional metal-organic frameworks (MOFs): MIP-206-Zr (1), MIP-206-Hf (2), MIP-206-OH-Zr (3), and MIP-206-OH-Hf (4). All four MOFs exhibit excellent thermal, hydrolytic, and acid-base stability, along with outstanding proton conductivity (σ). At 100 °C and 68% relative humidity (RH), their maximum σ values reach 1.13 × 10^-2, 3.40 × 10^-3, 1.47 × 10^-2, and 5.20 × 10^-3 S·cm^-1, respectively. Notably, stable conductivities in the range of 10^-4 -10^-3 S·cm^-1 are maintained even at 33% RH. Combined experimental and theoretical investigations elucidate the proton transport mechanism. Density functional theory (DFT)-derived electrostatic potential (ESP) analysis reveals that hydroxyl-functionalized MOFs possess more negative ESP minima than their IPA-based analogues, implying stronger proton affinity and richer binding sites. Hirshfeld surface analysis confirms that -OH groups promote O-H···O interactions, constructing a denser H-bonding network for efficient proton transfer. Activation energy calculations verify that proton conduction follows the Vehicle mechanism. With facile synthesis and low toxicity, these MOFs provide a robust theoretical and practical foundation for industrial applications in proton-exchange membrane fuel cells and other low-humidity electrochemical devices.