Inherent Ultrahigh Proton Conductivity of Two Highly Stable COOH-Functionalized Hafnium-Based Metal–Organic Frameworks

Although there has been some recent interest in the proton conductivity (σ) of highly stable carboxyl metal–organic frameworks (MOFs) made of tetravalent metal ions, given their potential applications in fuel cells and electrochemical sensing, research on MOFs constructed by hafnium(IV) ions needs to be expanded significantly. Based on this, we used two common and easily prepared phenylpoly(carboxylic acid) ligands, 1,2,4-phenyltricarboxylic acid and 1,2,4,5-phenyltetracarboxylic acid, to react with hafnium tetrachloride, respectively, creating two porous hafnium(IV)-based MOFs, UiO-66-COOH-Hf (1) and UiO-66-(COOH)2-Hf (2), with the same structure as UiO-66-Hf but with different numbers of free carboxylic groups. A series of stability assays revealed that the two MOFs had excellent structural rigidity, including thermal and water stability. More crucially, alternating current impedance experiments demonstrate that the σ of the two MOFs varies positively with humidity and temperature, reaching up to 10–3 S·cm–1 (1: 2.83 × 10–3 S·cm–1 and 2: 4.35 × 10–3 S·cm–1) under the right conditions (98% relative humidity and 100 °C). The latter roughly doubles the proton conductivity of the former, which is due to the difference in the number of free carboxyl groups, as confirmed by the structural analysis and proton conduction mechanism investigation. The high intrinsic σ of the two MOFs lays a solid foundation for their future application and affords new inspiration for developing high-performance proton-conductive materials.