Enhanced Water-Assisted Proton Conductivity of MOF-808(Hf) via Guest Molecule Encapsulation

This study successfully synthesized single-phase MOF-808(Hf) (<b>1</b>) and its postmodified derivatives, Im@MOF-808(Hf) (<b>Im@1</b>) and Tra@MOF-808(Hf) (<b>Tra@1</b>) via sublimation-induced loading of imidazole and 1,2,4-triazole. The guest molecules (Im/Tra) did not disrupt the <i>spn</i>-type three-dimensional porous framework of <b>1</b>; they only reduced the pore size and specific surface area by occupying the channels, confirming that the postmodification process was controllable and efficient. All three materials exhibited excellent comprehensive properties: high thermal stability (decomposition temperature >455 °C), good water stability, acid-base stability over a wide pH range (1.5-12.5), and strong water vapor adsorption capacity, fully meeting the core stability requirements for proton exchange membrane fuel cells. In terms of proton conduction, all samples showed notable performance, with proton conductivity (σ) increasing significantly with rising temperature and relative humidity (RH). Among them, <b>Im@1</b> performed best, achieving a σ of 0.127 S·cm^-1 at 98% RH/100 °C, 27 times higher than that of pristine <b>1</b>. Mechanistically, all samples realized efficient proton transport by the Grotthuss mechanism, except <b>Tra@1</b> at 68% RH, which followed the Vehicle mechanism. In summary, <b>1</b> and its postmodified composites hold substantial application potential for proton conduction in PEMFCs.