Enhancing the Proton Conductivity of MOF Materials under Varied Temperature and Humidity

Exploring approaches to enhance the stability of proton conductivity against environmental temperature and humidity variations is crucial for proton exchange membrane fuel cells (PEMFCs). Incorporating both proton sources and proton hopping sites into the framework of metal–organic frameworks (MOFs) offers a promising route to improve conductivity. In this study, a ligand substitution strategy was employed, adjusting both the rigidity of the ligand skeleton and the relative positions of the proton sources and hopping sites to tailor the MOF properties. Four ligands containing sulfonic groups (proton sources) and amino groups (hopping sites), that is, taurine (Tr), aminomethanesulfonic acid (AMSA), p-aminobenzenesulfonic acid (pASA), and 3-aminobenzenesulfonic acid (mASA), were used to modify MOF-808, yielding MOF-808-X (X = Tr, AMSA, pASA, or mASA). The proton conductivities of MOF-808-X and MOF-808 were investigated under 30%–100% relative humidity (RH) and 303–353 K. All four modified materials exhibited significantly higher proton conductivities than MOF-808. Among them, MOF-808-Tr reached the highest value of 1.95 × 10–1 S·cm–1 at 100% RH and 353 K. In addition, the modified materials showed enhanced stability against environmental changes. For instance, the conductivity change of MOF-808-pASA from 303 K/30% RH to 353 K/100% RH was only one-quarter of that observed for MOF-808. In summary, by integrating proton sources and hopping sites while tuning ligand rigidity and substituent positions, both the conductivity and the environmental stability of MOFs were effectively improved. This work demonstrates a practical structural modification strategy for enhancing the performance stability of proton-conducting MOFs.