Synergistic effect of MOF-Directed acid-base pairs for enhanced proton conduction

Developing novel proton-conducting materials is crucial to improve the energy conversion performance of proton exchange membrane fuel cells, especially under the condition of low humidity. Metal-organic frameworks (MOFs) have expressed an apparent potential in proton conduction applications due to their orderly and tunable structure. Previous studies mainly focus on the MOFs modified with homogeneous functional groups to tune the proton conductivities. In this study, an acid-base pair synergistic strategy, namely introducing the acidic and basic groups into mix-linker MOF simultaneously to obtain acid-base coexistence materials, was firstly proposed to improve the proton conduction performance at 75% RH. Herein, 10 kinds of ML-UiO-66-(A)(B)-X were synthesized and the effects of content and ratio of acid-base pairs in MOF matrix on proton-conducting properties were studied. It was found that ML-UiO-66-(NH2)(SO3H)-5:5 possessed the highest proton conduction in the ML-UiO-66 materials, with the value of 2.8 × 10-2 S·cm−1 at 353 K. In addition, at 303K, 75% RH, the proton conductivity of ML-UiO-66-(NH2)(SO3H)-5:5 reached 7.4 × 10−3 S·cm−1, which is three orders of magnitude higher than pristine UiO-66 (~980 times), two orders of magnitude higher than the base-functionalized UiO-66-NH2 (~529 times) and about 218% higher than the acid-functionalized UiO-66-SO3H at 97% RH. The results show that at relatively lower humidity 75%, ML-UiO-66-(NH2)(SO3H)-5:5 has the better proton conductivity than other UiO-66 materials at nearly 100% RH. Meanwhile, ML-UiO-66-(NH2)(SO3H)-5:5 shows high water and thermal stability under high humidity and high temperature, which ensures its application in complex environment. Performance and mechanism studies show that the strategy of introducing acid-base pair into MOF matrix may open an avenue to design novel proton-conducting materials with excellent properties.