Simultaneously anchoring free carboxyl and sulfonate groups into a metal-organic framework for high proton conductivity

Although the acid groups are regarded as one of the best species for proton conduction, introducing them into MOFs to enhance the proton conductivity still faces challenges due to their high coordination ability. Herein, we develop a coordination strategy to simultaneously introduce the free carboxyl and sulfonate groups into MOF-808 pores by grafting the monosodium 2-Sulfoterephthalate (SBDC) which contains one sulfonate group and two carboxyl groups onto the metal nodes. DFT calculations reveal the carboxyl group on the ortho position of the sulfonate groups shows a stronger coordination ability and binds onto the Zr6 cluster of MOF-808 in priority, leading to one free carboxyl group and one sulfonate group being preserved and dangling in the pores. The resultant MOF-808-XSBDC (X is the molar ratio of SBDC to MOF-808) series materials with free acid groups show an enhanced water adsorption behavior, thus significantly improving the proton conductivity. Particularly, MOF-808-5SBDC exhibits an ultrahigh proton conductivity, up to 4.11 × 10−2 S cm−1 at 80 °C and 100% RH, not only over two orders of magnitude higher than MOF-808, but also reaching the highest proton conductivity benchmark (>10−2 S cm−1) of MOFs. This work offers an effective way to simultaneously introduce the carboxyl and sulfonate groups into MOFs, providing vast opportunities to construct various proton conducting MOFs with the much better proton conducting performance.