Improving the proton conductivity of HKUST‐1 by hole expansion and ionic liquid introduction

Abstract

As a new type of proton conductor, metal–organic frameworks (MOFs) have attracted much attention because of their superior properties over conventional materials, such as the modifiability of framework, reversibility of coordination bond, high specific surface area, and porosity. It is predicted that the proton conductivities of MOFs can be improved by hole expansion and ionic liquid introduction. In this work, HKUST‐1 and LP‐HKUST‐1 were prepared, which were filled with different proportions of N‐methylimidazole triflate (MIM‐CF3SO3) to prepare composite materials MIM‐CF3SO3@HKUST‐1‐100% and MIM‐CF3SO3@LP‐HKUST‐1‐x (x = 25%, 50%, 75% and 100%). A total of seven kinds of materials were synthesized. The proton conductivities of all the materials at 75% RH were tested from 303 to 353 K. In this environment, MIM‐CF3SO3@LP‐HKUST‐1‐100% shows excellent proton conductivity (σ = 0.341 S·cm−1 at 353 K, 75% relatively humidity [RH]), being 7060 times that of HKUST‐1, and reaches the peak value of MOF family in recent years. Then, the conductivities of parts of the materials were tested in extreme environments, such as in high‐humidity environment (303–353 K, 100% RH), high‐temperature environment (373–423 K, N2 atmosphere), and low‐temperature environment (253–283 K, 75% RH). The results show that under all conditions above, the proton conductivity of MIM‐CF3SO3@LP‐HKUST‐1‐100% is the best, up to 0.341 S·cm−1 at 353 K and 75% RH, 0.179 S·cm−1 at 353 K and 100% RH, 1.31 × 10−3 S·cm−1 at 283 K and 75% RH, and 2.31 × 10−4 S·cm−1 at 423 K and N2 atmosphere, indicating that proton conductivity of HKUST‐1 is improved by hole expansion and ionic liquid introduction. Finally, the stability test showed that MIM‐CF3SO3@LP‐HKUST‐1‐100% was stable in all environments above. Moreover, the conductive mechanism of HKUST‐1 before and after introduction of ionic liquids was also discussed, providing a theoretical basis for the enhancement of proton conductivities of MOFs using ionic liquid introduction and hole expansion.