Abundant Hydrophilic Atoms Enhanced the Proton Conductivity of a Stable Metal–Organic Framework in a Wide Relative Humidity Range

Abstract

The low proton conductivity of proton-conducting solids at reduced relative humidity (RH) hinders the performance of fuel cells. Hence, we propose an assumption that a metal–organic framework (MOF) with a large portion of hydrophilic atoms may hold the adsorbed water molecules at reduced RH and thus maintain a high proton conductivity. As a proof, a Cu-based MOF whose ligand contains a large portion of O and N atoms is investigated and verified to be able to sustain the high water uptake capacity from 100 to 43% RH. Benefiting from the high and steady water uptake, the proton conductivity surpasses 10–3 S cm–1 even at 43% RH, higher than that of most reported proton-conducting MOFs at low RH. Moreover, the conductivity reaches up to 1.80 × 10–2 S cm–1 at 80 °C and 100% RH and is maintained up to 120 h, not only demonstrating an excellent stability but also reaching the highest proton conductivity benchmark (>10 – 2 S cm–1) of MOFs. Also, the synthesis time of this MOF is shortened to 7 h with a high yield of 99% at room temperature, superior to the reported proton-conducting MOFs. Molecular dynamics simulation further proves that the abundant hydrophilic atoms of the MOF keep the high water density in the pores in such a wide RH range, leading to the integrity of the hydrogen bonding networks to smoothly conduct protons. This work not only discovers a highly proton-conducting MOF in a wide RH range, but also provides a strategy to discover conductive MOFs by the portion of hydrophilic atoms in the MOFs, opening a cost-saving avenue to provide highly proton-conductive materials.