Exceptionally High Proton Conductivity in Cucurbituril-Based Metal–Organic Frameworks with Subnanometer Scale Channels

Abstract

Macrocyclic molecule-based metal–organic frameworks not only possess fascinating structures, but also exhibit abundant properties in various fields. Yet, their application potentials as solid conductors with high proton conductivity have not been thoroughly explored for proton exchange membrane fuel cells (PEMFCs). Herein, we report three isoreticular cucurbituril-based metal–organic frameworks (MOFs) [M2(L⊂CB[6]) STP2] (termed CUST-Fe, CUST-Co and CUST-Ni) constructed from neutral cucurbit[6]uril-based rotaxane ligands (L2+⊂CB[6]), anionic sulfonate-terephthalate ligands (STP3–) and different divalent transition metal ions (M = Fe2+, Co2+, Ni2+). The one-dimensional functional subnanometer channels of the MOFs along the c-axis endow them with the capability for superior proton transport. It is noteworthy that the proton conductivity of CUST-Ni reaches 8.8 × 10–3 S cm–1 at 60 °C and 97% relative humidity (RH), which is approximately 35 times higher than the optimal conductivity of CUST-Co. This is illustrated by the lower proton migration energy barrier of CUST-Ni than that of CUST-Co based on density functional theory (DFT) calculations. Both experimental and DFT calculation results indicate the important role of synergistic hydrogen bonding interactions in the subnanometer channels of the MOFs. This work offers new insight into the design of innovative MOFs with subnanometer scale channels as artificial mass transport materials.