Highly Anisotropic and Water Molecule-Dependent Proton Conductivity in a 2D Homochiral Copper(II) Metal–Organic Framework

Proton conductivity research on single crystals is essential to elucidate their conduction mechanism and guide the unidirectional crystal growth to improve the performance of electrolyte materials. Herein, we report a highly anisotropic proton-conductive 2D metal–organic framework (MOF) [Cu₂(Htzehp)₂(4,4′-bipy)]·3H₂O (1·3H₂O, H₃tzehp = N-[2-(1H-tetrazol-5-yl)­ethyl]-l-hydroxyproline) with definite crystal structures showing single-crystal to single-crystal transformation between the anhydrate (1) and trihydrate (1·3H₂O) phases. The hydrogen bonded chains consisted of well-defined lattice water molecules and hydroxyl functional groups of the Htzehp^2– ligand array inside the 2D interlayer spaces along the crystallographic a-axis ([100] direction) in 1·3H₂O. Temperature- and humidity-dependent proton conductivity was achieved along the [100] and [010] directions, respectively. The anisotropic proton conductivity of σ[100]/σ[010] in a single crystal of 1·3H₂O was as high as 2 orders of magnitude. The highest proton conductivity of 1.43 × 10^–3 S cm^–1 of 1·3H₂O at 80 °C and 95% relative humidity was observed among the reported 2D MOF crystals. The relation between the proton conductivity and structure was also revealed. The hydrogen bonded chain in 1·nH₂O plays a significant role in the proton transport. The time-dependent proton conductivity and single-crystal X-ray diffraction measurements demonstrated that 1·3H₂O is temperature- and humidity-stable and acts as an underlying electrolyte material for fuel cell applications.