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 [100] 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) [Cu-2(Htzehp)(2)(4,4'-bipy)]center dot 3H20 (1 center dot 3H20, H3tzehp = N-[2-(1Htetrazol-5-y0ethyl]-L-hydroxyproline) with definite crystal structures showing single-crystal. to single-crystal transformation between the anhydrate (1) and trihydrate (1 center dot 3H20) phases. The hydrogen bonded chains consisted of well-defined lattice water molecules and hydroxyl functional groups of the Htzehp2- ligand array inside the 2D interlayer spaces along the crystallographic a-axis ([100] direction) in 1 center dot 3H20. Temperature- and humidity-dependent proton conductivity was achieved along the [100] and [010] directions, respectively. The anisotropic proton conductivity of 6[100]/c[010] in a single crystal of 1.31120 was as high as 2 orders of magnitude. The highest proton conductivity of 1.43 X 10(-3) S cm(-1) of 1 center dot 3H20 at 80 degrees 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 P71E20 plays a significant role in the proton transport. The time-dependent proton conductivity and single-crystal X-ray diffraction measurements demonstrated that 1.31-120 is temperature- and humidity-stable and acts as an underlying electrolyte material for fuel cell applications.