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 Htzehp2– 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.