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

• Published in: Chemistry of materials Vol. 29; no. 5; pp. 2321 - 2331
• Main Authors: Li, Rong, Wang, Shuai-Hua, Chen, Xu-Xing, Lu, Jian, Fu, Zhi-Hua, Li, Yan, Xu, Gang, Zheng, Fa-Kun, Guo, Guo-Cong
• Format: Journal Article
• Language: English
• Published: American Chemical Society 14.03.2017
• ISSN: 0897-4756; 1520-5002
• DOI: 10.1021/acs.chemmater.6b05497

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 (1·3H 2 O, 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 (1·3H 2 O) 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 1·3H 2 O. 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 1·3H 2 O was as high as 2 orders of magnitude. The highest proton conductivity of 1.43 × 10–3 S cm–1 of 1·3H 2 O 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·nH 2 O plays a significant role in the proton transport. The time-dependent proton conductivity and single-crystal X-ray diffraction measurements demonstrated that 1·3H 2 O is temperature- and humidity-stable and acts as an underlying electrolyte material for fuel cell applications.