Metal-Organic Frameworks Containing Missing-Linker Defects Leading to High Hydroxide-Ion Conductivity

Abstract

The ionic conductivity properties of the face-centered cubic [Ni8 (OH)4 (H2O)2 (BDP_X)6] (H2 BDP_X=1,4-bis(pyrazol-4-yl)benzene-4-X with X=H (1), OH (2), NH2 (3)) metal-organic framework (MOF) systems as well as their post-synthetically modified materials K[Ni8 (OH)5 (EtO)(BDP_X)5.5] (1@KOH, 3@KOH) and K3 [Ni8 (OH)3 (EtO)(BDP_O)5] (2@KOH), which contain missing-linker defects, have been studied by variable temperature AC impedance spectroscopy. It should be noted that these modified materials exhibit up to four orders of magnitude increase in conductivity (-) values in comparison to pristine 1-3 systems. As an example, the conductivity value of 5.86 × 10(-9) S cm(-1) (activation energy Ea of 0.60 eV) for 2 at 313 K and 22% relative humidity (RH) increases up to 2.75 × 10(-5) S cm(-1) (Ea of 0.40 eV) for 2@KOH. Moreover, a further increase of conductivity values up to 1.16 × 10(-2) S cm(-1) and diminution of Ea down to 0.20 eV is achieved at 100% RH for 2@KOH. The increased porosity, basicity and hydrophilicity of the 1@KOH-3@KOH materials compared to the pristine 1-3 systems should explain the better performance of the KOH-modified materials.

Keywords: defects; impedance spectroscopy; ion conductivity; metal-organic frameworks; nickel.