Flexible side arms of ditopic linker as effective tools to boost proton conductivity of Ni8-pyrazolate metal-organic framework

Abstract

Two primitive metal-organic frameworks (MOFs), NiL1 and NiL2, based on Ni8O6-cluster and ditopic pyrazolate linkers, L1 (with rigid alkyne arms) and L2 (with flexible alkyne chains), were prepared. The proton conductivities of these MOFs in pristine form and imidazole-encapsulated forms, Im@NiL1 and Im@NiL2, were measured and compared. Upon introduction of imidazole molecules, the proton conductivity could be increased by 3 to 5 orders of magnitude and reached as high as 1.72 × 10−2 S/cm (at 98% RH and 80 °C). Also, whether imidazole molecules were introduced or not, Ni8O6-based MOFs with L2 in general gave better proton conductivity than those with L1 signifying that flexible side arms indeed assist proton conduction probably via establishment of efficient proton-conducting channels along with formation of highly ordered domains of water/imidazole molecules within the network cavities. Beyond the active Ni8O6-cluster, tuning flexibility of linker pendants serves as an alternative approach to regulate/modulate the proton conductivity of MOFs.