Protonic Conductivity of Nanocrystalline Zeolitic Imidazolate Framework 8

Abstract The synthesis and characterization of nanocrystals of the zeolitic imidazolate framework-8 (ZIF8) from zinc nitrate and 2-methylimidazole (Hmim) by a colloidal chemistry route is described in the search for novel protonic conductors. The ZIF8 powders have the sodalite cubic structure with a unit cell parameter slightly in excess of 1.7 nm, a crystallite size between 30 and 50 nm, and specific surface area between 800 and 2251 m 2 g −1 , associated to high levels of microporosity and some contribution of mesopores. The impedance spectra of ZIF8 powder compacts collected under 98% relative humidity (RH) depict at least 3 contributions, which could be ascribed to the bulk resistance at high frequency, and to external interfacial phenomena at intermediate and low frequencies. The conductivity data extracted from these data display strong humidity dependence, increasing by more than 4 orders of magnitude between 20 and 98% RH at 94 °C, ascribed to proton transport along adsorbed water molecules. This is also supported by the enhancement of 1 order of magnitude of the conductivity of samples with highest surface area, attaining a maximum of 4.6 × 10 −4 Scm −1 at 94 °C and 98% RH. The value is low, but in agreement with the reported low water uptake capacity of these materials. Surprisingly, the activation energy for conduction is as high as 110 kJmol −1 , which may denote a particular influence of the structure on proton diffusion. While the transport properties are still short for any technological application, these first results demonstrate the potential of the ZIF8 structure to combine exceptional thermal and hydrolytic stability with the potential to easily introduce high levels of proton transport by suitable functionalization.