Electrochemical synthesis of flower shaped morphology MOFs in an ionic liquid system and their electrocatalytic application to the hydrogen evolution reaction

Abstract

An important functional material, MOF-5, with unique flower shaped morphology, which is usually synthesized through hydrothermal or solvothermal methods at high temperature and pressure with high energy consumption, was successfully prepared by a mild in situ electrochemical synthesis method in a tunable ionic liquid (IL) system. In the reaction, H₂BDC (BDC = 1,4-benzene-dicarboxylate) was chosen as the organic ligand, and the ionic liquid was Bmim (Bmim = 1-butyl-3-methylimidazole) bromine which functioned as a templating agent. The π–π stacking interaction between the imidazole groups, and the ionic band between the Zn²⁺ and Cl⁻, cause the directional arrangement of the MOF-5 crystal. Results show that the reaction results in a more perfect MOF-5 crystalline phase in comparison to other methods. The product, MOF-5(IL), presents a distinctive flower shaped morphology with a diameter of about 10 microns, and possesses a homogeneous morphology, stable structure and high thermal stability (up to 380 °C in N₂ atmosphere). The electrochemical reaction in the ionic liquid Bmim bromine is a quasi-reversible redox reaction. The cyclic voltammetric curve of the MOF-5(IL) modified carbon paste electrode (CPE) illustrates that the flower shaped MOF-5(IL) has a better ability to catalyze the hydrogen evolution reaction than cubic MOF-5 prepared by other methods. The electrochemical method in the ionic liquid system can also be used to synthesize other MOF materials and nanomaterials by changing the metal ions, ligands and ionic liquid types.