Role of the Coordination of Metal Ions by Ammonia for the Anodic Electrosynthesis of MOF Films

Abstract

Metal-organic frameworks (MOFs), consisting of inorganic nodes coordinated by organic ligands, have received tremendous attention in the past decades due to their high surface area, high porosity, designable pore-aperture shape/size, and tunable building units (including metal centers and functional ligands). For most of their applications, such as separation, catalysis, sensors, and energy storage/conversion devices, MOFs in the form of films/membranes are desired. Anodic electrodeposition is a promising and cost-effective method to in-situ prepare metal-organic framework (MOF) films on metal substrates. However, the anodic synthesis of continuous zeolitic imidazolate framework (ZIF) films, an important subclass of MOF materials, has been proved challenging. Here, we report a facile anodic deposition method that enables the ultra-fast preparation of uniform and continuous ZIF-8 films with tunable thickness from the aqueous solution at room temperature by using the ammonia hydroxide-assisted deprotonation strategy (AHADS). The growth rate of the MOF films is flexibly controlled by the applied current densities, the concentration of the linkers, and the amount of added ammonia hydroxide. The roll-to-roll deposition of the ZIF-8 films on the zinc foils shows the possibility of this method for large-scale industrial synthesis. The control experiments and theoretical calculations reveal that the formation of Zn(NH3)4 2+ complex by adding excessive ammonia hydroxide is crucial to promote the coordination of 2-Methylimidazole (Hmim) with Zn2+. Afterward, the Zn2+ cation attracts electrons of Hmim and thus weakens the N-H bond, which decreases the free energy for the deprotonation of Hmim. As a result, the ZIF-8 films can be aqueously deposited at low pH under room temperature. Various pure ZIF films (ZIF-4, ZIF-7, and ZIF-67) with different metal nodes or organic linkers were successfully deposited via this strategy. In addition, ZIF-8 films deposited on the zinc foil show ability to decrease the overpotential for the zinc plating and stripping, suggesting its possibility for use in reversible zinc metal batteries. This work broadens the applicability of anodic deposition and indicates the potential applications of metal-supported MOF films in energy storage and conversion fields