Highly Crystalline Covalent Organic Frameworks Thin Films Synthesized by Thermal Evaporation and Solvent Vapor Annealing

Porous crystalline covalent organic frameworks (COFs) suffer from the difficulty of forming crystalline thin films, which hinders their application in membrane devices. Here, a strategy is developed for forming highly crystalline 2D COFs thin films via unique in situ thermal evaporation followed by solvent vapor annealing. Two different COFs are selected containing 1,3,5-triformylphloroglucinol (Tp), ionic triaminoguanidine hydrochloride (TG_Cl), and neutral 2,2′-bipyridine-5,5′-diamine (Bpy) monomers because of their common use in energy storage devices. The results show that the condensation rate between the monomers of Tp and ionic TG_Cl is faster than that of Tp and Bpy during the solvent vapor annealing, which leads to a homogeneous ionic TpTG_Cl crystalline thin film whereas the neutral TpBpy thin film consists of crystalline islands. AFM nanoindentation revealed a low stiffness of the ionic TpTG_Cl thin film compared to the neutral TpBpy thin film. Using polycrystalline TpTG_Cl and TpBpy thin films, Zn–air batteries (ZABs) are produced by covering Zn plates with the ionic TpTG_Cl (TpTG_Cl/Zn) and neutral TpBpy (TpBpy/Zn) thin films as anodes. As produced, TpTG_Cl/Zn-based ZAB exhibits a long cycle life of 91 h due to the high hydroxide ion conductivity and the Zn dendrites suppression.