High-Performance Piezoelectric Two-Dimensional Covalent Organic Frameworks

Organic piezoelectric nanogenerators (PENGs) are attractive in harvesting mechanical energy for various self-powering systems. However, their practical applications are severely restricted by their low output open circuit voltage. To address this issue, herein, we prepared two two-dimensional (2D) covalent organic frameworks (COFs, CityU-13 and CityU-14), functionalized with fluorinated alkyl chains for PENGs. The piezoelectricity of both COFs was evidenced by switchable polarization, characteristic butterfly amplitude loops, phase hysteresis loops, conspicuous surface potentials and high piezoelectric coefficient value (d33). The PENGs fabricated with COFs displayed highest output open circuit voltages (60 V for CityU-13 and 50 V for CityU-14) and delivered satisfactory short circuit current with an excellent stability of over 600 seconds. The superior open circuit voltages of CityU-13 and CityU-14 rank in top 1 and 2 among all reported organic materials-based PENGs. Two imine-linked two-dimensional covalent organic frameworks (COFs), designated as CityU-13 and CityU-14, were incorporated with 3,3,4,4,5,5,6,6,7,7,8,8,8-tridecafluorooctyl chains to induce intrinsic polarizability and piezoelectric properties. Utilizing these two COFs to assemble piezoelectric nanogenerators (PENGs), remarkable output open circuit voltage, short circuit current and stability were produced under the applied force without poling conditions. image