High-density active site COFs with a flower-like morphology for energy storage applications

Abstract

Redox-active covalent organic frameworks (COFs) are an emerging class of energy storage materials with notably abundant active sites, well-defined 1D channels and high surface areas. However, during the process of crystalline framework formation, error checking and proofreading will inevitably lead to internal structural defects. The morphological control of COFs is still a difficult task. Herein, solvent effects and pre-polymerization steps are used to control the preferential growth direction of the COF nucleus (extend along the conjugate plane in the a and b axis/vertically stacked along the c axis). As a result, the well-crystalline COFs with flake-flower (COFs-F) and rod-flower morphologies (COFs-R) are successfully prepared by controlling kinetic parameters. By comparing the electrochemical test results in different electrolytes, it is confirmed that the integration of carbonyl with anthraquinone blocks during the irreversible enolization process can achieve maximum utilization of the COFs skeleton. Benefiting from the high density redox active sites and more regular one-dimensional pores, COFs-R exhibits a specific capacitance of 486.3 F g⁻¹. Since the tight stacking along the c axis can relieve the collapse to a certain extent, after 10 000 cycles at 0.5 A g⁻¹, the capacitance of COFs-R is still maintained at 93.2%. The kinetic analysis shows that the capacitance contribution rate of COFs-R achieves 95.3% at 100 mV s⁻¹. Density functional theory (DFT) shows that COFs-R has a narrow band gap and a low energy barrier for electron transport. Due to the unique advantages of the morphology and high density of active sites, the COFs proposed in this work can provide a new candidate for the next generation of intelligent energy storage materials.