Skeleton engineering of isostructural 2D covalent organic frameworks: Orthoquinone Redox-active sites enhanced energy storage

Abstract

The integration of redox-active sites into the skeleton of open-framework materials is an efficient strategy toward high-performance organic electrodes for energy storage devices. In this work, stepwise introduction of ketone (KT) groups to the skeletons of isostructural two-dimensional (2D) covalent organic frameworks (COFs) was realized by the condensation of 2,4,6-triformylphloroglucinol (Tp, as nodes) with a series of ditopic diamines, which contained none, one, two, and four KT moieties in each linker units, respectively. The precise control of the redox functionalities at the molecular level, combined with regular built-in channels in these KT-Tp COFs endowed them with superior capacitances and excellent rate capabilities for energy storage. In particular, 2KT-Tp COF and 4KT-Tp COF electrodes exhibited high capacitances of 256 and 583 F g−1 at a discharge rate of 0.2 A g−1, respectively, which outperformed most reported COF-based electrodes. More importantly, exceptional long-term cyclabilities (> 92% capacitance retention) were achieved even after 20,000 cycles at a high current density of 5 A g−1 for these KT-Tp COFs. Our results demonstrated that orthoquinone moieties rendered enhanced performance than the redox COFs with isolated carbonyl groups.