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.

2D Redox‐Active Covalent Organic Frameworks for Supercapacitors: Design, Synthesis, and Challenges

“…in the skeletons. [17] Three redox-active COFs, namely, 1KT-Tp COF, 2KT-Tp COF, and 4KT-Tp COF, were newly synthesized by solvothermal Schiff-base condensation of 2,4,6-triformylphloroglucinol (Tp, as the knot) with different redox linkers including 2,7-diamino-9H-fluoren-9-one (1KT-BD, one carbonyl group), 2,7-diaminophenanthrene-9,10-dione (2KT-BD, two carbonyl groups), and 2,7-diaminopyrene-4,5,9,10-tetraone (4KT-BD, four carbonyl groups) (Figure 5a-c). The orthoquinone-based COFs exhibited good stability in strong acid.…”

“…Nitrogen-riched COFs are another important type of redoxactive COFs for electrochemical energy storage through Reproduced with permission. [17] Copyright 2020, Chinese Chemical Society. reversible faradaic processes.…”

“…Thirdly, various redox-active moieties can be readily introduced into the COF skeletons and the density of active sites can be facilely controlled by the pore-wall engineering strategy. [17] Last but not least, 2D redox-active COFs can be easily grown as thin films at different interfaces, [18] which facilitate the integration of COFs into electric devices. Most reported redox-active COFs involve carbonyl-containing structures, N/S-rich moieties, free radical species, etc.…”

“…d) Cyclic voltammograms for KT-Tp COFs and contrastive BD-Tp COF and BD-Tf COF electrodes. e) GCD curves KT-Tp COFs.Reproduced with permission [17]. Copyright 2020, Chinese Chemical Society.…”

Bottom-up wet-chemical synthesis of a two-dimensional porous carbon material with high supercapacitance using a cascade coupling/cyclization route

“…At the large current density, the curves show nearly symmetrical triangular shapes, suggesting fast ion and electron transport within the 2D-PCM electrodes. The specic capacitance calculated by the slope of discharge curve at the current density of 0.1 A g À1 is approximately 378 F g À1 , which is higher than many state-of-the-art 2D carbon-based materials including reduced graphene oxide (42 F g À1 ), 2D porous carbon nanosheets (244-304 F g À1 at 0.1 A g À1 ), 37 nitrogen-doped holey graphitic carbon (206 F g À1 at 0.1 A g À1 ), 38 2D conductive MOFs and 2D COFs (256-396 F g À1 at 0.2 A g À1 ) [39][40][41] (for more details see Fig. S17 † and for a comprehensive compilation of literature data see Table S1 †).…”