| Supercapacitor possesses ultralong cycle life, high specific capacitance, high charge-discharge efficiency and high output power, etc. It is a novel energy storage components and has great application background and tremendous marketable value. The preparation of high performance supercapacitor electrode materials has attended widely attention and deeply research in recent years. Anthraquinone is a kind of important redox organic material. It can be used to prepare electrode materials because of its advantages which are wide working voltage window, wide temperature range, low cost, etc. Due to the poor conductivity of anthraquinone, its development in technical field of electrochemical electrode materials is limited. To harness the electroactivity of anthraquinone as an electrode material, a great recent efforts have been invested to composite anthraquinone with carbon materials to improve the conductivity.As the efficient bottom-up approach to construct molecular nanostructures, controlling molecular self-assemblies and nanostructures at surfaces is one of the most imortant interdisciplinary realms involving surface science. Unlike halogen bonds, hydrogen bonds and other weak forces to construct a solid surface nanostructures, covalent organic framework is a solid surface in situ to form a covalent bond by the strong molecular interactions. This effectively overcome the poor stability problem which is caused by constructing nanostructures by the halogen bonds, hydrogen bonds and other weak forces. And it has made possible entirely new approaches to make the "bottom-up" approach to build a more stable supercapacitor electrode material.Here we report a non-covalent way to modify three-dimensional graphene with anthraquinone moieties through on-surface synthesis of two-dimensional covalent organic frameworks(COFs). Covalent organic framework has an important feature that the larger pore size can make anthraquinone moieties have sufficient contact with the electrolyte. Moreover, the good electrical conductivity of three-dimensional graphene improve poor electrical conductivity of anthraquinone moieties.We incorporate 2,6-diamino-anthraquinone(DAAQ) moieties into COF through Schiff-base reaction with benzene-1,3,5-tricarbaldehyde(BTA). The reaction conditions of preparation of the covalent organic framework and its surface morphology are confirmed by scanning tunneling microscope, atomic force microscope and x ray photo electron spectroscopy. The energy storage of covalent organic framework were studied by the cyclic voltammetry measurements and galvanostatic charge-discharge test. And the constant current charge and discharge mechanism is analyzed.Under the low temperature of 140℃, the prepared composite electrode material is amorphous. When the temperature rised to 205℃, the prepared covalent organic framework material appeared continuous and the coverage of covalent organic framework materials is more than 90%. This illustrates temperature has a big influence on on-surface shiff base reaction. The cyclic voltammetry measurements and galvanostatic charge-discharge test were carried out in the window of-0.4~-1.05V(vs. Ag/Ag Cl) and 1M KOH alkaline solution. When the scan rate is 20mv/s, the oxidation of the composite electrode material appear at-0.769 V, while its reduction appear at-0.888 V. When the current density is 2A g-1, The synthesized covalent organic framework-graphene composite exhibits large specific capacitance of 128 F g-1 with an improved accessibility of 13.45% of the anthraquinone moieties in an alkaline media. In two thousand galvanostatic charge-discharge test, the long-term galvanostatic charge-discharge cycling experiments revealed a decrease of capacitance, which was attributed to the electrostatic repulsion accumulated during charge-discharge circles. Moreover, isotropic expansion, contraction and expansion caused by repeatedly changing the nanostructure which result in the poor electrical conductivity between 2D COF layers. |
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