Design And Synthesis Of Non-covalently Functionalizing Graphene-based Materials By Organic Molecule For Electrochemical Capacitors

Electrochemical capacitors,also known as supercapacitors,are considered as supreme candidates for sustainable energy storage/conversion systems because of high power density,exceptionally long cycle life and superior safety coefficient.However,the intrinsically low energy density of electrochemical capacitors cannot reach the increasing demand of rail transit,smart grid and personal electronic device,etc.So how to increase the energy density of electrochemical capacitors while maintaining their exclusively high power density become the hotspot of current research.In recent years,as a new type of electrode material,organic molecules with electrochemical reversibility have exhibited the potential to bridge the gap between high energy density and power density.Compared with the traditional electrode materials,these organic molecules are expected to possess considerable energy density,primarily owing to the multi-electron faradaic reactions in a low molecular-weight charge storage unit.Meanwhile,their electrochemical properties can be reasonably tuned through changing the molecular structures or introducing different types of redox functional groups,which can provide more abundant material selection for electrochemical capacitors.And more importantly,as a kind of green and renewable materials,organic molecules will not face the threat of depletion of raw materials.However,the poor electroconductivity and the complexity of the electrochemical process all limit their application in electrochemical capacitor.In the present work,anthraquinone and its derivatives are used as electrochemical active materials to modify various types of graphene-based materials by non-covalent functionalization strategy.The prepared composite electrode materials exhibit excellent electrochemical properties.To evaluate the actual electrochemical capacitor performance,we assemble a series asymmetric or symmetric electrochemical capacitors by these electrode materials.The main content is as follows:?1?Through non-covalent functionalization strategy,anthraquinone?AQ?as the guest molecule was immobilized onto the surface of graphene framework?GF?without disrupting its sp² network.Under the?-?interaction model,the aromatic ring of AQ is inclined to parallel and close to the sp² network of GF and form a new composite structure consisting of two conjugated systems,which results in fast redox reaction rates of AQ due to low charge transfer resistance.Meanwhile,it will reveal higher overall capacitance than the bare GF because of the superposition between electrochemical capacitance and electric double-layer capacitance.The results show that the composite electrode material?AQ/GF?exhibits a high capacitance value(396 F g^-1 at 1 A g^-1),ultrahigh rate capability(64%capacitance retention at 100 A g^-1)and long cycle life?97%retention after 2000 cycles?.For further practical application,an asymmetric electrochemical capacitors has been assembled by using AQ/GF as negative electrode and GF as positive electrode.Maximum energy(13.2 Wh kg^-1)and power(9175.3 W kg^-1)densities have been obtained for the GF//AQ/GF device with a cell voltage of 1.6V.?2?Generally,the matching of the positive and negative electrodes of electrochemical capacitors plays an important role in the performance of energy storage.In order to further improve the energy density of the asymmetric capacitor on the basis of previous work,we firstly synthesized polyaniline nanotubes?PNTs?by a facile chemical self-assembly method,which exhibit a high energy storage performance in the positive potential region.Secondly,the porous nitrogen-doped carbon nanotubes?PNCNTs?with high specific surface area were prepared via a simple carbonization/activation route by using PNTs as the precursor.Through a low-temperature solvothermal process,AQ as the functionalizing molecule was anchored on PNCNTs.As a result,the as-prepared anthraquinone-functionalized porous nitrogen-doped carbon nanotubes?AQ@PNCNTs?exhibit excellent electrochemical performance,which are well able to match the PNTs for asymmetric electrochemical capacitors.Finally,capacitor was successfully constructed by integrating the PNTs?positive electrode?and the AQ@PNCNTs?negative electrode?and showed a maximum energy and power densities of 32.7 W h kg^-1 and 14.0 kW kg^-1.?3?The anthraquinone derivative Alizarin?1,2-dihydroxy-9,10-anthracenedione,AZ?as the functionalizing molecule was immobilized onto three-dimensional?3D?self-assembled graphene hydrogels?SGHs?through non-covalent functionalization.In the three-electrode system,the resultant AZ-functionalized SGHs?AZ-SGHs?electrode shows two pairs of mirror symmetry reversible peaks in the positive and negative potential regions,respectively,because two types of organic functional groups?hydroxyl and carbonyl?of AZ all possess electrochemical activity.When the resultant AZ-SGHs electrodes were integrated into a symmetric electrochemical capacitor,the electrode material shows a good self-synergy and potential self-matching behavior.As a result,the integrated device not only delivers high energy density(18.2 Wh kg^-1)in aqueous electrolyte solutions,but also exhibits excellent high current charge-discharge ability.?4?Through a simple one-step reflux method,Danthron?1,2-dihydroxy-9,10-anthracenedione,DT?was used as guest molecule to non-covalently functionalize reduced graphene oxide nanosheets?RGNs?and form DT-functionalized RGNs?DT-RGNs?composite electrode material.Due to introducing four multi-electron redox centers in DT molecular,the specific capacitance of DT-RGNs electrode is calculated to be 491 F g^-1 at 1 A g^-1 which increased by 30%for AQ-RGNS.Meanwhile,we also studied the effect of the intramolecular hydrogen bonds between carbonyl and hydroxyl on the electrochemical performance of DT-RGNs.Finally,we assembled DT-RGNs symmetric electrochemical capacitor and evaluated its electrochemical properties with a cell voltage of 1.1 V.As a result,the integrated capacitor exhibits the maximum energy density of 12.6 Wh kg^-1 at power density of 700 W kg^-1.