Preparation Of Aromatic Molecule-functionalized Graphene And Its Capacitive Performance In Ionic Liquid

The discovery and development of supercapacitors have enabled human beings to successfully tackle with the ongoing worldwide energy dilemma from the collection and transformation of natural renewable energy,such as wind energy and tidal energy.As one kind of environment-friendly and efficient electrochemical energy storage devices,supercapacitors have shown their distinguished superiority from batteries in power density,safety,and cycle life.Despite these advances,their applications in the field expecting high energy-power were largely impeded by the low energy density.Thus,the combination of advanced electrode materials with high capacitance and non-aqueous electrolytes with a wider electrochemical stability window is expected to be an effective strategy to overcome the energy limitation and achieve trade-off electrochemical performance of supercapacitors.Ionic liquids（ILs）with notable characteristics such as low vapor pressure,higher thermal stability,as one kind of organic molten,can provide a wide voltage window（2.5～4 V）,and thus have demonstrated their great potential in the manufacture of high-performance supercapacitors.In the paper,Emim BF₄ was chosen as the electrolyte to raise the energy content owing to its high conductivity,low melting point and viscosity.However,the electrode research focused on IL-based supercapacitors is still dominated by carbon-based materials,and the numbered reports of pseudocapacitance materials are mainly inorganic metal compounds.Differently,redox-active organic molecules with high electrochemical reversibility,can provide a large specific capacity,promising a good application in the preparation of high-performance electrode materials.Despite some advances,the low conductivity largely restricts the organic molecules fail to achieve full play of the capacitive performance and stability,as well as the practical applications.The introduction of conductive supporter such as carbon-based materials seems a better solution to address the above problem.Graphene as one single-layer sp² carbon ranks the best candidate because of its good electrical conductivity,large specific surface area,excellent mechanical strength,and can formπ-πinteractions with the aromatic molecules and the imidazole ring of ionic liquid cations.Moreover,graphene itself can contribute the electrical double-layer capacitance to maintain the high-power characteristic of the supercapacitor.Therefore,the combination of organic molecules and graphene is expected to maximize the merits of both electrochemical double-layer capacitance and pseudocapacitance,and significantly increase the energy density of supercapacitors.In terms of the large number of organic molecules to be chosen,aromatic molecules with typical structure and specific functional groups were selected to form the complex with graphene.Their electrochemical performance and possibly energy-harvesting mechanism were fully evaluated and explored in the Emim BF₄electrolyte.The mainly research for each work are focused on:1.Phthalocyanine（PC）and copper phthalocyanine（Cu PC）,one kind of porphyrin compounds with abundant pyrrolic-nitrogen in the macrocyclic ring,were used as the target molecules combining with graphene as the precursor to prepare the PC-r GA and Cu PC-r GA complex through one-step hydrothermal method in the presence of phosphoric acid.The introduction of phosphoric acid during the reaction:（i）to accelerate the dissolution of phthalocyanine and copper phthalocyanine at high-temperature to accelerate the anchoring of the phthalocyanine and copper phthalocyanine on the surface of graphene throughπ-πinteraction;（ii）to keep the phenolic hydroxyl group on the surface of the graphene from being removed;and（iii）to tailor the morphology of graphene-based composites to form a three-dimensional interconnected framework structure.Benefiting from the well wettability and favorable interaction of PC,Cu PC by ionic liquid Emim BF₄ electrolyte and the highly reversible faradaic redox reaction,the prepared PC-r GA and Cu PC-r GA electrode materials displayed both electric double layer capacitance and excellent pseudocapacitance.The specific capacitance of PC-r GA and Cu PC-r GA at 1 A g^-1 is 193 F g^-1 and 201 F g^-1,respectively.The maximum energy density released by the corresponding device based on PC-r GA and Cu PC-r GA are 106.8 W h kg^-1 and 111.4 W h kg^-1,respectively,and the capacitance retention is still competitive after 8000 charge/discharge cycles.The mechanism studies have shown that both the phthalocyanine skeleton and the metal center undergo a reduction reaction during the charging process,and the phthalocyanine skeleton carried out the reaction in prior.2.Considering the effective roles of different substituents on the electrochemical stability of aromatic molecules and the potential of heterocyclic aromatic hydrocarbons in electrochemical energy storageg,a series of anthraquinone molecules substituted by hydroxyl and amino groups at the ortho-positions were used to modify the surface of graphene by solvothermal method in the alcohol-water system.Compared to the weak redox response of unsubstituted anthraquinone（AQS）,AQS-functionalized graphene composites in the Emim BF₄ electrolyte,the dihydroxy and tetrahydroxy substituted anthraquinone（DHAQ,THAQ）at the ortho-position hybrid graphene electrode displayed an enhanced redox reaction and lower potential shift.This phenomenon indicates the positive role of the introduction of electrochemically active groups in changing the anthraquinone redox potential and improving the capacity utilization.Since the higher redox potential of amino group experience the faradaic reaction than the hydroxyl group,the integrated cyclic voltammetry area of the corresponding quinone（DAHAQ）functionalized graphene increases significantly when the dihydroxy group in THAQ at the ortho-position is replaced by the amino group.When the current density is 1 A g^-1,the specific capacitances of THAQ-/DAHAQ-GA electrode materials are 203 F g^-1 and 247 F g^-1,respectively.After 8000 cycles,the capacitance retention of THAQ-GA and DAHAQ-GA are still 75%and 85%,respectively,which are significantly better than the ortho-unsubstituted AQS-GA（64%）and dihydroxy-substituted DHAQ-GA（67%）.This work shows that the ortho-position multi-substituted anthraquinone derivatives have higher electrochemical stability and electrochemical reversibility,as well as the superiority of ortho-position more redox-active groups’substitution in regulating the energy storage of organic molecules via pseudocapacitance.3.Considering the Faradaic reactions arising from PC and Cu PC components in the PC-r GA and Cu PC-r GA composites only occurs at the negative electrode of the supercapacitor and the tuned electrochemical performances of organic molecules by different substituent groups,1,5-diamino-4,8-dihydroxyanthraquinone modified nitrogen-doped graphene electrode material（DAHAQ-NGA）was hydrothermally prepared with the optimized reaction temperature,reaction time,and solvent parameters.A pair of hydroxyl and amino groups located at the ortho-position of DAHAQ molecule can be oxidized in the Faradaic reaction while the paraquinone structure would experience the reduction.Therefore,the graphene modified by DAHAQ molecule can be used both as the positive and negative electrode material for fabricating supercapacitors,and meanwhile contribute pseudocapacitance.The specific capacitance of DAHAQ-NGA electrode obtained at 1 A g^-1in Emim BF₄ electrolyte is286 F g^-1.The assembled symmetrical supercapacitor can provide a maximum energy density up to 147 Wh kg^-1,as well as an excellent capacitance retention.The unique structure of DAHAQ and its synergy with IL electrolyte enable the DAHAQ-NGA composite a candidate material in the fabrication of advanced supercapacitors.4.Evaluating the influences of the synergistic effect between the aromaticity and functional groups of electrochemical active organic molecules on their faradaic reaction were launched to provide new insights into the design and preparation of novel high-capacitance composite materials based on the energy storage mechanism of pseudocapacitors and electric double-layer capacitors.1,4-Dihydroxybenzoquinone（DBQ）,1,5-diaminoanthraquinone（o-DAAQ）,and 2,6-diaminoanthraquinone（m-DAAQ）were chosen to decorate the graphene as electrode materials.The electrochemical behaviors of o-DBQ-GA/o-DAAQ-GA/m-DAAQ-GA were fully studied in the ionic liquid Emim BF₄ electrolyte.It was found that the CV curve of o-DBQ-GA electrode did not appear an expected redox peak.The possible reason for elucidating the phenomenon would be the limited aromaticity inherent from the benzoquinone,which leads to high-energy intermediates responsible for the difference in the electrochemical behaviors between o-DBQ-GA and o-/m-DAAQ-GA.The presence of intramolecular hydrogen bonds improve the stability of the transition state of o-DAAQ in the Faradaic reaction,and thus the o-DAAQ-GA electrode exhibited an improved capacitance and cycle life compared to m-DAAQ-GA.