Preparation Of N-Doped Carbon Aerogels Under Hypersaline Conditions For Supercapacitors

As a new type of energy storage device,supercapacitors?SCs?have been rapidly developed in recent years due to their long-cycle life,short charging-discharging time and mild application conditions.Among normal electrode materials for SCs,carbon aerogels?CAs?have been one of the most attractive candidates because they have advantages such as superior electrical conductivity,high surface area,wide pore size distribution and low cost.With the development of technology,carbon aerogels have demonstrated excellent performances in the fields of sensors,mobile communications,aerospace technology and absorption and so on.However,real-world applications of CAs are severely limited because of two reasons.One is that the hydrophobic surface of CAs is not good for ions to infiltrate and diffuse to electrode materials,which can greatly increase the contact resistence between eletrolyte ions and electrodes,leading to degraded supercapacitive performances.To enhance the corresponding properties,the carbon aerogel could be modified by doping hetero atoms?such as nitrogen,phosphorus,sulfur,etc.?The structure of nitrogen element is similar to carbon,so it can be effectively incorporated into the carbon skeleton.Another reason that hinders the application of the CAs is lying in complicated procedures to prepare them.A traditional way to fabricate them is evaporating the liquid embedded in gels under supercritical conditions,making the drying process highly dangerous and costly.A new strategy to solve such a problem is to prepare highly porous carbonaceous materials under hypersaline condition.The hypersaline environment,which is created by using hygroscopic inorganic salts with Lewis acid feature and strong enough polarizability,can stabilize the surface of the carbon materials,lessen the partial pressure of water and alter its construction so that reactions can potentially be performed under hypersaline.In this dissertation,nitrogen-doped carbon aerogels?N-CAs?were fabricated by solvothermal polymerization of phenol,formaldehyde and nitrogen precursors under hypersaline conditions and subsequent carbonization in N₂.Zinc chloride,which was utilized to offer the hypersaline condition,was used as a pore-forming agent,dehydration agent and porogen.Different organic amines such as urea,melamine and ethylenediamine?EDA?were selected as the nitrogen precursors.X-ray diffraction?XRD?,fourier transform infrared spectroscopy?FTIR?,raman and energy dispersive spectrometer?EDS?,transmission electron microscope?TEM?and Brunauer-Emmett-Teller?BET?,scanning electron microscope?SEM?and contact angle test were employed to study structures and morphologies of the as-prepared materials.The electrochemical properties of these materials were measured by cyclic voltammetry?CV?,galvanostatic charge-discharge?GCD?and electrochemical impedance spectroscopy?EIS?tests.1.By using urea as a nitrogen source,N-CAs were prepared by solvothermal polymerization of phenol and formaldehyde and subsequent carbonization at different temperatures.Characterization results show that the specific surface area of the N-CA with a carbonization temperature of 800°C is 828.9 m²/g.Electrochemical tests show that when the current density is 1 A/g,its specific capacitance is 323.3 F/g and it has outstanding cycling stability with a capacitance retention of 96.7%after 10000 cycles at 20 A/g in 0.5 M H₂SO₄ aqueous electrolyte.Moreover,symmetric supercapacitor?SSC?based on the N-CA electrodes could deliver an impressively high energy density of 13.9 Wh/kg at a power density of 700 W/kg.In addition,influences of the carbonization temperature on the structures of the obtained samples were investigated.With the increase of temperature,the disorder degree of N-CAs gradually increased while the nitrogen and oxygen contents decreased.2.Based on reports that melamine and formaldehyde converse to melamine resins at high temperatures,melamine was selected as the nitrogen precursor to prepare N-CAs.The N-CA obtained at a carbonization temperature of 800°C is hierarchically porous with honeycomb-like structure,and its specific surface area reaches 729.6 m²/g.When used as an electrode for SCs,it exhibits a high specific capacitance of 350.7 F/g at 1 A/g and outstanding cycling stability with 97.8%of the initial capacitance retained after 10000 cycles at 20 A/g in 0.5 M H₂SO₄ aqueous electrolyte.Moreover,SSC based on the N-CA electrodes could deliver an impressively high energy density of 26.8 Wh/kg at a power density of 800 W/kg.Further investigation discovered that as the carbonization temperature increased,the disorder degree of nitrogen-containing carbon aerogels gradually increased while the nitrogen and oxygen contents decreased.3.Based on reports that reactions of phenols,formaldehyde and EDA can produce nitrogen-containing intermediate polymer frameworks,so EDA was selected as nitrogen-precursor and catalyst for fabricating N-CA.The results show that the N-CA obtained at a carbonization temperature of 900°C has a specific surface area of is 1201.1 m²/g.Characterizations of this sample show that nitrogen-doping by EDA brings it greatly increased amounts of micropores and enhanced wettability,making it show superior capacitive behavior?426.1 F/g at 1 A/g in 0.5 M H₂SO₄?,outstanding cycling stability?104.4%capacitance retention after 10000 cycles?.The SSC based on this N-CA electrode could deliver an impressively high energy density of 32.4 Wh/kg at a power density of 800 W/kg.In addition,the adsorption performance of the sample was tested in common organic pollutants such as carbon tetrachloride,toluene and chloroform,and the adsorption capacity is 12-53 times of its own weight with long recyclability.