Construction Of Porous Carbon Materials With High Voltage Resistance And Its Application In Supercapacitors

Electric double layer capacitors（EDLCs）have good application prospects in new energy vehicles,urban rail transit and advanced energy storage systems due to their fast charging and discharging rates and long cycle life,but low energy density is the fatal flaw of EDLCs.The voltage window for organic-based EDLCs is typically at 2.7 V.Extending it to 3.2-3.3 V would increase the energy density by 40%-50%.This thesis is dedicated to modulating the type and number of oxygen-containing functional groups on the surface of porous carbon by secondary high-temperature treatment,increasing the chemical stability of the surface,improving the voltage window and thus enhancing the energy density of EDLCs.Highly condensed porous carbon（SGx）with good voltage resistance was prepared byπ-πconjugation in aqueous solution,KOH chemical activation and secondary high-temperature treatment using sulfonated pitch and graphene oxide as precursors.The prepared porous carbon material contains 3 wt.%of graphene oxide（SG3%）,which can well balance the contradiction between specific surface area and density,exhibiting a high specific surface area of 2700 m² g^-1 and a high theoretical density of 0.53 g cm^-3.In addition,the low oxygen content（9.82 at.%surface O atoms）and the stable surface chemistry（dominated by ether bonds）avoid side reactions between oxygen-containing functional groups and the electrolyte,thus improving the voltage resistance characteristics.The C_v and maximum energy density reached 80.6 F cm^-3(1 A g^-1)and31.1 Wh L^-1,respectively,in 1 M TEABF₄/PC electrolyte at 3.2 V.After 20,000 cycles of testing,almost no capacity degradation was observed.The oxygen-containing functional groups on the surface of the porous carbon nanosheets after the secondary high-temperature treatment were dominated by stable ether bonds and carbonyl groups,and the oxygen content was only 2.7 at.%at the lowest.The sample FG1%obtained a high conductivity of 108 S m^-1 by virtue of the special structure of continuous ultrathin porous carbon nanosheets（～20 nm）and high sp² carbon atom（87.4%）content.The sample FG1%was used as the electrode material of EDLCs in 1 M TEABF₄/PC electrolyte to further extend the voltage window to 0-3.3 V.The C_g and maximum energy density could reach 181.4 F g^-1 and 68.6 Wh kg^-1,and the multiplicity characteristic C_10/0.05 was 71.3%,and the capacity retention rate was still 88.9%after 10,000 cycles.In order to reduce the cost,highly voltage-resistant porous carbon（F-x）was prepared by simple chemical activation with secondary high-temperature treatment using only industrial humic acid as the precursor.Sample F-2 had a sp² carbon atom content of 89%,a surface C/O atom ratio of 17.6,a specific surface area of 3105 m² g^-1,a microporous ratio S_mic/S_BET of 88.6%,and an electrical conductivity of 72 S m^-1.Based on the above special structure,F-2 can also provide a large number of storage sites and rich and connected transmission channels for ions,and provide a short conduction distance for electrons,which is conducive to the construction of EDLCs electrodes with high conductivity,high voltage resistance and high capacity.The operating voltage of F-2 in 1 M TEABF₄/PC electrolyte can reach 3.3V,and the maximum energy density can reach 62.5Wh kg^-1(412 W kg^-1).