Mechanism Of Plasma Modification For Nitrogen/oxygen Co-doping On Lignin-based Porous Carbon

Currently,biomass-based porous carbon has drawbacks of long producing duration,high energy consumption,single pore structure,and low hydrophilic chemical groups content,which results in long preparation period,poor rate capability,and low specific capacitance of as-prepared electrodes and thus greatly limits its application in supercapacitors preparation.In this study,lignin was used as raw material to study the process and mechanism of its thermal conversion using real-time characterization method.Humidified microwave heating method was used to prepare lignin-based porous carbon with hierarchical structure and oxygen-enriched chemical structure within one step?5-50 min?.The porous carbon prepared under optimal process was used as a substrate to be futher modified by dielectric barrier discharge?DBD?plasma.Finally,lignin-based electrodes and a supercapacitor with good rate capability and high specific capacitance were successfully prepared.The results of this paper provide a fast recycling way for lignin to its industrial production of porous carbon-based electrodes with high performance and low production cost.The detailed results showed that:?1?A high carbon yield?31.8%,excluding the ash content?was achieved after the completely pyrolysis of lignin.This result proves that lignin used in this study is an excellent precursor for porous carbon preparation.The pyrolysis behavior of lignin can be described well using the first-order reaction model?Coats-Redfern?and the non-isothermal free model?Ozawa-Flynn-Wall?.?2?Humidified microwave heating method can be used to prepare porous carbon from lignin within one step?5-50 min?.The lignin-based carbon prepared under the microwave irradiation of 30 min and the weight ratio?KOH:lignin?of 3:1 possesses hierarchical structure?mesopore rate,65.58%?,oxygen-enriched chemical structure?oxygen content of 17.54%?,and super high values of specific surface area(2866.6 m²·g^-1)and total pore volume(2.022 cm³·g^-1).The as-prepared electrode demonstrated the specific capacitance of 216.3 A·g^-1 under the current density of 0.5 A·g^-1.The high retension rate?87.1%?of this specific capacitance value was also observed when the current density increased from 0.5 to 10 A·g^-1.Humidified nitrogen acts as a physical activating agent cooperated with KOH to produce pores.Under the current density of0.5 A·g^-1,the porous carbon samle prepared using humidified process?Lg-3:1-5?demonstrated higher specific capacitance by 8.4%than that of the sample prepared using dry nitrogen?U-Lg-3:1-5?.?3?Both NH₃ and water vapor DBD plasma under the duration of 5 min can further improve the pore structure of lignin-based porous carbon,showing the higher specific surface areas by10.2%and 11.6%as compared to that of untreated one,respectively.Also,these two modifications lead to the indroductions of various nitrogen-?maximum surface nitrogen content,4.88%?and oxygen-containing chemical groups?maximum surface oxygen content,33.43%?onto porous carbon surface.The prepared electrodes modified by NH₃ and water vapor DBD plasma can achieve the higher values of specific capacitanace by 34.6%and 31.9%in comparison to that of untreated one,respectively.The high retension rates?83.9%and 80.5%?of specific capacitance were observed when the current density increased from 0.5 to 10 A·g^-1.Especilly,the prepared porous carbon modified by NH₃·H₂O DBD plasma can achieve the highest value of specific surface area within a shorter duarion of 3 min,demonstrating a super high value of 3414 m²·g^-1,which is higher than that of untreated one by 19.1%.Also,this plasma modification would lead to the indroduction of various nitrogen-?4.12%?and oxygen-containing?27.38%?chemical groups.The as-prepared electrode can achieve the highest value of specific capacitanace of 298.8 F·g^-11 under the current density of 0.5 A·g^-1 which is higher than that of untreated sample by 38.1%.The high retension rate?85.2%?of specific capacitance was observed when the current density increased from 0.5 to 10 A·g^-1.The optimum NH₃·H₂O plasma modified porous carbon-based symmetric supercapacitor possesses a high specific capacitance of 257.6 F·g^-11 at the current density of 0.5 A·g^-1 and excellent cycle stability?remaining 95.3%of initial specific capacitance after 2000 cycles?.It remains a high power density of 1.4 kW·kg^-1·even at an energy density of 82.4 Wh·kg^-1.?4?The synergistic effect was observed during NH₃·H₂O DBD plasma modification,which would obtain the porous carbon with the highest specific capacitance.The possible affecting route was shown as follows:the molecule of NH₃·H₂O was degraded into NH₃ and H₂O which were pumped into the reacting chamber and excited by plasma generator into various nitrogen-and oxygen-containing particles.Then,these particles have synergistically effects on the physicochemical properties of porous carbon.NH₃·H₂O DBD plasma can result in the best electrochemical properties of prepared electrode in comparison to the two other modifications.The possible reasons were presented as follows:on the one hand,NH₃·H₂O DBD plasma can achieve the most developed porous structure.On the other hand,it leads to the introduction of both nitrogen and oxygen element into carbon structure which not only can improve the wettability of carbon surface but also participate in the Faradic reactions contributing pseudocapacitance.No linear relation between the physicochemical properties of carbon samples modified by DBD plasma and the specific capacitance of prepared electrodes,indicating that the value of specific capacitance is not determined by the single physicochemical property but the combined effects of these properties.