Study On Room-Temperature Oxidative Modification And Supercapacitive Performance Of Porous Carbon Electrode Materials

Supercapacitors,also called electrochemical capacitors,which is an important class of clean and sustainable energy storage device,can exhibit wonderful electrochemical performances such as high energy density,good power density,long cycling life,and fast charge/discharge processes.It is well understood that the electrode material is one of the key factors to determine the performance of supercapacitors.Up to now,as one of the primary electrode material for supercapacitors,porous carbon materials have attracted a great deal of attention due to their extraordinary properties such as high specific surface area(SSA),well-developed pore structure,low internal resistance,good chemical stability and low-cost.However,the carbon-based materials usually display a relatively low capacitance,which has become the crucial obstacle for their further application.Many researches have demonstrated that heteroatoms doping is an effective technical strategy to improve the capacitive performances of porous carbon materials.However,it is a remarkable fact that most of the ongoing researches mainly put their insights into the effects of the dopant type and the doping amount.As for the possible relevance between the type of the doped species and the corresponding contribution to the capacitance,it is presently not well understood.Furtheremore,how to design and achieve the optimal capacitive performances of carbon electrode materials by means of optimizing the type of doped species has been rarely reported.Therefore,in the present study,by taking O-doping as an example,the O species-doping at room temperature has been investigated on two different types of porous carbon materials–carbide-derived carbon(pure carbon)and soymeal-derived activated carbon(impure carbon),where the type and the amount of doped-O species can be tailored by the type of oxidizing agents.An attempt has been made to probe into the influence of the type of O species doped into carbon materials on the supercapacitive performances.Based on this principle,a new room-temperature oxidation system was designed to further improve the capacitance of porous carbon materials by optimizing the doped-O species.The main contents and results of this thesis are as follows:(1)The carbide derived carbon(CDC)was used as the precursor to prepare the O-doped samples by utilizing a room-temperature oxidation route with H₂O₂,HNO₃ and (NH₄)₂S₂O₈,respectively.The strategy is studied to probe into the influence of the type of O species doped into carbon materials on the supercapacitive performances.The results have shown that there were exist three O species(C-O,C=O and O-C=O)on all the oxidized CDC samples,but the amount and the proportion of O species are heavily affected by the type of oxidizing agent.(2)However,it is worth noting that,the different doped-O species have different contributions to the capacitive performances.With the content of the C=O species increasing,the oxidized CDC sample exhibit both higher specific capacitance and superior capacitance retention,demonstrating that the C=O species should be preferential for the achievement of superior electrochemical for CDC.For instance,the HNO₃-oxidized NOCDC presences the highest C=O species with a high specific capacitance of 245 Fg^-1at the current density of 0.5 Ag^-1,being remarkably increased by a factor of?2 compared to that of the pristine CDC.At the same time,the rate capability is also improved,even at a current density of 10 Ag^-1,the specific capacitance of NOCDC is still with a retention ratio of 86.84%,which is obviously higher than that of the pristine CDC(76.51%).Additionally,the specific capacitance of NOCDC is highly stable and about 93% can be remained after 10000 cycles,exhibiting an outstanding cycling stability.(3)In order to achieve a higher specific capacitance for the porous carbon electrode materials,the same strategy was used to oxidize the soymeal-derived activated carbon(AHC)with high specific surface area.The results have shown that both the microstructure,SSA and porosity of AHC decreased significantly after oxidizing with H₂O₂,HNO₃ and (NH₄)₂S₂O₈),respectively,and its specific capacitance were also become lower.The analysis suggests that the reduction of specific capacitance value for AHC should be related to the destruction of its pore structure caused by the oxidation process.In addition,same to the oxidized CDC,the three O species of C-O,C=O and O-C=O were also exist on all the oxidized AHC samples.For the three doped-O species,C=O species were also behaving a positive correlation with the supercapacitive performance of the oxidized AHC.(4)The effect of oxidation time on the structure of AHC was investigated with H₂O₂ as oxidant in the room-temperature oxidation system.The results have found that the structure of AHC was gradually destroyed with the extension of oxidation time,and the AHC structure was completely destroyed when the oxidation time reached 240h.According to the characterization analysis,the AHC were all transformed into carbon quantum dots(ACDs)with average particle size of about 2 nm and good fluorescence properties.Undoubtedly,this discovery will provide a convenient and efficient novel technique for the preparation of carbon quantum dots.(5)Based on the important understanding for the corresponding contribution of C=O species to supercapacitive performances.Herein,we propose a facile and effective method to improve the supercapacitive performance of CDC,which is realized by added the HF into HNO₃ as a regulator to compose a HNO₃/HF mixed acid system to optimize the doped-O species of CDC.Depending on the thermodynamic analysis,the HNO₃/HF mixed acid system can promote the formation of C=O species,demonstrating that this designed new route should be preferential for the improvement of superior electrochemical for CDC.The results have shown that the amount of the doped-O species can be tailored effectively by the HNO₃/HF mixed acid oxidation system.Specifically,oxidizing with the HNO₃/HF mixed acid system,the more C=O species would be retained(1.23 at.%)compared to the HNO₃single oxidation system(0.90 at.%).In addition,the HNO₃/HF-oxidized NF_1:1-CDC has a specific capacitance as high as 305 Fg^-1at 0.5 Ag^-1,and this value is 24.5%higher than the HNO₃single oxidized sample.Moreover,HNO₃/HF-oxidized NF_1:1-CDC exhibits the excellent rate capability with 90.2%retention rate from 0.5 to 10 Ag^-1,being 3.4% increased than that of the HNO₃ single oxidized sample.It indicates that the designed new room-temperature oxidation system will further improve the electrochemical performance of porous carbon materials by effectively promote the formation of C=O species.