【作者】 唐琴；

【导师】 周大利；

【作者基本信息】 四川大学 ， 材料学， 2022， 博士

【摘要】 作为一种具有高功率密度、能快速充放电、循环使用寿命长等优点的新型储能器件,超级电容器的开发利用受到了国内外学者的普遍关注。电极是超级电容器的核心部件,主要是产生双电层和积累电荷。电极材料的合理化学组成、高比表面积和发达的分级孔隙结构有利于增加电化学活性位点和电解质离子的快速迁移,提高材料的电化学应用性能。分级多孔碳及其复合物除具备上述特性外,还具有来源丰富、经济环保等优势,已成为超级电容器最具应用前景的电极材料之一。生物质废弃物,由于其本身特有的理化组成和独特的微观结构,是制备分级多孔碳及其复合物材料的理想前驱体。本研究分别选用湖北恩施当地具有地域特色的动植物废弃物竹笋壳绒毛、丝毛草绒毛、野生河虾头胸甲及豪猪背刺为原材料,借助各原料在自然生长过程中形成的独特天然有机/无机复合微结构和物相组成,分别采用KOH直接活化法、CO2活化法、预碳化结合KOH活化法等工艺路线制备了具有相应独特微结构和组成的多种超级电容器电极用分级多孔碳及碳基复合物。采用X射线衍射仪(XRD)、扫描电镜(SEM)、透射电镜(TEM)、比表面积测定仪(BET)、激光拉曼光谱仪(RAM)和X射线光电子能谱(XPS)等手段对所制的各分级多孔碳及碳基复合物的形貌及特性进行表征,利用电化学工作站综合测试系统研究了各种材料的电化学特性,并进行了相关机理探讨。主要研究结果如下:首次在当地竹笋壳绒毛中发现了天然SiC的存在,表明某些具有特定生物质组成的植物能在特殊的自然环境条件下生成SiC。利用竹笋壳绒毛自身含有天然碳化硅的特性,采用直接KOH活化碳化法制得了分级多孔碳/碳化硅复合物,制得的该竹笋壳绒毛基复合材料的形貌显示为泡沫状,具有“微孔-介孔-大孔”多级孔结构特征。在KOH和竹笋壳绒毛质量比为1.5:1及活化温度700℃的优化条件下,所制备分级多孔碳/碳化硅复合物(HPC/SiC-1.5)比表面积高,总孔体积较大且孔径分布合理。过高的KOH用量和过高的活化温度将导致刻蚀过度,产品收率大幅度降低和复合材料比表面积变小等。基于碳化硅的改性作用、高电子迁移率和稳定性,适宜的离子输运通道和高的比表面积,当用作超级电容器的电极材料时,HPC/SiC-1.5在1M Na2SO4电解质溶液体系中,1 A g-1电流密度下的比电容为234.2 F g-1,20 A g-1下的电容保持率为71.4%,在20 A g-1的电流密度下循环10000次后的容量保持率为90.8%,显示其具有优异的倍率性能和循环能力。组装的对称两电极体系在181.1 W kg-1和17.94 kW kg-1的功率密度下显示出25.20 Wh kg-1和17.44 Wh kg-1的高能量密度,高于文献报道的多数碳基材料。在当地丝毛草绒毛中也发现了天然SiC的存在。基于丝茅草绒毛自身含有天然碳化硅的特性和中空微管结构,采用先预碳化再KOH活化的方法制得了分级多孔碳/碳化硅复合物,形貌上显示为中空微管状,同样具有“微孔-介孔-大孔”多级孔结构特征。在碱碳比为1.5:1和活化温度为800℃的优化条件下,制备的分级多孔碳/碳化硅复合物(HPC/SiC-800-1.5)比表面积为1366.8 m2 g-1,总孔容达1.03 cm3 g-1,微孔主要分布在0.55～2.0 nm。其独特的中空多孔微管构造丰富了材料分级孔的层次结构,能为电解质离子提供更加丰富的储存场所和输运通道,结合碳化硅的协同改性效应,为形成双电层电容提供充足的活性位点。以所制样品HPC/SiC-800-1.5构筑的超级电容器用电极材料在6M KOH水系电解液中,表现出理想的电化学性能:在三电极体系中,0.5Ag-1电流密度下样品的最大质量比电容达372 F g-1,即使在高电流密度50A g-1的条件下,其比容量仍可高达290Fg-1,电容保持率为74%,而且于50Ag-1的高电流密度下循环充放电10000次后,其容量保持率仍可达91.4%,展示了材料优异的倍率性能和循环稳定性;组装的对称双电极电容器体系在6M KOH水系电解液中的能量密度为11.04 Wh kg-1,即使在5.63 kW kg-1的功率密度下,仍显示出9.4 Wh kg-1的较高能量密度。研究发现清江野生河虾头胸甲具有独特的“千层底”状分级叠层结构,其所含碳酸钙为结晶度良好的“螺柱”状方解石型碳酸钙,且呈三角形阵列状均匀嵌入有机质中。利用河虾头胸富含N、O元素的特性,采用CO2活化法和天然碳酸钙的模板作用,制备了具有“超大‘螺纹’孔-大孔-介孔-微孔”孔隙结构的高含量N/O原位共掺杂分级多孔碳片。其比表面积为1012.2 m2 g-1,总孔体积为1.20 cm3 g-1(其中微孔体积0.444 cm3 g-1,介孔体积0.756 cm3 g-1)。基于氮掺杂改变碳原子的电子分布进而提高碳材料的润湿性和提供部分赝电容作用,及含氧官能团提高材料的润湿性和为法拉第赝电容反应提供电化学活性位点,较高的N/O含量以及所含氮中高的结构氮掺杂比例(33.8%),叠层状分级多孔碳片在1M Na2SO4电解质溶液体系中亦展现出较为理想的超电容性能。在三电极体系中,在电流密度0.5 Ag-1的条件下,其最大质量比电容达380.2 F g-1。10 A g-1下循环充放电5000次,容量保持率为93.6%,呈现出优异的循环稳定性。但不足的是,随着电流密度的逐渐增加,材料质量比电容呈现出相对明显的衰减趋势,在10 Ag-1的电流密度下,材料的质量比电容仅为187.5 F g-1,容量保持率为49.3%,显示出较弱的倍率性能,这可能是因为其微孔孔容相对较低,以及其类吡咯型氮的高掺杂比例,在高电流密度条件下其电容量受到了一定的抑制,因而其倍率性能尚需进一步优化。基于当地豪猪背刺的超级微结构为片状结构单元的箱格组装特征和富含N、O元素特点,首次以废弃豪猪背刺为原材料,采用预碳化结合KOH活化法制得了超高比表面积、N/O原位共掺杂分级多孔碳片材料,该复合材料因豪猪刺碳化及活化过程中崩裂形成片层结构,片层结构中形貌规整的方形“凹坑”,丰富了孔道的分级层次,其和大孔共同充当电解质溶液的“储液池”。在活化剂KOH与豪猪背刺的预碳化产物的质量配比为2:1和热解温度为800℃的优化条件下,制得的分级多孔碳片(HPCS-800-2)具有3034.9 m2 g-1超高比表面积,1.873 cm3 g-1总孔体积(其中微孔体积为1.219 cm3 g-1,介孔体积为0.654 cm3 g-1),微孔集中于0.55～2.0 nm,介孔主要集中分布在2.0～4.5 nm。得益于豪猪背刺基分级多孔碳片的超高比表面积、适宜的离子输运通道以及N、O元素改善碳材料的溶液润湿性和提供部分赝电容作用,以其构筑的超级电容器用电极材料在6M KOH水系电解液中,表现出优异的电化学应用性能:三电极体系中,当电流密度为0.5Ag-1时,HPCS-800-2的最大质量比电容为392 F g-1,即使在30Ag-1的电流密度下,其电容仍可高达291 F g-1,容量保持率为73%,显示出良好的倍率性能。在高电流密度30A g-1循环充放电10000次后,样品电极的比电容仍能保持在约258 F g-1,其容量保持率高达88.7%。基于HPCS-800-2组装的对称双电极电容器体系中,在49.9 W kg-1和5.2 kW kg-1的功率密度下,能量密度分别为12.54 Wh kg-1和7.53 Wh kg-1。于电流密度5 A g-1下10000次循环充放电之后,电容保持率为90.7%,循环稳定性好。虽然豪猪背刺基分级多孔碳片中掺杂的氮含量(2.93%)较河虾头胸甲基分级多孔碳片(9.15%)中有所降低,但得益于其超高比表面积和大的微孔容积,以及其相对更高比例的结构氮掺杂量,两种材料在最大比电容量相近的情况下,豪猪背刺基分级多孔碳片却表现出更好的倍率性能。本研究为选择合适的生物废弃物作为前驱体,制备具有优异电化学应用性能的分级多孔碳及其复合物提供了新的思路,有利于环境保护和解决能源危机。更多还原

【Abstract】 As an energy storage device with the advantages of high power density,fast charging and discharging and long cycle life,the development and utilization of supercapacitor has attracted widespread attention of scholars at home and abroad.Electrode is the core component of supercapacitor,which mainly produces electric double layer and charge accumulation.The reasonable chemical composition,high specific surface area and developed hierarchical pore structure of electrode materials are conducive to the increase of electrochemical active sites and the rapid migration of electrolyte ions,so as to improve the electrochemical application performance of materials.Beneficial from the above features and rich sources,as one of the most promising electrode materials for supercapacitors,hierarchical porous carbon and its composites have become a hot topic in the research field of supercapacitors.Biomass waste,due to its special physical and chemical composition and unique microstructure,is an ideal precursor for the preparation of hierarchical porous carbon and its composites.In this study,some typical raw materials,such as the villi of bamboo shoot shell,the villi of twitch-grass,wild river shrimp carapace and porcupine spine,which are domestic biomass waste in Enshi City,Hubei Province with regional characteristics,were selected to prepare the hierarchical porous carbon and its composites.With the help of the special organic-inorganic hybrid micro structure and phase composition formed in the natural growth process of each raw material,several hierarchical porous carbon and carbon based composites with unique structure and composition for supercapacitor electrodes were fabricated by KOH direct activation method,CO2 activation method and pre-carbonization combined with KOH activation method.The structures and properties of the as-prepared hierarchical porous carbon and carbon matrix composites were characterized by XRD,SEM,TEM,XPS,Raman,EDS and BET.The electrochemical characteristics of the materials were studied by using the electrochemical workstation comprehensive test system,and the related mechanism was discussed as well.The main research results are summarized as follows:The existence of natural SiC was found for the first time in the villi of bamboo shoot shell(VBSS),indicating that some plants with specific biomass composition can produce SiC under special natural environmental conditions.Based on the features of porous microtubule and natural silicon carbide in the wasted villi of bamboo shoot shell,hierarchical porous carbon/silicon carbide(HPC/SiC)foam with pore structure of "micro-,meso-and macropore" was fabricated directly via a facile in-situ pyrolysis carbonization and KOH activation method,under the condition of low KOH consumption and low activation temperature.Too high KOH dosage and too high activation temperature will lead to excessive etching,lower product yield and lower specific surface area of composites.Under the optimized conditions of KOH to VBSS mass ratio of 1.5:1 and carbonization activation at 700℃,the prepared hierarchical porous carbon/silicon carbide composite(HPC/SiC-1.5)has a large surface area of 1357.9 m2 g-1,developed porous texture with total pore volume of 0.755 cm3 g-1,including microporous volume of 0.534 cm3 g-1 and mesoporous volume of 0.221 cm3 g-1,and a reasonable pore size distribution with an average pore diameter of 2.87 nm which micropores are mainly concentrated in 0.59～1.9 nm.Owing to the synergistic modification of silicon carbide,high electron mobility and stability,suitable ion transport channel,the obtained HPC/SiC-1.5 exhibits excellent electrochemical performance,such as a high specific capacitance of 234.2 F g-1 at the current density of 1 A g-1,a remarkable rate capability of 71.4%at 20 A g-1,and a prominent cycling performance with 90.8%retention after 10000 cycles in 1M Na2SO4 electrolyte in a three electrode system.The assembled symmetric supercapacitor demonstrates a high specific capacitance of 224 F g-1 at a current density of 0.2 A g-1.The high energy densities of 25.20 Wh kg-1 and 17.44 Wh kg-1 with corresponding power densities of 181.1 W kg-1 and 17.94 kW kg-1 have been obtained respectively,which is higher than most previously reported porous carbon materials.Natural SiC was also found in the villi of twitch-grass.Based on the properties of natural silicon carbide contained in the villi of twitch-grass and the hollow microtubule structure,the hierarchical porous carbon/silicon carbide composite was fabricated by pre-carbonization and KOH activation method.The morphology of microtubule was successfully retained and the pore structure of"micropore-mesopore-macropore" was formed,too.Under the optimized conditions of alkali carbon mass ratio of 1.5:1 and activation temperature of 800℃,the prepared hierarchical porous carbon/silicon carbide composite(HPC/SiC-800-1.5)has a specific surface area of 1366.8 m2 g-1,a total pore volume of 1.03 cm3 g-1(including microporous volume of 0.565 cm3 g-1 and mesoporous volume of 0.465 cm g-1)and an average pore size of 2.99 nm(micropores are mainly concentrated in 0.55～2.0 nm and mesopores in 2.0～3.1 nm).The unique structure of hierarchical porous carbon/silicon carbide microtubule provides a large storage space and fast transport channels for electrolyte ions.Combined with the synergistic modification effect of silicon carbide,it offers sufficient active sites for the formation of electric double-layer capacitance,which endows the electrode material for supercapacitor constructed with HPC/SiC-800-1.5 showing ideal electrochemical performance in 6M KOH aqueous electrolyte.In a three electrode system,the maximum mass specific capacitance at the current density of 0.5 A g-1 is 372 F g-1,which can still be as high as 290 F g-1 even at a high current density of 50 A g-1,with a capacitance retention of 74%.After 10000 cycles of charge and discharge at the high current density of 50 A g-1,the capacity retention ratio is highly to 91.4%,showing the excellent rate performance and cycle stability of the material.The energy density of the assembled symmetrical double electrode capacitor system in 6M KOH aqueous electrolyte is 11.04 Wh kg-1,showing a higher energy density of 9.4 Wh kg-1 even at the power density of 5.63 kW kg-1.The micro structure of the carapace of wild river shrimp in Qingjiang before and after deproteinization and decalcification were investigated in detail.For the first time,it was found that the shrimp carapace has a unique "thousand layer soles"-like hierarchical laminated structure.The calcium carbonate contained in it is "stud"calcite calcium carbonate with good crystallinity,which is evenly embedded in organic matter in a triangular array.High level N,O co-doped laminated-like hierarchical porous carbon sheet with "super threaded macropore-macroporemesopore-micropore" pore structure was successfully prepared via a facile CO2 activation method,using the wasted carapace of Qingjiang River shrimp as precursor,and natural calcium carbonate with uniform triangular array distribution in the carapace as in-situ template.The results show that the as-prepared high level N,O co-doped laminated-like hierarchical porous carbon sheet has a specific surface area of 1012.2 m2 g-1,a total pore volume about 1.20 cm3 g-1(including micropore volume of 0.444 cm3 g-1 and me soporous volume of 0.756 cm3 g-1),an average pore diameter about 4.71 nm(micropores concentrated in 0.55～2.0 nm,and mesopores distributed in 2.0～20 nm)and high content of in-situ doped nitrogen of 9.15%(w)and oxygen of 26%(w).Based on the fact that nitrogen doping changes the electron distribution of carbon atoms,so as to improve the solution wettability of carbon materials and provide partial pseudocapacitance,and oxygen-containing functional groups improve the wettability of materials and provide electroactive sites for Faraday pseudocapacitance reaction,laminated hierarchical porous carbon sheets with high N/O content and high proportion of structural nitrogen(33.8%)in doped nitrogen also show ideal supercapacitance properties in 1M Na2SO4 electrolyte solution.In the three electrode system,the maximum mass specific capacitance is up to 380.2 F g-1 at current density of 0.5 A g-1.And the capacity retention rate after 5000 cycles at 10 A g-1 is 93.6%,showing excellent cyclic charge-discharge stability.However,on the downside,with the gradual increase of current density,the mass specific capacitance of the material shows a relatively obvious attenuation trend.That is,at the current density of 10 A g-1,the mass specific capacitance of the material is only 187.5 F g-1,and the capacity retention rate is 49.3%,showing weak rate performance.This maybe due to its relatively low micropore capacity and its high proportion of pyrrole like nitrogen in doped nitrogen which inhibits its capacitance under the condition of high current density,thus its rate performance needs to be further optimized in future research.Based on the super microstructure of porcupine spines which is characterized by the box lattice assembly of sheet structure unit and rich in N and O elements,the N/O in-situ co-doped hierarchical porous carbon sheet(HPCS)with ultra-high specific surface area was fabricated by pre-carbonization and KOH activation method,using the waste porcupine spines as raw materials.The C/N/O matrix composite forms a lamellar structure due to the carbonization and cracking of porcupine spines during activation.It is found for the first time that there are not only interconnected macropores,mesopores and micropores in porcupine spine based HPCS,but also unique micron square pits,which enrich the multi-layer structure of pores and have the function of liquid storage of electrolyte solution as well.Under the optimized conditions of 2:1 mass ratio of activator KOH and the pre-carbonization product of porcupine spine and pyrolysis temperature of 800℃,the prepared hierarchical porous carbon sheet(HPCS-800-2)has ultra-high specific surface area of 3034.9 m2 g-1,large pore volume of 1.873 cm3 g-1(including microporous volume of 1.219 cm3 g-1 and mesoporous volume of 0.654 cm3 g-1),and average pore diameter about 2.57 nm(micropores mainly concentrated in 0.55～2.0 nm,mesopores mainly in 2.0～4.5 nm).Benefiting from the suitable ion transport channel,ultra-high specific surface area and N/O co-doping of porous carbon which endows improved solution wettability of carbon material and partial pseudo capacitance,the electrode material for supercapacitor constructed by HPCS-800-2 shows excellent electrochemical application performance in 6M KOH aqueous electrolyte,such as a high specific capacitance of 396 F g-1 at a current density of 0.5 A g-1 in a three electrode system,a high specific capacitance of 291 F g-1 and a excellent cycling performance with 88.7%retention after 10000 cycles even at a high current density of 30 A g-1,showing good rate performance.In addition,the assembled symmetric supercapacitor demonstrates a high specific capacitance of 316 F g-1 at a current density of 0.1 A g-1,and a specific capacitance of 217 F g-1 at a high current density of 10 A g-1.After 10000 cycles of charge and discharge at current density of 5 A g-1,the capacitance retention rate is 90.7%,exhibiting outstanding cycle stability.And under the power density of 49.9 W kg-1 and 5.2 kW kg-1,the energy density is 12.54 Wh kg-1 and 7.53 Wh kg-1 respectively,showing less energy loss.Although the nitrogen doping content(2.93%)in porcupine spine based hierarchical porous carbon sheet is lower than that in wild river shrimp carapace hierarchical porous carbon sheet(9.15%),due to its ultra-high specific surface area,large micropore volume and its relatively higher structural nitrogen doping proportion,the porcupine spine based hierarchical porous carbon sheet shows better rate performance when the maximum specific electric capacity of the two materials is similar.This study provides a new idea for selecting appropriate biomass waste as precursor to prepare hierarchical porous carbon and its composites with excellent electrochemical application performance,which is conducive to environmental protection and solving the energy crisis.更多还原