Interconnected Tubular Carbon Based Electrode Materials For Supercapacitors Operating Under Sub-ambient Temperatures

As a potential environment-friendly energy storage device,supercapacitor has the advantage of high instantaneous power,rapid charging-discharging and long cycle life.However,the relatively low energy density becomes the essential remaining challenge for its practical application.In addition,operation of supercapacitors under a sub-ambient temperature range with reasonable performance is of great importance for their outdoor applications.Considering the above-mentioned two technical bottlenecks,porous carbon materials with high specific surface area and unique interconnected tubular porous structure were designed and fabricated in this dissertation.The synthesized carbon materials were applied as electrode materials for supercapacitors based on electrolytes in water and organic solvents and the according effects of temperature,courrent collector,and other factors on the performance of supercapacitors were systematically analyzed.Furthermore,self-supporting flexible membrane electrodes were prepared for assembly of supercapacitors with high working voltage and high energy density under sub-zero temperatures.The major results are listed in the following parts.Nitrogen-doped porous carbon with interconnected tubular structure（NPTC_K）was synthesized by pyrolysis of poly（1-vinylimidazole）networks in the presence of titanate nanotubes and potassium nitrate,followed by removal of titanate nanotubes.The synthesized electrode material exhibits a gravimetric capacitance of 333 F·g^-1under 1 A·g^-1and excellent cycle stability(1.8%reduction after 5000cycles at 10 A·g^-1)in a three-electrode system in 6 mol·L^-1KOH aqueous solution at room temperature.The electric double-layer supercapacitor assembled with 6mol·L^-1of KOH solution and NPTC_Kexhibits an energy density of 10.8 Wh·kg^-1at a power density of 600 W·kg^-1at room temperature.When the temperature drops to-40℃,the operating voltage of the device can be increased to 0～1.7 V,and the energy density is 13.4 Wh·kg^-1at 850 W·kg^-1.However,the energy density of water-based electric double-layer supercapacitors is still at a low level.Introduction of transition metal compounds to carbon-based electrode materials is an effective to further improve their capacitance as transition metal compound can lead to an enhanced pseudocapacitance.Thus,nickel and cobalt sulfide compounds were introduced to the synthesized NPTC_Ksystem to form composite electrode materials（Ni Co₂S₄/NPTC_K）.At room temperature,Ni Co₂S₄/NPTC_Kcomposite shows a specific capacitance of 1090 F·g^-1at 1 A·g^-1in a three-electrode system using 6 mol·L^-1KOH aqueous solution as electrolyte.The acccordingly assembled asymmetric supercapacitor（Ni Co₂S₄/NPTC_Kas anode and NPTC_Kas cathode）has a high working voltage of 1.6 V,a high specific capacitance of 102.8 F·g^-1at a current density of 1 A·g^-1,and a high energy density of 36.6 Wh·kg^-1at a power density of 800 W·kg^-1.However,it has a serious capacitance performance degradation at-40℃.In order to obtain supercapacitor used under sub-ambient temperature with high energy density,symmetric supercapacitor assembled from the synthesized NPTC_Kelectrode,2 mol·L^-11-ethyl-3-methylimidazolium tetrafluoroborate（EMIMBF₄）/acetonitrile with higher stable voltage,and nickel foam delivers a higher working voltage and specific capacitance compared with the devices using aqueous electrolyte.At room temperature,the energy density reaches 32.7 Wh·kg^-1under a power density of 1300 W·kg^-1whereas the energy density is 48.3 Wh·kg^-1under 1.75 k W·kg^-1,and 18.6 Wh·kg^-1under 35.0 k W·kg^-1at-40^oC.In addition,the device retains 53.5%of its initial capacitance after 10,000 charge-discharge cycles at-40°C under 10 A·g^-1.For further improvement of the capacitance performance of ionic liquid supercapacitor under high current at low temperature,Nitrogen-doped porous carbon with interconnected large-size tubular structure（NPHTC_K）was prepared by using micrometer-sized titanate rods（Ti RDs）as hard templates.The results show that the material delivers a specific capacitance of 302 F·g^-1in a three-electrode system using 6 mol·L^-1KOH aqueous solution as electrolyte at room temperature under 1 A·g^-1and remains 228 F·g^-1even at a high current density of 50 A·g^-1.Moreover,even at high current density,both supercapacitors using KOH aqueous solution as electrolyte and using ionic liquid as electroylte assembled from NPHTC_Khave excellent capacitance performance and good cycling performance at both room temperature and-40℃.Finally,in order to eliminate the limitation of metal current collector on the working voltage of ionic liquid supercapacitor to obtain a higher energy density,the NPHTC_K-based,self-supporting flexible membrane electrodes with large surface area and reasonable mechanical strength were formed by a solvent casting technique using N’N-dimethyyformamide as solvent and poly（vinylidene fluoride）as binder.Supercapacitor assembled from the thus-formed membrane electrode as electrode and EMIMBF₄/acetonitrile as electrolyte exhibited a relatively wide electrochemical windows（0～3 V at room temperature and 0～3.8 V at-40℃）.Furthermore,the device delivers an energy density of 43.8 Wh·kg^-1under 1500W·kg^-1at room temperature and 55.6 Wh·kg^-1under 1900 W·kg^-1at-40℃,respectively.