Influence Of Nanocarbon Pore Structure On The Performances Of Supercapacitors And Its Mechanism

Regulation of pore structure is the key to develop advanced carbon-based supercapacitor materials with both high volumetric energy density and power density,is also a hot topic in the research of high-density porous carbon.In this dissertation,we proposed two effective strategies(i.e.,capillarity and ball-milling)to fabricate high-density collapsed carbon nanocages and greatly improve their volume performances.Based on the structure-function relationship between pore structures and capacitive performances,a distinct pore model system dominated by micro-,meso-and macropores was constructed to systematically study the underlying rules of direct-current equivalent series resistance,capacitance,and rate versus current density.It is revealed that the high-power energy output under large discharge is dominated by the diffusion-layer ions in the meso-and macropores of EDLC electrodes.The main contents are as follows:1.Compressing the surplus space within mesopores is a challenge to prepare high-density porous carbon with high volumetric performances.Based on the method of in-situ MgO template,we prepared high-density carbon nanocages with thinner shells through capillary,realizing the controllable compression of extra space of inside nanocages.The density of the optimized collapsed carbon nanocage is up to 1.32 g/cm3,which feature the uniform and narrow-distribution mesopores.Combining with the contribution of the high voltage window(4.0 V)of ionic liquids,the volumetric energy density of collapsed carbon nanocage supercapacitors achieved a record-high volumetric energy density of 73 Wh/L,which stays at the state-of-the-art level for compact carbon-based materials.2.The high-density collapsed carbon nanocages prepared by capillary is not in favor of mass production due to the low yield.Conversion of the normal carbon nanocages with high yield into high-density collapsed carbon nanocages is more realistic route.We proposed a new method of ball milling to directly obtain high-density carbon nanocages with high surface area and conductivity through compressing the inside space and hierarchical structure of nanocages.Compared with normal nanocages,the volumetric capacitance of the optimized sample(1.25 g/cm3)increased from 100 to 219 F/cm3,and the corresponding volumetric energy density from 3.5 to 7.6 Wh/L,These results demonstrated that this strategy is an effective to improve the density of normal carbon nanocages,which paved the road for the mass production of high-density carbon nanocages.3.Inspired by the structure-function relationship between pore structures and capacitive performances,three typical porous carbons dominated micro-,meso-and macropores are employed as model electrodes to reveal the underlying rules of direct-current equivalent series resistance,capacitance,and rate versus current density.We found that the reversed trend of areal capacitance versus current density.This behavior originates from the migration-rate differentiation of inner diffusion-layer ions relative to that of outer compact-layer ions,leading to the continuous decrease of diffusion-layer capacitance until less than the compact-layer capacitance.The migration-rate differentiation was enhanced by introducing meso-and macropores,slowing down the reduction of diffusion capacitance and dominating the output of high-power capacitive energy finally.These results pushed the transport mechanism of electrolyte ions one step further from micropores to meso-and macropores and revealed the differentiation behaviors of electrolyte under large-current discharge,as well as proposed a general high-power energy output model dominated by diffusion-layer ions for aqueous electrolytes.