Biomass Derived Hierarchically Porous Carbon Nanosheets And Assembling Of High-performance Supercapacitors

Carbon-based supercapacitors,as next generation energy storage devices,have attached significant attention due to their high power density,satisfactory rate capability,exceptionally long cycle life and excellent reliability.More importantly,the carbon-based electrode materials is the green and renewable resources,which can be derived from low-cost,easy preparation.However,carbon-based supercapacitors show an obvious shortcoming that the energy density of them is much lower than that of conventional batteries,which cannot fully meet the growing demand of the applications where high energy density is required.Thus,improving the energy density of carbon-based supercapacitors without sacrificing high power rate capability is still a significant work.We know that the energy density?E?of the device depends on the capacitance?C?and/or cell voltage?V?.That is E=0.5CV².Therefore,it is an effective method to improve the energy density through enhanced specific capacitance and maximizing operating voltage.Given this,the ultimate goal in this paper is to increase the specific energy density for supercapacitors,mainly including two aspects of research work.Firstly,new nano-structure electrode materials were designed and synthesized by a simple way.Meanwhile,we have investigated their supercapacitive properties to get high specific capacitance.Secondly,we screened appropriate electrolyte with higher decomposition voltage to assembled symmetric supercapacitors or constructed asymmetric supercapacitors composed of optimized positive and negative electrode to enlarge cell voltage,and to further realize the energy utilization.The concrete research content is as follows:1.A three-dimensional nitrogen-doped carbon nanosheets framework?N-CNF?has been obtained starting with cellulose acetate.The product is prepared through a so-called one-step method that carbonization,activation and nitrogen-doping occur simultaneously.The resultant N-CNF shows an architecture like graphene hydrogel with interconnected hierarchical porous structure,N-doping with high nitrogen content?8.7 wt%?and high specific surface area(1003.6 m² g^-1).The N-CNF electrode displays excellent electrochemical performances due to the unique architecture and pseudocapacitance contribution from heteroatoms.In the three-electrode configuration,the N-GNF achieves a high specific capacitance of 242F g^-1 at 1 A g^-1 and displays ultrahigh rate capability(83.4%capacitance retention at100 A g^-1)in 6 mol L^-1 KOH electrolyte.The symmetric supercapacitor?SSC?based N-CNF exhibits energy density as high as 60.4 Wh kg^-1(at a power density of 1750 W kg^-1)and 17.9 Wh kg^-1(at 850 W kg^-1)in ionic liquid and aqueous electrolytes,respectively.It is surprised that the single device filled by ionic liquid electrolyte is able to light easily 60 red light-emitting diodes?LEDs,2.2 V?in parallel after charging for only 10 s,showing an excellent energy storage/release performance.2.Hierarchically porous,nitrogen-doped and interconnected carbon nanosheets?HPN-CS?has been prepared from agaric through a one-step method,that is,simultaneous carbonization,activation and nitrogen-doping.Potassium hydroxide infiltrated into the cell walls of agaric acts as an in-built activating agent to induce a unique architecture of the resultant material.HPN-CS has average pore diameter of2.6 nm,specific surface area of 1565.6 m² g^-1 and high volume fraction of macro-/mesopores?71.7%?.It is noted that a lot of micropores with the simple pore structure are homogeneously distributed on the interconnected carbon nanosheets.The symmetric supercapacitor?SSC?based HPN-CS achieves a high operation voltage of2.0 V and energy density of 27.2 Wh kg^-1(at a power density of 1 kW kg^-1)in aqueous electrolyte of 2 M Li₂SO₄.Even at the power density of 50 kW kg^-1?50-times increase,a full charge-discharge within 3.2 s?,energy density still holds at 20.8 Wh kg^-1,indicating an excellent energy storage/release performance.In addition,the single device is able to easily light 60 light-emitting diodes?working voltage 2.0-2.2V?in parallel after charging for only 10 s,showing an outsta nding potential in the practical applications.3.The carbon nanotube/nickel hydroxide nanosheets?CNTs/N i?OH?₂?core-shell composites has been prepared through a facile chemical bath deposition.Combining excellent electrical conductivity of carbon nanotube and high specific capacitor of nickel hydroxide,as-prepared CNTs/N i?OH?₂ composites exhibits the high electrochemical performance.In the three-electrode configuration,CNTs/N i?OH?₂ electrode achieves a high specific(1251 F g^-1 at 1 A g^-1)and ultrahigh rate capability(78.4%capacitance retention at 100 A g^-1).To evaluate further the capacitive performance of the as-prepared CNTs/N i?OH?₂ composites in a full cell set-up,an asymmetric supercapacitor?ASC?is assembled by using the CNTs/N i?OH?₂as the positive electrode and hierarchically porous,nitrogen-doped graphene-like carbon nanosheets?HPN-GCN?as the negative electrode in KOH electrolyte.The as-assembled asymmetric device presents an energy density as high as 34 W h kg^-1along with power density of 0.8 kW kg^-1,even at the power density of 16 kW kg^-1,energy density still holds at 22.2 W h kg^-1,indicating an outstanding rate capability.4.The bimetallic?N i,Co?hydroxide is uniformly grown on the electro-etched carbon cloth?CC?by a facile coelectrodeposition method,and then the honeycomb-shaped N iCo₂O₄/CC?HSNC?composite is formed by transforming the hydroxide precursor into its bimetallic oxides through the subsequent thermal treatment.The special structure of the HSNC as binder-free electrode is responsible for its excellent electrochemical performance with carbon-like power feature.The experimental results show that HSNC electrode exhibits a high specific capacitance with remarkable cycle stability(94.3%after 10000 cycles at 10 A g^-1)in the three-electrode configuration.To evaluate further the capacitive performance of the as-prepared binder-free electrode in a full cell set-up,an asymmetric supercapacitor?ASC?is assembled by using the HSNC as the positive electrode and reduced graphene oxide/carbon cloth?r GO/CC?as the negative electrode in KOH electrolyte.The as-assembled device presents an energy density as high as 32.4 W h kg^-1 along with power density of 0.75 kW kg^-1,comparing with nickel-metal hyoride battery?Ni-MH?batteries(30.0 W h kg^-1 at 0.35 kW kg^-1).Even at the power density of 37.7kW kg^-1?50-time increase,a full charge-discharge within 3.5 s?,energy density still holds at 17.8 W h kg^-1,indicating an outstanding rate capability.Furthermore,the as-fabricated device exhibits a long cycle lifetime(76.5%after 10000 cycles at 3 A g^-1)with a cell voltage of 1.5 V.