Preparation And Electrochemical Performance Of Carbon-based Materials

As a new type of energy storage device,supercapacitors and sodium ion battery have attracted extensive attention worldwide.Supercapacitor has the advantages of long cycle life,fast charge and discharge rate,higher energy density than conventional capacitors,and higher power density than batteries.Compared with the existing battery system,Sodium ion battery has the advantages of abundant resources,low cost,and environmental friendliness,etc.In this paper,nitrogen-doped porous biomass carbon material was prepared by pyrolysis method using shrimp shell as raw material and KOH as activator.It was first applied to supercapacitor.The main conclusions are as follows:（i）After a high-temperature activation reaction of 2 h,the nitrogen-doped porous biomass carbon material exhibits a continuously porous and highly overlapping and curled nanosheet morphology,and a clear three-dimensional open structure can be observed on the surface,mainly including micropores and mesopores structure.This structure is more conducive to the storage and rapid transport of electrolyte ions and electrons;（ii）The heteroatoms contained in the material are pyridine-type N heteroatoms.This type of heteroatoms has electron vacancies,which is more conducive to increasing the conductivity of the material and reducing the resistance.The surface of the material also contains some oxygen-containing groups,these oxygen-containing groups help to improve the electrolyte’s wettability to the material,and further increase the effective contact between the electrolyte and the material,thereby improving its electrochemical performance;（iii）The material has a specific surface area of 2300 m²/g,and a pore volume of1.63 cm³/g.The larger specific surface area and pore volume can provide more effective capacitance,which greatly enhances the energy storage capacity of the material;（iv）The electrochemical performance of the material was tested using a three-electrode system.In the 0.5 M H2SO4 acid electrolyte,the double-layer capacitance characteristics of the material are relatively poor,showing some pseudo-capacitance characteristics,probably due to the redox reaction of some hetero-atom functional groups on the surface of the material under acidic conditions.At a scan rate of 2 mV/s,the specific capacitance is 442 F/g.At a current density of 10 A/g,the retention of the capacitor is 87.2%after 30000 cycles.In the 2 M Li2SO4 neutral electrolyte,the double layer capacitance of the material is very good.At a scan rate of 2 mV/s,the specific capacitance is 191.18 F/g.At a current density of 10 A/g,the capacitance retention rate was 89.6%after 30000 cycles.The specific capacitance in the Li2SO4 neutral electrolyte is relatively small,probably because Li⁺is smaller than the H⁺atom radius and cannot pass through the smaller-diameter channel,and thus cannot effectively store the capacitance.It is worth noting that in Li2SO4,the material’s capacity retention rate is high,which proves that the stability of the material in the neutral electrolyte is very high.Based on the above conclusions,the nitrogen-doped porous biomass carbon material with shrimp shell as a precursor has excellent electrochemical performance as an electrode material of a super capacitor.Two different morphology transition metal alloy materials:WTe₂,and a carbon-based alloying material:RGO@WTe₂,were prepared by hydrothermal method and CVD method.For the first time they were used as sodium ion anode materials to study their potential in energy storage.The main conclusions are as follows:（i）The WTe₂ obtained by the hydrothermal reaction and high-temperature oximation reaction of the precursor WCl6 is a nanorod（WTe2 NRs）with a particle size of approximately 200-300 nm;it is obtained from the precursor（NH₄）₆H₂W₁₂O₄₀.xH₂O via the same reaction.The WTe2 is nanoflower-like（WTe2 NFs）with a particle size of approximately 2 um;The precursor is WCl6 and graphene oxide（GO）,RGO@WTe2obtained by hydrothermal reaction and high-temperature oximation reaction is a nano-block coated with porous reduced graphene oxide（RGO）with a particle size of approximately 1 um.The three morphologies of WTe₂ have high crystallinity and are single crystal structures;（ii）At a current density of 0.1 A/g,the initial specific discharge capacity of WTe₂NRs is 442 mA h/g,the corresponding initial charge specific capacity is 318 mA h/g,and the irreversible specific capacity is 124 mA h/g.The Coulomb efficiency is 71.95%,the reversible specific capacity is high up to 221 mA h/g after 100 cycles,and the initial discharge specific capacity of WTe₂ NFs is 324 mA h/g,and the initial charge specific capacity is 254 mA h/g.Irreversible ratio The capacity is 70 mA h/g,the first cycle Coulomb efficiency is 78.34%,and the reversible capacity after 100 cycles is 55 mA h/g.As an anode material,the electrochemical performance of WTe₂ NRs is significantly better than that of WTe₂ NFs,which is mainly due to the more advantageous morphology of WTe₂ NRs.（iii）Compared with the WTe₂ NFs with the same WCl₆ precursor but with no RGO introduced,the initial specific discharge capacity of RGO@WTe₂ is 442 mA h/g,the initial charge specific capacity is 318 mA h/g,the irreversible specific capacity is 124mA h/g,the first cycle Coulomb efficiency was 71.95%,and after 100 cycles the reversible specific capacity was as high as 221 mA h/g.After 200 cycles,the reversible capacity was still as high as 160 mA h/g.In contrast,the main reason for the better electrochemical performance of RGO@WTe₂ is that the porous structure of RGO facilitates the infiltration of the electrolyte on the electrode surface,while RGO itself has a large number of active sites for storing sodium,these significantly improved electrodes.Specific capacity:In the cycle process,RGO may act as a buffer body,to a certain extent,ease the volume expansion of the electrode during the cycle,thereby improving the cycle performance;In addition,RGO as a good electronic conductor,beneficial to electronic Transport and proliferation.