Design, Synthesis, Structural Analysis Of Porous Biomass Carbon And Its Application In Novel Energy Storage Devices

With the rapid development of society,fossil fuels are gradually depleted.And using fossil fuels result in serious harm to human living environment.Therefore,the development of the clean and renewable energy has great significance to human society.As a device for electrochemical energy storage and conversion,lithium batteries have been widely used in various fields.However,with the progress of science and technology,the demand for energy storage equipment continues to increase.Some defect of lithium-ion batteries are gradually exposed,such as transient underpower and high cost.Therefore,it is imperative to study new environmentally friendly energy storage equipment.As a new energy storage device,supercapacitors have gradually attracted a lot of attention due to its advantages,such as clean and non-polluted,low cost,long service life,high power density and energy density.Supercapacitors are usually divided into electric double-layer supercapacitors,pseudocapacitors and hybrid supercapacitors.Among them,Hybrid supercapacitors have wider voltage range,higher energy densities and better cyclic stability than the other two supercapacitors.Hybrid supercapacitors typically use the electric double layer principle to store energy on the negative electrode,while the positive electrode stores energy through the redox reaction.At present,because the specific capacitance of the negative electrode is much smaller than that of the positive electrode,the overall specific capacitance of the hybrid supercapacitor is relatively small.Therefore,how to effectively increase the specific capacitance of the capacitive electrode material becomes a key technology for improving the energy storage of the hybrid supercapacitor.As a common negative electrode material,carbon materials have been extensively studied due to their advantages such as low cost,good stability and excellent electrical conductivity.The energy storage mechanism of carbon materials is mainly to form an electric double layer at the interface between the electrode material and the electrolyte through electrostatic or adsorption,so as to achieve the purpose of storing energy.According to the mechanism,there is a great relationship between the energy storage properties of carbon materials and specific surface area of carbon materials.Therefore,the common method for improving the energy storage properties of carbon materials is generally to prepare a high specific surface area material with porous structure.In recent years,biomass carbon materials have gradually become one of the hot carbon materials.The first reason is that the sustainable and renewable biomass is not only low in cost and abundant in resources,but also are composed of fiber-based polymers with specific structure and rich carbon content,which is propitious to preparing the 3D biomass carbon.The other reason is that using biomass to prepare carbon materials can effectively improve the utilization of natural resources and reduce environmental pollution.Moreover,the biomass with porous structure is conducive for the formation of the porous carbon material and it is the best three-dimensional template for the preparation of biomass carbon,so as to increase the specific surface area of carbon materials and shorten the diffusion distance of electrolyte ions to improving the diffusion rate of ions.And the electrochemical properties of carbon materials can be further increased.In view of the above,this work this paper conducts a series of electrochemical studies on biomass carbon materials.The details are as follows:1)Designing,synthesis and electrochemical performance of wheat straw-derived biomass carbon materialsWheat straw was used as biomass raw material to prepare the three-dimensional porous biomass material by the cross-linking process with citric acid and KOH activation treatment.The obtained material has abundant pore structure and the highest specific surface area can reach 2559.6 m2 g-1.And the porous biomass material shows excellent electrochemical performance.2)The performance and application of liquid or solid symmetric supercapacitors based on wheat straw-derived biomass carbon materialsThe electrodes based on the wheat straw-derived biomass carbon material were used as the positive electrode and the negative electrode of the liquid symmetric supercapacitor with 3 mol L-1 KOH solution.The liquid supercapacitor has a capacitance retention rate of 94.71% after 20,000 cycles,showing excellent cycle life.The straw-derived biomass carbon electrode was assembled into a solid supercapacitor using a polyvinyl alcohol?PVA?-KOH gel as a solid electrolyte.Solid-state symmetric supercapacitors based on wheat straw-derived biomass carbon not only have higher energy density but also have good flexibility and low temperature resistance.3)The performance and application of hybrid supercapacitors based on porous biochar materials and Ni?OH?₂-Co₂?OH?₃Cl double-layer compositesThe Ni?OH?₂-Co₂?OH?₃Cl double-layer nanocomposite supported by nickel foam was successfully prepared by a simple hydrothermal method.The structural characterization shows that the lower layer of Ni?OH?₂-Co₂?OH?₃Cl is Ni?OH?2 with the cross-linked nanosheet and the upper layer is Co₂?OH?₃Cl like flowers.According to the electrochemical performance tests,the specific capacity of the Ni?OH?₂-Co₂?OH?₃Cl is higher than the crosslinked Ni?OH?2 nanosheets.More importantly,the hybrid supercapacitor based on Ni?OH?₂-Co₂?OH?₃Cl and chitosan-derived biomass carbon exhibits a wide voltage range,a high energy density and excellent cycle stability.All above results show that the hybrid supercapacitor based on Ni?OH?₂-Co₂?OH?₃Cl double-layer nanocomposite and chitosan-derived biomass carbon is a potential energy storage device.