Preparation And Electrochemical Performance Of Sodium Alginate-Based Carbon Composite

The extensive use of fossil fuels has led to increasingly serious environmental problems,and the task of researching clean and sustainable energy storage devices is imminent.Supercapacitors have become a new type of energy storage device with great potential because of their fast charge and discharge speed,high efficiency,and long cycle life.As a commonly used electrode material for supercapacitors,the research on biomass-based carbon composites has become a hot spot and is an important driving force for renewable energy storage technology.Sodium alginate is a biomass raw material with a simple extraction process.The molecule contains a large number of carbon elements and-OH and-COOH groups.During the high-temperature carbonization process,CO2 and gaseous H2O will be generated,which will leave rich pore structure on the surface of the material.In addition,the oxygen atoms on the SA hydroxyl group can be chelated with metal cations.Using this characteristic,a carbon/metal oxide composite aerogel material with a threedimensional network "egg box" structure can be obtained.Manganese and its oxide(MnOx)have ultra-high theoretical specific capacitance and power characteristics,but are often limited by its poor electrical conductivity and mechanical stability.In recent years,it is usually solved by nanometerization or composite with carbon substrate.Nitrogen atom doping is also one of the important modification methods of carbon materials.It can trigger a variety of Faraday reactions,thereby significantly increasing its specific capacity,so that carbon materials have both electric double layer capacitance and pseudocapacitance characteristics.This study uses the chelating properties of sodium alginate to design a carbon/manganese oxide composite material with a three-dimensional network structure.The manganese oxide is successfully dispersed into uniform nanoparticles and embedded in the carbon substrate,which solves the long-term work of metal oxides.The problems of accumulation and volume expansion in the system.This study also used the electrostatic combination between sodium alginate and pyrrole molecules to prepare a nitrogen-doped carbon nanosphere with good electrochemical properties.Using sodium alginate as the carbon source and Mn(NO3)2 as the manganese source,the light pink manganese alginate hydrogel was prepared by the drop ball method.After treated by freeze drying,700? carbonization and oxidation treatment at different temperatures,the SAC/Mn3O4 composite aerogel material was prepared successfully.Sodium alginate-based carbon successfully dispersed Mn3O4 into uniform nanoparticles,and the threedimensional network structure of the composite aerogel provides many mesopores for the material,which can increase the effective specific surface area,and is beneficial to the infiltration of the electrolyte and transmission.The electrochemical test of composite aerogel as a supercapacitor electrode material shows that its various components have a good synergistic effect.It not only has the high specific capacity of Mn3O4,but also maintains the excellent rate performance of the carbon material.Among them,the SAM-300 sample has the highest electrochemical performance,with a specific capacitance of 250F g-1;after 10,000 long cycles at a current density of 5Ag-1,the capacitance retention rate reaches 90.3%,which is compared with pure Mn3O4.Cycle stability has been effectively improved.Hydrophilic functional groups are grafted on the surface of CNTs through mixed acid(H2SO4/HNO3)pretreatment to improve the dispersibility in water,and they are doped into manganese alginate hydrogel.Further,the SAC/Mn3O4/CNTs composite aerogel material was prepared by freeze-drying,carbonization and oxidation processes.The test results show that the addition of a small amount of CNTs can effectively increase the capacitance of the composite material.Among them,the best doping ratio is mSA:mcNTs=1:1,and the specific capacity of the sample CMC-1 reaches 400.3 F g-1 at a current density of 0.5A g-1,and it is carried out at a current density of 5A g-1 after 10,000 long cycles,the capacitance retention rate reached 96.85%,and its cycle stability was significantly improved compared with the samples without CNTs.When the doping amount is further increased,the electrochemical performance of the composite material decreases.Studies have shown that the addition of CNTs affects the pore distribution of composite materials,the proportion of micropores increases,and the proportion of mesopores decreases.In terms of electrochemistry,the CNTs in the composite material can not only improve the electrical conductivity,but also provide a support for fixing the structure,thereby enhancing its mechanical properties;but at the same time,excessive doping of CNTs can also destroy the three-dimensional network structure,leading to its collapse and agglomeration.Using sodium alginate as the structural template,PPy/SA nanospheres were successfully synthesized through the oxidative polymerization of pyrrole,and further heat-treated in N2 at different temperatures(600?,700?,800?,900?)obtain nitrogen-doped carbon nanospheres.The test results show that the addition of SA transforms PPy from an irregular multi-level dendritic type to a dispersed nano-spherical type,which effectively increases the specific surface area of the material and the contact area with the electrolyte.It provides an important morphological basis for the Faraday reaction.The increase of heat treatment temperature affects the order of carbon,the content of N and the form of existence.Among them,SPC-700 has the best electrochemical performance,with a specific capacity of 162 F g-1 at a current density of 0.5A g-1.After 10,000 long cycles at a current density of 5A g-1,the capacitance remains nearly 100%.