Microstructure Control And Capacitance Properties Of Sodium Alginate Based Carbon Aerogels And Their Composites

Supercapacitor is a new type of energy storage device which can efficiently and safely store clean and sustainable energy such as wind energy and solar energy.Sodium alginate（SA）is a kind of marine biological material,which is rich in hydroxyl and carboxyl groups,and can rapidly ion exchange and cross-link with multivalent metal cations to obtain hydrogels with microstructure similar to“egg-shell”.Further treatment can be used to prepare porous carbon aerogels and metal oxide uniformly loaded composites,which are important candidates for high performance electrode materials.In this paper,three kinds of supercapacitor electrode materials based on SA were prepared from the perspective of microstructure control.The main results are as follows:（1）Porous carbon aerogels were prepared by using SA chelating property,introducing Zn²⁺to cross-link into hydrogels,and then freeze-drying and carbonization at 950°C.When the concentration of Zn²⁺is 0.5 M,the pore structure of the material is rich,and the specific surface area is 1261.9 m² g^-1.The specific capacitance of the three-electrode system is 175.7 F g^-1 at 0.5 A g^-1,and the capacitance does not decay after 10000 cycles at 5 A g^-1.（2）Using SA as carbon source,（NH₄）₂HPO₄ as nitrogen source and phosphorus source,nitrogen/phosphorus co-doped carbon aerogels were prepared by blending,freeze drying,carbonization at 800°C and pickling.When the mass ratio of the two materials is 1:1,the material(NPCA_1-1)has the most suitable nitrogen doping amount（6.8 at%）and phosphorus doping amount（4.9at%）.The specific capacitance of NPCA_1-1 is 220 F g^-1 at 0.5 A g^-1,and the capacitance can be maintained by 92.1%after 10000 cycles at 5 A g^-1.The symmetrical capacitor system can work reversible under the wide voltage window of 0-1.3 V,showing 9.1 Wh kg^-1 energy density at 161 W kg^-1,and the capacitor can maintain 81.8%after 10000 cycles at 10 A g^-1.（3）The first network was constructed by intermolecular hydrogen bonds between SA and gum arabic（GA）,and the second network was constructed by chelating crosslinking between SA and Mn²⁺.The carbon-based Mn₃O₄composites with dual network structure were prepared by freeze-drying,carbonization and oxidation treatment.The carbon content was higher and the structure was more stable.The specific capacitance of the obtained material（GA/SA@Mn₃O₄-300）at 300°C was 338.4 F g^-1 at 0.5 A g^-1,and the stability was good(the capacitance loss was only 9.5%after 10000 cycles at 5 A g^-1).（4）Using NPCA_1-1 as negative electrode and GA/SA@Mn₃O₄-300 as positive electrode to assemble asymmetric capacitors,GA/SA@Mn₃O₄-300//NPCA_1-1 can work reversiblely in a wide voltage window of 0-1.4 V.It has a high energy density of 16.3 Wh kg^-1 at 174 W kg^-1,and the capacitance can remain 86.9%after 10000 cycles at 10 A g^-1.The energy storage and power output ability are excellent,which has certain commercial potential.