Preparation And Supercapacitor Properties Of Shrimp Shell-derived Carbons And Their MnO₂ Composites

In response to the call of green and sustainable development,supercapacitor has become a hot research topic as a typical energy storage device.As we all know,the electrode material determines the performance of supercapacitor.The fast charge discharge response of carbon materials makes it the first choice of electrode materials for supercapacitors.However,commercial carbon-based supercapacitors still have the disadvantages of high cost and low energy density.Therefore,it is urgent to develop a cheap and highly active carbon material or composite material to take the place of the activated carbon.The source of carbon material is also very important.Shrimp shell is a very potential renewable biomass resource,which has a huge reserve in the nature.Shrimp shells can be directly converted into carbon materials or synthesize chitosan extracting chitin from shrimp shells to prepare materials.The purpose of this paper is to prepare high-performance carbon materials and Mn O₂/C composite materials for supercapacitors by developing novel and simple methods using renewable shrimp shells and chitosan.The main research contents are as follows:?1?Using shrimp shell as carbon precursor,a series of shrimp shell carbon materials were prepared by simple impregnation and different combination of eutectic molten salts as carbonization medium.When the composition of eutectic salt is Zn Cl₂/KCl?mole ratio of 48:52?,the prepared shrimp shell carbon material is nitrogen,oxygen and sulfur co-doped carbon foam showing the best physical and electrochemical properties with high specific surface area(1654.8 m² g^-1),rich heteroatom doping,high specific capacitance(0.5 A g^-1 for 250.3 F g^-1)and 90%cyclic capacitance retention.The carbon-based aqueous symmetrical supercapacitors assembled with the electrode material also shows excellent electrochemical performance.The capacitance of the device can be maintained about 88%of the initial capacitance at 1 A g^-1for 10000 cycles.The energy density can reach 7.5 Wh kg^-1 when the power density is 250.2 W kg^-1??2?The reticular N-doped porous carbon aerogel derived from chitosan using chelation reaction with Zn²⁺and freeze-drying followed by carbonization.The original3D network structure of Zn-chitosan complex can be well reserved under freeze-drying procedure.During the high-temperature calcination process,zinc chloride further activated the carbon skeleton and promoted the development of carbon skeleton channels.The material shows the best physical and electrochemical properties when the ratio of chitosan to Zn Cl₂ is 1:3.The as-synthesized N-doped porous carbon aerogel presents high N contents?6.58%?and extremely large specific surface area(2529 m² g^-1)associated with uniform mesopores size distribution.Benefiting from the advantages of its microstructure,the carbon material delivers excellent specific capacitance(267 F g^-1 at 1A g^-1)and cyclability with 100%capacitance retention after 10000 cycles in 6 M KOH electrolyte.The as-fabricated symmetrical supercapacitor delivers a high specific capacitance,good cycle stability and high energy density.The as-fabricated flexible micro-supercapacitor delivers a high areal capacitance of 25.6 m F cm^-2 at a scan rate of 5m V s^-1,outstanding energy density of 0.78?Wh cm^-2 at power density of 37.48?W cm^-2,exceptional cyclability with only 1%capacitance fading after 10000 cycles,as well as superior flexibility with capacitance retention of 99%from bending test.Therefore,the strategy clears the way for the development of carbon aerogel as new material applied in supercapacitors and flexible micro-supercapacitors.?3?A series of Mn O₂/C composites were prepared by the one pot wet chemical reaction using the nitrogen doped carbon aerogels as carbon substrates and reducing agents.The morphology and crystal phases of the composites could be controlled by controlling the reaction time.The transformation of morphology and crystal form comes from the change of cation concentration in the system.The absence of cation induces the transition of metastable layered structure to acicular structure with the continuous wet reaction.Compared with 1D nanowire,Mn O₂ with 2D layered structure has more available electrochemical active surface,the entry of electrolyte ions,and the increase of contact area between the electrode material and the electrolyte,thus providing rich electroactive sites for pseudo capacitance reaction.The composites exhibited the best electrochemical performance when the reaction time was 8h.The synergistic effect of reticular porous nitrogen doped carbon aerogels and self-assembled flower spherical Mn O₂ of cross-linked ultrathin nanosheets enhanced the electrochemical performance.The capacitance can reach 273.5 F g^-1 at 0.5 A g^-1.The capacitance of the device can be maintained about 71.1%of the initial capacitance at 2 A g^-1for 5000 cycles.It is a new excellent electrode material for supercapacitor.