Fabrication And Electrochemical Performance Study Of Manganese Oxides/Carbon Materials Composites

Manganese oxides have attracted tremendous interests in supercapacitor due to their high theoretical capacitance,good environmental compatibility and abundant reserves,but the lower energy density limit their further development.According to the formula E=0.5CV~2,the improvement of energy density can be solved from two aspects:specific capacitance and potential window.Although nanomaterials and carbon composite can improve the specific capacitance,the agglomeration phenomenon reduces specific surface area and restricts the specific capacitance.Moreover,carbon composite only connected by van der Waals force or electrostatic adsorption,which result small interface connection area,weak combination and high internal resistance,causing the limited specific capacitance.Although the current potential window can larger than 1V,which needs to sacrifice its specific capacitance,thus restricting the further improvement of energy density.In this regard,this paper uses the Mn-O-C bond to assist the preparation of high-performance manganese oxide/carbon composite electrodes,and the agglomeration phenomenon can be solved through the multi-shell structure or vertical-aligned structure.By using surface oxygen vacancies or plasma assisted method,the Mn-O-C bond in interface can be realized,which breakthroughs the small connection area,high resistance and weak bonding strength between interface,revealing the influence mechanism between Mn-O-C bond and connection impedance and improving the specific capacitance and stability.Moreover,a built-in electric field is formed at the heterogeneous interface of manganese oxide/graphene quantum dots constructed by Mn-O-C bond,which reveals the connection between the built-in electric field with Fermi energy level,realizing the increase of specific capacitance and potential window,and significantly improving the energy density.To solve the limited specific surface area of manganese oxides,a new method for preparing multi-shell manganese oxides hollow particles was proposed.The Mn CO3 microcubes were used as the basic frame and KMn O4 as oxidizer.The shell number could be controlled.Multi-shell structure significantly increased the electrochemical active site,the Rct is 3.15Ω,the maximum specific capacitance value is 608 F/g,and the electrode can maintain 65%of the initial capacitance after 5000cycles.When assembling asymmetric supercapacitor(ASC),it exhibits the high energy density of 55 Wh/kg.To further solve the limitation of the preparation of more-shell structure and improve cycling stability,the calcination of template was used to realize the preparation of the quadruple-shell Mn3O4 hollow sphere,which can maintain 80%of the initial capacitance after 5000 cycles,demonstrating the improvement of cycling stability.In order to overcome the small connection area and high impedance of composite electrode,the oxygen vacancies were introduced to assist the formation of Mn-O-C bond in Mn3O4 hollow spheres/graphene composite electrode,which improves the conductivity and stability of the electrodes.Oxygen vacancies can generate a positive area and hence anchor with negatively charged OH groups on GO to form the Mn-O-C bond,accelerating the electronic transmission in interface and promoting the interface connection mode change from point contact to surface contact,and the Rct of the composite was reduced to 1.1Ω.The Mn-O-C chemical bond at the interface can promote the redox reaction and improve the specific capacitance to 802 F/g.As a structural buffer,graphene improves the stability and the electrode can maintain 98%of the initial capacitance after 10000 cycles.When assembling the ASC,the device exhibits the maximum energy density of 72.3Wh/kg.A new strategy of plasma assisted in-situ preparation of vertical-aligned manganese oxide/carbon composite electrode was proposed.The conductive path between the manganese oxide composite electrode and the current collector was optimized due to the formation of vertical-aligned structure.CNTs are catalyzed by metal Co nanoparticles to obtain CNTs-Mn O2 nanosheets with mesoporous structure,the Rct was reduced to 0.8Ω.CNTs expand the 2D nanosheet structure into a 3D cross-linked network structure,which enhance electron transfer and prevent the collapse of nanosheets.The vertical-aligned CNTs-Mn O2 obtained the maximum specific capacitance of 1131 F/g.After 10000 cycles,the specific loss is 5.6%,which is mainly due to shedding of CNTs.To solve the weak interface bonding in vertical-aligned electrodes,the in-situ preparation of vertical-aligned graphene-Mn O2nanoparticle was realized with the assistance of plasma.Mn-O-C bond was effectively formed to improve the interface bonding strength and improve the stability.The introduction of graphene prevents the agglomeration of nanoparticles and increases the electrical conductivity and structural stability.The specific capacitance can reach up to 1176 F/g,and the capacitance loss is only 1.9%after 10000 cycles.The graphene quantum dots/Mn O2(GQDs/Mn O2)heterostructure electrodes were prepared by the in-situ plasma-assisted process.The effect between the in-situ formed heterostructure and the potential window of the electrode was clarified.The results show that GQDs optimizes the charge conduction path,its edge structure provides active sites.The specific capacitance of GQDs/Mn O2 is improved by～700F/g(1094 F/g)compared with the original Mn O2 at 1 V.Mn-O-C bond promotes the formation of heterogeneous structures,and the built-in electric field generated at the interface is opposite to the external electric field potential,the potential window of GQDs/Mn O2 can be widened from 1 V to 1.3 V,and the specific capacitance value can be further increased to 1170 F/g at 1.3 V.Using GQDs/Mn O2 as the positive electrode and N-doped graphene as the negative electrode,the ASC device can deliver the highest energy density of 118 Wh/kg.