Construction And Capacitive Properties Of Three-dimensional Porous Cobalt-based Materials

In the era of the Internet of Things（IOT）,supercapacitor（SC）,as one of the most promising energy storage devices,meets the human demand for energy supply methods with its environmentally friendly,high-power density and excellent rate performance,etc.The electrode material is the key factor to determine the electrochemical properties of SC,and the rational construction of high-performance electrode materials is essential for the future development of SC.Cobalt-based materials are widely used in the field of SC due to their unique structure,high theoretical specific capacity,abundant resources and low price,but these disadvantages such as poor conductivity and rare active sites seriously limit its commercial application.To address the above scientific issues,this paper proposes the construction of a series of high-performance three-dimensional（3D）porous cobalt-based materials through structural,defect and heterogeneous interface strategies.A variety of analytical and testing methods are used to conduct detailed physical characterization and performance studies of the prepared electrode materials,and the factors affecting the capacitive performance of the electrode materials are systematically investigated by combining with density function theory calculations.The specific research contents and results are as follows:1.The Ni Co-LDHs flower-like microspheres with 3D hierarchical porous structure（denoted as Ni Co-LDHs FMs）are constructed via solvothermal method using polyvinylpyrrolidone（PVP）as the structure guide,cobalt acetate（Co（CH₃COO）₂·4H₂O）and nickel acetate（Ni（CH₃COO）₂·4H₂O）as the metal source,which assembles with activated carbon（AC）to form asymmetric supercapacitors（ASC）.The precise modulation of the material structure by PVP is systematically investigated,and the results show that PVP effectively prevents the agglomeration of metal ions under the spatial potential barrier or electrostatic repulsion,allowing the two-dimensional（2D）Ni Co-LDHs nanosheets to be tightly assembled together,thereby constructing a 3D hierarchical porous structure.Benefiting from abundant active sites and excellent specific surface area,the PVP-tuned Ni Co-LDHs FMs material displays a high specific capacity of 1555 F/g at 1 A/g and a capacitance retention of 93%after 3000 cycles.The assembled ASC device achieves an energy density of 63.40 Wh/kg at a power density of934 W/kg.And the device maintains an initial specific capacity of 93.50%after 3000cycles.2.The 3D hollow porous prismatic cone-prismatic flower-like cobalt tungstate material（3D-H Co WO₄/NF）（H denotes as hollow porous structure）grown in situ on NF is constructed for the first time by anion substitution by solvothermal effect using cobalt-based metal organic framework（ZIF-67）as the cobalt source（Co）,sodium tungstate（Na₂WO₄·2H₂O）as the tungsten（W）source,and nickel foam（NF）as the conductive substrate.The introduction of tungstate into the interior of ZIF-67 with high specific surface area make the material to expose abundant active sites while obtaining a solid framework.And the 3D conductive network constructed by assembling the material with NF provides a short charge transfer path.The results reveal that the hollow porous"petals"are assembled into a connected"flower"structure,which is a spontaneously transformation process under thermal treatment without additionally adding the inducing agents.Thanks to the unique 3D structure,the high valence W elements and the 3D conductive network provided by NF,the shape of cyclic voltametric curves remains undistorted at a high scan rate of 350 m V/s,proving the3D-H Co WO₄/NF material has an excellent rate performance.Meanwhile,the specific capacity of the material can reach 1395 F/g,and the ASC assembled with AC can still maintain 94.70%of specific capacity after 3000 cycles,with an energy density of 29Wh/kg at a power density of 404 W/kg.3.Based on the kirkendall effect,the molybdenum trioxide（Mo O₃）is converted in situ into D-H-Ni Mo O₄@Co Mo O₄composites with oxygen（O）defects（D denotes as O defects,H denotes as hollow porous structure）by solvothermal and thermal reduction treatment using ammonium molybdate(（NH₄）₆Mo₇O₂₄·4H₂O),Ni（CH₃COO）₂·4H₂O and cobalt chloride（Co Cl₂·6H₂O）as raw materials.Compared with the structural modulation,O defects can efficiently modulate the electronic structure and induce charge imbalance distribution,thus accelerating the electron transfer within the material.The results show that PH₃induces partial reduction of Mo⁶⁺to Mo⁵⁺,which in turn generates O defects and reduces the resistance during ion embedding/detachment.Meanwhile,the in-situ composite of highly electrochemically active Co Mo O₄and high conductivity Ni Mo O₄can enable the material to effectively inherit multiple advantages of both components and increase the loading of active substances.The experimental results show that D-H-Ni Mo O₄@Co Mo O₄possesses a higher specific capacity（1329F/g）compared with the defect-free and single-component materials,and the capacitance retention is 95.80%after 3000 cycles.Moreover,the constant current charge/discharge curves display that the material has a smaller voltage drop value and its impedance is1.28Ω,which is much lower than that of the defect-free material（7.27Ω）.The ASC device assembled from D-H-Ni Mo O₄@Co Mo O₄and AC obtains an energy density of34.13 Wh/kg at a power density of 401 W/kg and a capacitance retention of 95.10%after 3000 cycles.4.Sulfur（S）-defective 3D porous sheet-like Co₃S_xP_y@Ni₃S_xP_yheterogeneous composites in situ assembled on NF（denoted as V_s-NCSP-X/NF,V_sdenotes S-defects,X denotes sulphuration time）are constructed by hydrothermal and high-temperature calcination with ZIF-67 as the precursor,nickel nitrate（Ni（NO₃）₂·6H₂O）as the nickel（Ni）source,and NF as the conductive substrate,supplemented with thioacetamide and sodium hypophosphite.Compared with previous experiments,the theoretical capacitance and electrochemical activity of sulfur（S）and phosphorus（P）compounds in this experiment are better than those of oxide/hydroxide,and the heterogeneous composites can not only improve the intrinsic electronic conductivity of the materials by inducing local charge redistribution,but also enhance the structural stability of the materials through strong covalent bonds between the interfaces.It is found that that the substitution of some S atoms by P atoms introduces defect sites without destroying the initial crystal structure,which enhances the covalent properties of the material,thus improving the cycle life and electrochemical activity.The electrochemical results show that the V_s-NCSP-10/NF material has an extremely high specific capacity（4930 F/g）,and the specific capacity only decays by 4%after 3000 cycles,and the capacitance retention is as high as 94%even after 5000 cycles.Moreover,the charge transfer impedance of V_s-NCSP-10/NF is negligible,indicating that it possesses a faster electron/ion diffusion rate.Meanwhile,density function theory calculations confirm that the density of states（DOS）at the Fermi energy level of the V_s-NCSP is higher than that of the Co₃S_xP_ymaterial（denoted as V_s-CSP）without heterogeneous interface strategy modulation,indicating that the heterogeneous interface can effectively improve the conductivity of the material.The ASC device assembled with V_s-NCSP-10/NF and AC possesses a very high capacitance retention（99%（3000 cycles）,96.50%（5000 cycles））and obtains the energy density of 30 Wh/kg at a power density of 818.20 W/kg.