Study Of Synthesis And Energy Storage Mechanism For Nickel-cobalt Based Supercapacitor Materials

Supercapacitors have excellent power performance,can be rapidly charged and discharged in a short period of time,combined with long service life,low maintenance cost and excellent cycle stability.These remarkable electrochemical advantages make them shine in the field of energy device applications.Electrode materials directly affect the energy storage performance of devices.Nickel-cobalt-based electrode materials have good electrical conductivity and redox ability,wide sources,low production costs,and environmental friendliness,which enhance their research value.Among them,nickel-cobalt-based sulfides and basic carbonates（hydrotalcites）have become the current research hotspots.However,due to the morphological destruction during charge and discharge,it is easy to lead to poor cycle stability and slow kinetic behavior,which limits the commercial application.In response to this issue,based on single-component nickel-cobalt-based materials,this work used theoretical calculations to predict material properties,and developed new process routes,and designed heterostructures to synthesize new hierarchical structure electrode materials,which effectively improved the structural and electrochemical drawbacks of traditional electrode materials,and achieved synergistic improvement in device performance.The major findings are as follows:（1）The self-supported Ni Co₂S₄ nanowire electrode material was designed and fabricated.The excellent electrochemical performance of nanowire electrodes was the result of combined effects of the complete sulfurization reaction and the synergistic regulation between bimetals:the fully sulfurized Ni Co₂S₄ nanowires had high purity,high content of active substances per unit volume and intermetallic ions.The regulation effect of stimuli could effectively improve the redox activity and cycle stability.The initial specific capacitance was as high as 1316.0 F g^-1,with only 8.7%capacitance attenuation after 1000 cycle test.（2）Based on density functional theory,the crystal models of five electrode materials with different vulcanization and single/bimetallic structures were established,and the electrical conductivity of the electrode materials was studied by analyzing the energy band structure and density states.The fully vulcanized Ni Co₂S₄ nanowire electrode had the most divergent band structure and the gentlest density of state distribution.The material had strong electron non-local properties and small electron effective mass,that was,strong electron migration ability,indicating that this material had the strongest electron mobility.The calculated conductivity matched with the experimental results,which theoretically verified the electrochemical performance of the fully vulcanized Ni Co₂S₄nanowire electrode,and provided a substrate material for the subsequent electrode material preparation.（3）The Ni Co-LDHs nanosheet powder electrode material was prepared by a micro-impinging stream reactor,and the nanosheets were loaded onto the self-supported Ni Co₂S₄ nanowires through structural design to obtain the self-supported Ni Co₂S₄@Ni Co-LDHs wicker-like core-shell structure electrode material.The electrode prepared by special structural design improved the low specific capacitance of the self-supported Ni Co₂S₄ nanowire electrode,and also improved the rate capability and cycle performance of the Ni Co-LDHs nanosheet electrode.The synergistic effect of the two heterogeneous materials greatly improved the capacitive performance of the composite electrode,accompanied with a specific capacitance of 1927.0 F g^-1.Compared with the Ni Co₂S₄/NF and Ni Co-LDHs electrodes,the capacitance has increased by 46.6%and 33.8%,respectively.The capacitance retention after 1000 and 2000 cycles was97.6%and 82.3%,indicating excellent cycle stability.The assembled hybrid supercapacitor device could widen the operating voltage window to1.8 V without significant polarization.The device had a good power and energy density synergistic effect(P=600.6 W kg^-1,E=87.5 Wh kg^-1;P=12013.3 W kg^-1,E=60.4 Wh kg^-1).（4）A three-dimensional Ni Co₂S₄@Mn O₂ honeycomb structure electrode material was synthesized successfully by loading Mn O₂nanosheets on Ni Co₂S₄ nanowires in the form of simple co-precipitation in a countercurrent-flow micro-channel reactor.The addition of Mn O₂effectively widened the working potential window of the electrode and improved the cycle stability.The assembled HSC device exhibited an exceptionally long lifetime.5,000 cycles test were carried out under 5 A g^-1,with only 3.2%capacitance attenuation.（5）A coil-type microreactor was built and combined with hydrothermal operation,and Cd Se quantum dots were synthesized continuously in an aqueous environment.Using the same microfluidic preparation scheme,Cd Se quantum dots were successfully loaded onto the surface of Ni Co-LDHs nanosheets.Under the optimal conditions,the concentration of quantum dot solution was 0.312 mmol L^-1,and the diameter of Cd Se quantum dots was about 1.92 nm.Compared with the traditional batch reactor preparation scheme,the microreactor preparation of water-soluble Cd Se quantum dots solved the traditional problems of low product quality,poor repeatability and low yield.Synthesis of quantum dots in aqueous phase also greatly reduced the harm of organic reagents to the environment and human body.The prepared Ni Co-LDHs@Cd Se composite electrode had lower specific capacitance with 608.4 F g^-1.However,the cycle performance and rate performance were prominent which provided an ideal route for the development of new electrode materials.