Construction And Propertiies Of Silicon Nanowires Array/Nickel Cobalt Bimetallic Compound Electrode Materials

Supercapacitors（SC）,also known as electrochemical capacitors,are excellent energy storage systems with robust lifetime,fast charging capability and high specific power capacity.Nowadays,SC have attracted considerable attention in consumer electronics such as hybrid electric vehicles and portable electronics.However,the relatively low energy density severely limits their further application.In recent years,much research in the field of SC has focused on the use of hybrid materials that not only overcome the shortcomings of each component,but also yield synergistic effects to obtain high-performance SCs.Transition metal compounds have been widely used as electrode materials for SC due to their abundant sources,low cost,and high electrochemical activity.In particular,binary transition metal compounds have higher electrochemical activity and specific capacitance than monometal transition metal compounds due to their multiple oxidation states.In addition to the prominent electrode material,the structure of the electrode material also plays a key role in the electrochemical properties of SCs.The 3D silicon structure with a large specific surface area can not only be fabricated by a simple solution method,but also can be combined with the modern silicon industry,which is helpful for the integration of the prepared electrode with semiconductor devices.It is an excellent electrode material substrate for supercapacitors.In order to prepare the SC with excellent electrochemical performance,this paper employs three-dimensional silicon as the substrate,uses the electrochemical deposition method to modify the highly conductive metal layer as the charge collection layer on its surface,and then through the electrodeposition method to prepare the transition metal nickel-cobalt compound（Ni-Co）on the three-dimensional silicon substrate.After continuous improvements,silicon-based supercapacitors with excellent electrochemical properties were finally fabricated.1.Preparation and properties of SiNWs-Ni/NiCo-LDH electrode materialsSilicon nanowire arrays（SiNWs）with controllable morphology were fabricated by metal-assisted catalytic etching（MACE）and tetramethylammonium hydroxide solution（TMAH）.To improve the stability and conductivity of the electrode,SiNWs were modified with Ninanoparticles（SiNWs-Ni）,and nickel-cobalt bimetal hydroxide（NiCo-LDH）was grown in situ on the SiNWs-Nisubstrate by electrochemical deposition.In order to further improve the specific capacitance and conductivity of the composite electrode material,graphene oxide（GO）was doped into NiCo-LDH to prepare SiNWs-Ni/NiCo-LDH/GO composite electrode material.By optimizing the ratio of nickel-cobalt,GO concentration and electrodeposition time,the specific capacitance of the SiNWs-Ni/NiCo-LDH/GO composite electrode can reach 2821.9 m F cm^-2 under the current density of 1 m A cm^-2.Using SiNWs-Ni/NiCo-LDH/GO electrode as positive electrode and AC as negative electrode,an asymmetric all-solid SiNWs-Ni/NiCo-LDH/GO//AC supercapacitor was assembled,and its power density can reach 53.2 Wh kg^-1.2.Preparation and properties of SiNWs-Ni/NCS electrode materialsNickel-cobalt sulfide（NCS）has a higher electrical conductivity and stability than nickel-cobalt dihydroxide,as well as more electrochemical active sites.Therefore,the SiNWs-Ni/NCS composite electrode material is fabricated based on the previous experiments presented in this section.In order to further improve the stability and conductivity of the electrode material,polypyrrole（PPy）was doped into the NCS composite material to prepare a layered multi-shell SiNWs-Ni/NCS-PPy composite electrode.By studying the influence of the concentration of surfactant,pyrrole content and electrodeposition time on the electrochemical properties of the composite electrode,ultimately,the as-prepared electrode demonstrated a remarkable specific capacitance(1602 F g^-1 at 1 A g^-1),an excellent rate capacity(1348 F g^-1 at 30 A g^-1)and a high cycling stability（91.6%of initial capacitance after 10000 cycles）.3.Preparation and properties of SiNWs-Ni/NCSe electrode materialsSelenium（Se）not only has the same valence electrons and oxidation number as oxygen（O）and sulfur（S）,but also has the same electrochemical properties and abundant chemical active sites,and its electronic conductivity is much higher than that of sulfur and oxygen.Nickel-cobalt selenide（NCSe）was prepared on SiNWs modified with nickel nanoparticles to form a unique 3D layered core-shell SC electrode.In order to improve the electrochemical performance of the electrode,reduced graphene oxide（RGO）was incorporated into NCSe composite,and the growth velocity,RGO concentration and electrodeposition time on the electrochemical performance of the electrode material were investigated.Benefiting from the synergy effect between NCSe and RGO,high-speed electron transport channels of SiNWs-Ni,and abundant electroactive sites of porous NCSe layer composed of small nanoparticles built up from countless ultrathin nanosheets,the synthesized SiNWs-Ni/NCSe-RG electrode achieved a peak capacitance of 2268 F g^-1(1973 m F cm^-2)at 1 A g^-1 and an excellent rapid-charging capability(1540 F g^-1at 30 A g^-1).Furthermore,a solid asymmetric SC was fabricated by employing SiNWs-Ni/NCSe-RG as the positive electrode,demonstrating a superior energy density of 53.23 Wh kg^-1 at the power density of 849.97 W kg^-1.