Construction And Pseudocapacitive Behavior Of Nickel-based Compound Nanostructured Electrodes

Supercapacitor?SC?,based on the electrochemical reactions occurred at the interface of electrode and electrolyte,is a new kind of energy storage devices between traditional physical capacitors and rechargeable batteries.Due to its high energy and power density,excellent cycling stability,and fast charge/discharge process,SC has promising potential for high-power devices,electric vehicles,smart electronic mico-devices,and so on.Recently,the key research of SCs focuses on how to improve the electrochemical performance of electrode materials.This dissertation is mainly concentrated on elaborate design and controllable synthesis of nickel compounds nanostructures for SC electrode materials,such as porous NiS square rods,Al-doped mesoporous NiS nanoflowers,Ni nanoparticles decorated Ni_0.2Mo_0.8N nanorod arrays,and hierarchical Ni-Co-Se nanostructure.Moreover,the effect of porosity,available surface area,and chemical composition of electrode materials on the electrochemical performance is extensively investigated in detail.Accordingly,the asymmetric SCs fabricated by using optimized electrode materials can achieve outstanding electrochemical performance.The main results are outlined as following:To reveal the effect of porosity and surface area of electrode materials on their pseudocapacitive performance,the porous NiS₂ square rods were controllably synthesized by a self-assembly and Ostwald ripening method and annealing treatment,whose pseudocapacitive performance was investigated to be semi-infinite diffusion-limited redox reaction.In detail,the Ni-based organic precursor was synthesized,and its growth mechanism was revealed by time and reagent dependent experiments.By optimizing the reaction conditions,the uniform and well-structured square rods were successfully synthesized.Then the precursor was annealed in Ar to obtain porous nickel disulfide under the precise control of annealing temperature and heating rate,so that the gas generated from the decomposition of organics could drill numerous holes within the materials to form pores.It was found the NiS₂ rods obtained by this strategy possessed high surface area(59.2 m²g^-1)and suitable pore size distribution?²4.4 nm?,providing much more electroactive sites to contact with electrolyte ions and showing high specific capacitance(1020.2 F·g^-1 at 1A·g^-1).Moreover,an asymmetric SC fabricated by using NiS₂ rods as positive electrode and graphene?RGO?as negative electrode delivered higher energy density than symmetric RGO//RGO device at the same power density.To increase the available surface area of electrode materials,the mesoporous flower-like nanomaterial with high surface area was further synthesized through Kirkendall effect.The abundant 3D ionic channels in the nanoflowers brought in surface-controlled pseudo-capacitive processes,resulting in the improvement of rate performance of electrode materials.The synthesis of the nanostructural electrode material was involving a two-step hydrothermal method.During the first hydrothermal reactions,Ni-Al subcarbonate nanoflowers with large surface area were controllably grown by a precise concentration of aluminium nitrate,whose growth mechanism was also investigated.Followed by hydrothermally reacting with Na₂S,large numbers of pores were generated within the materials due to Kirkendall effect and dissolution of redundant Al in the precursor,contructing numerous holes and channels in the sulfides.The plentiful 3D ionic channels in NiS nanoflowers shortened the diffusion length for electrolyte ions and benefited the enhancement of electrochemical performance of electrode materials.Furthermore,the rest of Al as a dopant in NiS increased the charge density of nickel that surrounded by the Al atom,leading to slight enhancement of electrochemical performance.Therefore,the Al-doped mesoporous NiS nanoflowers showed high specific capacitance of 1395 F·g^-1 at 1A·g^-1.The electrochemical performances of SCs fabricated by using different nickel sulfides as positive electrode and activated carbon as negative electrode were also investigated.It is well known that the ease of electrochemical reactions?i.e.,intrinsic electroactivity?occurred on the surface of electrode materials has significate influence on the pseudo-capacitive performance.The electrode materials with high electrochemical activity were successfully synthesized to unveil the electrochemical energy storage mechanism of transition metal nitride.Specifically,the Ni-Mo oxides nanorods precursor was grown on nickel foam by a simple and common hydrothermal method,which was then annealing in NH₃ or air to obtain Ni-Mo-N or NiMoO₄ electrode,respectively.The electrochemical measurements showed that Ni-Mo-N electrode had higher areal capacitance(4892 mF·cm^-2at 2 mA·cm^-2),lower solution resistance,and lower charge transfer resistance,implying its higher electroactivity than that of NiMoO₄ electrode.Moreover,Ni-Mo-N electrode after electrochemical tests was characterized and analyzed by XPS and FESEM in detail.It was found that a layer of Ni-based?oxy?-hydroxides nanosheets with high electroactivity was formed at the surface of nanorods employed as the diffusion channels for electrolyte ions,while the Ni-Mo nitride nanorod core under the Ni-based?oxy?-hydroxides was served as conductive network for the fast transfer of faradaic electrons.This dual-channel nanostructure for both electrolyte ions and electrons resulted in the excellent electrochemical performance of Ni-Mo-N electrode.Thus,ideal electrode materials should possess abundant 3D ionic diffusion channels and outstanding electrochemical activity.As a result,hierarchical CoSe₂ dodecahedrons@?Ni,Co?Se₂ nanotubes nanostructure was elaborately designed and successfully synthesized on carbon cloth to achieve superior electrochemical performance(1823 F·g^-1 at 2 mA·cm^-2).Meanwhile,Fe₃O₄/carbon nanorods composite on carbon cloth was obtained by cation-exchange process and annealing treatment,showing a specific capacitance of 377 F·g^-1 at 2mA·cm^-2.An asymmetric SC utilized above materials as positive and negative electrode materials,respectively,delivered high specific capacitance of 197 F·g^-1 at 3 mA·cm^-2 and high energy density of 77.6 Wh·kg^-1 at a power density of 327 W·kg^-1.Considering the effect of dual-channel nanostructure on the electrochemical performance of electrode materials,Ni-Co-Se hierarchical electrode achieved excellent pseudocapacitive performance,and a SC fabricated by the electrode materials also exhibited superior electrochemical performance.In summary,this dissertation focused on the effect of 3D ionic channels constructing by high surface area and proper porosity and high intrinsic electroactivity benefiting fast electrons transport and redox reactions on the pseudocapacitive performance of electrode materials.Based on the results,optimized electrode materials with excellent electrochemical performance could be designed and synthesized,laying the foundation for supercapacitor applications.