| Electrochemical capacitors(ECs), as a desirable energy storage device between traditional static capacitors and secondary batteries, have become a research hotspot in electrochemical energy storage applications, due to their high power density, long cycle life and reliable safety, and so on. As an important component of the device, the microstructure and composition of the electrode material directly determine the whole electrochemical performance of the device. Binary transition metal oxide, nickel cobaltite(NiCo2O4) with typical pseudo-capacitance, has recently been extensively investigated and considered as one promising electrode material for next-generation high-performance, low-cost ECs because of its appealing electrical conductivity and high theoretical specific capacitance(SC). Based on above considerations, in this thesis, we mainly focus on the rational design and controllable synthesis of electroactive NiCo2O4 with well-defined micro-/nanostructures and self-supported structures with the final aim to fabricate intriguing NiCo2O4 electrode materials with convenient ions and electrons transportation, and abundant electroactive sites for efficient energy storage. The detailed research contents of the thesis were described as follows:(1) A template-free strategy was established to fabricate ultra-layered mesoporous NiCo2O4 nanowires(NWs) constructed from single-crystalline nanosheet(NS) building blocks. The formation mechanism and supercacitive performance of ultra-layered mesoporous NiCo2O4 NWs were also explored in detail. These NSs were stacked along the thickness direction, forming an ultra-layered mesoporous structure. And the nickel foam substrate played an important role in the formation of the unique microstructure. The unique ultra-layered mesoporous structure was efficient to improve the electrochemical performance by rendering rich electron/ion diffusion channels, shortening the diffusion distance and reducing charge transfer resistance, which resulted in the high electrochemical utilization. The novel NiCo2O4 NW electrode delivered a desirable SC of 401 F/g at a current density of 1 A/g, and even 301 F/g at 8 A/g. In addition, the SC degradation of ~10% after 5000 continuous cycles at a current density of 6 A/g indicated the large SCs and good electrochemical stability of the ultra-layered mesoporous NW electrode at high rates.(2) An efficient template-engaged synthetic strategy was developed to synthesize hierarchical NiCo2O4 mesoporous hollow sub-microspheres by using silica spheres as a template. Furthermore, the formation mechanism and structure-electrochemical performance relationship was systematically studied. The mesoporous hollow sub-microspheres are constructed by lots of delicate NSs as the subunit, of which the thickness is as thin as a few nanometers. And it is due to the existence of these ultrathin NSs that the hollow NiCo2O4 sub-microspheres with wrinkle-like rough surfaces are evident. The configuration is not only very beneficial to the rapid penetration of the electrolyte, but also effective increases the contact area between the electrolyte and NiCo2O4, to improve its storage capacity. When utilized as an appealing electroactive material for ECs, the as-prepared hierarchical NiCo2O4 hollow sub-microsphere electrode delivered a SC of 678 F/g at a current density of 1 A/g, and kept it as high as 540 F/g even at 10 A/g. Furthermore, desirable cycling stability of 13% SC degradation over continuous 3500 cycles at 10 A/g was observed.(3) A simple cost-effective and scale-up polymer-assisted chemical solution synthesis of a 3D interconnected hierarchical porous network-like NiCo2O4 framework electrode was successfully developed. Owing to the synergetic effect of ethylenediaminetetraacetic acid and polyethyleneimine, the three-dimensional(3D) interconnected hierarchical porous network-like framework composing of nanosized building blocks with 20-30 nm was obtained, and delivered a continuous electron transport path, convenient electrolyte penetration-diffusion and large electrode-electrolyte interface simultaneously, which was of great significance for possessing large SC and a striking cycling stability. The unique network-like NiCo2O4 electrode delivered a desirable SC of 587 F/g at 2 A/g and exhibits a stable cycling behavior at each current density. When a continuous cycle is carried out from 2 A/g to 16 A/g, and turned back to 2 A/g again, 98% of the maximum SC at 2 A/g still can be recovered.(4) An efficient two-step strategy involving a co-electrodeposition and subsequent thermal calcination was proposed to grow ultrathin mesoporous NiCo2O4 NSs on Ni foam with strong adhesion for high-performance ECs. The porous NiCo2O4 NSs were only 2-4 nm in thickness and exhibited folding silk-like morphology. This self-supported hybrid electrode of ultrathin mesoporous NiCo2O4 NSs supported on Ni foam had a wealth of electroactive sites and rich mesoporous porosity. In addition, the self-supported hybrid electrode made the electroactive NiCo2O4 contact with both the electrolyte and the substrate directly, and enhanced the penetration and diffusion of the electrolyte, which effectively improved the electrochemical utilization of the NiCo2O4. The unique self-supported ultrathin mesoporous NiCo2O4 NSs electrode exhibited excellent pseudo-capacitance of 2010(~1.61 F/m2), 1450 F/g at current densities of 2 and 20 A/g, respectively. After continuous cycling for 2300 cycles at various current densities ranged from 2 to 20 A/g, ~94% of the initial SC at 2 A/g was still delivered and maintained for another 100 cycles without noticeable diminishment. |
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