Studies On Synthesis And Electrochemical Properties Of Three-dimensional Hierarchical Nanostructured NiCo₂O₄ Microspheres

With increasing depletion of traditional fossil fuels and environmental pollution problems, new renewable energy sources and energy storage materials have been attracting more and more research interest. Direct methanol fuel cells (DMFCs) and supercapacitors (SCs) have attracted considerable research interest in electrochemical conversion and storage areas due to their high energy conversion efficiency, high power density, long cycle life and environment friendly and so on. NiCo2O4 as a mixed-transition-metal oxides due to it has two solid state redox couples can generate two active sites for electrode reaction, which is in favour of improving the electrochemical performance of electrode materials, meanwhile NiCo2O4 relatively abundance, low cost and environmental benign has been widely used in electrocatalysis and supercapacitors field. However, it is still highly desirable to further improve the electroactivity and stability of the NiCo2O4 matrials. In the present thesis, a facile template-free solvothermal method with no any additive alkali and followed an annealling in air was developed to prepare a series of novel 3D hierarchical NiCo2O4 mesoporous ultrathin nanosheets microspheres, which can greatly improve the performance of NiCo2O4 electrode materials. Methanol oxidation reaction (MOR) and supercapacitors (SCs) were used for evaluating the electro-chemical performance of the as-prepared electrode materials. In this thesis, the major contents and innovations are as follows:1. A facile template-free solvothermal method without any additional alkali was developed to fabricate a series of 3D hierarchical nanosheets hydoxylized Ni, Co nitrate microspheres precursor using nickel and cobalt nitrate as metal sources and water-IPA(isopropanol)-methanol ternary mixed solvent as reaction medium. Synthesis parameters studies indicate that the IPA mianly contributes to the formation of a hollow structure, and methanol is mainly to the creation of the precipitant OH" ions and the followed formation of primary nanosheets. Keeping the usage of IPA as 68 mL, when the Vmethanol and Vwater is 11 and 4 mL, respectively, a 3D hierarchical nanosheets hollow microspheres precursor was obtained; when Vmethanoi and Vwater is 9 and 6 mL, respectively, a 3D hierarchical nanosheets solid microspheres precursor was obtained. Moreover, based on time-dependent morphology monitoring of the precursor, a possible formation mechanism as inside-out Ostwald ripening process of the 3D hierarchical hollow microspheres precursor is proposed2. The above hollow and solid precursors are annealed to obtain the 3D hierarchical NiCo2O4 nanosheets hollow microspheres and solid microspheres, respectively. The comprehensive characterizations show that the hollow microsphere is self-assembled by the mesoporous ultrathin nanosheets consisting of ultrafine NiCo2O4 nanoparticles (11.9 nm), and possesses high specific surface area of 93.4 m2·g-1 and pore volume of 0.368 cm3·g-1, which are greatly larger than corresponding solid one (69.4 m2·g-1,0.294 cm3·g-1).3. The electrocatalytic methanol oxidation results indicate that NiCo2O4 hollow microspheres possess much higher MOR performance than Co3O4 and NiO solid microspheres, and obviously high than NiCo2O4 solid microspheres. The NiCo2O4 hollow microsphere has current density of 95 A·g-1, the overpotential of ?0.27 V (vs. SCE) and 90% retention of current density after 500 cycles.4. The supercapacitors tests indicate that NiCo2O4 hollow microspheres possess more excellent supercapacitors performance than NiCo2O4 solid microspheres. The NiCo2O4 hollow microspheres show specific capacitance (Cs) at 1 A·g-1 of 1701 F·g-1, the rate capability of -61.5% at 15 A·g-1, and 78.2% retention of Cs after 1000 charge-discharge cycles even at 10 A g-1.5. The NiCo2O4 hollow microspheres possess excellent electrochemical performance can be explained as follows:1) The high conductivity can effectively accelerate electron transfer in electrode and electrode/electrolyte interfaces; 2) Large specific surface area can not only afford enough active sites for electrode reaction but also greatly improve the electrode/electrolyte contact area; 3) Numerous open channel between neighboring mesoporous ultrathin nanosheets can offer many facile diffusion paths for ion/electrolyte; 4) The hollow microspheres can serve as an "ion-buffering chamber" accommodating the resultant strain originated from high rate insertion and extraction of OH- ions.