The Research On The Synthesis Of Metal Oxide Nanoarray Structure And Their Pseudocapacitve Performance

With ever-increasing energy and power demands in applications such as mobile electronics and electric vehicles, supercapacitors as new energy storage devices, have attracted considerable attention due to their fast recharge ability, high power performance, long cycle life, and low maintenance cost. However, the low energy density of supercapacitors prevents their use in many important applications. To overcome this limitation, current research works mainly focus on increasing their energy density to make them comparable to batteries. One effective method is to directly grow smart integrated architectures from metal oxides on conducting substrates as binder-free electrodes for supercapacitors. In this way, many competitive advantages such as rich accessible electroactive sites, fast electron-transport access, easy diffusion of the electrolyte and fascinating synergetic properties of components can be simultaneously achieved to deliver a high specific capacitance, desirable cycle life and good rate performance. The main targets of this paper are as follows: Metal oxide nanoarray structure is designed rationally and fabricated by a facile and green method, and the growth process of structures and effect on electrochemical properties of different kinds of nanoarray materials are systematically researched. The advantages of nanoarray structured electrode materials in supercapacitors are researched. Main research contents are as follows:（1） In this chapter, CoMoO₄ nanoplate arrays（NPAs） were grown directly on Ni foam via a template-free hydrothermal route. The morphology of CoMoO₄ NPAs was examined by scanning and transmission electron microscopy and the phase structure of nanoplates（NPs） was analyzed using X-ray diffraction spectroscopy. Based on a series of time-dependent experiments, a possible growth mechanism for this structure was proposed. The CoMoO₄ NPAs supported on Ni foam could be directly used as integrated electrodes for electrochemical capacitors. Such unique array architectures exhibited remarkable electrochemical performance with a high specific capacitance of 1.26 F cm^-2 at a charge and discharge current density of 4 m A cm^-2 and 0.78 F cm^-2 at 32 m A cm^-2 with an excellent cycling ability（79.5% of the initial specific capacitance remained after 4000 cycles）. The superior electrochemical performances could be attributed to the open network structure constituted of interconnected CoMoO₄ NPAs directly grown on current collectors that could improve electron transport and enhance electrolyte diffusion efficiency（2） A facile and scalable strategy was developed to construct NiMoO₄ NW arrays on Ni foam with high electrochemical performance for supercapacitors. The as-prepared NiMoO₄ NW array electrode delivered a high capacitance of 1.96 F cm^-2(1308 F g^-1) even at a very high current density of 112 m A cm^-2(74.7 A g^-1), desirable rate performance and electrochemical stability. Such intriguing capacitive behavior is attributed to the one dimensional array structure and synergistic effects between both nickel and molybdenum ions in the binary oxide. Our work confirms the feasibility of the rational design of novel advanced electrochemical pseudocapacitor materials. In addition, the electrode design concept can be easily generalized to other binary or ternary metal oxides with unique array structures on conducting substrates for large scale supercapacitor applications.（3） NiMoO₄ nanowire array grown radially on carbon cloth with good electrochemical properties have been synthesized by a cost effective hydrothermal procedure. The NiMoO₄ NW arrays supported on carbon cloth was directly used as integrated electrodes for electrochemical capacitors. The NiMoO₄ NW arrays yielded high-capacitance performance with a high specific capacitance of 1.27 F cm^-2(1587 F g^-1) at a charge and discharge current density of 5 m A cm^-2 and 0.76 F cm^-2(951 F g^-1) at 30 m A cm^-2 with a good cycling ability（76.9% of the initial specific capacitance remains after 4000 cycles）. An aqueous symmetric supercapacitor device with a maximum voltage of 1.7 V has been fabricated, delivering both high energy density(70.7 Wh kg^-1) and power density(16,000 W kg^-1 at 14.1 Wh kg^-1). These results show that the NiMoO₄ nanowire arrays with large surface area, combined with the flexible carbon cloth substrate can offer great promise for large-scale supercapacitor applications.（4） Well-ordered manganese oxide（MnO₂） nanosheet arrays（NSAs） were grown directly on carbon cloth via a simple in-situ redox replacement reaction between potassium permanganate（KMn O₄） and carbon cloth without any other oxidant or reductant addition. The morphology of MnO₂ NSAs was examined by scanning and transmission electron microscopy and the phase structure of nanosheets（NSs） was analyzed by X-ray diffraction spectroscopy. Based on a series of time-dependent experiments, a possible growth process for this structure was proposed. The MnO₂ NSAs supported on carbon cloth were directly used as integrated electrodes for electrochemical capacitors. The ordered MnO₂ NSAs yielded high-capacitance performance with a high specific capacitance of 2.16 F cm^-2 at a charge and discharge current density of 5 m A cm^-2 and 1.01 F cm^-2 at 20 m A cm^-2 with a cycling ability（61.4% of the initial specific capacitance remains after 3000 cycles）. The MnO₂ nanosheet arrays with large surface area and high degree of ordering, combined with the flexible carbon cloth substrate can offer great promise for large-scale supercapacitor applications.（5） A novel composite structure of core/shell NiMoO₄@MnO₂ array was directly synthesized on carbon cloth by a facile two-step hydrothermal route for supercapacitors. The smart combination of NiMoO₄ and MnO₂ shows a synergistic effect for capacitors with greatly enhanced performance. The NiMoO₄@MnO₂ array electrode yields high-capacitance performance with a high areal capacitance of 3.90 F cm^-2 at a charge and discharge current density of 8 m A cm^-2 and 3.22 F cm^-2 at 24 m A cm^-2 with a desirable cycling ability（90.5% of the initial specific capacitance remains after 4000 cycles）. Such core/shell composite nanoarchitectures exhibit remarkable electrochemical performance with high capacitance and excellent long-term cycling stability, which could be promising pseudocapacitive electrode materials for high performance supercapacitors.