| A rapid development of flexible electronics such as roll-up displays, hand-held portable devices, and sensor networks has promoted the needs for flexible energy sources. Owing to the high power density, long cycling stability and fast charging/discharging, flexible supercapacitor (SC) is considered to be a good candidate for flexible electronics. Recently, because of the high theoretical capacitance, pseudocapacitor that based on the transition metal oxides has attracted our attention. With the development of nano science and technology, researchers are realizing that the performance of pseudocapacitor is largely hindered by the electron transport and ion diffusion, leading to the reported values are far behind the theory.This doctoral dissertation firstly reviews the developments of flexible electronics and SCs, focuses on energy storage mechanism, classification and development of pseudocapacitor, then propose the structure design and modification of transition metal oxides to solve the poor electron/ion transport, hoping to obtain a high energy/power density within high mass loading electrode. Finally, we could fabricate a high performance flexible solid-state SC to drive flexible electronics. The main results are as following:(1) In order to solve the poor electron conductivity of transition metal oxides, core-shell structure is achieved based on the growth of metal oxide on the surface of high conductive materials, leading to a three-dimensional electron conducting path. When using carbon fiber/MnO2 core-shell structure nanoelectrodes to fabricate a solid-state SC, it shows high electron conductivity and capacitive performance. The discharge current shows a linear relationship with scan rates up to 20 V/s, and the CV curves retain the almost rectangular shape with little variance even at an ultrafast scan rate of 50 V/s. Meanwhile, it has been found that the thickness of metal oxide is increasing when add the mass loading of metal oxides, resulting in the decrease of electron conductivity. For example, we use electrochemical deposition method to grow MoO3 on high conductive WO3, when increase the mass loading of MoO3, the areal capacitance is increasing while its gravimetric counterpart is decreasing, in another words, the ultilization of active material is largely confined. To solve the poor electron conductivity especially in the high mass loading electrode, we need to modify the materials essentially.(2) We use doping and hydrogenation methods to tune the carrier density of metal oxide, focusing on solving the poor electron conductivity essentially. According to the first principle theory, for n-type oxide such as MnO2 and MoO3, after modification the Fermi level increases and approaches to conduction band, sometimes it introduces defect level. All of these changes will result in the enhancement of carrier density and largely increase the electron conductivity. Our experiments confirm the enhancement of conductivity with up to two orders of magnitude. High mass loading (more than 2 mg/cm2) V-doped MnO2 electrode shows a high capacitance of 439 F/g and hydrogenated MoO3 achieve a high volumetric capacitance of 291 F/cm3.(3) In order to solve the issue of ion diffusion, for layered structure materials, we propose a cation preintercalated method to increase the interlayer spacing. We preintercalate K+ into the interlayer of MoO3 (KyMoO3-x) and find that different cations (Mg2+, Na+, K+ and Li+) are able to intercalate along the (010) facet of KyMoO3-x with high intercalation capacitance (374 F/cm3 at 0.5 A/g in 5 M LiCl). Using seawater as electrolyte, high volumetric capacitance (188 F/cm3 at 0.S A/g) and good rate handling are also achieved. For nonlayered structure materials, we report a general strategy that uses the surfaces of water-soluble salt crystals as growth templates and is applicable to various transition metal oxides, such as hexagonal-MoO3, MoO2, MnO and hexagonal-WO3. The planar growth is proposed to occur via a match between the crystal lattices of the salt and the growing oxide. Restacked two-dimensional h-MOO3 exhibits high pseudocapacitive performances (e.g.,600 F/cm3 in H2SO4 and 300 F/cm3 in an A12(SO4)3 electrolyte). In organic electrolyte, it achieves 996 C/g at 2 mV/s which is approching the theoretical value.(4) The current strategy of electrode fabrication has two problems:the use of binder will decrese the conductivity and the utilization of mass and volume in the electrode is very low. To solve these issues, we report a flexible freestanding electrode that is fabricated by simply vacuum-filtering the dispersion of active materials and carbon nanotubes (CNTs), sufficiently utilizing the synergistic effects from the high electrochemical performance of active materials and the high conductivity and mechanical consolidation of the CNTs. We also study the influence of species and amounts of functional groups on the conductivity and electrochemical performance of CNTs.(5) We successfully fabricate the flexible asymmetric solid-state SC, and use it to drive a high power consumption wireless transport system. Combined with solar cell, triboelectrical generator and other energy harvesting system, we could collect the energy from environment and store them into SCs to fabricate a self-powered system. |
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