Fabrication Of LDHs-Based Hybrid Electrodes And Study On Their Electrochemical Performance

Layered double hydroxides (LDHs) are a class of two dimensional (2D) layered materials with tunable intrinsic physicochemical properties. By virtue of their flexibility and diversity in chemical composition, morphology and intercalated structure, LDHs provide great opportunity for the development of highly-efficient electrochemical materials. However, their inherent low conductivity and the large ion-diffusion resistance result in moderate capacitance and poor rate capability as supercapacitor materials, which significantly limit their practical application in energy storage and conversion systems. To solve these problems, the rational design and construction of hybrid electrodes with enhanced electric conductivity and facile electron-ion transport perperties would be a tendency for LDHs-based electrode materials. In this dissertation, several LDHs-based hybrid electrodes were successfully fabricated by electrostatic assembly, calcination-rehydration, topological transformation and ion-exchange methods, respectively. The electron and ion transport behavior of these electrodes have been optimized by viable strategies at the mesoscale or molecular scale, achieving high-performance electrochemical electrode materials. The influence of the interlayer ion diffusion on electrochemical property has been studied by a combination study based on experiment and theoretical simulation. Several electrodes with ingenuity architecture have been achieved, which exhibited excellent performance in energy storage/conversion and electrochromism field. The detailed researches are as follows:1. Optimization of electron transport property of LDHs-based hybrid electrodesA NiAl-LDH@carbon nanoparticles (CNPs) hybrid electrode was fabricated by a facile and cost-effective approach, which involves the growth of vertically aligned LDH NPAs via a hydrothermal reaction followed by the coating of CNPs through vacuum filtration process. The resulting hybrid material exhibits excellent capacitive performance, including a high specific capacitance (1146 F/g,10mV/s), good cycling capability and a high energy density (47.7 Wh/kg). The improved capacitance and rate capability can be attributed to the enhanced electrical conductivity resulting from the incorporation of CNPs and the facile ion diffusion pathway provided by the 3D macroporous structure. Therefore, this asymmetric supercapacitor holds great promise for application in flexible and wearable energy storage devices.Porous CoNi2S4 nanosheets array electrode on carbon fibers (CFs) was fabricated by sulfidation treatment on CoNi-LDH electrode at a high temperature. The improved electric conductivity and pore structure provide fast electron and ion transport during the electrochemical redox process. As a result, the CoNi2S4@CFs exhibits a largely enhanced specific capacitance (1742 F/g) relative to the CoNi-LDH@CFs electrode (1402 F/g), as well as a good rate capability. Furthermore, a micro-wire shape supercapacitor device was assembled by using CoNi2S4@CFs, CFs and PVA/KOH as cathode, anode and electrolyte, respectively, which showed a high specific capacitance (232 mF/cm2) and good cycling stability.2. Regulation on the ion transport of LDHs-based electrodes with various intercalated structureHigh-performance electrochromic films based on exfoliated LDH nanosheets and Prussian blue (PB) nanoparticles were fabricated via layer-by-layer assembly technique. The resulting (LDH/PB)n electrodes exhibit electrochromic behavior arising from the reversible K+ion migration into/out of the PB lattice, which induces a change in the optical properties of the electrode. An electrochromic device (ECD) based on the (LDH/PB)60-ITO/KC1 electrolyte/ITO sandwich structure displays superior response properties (0.91/1.21 s for coloration/bleaching), a comparable coloration efficiency (68 cm2/C) and satisfactory optical contrast (44.6% at 700 nm), in comparison with other inorganic material-based ECDs reported previously.A hierarchical CoAl(OH)-LDH (H-OH-LDH) electrode was prepared via a continuous calcination-rehydration treatment of plate-like CoAl(CO3)-LDH (P-CO3-LDH) array on nickel foil substrate. The H-OH-LDH electrode shows a well-defined hierarchical structure with greatly increased accessible interlaminar surface area, leading to improved electrochemical energy storage ability. Most significantly, the interlayer space of H-OH-LDH acts as electrolyte micro-reservoir to store OH-ions, which dramatically decreases the diffusion resistance of OH" to the inner surface of LDH lamella, and consequently results in an ultrahigh rate capability (capacitance reservation of 66% when the current density increases from 1 to 100 A/g).NiAl-LDH nanoplates arrays with different interlayer ions were prepared by a facile two-step route:in situ growth of vertically aligned NiAl(NO3)-LDH nanosheets array (NSA) via a hydrothermal reaction followed by a ion-exchange reaction. Three types of ions (nitrate, pentane sulfonate and dodecyl sulfonate) were induced into the LDH gallery with the interlayer spacing of 0.89 nm,1.74 nm and 2.54 ran (defined as NiAl(NO3)-LDH, NiAl(PS)-LDH and NiAl(DS)-LDH), respectively. Both the capacitance and rate capability enhance with the increase of interlayer spacing. From the structural perspective, a large spacing between adjacent monolayers is essential for the ion transport, which provides a good microenviroment for the redox in molecular scale even at high scanning rate.