Design Of Structured LDHS-Based Electrodes With Enhanced Electrochemical Properties

The ordered and structured design of functional electrodes meld high surface area for heterogeneous reactions with conductivities for electron and ion transport, which exhibits a promising research prospect for further improvement in the interfacial activity of electrochemical storage and conversion materials. For transition metal-based compounds with high electrochemical activities, it is still challenging to achieve well-defined structured systems due to their poor adjustability of structure and composition. By virtue of the versatility and plasticity, layered double hydroxides (LDHs) offer the opportunity to develop novel hybrid materials with controllable architectures. In this dissertation, based on LDHs-based materials as the building units and active centers to optimize the dispersivity of components and capability of electron/ion transport, we have designed a series of structured configurations by in situ growth and assembly methods with controllable hybridization spatial orientation and tunable topography parameter. Available strategies have been enriched to design and fabricate ordered and structured LDHs on different microscopic scales. Several functional hybrid electrodes with novel structures have been developed, which yield excellent electrochemical energy storge (supercapacitor) and conversion (electrocatalysis) performance. On the basis of building up LDHs-based structured configurations, by using a combination of theoretical simulations and experimental studies, we have analyzed the influence of intrinsic properties of LDHs host layers and confined interlayer domains on the electrochemical performance, and investigated the relationships between and among design, structure and performance of energy storage and conversion materials. The detailed research contents are as follows:1. Hierarchically structured LDHs by in situ growth and theirelectrochemical propertiesWe develop a new and facile in situ growth route with general applicability for LDHs. Hierarchical structures of LDHs with perpendicular orientation (ab-plane) can be firmly grown on various substrates, including conductive glass and micro-/nano-scaled carbon materials. Several hierarchical nanostructured composites including noble metal/LDH, metal oxide/LDH, LDH/carbon fibers (CFs) and LDH/carbon nanotubes (CNTs) have been fabricated.Hierarchical CoAl-LDH strutures with vertical orientation have been synthesized on the smooth surface of conductive glass by using the in situ growth method, which can serve as three-dimensional building units. In light of exogenous route, a Pd-based electrocatalyst with finely-dispersed PdNPs anchoring onto the vertically-aligned CoAl-LDH nanowalls has been fabricated. Based on the elaborate network architecture and the effective exposure of active sites, the resulting composite yields a largely improved catalytic activity towards ethanol electrooxidation in comparison with the commercial Pd/C catalyst.Moreover, we have further extended the in situ growth method to construct hierarchical LDHs on the surface of micro-/nano-scaled carbon materials. CoAl-LDH nanowalls have been grown on flexible CFs, which can act as a reductant and the porous framework for anchoring and dispersing the nanoscaled MnO2. Because the sophisticated configuration guarantees effective electron/ion transport and the synergetic contribution from individual constituents, the resulting MnO2/LDH/CFs composite exhibits a highly boosted specific capacitance (944 F/g at a current density of 1 A/g).Controllable fabrication of electrochemically active species has been made by using the LDHs as the homogeneous matrix for transition metals. We have explored the critical role of LDH host layers in the integration of electrochemical activities. Well-defined core-shell structures of CoMn-LDH/CFs and NiMn-LDH/CNTs have been further constructed. Owing to the unique structure and homogeneous microenvironment, the as-prepared CoMn-LDH/CFs and NiMn-LDH/CNTs electrodes exhibit excellent capacitive performance (CoMn-LDH/CFs:1079 F/g at a current density of 2.1 A/g; NiMn-LDH/CNTs:2960 F/g at a current density of 1.5 A/g). A combined experimental and theoretical study demonstrated the homogenous and ordered arrangement of metal units within the LDH layers enhances the charge transfer and synergistic reaction activity. Directing toward the advantages of these structures, flexible solid-state supercapacitor devices have been further prepared and investigated.2. Orderly structured LDHs by assembly and their electrochemical propertiesWe have designed multilayered and ordered hybrid structures to improve the interfacial homogeneity and synergistic effect, aiming at underlining the microscopic size effect. We have also focused on the relationship between the elaborate dimensional features and electron/ion transport properties.Based on the two-dimensional LDHs monolayers with atomic thickness, we designed ordered hybrids with heterogeneous multilayers by assembly route. A molecular-scale hybrid system has been developed by the heteroassembly of CoNi-LDH monolayers and the conducting polymer (PEDOT:PSS) for all-solid-state flexible supercapacitor devices. From the structural perspective, an intimate and homogeneous interfacial bonding between the LDH monolayers and the interlayer PEDOT:PSS is essential for superior charge storage properties:the LDH monolayers provide redox species and a restricted microenvironment for the accommodation of the conducting polymer with desirable spatial arrangement and doping level; while sandwiched PEDOT:PSS layers enable effective charge transport and electrode integrity. The resulting CoNi-LDH/PEDOT:PSS hybrid materials possess enhanced overall performance of charge storage with a high capacity and excellent rate capability (capacitance loss<17% at 30 A/g).We have further explored the influence of the size effect of LDHs host layers on the electrical properties of noble metal nanoparticles. The ordered LDH/Au nanoparticles (AuNPs) multilayered films was prepared via layer-by-layer (LBL) assembly technique, in which the AuNPs are highly dispersed with a monolayer arrangement in the LDH gallery. The long-range ordered structure leads to the formation of the plasmon coupling of adjacent AuNPs layers, which facilitating electron transfer across the multilayers. Moreover, the LDH provide a confined and stable interlayer for the maintaining and enhancing the activity of AuNPs, so that the obtained modified electrodes display remarkable electrocatalytic behavior towards glucose.