Ldhs-based Core-shell Nanowire Arrays:Design,Fabrication And Applications In Electrochemical Energy Storage/conversion

The development of highly-efficient electrode materials for electrochemical energy storage and conversion holds the key to the widely utilization of clean and renewable power sources. Electrodes based on core-shell nanowire arrays with large active surface area, superior electron transportation and ion diffusion properties play a key role; and synergistic effect between different components provides a versatile platform for the enhancement of energy storage and conversion. Making full use of each component to combine their advantages and achieve a synergistic effect is curcial for the electrochemiacal performance of core-shell nanowire arrays.However, due to the complexity in the composition and structure of core-shell nanowire arrays, the structure design, controllable synthesis and performance improvement of electrodes remain a big challenge. Therefore, the seeking on materials with high electrochemical activity, controllable structure/composition and energy storage/conversion mechanism are highly desirable for the development of high-performance core-shell nano wire arrays.Layered double hydroxides (LDHs) with the merits of controllable composition and highly-despersed active sites, have been recognized as one of the most promising electrode materials for electrochemical energy storage and conversion. Although much progress has been made in last decade, deep studies on the construction of LDHs-based mutli-dimensional nanostructures to improve the electron transportation and surface activity are crucial for further enhancement of electrochemical performances of LDHs. In this dissertation,several LDHs-based core-shell nanowire arrays were fabricated by in situ growth or electrodeposition of 2-dimansional LDHs nanoplatelets on the surface of 1-dimensional nanowire arrays. The improvement of the electrochemical performances of LDHs-based electrode materials in the field of supercapacitor, oxygen evolution reaction (OER), and photoelectrochemical(PEC) water splitting was achieved by the rational design and fabrication of LDHs-based core-shell nanowire arrays. In addition, the synergistic effect between different components of LDHs-based core-shell nanowire arrays was thoroughly investigated; the structure-function relationship and the mechanism for the enhanced electrochemical performance were revealed. The detailed research contents are as follows:(1) Co3O4@NiAl-LDH core-shell nanowire arrays for supercapacitor NiAl-LDH was in situ prepared on the surface of Co3O4 nanowires by a step-wise sol-gel process and hydrothermal growth. The NiAl-LDH nanosheets which interconnected with each other and vertically anchored on Co3O4 nanowires form a highly porous morphology. The resulting Co3O4@NiAl-LDH core-shell hierarchical nanowire arrays electrode exhibits promising supercapacitance performance with largely enhanced specific capacitace (1772 F g-1 at 2 A g-1) and rate capability (61.4% retention at 2 A g-1), much superior pristine Co3O4 nanowire arrays. The improvement in electrochemical behavior is attributed to the hierarchical porous architecture of NiAl-LDH shell and the strong core-shell binding interaction, which enables a sufficient exposure of active species and facilitates the chaege transportation process.(2) ZnCo2O4@NiFe-LDH core-shell nano wire arrays for oxygen evolution reaction (OER)Herarchical ZnCo 2O4@NiFe-LDH core-shell nanowire arrays were fabricated by electrodeposition of NiFe-LDH nanoplatelets on ZnCo2O4 nanowire arrays. The resulting ZnCo2O4@NiFe-LDH core-shell electrode exhibits excellent OER performance, including lower overpotential (245 mV at J=10 mA cm-2) and increased current density (22.5 mA cm-2 at η=300 mV),not only superior to pristine ZnCo2O4 nano wire arrays and NiFe-LDH nanoplatelets arrays, but also better than the commercial Ir/C catalyst. The improvement in OER performance is attributed to the hierarchical morphology of the core-shell nanowire arrays, which benefits the exposure of surface active species and facilitates the electron transportation and ion diffusion. In addition,the interaction between ZnCo2O4 core and NiFe-LDH shell further enhances the OER activity of NiFe-LDH and improves the interfacial charge transfer.(3) TiO2/rGO/NiFe-LDH core-shell nanorod arrays for photoelectrochemical (PEC) water splittingHierarchical TiO2/rGO/NiF e-LDH core-shell nanorod arrays were fabricated by spin-coating of graphene nanosheets on the surface TiO2 nanorod arrays, followed by a subsequent electrodeposition of NiFe-LDH nanoplatelets.The TiO2/rGO/NiFe-LDH core-shell nanorod arrays are demonstrated with fine control over the composition and morphology; rGO and LDH are uniformly anchored onto the surface of TiO2 nanorod arrays. The ternary TiO2/rGO/NiFe-LDH nanorod arrays photoanode displays excellent performance for PEC water splitting, with largely enhanced photocurrent density (1.74 mA cm-2 at 0.6 V),photoconversion efficienty (0.58% at 0.13 V) and stability (a stable O2 production within 5 h continuous reaction) compare to pristine TiO2 and dual TiO2/rGO and TiO2/NiFe-LDH nanaorod arrays. It is worthy mentioning that the photocurrent density of TiO2/rGO/NiFe-LDH, which achieves 93% of the theotetical limit of TiO2, is superior to previously reported TiO2-based photoanodes in benign and neutral media. An experimental-computational combination study reveals that rGO with a high work function and superior electron mobility accepts photogenerated electrons from TiO2 and enables fast electron transportation; while NiFe-LDH acts as a cocatalyst which acceletrates the surface water oxidation reaction. The synergistic effect between rGO and NiFe-LDH simultaneously enhances the charge separation and water oxidation efficiency, and therefore improves the PEC performance. In addition, this strategy is also demonstrated in other ternary nanowire array photoanodes (a-Fe2O3/rGO/NiFe-LDH and W03/rGO/NiFe-LDH) with promising PEC performance.