Design Of Layered Double Hydroxide Based Nanostructured Electrocatalysts And The Performance

Heterogeneous catalysis is widely used in human life, such as in fields of chemical industry, energy, biology and environment. Heterogeneous catalysts, suffer from low conductivity and activity. In this dissertation, we demonstrate the methodology of synthesis and structure modification and optimization of layered double metal hydroxides (LDH)-based catalyst by thermal ammonolysis and oxidation as well as precursor particle size control. And the LDH precursor becomes an alternative of high conductivity, activity, and specific surface area. The size regulation of LDH can be realized by controlling the ultrasonic exfoliate condition, and the ultrafine LDH nano dots can be obtained. By the combination of experimental and theoretical calculation, the LDH precursor of nitride, oxide, and LDH nano crystal structure, morphology, and valence state of elements, the interaction of intercalating metals were studied and the effects of catalyst structure, morphology and composition of the influence of water splitting by electrocatalysis; In this paper, a series of transition metal electrocatalysts for water splitting of high catalytic activity using LDH as precursors, which provided experimental and theoretical basis for the development of a new type of high performance electrocatalysts.The main research contents and results of this paper are as follows:(1) We report the successful synthesis of Ni3FeN-NPs by thermal ammonolysis of an ultrathin NiFe-LDH precursor, and the subsequent identification of Ni3FeN-NPs as a highly efficient electrocatalyst for overall water splitting. The excellent performance of Ni3FeN-NPs derives from the intrinsic metallic character and unique electronic structure of this compound which improve the electrical conductivity and adsorption of H2O, and also the nanosized particle effects which increase the availability of active sites for electrocatalysis. Ni3FeN-NPs exhibited enhanced HER and OER catalytic activity compared with Ni3FeN-Bulk, Ni3N, NiFe-MMO and commercial Pt/C. This work prompts further investigations into bimetallic nitride systems for overall water splitting.(2) Ultrafine NiO nanosheets with a particle size of?4.0 nm and thickness of ?1.1 nm dispersed in TiO2 with excellent OER performance in aqueous alkali media have been obtained by calcination of monolayer NiTi-LDH nanosheets. TEM, EXAFS, ESR and DFT calculations established that the atomic-thick NiO nanosheets in Mono-NiTi-MMO expose a high percentage of reactive {110} facets containing Ni3+active sites as well as local structural distortions, which promote H2O adsorption and increase the charge-transfer efficiency, resulting in the significantly improved H2O oxidation activity. This work demonstrates the potential of novel ultrafine and ultrathin oxides materials for electrocatalytic applications. Using this same synthetic strategy, other metallic oxide nanosheet electrocatalysts with controllable exposed high-energy facets and desirable eg orbital occupancy, such as FeOx, CoOx, MnOx, could be obtained.(3) ZnCo-LDH was synthesized by coprecipitation method, and the single layer LDH nano dots was exfoliated to a particle size of 3.5 nm and the thickness of ?0.5 nm with the method of ultrasonic. The material has a larger specific surface area, which can be exposed to more active sites. Compared to the bulk LDH precursor, the current densities of nano dots was increased 16 times of the OER performance. In addition, the high performance of nano dots could be associated with their ultrathin structure which shortens ion diffusion pathway length and the surface defects which could promote the H2O adsorption. Using this synthetic strategy, other LDH nano dots could be designed, such as the LDH which comprise of Fe, Co, Ni, Mn and other transition metals.(4) As a cocatalyst, ZnCo-LDH nano dots could promote the photoelectrocatalytic oxygen evolution of TiO2-nanotube arrays (TiO2-NTAs). The nano dots were load to TiO2-NTAs by electrodeposition because of the electropositive of nano dots. By analyzing the different loading conditions, the optimal conditions of the loading of the ZnCo-LDH nano dots was at the voltage of 8 V, and with the loading time of 2?5 min. This method provides an application of LDH as cocatalysts, compared to the bulk LDH, the nano dots show some advantages in terms of size and activity.