Preparation And Design Of Co、Fe_3-x、O₄（x=1, 2） Based Spinel Oxide For The Electrochemical Analysis

The rechargeable metal-air battery has recently attracted intensive attention due to its extremely high theoretical energy density, low cost and environmental benignity. It shows potenital application in portable electronics, communication apparatus and electric vehicles. However, the practical application of metal-air battery is still restricted by the sluggish electrochemical reaction dynamics at the air electrode. Electrocatalysts can effectively alleviate this problem. So reasonable designing and synthesis of environmental friendly, efficient and low-cost catalysts for oxygen reduction reaction（ORR） and oxygen evolution reaction（OER） have become the heart of the study and development of metal-air battery. In this thesis, we have studied the low-cost cobalt ferrite spinel based materials Cox Fe3-x O4（x=1, 2）. We tailored the catalytic performance of Cox Fe3-x O4 by controlling the microstructure, crystal orientation, covalent coupling between Cox Fe3-x O4 spinel oxides and porous carbon or conductive metal oxide. We developed methods for the synthesis of Cox Fe3-x O4/carbon and Cox Fe3-x O4/conductive oxide nanomaterials and elucidated the influence of structure, crystal orientation and the carbon/conductive metal oxide supports on the catalytic performance of Cox Fe3-x O4 spinel oxides. This thesis consists of the following five parts:Firstly, we designed and synthesized the Fe Co₂O₄ spinel oxide with hollow cubic structure. The Fe Co₂O₄ hollow cubes（300-400 nm） are composed of orderly assembled nanoparticles with size ranging from 5 to 10 nm, and possess a relatively high specific surface area（91.74m2/g）. Mesopores and macropores exist in the hollow cubic structure, which can facilitate the efficient transport of O2 and electrolyte in the triple-phase（solid–liquid–gas） region. The Fe Co₂O₄ hollow cubes demonstrate higher catalytic activities and stabilities toward ORR and OER in alkaline solution as compared with Fe Co₂O₄ nanoparticles. When used as the catalyst for the cathode of Li-O2 batteries in nonaqueous electrolyte, the hollow cubic structure can provide more voids for the deposition of discharge products（Li2O2）. This relieves the clogging problem and reduces the polarization during the charge and discharge processes, improving the capacity and cycle stability. The results reveal the special hollow cubic structure is beneficial for the the catalytic activity improvement of the Fe Co₂O₄ spinel oxide.The effect of crystal orientation was explored on the catalytic performance of Co Fe₂O₄ spinel oxide. Firstly, Co Fe₂O₄ with cubic and octahedral morphology, which are predominantly enclosed by {100} and {111} planes, respectively, were synthesized. Then the catalytic activities of Co Fe₂O₄ enclosed by {100} and {111} planes were studied. The relationship between crystal orientation and electrocatalytic activity was investigated. The difference in the atom arrangement and surface energy between {100} and {111} planes give rise to the discrepancy in exposed active sites and adsorption energy for O2. The {111} planes demonstrate more enhanced electrocatalytic activities and stabilities for ORR and OER.In order to address the issue of low electronic conductivity in Fe Co₂O₄, we designed and fabricated a hybrid with Fe Co₂O₄ nanoparticles loading on the surface of hollow reduced graphene oxide spheres（FCO/Hr GOS）. The Fe Co₂O₄ nanoparticles combine with hollow reduced graphene oxide spheres（Hr GOS） via the covalent coupling in the hybrid, which can not only improve the electronic conductivity of Fe Co₂O₄ nanoparticles, but also alter the element bonding state and adjust the electron density around Fe/Co, so as to accelerate the catalytic reaction. Meanwhile, the covalent coupling can also prevent the detachment of Fe Co₂O₄ nanoparticles from Hr GOS during cycling process. Moreover, the special 3D hierarchical mesoporous architecture arised from Hr GOS facilitates the transport of O2 and electrolyte. Benefiting from the above factors, the FCO/Hr GOS hybrid demonstrates superior electrocatalytic activities, and more enhanced electrochemical stabilities for ORR and OER.On the basis of the above work, we further investigated the impact of heteroatom doping in graphene on the covalent coupling between spinel oxides and graphene and the resulting catalytic activity. Firstly, we synthetized the hybrid with mesoporous monodispersed Co Fe₂O₄ nanospheres（⁷0 nm） loaded on graphene sheets. The doping of N in graphene provides the material with more active sites. The Co Fe₂O₄ nanospheres are deposited on the both sides of graphene, which can effectively suppress the agglomeration of Co Fe₂O₄ nanospheres and restacking of graphene in the hybrid and allow a higher loading of Co Fe₂O₄. Secondly, we designed and synthesized the hybrid with Co Fe₂O₄ nanoparticles deposited on nitrogen/sulfur dual-doped three dimensional（3D） reduced graphene oxide networks（CFO/NS-r GO）. The doping of S and N can not only provide the 3D graphene networks（NS-r GO） with abundant defects, but also play a significant role in engineering the covalent binding between Co Fe₂O₄ and the NS-r GO. The covalent coupling between Co Fe₂O₄ and NS-r GO via S exists as the –C–SOx–M–. The Co and Fe in CFO/NS-r GO are preferentially bonded to pyrrolic N. The dual-doping of N/S in NS-r GO, covalent coupling between Co Fe₂O₄ and NS-r GO and 3D reduced graphene oxide networks structure endow the CFO/NS-r GO hybrid with prominent electrocatalytic activities and stabilities for ORR and OER.To address the issue of carbon oxidation at high potential during the catalytic process, the conductive metal oxide–the tin doped indium oxide（ITO） was used as the support for Co Fe₂O₄ spinel oxide. And we synthesized the hybrid with Co Fe₂O₄ nanoparticles supported on the surface of ITO. Moreover, the degradation mechanism during ORR and OER process was also been investigated for the materials. Due to the eminent electronic conductivity and electrochemical stability of ITO at a high potential, the CFO/ITO hybrid exhibits improved catalytic activities and electrochemical stabilities as compared to the hybrid with Co Fe₂O₄ nanoparticles supported on super P carbon black（CFO/Super P）. The degradation of CFO/ITO and CFO/Super P in ORR and OER activities under the long-term operation mainly originate from the following several aspects: the detachment of CFO from the support, the partial dissolution of CFO, the dissolution of Sn in the ITO or the oxidation of Super P at high potential.