Three-Dimensional Electrodes Based On Transition-Metal-based Compounds And Electrocatalytic Water Splitting

Transition metal compounds have been widely utilized in many fields such as photocatalysis,supercapacitors,Li-ion battery,fuel cells and electrocatalysis due their superior conductivity and good stability,therefore attracting much attention from the researchers.They are remarkable electrocatalysts because most of the them are accessible to oxygen and hydrogen.Moreover,they are easy to modify and improve in the structures and the synthesizing methods are easier as well.It is a good choice to replace the noble metal based electrocatalysts with the transition metal-based electrocatalysts.It is urgent to explore the design and fabrication of the transition metal based electrocatalysts.Herein,some types of transition metal compounds are designed and fabricated as the bifunctional electrocatalysts towards HER and OER making up the overall water splitting device.A few progresses on electrocatalytic water splitting have been made.The transition metal based electro catalysts have been modified boosting the electrocatalytic performance.The main contents are as following:1.Ni（NO3）2·6H₂O and Fe（NO3）3·9H₂O with NH4F and urea are used as the source to produce NiFe-precursors and Ni-precursors ultrathin nanosheets on the carbon cloth with the later thickness of 10-30 nm via hydrothermal method.The interconnected nanosheets on the carbon formed flower-like structures.Ni2P and Ni5P4 were obtained through phosphorization for the Ni-precursors on the condition of Ni/P=2:1 and 5:4.Surprisingly,Fe doped Ni2P was obtained through phosphorization for NiFe-precursors on the condition of Ni/P=5:4.The different types of phases for impact on the electrocatalytic performance and the doping effect on the phases were studied by comparing the synthesized electrocatalysts towards HER and OER.The results suggested that the intrinsic activity of the electrocatalysts has been enhanced with the increasing amount of P.Meanwhile,the doped cation changed the density of the electrons playing in a vital role in improving the activity of the electrocatalysts.2.NiFe-precursors were synthesized by the electrochemical deposition using Ni（NO3）2·6H₂O and Fe（NO3）3·9H₂O solution as electrolyte and carbon cloth as working electrode.The formation of bimetallic Ni3FeN has been studied.The precursors were annealed at 400 ℃ in different atmospheres（N2,NH3 and N2/H₂）obtaining the NiFeOx,Ni3FeN and Ni3Fe indicating Ni3Fe as the intermediate determined the final morphology of Ni3FeN nanoframework.Compared with Ni3N and Fe2N,the synergistic effect of both cationic atoms contributes to enhancing the electrocatalytic performance.Electrochemical measurement revealed the composite not only delivers improved electrocatalytic activity but also shows remarkable stability.3.The fibrous bifunctional electrocatalysts towards HER and OER prepared by assembling multi-component nitrides-oxides hetero-nanostructures on graphite fiber through a facile electrochemical deposition method following nitridation process.Benefiting from the collaborative advantages of metallic characters of all the three components,Ni3N,CoN and NiCo₂O₄,the unique valence and electronic states of them,the special nanoflake-nanosphere network structures with interface electric field effect,and high conductivity of graphite fiber,as well as the outstanding environmental stability of all the parts of the hybrid fibrous electrocatalysts.The theory calculation results reveal that the elevated activities in electrochemical reactions could be correlated to the roles of Ni3N and CoN derived from the in situ nitridation for NiCo₂O₄ by analyzing relationship between the electrons transferring and the Femi level changes comparing oxides with oxides/nitrides.The results indicated that Ni3N mainly contributes to HER process and CoN is mainly beneficial for OER process.In summary,this thesis designs and synthesizes three-dimensional transition-metal-based electrocatalysts.The performance of electrocatalysts has been enhanced through doping and constructing heterostructures to change the intrinsic structures.The improved performance also arises from the increased electrochemical active sites because of the expanded surface areas of three-dimensional structures.The above results provide a new method of transition-metal-based compounds as electrocatalyst synthesizing and design and have an important impact on research on improving the electrocatalytic performance.