Synthesis And Electrochemical Properties Of Transition Metal Carbides And Their Composites

Hydrogen generation by electrocatalytic water splitting is a promising technology;however,it is still a huge challenge to develop economical and efficient catalysts to promote the large-scale industrialization.Transition metal carbide is one of the most promising noble metal-free electrocatalyst,because it has a d-band electronic structure which is similar to platinum,and its optimization can improve its hydrogen adsorption kinetics,leading to better electrocatalytic activity.However,large particles and agglomerations are easily formed due to the preparation of carbide materials by carbonizing the precursor at high temperature,which will greatly reduce carbides'catalytic activity.To address these issues,this thesis mainly takes molybdenum carbide and tungsten carbide as research objects,designs reasonable nanostructures utilizing spray drying method which can be mass-produced,prepares molybdenum carbide/carbon nanotubes,tungsten carbide/carbon nanotubes and their composites,and investigates their electrochemical properties in detail.The main research contents and conclusions are as follows:1.Molybdenum carbide nanoparticles loaded with three dimensional?3D?carbon nanotubes?CNT?networks were prepared by spray-drying followed by carbonization?Mo₂C/CNT-SC?,and their hydrogen evolution properties were studied.The research shows that the Mo₂C/CNT-SC_1.1 synthesized from the raw material ratio of C/Mo?molar ratio of 10:1?and carbonation temperature of 700?has the best hydrogen evolution performance:the onset overpotential is²0 mV?vs RHE?;the Tafel slope is53.6 mV dec^-1;It was stable under the i-t test over 35 h.The main reason for the excellent performance of Mo₂C/CNT-SC is that the ultra-small Mo₂C nanoparticles are embedded in the highly conductive 3D porous CNT network structure,leading to a great number of exposed active sites.The tight connection between Mo₂C and CNT enhances the efficiency of charge transfer.2.We synthesized W₂C nanodot-decorated CNT networks?W₂C@CNT-S?via a facile and scalable spray drying method followed by a carbonization process.The effects of different preparation methods and a different ratios of raw materials on catalytic performance were investigated.The research shows that W₂C@CNT-S8 has the best hydrogen evolution performance:the onset overpotential is 60 mV vs RHE,and the Tafel slope is 57.4 mV dec^-1?acidic solution?;The onset overpotential is 40 mV vs RHE,and the Tafel slope is 56.2 mv dec^-1?alkaline solution?.The main reasons for the excellent performance are the 3D porous structure formed by spray-drying process,and the highly conductive CNT nanostructure modified by extremely small W₂C nanodots.3.Metallic tungsten/Tungsten carbide/Carbon nanotube multiphase heterogeneous composites?W-W₂C/CNT?were prepared by spray drying and controlled carbonization process.The effects of different ratios of W/W₂C on the hydrogen evolution performance of the whole material were investigated.The results show that W-W₂C/CNT-6 heterogeneous composites exhibit excellent hydrogen evolution in both acidic and alkaline media:ultra-small onset overpotential?acidic media:40 mV vs RHE;alkaline media:20 mV vs RHE?and small tafel slope(acidic media:56 mV dec^-1;alkaline media:51 mV dec^-1).The excellent performance in both acidic and alkaline media is mainly attributed to the three-dimensional porous conductive network and the ligand effect of metallic tungsten and the synergistic effect between multiple components.