Study On Cotrolled Preparation And Electrocatalytic Performance Of Tungsten-based Nanomaterials

With the increasing consumption of non-renewable resources,the development of new energy is more and more urgent.However,many problems exist in the application of the commercial catalysts in new energy,such as high cost,poor performance and short life.Due to its excellent conductivity,structural stability and platinum-like catalytic activity,tungsten oxide/carbide nanomaterials can be used as carriers and co-catalysts for electrode catalysts used in electrolysis of water,fuel cell and air cell.In this thesis,tungsten oxide?WO3?with different morphology and size was prepared by using CNTs as template,and the influence of morphology and size on the hydrogen oxidation reaction?HOR?of WO3 was investigated.In situ carburizing reduction of WO3 was carried out by using ethanol/methanol as carbon source through temperature programming method.The carbon layer was coated on the surface of tungsten carbide?WC@C?,and its performance of hydrogen evolution reaction?HER?was studied by loading Pt nanoparticles?Pt NPs?.By adjusting the ratio of ethanol/methanol,a lot of ultrathin coiled carbon nanosheets deposited on the surface of tungsten carbide?WC@UC-CNS?.Finally,after acid etching of WC@UC-CNS and removing WC,Pt NPs were deposited on the surface of UC-CNS,its catalytic performance of methanol oxidation reaction?MOR?as analyzed.The obtained results in the study are shown as follows:?1?The CNTs were purified by calcination and acid washing to obtain pure CNTs without impurities,such as catalyst iron particles and free carbon.The CNTs were then dispersed in ethylene glycol solution as a template.H2WO4 particles could be deposited on CNTs by liquid phase method after sodium tungstate?Na2WO4·2H2O?and acid solution added in the solution.By adjusting the molar ratio of CNTs and Na2WO4·2H₂O,the morphology and size of H2WO4,H2WO4/CNTs composites with different morphologies?plates,short rods and long rods?and sizes?20-200 nm?could be synthesized.?2?The H2WO4/CNTs was placed in a tubular furnace under nitrogen atmosphere and kept at 600?for 2 h.The crystallization water in H2WO4/CNTs was removed to obtain the WO3/CNTs product.WO3/CNTs inherited the morphology of H2WO4/CNTs and did not change significantly in size.Comparing the HOR performance of WO3/CNTs with flakes,short rods and long rods,it was found that shorter rod-shaped WO3/CNTs with smaller size and higher specific surface area have the best HOR performance.After 1000 cycles of cyclic testing,its peak current density can still reach 42.73 mA cm^-2,even higher than that of the short rod-shaped WO3(4.49 mA cm^-2)obtained by heat preservation and dehydration in air.The results show that WO3/CNTs with excellent HOR performance are due to small sized WO3 and high conductive CNTs.?3?The optimal carbonization temperature,time and volume ratio of ethanol/methanol were explored by in-situ reduction and carbonization of WO3?a dehydrated product in the air?by using ethanol/methanol.It was found that the carbon source was injected in the furnace from the temperature rise of 600?,and kept at 1000?for 2 h,the ratio of ethanol to methanol was 10:90,the final carbide product was WC that covered by graphitized carbon layer,namely WC@C.Microwave method was used to load Pt NPs on WC@C,and graphitized carbon layer played a role in fixing and dispersing Pt NPs.It shows that WC@C/Pt possesses excellent performance of HER.When the current density reaches 10 mA cm^-2 in acidic solution,it needs the overpotential of 43 mV,while in alkaline solution,it needs the overpotential of 52 mV.The ethanol with high molecular weight of carbon can pyrolyze and provide excessive carbon when the volume ratio of ethanol/methanol is 20:80.Under the catalytic action of WC,excess carbon is formed on the surface of WC particles,which is named as WC@UC-CNS.When the WC@UC-CNS was placed in HF/HNO3/H2O solution,WC was etched,the UC-CNS with high graphitization degree?IG/ID=1.43?and high specific surface area(547 m² g^-1)was obtained.Pt particles were loaded on the surface of UC-CNS to obtain UC-CNS/Pt catalyst.The electrochemical active specific surface area?ESCA?reached121.98 m² g^-1 in the 0.5 M H2SO4 electrolyte,and the peak current density of MOR in the medium reaches 91.7 mA cm^-2 in 0.5 M H2SO4/1 M CH3OH electrolyte.Even after 14,000cycles at 1.605 V polarization voltage,it still maintains high electrocatalytic activity.