Growth Of Three-dimentional Graphene On Carbon Fibers And Preparation And Catalytic Performance Of Its Composites

The most advanced catalyst for water splitting electrocatalysis is noble metal catalyst.However,it is difficult be applied on a large scale due to its scarcity,poor stability and size limitation.Therefore,it is urgent to develop new non-precious metal catalysts.In-situ growth of a vertically oriented two-dimensional active material on the surface of three-dimensional porous conductive substrates can maximize the specific surface area,increase the conductivity,provide more active edge sites,promote the bubble release,and release the active sites on the surface of catalysts,which is considered as a promising material for water splitting electrocatalysis.In this thesis,vertically oriented three-dimensional graphene nanosheets with high specific surface area,high electrical conductivity,and high stability are grown on the surface of carbon fibers by chemical vapor deposition（CVD）.MoS₂nanosheets are grown on the surface of the 3DVG by CVD using molybdenum chloride and sulfur powder as the growth sources.The structure of the 3DVG/MoS₂nanosheets can be controlled by changing the ratio of growth sources,reaction temperature,and holding time.The results show that at 450°C,the thickness of MoS₂ bulks decreases with the increase of holding time.As the temperature increased to 500°C,the MoS₂ nanosheets become bigger and denser.When the temperature further increased to 550°C,the size of MoS₂ nanosheets no longer changes.The nanosheets are vertically grown on the substrate surface and the active edge sites are fully exposed.When fixing the temperature and holding time at 550°C and 10 min,the effect of the different mass ratio of molybdenum chloride to sulfur powder on the hydrogen evolution performance is studied.In a standard three-electrode system in 1 M KOH solution,the sample prepared at 1:3 mass ratio shows the highest performance with an overpotential of 209 mV and tafer slope of67.5 mV/dec.NiCoFe（OH）x nanosheets are grown on the surface of 3DVG/MoS₂ nanosheets by electrodeposition in mixed solution of Ni（NO₃）₂,Co（NO₃）₂,and Fe（NO₃）₃.The effects of Fe（NO₃）₃ concentration and electrodeposition time on the electrochemical performance were studied.With the increase of Fe（NO₃）₃ concentration,the oxygen evolution activity of the 3DVG/MoS₂/NiCoFe（OH）x nanosheets increases firstly and decreases afterwards.The optimum concentration of Fe（NO₃）₃ is 0.05 mol/L.Under this concentration,as the electrodeposition time increases from 300 s to 600 s and then to 900 s,the oxygen evolution activity increases first and then decreases.The sample electrodeposited for 600 s possesses the highest oxygen evolution performance with an overpotential of 90 mV and tafer slope of 44.4 mV/dec.Too short deposition time results in very small loading of the catalysts and insufficient active sites.On the contrary,too long deposition time will cause the agglomeration of nanosheets,cover the active edge sites,and decrease the conductivity.The3DVG/MoS₂/NiCoFe（OH）x nanosheets under this condition are phosphatized by sodium hypophosphite.The obtained 3DVG/MoS₂/NiCoFePx nanosheets possess excellent hydrogen evolution performance.The overpotential is as low as 44 mV and the tafer slope is 37.9 mV/dec.The two catalysts for hydrogen evolution and oxygen evolution are integrated in a two-electrode system for testing the overall water splitting electrocatalysis in a 1 M KOH solution.The polarization curve shows that a high current density of 100 mA/cm² is obtained at a very low potential of1.579 V.Chronopotentiometry curves show the materials have good catalytic stabilities in oxygen evolution reaction,hydrogen evolution reaction and overall water splitting.After working for 100 h under the constant current density of 100mA/cm²,there is only a small increase in voltage,indicating the excellent application potential of the new 3DVG based catalytic materials.