Synthesis And Electrocatalytic Performance Study Of Transition Metal Compound/nitrogen-doped Carbon Nanosheets Composites

Hydrogen is the most promising alternative to fossil energy to resolve the energy crisis and reduce air pollution.At present,the catalyst system represented by Pt has excellent hydrogen evolution activity,its high cost and low earth abundance limit the industrialization process of this kind of catalyst.Therefore,the construction of low-cost and high-efficiency catalytic systems is currently a hot research direction.Transition metal carbide?TMC?and transition metal phosphide?TMP?have similar electrocatalytic properties to precious metals,but their electrical conductivity and specific surface area are not satisfactory.The construction of carbon composite system is an effective way to improve the catalytic activity of materials and reduce costs.Therefore,in this study,a low-cost and nitrogen-rich natural polymer guar gum was used as a carbon source to prepare transition metal compound composite nitrogen-doped carbon nanosheets with high activity and excellent stability by adjusting the microstructure of carbon matrix,controlling the active component phase and catalytic activity.The specific work content is as follows:?1?Using natural polymer guar gum as the carbon source and ammonium molybdate tetrahydrate as the molybdenum source,freeze-drying to form a sheet-shaped precursor,and ultrafine molybdenum carbide/Nitrogen doped carbon composite system?nMo_xC/NC?was prepared by high-temperature solid-phase reaction under an argon atmosphere.The strong coordination of hydroxyl groups and molybdate in guar gum limits the aggregation of Mo_x C particles,and its N-doped carbon matrix can provide a fast electron transmission channel for Mo_x C.In addition,by adjusting the mass ratio?n?of ammonium molybdate tetrahydrate to guar gum,molybdenum carbide in different phases can be obtained.When n is equal to 0.5,the MoC phase is mainly obtained.and when n is equal to 1.5,it tends to generate a highly active Mo₂C phase.When n is equal to 1,the product Mo_xC/NC has the best catalytic activity.In an alkaline medium,the overpotential of Mo_xC/NC to reach a current density of 10 mA/cm² is 163mV,and the slope of Tafel is 64.8 mV/dec,the activity of the catalyst did not decrease significantly after 1000 CV tests.?2?To further solve the problem of low specific surface area of Mo_xC/NC,using NaCl as a hard template,Mo₂C/nitrogen-doped porous carbon nanosheets?xMo₂C/NCS-y?were synthesized.The particle size of Mo₂C and the structure of the carbon matrix can be adjusted by adjusting the concentration of ammonium molybdate tetrahydrate and the amount of sodium chloride added.Not only does NaCl facilitate the formation of porous carbon nanosheets structures,but also its molten salt promotes the formation of high crystallinity?-Mo₂C.In alkaline media,Mo₂C/NCS-16 exhibits excellent HER performance,reaching an overpotential of 142 mV at a current density of 10 mA/cm²,a Tafel slope of 56.1 mV/dec,and the catalyst has excellent HER durability.The excellent hydrogen evolution performance of Mo₂C/NCS-16 is attributed to the synergistic effect of high catalytic activity?-Mo₂C and large specific surface area porous nitrogen-doped carbon nanosheets.?3?In order to solve the problem of single catalytic function of carbon based catalyst derived from guar gum,on the basis of the system of guar gum/transition metal salt/sodium chloride,a bifunctional electrocatalyst of cobalt phosphide composite nitrogen doped porous carbon nanosheets?CoP/NCS-x?was synthesized by high temperature solid state reaction and low temperature phosphating method.The introduction of NaCl not only promoted the porosity of guar gum derived carbon nanosheets,but also promoted the formation of cobalt nanosheets.The formation of cobalt nanosheets is conducive to the full exposure of the active sites,as well as to the improvement of phosphating uniformity.CoP/NCS-16 has both high HER and OER catalytic activity in alkaline media.The HER and OER overpotentials at the current density of 10 mA/cm² were 132 mV and 320 mV,and the Tafel slopes were 57.0 mV/dec and 73.9 mV/dec,respectively.Meanwhile,CoP/NCS-16 exhibited excellent durability.