Preparation Of Nitrogen Doped Porous Carbon Composites With Transition Metal Loading And The Electrocatalytic Mechanism For Hydrogen Evolution

Because of zero emission,high energy density and combustion value,hydrogen has attracted more and more attention.Hydrogen production through water splitting with widely resources,high purity and environmental friendliness has become a development potential technology.The development of highly active and stable catalysts is the essential to the hydrogen production technology.However,at present,the commercial catalysts for water splitting are noble metals,such as,Pt,Ir and Rh,which presented the shortcomings of low activity,stability and atomic efficiency.Porous carbons can be used as a environmental recyclable catalyst support with high surface area,high porosity,easy modification and good electrical conductivity.The catalyst support application of heteroatom doping and modification porous carbon in the water splitting field can provide excellent catalytic active and stability.In this work,porous carbon is prepared through a novel method using bio-mass and organic polymers.The catalytic activity and stability are improved through the construction of transition metal heterocrystalline structure and porous defective carbon.To investigate the effect of materials morphology,heterocrystalline structure and defective structure on the catalytic activity through XRD,XPS,Raman,SEM,TEM,EPR,bet measurement and electrochemical analyst.The obtained research results are as follows:（1）To solve the problems of low catalytic activity,poor stability and complicated preparation process of porous carbon matrix,we adopted a one-pot hydrothermal method with melamine,pectin,nickel nitrate and cobalt nitrate as precursors to prepare nickel-cobalt alloys with N-doped porous carbon microspheres coating(CN_Ni,Co).The Ni/Co alloys with N-doped porous carbon microsphere coating exhibited a low overpotential of 240 mV at 10 mA/cm²and a low Tafel slope of 55 mV/dec.To improve the active sites and stability of CN_Ni,Co,we prepared N-doped porous carbon microsphere with Ni₂P/CoP loading(CN_Ni2P/CoP)through a two-step hydrothermal method:ⅰ)melamine and pectin were used to obtain N-doped carbon microsphere;ⅱ)N-doped carbon microsphere,Ni（NO₃）₂and Co（NO₃）₂were precursors to prepare CN_Ni2P/CoP.CN_Ni2P/CoPhad a low overpotential of 160 mV at 10 mA/cm²and the Tafel slope was 57 mV/dec,which were attributed to the plentiful of active sites from carbon mocrosphere and the presence of heterocrystalline structure between different metal compounds.（2）Using melamine,pectin,nickel nitrate,ammonium dihydrogen phosphate and ammonium tetratholybdate as precursors to prepare N-doped porous carbon with Ni₂P and MoS₂loading through a handy hydrothermal method.The obtained catalysts possessed a low overpotential of 170 mV at a current density of 10 mA/cm²,and a low Tafel slope of75 mV/dec.In addition,the catalysts exhibited a high exchange current density of 0.76mA/cm²and an excellent TOF value,verifying a good hydrogen evolution reaction kinetics.The outstanding catalytic activity could be attributed to the abundant of active sits from N-doped carbon microsphere and the unique heterocrystalline structure between Ni₂P and MoS₂.DFT calculation result also demonstrated that the heterocrystalline structure could induce the charge redistribution to reduce the H adsorption free-energy(ΔG_H*)and facilitate the catalytic reaction.（3）To investigate the effect of heterocrystalline structure and improve the catalytic activity,we adopted phytic acid,chitosan,nickel nitrate and ammonium molybdate as precursors to obtain 3D N-doped porous carbon with MoP/Ni₂P loading.The onset potential was 5 mV,the overpotential at 10 mA/cm²was 78 mV and the Tafel slope was 64mV/dec,which were attributed to the unique heterocrystalline structure and porous microstructure.The experimental and DFT calculation results confirmed the effect of heterocrystalline structure on the catalytic activity.（4）In order to study the influence of the defect degree of carbon matrix,we prepared defective porous carbon matrix supported molybdenum compounds（Mo₂C,MoP and MoS₂）using nitrogen-containing organic monomers tribenzoic acid,2-aminobenzoic acid,polyvinylpyrrolidone and ammonium chloride as precursors.The defective N-doped porous carbon with Mo₂C,MoP and MoS₂loading exhibited a low overpotential at 10mA/cm²（87 mV,67 mV and 102 mV,respectively）and a low Tafel slope（109 mV/dec,45mV/dec and 52 mV/dec,respectively）.The plentiful of defect and the synergistic effect between N dopant,adjacent C atoms and molybdenum compounds contributed to the excellent catalytic activity.DFT calcalation verified the the synergistic effect:the interaction between Moand the adjacent C induced the electron cloud from Moatom unfilled d-orbit expansion,which facilitated the adsorption of H*and reduced the H adsorption free-energy(ΔG_H*).