In-situ Construction And Surface Control Of Nitrogen-doped Carbon Composite Supports And Its Pt-loading Catalytic Behavior

With the rapid development of human society,our demand for energy is increasing and the environmental pollution is becoming more and more serious.The energy crisis and environmental pollution have gained considerable attention.Therefore,the development of environmentally friendly,renewable and efficient energy has become an urgent problem to be solved in modern society.Direct methanol fuel cell has been widely used for its advantages of high energy density,high conversion efficiency and environmental friendliness.However,the anode of direct methanol fuel cell is still faced with huge challenges such as high cost of Pt catalyst,slow reaction kinetics,easy poisoning by strongly intermediate species and poor stability.Therefore,the development of new anode catalysts with high catalytic activity,high stability and strong resistance to CO poisoning by means of surface regulation and in situ construction is one of the research hotspots.Due to the advantages of wide sources,large specific surface area,good electrical conductivity,carbon materials are considered to one of the most promising catalyst supports.However,due to the low intrinsic state activity of carbon materials,it is necessary to use the strategy of surface control,such as heteroatom doping(B,N,P,S,etc.),rich defect sites and surface function,to improve its physicochemical and electrochemical properties.On account of in-situ carbon composites are more uniform and can shorten the electron transport path and accelerate the electron transport,for the above reasons,the following work is carried out in this paper:Firstly,we report that different distribution of in-situ carbon coated hollow CoS2nanocages can be used ZIF-67 as the precursor,synthesized only by adjusting sulfuration time,followed by calcination,and Pt/CoS2-C@TCL,Pt/CoS2@TCL and Pt/CoS2catalysts were prepared by electrodeposition of Pt.Through physical characterization,it can be proved that the inside and outside of CoS2-C@TCL are in situ coated with a thin layer of carbon,and the CoS2-C@TCL have hollow and porous structure,which are benefit to the entry of electrons and electrolyte solution into the material and reduces the transfer resistance of ions and electrons.The electrochemical test showed that the mass activities of Pt/CoS2-C@TCL catalyst in acidic and alkaline medium are 1238 mA mg^-1Pt and 2045 mA mg^-1 Pt,which are 4.3 and 2.6 times superior than the commercial Pt/C,and much higher than that of Pt/CoS2@TCL and Pt/CoS2 catalysts.The accelerated aging test of cyclic voltammetry showed that the ECSA retention rate of Pt/CoS2-C@TCL catalyst was significantly higher than that of other catalysts both in acidic and alkaline medium.The above results showed that the interconnected between carbon layer coated CoS2 particles and thin carbon layer outside of the CoS2 nanocages not only improve the conductivity of the support,reduce the contact resistance and diffusion resistance,but also provide a bridge for the transport of electrons.In addition,the internal CoS2 as a support for the nanocages and the external carbon layer,protecting the structural integrity of the supports.Therefore,the stability of the catalyst is improved.Secondly,in-situ bamboo-like Co@N-CNTs,Ni@N-CNTs and FeC@N-CNTs supports derived from high temperature calcination dicyandiamine and salt of acetylacetonate(Co,Ni,Fe),then defects riched Pt/Co@N-CNTs,Pt/Ni@N-CNTs and Pt/FeC@N-CNTs catalysts were prepared by microwave assisted glycol method.The electrochemical tests showed that the mass activity and specific activity of Pt/Co@N-CNTs catalyst are 1652 mA mg^-1Pt and 4.82 mA cm^-2Pt under alkaline conditions,which are 2.19 times and 1.41 times higher than that of commercial Pt/C catalyst,and much higher than that of Pt/Ni@N-CNTs and Pt/FeC@N-CNTs catalysts.Due to the dominated pyridinic-N,enriched defect,abundant wrinkle of carbon layer,as-fabricated Co@N-CNTs provide optimal surface of backbone for Pt deposition with narrowest particle size distribution and minimum size,which can improve the utilization rate of Pt.Furthermore,N doped endows the catalyst with excellent electron transport capacity.In addition,the strong interaction between Pt and the support enhanced the intrinsic activity of each active site,thus enhanced the catalytic activity of methanol oxidation of Pt/Co@N-CNTs.Be beneficial for the narrow particle size distribution of Pt nanoparticles on Pt/Co@N-CNTs,the close distance between Pt and Pt nanoparticles is conducive to the adsorption-OH on Pt oxidize CO intermediate on adjacent Pt.Therefore,Pt/Co@N-CNTs also have better anti-co toxicity and stability.Thirdly,we reported the in-situ formation of long defect-rich nitrogen-doped carbon nanotubes(NCNTs)coupled with porous TiO2/NC composite(TiO2@NC-NCNTs)derived from high temperature calcination the different proportions spindle-type bimetallic(Ti/Co)MOF microrods,and melamine as carbon and nitrogen sources,then Pt/TiO2@NC-NCNTs catalyst were prepared by microwave assisted glycol method.For MOR,the optimized Pt/TiO2@NC-NCNTs-7 catalyst exhibits superior mass activity of577 mA mg_Pt^-1(in acidic medium)and 3100 mA mg_Pt^-1(in alkaline medium),which is2.02 and 4.08 times superior than the commercial Pt/C catalyst,and Pt/TiO2@NC-NCNTs-7 shows excellent CO tolerance,better cyclic stability and fastest reaction kinetics.Pt/TiO2@NC-NCNTs-7 catalyst also exhibits a more positive onset potential(1.01 V)and upper limiting current density(5.03 mA cm^-2)than commercial Pt/C(0.96V and 4.75 mA cm^-2)toward oxygen reduction reaction.The excellent electrochemical performance are attributed to proper introduction of Co in MOFs produces both more defects and high graphitization in N-doped carbon,which not only endows the nanocatalyst excellent charge/electron transport ability but also provides a large amount of active sites to Pt NP deposition,further improving stability and catalytic activity in the electrochemical process.High corrosion resistance and rich hydroxyl groups enable TiO2to avoid poisoning by CO intermediates and retain the structural integrity of the whole catalyst under harsh electrochemical conditions.In summary,the catalyst of Pt/TiO2@NC-NCNTs-7 has moderate of defect sites,the highest content of pyridinic-N and Pt⁰,so it exhibits superior catalytic activity toward to MOR and ORR.