Preparation And Electrocatalytic Activity Study Of Hollow Carbon Sphere-based Nanocomposites

Energy resources are the material basis for the survival and development of human society.New energy conversion devices such as zinc-air batteries and proton exchange membrane fuel cells have received extensive attention from scientific researchers because they are highly efficient and environmentally friendly,do not undergo a combustion process,and are not restricted by the Carnot cycle.Oxygen reduction reaction（ORR）is the common and core reaction of these devices.Due to its slow kinetics,suitable electrochemical catalysts are needed to lower the energy barrier required for the reaction.Hollow carbon spheres are an ideal carrier material due to their large specific surface area,good electrical conductivity and easy modification.In this paper,based on nitrogen-doped carbon hollow spheres,three different types of hollow carbon sphere-based composite catalysts were designed and synthesized according to the needs of different catalytic environments,and electroactivity of ORR and the battery performance of the assembled zinc-air battery were studied respectively.The main work of this paper is as follows:（1）Using self-made silica spheres（Si O₂）nanospheres with a diameter of 220 nm as the hard template,a layer of dopamine（DA）was polymerized on the surface.Using the electrostatic attraction between polydopamine（PDA）and graphene oxide（GO）,a layer of graphene oxide was wrapped on the outer layer of polydopamine.Subsequently,carbonization and etching processes were carried out to remove the silicon dioxide spheres to obtain the nitrogen-doped carbon hollow spheres modified by metal-free reduced graphene oxide（r G@NHCS）.By testing the ORR activity in alkaline and neutral electrolyte solutions,the optimal GO modification amount（5 r G@NHCS）was determined.Studies have shown that the modification of reduced graphene oxide improves the degree of graphitization of the carbon shell layer of the hollow carbon spheres,and the ultra-thin carbon wall（～7 nm）is conducive to mass transfer of ORR,which makes the ORR activity of 5 r G@NHCS comparable to Pt/C in alkaline/neutral media.Alkaline/neutral zinc-air batteries were assembled using the prepared samples as the catalysts of air electrodes,which have a stable and higher voltage platform than Pt/C at different current densities of 10,20,50 and 100 m A·cm^-2.（2）Using self-made SiO₂ nanospheres with a diameter of 220 nm as the hard template,a certain amount of cobalt acetate（Co（CH₃COOH）₂.4H₂O）and nickel acetate（Ni（CH₃COOH）₂·4H₂O）were added during the process of polymerizing DA on its surface.After carbonization,it was mixed with dicyandiamide,sucrose,cobalt acetate,and nickel acetate and then grounded well to form uniform mixture.After carbonization at high temperature,the Si O₂ hard template,inert metal ions and groups were removed by etching.By adjusting the mass ratios of raw materials,a series of composite materials with different proportions of hollow carbon spheres and tubular carbon coexisting were obtained.The ORR activity of the prepared catalysts was tested in alkaline medium,and the results showed that the sample with the most tubular carbon content has the most positive onset potential（0.91 V vs RHE）,while the sample with the most spheres has the greatest limiting diffusion current(5.23 m A·cm^-2).Furthermore,Co Ni/NHCS-TUC-3 provides the lowest potential difference(ΔE=E _j=10-E_1/2)of 0.806V.The ORR and OER electrochemical activities were combined to determine the optimal ratio of hollow carbon spheres to tubular carbon（sphere:tube=1:3）.Namely,the sample（Co Ni/NHCS-TUC-3）is the suitable catalyst of the air electrode for a rechargeable alkaline zinc-air battery.After cycling for 70 h at10 m A·cm^-2,the voltage efficiency of the battery with Co Ni/NHCS-TUC-3 remains as high as 55.6%.（3）The above CoNi/NHCS-TUC-3 sample was immersed in aqueous platinum tetrachloride（Pt Cl₄）solution to reduce Pt ions into Pt nanoparticles.After filtering,the obtained solid was then pyrolyzed to prepare Pt-Co-Ni co-doped hollow carbon sphere/tubular carbon composite catalyst with Pt content of less than 4 wt%（Pt-Co-Ni/NHCS-TUC-600）.By constructing a reasonable spatial structure of the catalyst and introducing transition metal co-doping,the Pt mass activity of Pt-Co-Ni/NHCS-TUC-600 for ORR was greatly improved.ORR electrochemical activity tests showed that the Pt-Co-Ni/NHCS-TUC-600 reveals excellent catalytic performance comparable to commercial Pt/C,which is specifically expressed as the ORR onset potential of 0.93 V,half-wave potential of 0.84 V,limiting diffusion current density of 6.97 m A·cm^-2,and extremely high cycle stability.This would be ascribed to the synergistic effect between reasonable structure and transition metal co-doping.