| The rapid industrialization process has brought global challenges of energy crisis and environmental pollution,which may be solved by clean and renewable energy.In recent decades,electrocatalytic reaction has attracted extensive attention because of its important role in environmental protection and energy storage and utilization,including fuel cell,metal air cell,water electrolysis and so on.Among various nano catalysts,compared with non noble metal catalysts,noble metal based catalysts show excellent catalytic activity and stability in most electrocatalytic reactions.Among them,two key semi reactions are involved in the electrolysis of fuel cell,metal-air battery cell and water electrolysis,namely oxygen reduction reaction(ORR)and hydrogen evolution reaction(HER).So far,precious metal Pt is still the best catalyst for these two semi reactions,however,Pt has the disadvantages of scarce reserves,high price,and easy to reunite.Therefore,it is particularly important to reduce the amount of Pt,reduce the size of Pt particles and improve the utilization efficiency of Pt atoms.Graphene is a single-layer two-dimensional carbon nano sheet,which has excellent properties such as high specific surface area,excellent conductivity,good thermal stability and chemical stability.Graphene and graphene derivatives have been used as substrates for the synthesis of various noble metal nanocomposites,showing excellent properties in the application of electrocatalytic energy conversion.Based on the above,in this paper,nitrogen doped hollow graphene spheres were synthesized by using polystyrene spheres(PS~+)as template,then introducing melamine as the nitrogen source in the process,nitrogen-doped hollow graphene spheres were synthesized as the substrate material,and their morphology was similar to that of honeycombs.N-doping and spherical hollow structure allow graphene to better exert its unique properties and provide abundant rivet sites for the loading of precious metals in the later stage,which helps to uniform Ly disperse Pt particles and reduce particle size.The research content of this paper mainly includes the following two parts:1.Application of Pt-supported nitrogen-doped honeycomb graphene spheres as efficient bifunctional catalysts for ORR and HER in acidic and alkaline electrolytes:According to the aforementioned,we successfully synthesized NHGSs(N-doped honeycomb Hollow graphene balls)as the base material,in order to make graphene better exert its unique properties such as high specific surface area and high conductivity,the conditions of graphene substrates have been explored.The results show that the structure and electrochemical performance of the sample NHGSs-2g PS~+are the best(use 2g PS~+has the best effect).Afterwards,chloroplatinic acid was chemically reduced by sodium borohydride as a reducing agent to obtain NHGSs/Pt loaded with Pt nanoparticles.The results show that the doped N atoms have a riveting effect on the Pt nanoparticles,so the N-doped graphene substrate can firmly attach the Pt and control the nucleation size of the particles,thereby improving the catalytic activity and durability of the material.In addition,the introduction of N atoms will also play a positive role in the performance improvement of HER and ORR.Using the obtained electrode material NHGSs/Pt(with gradient change of Pt loading)as ORR and HER catalysts,the electrochemical performance of NHGSs/Pt material in acidic and alkaline electrolytes was studied by cyclic voltammetry,rotating disk electrode method,etc.Since the introduction of N atoms provides conditions for the staking of Pt and the dispersion of nanoparticles,thereby improving the utilization rate of noble metals,in addition,N atoms can also improve the electrochemical performance,so the obtained catalyst NHGSs/Pt-2 has a practical loading capacity of NHGSs/Pt-2.In the case of only 10%Pt,the catalytic performance of ORR and HER under acidic and basic conditions exceeds to the commercial catalysts with 20wt%Pt/C.2.On the basis of work 1,we continued to explore the effect of heteroatom doping on the oxygen reduction and hydrogen evolution performance of hollow graphene sphere-supported Pt catalysts by using the optimal conditions of Pt loading:hollow graphite with high specific surface area was still selected.Four different substrate materials were synthesized by introducing heteroatoms(N,Fe,and Co)into olefin spheres(HGSs)as substrates,and they were loaded with Pt particles using an equal amount of Pt precursor.The obtained results were investigated and discussed in detail.Relationship between ORR and HER performance of electrode catalysts and doped heteroatoms,such as nitrogen(N),iron(Fe),and cobalt(Co)atoms.The results show that the introduction of Fe and Co can form very efficient heteroatom active sites Fe/Co-Nx-C in the substrate material,which can reduce the amount of Pt.In addition,as the riveting site of Pt loading,N atoms can achieve more uniform loading of noble metal Pt particles and smaller particle size regulation,which can effectively improve the catalytic activity of electrode materials and reduce the amount of Pt used.In addition,the doped N atoms are also closely related to the formation of Fe/Co-Nx-C active sites.The onset potential,limiting current density,half-wave potential,and Tafel slope of the sample FeCo-Nx HGSs/Pt with a Pt loading of about 10%in both acidic and alkaline electrolytes can all exceed or be comparable to the commercial catalyst Pt/C(20 wt%).In addition,the(η100)and Tafel slope of the electrode material FeCo-Nx HGSs/Pt are superior to the commercial catalyst Pt/C(20 wt%)in both acidic and alkaline electrolytes.Thereby,the purpose of reducing the amount of precious metals without reducing the catalytic performance is achieved. |
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