The Study On Surface Dealloying Of Pt-Co Alloy And Its Electrocatalytic Performance For Oxygen Reduction Reaction

Proton exchange membrane fuel cell（PEMFC）is an energy conversion device that directly converts the chemical energy in the fuel into electrical energy.It is a new type of high-efficiency and clean power generation technology.However,its cathodic oxygen reduction（ORR）reaction kinetics is slow,and it relies heavily on the scarce and high-cost precious metal Pt.Increasing the amount of Pt-based catalysts will increase the cost of fuel cells,which will seriously hinder the further development of fuel cells.At present,researchers have devoted a lot of efforts to the research of fuel cell cathode oxygen reduction reaction catalysts,which are mainly divided into precious metals and non-precious metals.However,the ORR activity of non-precious metals is not as good as precious metals as a whole,and the fuel cells are strongly acidic and high potential conditions will also make non-precious metals unstable,so there is still a considerable gap between practical applications.Pt-based alloys are currently the catalysts with the highest catalytic activity.Doping Pt with non-noble metals（Fe,Co,Ni,etc.）can help reduce the cost of the catalyst and further improve the catalytic activity,which has very important scientific research and practical significance.In this thesis,the Pt-Co alloy was studied to obtain a catalyst with ORR activity better than commercial Pt/C.The Pt₃Co/C catalyst with ORR activity better than commercial Pt/C was prepared by the impregnation-gas-phase reduction method and electrochemical dealloying process,and the optimal reduction conditions were explored.By changing the support,a Pt₃Co/C catalyst with excellent performance in fuel cells is obtained.Through chemical dealloying of the catalyst,the best chemical dealloying conditions are explored,and the ORR activity is similar to that after electrochemical dealloying.The main research content and results of the thesis are as follows:（1）H₂Pt Cl₆·6H₂O and Co Cl₂·6H₂O were used as the metal precursors,Ketjen black EC-300J was used as the carbon-based material,an alloy catalyst（30 wt.%Pt₃Co/C）uniformly supported on the carbon material was prepared by the method of impregnation-gas-phase reduction,and the ORR catalytic activity of Pt₃Co alloy was enhanced by electrochemical dealloying.The effects of different reduction temperatures on the catalytic performance of the ORR were studied.The reduction conditions were H₂ reduction at 200°C for 2 h and argon at 600°C for 7 h,the ORR activity was the best.The half-wave potential of the catalyst was 0.921 V（vs.RHE）,the mass activity was 0.30 A/mg,and the specific activity was 0.53 m A/cm².Both the mass activity and the specific activity were 2.5 times that of 40%JM Pt/C.Through TEM,it could be seen that the Pt₃Co alloy nanoparticles are intact and uniformly dispersed on the carbon support,with an average particle size of about 2.89 nm.（2）In order to obtain a Pt₃Co alloy catalyst that performs better in fuel cells,the original metal precursor and the impregnation reduction method are still used,Vulcan XC-72R carbon black was added and the carbon support ratio was adjusted to XC72:EC300=1:1.At the same time,the reduction conditions were changed to H₂ reduction at 300°C for 2 h,and Ar reduction at 600°C for 7 h,and finally a mixed support Pt₃Co alloy catalyst was obtained.By the electrochemical dealloying process,the half-wave potential of the catalyst reached 0.915 V（vs.RHE）,the mass activity was 0.23 A/mg,and the area activity was 0.34 m A/cm²,which was significantly improved compared to40%JM Pt/C.Through TEM,it could be seen that the Pt₃Co alloy nanoparticles are intact and uniformly dispersed on the carbon support,with an average particle size of about 1.96 nm.Exploring its performance in fuel cells,it was found that the power density is much higher than 40%JM Pt/C at low and high current density.（3）The chemical dealloying method was used to compare the effects of different chemical alloying methods on the catalyst ORR activity.HNO₃,CH₃COOH,H₃PO₄three different acids and buffer solution of different p H were used as the chemical dealloying solvents to chemically dealloying the synthesized Pt₃Co/C.We found that the ORR catalytic performance after chemical dealloying with H₃PO₄ and Na H₂PO₄/Na₂HPO₄ buffer solution with p H=6 was close to that of electrochemical dealloying,and both were higher than 40%JM Pt/C.At the same time,the Pt₃Co/C after dealloying with a buffer solution of p H=6 has better stability than 40%JM Pt/C.Through TEM,it could be seen that the alloy nanoparticles are intact and uniformly dispersed on the carbon support.The average particle size were about 1.90 nm and 1.79nm,respectively.At the same time,the temperature,concentration,time,atmosphere and other conditions of chemical dealloying were changed.For other different chemical alloying conditions,the corresponding optimal conditions were also found.