Controlled Synthesis Of Refractory Metal Doped Pt Alloy And Its Origin Of High Electrocatalytic Activity

Oxygen reduction reaction(ORR)is of great significance for the development of new energy technologies such as fuel cells.However,the mechanism of ORR is complex and its kinetic is slow.So the catalysts are required to overcome this energy barrier.Currently,most of the catalysts available for ORR have very high precious metal Pt loading,such as commercial Pt/C catalyst.However,the precious metal Pt is a scarce resource and expensive,which is one of the reasons for the high production cost of the fuel cell,so that leads to the delay in the commercial application of the fuel cells.In order to reduce the amount of Pt and develop a high-performance catalyst so that promote large-scale application of fuel cells,the subject developed Pt-based binary and termary alloy catalysts are used as a substitute for commercial Pt/C.The Pt-based alloy catalyst can not only greatly reduce the amount of precious metal Pt,but also further improve the performance of the catalysts for ORR.In this paper,a series of different Pt-based alloys were prepared,and their morphology,structure and ORR catalytic properties were explored.Finally,the reasons for the enhanced performance of the catalysts for ORR were analyzed by XPS,XAS and DFT.This topic will help to further clarify the complex kinetics of the oxygen reduction reaction and have a certain guiding role in the preparation of a new generation of low-cost and high-performance electrocatalysts for ORR.The results obtained are as follows:(1)Pt-Pr/C and Pt-Bi/C binary alloy catalysts were prepared by improved polyol thermal reduction method,and the optimum pH value and optimum alloy composition ratio were determined.the optimum pH value for Pt-Pr/C system is 7,and the optimum alloy composition ratio for Pt-Bi/C system is Pt:Pr=3:1;The optimum pH value for Pt-Bi/C system is 10,the optimum alloy composition ratio for Pt-Bi/C system is Pt:Bi=3:1.The obtained binary alloy nanoparticles are uniformly dispersed on the carrier graphene with a particle diameter of 2-7 nm.The activity of the obtained Pt-Pr/C binary alloy catalyst far exceeds that of commercial Pt/C.In 0.1M HClO4 solution,the specific activity of Pt-Pr binary alloy is 1.11 mA·cm-2,which is 5 times higher than that of the commercial Pt/C(0.21 mA·cm-2),the mass activity is 0.91 A·mgPt-1,about 479%higher than that of the commercial Pt/C(0.19 A·mgPt-1).XPS,XAS,DFT demonstrate that the activity enhancement can be explained from the two aspects of electronic effect and space effect.The low electronegativity of the lanthanide Pr reconstructs the electronic arrangement around the Pt atoms and weakens the binding energy between Pt and ORR intermediates(OHad);and the larger atomic radius of Pr changes the Pt atomic arrangement,further reducing the binding energy between Pt and OHad.In addition,DFT shows that the binding energy between Pr and OHad is stronger than that of Pt and OHad.These factors make the Pt atom have more electrochemical active sites,which is beneficial to the ORR.This study proposes to introduce the early transition metal Pr into the Pt lattice as a high-performance electrocatalyst,and also provides a general search for other metals that can improve the electrocatalytic performance drastically.(2)There is still room for further enhanced activity of the binary alloy catalyst for ORR.Pt-Pr-W/C and Pt-Co-W/C termary alloy nanoparticles were prepared by the same polyol thermal reduction method using the optimum pH value and the best alloy composition ratio discussed in the preparation of the binary alloy.The obtained nanoparticles are uniformly dispersed on the graphene and have a particle diameter of about 2-4 nm.The activity of the Pt-Pr-W/C ternary alloy is comparable to that of commercial Pt/C.However,in 0.1 M HClO4 solution,the obtained Pt-Co-W ternary alloy exhibited a surprisingly high specific activity of 3.41 mA·cm-2,which is 4.3 times higher than that of the well-known Pt-Co binary alloy(0.80 mA·cm-2)and 13 times more than that obtained by the commercial Pt/C(0.27 mA·cm-2).The mass activity of the alloy was 2.25 A·mgPt-1,which is 4.2 times higher than that of the Pt-Co binary alloy(0.53 A·mgPt-1)and 12 times higher than that obtained by the commercial Pt/C(0.19 A·mgPt-1).Tungsten(W),as a refractory 5d transition high-valency metal with unique physical and chemical properties,such as strong electronegativity,a highly unsaturated 5d orbital,and a high valence state coordinated with an oxygen-containing group(for example,O,OH,OOH).XPS,photoluminescence and other means show that the synergistic effect of W and Co makes the OH adsorption energy △EOH of the catalyst closer to the peak of Sabatier volcano 0.1 eV.The addition of W makes the adsorption of Pt-Co with the reactant O2 become stronger,and the desorption of the intermediate product OHad is also enhanced,so the ORR activity is greatly improved.This study shows that the addition of refractory transition metals to the Pt-Co binary system greatly improves the electrocatalytic performance of the catalyst.Therefore,the introduction of a similar refractory transition metal like W in the Pt-Co system can provide a manufacturing idea for the next generation of high-activity catalysts.(3)Finally,we verify the conclusion that the refractory transition metal is added to the Pt-Co binary alloy to greatly improve the ORR activity.Molybdenum(Mo),as the refractory transition metal,is introduced into the Pt-Co system,meanwhile,Bi,not the refractory transition metal,is also doped into the Pt-Co system.Pt-Co-Mo/C and Pt-Co-Bi/C ternary alloy nanoparticles were prepared by the same preparation conditions and preparation methods of Pt-Co-W/C ternary alloy.Obtained alloy nanoparticles were uniformly dispersed on graphene,and the particle size was about 2-5 nm.Electrochemical performance results show that the addition of Mo does greatly improve the electrocatalytic performance of Pt-Co binary alloy for ORR.The mass activity of Pt-Co-Mo/C was 1.23 A·mgPt-1,which is 4.2 times higher than that of the Pt-Co binary alloy(0.53 A·mgPt-1)and 12 times higher than that obtained by the commercial Pt/C(0.19 A·mgPt-1).The specific activity of Pt-Co-Mo/C is 1.04 mA·cm-2,which is 4.3 times higher than that of the well-known Pt-Co binary alloy(0.80 mA·cm-2)and 13 times more than that obtained by the commercial Pt/C(0.27 mA·cm-2).Although there is not higher than that of Pt-Co-W and the activity of Pt-Co-Bi is much lower than that of commercial Pt/C.The order of activity is Pt-Co-W>Pt-Co-Mo>Pt-Co>Pt/C>Pt-Co-Bi.The study further proves that the addition of refractory metals W and Mo to the Pt-Co binary alloy system greatly increases the activity of the catalysts for ORR,which is the electronegativity of such transition metals,the electron density of d-band and the binding energy between these transition metals and OHad play synergistic effect on the electrocatalytic activity of the Pt-based alloy catalyst.It may be considered to add refractory transition metals with similar properties as W to the Pt-Co binary system,such as Ta and Hf,and the activity may be greatly improved,which provides another solution of preparing low-cost and high-performance electrocatalysts for ORR.