The Application Of Ag/Pt Bimetallic Materials In The Cathode Of Fuel Cells

Fuel cells have a lot of advantages,such as high energy density,high energy conversion efficiency and no pollution,which can be helpful in solving the energy crisis and the environmental pollution problem.However,the high cost and the complicated preparation process of the fuel cells hinder their development.Therefore,how to reduce the cost of the fuel cells is a key issue in the research field of the fuel cells.Now,the catalytic performance of noble metal catalyst like Pt/C used in the cathode of the fuel cells to catalyze oxygen reduction reaction(ORR)cannot meet the need of the practical application.Therefore,a large number of expensive catalysts have to be used in cathode,which raises the price of the fuel cells.In order to solve this problem,scientists have proposed many ways to reduce the usage of Pt in the Pt based catalysts without lowering their catalytic activity.The doping of other metal in pure Pt catalyst has been proved to be effective.By adding other cheaper metals,the number of Pt atoms inside the catalyst that cannot participate in the ORR catalyis will be reduced,which improves the utilization of Pt atoms and thus enhances the catalytic performance of Pt based catalysts.The lattice parameters of Ag metal are similar to those of Pt metal,so they can easily grow together,and the price of Ag metal is much lower than that of Pt metal.Therefore,the combination of Ag and Pt can significantly decrease the cost of Pt based catalysts,and the catalytic performance may be improved.This thesis explores the feasibility of Ag/Pt bimetallic materials as the cathode catalysts for fuel cells,and studies the preparation and electrocatalytic properties of a series of Ag/Pt bimetallic oxygen reduction catalysts.The research contents can be split into the following parts.(1)A one pot method was applied to synthesize the Ag-Pt alloy oxygen reduction catalyst supported on carbon support.The formation of the Ag-Pt alloy was confirmed by XRD,EDS,elemental analysis and XPS characterization.The electrochemical characterization of the prepared catalyst showed that the best catalytic activity of the Ag-Pt/C sample was 2 times higher than that of the commercial Pt/C catalyst,and the stability was also improved in some degree.And the membrane electrode assembly(MEA)made by the Ag-Pt/C exhibited better performance than that of the MEA made by the commercial Pt/C catalyst.The reason for the enhancement is that although the addition of Ag atoms reduces the amount of Pt atoms that cannot reach oxygen in the bulk phase,which raises the utilization of Pt atoms.And the oxygen adsorption energy of Ag-Pt alloy for oxygen reduction reaction is slightly lower than that of pure Pt surface,demonstrated by the theoretical calculation results,which enhances the ORR activity of the Ag-Pt alloy too.(2)The result of the previous chapter turned our attention to the Ag@Pt core-shell structure.We applied a sequential reduction method to prepare Ag@Pt core shell structural nanoparticles,and confirmed the formation of Ag@Pt core shell structure by TEM,XRD,EDS,elemental analysis and XPS.After being loaded on the carbon,the Ag@Pt nanoparticles showed an ORR activity that far less than that of commercial Pt/C catalysts,which was only half of that of Pt/C.And compared with commercial Pt/C catalysts,the Ag@Pt catalyst showed no improvement in stability.The theoretical calculation results showed that the oxygen adsorption energy of Pt shell deposited on the Ag substrate is too high,which reduces the desorption rate of oxygen intermediate during the process of the ORR catalysis process,and hence lowers the overall catalytic rate.The cause for the elevated oxygen adsorption energy is that the lattice parameter of the Ag substrate is larger than that of the Pt surface.Therefore,the surface of Pt is affected by the tensile effect of Ag substrate,which was confirmed by the high-resolution Pt4f spectra of XPS.These results show that the Ag@Pt structure is not suitable for catalyzing oxygen reduction reaction.(3)Learning from the result of the last chapter,we prepared the hollow porous Ag/Pt bimetallic nanoparticles(HP-Ag/Pt)with an average particle size of 17 nm by the galvanic replacement between Ag metal and Pt ions.We used TEM to determine the hollow structure of the sample,and then XRD,EDS,elemental analysis and XPS were used to further prove that the outer shell of the HP-Ag/Pt is pure Pt and the inner shell is Ag-Pt alloy.After being loaded on carbon,the HP-Ag/Pt nanoparticles showed a great mass activity which was 3 times higher than that of commercial Pt/C catalyst,and its catalytic stability was also significantly higher than that of commercial Pt/C catalyst.The Pt4f spectra of XPS confirmed that the Pt surface of HP-Ag/Pt structure no longer suffers the strong tensile effect from the Ag-Pt substrate.Instead,the oxygen adsorption energy of Pt surface of HP-Ag/Pt structure is lower than that of the pure Pt surface,which improves the balance between the adsorption of oxygen and the desorption of oxygenated intermediates during catalyzing ORR.Therefore,the HP-Ag/Pt showed better ORR activity.This inference is proved by DFT calculation.The results in this chapter demonstrate that by controlling the surface structure,the Ag/Pt bimetallic materials can overcome the side effects of Ag on Pt and show excellent ORR catalytic activity and stability.(4)The size of the nanocatalyst prepared in last section is much bigger than the size of commercial Pt/C catalyst,so the specific surface area of the hollow Ag/Pt nanoparticles can be further improved.We introduced the surfactant during the preparation to synthesize the hollow porous Ag/Pt bimetallic nanoparticles(Ag-Pt@Pt)with much smaller size at room temperature(about 6 nm).The electrochemical characterization showed that the Ag-Pt@Pt possesses better ORR activity than HP-Ag/Pt.When being loaded on reduced graphene oxide,the Ag-Pt@Pt showed an excellent ORR activity which was 5.3 times more than that of the commercial Pt/C catalyst,and its catalytic stability is also much higher than the commercial Pt/C catalyst.This is due to the smaller size of the nanoparticles brings higher electrochemical surface area,so there are more catalytic active sites on the surface of Ag-Pt@Pt nanoparticles.Using DFT calculation,we have proved that the surface of Ag-Pt@Pt has a better catalytic activity for ORR.In addition,graphene has higher conductivity and stability compared to ordinary carbon support,so the catalytic activity and stability of Ag-Pt@Pt/rGO sample can be further improved.These results indicate that smaller size brings a higher surface area and improves the catalytic performance of the same Ag/Pt bimetallic surface structure,and the application of suitable carrier can further improve the catalytic performance of the Ag/Pt material.(5)Considering the Ag-Pt@Pt bimetallic materials can be synthesized under mild condition at room temperature,which would not damage the Nafion.So we load Ag/Pt bimetallic nanoparticles onto Nafion using a simple and repeatable method,and the Pt loading can be easily controlled.The binding of Pt to the Nafion membrane is so stable that no obvious falling off of Pt can be observed after being sonicated for half an hour.And the thickness of the grown Pt layer is only a few nanometers,which brings the high utilization of Pt atoms.And the Nafion-Ag/Pt membrane can be easily manufactured into MEA by hot-pressed with the bare carbon paper,which greatly reduces the difficulty of preparing the MEA.The MEA made from the Nafion-Ag/Pt outputs a max power density that about 1.3 times higher than that of the traditional MEA made by gas diffusion layer method,and the Pt loading of the MEA made from the Nafion-Ag/Pt is also lower(9.8%of the usage of traditional GDL MEA).These results demonstrate that the Ag/Pt bimetallic materials can be used to decrease the difficulty of the preparation of MEA and thus reduce the cost of the fuel cells.In conclusion,in this thesis,a series of Ag/Pt bimetallic ORR catalysts were synthesized by controlling the morphology and designing the surface structure,which solve the problems of the application of Ag/Pt bimetallic material in the fuel cell cathode.Moreover,well-designed Ag/Pt bimetallic materials possess better ORR catalytic performance and lower cost compared to the pure Pt,which may replace the commercial Pt/C catalyst in the future.Therefore,the Ag/Pt bimetal materials can be applied to reduce the cost of fuel cells by preparing alternative cheaper catalyst and reducing the difficulty of MEA preparation,which is potential in the applications of the fuel cells.