Study On Surface Modification And Oxygen Reduction Performance Of Platinum-cobalt Catalysts For Fuel Cells

The proton exchange membrane fuel cell（PEMFC）developing with hydrogen energy as a new energy is currently one of the key research fields of researchers.It can generate electricity without burning,producing only water and heat,making them sustainable and low-carbon.The research difficulties of PEMFC mainly focus on how to improve the reaction rate of cathode oxygen reduction reaction（ORR）.Using catalysts is an effective way to rapidly increase the ORR reaction rate.The most widely used carbon-supported platinum（Pt）catalysts are faced with many problems such as scarce platinum resources and poor catalytic performance.Therefore,to commercialize fuel cell,the preparation of low-cost,efficient and durable electrocatalysts is the key.It was found that the catalytic performance of platinum-based catalysts can be effectively improved by generating platinum-based catalysts,adjusting the electronic structure,and using functional modification species.This dissertation is mainly devoted to the surface modification of ordered Pt Co catalysts to appropriately control the electronic structure and elemental composition of the catalyst surface.And by adding different modified species,the catalytic performance can be improved as well as the utilization rate of Pt atoms.Based on the above research strategy,the main research contents of are as follows:（1）In order to improve the shedding of non-precious metals in acidic electrolytes,ionic liquid was used to embellish the catalysts to obtain fct-Pt Co/C@ILs catalysts with ionic liquid layers.A conductive ionic liquid film could be formed on the surface of the catalyst and this ionic liquid layer does not change the crystal form and the degree of dispersion of the catalysts.Electrochemical tests showed that the catalyst with ionic liquid layer does not affect the mass transfer process,the mass activity of catalyst with 5 wt%ILs loading are 1.04 A mg _Pt^-1,the oxygen reduction performance was improved.After 10000 cycles of stability test,it was found that the mass activity of the catalysts modified by ionic liquid only lost 9.6%,and the original morphology and durability of the catalysts were maintained.（2）In order to improve the catalyst particle size growth during high temperature calcination and non-precious metal easily corroded in electrochemical tests,fct-Pt Co/C@NC catalysts with molten salt-assisted dopamine confinement modification were prepared.It was found that under the protection of polydopamine and potassium chloride,the size of metal particle was significantly suppressed during high temperature calcination.XPS test demonstrated that the nitrogen-doped carbon shell can modulate the electronic structure of catalyst surface,it plays an important role in promoting ORR activity.Electrochemical characterization displayed that the mass activity of catalyst is 1.24 A mg _Pt^-1;After ADT test,the mass activity and specific activity loss are 16%and 7.7%respectively,which effectively maintained the structural stability.（3）In order to simplify the preparation process of catalyst and further improve the catalytic performance,fct-Pt Co/C@NC catalyst with rich surface graphitic nitrogen and porous carbon shell was obtained by impregnation-reduction method using dicyandiamide with high nitrogen content and citric acid as carbon source.The physical characterization showed that the high-temperature carbonization of DCD can generate graphitic carbon,which facilitates the ORR;The pore structure of catalyst is predominantly mesopores,which facilitates mass transfer during the reaction.Electrochemical test showed that the best mass activity was 1.60 A mg _Pt^-1 achieved at a mass ratio of dicyandiamide to citric acid of 3:1;After 10000 cycles of the durability test,the E_1/2 of fct-Pt Co/C@NC was only declined of 5 m V,and the ECSA loss was10%,confirming that this scheme can significantly improve the stability of the catalysts.