Durability Study Of Pt-Cu Electrocatalysts For Oxygen Reduction Reaction

The development of renewable energy to replace traditional fossil fuel towards carbon neutrality is one of the most important research fields for the development of energy materials.The use of hydrogen energy is characterized by zero carbon emissions,and therefore attracts much attention.Proton exchange membrane fuel cell（PEMFC）,which uses hydrogen as a fuel to convert chemical energy into electrical energy,has received a lot of attention and development in recent years.One of the cores of PEMFC is the electrochemical reaction between the cathode and the anode,where the kinetic hysteresis of the oxygen reduction reaction（ORR）at the cathode limits the efficiency of the PEMFC.Therefore,the development of ORR catalysts is one of the current research priorities,whereas the research of Pt-based catalysts has received much attention due to its excellent intrinsic catalytic activity.Pt-M catalysts are formed by alloying Pt with transition group metals（M）,which can both enhance catalytic activity and reduce the amount of Pt metal used.Among different Pt-M electrocatalysts,Pt-Cu has gained importance in recent years due to its high catalytic activity and low cost.However,for practical applications of PEMFC,Pt-based catalysts need to have both high activity and high cycling stability.In the case of PEMFC-powered vehicles,for example,the internal operating conditions include low potential under PEMFC loading and high potential under switching conditions.Although durability studies have been conducted on commercial Pt nanocatalysts,the durability of Pt-M at different potentials is still lacking and the degradation mechanism is still unclear.This thesis therefore investigates the durability of Pt-Cu catalysts at high and low potentials by means of structural characterization,microscopic characterization,electrochemical testing and accelerated durability testing.Based on the synthesis of highly catalytically active Pt-Cu catalysts with special hexagonal structures,the degradation mechanism of the loaded Pt-Cu catalysts was further investigated by correlating the changes in ORR catalytic performance with structural and microstructural changes at different potentials and cycle numbers.The research results include:（1）Polyhedral Pt-Cu nanocrystals,concave Pt-Cu nanocrystals and hexagonal Pt-Cu nanocrystals were prepared by the solvothermal method.The hexagonal Pt-Cu nanocrystals showed the best performance,with the carbon loaded PtCu_2.5/C catalysts showing ORR mass activity and specific activity of 0.56 A·mg _Pt^-1 and 2.36 m A·cm^-2_Pt,respectively,which were 6 and 17 times higher than those of commercial Pt/C.（2）Cycling stability at low potential and microstructural characterization of PtCu_2.5/C catalysts.After 15,000 cycles at low potentials of 0.6-1.0 V_RHE,the hexagonal PtCu_2.5/C catalyst showed a loss of 53.10%on kinetic current,a 20%increase in overpotential,a mass activity loss of 53.03%,a specific activity loss of 41.43%and an electrochemically active surface area loss of 19.80%,indicating a lack of durability of the catalyst.Elemental analysis showed that the elemental loss of Cu gradually increased with the number of cycles,decreasing from 71.35%to 59.51%after 15,000cycles.TEM study and 3D reconstruction showed that there were more pores on the surface and inside the Pt-Cu particles,with the number of pores increasing with the number of cycles.The porous structure leads to an increase on surface area of curved surfaces and concave surfaces with negative curvature,and also an increase on surface defects such as curved edges and steps,resulting in an increase in the number of low-coordinated atoms on the catalyst surface.Combined with the performance and microstructural characterization,the degradation mechanism of PtCu_2.5/C catalysts under low potential cycling is proposed:the loss of Cu during cycling leads to a weakening of the ligand effect and strain effect;the increase on the number of low coordination atoms on the surface leads to an upward shift of the d-band center,which enhances the adsorption of oxygen-containing intermediates on the surface sites of the particles,decreasing the ORR rate.（3）Cycling stability at high potential and microstructural characterization of PtCu_2.5/C catalysts.2000 cycling at 1.1-1.6 V_RHE high-potential conditions resulted in a kinetic current loss of 75.83%,an increase in overpotential of 0.07%,a mass activity loss of 75.92%,a specific activity loss of 54.58%and an electrochemically active surface area loss of 46.99%,with a significant lack of durability of the catalyst.The TEM and 3D reconstruction showed that the Pt-Cu particles had developed a large number of connected holes after 500 cycles and the number of low coordination atoms on the catalyst surface had increased significantly.After 2000 cycles,not only did the number of cavities increase,but also oxidation products such as carbonyl groups appeared on the surface of the carbon carrier,with new characteristic peaks in the 0.55-0.70 V_RHE potential range.The degradation mechanism of the PtCu_2.5/C catalyst under high-potential cycling is suggested by the combination of performance and microstructural characterization:the loss of Cu during cycling leads to a weakening of the ligand and strain effects;the increase in the number of low coordination atoms on the surface leads to an upward shift of the d-band center,which enhances the adsorption of oxygen-containing intermediates on the surface sites of the particles and reduces the ORR rate;and the oxidation of the carbon carrier leads to a decrease in its electrical conductivity.The oxidation of the carbon carrier leads to a decrease in its electrical conductivity,and the oxidation products encapsulate the Pt-Cu particles,thus hindering the adsorption of Pt onto O and further degrading its catalytic performance.The present study is important for the durability study of Pt-Cu/C catalysts and explains the different degradation mechanisms of hexagonal-shaped Pt-Cu/C catalysts at high/low potentials,which is an important reference for the exploration of degradation mechanisms of related Pt-M/C catalysts.