Reaserch On Preparation And Performance Of High Performance Catalysts For Hydrogen Fuel Cells

Achieving carbon neutrality and building a clean energy society are the direction and goals of social development at this stage.The search for cleaner and more efficient energy supply devices has become a top priority.Proton exchange membrane fuel cell（PEMFC）is a very important type of portable power source with its advantages of high stability and high output power.It is widely used in commercial cars,heavy trucks,forklifts and portable lighting equipment.However,most of the PEMFC catalysts currently used are carbon-supported platinum or carbon-supported platinum-based alloy catalysts.Due to the high price of the catalyst and the attenuation of activity of catalytic oxidation reduction reaction（ORR）during use,it is necessary to find a low-platinum ORR catalyst with higher activity and stronger stability.The synthesis of small-size,super-dispersed,high-active surface polyhedral structure,and low-platinum catalyst is the current key issue for the realization and high performance of platinum-based catalysts.Based on this,we launched the preparation and performance research of high-performance hydrogen fuel cell catalysts.The main progress are as follows:Firstly,a series of Pt_xCo_y/C catalysts were synthesized by the co-precipitation reduction method,and the microstructure characterization,composition analysis and electrochemical performance test were carried out to conduct in-depth research and strengthen the understanding of the bimetal effect.Among them,the ORR mass activity（MA）of Pt₁Co₁/C catalyst is 1.3 times higher than that of commercial carbon-supported platinum（JM20Pt/C）,and the half-wave potential is positively shifted by 12m V relative to JM20Pt/C.Accelerated aging test（ADT）in 2000 cycles showed slightly better stability of Pt₁Co₁/C.The reason for the increase in catalytic activity is:the presence of Co changes the electronic state of Pt on the surface,and weakens the adsorption of oxygen-containing intermediates,which accelerates the rate of water generation,thereby improving the catalytic performance of ORR.Secondly,on the basis of the above-mentioned co-precipitation reduction method,combined with the subsequent post-treatment process,small-diameter polyhedral platinum-cobalt alloy nanoparticles（H-Pt Co/C）were synthesized.It is observed by STEM that the H-Pt Co/C alloy nanoparticles present a regular polyhedral structure,with an average diameter of 3.47 nm,and are surrounded by crystal planes of the{111}and{100}families.The synthesis of small particle size greatly increases the utilization rate of Pt,and the Pt{111}crystal plane has higher ORR catalytic activity among all low-index crystal planes.These structural advantages are demonstrated in electrochemical tests.The ORR catalytic activity of H-Pt Co/C is 6 times that of JM20Pt/C,and it also shows excellent stability in the aging test of 10,000 cycles.The mechanism for the improvement of the ORR catalytic activity of H-Pt Co/C is that the synthesis of small particles increases the actual utilization of Pt.In addition,the formation of polyhedral structure is also very helpful to the improvement of activity.Pt（111）of Pt-based alloy has suitable oxygen-containing intermediate adsorption energy,which greatly improves the reaction rate of ORR.The reason for the improved stability is that polyhedral structure is surrounded by low free energy crystal planes.The low energy surface makes the alloy particles more stable during the catalytic ORR process and avoids the dissolution of high Gibbs free energy atoms on the surface.Which ensuring the stability of H-Pt Co/C.Finally,using the impregnation reduction method to explore the synthesis of phosphorus-doped Pt/C series catalysts（Px-Pt/C,x=0.5,1.0,1.5,2.0,3.0）.By using XPS technology to analyze the Pt electronic state of the catalyst,it is found that the phosphating can make the Pt4f orbital shift,but as the doping amount of P continues to increase,the forward shift will become very small.We analyze that the doping of P in the Pt lattice is not unlimited.When a threshold（～41%）is reached,the doping cannot be continued.The results of testing the ORR catalytic activity of the series of catalysts by electrochemical testing technology found that P doping can indeed improve the ORR catalytic activity of Pt,and the mass activity of P_1.5-Pt/C is 1.6 times that of JM20Pt/C.In terms of stability,the average particle size of P_1.5-Pt/C particles increased from3.02nm to 3.16nm after 10,000 cycles of accelerated aging test,ECSA increased by2.92%,MA decreased by 43.87%.While the average particle size of Pt particles of JM20Pt/C is coarsened from 3.17 to 3.85,ECSA is reduced by 33.37%,and MA is lost by 53.33%.P_1.5-Pt/C exhibits better stability than JM20Pt/C.The improvement mechanism of P doping on activity and stability is that the presence of phosphorus can promote timely desorption of oxygen-containing intermediates from the surface of Pt,avoid the poisoning of surface catalysts,and thus ensure the continuous ORR catalytic activity of the catalyst.This may be due to the geometric and electronic changes of Pt caused by the introduction of non-metallic phosphorus.In addition,Pt and P form an alloy-like structure,which weakens the repulsive force between Pt atoms,reduces the dissolution of Pt on the surface,and avoids Oswald ripening during accelerated aging,which also contributes to stability.