Structural Regulation And Electrochemical Properties Of Low Platinum Carbon-based Electrocatalytic Materials

In recent years,energy shortage and environmental pollution are two major problems facing the development of human society.Therefore,it is urgent to explore and develop low-cost and efficient energy conversion technology.Among them,oxygen reduction（ORR）is an important electrode reaction in electrochemical energy conversion devices.The ORR electrocatalyst is the key material to realize this reaction process.High-performance ORR electrocatalyst has high catalytic activity,which can effectively reduce the reaction overpotential,accelerate the reaction kinetics process and improve the reaction efficiency.At the same time,it has better stability,so it can improve the durability of ORR electrode.Therefore,improving the activity and stability of catalyst and reducing the cost of catalyst are the main tasks to achieve the wide application of electrochemical energy conversion devices.At present,platinum（Pt）-based precious metal catalysts are available for commercial use,but their high price and non-renewable nature limit their large-scale commercial use.Therefore,the development of low Pt,high efficiency and stable ORR electrocatalysts is of great significance to advance the research frontier of novel high efficiency electrochemical energy conversion devices and realize the wide industrial application of catalysts.The results show that alloying strategy can accurately design and regulate the phase structure of catalyst to change the internal structure of catalyst,and the active component sites and content of catalyst can improve the performance and stability of catalyst.Therefore,based on the above research background,this paper aims to design and prepare low platinum and high efficiency catalysts.The main research contents and results are as follows:（1）Alloying strategy can reduce the amount of precious metal Pt,and metal element Iron（Fe）can be used to change the electronic structure of Pt and improve the catalytic performance.Carbon nanosized onion（CNOs）supported Pt-Fe alloy particle electrocatalyst（Pt-Fe@CNOs）was prepared by discharging graphite rods in metal salt solution.The Fe Pt and Pt₃Fe intermetallic compounds with different phase structures were obtained by adjusting the annealing temperature.It is noteworthy that no organic molecules were used as surfactants or solvents in the whole process.The Fe Pt intermetallic compound on carbon nano-onion（Fe Pt@CNOs-600）was prepared at 600℃with excellent ORR and hydrogen evolution（HER）catalytic activity.Fe Pt was a face-centroid tetracyclic（fct）L1₀ ordered phase structure with an average particle size of 3.29±0.63 nm.And the distribution in carbon nano onion matrix is uniform.For the ORR under alkaline conditions,the half-wave potential is 0.88 V,the initial potential is 0.99 V,the Tafel is 52 m V dec^-1,and it can maintain good stability,which is better than the performance of commercial Pt/C.For HER under acidic conditions,Fe Pt@CNOs-600requires only 27 m V crossing potential at 10 m A cm^-2,which is also superior to commercial Pt/C.The study of structure-activity relationship shows that the phase structure of the catalyst can be precisely regulated by high temperature annealing,which can transform the alloy state into intermetallic compound,and the interaction between the nanoparticles and the carbon support can be strengthened,thus improving the catalytic efficiency and stability of the catalyst together.（2）In order to further reduce the use of precious metal Pt,palladium（Pd）was used instead of Pt,and carbon nano-onion supported Pd-Fe alloy particle electrocatalyst（Pd-Fe@CNOs）was synthesized by discharging graphite rod in metal salt solution.The Fe Pd intermetallic compound electrocatalyst（Fe Pd@CNOs-600）was prepared by adjusting the annealing temperature and annealing temperature at 600℃.In alkaline electrolyte,its ORR activity is improved,the half-wave potential is 0.89 V,the initial potential is 1.01 V,the Tafel slope is 59m V dec^-1,and it can maintain good stability.The structure-activity relationship study showed that there was a suitable interatomic distance between Fe-Pd in the Fe Pd alloy structure,which was conducive to the reduction and adsorption of molecular oxygen.The introduction of Fe weakened the adsorption of Pd to oxidized species,thus improving the ORR performance.（3）To further reduce the use of precious metals,a novel metal-free nitrogen（N）-doped graphene nanospheres（NGNs）ORR electrocatalyst was designed.A defect control strategy was adopted to take graphene nanospheres（GNs）with more defects as the matrix.Due to the large number of edge defects exposed in GNs,the N doping content could reach 14.01at.%,and the N loading could be controlled by changing the annealing temperature.The results show that the sample NGNs-900 obtained at 900℃has a high specific surface area and abundant pore structure.The half-wave potential and limiting current density can reach 0.872 V and 4.25 m A cm^-2,respectively.The durability of Pt/C catalysts can be maintained for 10 h,which is better than commercial Pt/C catalysts and metal-free carbon catalysts reported in the past.Based on the experimental study and density functional theory（DFT）calculation,it is found that the synergistic effect of graphite-N and pyridine-N in NGNs catalyst is the essential reason for improving ORR performance.In conclusion,in this study,using the idea of alloying phase regulation and defect regulation,the small size,uniform distribution and excellent comprehensive performance of carbon nano onion supported alloy particle electrocatalyst Fe Pt@CNOs-600,Fe Pd@CNOs-600 and metal-free NGNs-900 catalyst were successfully prepared.The active site and mechanism of the catalyst were also discussed.These new catalysts are economical,environmentally friendly and efficient,and have great potential to replace precious metal-based catalysts.The study also demonstrated the important role of alloying,phase regulation and defect engineering regulation in the performance optimization of catalysts,and provided a new idea and effective experimental reference for the design and application of new and efficient catalysts with low platinum.