Construction And Properties Of Palladium-based And Platinum-based Oxygen Reduction Catalysts

With the demand for the development of alternative renewable and sustainable energy sources and the continuous and urgent need and interest in basic application development,feasible fuel cell configurations have been established for different types of platforms.Among them,proton exchange membrane fuel cell（PEMFC）is one of the most promising clean energy technologies that have been developed.Its efficiency is as high as about 60%.Compared with traditional fossil fuels,it has higher energy conversion efficiency and the ability to use clean fuels without emitting pollutants.Despite these advantages,its cost issues limit the popularity of PEMFC technology to the market.In particular,the most widely used platinum（Pt）catalyst in PEMFC requires about 30 grams,which is worth thousands of dollars,and accounts for a significant portion of the total cost of PEMFC.Therefore,it is urgent to develop new catalysts to reduce production costs.Based on the research background of catalysts,two noble metal-based catalysts have been designed and synthesized for oxygen reduction reaction（ORR）performance testing.One is a composite material（Pd₃P@NPC）prepared by SiO₂ sacrificial template method and high temperature thermal decomposition method,in which the palladium phosphide nanoparticles are fixed on the three-dimensional bubble-like porous heteroatom-doped carbon film.ICP-AES,TEM and other characterization methods show that the loading of Pd in Pd₃P@NPC catalyst is low（5.20 wt%）.Moreover,the average size of the anchored palladium phosphide nanoparticles in Pd₃P@NPC is about4.9 nm,which has nano-size effect and can expose more active sites to increase catalytic activity.In Pd₃P@NPC,the conductivity of palladium phosphide nanoparticles and carbon substrate are very good,which can speed up the transmission of electrons.According to XPS detection,it is found that there is a strong electronic interaction between the nanoparticles and the substrate,which can not only adjust the electronic structure of atoms and optimize ORR activity,but also make the nanoparticles load on the substrate without falling off and aggregation,ensuring good stability.The ORR performance test results of Pd₃P@NPC catalyst in a standard three-electrode system containing 0.1 M KOH alkaline medium display that the half-wave potential of Pd₃P@NPC is 0.885 V,36 m V higher than Pt/C,and the mass activity is 8.18 times higher than Pt/C.After undergoing 40 000 potential cycles,the mass activity of Pd₃P@NPC lost only 14.48%compared with that before the stability test.Its excellent ORR activity is due to the synergistic catalysis of the two parts of the catalyst.The other noble-metal catalyst is a composite material（Pt@V₈C₇）with a lamellar nanomesh structure supporting metal nanoparticles synthesized by hydrothermal method and high-temperature pyrolysis method.The physical characterization techniques determin that the loading of Pt in Pt@V₈C₇ is 6.82 wt%,the size of Pt nanoparticles is about 1-3 nm,and its nano-size effect could expose more active sites for enhancing catalytic activity.In the Pt@V₈C₇ catalyst,the good conductivity of Pt nanoparticles and the V₈C₇ substrate can accelerate the electron transport during the reaction process.The XPS test reflects the strong electronic interaction between the particles and the substrate,adjusting the electronic structures of the Pt atoms,thereby substantially improving the ORR activity.The interaction between the two parts also ensures that Pt nanoparticles can be firmly anchored to V₈C₇ during the catalysis process without falling or accumulating.In addition,the ORR catalytic performance of the Pt@V₈C₇ catalyst is tested on a rotating disk electrode using a standard three-electrode system.The results show that the catalytic performance of Pt@V₈C₇catalyst is superior to that of commercial Pt/C and has reached a new level of activity.Specifically,in 0.1 M HCl O₄ acidic medium,the half-wave potential of Pt@V₈C₇appears at 0.903 V,which is 51 m V higher than the half-wave potential of Pt/C,and its mass activity is maintained at 0.513 m Aμg_Pt^-1.After 20 000 potential scanning cycles,its mass activity is still very high and is 9.79 times higher than that of Pt/C after the corresponding test.Its excellent ORR catalytic activity is achieved by the synergistic catalysis between Pt nanoparticles and V₈C₇ substrate in the Pt@V₈C₇ catalyst.Through the research content of this paper,the purpose of ensuring that the catalytic performance of ORR is not reduced or even improved on the premise of reducing the amount of precious metals has been realized.