Nitrogen Doped Carbon Encapsulated Platinum Based Electrocatalysts And Their ORR Performance

Proton exchange membrane fuel cells（PEMFCs）are the mainstream clean energy utilization devices.The Pt-based catalyst at cathode,as a significant part of PEMFCs,is the most widely used ORR catalysts,but due to the expensive price and poor long-term durability,numerous researches and development have been carried out to enhance their electrocatalytic properties by using less platinum（Pt）.The optimization and improvement of the stability of Pt-based catalysts at cathode are mainly studied in this thesis.A N-doped carbon encapsulated Pt nanocrystalline structure catalyst are designed to be fixed firmly on the carbon support.The main method is coating the noble metal nanoparticles with a thin nitrogen-doped carbon layer,which can inhibit the migration,dissolution or detachment of Pt or Pt alloy particles.Thus,the catalyst shows better stability.The main research contents and results are listed as follows:First,the N-doped carbon encapsulated Pt nanocrystalline structure catalyst is designed and synthesized for oxygen reduction reaction.PANF is used as the source of nitrogen-doped carbon thin layer.The uniform noble metal nanoparticles wrapped with fibers are prepared by ethylene glycol reduction method and mixed with PANF in the colloid.Then the particles are loaded on XC-72.Finally,heat treatment is carried out to obtain Pt@NC/C catalyst.The study find that the introduction of nitrogen-doped carbon structure can enhance the interaction between Pt particles and carbon support,and immobilize Pt particles on the surface of carbon support,thereby improving the stability of the catalyst.Electrochemical test results show that the electrochemical surface area of Pt@NC/C catalyst is 98.3 m²/g_Pt,which is 1.17 times of Pt/C(84.2m²/g_Pt),indicating that the active site of Pt surface is better utilized.At the same time,the mass activity of the Pt@NC/C catalyst calculated is 116.5 mA/mg,which is higher than that of the commercial Pt/C（109.6 mA/mg）.For the electrochemical stability,after8000 cycles,the loss of ECSA and MA of the as-prepared catalyst are only 27.4%and35.2%,much lower than Pt/C（49.3%and 63.5%）.Furthermore,this method was applied to a N-doped carbon encapsulated Pt₃Co catalyst to synthesize a Pt₃Co@NC/C electrocatalyst.The performance tests show that the ECSA and MA of Pt₃Co@NC/C are 101.4 m²/g_Pt and 121.7 mA/mg,respectively,which are 1.2 times and 1.1 times better than that of Pt/C(85.6 m²/g_Pt and 116.3mA/mg).As for the electrochemical stability of the catalyst,after 8000 cycles of accelerated experiments,the ECSA and MA of Pt3Co@NC/C decrease by 25.2%and26.5%,respectively,which is less than that of Pt/C.Moreover,the average diameters of Pt3Co in Pt3Co@NC/C increase from 2.8 nm to 5.7 nm.For Pt/C,the average diameters increase from 2.7 nm to 7.0 nm,which also shows the inhibition effect of the thin carbon layer on Pt₃Co nanoparticle growth.