Synthesis And Performance Of PtNi₃-Based Alloy Catalyst Toward The Oxygen Reduction Reaction

The depletion of traditional energy and the improvement of environmental awareness force people to seek for a cheap,clean and sustainable new energy.Fuel cells are considered as one of the most promising new energy technologies for high energy density,high energy conversion efficiency and environmental friendliness.However,the oxygen reduction reaction(ORR)is kinetically sluggish and highly dependent on the noble Pt metal with small reserve,high price and easy deactivation by carbon monoxide,which seriously hinders the large-scale commercial applications of fuel cells.Pt-Ni alloy system,especially the PtNi3 alloy with up to 75%Ni content and excellent ORR activity(calculation result of density functional theory),have great potential to become one of the most promising cathode catalyst materials in the future.This thesis mainly carried out the following works:(1)Preparation of PtNi3/C alloy catalyst with uniform distribution of nanoparticles via microwave-assisted ethylene glycol reduction method.Electrochemical tests showed that the half-wave potential(E1/2)of PtNi3/C was up to 0.924V(vs.RHE),which was 28 mV higher than that of commercial Pt/C catalyst(JM-Pt/C).The mass activity(MA)and specific activity(SA)of PtNi3/C reached 0.62 A·mg-1pt and 1.48 mA·cm-2,which were 2.3 times and 4.5 times of JM-Pt/C,respectively.According to the analysis results,compared with Pt/C,the improved ORR activity of PtNi3/C is mainly attributed to the merits of the electronic effects and geometric effects.When the accelerated durability test(ADT)of PtNi3/C alloy catalyst was carried out in acidic solution,it was found that the E1/2 of PtNi3/C shifted negatively by 66 mV,the electrochemical surface area(ECSA)decreased by 29%,and the MA and SA significantly reduced by 87.1%and 81.1%,respectively.The PtNi3/C alloy catalyst showed very poor durability.(2)Based on the study of PtNi3/C,we further modified it by introducing an N-doped graphite carbon(NGC)layer to prepare the PtNi3/C@NGC catalyst by means of polydopamine coating and carbonization in an inert atmosphere.Electrochemical tests showed that the E1/2 of PtNi3/C@NGC was up to 0.940V(vs.RHE),which was 16 mV higher than that of PtNi3/C.The MA and SA reached 1.16 A·mg-1 pt and 2.90 mA·cm-2,which were 1.9 times and 2.0 times of PtNi3/C,respectively.When the ADT was employed in acidic solution,it was found that the PtNi3/C@NGC catalyst showed superior durability with negligible attenuation of E1/2,ECSA,MA and SA after ADT.We attribute the improved ORR activity to the merits of the electronic effects and geometric effects,and the highly enhanced durability to the merits of the Pt-rich surface layer that could prevent Ni dissolution in acidic solution to maintain the structural stability of the catalyst.This thesis have made progress in improving the activity and durability of ORR catalysts based on the PtNi3 system,which may provide some guidance and reference for solving similar problems for other Pt-M(transition metal)alloy catalysts.