Study On Pt-based Transition Metal Alloy Catalyst And Its Performance In Catalyzing Oxygen Reduction Reaction Of Hydrogen Fuel Cell

Currently,Pt/C catalysts are still commonly used in hydrogen fuel cells,and the catalytic performance of the catalysts should be improved further to reduce the cost of fuel cells.It has been proved that Pt-based transition metal alloy catalysts show excellent catalytic performance in the oxygen reduction reaction.But most of the transition metal ions can occur Fenton reaction to the hydrogen peroxide generated during the oxygen reduction reaction,generating potent oxidizing radicals.These free radicals can cause the decomposition of the Nafion membrane and reduce the stability of the membrane electrode.Mn ion can only cause the relatively weak Fenton reaction and it has an excellent inhibitory effect on the Fenton reaction caused by transition metals such as Fe and Co.Meanwhile,the addition of manganese in the catalyst can improve the catalytic performance effectively.Still,it isn’t easy to prepare Pt Mn-based alloy catalysts because of the significant difference in redox potentials between Mn and Pt.Therefore,it is of great significance to explore the effective synthesis method of Pt Mn-based alloy catalysts and evaluate the practical performance of Pt Mn-based alloy catalysts in PEMFC membrane electrodes.In this thesis,the Pt Mn-based alloy catalysts and its catalytic performance in oxygen reduction of hydrogen fuel cell were studied.The adequate preparation of 40wt%Pt Mn/C alloy catalysts and 50wt%Pt Co Mn/C alloy catalysts with high loading and high dispersion has been achieved,and the series characterization and analysis of catalysts and the evaluation of hydrogen fuel cell membrane electrode performance have been completed as follows:（1）Pt Mn/C catalysts with an average particle size of 3.54 nm and excellent catalytic performance were prepared by a double heat treatment strategy.The precursor was firstly decomposed in an inert atmosphere and then reduced in a reducing atmosphere.The Pt Mn/C catalysts were characterized by HRTEM,Mapping,and EDS,which indicated that the catalysts had high dispersibility and high alloying structure.The effects of preparation conditions such as the type of carbon carrier,and the ratio of Pt and Mn on the microstructure and performance of Pt Mn/C catalysts were investigated.The experimental results showed that the performance of Pt Mn/C catalysts was more obviously influenced by the type of carbon carrier and the amount of solvent.In contrast,it was little affected by the medium acid treatment.The highest mass activity of Pt Mn/C catalyst was250.5 A/g _Pt(118.2 A/g _Ptfor Pt/C catalyst)when the Pt,Mn ratio was 1:1.The peak power density of Pt Mn/C catalyst reached 1343 m W/cm²（1074m W/cm²for Pt/C catalyst）under the same test conditions.After 10000turns of stability test,the peak power density of Pt Mn/C catalyst decreased by 16.3%.（2）The effect of heat treatment conditions on the structure and properties of Pt Mn/C catalysts prepared by the aniline method was investigated further by varying temperatures of the two stages of heat treatments.A series of Pt Mn/C catalysts were prepared by changing only the first heat treatment temperature from 700°C to 1100°C.The results showed that the best catalytic activity(mass activity of 250.5 A/g _Ptand peak power density of 1343 m W/cm²)was achieved at the heat treatment temperature of 900°C with the increase of the first heat treatment temperature.Combined with the electrochemical tests and characterization analysis results,the effect of different pyrolysis temperatures in an inert atmosphere on the catalyst structure and performance was elucidated,and various degrees of pyrolysis resulted from different pyrolysis temperatures.In the experiments,the effect of second heat treatment temperature with reducing atmosphere on the micro-structure and performance of Pt Mn/C catalyst was investigated,the catalyst activity gradually decreased(mass activity decreased from 250.5 A/g _Ptto 20.8 A/g _Pt)with the second heat treatment temperature（700～1000℃）increased.Combined with XRD,TEM,and other characterization analyses,it can be seen that the alloying degree of the catalyst was significantly enhanced in the second heat treatment condition.Still,the nano-particle agglomeration was intensified,which caused the decrease of catalytic activity.（3）Pt Co/C catalysts have excellent catalytic properties,and the dissolution of cobalt can trigger the Fenton reaction in the reaction process.The addition of manganese can inhibit the Fenton reaction and the catalyst’s performance can be improved.In this paper,Pt Co Mn/C catalysts with highly alloyed and dispersed were prepared by using sodium borohydride method and acid leaching.The highest mass activity of 121.7 A/g _Ptwas achieved when the cobalt precursor addition was 4.3times the theoretical cobalt content（4.1wt%）in the synthesis process.Then the effect of heat treatment temperature on the catalyst performance was investigated further.The best catalytic activity was achieved when the heat treatment temperature was 800°C,with the mass activity increased to 142.6 A/g _Ptand the peak power density reached 1292m W/cm².Meanwhile,the effect of Pt/C catalyst on Pt Co Mn catalyst was investigated,and a highly homogeneous Pt/C catalyst with an average particle size of 1.83 nm（Pt/C catalyst with a particle size of 3.17 nm）was prepared by using the formaldehyde method.It was shown that the high homogenous Pt promoted the concentration of the particle size distribution of the ternary alloy nanoparticles to the lower nanoparticle size,and the mass activity was effectively enhanced to 161.0 A/g _Pt.