Synthesis And Electrocatalytic Performance Of Highly Active And Durable Pt-based Electrocatalysts

Proton exchange membrane fuel cell is a kind of the clean power with good development prospects due to high energy conversion rate,low-temperature operation,and environmentally friendly.In this dissertation aiming at the problems existing in the electrochemical activity and durability of fuel cell Pt-based catalysts,the electrochemical performance of Pt-based catalysts are enhanced by using metal oxide as supports.At the same time,shape-controllable and surface-clean Pt-Ni alloy catalysts are synthesized through the assistance of small molecules in the none-surfactant solvents,which improve the electrocatalytic activity of catalysts.The electrocatalytic activity and electrochemical durability of Pt-based catalysts supported on SnO₂ were investigated.Pt/SnO₂-C catalysts were synthesized by depositing Pt nanoaprticles（NPs）with ethylene glycol method on SnO₂-C composite supports which were prepared through high temperature hydrolysis method.The results of transmission electron miscroscopy（TEM）and High resolution TEM（HRTEM）showed that Pt NPs on existed on two phase interface locations of SnO₂ NPs and C,forming Pt-SnO₂-C triple junction structures compared to single Pt/C catalysts.Electrochemical measurement results showed that,compared with traditional Pt/C catalysts,oxygen reduction reaction（ORR）of Pt/C catalysts modified by SnO₂ is increased nearly 1 times and the durability is also enhanced about 1 times.X-ray photoelectron spectroscopy（XPS）confirmed the electron effect of SnO₂ and Pt located at Pt-SnO₂-C triple junction structures facilitated the performance improvement of Pt/C catalysts.Pt NPs were anchored by SnO₂ on the suppports due to the interaction between SnO₂ and C,which improved the durability of Pt catalysts.Moreover,the high resistance to electrochemical oxidation corrosion of SnO₂ effectively prevented the departure of Pt nanoaprticles due to the support corrosion under high voltage.Pt/NbO₂-C catalysts were synthesized by depositing Pt NPs on NbO₂-C mixture supports which were prepared through simple high-speed ball grinding.The mixture support of NbO₂ and C could provide two phase interface locations of metal oxide and C,facilitationg the deposition and dispersion of Pt NPs on these locations and formation of Pt-NbO₂-C triple junction structures.Compared with traditional Pt/C catalystss,methanol oxidation reaction（MOR）onset potential of NbO₂-modified Pt/C catalysts is shifted negatively about 105 m V,and its ability of resistance to poison and electrochemical stability were also improved.XPS confirmed the electron transfer from Nb to Pt,which lead to the increase of Pt electron density.This electron effect weakened the absorbtion of MOR intermediate CO species on Pt surface,and OH funtion group absorbed on NbO₂ accelerated CO oxidation,facilitated the recovery of Pt active sites,which improved the MOR electrocatalytic activity and electrochemical durability of Pt catalysts.A new method to synthesize octahedral/truncated octahedral Pt₃ Ni alloy nanocrystals with（111）preferential crystal planes by use of propionic acid（PA）as regulators in none-surfactant N,N-dimethylformamide（DMF）solvents was proposed further from the perspective of catalyst metal.And it’s found that cubic Pt₃ Ni alloy with（100）preferential crystal planes could also be synthesized by adjusting the precursor concentration ratio.The mechanism of controlling shape and size in this solvent system by adjusting volume ratio of DMF/PA and the type of carboxylic acid was revealed.When the precursor concentration ratio was different,the aldehyde produced by the reaction between DMF and PA under the high temperature and high pressure acting as weak surfactants could be selectively adsorbed on the different crystal plane of Pt-Ni alloy,which facilitated the formation of Pt-Ni alloy nanocrystals with different shape.Electrochemical measurement results showed that ORR kinetic current density at 0.9 V of Pt Ni alloy catalysts containing more octahedron/truncated octahedron is 2.56 m A/cm2,which is 13 times that of commercial Pt/C catalysts.MOR forward peak current density of cubic Pt₃ Ni alloy catalysts is 3.85 m A/cm2,which is 4.6 times that of commercial Pt/C catalysts.A new method to synthesize shape-controlled Pt₃ Ni alloy nanocrystals in new solvents-formylpiperidine（FPD）in combination with the assistance of CO gas through fast pyrolysis was put forward in order to simplify the synthesis system and shorten the preparation period.The result of TEM and XPS showed that shape,size and surface composition of Pt-Ni alloys could be effectively adjusted by changing the duration of exposure to CO gas at the reaction early stage,and the mechanism of controlling shape was revealed.Metal atom complex by the interaction amide solvents,CO with metal precursors was formed during the nucleation growth process.The form and length of this kind of complex could change wi th the duration of exposure to CO,which could adjust the deposition rate of Pt and Ni atoms on different crystallographic plane.When duraion of exposure to CO at the synthesis reaction early stage was 10 minutes,the form and length of metal atom complex facilitated the preferential depostion of Pt and Ni atoms on（100）crystallographic plane.Therefore,more octahedral or truncated octahedral Pt₃ Ni alloy with（111）preferential crystallographic plane was synthesized.Electrochemical measurement results showed that its ORR activity（1.12 m A/cm2@0.9 V）is up to 5 times of commercial Pt/C catalysts.Further,SnO₂ modified Pt₃ Ni alloy catalysts containing octahedron/truncated octahedron were prepared to improve its stability.The results of HRTEM showed that the newly added SnO₂ is adjacent to Pt₃ Ni.Electrochemical measurement results showed that,compared to the same shaped Pt₃Ni/C catalysts,ORR activity of SnO₂-modified Pt₃ Ni alloy catalysts increased 1.4 times（2.67 m A/cm2@0.9 V）,and the stability increased 1.5 times.XPS confirmed the electronic interaction between SnO₂ and Pt₃ Ni changed the Pt electron configuration,which facilitated the improvement of ORR electrocatalytic activity.Moreover,electron effect between SnO₂ and Pt₃ Ni aggravated Pt surface segregation,making surface Pt concentration of Pt₃ Ni alloy increase,which is one of reasons for the enhanced stability.