Fine Structure And Electrocatalytic Properties Of Pt/CN_X Nanocatalysts Doped With Ti Compounds

If the cost of hydrogen fuel cell vehicles reduces as quickly as lithium battery electric vehicles and accepted by the society,the consumption of Pt must be reduced to the level of precious metals used in gasoline vehicles exhaust purifiers,which is less than 0.1 g/kW.However,represented by Toyota Mirai hydrogen fuel cell vehicle,it has made great progress but still unresolved,which has become one of the most difficult scientific and technological problems in modern times.The scientific reason is that due to the complex influencing factors,the relationship between the composition,structure and electrocatalytic activity of Pt catalyst and support materials is still unclear,especially due to the limitation of previous analysis methods,the fine structure of Pt catalyst and support materials is not deeply understood.In this paper,we adopted synchrotron radiation technique to characterize the fine structure and took Pt/CNx nanocatalyst of Pt particles on CNx nanowires as the research object.Based on the analysis of the correlation between its fine structure and hydrogen oxidation reaction(HOR)electrocatalytic activity,the scientific issues such as the size of ultrafine Pt nanoparticles,the fine structure of TiO2-doped composite Pt/TiO2-CNx nanocatalysts with different crystalline phases and content,the fine structre of TiN-doped composite Pt/TiN-CNx nanocatalyst and HOR electrocatalytic activity and stability were systematically studied,and the interaction mechanism between various components was discussed.Polypyrrole was synthesized by chemical method and then sintered at 800℃to prepare CNx nanowires support with a diameter of about 40～60 nm,which was interwove into three-dimentional mesh-like structure.The ultrafine Pt nanoparticles of 2.03±0.26 nm were homogeneously loaded on CNx nanowires by ethylene glycol in-situ reduction method,and Pt/CNx nanocatalyst with high HOR electrocatalytic activity was successfully obtained.The analysis of the relationship between composition,structure and properties shows that polypyrrole is transformed into CNx nanowires with numerous pyridinic-N and graphitic-N structures under high temperature sintering.The electrons on these N-containing functional groups form C=C π*,C-N π*and other orbitals,which not only increases the carrier concentration of the support and improves its conductivity,also makes the N atoms have stronger electron donating ability and provide electrons to 5d-band vacancies of Pt atoms.As a result,Pt atoms on Pt/CNx have less coordination number and greater disorder of Pt-Pt bond than Pt/C,and the content of electrocatalytic active component metallic Pt0 is higher,that is,there is a stronger metal-support chemical interaction between Pt nanoparticles and CNx nanowires,resulting in higher electrocatalytic activity and stability.The HOR exchange current density of Pt/CNx is 0.758 士 0.055 mA·cm-2Pt,which is 2.4 times higher than that of traditional Pt/C,and its stability is improved by 5%.However,the improvement range is small.In order to further improve the stability of Pt catalyst,the optimization of support material doped with titanium compounds was investigated.TiO2 nanoparticles with different anatase and rutile(A/R)contents were prepared by heat treatment at different temperature in the range of 700～900 0C using anatase(A)～rutile(R)phase transformation,and Pt/TiO2-CNx nanocatalysts doped with different TiO2(A/R)content were obtained.The results show that Pt/TiO2-CNx exhibits a "positive" nanometer size effect even if the Pt metal is ultrafine nanoparticles at 1.8～2.8 nm,that is,the HOR electrocatalytic activity increases with the decrease of Pt nanoparticles size.The test results also reveal that the rutile content in TiO2-CNx supports has a "volcano-type" effect on the specific surface area of the supports and the electrochemical surface area of the catalysts.The specific surface area of TiO2(25%R)-CNx and the electrochemical surface area of Pt/TiO2(25%R)-CNx are the largest.Meanwhile,the relationship between HOR current density of Pt/TiO2-CNx and rutile content also presents a "volcano-type".Pt/TiO2(25%R)-CNx catalyst has the highest HOR electrocatalytic activity with the exchange current density of 0.962±0.055 mA·cm-2 pt,which is 3 times and 1.3 times as much as Pt/C and Pt/CNx,respectively.Moreover,Pt/TiO2(25%R)-CNx catalyst also exhibits the highest stablity,which is 35%higher than Pt/C and 30%higher than Pt/CNx,showing substantial improvement.The mechanism of TiO2 doping improving the electrocatalytic activity and stability of Pt catalyst was analysed.X-ray absorption fine structure(XAFS)analysis shows that Ti-O coordination decreases first and then increases with rising the rutile content in TiO2-CNx support,and the coordination in TiO2(25%R)-CNx is the smallest,indicating that Ti forms a certain degree of bonding with C=C π*and C-N π*orbital electrons in CNx nanowires,which consolidates the stability of TiO2(25%R)-CNx support.At this point,the coordination of Pt-Pt is the smallest and bond length is the shortest,that is,the size of Pt nanoparticles decreases.Transmission electron microscopy analysis shows that the diameter of Pt nanoparticels in Pt/TiO2(25%R)-CNx is the smallest with the average size of 1.83±0.26 nm.X-ray photoelectron spectroscopy analysis shows that the content of Pt0 is increased by 11%and 3%compared with Pt/C and Pt/CNx,respectively.The mechanism of "volcano-type" effect on the electrocatalytic activity and stability of Pt/TiO2-CNx nanocatalysts doped with TiO2 is comprehensively deduced as follows:in Pt/TiO2(25%R)-CNx,Ti has the strongest interaction with C=C π*,C-Nπ*orbital electrons in CNx nanowires,giving rise to the smallest size of Pt nanoparticles and the largest content of Pt0,which improves the electrocatalytic activity and stability,resulting in "volcano-type" effect.The HOR electrocatalytic activity and fine structure of Pt/TiN-CNx nanocatalysts with 10%,20%and 30%mass percentages of TiN and pyrrole monomer were investigated.The HOR exchange current density of Pt/TiN(20%)CNx is 0.94±0.14 mA·cm-2 Pt,which is 3 times of Pt/C and 1.2 times of Pt/CNx,respectively,and its stability has a 15%and 10%improvement over Pt/C and Pt/CNx,respectively.XAFS analysis shows that after 6-coordination TiN doping,Ti-N changes from 4.9 coordination with Pt free to 4.4 coordination with Pt nanoparticles deposition,which is seriously distorted.Simultaneously,the binding energy of Ti LⅡ,Ⅲ-edge and N K-edge in CNx nanowires are positively shifted,and the content of Pt0 is 17%and 9%higher than that of Pt/C and Pt/CNx,respectively,indicating that electrocatalytic activity and stability are improved by the coaction of Ti,Pt and N due to TiN doping.