Electrocatalytic Performance Of Pt Catalyst Loaded On Nitrogen-doped Carbon Nanotubes

In the fields of water electrolysis and direct alcohol fuel cells,currently,noble metal Pt is the best electrocatalyst towards hydrogen evolution reaction(HER),oxygen reduction reaction(ORR),methanol oxidation reaction(MOR)and ethanol oxidation reaction(EOR).However,the defects of noble metal Pt catalyst,such as high cost and low reserves,have severely hindered its large-scale application in the fields of water electrolysis and alcohol fuel cells.It would be an effective strategy to promote the practical application of Pt-based catalysts through enhancing the catalytic performance and reducing Pt usage.In this paper,to promote the targeted design and large-scale practical application of high-performance and low-cost Pt-based catalysts,the HER,ORR,MOR and EOR electrocatalytic performances of a small amount of Pt catalyst loaded on N-doped carbon nanotubes,the active N species which enhances electrocatalytic performances of Pt catalyst,and the resistance to ethanol penetration were systematically investigated by the experiments and theoretical calculations.The specific contents are as follows:(1)Influences of N-doped carbon nanotubes on the HER performance of a small amount of loaded Pt catalystN-doped carbon nanotubes(N-CNTs)were prepared by chemical vapor deposition(CVD).The distributions of N species in N-CNTs were adjusted by changing heating temperature and time under vacuum condition and combining XPS characterization.Then,a small amount of Pt catalyst was loaded on the surface of each prepared carbon nanotubes according to the same theoretical loading.The morphology,structure and element content of as-prepared Pt-based catalysts were characterized by SEM,TEM and XPS.And the HER activities and stabilities of Pt-based catalysts were estimated by electrochemical measurements in acidic and alkaline media,respectively.Results indicated that the HER activity and stability of Pt/N-CNTs with more graphite nitrogen and 8.6 wt% Pt loading even exceeded that of 40 wt% commercial Pt/C catalyst in alkaline medium and were comparable to 40 wt% commercial Pt/C catalyst in acidic medium.At the same time,the correlations between the intrinsic activities(TOF)of Ptbased catalysts and the contents of different N species further revealed that graphite nitrogen is the main active N species which improves HER performance of Pt catalyst,confirmed by density functional theory(DFT)calculations.(2)Influences of N-doped carbon nanotubes on the MOR and EOR performances of a small amount loaded Pt catalystMOR and EOR activities and stabilities in alkaline medium of a series of Pt-based catalysts prepared above were estimated by electrochemical measurements as well.Results indicated that the MOR and EOR activities and stabilities in alkaline medium of Pt/N-CNTs with more graphite nitrogen and 8.6 wt% Pt loading exceeded that of 20 wt% commercial Pt/C catalyst.At the same time,the correlations between MOR and EOR TOF values of Pt-based catalysts and the contents of different N species further demonstrated that graphite nitrogen is also the main active N species which enhances MOR and EOR performances of Pt catalyst.(3)The electrocatalytic selectivity of little Pt catalyst loaded on N-doped carbon nanotubes for EOR and ORRWith the N-CNTs prepared above as carriers,different amounts of Pt were loaded on their surface.The morphology,crystal plane and actual Pt loadings of Pt-based catalysts were characterized by TEM,XRD and LA-ICP-MS.And their EOR and ORR activities were estimated by electrochemical measurements in alkaline medium,respectively.Results showed that when the Pt loading was 0.07 wt%,Pt/N-CNTs1 showed almost no ethanol oxidization activity,but excellent ORR performance.Meanwhile,EOR and ORR peak potentials of Pt-based catalysts shifted,probably caused by the exposed quantity of Pt(111)crystal facet.DFT calculations demonstrated that the adsorption energy of oxygen molecule was about 3.79 times that of ethanol molecule on the surface of little Pt catalyst loaded on the surface of electron-donated N-CNTs,indicating that little Pt catalyst selectively catalyzed oxygen molecule when ethanol and oxygen molecules coexisted,meaning the mixed potential would not occur although ethanol molecules penetrated from the anode to cathode.