| Energy crisis and environmental pollution are two urgent problems facing human society at present.Fuel cell based on the reaction between oxygen and hydrogen is environmentally friendly and has high power density.Fuel cell is expected to solve the problems caused by the use of internal combustion engine based on fossil fuels and has a good development prospect.However,due to the sluggish kinetics of oxygen reduction reaction（ORR）in the cathode of fuel cell,a large amount of platinum-based precious metal catalysts are needed to accelerate the reaction,which limits its large-scale application.It is urgent to develop efficient and stable low-platinum and even cheap metal catalysts.In addition,in order to deal with the problem of organic environmental pollution in industrial production,advanced oxidation process can be used to use oxygen as a raw material to produce hydroxyl radicals through for degrading organic pollutants.Similarly,the process needs to be driven by a specific electrocatalyst.The above processes are all based on oxygen reduction reaction.In this thesis,two kinds of nanocomposite electrocatalysts were designed to solve the above problems.The following work has been carried out:（1）A titanium supported platinum（Pt）nanoparticle ORR catalyst was designed and synthesized by heat treatment of supported followed by atomic layer deposition（ALD）.In this thesis,Ti O2 was treated at high temperature in the ammonia atmosphere to form titanium nitride（Ti OxNy）in the outer layer and Ti N phase in the core of nanoparticle.Ti OxNy layer is rich in oxygen vacancies.The vacancies not only can act as ORR active sites,but also anchor Pt atoms during the ALD process to form highly dispersed ultra-fine Pt particles.The firm attachment between Pt and support can inhibit the dissolution and aggregation of Pt and display enhanced stability during long-period ORR reaction.In terms of activity,we found that when the bulk particles of Ti O2 were completely transformed into Ti N,the activity of ORR increased greatly.This is mainly because the electrical conductivity of Ti N is significantly higher than that of Ti O2,and the oxygen vacancies improved the dispersion of Pt particles and the atomic efficiency of Pt.The XPS results confirmed that there was a strong metal-support interaction between Pt and titanium-based support with rich oxygen vacancies.With the increase of the ammonization degree of Ti O2,the Pt 4f peak shifts to high binding energy,indicating electron transfer from Pt to the support with the increasing density of vacancies.Although the strong interaction can stabilize Pt,the oxidic Pt was not conducive to the occurrence of ORR.With the increase of the number of ALD deposition circles,Pt particles grew into larger ones,the proportion of interfacial Pt atoms decreased.Meanwhile Pt particles became more metallic and electronically rich,which benefited ORR.Thus,the mass activity of Pt was significantly enhanced.Inspired by this,we directly used commercial Ti N as the support,and found that Ti OxNy layer with oxygen-rich vacancies was formed on the Ti N surface,which provides rich nucleation sites for Pt deposition.If Ti N was completely oxidized to Ti O2,the performance of the catalyst decreased significantly,which proved the enhancement effect of highly conductive Ti N on ORR.The corresponding XPS analysis results of Ti N were consistent with those of ammoniated Ti O2.Finally,the Ti OxNy-supported Pt catalyst ORR delivered excellent performance by optimizing ALD deposition.The Pt mass activity is 1.9 times higher than that of commercial Pt/C,which is expected to be used in fuel cell devices.（2）A carbon-based transition metal nanocomposite catalyst was designed and synthesized.The electron transfer number of ORR changed from 3.5 to 3.0 by adjusting the type and concentration of surface oxygen functional groups.Cobalt nanoparticles formed by pyrolysis of cobalt salt can act as the catalytic center for the carbon nanotubes（CNT）when pyrolysis of the mixture dicyandiamide and cobalt salt.Dicyandiamide was the carbon and nitrogen source.The graphite nanofragments formed from thermal decomposition of dicyandiamide pyrolysis migrated to the cobalt center to drive the CNT growth.As a results,Co N@CNTs catalyst was obtained.SEM and TEM characterization results confirmed that the catalyst had a hollow tubular structure.And it was found that N and O were uniformly distributed on the tubular structure.Electrochemical tests show that the electron transfer number of the catalyst in the ORR process was about 3.5,which is not ideal.According to the literature report,the electron transfer number of CNT is negatively related to the oxygen content.Thus,we added the oxygen-rich organic polyethylene oxide（PEO）into Co N@CNTs,which was then pyrolyzed together.XPS analysis showed that the oxygen content of the composite catalyst increased significantly.Electrochemical tests also confirmed that the electron transfer number of the composite catalyst decreased after the introduction of PEO,The O1s XPS results showed that the concentration of ester epoxy group increased after the introduction of PEO,which agrees with the change of the electron transfer number.This suggests that the ester epoxy group is the active site.In addition,it was also found that the ORR activity decreased slightly,which was related to the number of defects in the catalyst.The pyrolyzed PEO product may cover the original defects on the Co N@CNTs,which reduced the adsorption capacity of O2.Finally,the electron transfer number of the composite catalyst in the ORR process reached around 3 in the potential range of 0.2-0.7V（vs.RHE）,and showed decent ORR activity,excellent methanol tolerance and stability.The modified Co N@CNTs,catalyst has potential in the degradation of organic pollutants in acidic wastewater. |
