Preparation Of Ag/MnO₂ Catalyst And The Change Of Reaction Path By CHNTs For Oxygen Reduction Reaction

Proton exchange membrane fuel cells（PEMFCs）have application potential in the field of fuel cells due to their low environmental pollution and high conversion efficiency.The cathodic oxygen reduction reaction（ORR）process involves the transfer of 2-electron and 4-electron reactions,possessing complicated reaction mechanism.In alkaline media,the intermediate HO₂^-through 2-electron path,which is highly oxidizing and corrosive,has a negative effect on the catalyst and the proton exchange membrane.Therefore,the 2-electron path should be avoided as much as possible,which mainly depends on the catalysts.Although platinum（Pt）catalyst can realize pure 4-electron transfer,its high cost and low reserves limit the commercial applications.Studies have found that transition metal manganese oxides（especially MnO₂）are highly active for ORR in alkaline media,however,their conductivity and stability are poor.Therefore,this thesis combines Ag and carbide halloysites（CHNTs）with MnO₂as composite catalysts to improve the conductivity and catalytic activity of MnO₂.Meanwhile,it is worthy of emphasizing the effect of the MnO₂ morphology,the introduction of Ag and CHNTs on the catalytic performance and reaction path for ORR.The research content of this paper is as follows:MnO₂ cataysts with different morphologies were prepared using hydrothermal method by controlling the reaction time and concentration of precursor,and Ag/MnO₂composite catalysts were further constructed through doping Ag on MnO₂ surface by a reduction method to improve the conductivity of the catalyst.It is found by characterization techniques and electrochemical tests that the Ag/MnO₂ composite catalysts exhibited an improvement of the catalytic activity for ORR,which is attributed to the formation of defect sites in MnO₂ by the doping of Ag nanoparticles.Specifically,the defect sites induce the existence of more oxygen adsorption sites on the surface of catalytsts,which is benefical to the adsorption and activation of oxygen molecule.In contrast,the Ag/MnO₂ cataysts with nanowire morphology illustrated the largest electrochemical specific surface area,resulting in highest catalytic activity for ORR.Based on nanowire MnO₂ with better ORR performance,Ag/MnO₂ catalysts with different content of Ag were prepared by adjusting the doping amount of Ag,and the influence of the content of Ag on the defect sites of MnO₂ was explored.It is found that the Mn³⁺gradually increases with increasing the addition of Ag,suggesting the increase of the surface defect sites and oxygen adsorption sites.In specific,the 51wt.%Ag/MnO₂ catalyst has the largest specific surface area and electrochemical active area,leading to exposing more active sites of the catalytic reaction.Therefore,the 51 wt.%Ag/MnO₂ catalyst has the best ORR catalytic activity among the prepared samples.The MnO₂-CHNTs composite support was prepared,and then Ag/MnO₂-CHNTs composite catalyst was constructed by impregnating Ag on the MnO₂-CHNTs composite support.A series of characterization and electrochemical tests were used to investigate the structure,surface element valence and catalytic performance of the catalyst.The study found that the addition of CHNTs effectively promoted the dispersion of MnO₂ nanowires.More importantly,the constructed three-component interface of Ag,MnO₂ and CHNTs efficiently suppressed the 2-electron path as well as facilitated the 4-electron path.Finally,the introduction of CHNTs changed the reaction path of ORR as well as enhanced the stability of the cataltysts.