Preparation And Performance Of Manganese Dioxide-Based Composites Oxygen Reduction Electrocatalyst

Proton exchange membrane fuel cell?PEMFC?is a kind of promising,safe,environmental and friendly electrochemical energy converter,in which the oxygen reduction reaction?ORR?plays a key role.The platinum-based catalysts can effectively reduce the large overpotential of ORR,but its scarce reserves and high cost have seriously weakened their large-scale application in PEMFC.Therefore,it is of great significance for the sustainable development of PEMFC to explore new catalysts with low cost,high activity and stability.Manganese dioxide is widely considered as an excellent candidate material for ORR owing to its advantages of abundant resources,cheap availability and great potential for catalyzing ORR.After combined with nitrogen-doped carbon materials or precious metal nanoparticles,both enhanced electrical conductivity and improved stability are achieved for manganese dioxide.In this paper,manganese dioxide was as the object,and carried out relevant experiments by combining with nitrogen-doped carbon materials and metal silver to construct ORR catalysts of different composite forms.The details are as follows:1.Using ammonium persulfate as oxidant,pyrrole was polymerized on the surface of MnO₂ nanotubes.Then,polypyrrole?PPy?was used to reduce silver nitrate in the presence of polyvinylpyrrolidone and the silver nanoparticles were grown on the surface of the polypyrrole.Further pyrolysis treatment on the the obtained product led to the formation of MnO₂/N-C/Ag.The MnO₂/N-C/Ag displayed an onset potential potential of 0.896 V vs.RHE,half-wave potential of 0.804 V vs.RHE?reversible hydrogen electrode?,and a current density of 4.769 m Acm^-2 at a potential of 0.4 V,demonstrating satisfactory catalytic activity towards ORR.In addition,it showed stronger resistance to methanol and stability compared with commercial Pt/C.After cycling for 20000 s,6 % current loss was observed for MnO₂/N-C/Ag.The synergistic effect between Ag nanoparticles,the N-doped carbon derived from polypyrrole and MnO₂ nanotubes,promoted the generation of a large number of Mn³⁺/Mn⁴⁺ and relevant oxygen vacancies of the MnO₂/N-C/Ag.2.Using one-dimensional conductive polymer PPy nanotubes as precursors and KMnO₄ as oxidants,MnO₂ nanoparticles were generated on PPy surface through facile one-step reaction.Then PPy/MnO₂ was converted into MnO₂ anchored nitrogen-doped carbon?N-C/MnO₂?composite via further pyrolysis treatment.As a result,the nitrogen-doped carbon/MnO₂ showed satisfactory catalytic activity during ORR process.The N-C/MnO₂ presented an onset potential of 0.923 V vs.RHE and half-wave potential of 0.832 V vs.RHE.The current density of 4.730 m Acm^-2 at a potential of 0.4 V was also observed.Apart from that,the N-C/MnO₂ showed stronger resistance to methanol and better stability than commercial Pt/C.The only 7 % current loss after 20000 s cycles was observed.The excellent catalytic performance of N-C/MnO₂ was attributed to the nitrogen-doped carbon and MnO₂,where the former one led to more oxygen vacancies in MnO₂ during carbonization,and the existence of the later one contributed to generate a large number of active sites in nitrogen-doped carbon components.