Cathode Electrocatalysts Of Direct Methanol Fuel Cell Based On Manganese Oxides

Manganese oxides are potential cathode non-platinum catalysts because of theirprominent advantages of high oxygen reduction catalytic activity and chemical stability,high abundance, low cost, environmental friendliness and so on. Nevertheless, thecatalytic activities of different manganese oxides memain controversial, and usually berestricted by their low electrical conductivity, which is unfavorable for fast electrontransfer. In this paper, different manganese oxides were combined with carbon nanotubes(CNTs) to take full advantage of their catalytic activities.Composite catalysts containing MnO2and CNTs were rapidly synthesized usingmicrowave-assisted deposition method. The unique structure of birnessite MnO2nanoflakes evenly growing on the surface of raw CNTs was in favour of the contactbetween electrolyte and catalyst active sites, exhibiting a high oxygen reduction reactioncatalytic activity in alkaline solution, which was almost equivalent to that of the Pt/CNTsno matter in the initial potential, half wave potential or the limiting current density whenthe content of MnO2is48.2%. Moreover, as the etching effect of KMnO4to CNTs walls,MnO2nanoflakes grow into the walls of CNTs and not easy to fall off, contributing to agood cycle stability.Mn3O4/MnCx/CNTs composite catalyst was prepared by treating the mixture of CNTsand metal manganese under a vacuum enviromment of10-1Pa at600C for40min,followed by further heat treatment. Mn3O4and MnCxevenly covered on the surface ofCNTs. The MnCxintermediate layer between Mn3O4and CNTs formed low ohmic contactresistance and increased the conductivity of the catalyst. This kind of catalyst was of highcatalytic activity for oxygen reduction reaction. The number of electrons transferred perO2molecular was calculated as high as3.9. Because the MnCxintermediate layer madethe contact between the CNTs and Mn3O4chemical bonding, Mn3O4/MnCx/CNTs showedfar higher than that of Pt/CNTs.MnO/C-N/CNTs composite catalyst was prepared using CNTs as carbon source, milkpowder as nitrogen source and KMnO4as manganese source under the condition of high-temperature and nitrogen protection. The influences of C-N content and pyrolysistemperature on catalytic activity were futher studied. The catalyst exhibited excellentcatalytic activity of oxygen reduction reaction because the laminar nitrogen dopedamorphous carbon materials origined from carbide the milk powder can promote thereduction of O2to HO2-and weak the agglomeration of CNTs, MnO can falilitate thedecomposition of H2O and CNTs can increase the conductivity of MnO/C-N/CNTs.Moreover, this catalyst processed relatively high stability.