Preparation Of Transition Metal And Nitrogen Co-Doped Carbon Materials For Oxygen Reduction Reaction

Development and research of the high performance non-precious metal catalysts for oxygen reduction reaction(ORR) is the key to the commercialization of fuel cells. Among them, transition metal/nitrogen-carbon(M/N-C) materials have been considered as the most potential catalysts toward ORR due to their efficient catalytic activity and low cost. However there are still some problems:(1) their catalytic activity is still less than commercial Pt/C, especially in acid system;(2) the interface force between the carbon support and active sites is weak;(3) the reason for the differences of catalytic activity under the acid and alkaline system is not clear and lack of system research. In view of the above problems, we in-situ constructed M/N-C catalysts through one-step pyrolysis, effectively enhancing the interface force between carbon support and active sites with the catalytic activity comparable to the commercial Pt/C under alkaline condition. Among them, the binary metal doped M/N-C catalyst showed good catalytic activity under acid system. In addition, the pH effect was explored systematically and the specific research is as follows:Firstly, Co, N co-doped bamboo-like large size carbon nanotubes(CoN-BCNTs) was prepared via simple in-situ pyrolysis of cobalt, nitrogen and carbon precursor. The growth mechanism and catalytic activity of CoN-BCNTs were characterized and analyzed systematically. As a result, CoN-BCNTs showed a good catalytic performance for ORR(E1/2=0.80 V,Jm=-4.70 mA·cm-2) which can match the state-of-the-art 30wt% Pt/C catalysts in alkaline medium. Moreover, CoN-BCNTs exhibited an excellent stability and a good methanol tolerance. The outstanding performance of CoN-BCNTs can be attributed to the high nitrogen dopant and special structure of bamboo-like larger size carbon nanotubes.Secondly, we employed Fe as the transition metal source which showed excellent activity towards ORR to construct more efficient catalysts. Then Fe, N co-doped hollow carbon sphere with mesoporous structure(FeN-HCS) was prepared though the same synthesis strategy as CoN-BCNTs. The intrinsic activity of FeNx and the rational structure of hollow carbon sphere with mesoporous structure endowed the outstanding ORR activity of FeN-HCS(E1/2=0.83 V,Jm=-5.64 mA·cm-2) and stability in alkaline medium. And the performance of FeN-HCS is even superior to that of 30wt% Pt/C catalysts. Furthermore, the effect of the second heat treatment has been investigated. We found that the second heat treatment played a key role in the formation of hollow carbon sphere with mesoporous structure.Finally, we introduced binary transition metal and graphene to improve the M-N/C ORR catalytic performance in acidic medium. The Fe, Co, N co-doped hollow carbon sphere supported on graphene(FeCoN-HCS/G) was prepared by the same preparation method. Thanks to the synergistic effect of binary metal and the optimization of the structure, FeCoN-HCS/G exhibited the enhanced ORR performance in the acidic medium. In addition, we also studied the differences of the catalytic performances of the three catalysts under both alkaline and acidic medium and put forward to the possible reason. Our research can provide a new way for the catalyst design in both alkaline and acidic medium.