Abstract: The development of high-performance and low-cost catalytic materials for the oxygen reduction reaction (ORR) has been a major task for the large-scale application of fuels cells.^[1] Currently, platinum and platinum-based alloys catalysts are generally used as effective cathode catalysts for oxygen reduction reaction (ORR). However, the high price and degradation of Pt catalytic activity severely with time restrict the practical application. Thus, alternative catalysts based on metal-free materials, or nonprecious metals have attracted much attention. Among the metal-free materials for ORR, carbon doped with nitrogen is a promising catalyst because the long-pair electron from nitrogen can active the π electrons in carbon for ORR; thus, O₂ molecules are reduced on the positively charged C atoms that neighbor N atoms. Recently, the N-doped carbons, after further supporting the transition metal-based materials such as Co₃O₄, exhibit comparable ORR activity to commercial Pt/C.^[2] The enhancement of the activity is the evidence of synergetic effect between the active nanoparticles and carbon support.

In this work, we have focused on the research work on the preparation of nitrogen-doped carbon encapsulated cobalt-based nanoparticles (Co₃O₄@N-doped carbon) as a synergistic catalyst for ORR in an alkaline solution. We employed an impregnation and calcination method to prepare Co₃O₄ nanocrystals supported on the surface of carbon (Vulcan XC-72R). Subsequently, the obtained Co₃O₄/C was mixed with melamine which is used as nitrogen and carbon source. The mixture was heated at 700 ℃ for 1 h in argon atmosphere. During this procedure, most Co₃O₄ particles are reduced to cobalt by carbon matrix, and residual cobalt oxide (Co₃O₄) is on the surface of these cobalt particles. The morphological and electronic structure of the resulting Co₃O₄@N-doped carbon hybrid catalysts were characterized by transmission electron microscopy (TEM), X–ray diffraction (XRD). The results show that the prepared nanocrystals are uniformly dispersed on surface of the support. The average diameter of nanocrystals is about 10 nm. The obtained hybrid catalyst exhibits improved electro–catalytic activity for ORR in 1 M NaOH solution, which is close to that of a commercial Pt/C (E-TEK) under the same condition. The enhanced activity toward ORR on this hybrid catalyst can be explained by strong electronic interaction between particles and nitrogen-doped carbon. Moreover, in this material, three common nitrogen bonding configurations are identified within the carbon lattice, including quaternary N (or graphitic N), pyridinic N, and pyrrolic N respectively. The active nitrogen content among these hybrid materials is estimated to be 4.13 at.%, which can effectively increase the electrochemically active surface areas of the catalysts.

Reference:

[1]Y. Liang, Y. Li, H. Wang, J. Zhou, J. Wang, T. Regier and H. Dai, Nat. Mater. , 2011,10:780-786  

[2]Wu J, Xue Y, Yan X, Yan W, Cheng Q, Xie Y, Nano Research, 2012,5:521-530