Nitrogen-doped Graphitic Carbon As Efficient Electrocatalyst For Oxygen Reduction

Proton exchange membrane fuel cells（PEMFCs） have been considered to be the most promising chemical power sources of electric cars and other civil occasion in the future due to many advantages such as high energy conversion efficiency, zero emissions and quick start at room temperature. But the high cost, resource scarcity and poisoning of electocatalyst seriously restrict their commercialization. Therefore, low Pt and free Pt catalysts have been recognized as the research emphasis in the field of electocatalyst. However, for the long-term benefits, developing catalysts with low cost, rich resources and good catalytic performance that can completely substitute Pt/C is the fundamental way to realize the practical application of PEMFCs.Therefore, we have designed two kinds of nitrogen doped carbon free of Pt catalysts.Aiming at overcoming the shortcomings of graphene in the catalytic activity, mass transfer and conductivity between the graphene layers, we desigened a catalyst, nitrogen-doped-graphene supported Co₃O₄ particles catalysts, in which the oxidized graphene（GO） was reduced by organic amine, whose amino functional groups can graft to GO through graphene defects. Then, amino functional groups were converted to schiff base functional groups by condensation reaction of amine and aldehyde. Simultaneously, non-noble metal oxides were anchored under the condition of the reflux condensation. On the one hand, using ligament of Schiff-base-metal formed between layers of graphene is an ingenious way to build three-dimensional crosslinked stereo structure of N-doped graphene catalyst, resuting in the improved electronic conductivity and mass transfer rate. On the other hand, a large number of Schiff-base-metal active sites have synergetic effect with Co₃O₄ so that they can significantly enhance the ORR catalytic activity.To obtain a nitrogen-doped carbon nanotube, which is supposed conducive both in conductivity and in activity for catalysis of oxygen reduction reaction, we take the tubular MnO₂ as a template and catalyst to synthize tubular polyaniline（PANI） by in situ oxidative polymerization of aniline on the MnO₂ tubes. To further improve the graphitization, we applied SiO₂ GEL enclosing PANI-wrapped MnO₂ tubes, which was heated at 900 oC twice before and after SiO₂ GEL removal. The final product is named CN-SiO₂-HT2. The catalysts that was obtained by directly heating PANI-wrapped MnO₂ tubes at 900 oC once is named CN-HT1 for comparison. In this work, Scanning electron microscope（SEM）, X-ray diffraction spectrum（XRD） and electrochemical methods were used to search the nano structure and electrochemical activity of the CN-SiO₂-HT2 catalysts. The MnO₂ can not only serve as the template for forming shaped PANI precursor, but also acted as the chemical oxidative initiator for aniline polymerization. During the reaction, the MnO₂ template can be spontaneously reduced by aniline monomer into soluble Mn2+ in acidic environment, making it easy to be removed. What’s more, its residue of MnO₂ can also serve an assistant catalyst for the ORR. In addition, the use of SiO₂ effectively prevents the collapse and reunion of the polyaniline during high temperature carbonization, resulting in an improved catalyst production and a high efficiency of nitrogen-doping, which can eventually enhance the ORR activity of the catalyst.