At present, we cannot well understand this phenomenon, but we presume that it is because of the serious reunion of the metallic cobalt particles since XRD results have revealed much larger C646 solubility dmso crystallite size of metallic cobalt in these catalysts than in those prepared with cobalt acetate and cobalt nitrate as precursors. These results disclose that small Co particles and the uniform dispersion are beneficial for obtaining a high-performance Co-PPy-TsOH/C catalyst towards ORR, while large cobalt particles and the agglomeration

deteriorate the catalytic performance. Figure 5 TEM images of Co-PPy-TsOH/C catalysts prepared from various cobalt precursors. (a) Cobalt acetate; (b) cobalt nitrate; (c) cobalt oxalate; (d) cobalt chloride. Figure 6 demonstrates Raman spectra of the Co-PPy-TsOH/C catalysts prepared from various cobalt precursors. As in our previous work [10, 23], the characteristic peaks generally observed in the wavenumber range from about

900 to 1,150 cm−1 for PPy and 1,370 cm−1 for antisymmetric in-ring C-N stretching [31, 32] disappeared in all the obtained catalysts, while only two peaks representing the disorder-induced band (D band, 1,327 cm−1) and the graphite band (G band, 1,595 cm−1) URMC-099 for carbon can be found, indicating the decomposition of PPy and insertion of nitrogen into the carbon layers during high-temperature pyrolysis. Usually, the graphitization degree of carbon materials can be estimated with the ratio of the G band and D band intensities (I G /I D ), the higher the ratio, the larger the

graphitization degree [33]. For the studied catalysts in the present work, the values of I D and I G extracted from Figure 6 along Thymidine kinase with the calculated values of I G /I D are listed in Table 2. An inverse order of the graphitization degree is exhibited to that of catalytic performance, resulting from the reconfiguration of nitrogen-impregnated graphitic carbon. So, it could be summarized that the graphitization degree of carbon in the Co-PPy-TsOH/C catalysts plays significant role on the catalytic performance towards ORR, the lower the graphitization degree, the better the catalytic performance. It is worthwhile to note that this relationship between the graphitization degree of carbon and the catalytic properties of Co-PPy-TsOH/C catalysts is just opposite to that drawn by Choi et al. [34] for nitrogen-containing carbon-based catalyst for ORR. We cannot, at present, well understand this discrepancy, but we believe one of the probable reasons is the different preparation of the catalysts and the different carbon and nitrogen sources used, resulting in different microstructure. In Choi et al.’s research [34], the catalysts were prepared through pyrolysis of polymer, dicyandiamide, with/without metal precursors where the polymer was used as the source for both carbon and nitrogen.