fig4

Figure 4. (A) XPS measurement spectra, (B and C) Fe 2p XPS spectra of Fe-N-C and Fe/Ni-N-C, (D and E) Ni 2p XPS spectra of Ni-N-C and Fe/Ni-N-C, (F-H) N 1s XPS spectra of Ni-N-C, Fe-N-C, and Fe/Ni-N-C, and (I) schematic diagram of different N types.