Fig. 1. Schematic diagram of the oxygen-containing functional modification for original CP.

Fig. 2 High resolution C1s spectra of original CP (a) and OCP (b). Contact angle of original CP (c) and OCP (d). SEM images of original CP (e) and OCP (f).

Fig. 3. Schematic diagram for in-situ growth of the Co@NCNTs onto the OCP.

Fig. 4 (a-c) SEM images of the N-CNTs-20@Co/OCP. (d-f) TEM of images of the N-CNTs-20@Co/OCP. (g) HRTEM images of the N-CNTs-20@Co/OCP. (h) Corresponding to the SAED patterns of the N-CNTs-20@Co/OCP. (i?l) EDS mapping images of the N-CNTs-20@Co/OCP.

Fig. 5. (a) XRD patterns of N-CNTs-20@Co/OCP and OCP. (b) Raman scattering spectra of N-CNTs-20@Co/OCP, CNTs-20@Co/OCP and OCP. (c) High resolution Co 2p spectra. (d) High resolution C 1s spectra. (e) High resolution N 1s XPS spectra. (f) Corresponding nitrogen species content.

Fig. 6. (a) CV curves of N-CNTs-y@Co/OCP in N2-(dash line) and O2-(solid line) saturated 0.1 mol L-1 KOH. (b) ORR polarization curves of N-CNTs-y@Co/OCP in O2-saturated 0.1 mol L-1 KOH solutions. (c) LSV polarization curves and corresponding K-L curves of N-CNTs-20@Co/OCP in 0.1 mol L-1 KOH. (d) Plots of the peak current density as a function of the square root of the capacitive current at 0.4 V as scan rate for N-CNTs-y@Co/OCP. Durability tests of N-CNTs-20@Co/OCP (e) and Pt/C (f) in 0.1 mol L-1 O2-saturated KOH, respectively.

Fig. 7 DFT calculations on Co effect. Calculated free energy profiles for ORR elementary steps with active sites at top side (a) and Sidewall (b). Same carbon has been explored for CNT and Co@CNT for comparison. Overpotential has been labelled as blue numbers in unit of V when external potential U = 1.23 V has been applied. Charge density difference between Co cluster and CNT was plotted with Co4 cluster located at capped side (c) and middle channel (d). Blue and yellow isosurfaces indicate electron donation and accumulation with an isovalue of 0.003 e ??3, C, H and Co are shown as brown, white and blue balls.