The activation process of Co-N-C led to incorporation of Fe ions, as confirmed by inductively coupled plasma atomic emission spectroscopy (ICP-AES). In a typical sample, the loadings of Co and Fe were 9.4(±1.4) ×10 -8 mol/cm 2 and 8.6(±1.6) ×10 -9 mol/cm 2 after activation, respectively. According to previous reports, trace amount of iron ions are essential for the high activity of NiOx and CoOx pre-catalysts. To verify the role of Fe in the activity of Co-N-C, a number of experiments have been conducted (Fig. and). First, the Co-N-C was tested in an iron-free KOH solution (1 M, obtained by treating with Ni(OH)2, see Supporting Information for details), the initial activity was low, with an overpotential of 495 mV at 10 mA/cm 2 and a Tafel slope of 72 mV/dec. The activity could be improved by prolonged electrolysis to reach an overpotential of 443 mV at 10 mA/cm 2 and a Tafel slope of 58 mV/dec. The activities are much lower than that of the activated Co-N-C in a "normal" KOH solution which contains a trace amount of iron ions (0.18 ppm). Addition of 10 ppm of Fe(NO3)3•9H2O into the "normal" KOH further improved the activity. The overpotential at 10 mA/cm 2 was decreased to 309 mV. These results (Fig. ) support the hypothesis that iron incorporation is essential for the high activity of Co-N-C. A new reduction peak at 1.189 V emerged after activation in Fe-containing "normal" KOH, while the corresponding oxidation peak was hidden by background capacitive current (Fig. ). This feature was absent for the catalyst activated in Fe-free KOH. This new redox feature indicates the presence of a new Co species formed upon Fe incorporation. The activated Co-N-C catalyst is then labeled as Co-Fe-N-C. .