The activity of Co-Fe-N-C depends critically on the number of iron sites. The current density at 1.55 V vs. RHE is roughly proportional to the loadings of Fe (Fig. ). The loadings of Fe could be roughly controlled by changing the activation time and the Fe concentration in KOH. Interestingly, the absorption of Fe ions from Fe-containing solution is reversible. When OER electrolysis was conducted over a long period of time (2 h) using Co-Fe-N-C in a Fe-free KOH solution, the amount of Fe in the catalyst decreased, and the OER activity also became worse. The activity could be recovered in KOH solutions containing even a trace amount of Fe ions, suggesting a fast equilibrium between absorption and desorption of Fe ions (Fig. ).