I am taking CV from N-doped graphene samples in order to study their ORR performance. Some of the electrodes reach a flat plateau in their current density but some not. I searched and found some discussions about it and generally people say that it is due to lack of electrochemical activity of the electrode. However, most of the time the electrode without flat plateau gives higher cathodic current than the ones with specific mass transport limiting current. I would appreciate it if anyone can explain me the reason behind this situation.