The redox reaction is divided into two half reactions: the hydrogen oxidation reaction (HOR) occurring at the anode and the oxygen reduction reaction (ORR) occurring at the cathode. HOR has zero free energy change while ORR releases ideally 4.92 eV which amounts the total free energy change of the entire electrochemical reaction. Even though both HOR and ORR reactions need catalysts, ORR has one intrinsic problem, i.e., it is highly inefficient. In acidic media, the kinetics of the ORR is 6 times slower than HOR. Thus, in order to boost the sluggish ORR kinetics an extra potential known as overpotential must be applied. This overpotential causes a significant voltage drop from the ideal open-circuit voltage of 1.23 V referenced to the standard hydrogen electrode (SHE). Consequently, very active catalysts become mandatory to minimize this undesired overpotential. To date, commercial platinum (Pt) supported on carbon black (Pt/C) remains the state-of-the-art catalyst choice. However, even Pt requires large amounts of catalyst loading to reach a reasonable efficiency. Unfortunately, the scarcity and high cost associated with this novel metal represents the main bottleneck for the large-scale application and commercialization of PEMFCs. In order to increase the viability of PEMFCs, Pt content has to be reduced. The U.S. Department of Energy (DOE) has established a target limit of less than 0.125 mgPt cm -2 . Conventionally, the ORR activity is evaluated in terms of the intermediates' binding strength with an activity-estimation plot, i.e., volcano plot. The ORR volcano plot follows the Sabatier principle: the catalyst should not interact neither too strongly nor too weakly with the ORR intermediates: hydroperoxyl (OOH*), atomic oxygen (O*) and hydroxide (OH*), where the asterisk (*) denotes an adsorbed species. If the catalyst binds OH* too strongly, OH* reduction to water will be hindered. Additionally, the availability of active sites will be diminished, causing catalyst poisoning. In contrast, if OH* adsorption is too weak, OOH* adsorption will be weak as well due to the scaling relationship between OH* and OOH*, and thus, O2 reduction to OOH* will be hampered. An optimal catalyst should maintain a balance between an efficient O2 activation and OH* removal. More generally, the binding strength of all intermediates should be close to the equilibrium values of 1.23n eV (n = 1-4) at 0 V (vs SHE). Otherwise, one of the reaction steps will become endergonic and will give rise to an overpotential. Pt is located near the vertex of the mentioned volcano plot for ORR and thus it is the best catalyst for this electrochemical reaction.