The overall ORR activity of catalysts depends on both the intrinsic activity of the metal centers and efficiency of the mass transport process. Owing to the high mass loading of SACs used for MEA applications, mass transport becomes the key factor that limits the overall cell performance. Although the sluggish reaction kinetics could be enhanced via the modulation of the structure of the active moiety, the slow diffusion rate of O, from the bulk electrolyte to the active center still limits the overall ORR rate, and consequently, the total output power.