Porous Carbons as Oxygen Reduction Electrocatalysts

Abstract

In search for new alternative sources of energy carbon materials placed themselves as a valuable alternative to replace efficient noble metal- or transition metal-based catalysts. Even though the search for this kind of efficient catalysts started from the discovery and wonders of graphene, soon it has been established that to bring a catalytic activity for oxygen reduction to graphene an introduction of heteroatoms to its matrix or defect engineering is a must. The intent of this review is to present a brief journey on the development of heteroatom -doped "flat" carbons for an oxygen reduction electrocatalysis, followed by an indication of the role of defects as a general driving force, through heteroatom-doped porous carbons to heteroatom-free porous carbons. The emphasis is on importance of porosity, no necessary that 3-D engineered. The development of that concept is presented in the chronological order. Since to summarize all works on this topic sounds as a rather impossible task, and numerous reviews on the specific aspects of carbon-based metal-free ORR catalysts have been published recently, our intention is to stress the works where other factors than an alteration in the electronic/structure affect or could affect the ORR efficiency. These other factors include oxygen accessibility to small pores and its adsorption there. Even though the development of pores certainly is associated with an increase in the level of defect, the pore- influenced oxygen reduction mechanisms addressed here differs from that on the electronic defects-based catalytic sites.