Revealing the impact of small pores on oxygen reduction on carbon electrocatalysts: A journey through recent findings

Oxygen reduction reaction, ORR, is an important process affecting the performance of fuel cells. To be effective, efficient catalysts are needed. Recently, metal-free carbon materials have been extensively investigated for this purpose. The interest of scientists evolved from concentrating on nonporous graphene or CNT to focusing on porous carbons. The latter includes a range of materials with heterogeneous pore sizes. They can provide a performance comparable to that of Pt/C in terms of an electron transfer number, current, kinetics, or even an onset potential. It has been established that heteroatom-based catalytic centers (N, S, P, B), defects, and conductivity markedly increase the ORR performance. Regarding the porosity, the main consensus is that it is important for the dispersion of catalytic centers and mass transfer. In this review, some recent literature results on porous carbons, with or without heteroatoms, are critically analyzed with an intent to reveal the effect of micropores in these carbon catalysts on the efficiency of ORR. It is suggested that micropores, by providing strong adsorption forces for O2 molecules, promote bond splitting and a 4 electron transfer. This process occurs in parallel to that catalyzed by heteroatoms/defects and thus advances the ORR efficiency.