The extent of carbon surface oxygen affinity and its effects on the activity of metal-free carbon catalysts in the oxygen reduction reaction: the interplay of porosity and N-, O- and S-enriched surface chemistry

Abstract

Highly porous carbon black, Black Pearl 2000, was modified with urea and thiourea to introduce only N- or N- and S-containing functional groups, affecting its activity in the oxygen reduction reaction. The samples were exposed to heating at 450 and 850 °C to vary surface features. Their chemistry and porosity were evaluated using various physical and chemical methods. An almost identical porosity of the urea-modified samples allowed for the evaluation of the effects of their surface chemistry on the ORR, assuming similar contributions of their micropores activity to an adsorption-induced oxygen reduction process. Even though the results suggested that pyridones were involved in the oxygen reduction activity, the number of electron transferred (∼4) and the onset potential were found to be directly dependent on the affinity of the surface to transport and adsorb dissolved oxygen, either into ultramicropores and onto N- and/or S- and N-based catalytic centers. The introduction of N and S led to a high catalytic activity. Sulfur, mainly in hydrophobic thiophenic species, compensated for the smaller absolute amounts of nitrogen. For the thiourea-modified series, the effect of ultramicropores in advancing the ORR was clearly indicated by the higher activity with fewer catalytic sites on the surface.