Probing the Oxygen Reduction Reaction Active Sites over Nitrogen-Doped Carbon Nanostructures (CNx) in Acidic Media Using Phosphate Anion

To probe the active sites of nitrogen-doped carbon nanostructures (CNx), the effect of dihydrogen phosphate (H2PO4–) anion on their oxygen reduction reaction (ORR) performance was investigated by adding increasing concentrations of phosphoric acid in half-cell measurements. A linear decrease in specific kinetic current at 0.7 V was noted with increasing phosphate anion concentration. It was also found that the adsorption of phosphate species on CNx was strong and the corresponding ORR activity was not recovered when the catalyst was reintroduced to a fresh HClO4 solution. Trends similar to those noted upon addition of H3PO4 to the half-cell were observed when CNx catalysts were soaked in phosphoric acid. Adsorption of dihydrogen phosphate ions on the surface of CNx exposed to phosphoric acid was verified by transmission infrared (IR) and Raman spectroscopy as well as X-ray photoelectron spectroscopy (XPS). XPS results also showed a decrease in the surface concentration of pyridinic-N species accompanied by an increase of equal magnitude in the surface fraction of quaternary-N species, which would include the pyridinic-NH sites. A linear correlation was observed between the loss in pyridinic-N site density and that in ORR activity. The observed poisoning phenomenon is consistent with the two possible active site models, i.e., pyridinic-N sites, which would be rendered inactive by protonation, or the C sites neighboring pyridinic-N species. These latter species would be poisoned by a site blocking effect if they strongly adsorb the phosphate ions. Strong adsorption of negatively charged phosphate ions on neighboring C atoms would also stabilize the pyridinic-NH sites. By identifying a poison that can be used as a probe, this study provides a first step toward identification and quantification of active sites in CNx catalysts.