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

• Published in: ACS catalysis 2016-10, Vol.6 (10), p.7249-7259
• Main Authors: Mamtani, Kuldeep, Jain, Deeksha, Zemlyanov, Dmitry, Celik, Gokhan, Luthman, Jennifer, Renkes, Gordon, Co, Anne C, Ozkan, Umit S
• Format: Article
• Language: English
• Subjects: Chemistry
• ISSN: 2155-5435; 2155-5435
• DOI: 10.1021/acscatal.6b01786

To probe the active sites of nitrogen-doped carbon nanostructures (CN x ), 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 CN x 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 CN x catalysts were soaked in phosphoric acid. Adsorption of dihydrogen phosphate ions on the surface of CN x 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 CN x catalysts.