Identification of catalytic sites in cobalt-nitrogen-carbon materials for the oxygen reduction reaction

• Published in: Nature communications 2017-10, Vol.8 (1), p.957-11, Article 957
• Main Authors: Zitolo, Andrea, Ranjbar-Sahraie, Nastaran, Mineva, Tzonka, Li, Jingkun, Jia, Qingying, Stamatin, Serban, Harrington, George F., Lyth, Stephen Mathew, Krtil, Petr, Mukerjee, Sanjeev, Fonda, Emiliano, Jaouen, Frédéric
• Format: Article
• Language: English
• Subjects: 639/638/161/886; 639/638/161/893; Absorption spectroscopy; Active sites; Carbon; Catalysis; Catalysts; Chemical reduction; Chemical Sciences; Chemical synthesis; Cobalt; Density functional theory; Electrode potentials; Electron states; Humanities and Social Sciences; Iron; Magnetic permeability; Magnetic susceptibility; Metal particles; Metals; multidisciplinary; Nitrogen; Oxygen; Oxygen reduction reactions; Pyrolysis; Redox potential; Science; Science (multidisciplinary); Single atom catalysts; Spectroscopy; X-ray absorption spectroscopy
• ISSN: 2041-1723; 2041-1723
• DOI: 10.1038/s41467-017-01100-7

Single-atom catalysts with full utilization of metal centers can bridge the gap between molecular and solid-state catalysis. Metal-nitrogen-carbon materials prepared via pyrolysis are promising single-atom catalysts but often also comprise metallic particles. Here, we pyrolytically synthesize a Co–N–C material only comprising atomically dispersed cobalt ions and identify with X-ray absorption spectroscopy, magnetic susceptibility measurements and density functional theory the structure and electronic state of three porphyrinic moieties, CoN 4 C 12 , CoN 3 C 10,porp and CoN 2 C 5 . The O 2 electro-reduction and operando X-ray absorption response are measured in acidic medium on Co–N–C and compared to those of a Fe–N–C catalyst prepared similarly. We show that cobalt moieties are unmodified from 0.0 to 1.0 V versus a reversible hydrogen electrode, while Fe-based moieties experience structural and electronic-state changes. On the basis of density functional theory analysis and established relationships between redox potential and O 2 -adsorption strength, we conclude that cobalt-based moieties bind O 2 too weakly for efficient O 2 reduction. Nitrogen-doped carbon materials with atomically dispersed iron or cobalt are promising for catalytic use. Here, the authors show that cobalt moieties have a higher redox potential, bind oxygen more weakly and are less active toward oxygen reduction than their iron counterpart, despite similar coordination.