Abstract
Amorphous and graphitized nitrogen-doped (N-doped) carbon spheres are investigated as structurally well-defined model systems to gain a deeper understanding of the relationship between synthesis, structure, and their activity in the oxygen reduction reaction (ORR). N-doped carbon spheres were synthesized by hydrothermal treatment of a glucose solution yielding carbon spheres with sizes of 330 ± 50 nm, followed by nitrogen doping via heat treatment in ammonia atmosphere. The influence of a) varying the nitrogen doping temperature (550–1000 °C) and b) of a catalytic graphitization prior to nitrogen doping on the carbon sphere morphology, structure, elemental composition, N bonding configuration as well as porosity is investigated in detail. For the N-doped carbon spheres, the maximum nitrogen content was found at a doping temperature of 700 °C, with a decrease of the N content for higher temperatures. The overall nitrogen content of the graphitized N-doped carbon spheres is lower than that of the amorphous carbon spheres, however, also the microporosity decreases strongly with graphitization. Comparison with the electrocatalytic behavior in the ORR shows that in addition to the N-doping, the microporosity of the materials is critical for an efficient ORR.
Highlights
Fuel cells and metal–air batteries are important renewable energy technologies
No remaining catalyst particles are observed for the graphitized samples via energy dispersive X-ray spectroscopy (EDX) and X-ray photoelectron spectroscopy (XPS)
The N-doped carbon spheres (NCS) synthesized in the present work are characterized by a well-defined spherical shape and smooth surface
Summary
Fuel cells and metal–air batteries are important renewable energy technologies. Both rely on the oxygen reduction reaction (ORR). We had reported on core–shell titanium (oxy)nitride and tantalum (oxy)nitride@N-doped carbon composite spheres, which were based on a similar conducting carbon core as investigated in the present study, and their performance as ORR catalysts [34,35]. For these systems, which turned out to be promising ORR catalysts, we found that nitriding the metal-oxide shell concomitantly results in drastic structural changes and nitriding of the carbon core. A more detailed account of the electrochemical properties and of the ORR activity and mechanism is given elsewhere [27]
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