Influence of Calcination Treatments on the Formation of N-Doped Carbon Nanofibers and Their Electrochemical Properties

Tuesday, 3 October 2017
Prince George's Exhibit Hall D/E (Gaylord National Resort and Convention Center)
R. Ojeda-López (Universidad Autónoma Metropolitana - Iztapalapa), G. Ramos-Sánchez (Universidad Autónoma Metropolitana Iztapalapa), J. M. Esparza-Schulz, I. J. Pérez-Hermosillo, A. Domínguez-Ortiz, and I. González (Universidad Autónoma Metropolitana - Iztapalapa)
Carbon nanofibers (CNFs) are materials with potential applications as electrode supports for fuel cells, mainly due to their interesting characteristics: i) high surface area, ii) controlled chemical properties, and iii) high electric conductivity. CNFs can be modified by nitrogen doping; this procedure allows the formation of specific sites on the nanofibers surface which, when used as support, lead to controlled size and dispersion of the active material; moreover, the presence of N atoms as dopant causes the modification of the electronic density close to the doping site, specially diminishing the electron density in neighboring C atoms which also catalyze the fuel cell reactions.

In this work we propose a different alternative, the production of the support with the intrinsic formation of dopants in CNFs. N-doped carbon nanofibers were synthetized by electrospinning of polyacrylonitrile (PAN) followed by a two-step calcination process: stabilization and carbonization, the first was carried out under air atmosphere at 553 K while carbonization was conducted in inert atmosphere at five different temperatures, 873, 973, 1073, 1173 and 1273 K. X-ray Photoelectron spectroscopy experiments indicate that depending on the carbonization temperature this process produces different amounts of nitrogen species in the materials. For carbonization temperatures of 873 and 973 K, nitrogen species are mainly found in N-6 form (piridinic) while a small amount of nitrile groups is still present. For the materials with temperature treatment above 1073 K, N-Q (quaternary or graphitic nitrogen) becomes the most abundant nitrogen species, this indicate than the formed N-Q is much more stable than N-6 at higher temperatures.

The stability of the material was evaluated by cyclic voltammetry in acid media in anodic direction. The results indicate that all CNFs present a less positive oxidation onset potential in comparison to carbon Vulcan. This result agrees with thermogravimetric analysis in which N doped CNF decomposition (CO2 formation) is obtained at higher temperatures than commercial supports. Moreover, the CNFs were also evaluated as electrocatalysts for the oxygen reduction reaction (ORR) in alkaline conditions. The preliminary results show that as the quantity of N-Q augments, the catalytic activity concomitantly increases as well, i.e. the onset potential for oxygen reduction increases.