Towards understanding the active sites for the ORR in N-doped carbon materials through fine-tuning of nitrogen functionalities: an experimental and computational approach

Abstract

The design of advanced N-doped carbon materials towards oxygen reduction reaction (ORR) catalysis is only possible if the nature of the active sites is fully understood. There is an important piece of research seeking to overcome this challenge through experimental or theoretical results. However, the combination of both approaches is necessary to deepen into the knowledge about this subject. This work presents excellent agreement between experimental results and computational models, which provides evidence about the nature of the most active sites in N-doped carbon materials. N-doped carbon materials have been experimentally obtained through double stage treatment of polyaniline in distinct atmospheres (both oxygen-containing and inert atmosphere) at different temperatures (800-1200ºC). According to temperature programmed desorption (TPD), Raman spectroscopy, N 2 -adsorption isotherms at -196ºC and Xray photoelectron spectroscopy (XPS), this synthesis method provides the selective formation of nitrogen species, without important changes in structural order or porosity. ORR catalytic tests evidence the highly efficient catalysis, with platinum-like performance in current density and onset potential, of N-doped carbon materials selectively containing graphitic-type nitrogen species. Computational chemistry, through DFT calculations, shows that edge-type graphitic nitrogen is more effective towards ORR catalysis than pyridinic, pyrrolic, pyridonic, oxidized and basal-type graphitic nitrogen species.After the electron supply, the proton must be introduced into the model structure to complete the first reduction stage. Fig. 7a illustrates this process. The proton is attracted towards the oxygen atoms that have collected the electrons forming an intramolecular hydrogen bond.

Hydrothermal Carbon/Carbon Nanotube Composites as Electrocatalysts for the Oxygen Reduction Reaction

“…These modifications can be achieved by the incorporation of heteroatoms like nitrogen, sulfur, phosphorus, or boron and/or advanced carbon materials like graphene oxide or carbon nanotubes (CNTs) [9,12,13,17,[25][26][27][28]. Nitrogen is the most promising heteroatom for ORR, as pyridinic nitrogen favors the bonding of oxygen to the adjacent carbon, while the presence of quaternary nitrogen favors oxygen dissociation [7,29]. Therefore, the appropriate nitrogen functionalities seem to be essential to improve the electroactivity of the carbon materials towards the ORR.…”

Polyaniline-Derived N-Doped Ordered Mesoporous Carbon Thin Films: Efficient Catalysts towards Oxygen Reduction Reaction

“…According to the physicochemical and electrochemical characterization, the enhancement in the catalytic activity of the most active samples can be associated with the presence of an N-doped carbon framework within the porous structure of the Silica and Titania, which exhibits a quaternary nitrogen-enriched surface. Indeed, edge-type quaternary nitrogen species have been proposed as highly efficient active sites towards ORR with high selectivity towards water formation due to its low kinetic barriers and bridging binding mode for oxygen molecule chemisorption [ 21 , 22 , 23 , 24 , 53 ]. Nevertheless, the higher amount of N-doped carbon material in the samples obtained with the highest polymerization times can also explain the enhanced electrocatalysis.…”

“…The quaternary-type nitrogen species of G/Si_PANI_5min_900 sample have transformed into, mainly, pyridone species after the etching process. This transformation can only be understood if the quaternary-type nitrogen species are located at the edge of the carbon framework, since the oxidation of a basal-plane quaternary nitrogen species is thermodynamically unfavorable [ 53 ]. The results are in agreement with the higher catalytic activity of edge-type quaternary nitrogen species with respect to pyridone-type nitrogen functional groups.…”

Understanding the self-templating of hierarchically porous carbon electrocatalysts using Group 2 coordination polymers

“…59 Their relative proportions vary between the four carbons, but the fraction of pyridinic nitrogen, an important contributor to ORR activity, 60 is overall quite constant (1.1-1.7 wt%,) and sufficient for effective ORR electrocatalysis. 1,2,[61][62][63] The similarities in carbon composition along the series, in contrast to the far-swinging differences in porosity, suggest that the ORR activity of these catalysts will depend chiefly on their microstructure. To investigate this dependence, we studied the activity of NC-M carbons towards the ORR, using a rotating ring-disc electrode in an O 2(g) -saturated 0.1 M KOH electrolyte.…”