Formation of Active Sites for Oxygen Reduction Reactions by Transformation of Nitrogen Functionalities in Nitrogen-Doped Carbon Nanotubes

Abstract

Heat treating nitrogen-doped multiwalled carbon nanotubes containing up to six different types of nitrogen functionalities transforms particular nitrogen functionalities into other types which are more catalytically active toward oxygen reduction reactions (ORR). In the first stage, the unstable pyrrolic functionalities transform into pyridinic functionalities followed by an immediate transition into quaternary center and valley nitrogen functionalities. By measuring the electrocatalytic oxidation reduction current for the different samples, we achieve information on the catalytic activity connected to each type of nitrogen functionality. Through this, we conclude that quaternary nitrogen valley sites, N-Q(valley), are the most active sites for ORR in N-CNTs. The number of electrons transferred in the ORR is determined from ring disk electrode and rotating ring disk electrode measurements. Our measurements indicate that the ORR processes proceed by a direct four-electron pathway for the N-Q(valley) and the pyridinic sites while it proceeds by an indirect two-electron pathway via hydrogen peroxide at the N-Q(center) sites. Our study gives both insights on the mechanism of ORR on different nitrogen functionalities in nitrogen-doped carbon nanostructures and it proposes how to treat samples to maximize the catalytic efficiency of such samples.

Straightforward synthesis of nitrogen-doped carbon nanotubes as highly active bifunctional electrocatalysts for full water splitting

“…The N1s spectra of the pyrolyzed NMWNT material is deconvoluted to the three main peaks. The first two peaks located at $400.7 and $398.4 eV are attributed to the graphitic and pyridinic nitrogen, respectively, while the third peak at $402.6 eV corresponds to the protonized imine nitrogen [54][55][56][57]. All spectra require an additional wide component around 406 eV representing various oxides and satellite structures.…”

Highly efficient cathode catalyst layer based on nitrogen-doped carbon nanotubes for the alkaline direct methanol fuel cell

“…graphene [10,[23][24][25][26][27][28][29][30][31][32][33], carbon nanotubes [11,20,21,[34][35][36][37][38][39][40][41][42][43], carbon nanofibers [12,[44][45][46][47], mesoporous carbon [15,22], graphitic carbon [48,49], carbon spheres [19,[50][51][52][53], carbon nanocages 4 [54], flower-like carbon [55], carbon aerogel [56,57], vesicular carbon [58], nanodiamonds [59]) relatively few have been tested in actual fuel cell conditions [58,[60][61][62]…”

Nitrogen-doped graphene with enhanced oxygen reduction activity produced by pyrolysis of graphene functionalized with imidazole derivatives

“…These results are in agreement with the reported literature that pyrrolic-N sites transform to pyridinic-N at moderated temperatures (400-600 °C) via a dynamic surface arrangement. Further, at higher temperatures (T > 600 °C) pyridinic-N sites transform into graphitic-N through a ring condensation mechanism, indicating that graphitic-N is the most thermally stable structure [55,124,126]. The thermal transformation of nitrogen functionalities for PANI treated samples is schematically shown in Figure 5.8.…”

“…With increasing annealing temperature from 600 °C to 900 °C, the electrocatalytic activity was enhanced as indicated by the higher current density, more positive onset and peak potentials (Table 5.2). It can be due to the higher graphitization degree of the N-doped layer (lower D/G ratio) and larger contribution of graphitic-N and pyridinic-N sites (from XPS data), which promote the electron transfer and cause a positive shift of the reduction potential [126]. The enhanced electrocatalytic activity of N-FWCNTs was quantitatively evidenced by RDE experiments ( Figure 5.4 c&d) and the electron transfer number calculated from the K-L equation.…”