Nitrogen-Doped Carbon Nanofibers for the Oxygen Reduction: The Role of Iron As Growth Catalyst
Authors: Buan Marthe Emelie Melandsø , Muthuswamy Navaneethan , Cognigni Andrea , Walmsley John Charles , Chen De , Rønning Magnus 
Published: February 2020

Nitrogen-doped carbon nanofibers (N-CNFs) prepared in the presence of Fe have shown promising activity for the oxygen reduction reaction (ORR), the cathode reaction in fuel cells 1–3. However, in acidic electrolyte the ORR-activities reported so far has not been as high as for Pt/C. This could be due to low active site density and poor accessibility of the active sites to the oxygen molecules. In addition, the nature of the active sites and the role of the transition metal used as carbon growth catalyst still remain unclear. Here, a systematic evaluation of the ORR on N-CNFs has been performed by tailoring the physicochemical properties of the N-CNFs using a catalytic chemical vapor deposition method. The role of iron has been further explored by post-treatment of the N-CNFs with nitric acid combined with careful analysis using XPS, EELS and XAS.

The N-CNFs were prepared by decomposing CO and NH3 over 20 wt% Fe supported on expanded graphite. By varying the synthesis conditions such as synthesis temperature and NH3-partial pressure the N-content, N-groups and microstructure of the N-CNFs could be tailored. Characterization of the elemental composition and N-groups present at the surface of the N-CNFs was done by XPS. The N-CNFs grown at 650⁰C were treated with concentrated nitric acid and annealed at 900⁰C in order to remove metallic Fe and oxygen functional groups respectively. The remaining iron was then characterized by EELS, and in-situ under reducing conditions using EXAFS/XANES. The electrocatalytic activity for the oxygen reduction was examined by performing linear sweep voltammetry in 0.5M H2SO4. Detection of H2O2formed during the ORR was enabled using a rotating ring disk electrode.

By increasing the CVD synthesis temperature from 650⁰C to 750⁰C the N-CNF yield increased while the N-content in the surface of the N-CNFs decreased from 4.8at% to 1.4at% respectively, see Fig. 1a. Furthermore, at 750⁰C the N-CNFs contained more quaternary nitrogen, while synthesis at lower temperatures favored the presence of pyridinic nitrogen groups. This could be related to the microstructure of the N-CNFs since the N-CNFs grown at lower temperatures showed a fishbone structure, while the N-CNFs grown at 750⁰C were similar to multiwalled CNTs and thus contained fewer edges. Furthermore, increasing the partial pressure of NH3resulted in N-CNFs with higher N-content and a higher percentage of pyridinic nitrogen incorporated in the N-CNFs. Testing of the N-CNFs for the oxygen reduction in acidic electrolyte, Fig. 1a, revealed that the N-CNFs with the highest nitrogen content were the most active oxygen reduction catalysts.

Harsh post treatment with nitric acid was used to remove Fe from the N-CNFs. TEM imaging after acid treatment and annealing, Fig. 1b, showed clean N-CNFs with no visible iron particles. However, EELS analysis (Fig. 1c-e) revealed a weak, but consistent and homogeneous Fe signal in the N-CNFs. In addition, the activity and selectivity for the oxygen reduction was preserved after acid treatmet. Further analysis with XAS showed that the iron remaining in the N-CNFs was not iron oxide or metallic iron, but rather iron coordinated to nitrogen or carbon.

In conclusion, the optimal CVD synthesis temperature for achieving an active ORR-catalyst based on N-CNFs was 650⁰C. Furthermore, a high NH3 partial pressure gave high nitrogen content in the surface of the N-CNFs and was beneficial for both the oxygen reduction activity and the selectivity towards H2O. The best N-CNFs catalyst contained 5.8 at% nitrogen and showed an ORR onset potential of 0.96V vs. RHE. Characterization of the post-treated N-CNFs with EELS and XAS showed that iron was incorporated into the N-CNF structure during synthesis. This suggests that Fe-Nxmoieties can be formed during N-CNF growth from Fe and could be the active sites for the ORR on N-CNFs.

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Figure 1