Nitrogen-Coordinated Iron-Carbon and Nitrogen-Doped Carbon-Encapsulated Iron Nanoparticle Electrocatalysts for the Oxygen Reduction Reaction

Abstract

The development of fuel cells is hampered by the sluggish oxygen reduction reaction (ORR), which is responsible for high overpotentials, even on Pt-based electrocatalysts. Therefore, it is mandatory the search for more active materials and, at the same time, composed by earth-abundant elements for attaining the widespread commercialization of the fuel cell systems. Non-noble metal electrocatalysts based on iron, nitrogen and carbon composites have gained considerable attention mainly due to recent discoveries of two different active structures: (i) nitrogen-coordinated iron-carbon materials (represented here by Fe-N-C), in which the active center contains the nitrogen-iron coordination and; (ii) nitrogen-doped carbon-encapsulated iron nanoparticles (Fe@N-C), in which the electrocatalyst is formed by iron nanoparticles protected by a graphitic shell of carbon layers, doped with nitrogen atoms [1,2]. In the present study, non-noble metal electrocatalysts composed by iron, nitrogen and carbon were synthesized by the pyrolysis of a mixture of Vulcan carbon, iron chloride and a nitrogen precursor in N2 atmosphere. The results showed that when the pyrolysis was conducted at 700 oC, the use of imidazole as the nitrogen precursor resulted in the formation of the encapsulated nanoparticle structure, as revealed by the presence of a peak at ca. 2.1 Å (metallic Fe nanoparticles) in the Fourier Transform (FT) of the EXAFS oscillations. On the other hand, when phenanthroline was used as the nitrogen precursor, a FT peak centered by ca. 1.5 Å was obtained, evidencing the formation of the nitrogen-iron coordinated structure. When the pyrolysis was conducted at 1050 oC, the resulting structure was the same for the case of imidazole, but a mixture of Fe@N-C and Fe-N-C was obtained for phenanthroline. The ORR polarization curves for the materials prepared at 700 oC showed much higher activity for the Fe-N-C structure, but with similar and low stability for both materials. The pyrolysis at 1050 oC also showed higher activity for the material prepared with phenanthroline, but the stability was considerably improved, with similar behavior for both electrocatalysts. An additional pyrolysis step at 950 oC in NH3 atmosphere resulted in an increased ORR activity and stability for both electrocatalysts, but still with much higher activity for the Fe-N-C-containing material. Interestingly, the EXAFS results showed that the prolonged NH3 treatment of the Fe@N-C structure conducted to the appearance of the FT peak at 1.5 Å (nitrogen-iron coordination), and the ORR polarization curves indicated further increase on its original activity and stability. These results evidenced that the increase in the overall nitrogen content of the electrocatalyst by using a efficient iron chelating agent (phenanthroline), or via NH3 treatment, improved the ORR activity due to the increase in the iron-nitrogen interaction for both Fe-N-C and Fe@N-C initial structures. (Further enhancement in the ORR activity may come from the increase in the nitrogen doping level of the graphitic matrix). The increase in the nitrogen content for both Fe-N-C and Fe@N-C structures may promote a more embraced carbon matrix and, so, it may avoid the demetallation phenomenon, increasing the stability during the ORR. [1] U.Tylus, Q. Jia, K. Strickland, N. Ramaswamy, A. Serov, P. Atanassov, S. Mukerjee, J. Phys. Chem. C 2014, 118, 8999−9008. [2] K. Strickland, E. Miner, Q. Jia, U. Tylus, N. Ramaswamy, W. Liang, M-T. Sougrati, F. Jaouen, S. Mukerjee, Nat. Commun. 6:7343 (2015) 1-7. Acknowledgements The authors gratefully acknowledge financial support from FAPESP (2013/16930-7, and 2016/13323-0) and CNPq.