Defective Nitrogen-Doped Graphene Foam: Clarifying the Role of Nitrogen in Non-Precious ORR Catalysts

The energy industry is set to be revolutionized by polymer electrolyte membrane fuel cells (PEMFCs). Commercial PEMFC stationary units for industrial / home use are already on sale in Japan, and PEMFC vehicles will be commercialized in various countries in 2015. Platinum-decorated carbon black electrocatalyst powder is a crucial aspect of PEMFC technology. However, Pt is expensive, and limits PEMFC durability due to aggregation, dissolution, ripening, and carbon corrosion. Pt-free catalysts are therefore desirable for next-generation fuel cells.

                Non-precious, Pt-free catalysts for the electrochemical oxygen reduction reaction (ORR) in acid media have been the subject of intense research for the past few decades. One of the most popular materials in this field are pyrolysed mixtures of Fe/C/N-containing precursors, subjected to various heating, acid washing, and milling procedures. The mechanism for the ORR in these catalysts is not well understood, and is still debated. There are two main camps; those who see Fe as part of the catalytic center; and those who see Fe as a generator of active sites, whilst nitrogen would play the active role in the ORR. The history of this field is well-summarized in a recent review.¹

                Resolution of this debate is hampered by the complicated chemical structure in these materials; the possible combinations of Fe, N, and C atoms that could be ORR active are many. Therefore our approach is to simplify the system by removing Fe from the equation, and clarifying the fundamental catalytic activity of nitrogen-doped carbons.

                Initially we synthesized carbon nitride, and pyrolysed carbon nitrides supported on carbon black and carbon nanotubes. These materials had some inherent ORR activity, but Fe contamination was an issue,^2,3,4 as with most materials approaches to this problem. We therefore developed synthesis of a truly metal-free nitrogen-doped graphene foam (GF_N), which showed high inherent 4-electron oxygen reduction in acid medium.^5,6

                Here we synthesize GF_N by combustion of nitrogen-containing sodium alkoxide, followed by washing, 1000˚C heat treatment in N₂ and H₂, and graphitization at 1400˚C. This is a 3D carbon with micron-scale pores encapsulated by thin defective graphene walls with a thickness of around 2 nm, with a surface area of > 700 m²/g and a nitrogen content of around 0.5 at.%. (Fig. 1). The material was confirmed to be Fe-free by ICP analysis.

                Linear sweep voltammograms (LSVs) of GF_N (Fig. 2) show relatively high activity for an entrirely metal free catalyst (maxinum current density, -4 mA/cm², mass activity (at 0.6 V), 2.79 A/g_cat). The onset potential (at -10 µA/cm²) is around 0.85 V, similar to many Fe-containing catalysts. The electron transfer cooeficient (calculated from the ring electrode) is 3.6, incdicating majority 4-electron transfer. We conclude that Fe-free active sites may contribute significantly to the 4-electron ORR in Fe/C/N-based catalysts (although this does not mean that Fe-centers have no activity).

Figure 1. (a) Schematic showing some PEMFC degradation mechanisms; (b) nitrogen-doped graphene schematic; (c) SEM image of GF_N; (d)  XPS N1s spectrum; linear sweep voltammograms for GF_Nin acid, at various rotation speed; (e) disc electrode, (f) ring electrode. 

                The International Institute for Carbon-Neutral Energy Research (WPI-I2CNER) is supported by the World Premier International Research Center Initiative (WPI), MEXT, Japan.

1. J-. P. Dodelet, Electrocatalysis in Fuel Cells, Springer (2013):271

2. Lyth, S. M., et al.  J. Phys. Chem. C113.47 (2009): 20148

3. Lyth, S. M., et al. J. Electrochem. Soc. 158.2 (2011): B194

4. Lyth, S. M., et al. J. Nanosci. & Nanotech. 12.6 (2012): 4887

5. Lyth, S. M., et al. e-J. Surf. Sci. & Nanotech. 10 (2012): 29

6. Liu, J, et al. J. Electrochem. Soc. 161.4 (2014):F544-F550