Impact of Nitrogen Precursor’s N/C Ratio on ORR Activity in Fe/N/C Catalysts for PEFC Applications

Among the non-precious metal catalysts, metal-nitrogen doped carbon materials (M/N/C) have been considered a suitable candidates in recent decades due to their high activity in acid medium. Yeager and co-workers revealed that it is possible to avoid using expensive macrocycles by heat-treating, in an inert atmosphere, a cost-effective metal precursor and a nitrogen precursor impregnated on an amorphous carbon black [1]. Following this approach, many methods have been explored to prepare high active M/N/C catalysts.

In our approach, to synthesize the iron based M/N/C (Fe/N/C) catalyst, a nitrogen source, a high surface area carbon source (Ketjenblack® 600JD) and an iron precursor (iron acetate) are pyrolyzed together in a closed container (quartz ampoule). This method is designed to increase the residence time of evaporated and decomposed products on the carbon under pressure, thereby increasing the density of nitrogen-based catalytic sites.

In our previous nitrogen source optimization studies, the most active catalysts were obtained using melamine, having a N/C ratio of 2 [2]. This enhanced activity was attributed to the increase in bulk nitrogen content and hence increased density of active catalytic sites, without significant pore blockage via carbon deposition. In another study, it is found that the best ORR performance was obtained with 1.2 wt% nominal Fe with respect to the entire mixture [3]. As nitrogen is believed to be a part of the catalytic site, increasing the surface nitrogen density in the presence of metal could potentially increase active site density.

Based on results obtained from our previous studies, however, it is concluded that when increasing the N/C ratio of nitrogen precursor, the nitrogen retention and ORR activity increases. To test this, Fe/N/C catalysts were prepared nitrogen precursors of very high N/C ratio, namely, 3-Aminotriazole (N/C=2), 3, 5-Diaminotriazole (N/C=2.5) and 5-Aminotetrazole (N/C=5 and these results are compared with melamine (N/C=2).

Fig. 1 shows the comparative ORR performance curves for Fe/N/C catalysts prepared with various nitrogen sources in oxygen saturated 0.5M H₂SO₄. Among the nitrogen sauces, the Fe/N/C catalyst prepared with melamine shows enhanced ORR activity in relation to other azole precursors. However, there is only a marginal difference in performance was observed for Fe/N/C catalyst prepared triazole compounds.  Surprisingly, in the case of tetrazole, although it possesses the high nitrogen content (N/C=5) in relation to others, it shows extremely ORR low activity.  The results clearly indicate that, for nitrogen precursors of high N/C ratio, the pathway of decomposition and active site formation becomes critical. 

Acknowledgement

We gratefully acknowledge the financial support from the U.S. Department of Energy (EERE), under a Non PGM Catalyst development effort (Contract no EE 0000459) lead by Northeastern University

  References

1. S. Gupta, D. Tryk, I. Bae, W. Aldred, and E. Yeager, J. Appl. Electrochem., 19 (1989) 19.

2. V. Nallathambi, N. Leonard, R. Kothandaraman and S. Calabrese Barton, Electrochem. Solid-State Lett., 14 (2011) B55.

3. S. Ganesan, N. Leonard, and S. Calabrese Barton, Electrochem. Soc. Trans., 58 (2013) 1691.