Pyrolysis Pressure Dependence of MNC Catalysts for Oxygen Reduction

Reducing or replacing precious-metal catalyst content, without loss of performance, is the focus of much current electrocatalysis research in proton exchange membrane fuel cells (PEMFCs).¹ One particular approach is using non-precious transition metals (such as iron, cobalt, copper) combined with nitrogen doped carbon. These transition metal-nitrogen-carbon (MNC) electrocatalysts show promising activity for ORR in both acid and alkaline environments.

In this study, MNC catalysts were synthesized by pyrolyzing a mixture of metal salt (iron acetate), nitrogen precursor (melamine) and high surface area carbon (Ketjenblack) in a closed reusable vessel reactor. This approach was designed to increase the density of nitrogen-based active sites because of the improved equilibrium conditions that occur at high pressures.^2-4 Here, we studied the effects of loading (mass/volume) of precursor materials in the pyrolysis reactor, which directly impacts pyrolysis pressure. For these studies, iron and nitrogen content were fixed at 1.2 wt% and 25 wt% respectively. As shown in Figure 1, higher loadings increased gas phase pressure and improved ORR activity. This trend suggests that the limiting reaction may be an adsorption process driven via high partial pressure of volatile intermediates from the nitrogen precursor. In this talk, possible mechanism of active site formation will be discussed. Additionally, the effects of different nitrogen (such as ammonium carbamate and bipyridine) and metal precursors (such as Co and Mn) on ORR activity will be presented. In addition to traditional rotating disc electrode studies, square wave-voltammetry technique will be used to probe metal oxidation-reduction couples that provides an estimation of a number of metal-centered complexes in MNC catalysts. In order to evaluate MNC catalysts for real-world performance, fuel cell membrane electrode assemblies (MEA) of 5- 25 cm² electrode areas in H₂/air and H₂/O₂will also be presented.

 Acknowledgement

We gratefully acknowledge financial support from the U.S. Department of Energy (DOE), under Non-PGM Catalyst development effort lead by Northeastern University (Prof. Sanjeev Mukerjee, P.I.).

References

1.         M. K. Debe, "Electrocatalyst approaches and challenges for automotive fuel cells," Nature, 486(7401), 43-51 (2012). doi:10.1038/nature11115

2.         R. Kothandaraman, V. Nallathambi, K. Artyushkova and S. C. Barton, "Non-precious oxygen reduction catalysts prepared by high-pressure pyrolysis for low-temperature fuel cells," Applied Catalysis B: Environmental, 92(1–2), 209-216 (2009). doi: 10.1016/j.apcatb.2009.07.005

3.         V. Nallathambi, N. Leonard, R. Kothandaraman and S. C. Barton, "Nitrogen Precursor Effects in Iron-Nitrogen-Carbon Oxygen Reduction Catalysts," Electrochemical and Solid-State Letters, 14(6), B55-B58 (2011). doi:10.1149/1.3566065

4.         S. Ganesan, N. Leonard and S. C. Barton, "Impact of transition metal on nitrogen retention and activity of iron-nitrogen-carbon oxygen reduction catalysts," Physical Chemistry Chemical Physics, 16(10), 4576-4585 (2014). doi:10.1039/c3cp54751e