Preparation of Metal Nitrogen Carbon Electrocatalysts By High Pressure Pyrolysis for Oxygen Reduction Reaction in Acid and Alkaline Media

Due to their cost and scarcity, finding alternatives to platinum and platinum based precious metal electrocatalysts for oxygen reduction reaction (ORR) has been attracted by many researchers in recent years.¹ Non-precious metal based catalysts supported on nitrogen-doped carbon, namely MNC catalysts, have shown reasonable activity and stability in acidic and alkaline media. Among them, Fe based MNC catalysts exhibited highest ORR activity, to date.^2,3

In this study, we prepared MNC catalysts by pyrolyzing a mixture of metal salt (iron, cobalt and manganese precursors), nitrogen precursor (melamine, ammonium carbamate and bipyridine) and high surface area carbon (Ketjen Black) in a stainless steel closed reactor with an attached analog pressure gauge. We showed that high pressure pyrolysis (HPP) approach was used to increase the density of nitrogen-based active sites because of the improved equilibrium conditions that occur at high pressures.^4-6With HPP method, we obtained up to ~ 400 psi pressure with a mass loading of 3.5 g precursor.

Figure 1 shows a representative ORR performance of a Fe-N-C catalyst in acid and alkaline media. Polarization curves obtained at room-temperature in oxygen saturated 0.5 M H₂SO₄and 0.1 M KOH with a 1200 rpm at a 0.5 mVs^1- scan rate. For the synthesis of MNC catalyst, iron and nitrogen content were fixed at 1.2 wt% and 25 wt% respectively. It is obvious that the Fe-N-C exhibits a higher ORR performance in alkaline. 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. Additionally, the effects of the nature and composition of different nitrogen and metal precursors (cobalt and manganese) and carbon supports (carbon nanofibers and mesoporous carbon) on ORR activity will be presented. In order to evaluate MNC catalysts for real-world performance, fuel cell membrane electrode assemblies (MEA) of 5 cm²electrode areas for alkaline and proton exchange membrane fuel cells will also be presented.

References

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

2. G. Wu and P. Zelenay, "Nanostructured nonprecious metal catalysts for oxygen reduction reaction", Acc. Chem. Res., 46, 1878–1889 (2013). doi:10.1021/ar400011z.

3. A. Serov, K. Artyushkova and P. Atanassov, "Fe-N-C oxygen reduction fuel cell catalyst derived from carbendazim: Synthesis, structure, and reactivity", Adv. Energy Mater., 4, 1–7 (2014). doi:10.1002/aenm.201301735.

4. 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:http://dx.doi.org/10.1016/j.apcatb.2009.07.005

5. 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:10.1149/1.356606 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

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.).