Developing a non-precious metal catalyst (NPMC) to replace Pt on the cathode of polymer electrolyte fuel cells is currently one of the most important challenges in electrocatalysis.¹ In the recent years, incorporating transition metals, such as iron or cobalt into a nitrogen-doped carbon nanomaterial, thus forming active sites for the oxygen reduction reaction (ORR), has been shown to be a promising strategy for NPMC design. The debate on the exact active sites and the mechanism of the ORR in this type of catalysts is still ongoing, but many advances have been made by studying the effect of catalyst precursors (the sources for the carbon, nitrogen and transition metals) on the ORR activity of the final catalyst. Carbide-derived carbons (CDC) are carbon materials synthesized by removal of metals from metal carbides. By careful selection of synthesis conditions, the CDCs can be tuned to have specific pore size distributions and degrees of disorder. As a continuation of our previous work,^2,3 where we studied nitrogen and transition metal doped highly microporous CDCs (M-N-CDC), we have now incorporated multiwall carbon nanotubes (MWCNT) into the M-N-CDC catalyst to form a composite catalyst (M-N-comp).⁴

The catalysts were synthesized in a two-step pyrolysis procedure: first the CDC was ball-milled along with iron(II)acetate and 1,10-phenanthroline. This mixture was pyrolyzed and then MWCNTs along with dicyandiamide and another amount of iron(II)acetate were added. The surface morphology and elemental composition of the final catalysts were investigated with scanning electron microscopy, N₂ physisorption, inductively coupled plasma mass spectrometry and X-ray photoelectron spectroscopy along with rotating disk electrode (RDE) and single-cell anion-exchange membrane fuel cell (AEMFC) studies to give a thorough characterization of the physico-chemical properties of the catalysts and the relationships between them. The addition of MWCNTs was revealed to add mesoporosity as the MWCNTs filled the space in between the highly microporous CDC grains, thus increasing the ORR activity by providing more active sites and also facilitating mass transport. The ORR onset potential for the best catalyst, which had a CDC-to-CNT ratio of 2:1 in RDE mode was 0.99 V vs RHE in 0.1 M KOH and the maximum power density using this catalyst in a H₂/O₂ AEMFC with Tokuyama A201 anion exchange membrane was 120 mW cm^‒2. However, the catalyst with lowest CDC-to-CNT ratio (1:2), which had more negative onset potential in RDE testing, performed better at higher current densities in the AEMFC and reached a maximum power density of 160 mW cm^‒2, likely due to better mass transport in the thick catalyst layer. The best catalysts exceeded the activity of commercial Pt/C in both RDE and AEMFC testing, showing the excellent potential of CDC-based catalysts to replace costly Pt in AEMFC devices.

References

1. A. Sarapuu, E. Kibena-Põldsepp, M. Borghei, and K. Tammeveski, Electrocatalysis of Oxygen Reduction on Heteroatom-doped Nanocarbons and Transition Metal-Nitrogen-Carbon Catalysts for Alkaline Membrane Fuel Cells, J. Mater. Chem. A, 6, 776-804 (2018).
2. S. Ratso, I. Kruusenberg, M. Käärik, M. Kook, R. Saar, P. Kanninen, T. Kallio, J. Leis, and K. Tammeveski, Transition Metal-Nitrogen Co-doped Carbide-Derived Carbon Catalysts for Oxygen Reduction Reaction in Alkaline Direct Methanol Fuel Cell, Appl. Catal. B Environ., 219, 276–286 (2017).
3. S. Ratso, I. Kruusenberg, M. Käärik, M. Kook, L. Puust, R. Saar, J. Leis, and K. Tammeveski, Highly Efficient Transition Metal and Nitrogen Co-doped Carbide-Derived Carbon Electrocatalysts for Anion Exchange Membrane Fuel Cells, J. Power Sources, 375, 233–243 (2018).
4. S. Ratso, M. Käärik, M. Kook, P. Paiste, V. Kisand, S. Vlassov, J. Leis, and K. Tammeveski, Iron and Nitrogen Co-doped Carbide-Derived Carbon and Carbon Nanotube Composite Catalysts for Oxygen Reduction Reaction, ChemElectroChem (2018). doi:10.1002/celc.201800132.