Transition Metal and Nitrogen Co-Doped Carbide-Derived Carbon Catalysts for Oxygen Reduction Reaction in Alkaline Direct Methanol Fuel Cell
Authors: Ratso Sander , Kruusenberg Ivar , Käärik Maike , Kook Mati , Saar Rando , Kanninen Petri , Kallio Tanja , Leis Jaan , Tammeveski Kaido 
Published: February 2020

Finding viable alternative catalysts for the oxygen reduction reaction (ORR) is a key issue in modern electrocatalysis. Traditional Pt/C catalysts are not viable in the long term and thus much effort has been focused on non-precious metal catalysts (NPMCs). Metal-nitrogen doped carbon materials (MNCs) have emerged as the main candidate for replacing the platinum-based catalysts. Among these materials, carbide-derived carbon (CDC), which is a type of carbon materials produced by removing the metal atoms from a carbide lattice, stand out as exceptional candidates due to easily variable porosity. The specific surface areas of CDC materials can reach around 1000-2000 m2 g−1 and the textural characteristics can be tuned by selection of carbides and synthesis conditions, with reproducible large-scale syntheses already shown.1

In this work, we propose a method to introduce nitrogen and metals into a carbide derived carbon material with little loss of porosity using dicyandiamide (DCDA) and either cobalt or iron salts (Figure 1b). The CDC is derived from titanium carbide and then doped by pyrolyzing a mixture of the CDC, DCDA and either CoCl2 or FeCl3. The textural properties of the catalysts are probed with N2 physisorption, the morphology with scanning electron microscopy, the surface elemental composition with X-ray photoelectron spectroscopy (XPS) and the structure with Raman spectroscopy. The catalysts‘ activity towards ORR along with the stability during 1000 cycles and methanol tolerance in 0.1 M KOH is assessed using the rotating disk electrode (RDE) method and compared to that of commercial Pt/C catalysts (Figure 1a). The catalysts are also utilized in a alkaline direct methanol fuel cell (ADMFC)2 and an H2/O2 anion exchange membrane fuel cell (AEMFC).
The catalysts are shown to retain their microporous structure after the pyrolysis with successful doping of up to 5.3 at.% of nitrogen into the surface layer of the materials as determined by XPS. The catalysts show excellent activity in 0.1 M KOH rivaling that of commercial platinum-based catalysts and are very stable during 1000 potential cycles, with the iron-based catalysts somewhat more active. Up to 3 M methanol concentration also has very minimal effect on the activity of the catalysts. In an ADMFC using the Fuma-Tech FAA3 membrane the MNC catalysts show better performance than Pt/C (Figure 1c) and in an AEMFC with the Tokuyama A201 membrane as the polymer electrolyte they also rival the Pt/C catalyst.
Overall, we show that transition metal and nitrogen-doped CDCs present a viable alternative to commercial Pt/C catalysts for oxygen reduction in alkaline conditions and alkaline membrane fuel cells.

References

1. S. Ratso, I. Kruusenberg, M. Käärik, M. Kook, R. Saar, M. Pärs, J. Leis and K. Tammeveski, Carbon, 113, 159–169 (2017).
2. S. Ratso, I. Kruusenberg, M. Käärik, M. Kook, R. Saar, P. Kanninen, T. Kallio, J. Leis and K. Tammeveski, Appl. Catal. B Environ., 219, 276–286 (2017).

Figure 1. Comparison of ORR polarization curves in O2-saturated 0.1 M KOH (a), pore size distributions (b) and ADMFC power densities (c) for the transition metal doped materials and commercial Pt/C.

Figure 1