The oxygen reduction reaction (ORR) is one of the most important electrochemical reactions, used in metal-air type batteries and different types of fuel cells. The traditionally used catalyst, platinum, is expensive and scarcely available and thus many new catalysts have emerged, among which are heteroatom-doped carbon materials. Various carbon nanostructures have been widely used as base materials for heteroatom doping due to their high electrical conductivity, porosity and stability in electrochemical conditions. Owing to their porous structure, reactants and products are easily transferred to and from the active sites. Among these materials, carbide-derived carbon (CDC), which is a carbon material produced by removing metal atoms from a carbide lattice, stands out as an exceptional way to gain a base material with tunable properties. The specific surface area of CDC materials is typically in the range of 1000–2000 m² g⁻¹, the pore size is easily tunable via selection of different starting carbides and synthesis temperature with reproducible results in large scales. Previously, we have studied ORR electrocatalysis on undoped CDC materials in alkaline media. These materials catalyzed a 2e^‒ reduction of O₂.¹

In this work, we demonstrate a pyrolysis method to synthesize nitrogen-doped carbide-derived carbon catalysts using titanium carbide as a base material and dicyandiamide (DCDA) as a nitrogen source. The CDC materials are obtained using chlorination to remove the titanium and then pyrolyzed along with DCDA at 800 °C for 2 h in a tube furnace to substitute nitrogen into the carbon lattice.²

The physical characteristics of the catalyst materials were studied using N₂ physisorption, X-ray diffraction, Raman spectroscopy, X-ray photoelectron spectroscopy and scanning electron microscopy, which revealed that the materials had been successfully nitrogen doped, had a very high specific surface area and were highly amorphous. SEM revealed highly porous materials consisting of particles of various sizes (Figure 1a). These catalysts were then tested using the rotating disk electrode (RDE) method in alkaline solution toward the ORR. The mediocre activity of the CDC materials was increased by the nitrogen-doping procedure, which further increased the already high specific surface area of the materials and produced nitrogen-containing groups on the surface of the catalysts. The catalysts produced via the nitrogen doping of carbide-derived carbon are superior to the catalyst synthesized by chlorinating titanium carbonitride, but still not as active as 20% Pt/C (Figure 1b). However, the process for CDC synthesis is already used commercially and adding nitrogen doping to it would be just one more step. The N-doped CDC catalysts also showed excellent stability over 1000 potential cycles, which is an important property for fuel cell applications.

Overall, the nitrogen-doped CDC catalysts are promising cathode materials for alkaline membrane fuel cell as they are stable and highly active toward ORR. Work is in progress to test these materials in fuel cell conditions.

References

1. I. Kruusenberg, J. Leis, M. Arulepp, and K. Tammeveski, J. Solid State Electrochem., 14, 1269 (2010).

2. S. Ratso, I. Kruusenberg, M. Käärik, M. Kook, R. Saar, M. Pärs, J. Leis, and K. Tammeveski, Carbon, 113, 159 (2017).