Benefits of Using Rapid Microwave Heating in the Synthesis of Metal-Free Carbon Electrocatalysts

Heat treatment constitutes a key synthesis step for nitrogen-doped carbons, which have been investigated for applications to numerous electro- and thermo-catalytic reactions. However, the conventional approach relying on bulk heating limits our ability to tailor catalyst properties due to the fast gasification of carbon-based materials in reactive atmospheres such as NH₃. In this paper, we report the advantages of utilizing microwave technology for improving the catalytic performance of nitrogen-doped carbon via selective and rapid heating. We synthesized two distinct sets of nitrogen-doped carbons from graphene oxide: one exhibiting micropores approximately 1 nm in width (HS) and another characterized by their scarcity of micropores (NS). Despite their comparable nitrogen contents, HS exhibits significantly higher activity than NS in the electrochemical oxygen reduction reaction (ORR) in an acidic electrolyte. These samples underwent further heat treatment in NH₃ or N₂ using a conventional tubular furnace or a single-mode microwave reactor apparatus. Our results demonstrate that microwave heating limits the gasification of carbon-based materials, which effectively permits heating up to an average temperature of 1400 °C in NH₃. Microwave heating of HS in NH₃ enhances its pore hydrophobicity while maintaining the microporosity, substantially improving the ORR activity. Conversely, microwave heating of HS in N₂ considerably diminishes its ORR activity, despite enhancing pore hydrophobicity and maintaining an adequate nitrogen species presence. These findings emphasize the crucial role played by the coexistence of carbon defects and specific nitrogen sites generated through heating in a NH₃ atmosphere within hydrophobic micropores for high ORR activity. In summary, our data highlight the distinct advantages of the high-temperature treatment of nitrogen-doped carbons in NH₃ using microwave heating. This approach enables us to adjust the reactive microenvironments, thereby improving catalytic performance without sacrificing crucial active sites within micropores for ORR catalysis.