Understanding the N-Doping Process of Graphene to Optimize Its N Species Proportion

Current global challenges, such as alternative clean energy sources or environmental remediation issues, have turned their attention into electrochemical reactions are important due to their electrical and thermal properties, but their chemical stability represent a task to overcome. Recent advances have highlighted nitrogen-doped carbonaceous materials as an alternative to metallic electrocatalysts, owing to their earth-abundant availability and improved resistance to poisoning effects by side products.<br/>Nitrogen doping introduces modifications to the carbon network causing redistribution of charge locally, creating active sites that facilitate various electrochemical reactions. Nitrogen atoms can be incorporated into the sp² carbon network in several configurations, such as pyridinic, graphitic, pyrrolic nitrogen, or even as oxidized nitrogen species. Each configuration contributes uniquely to the catalytic properties of the material giving place to different pathway selectivity of the electrochemical reactions. For instance, pyridinic nitrogen presents selectivity for the 4-electron pathway for the oxygen reduction reactions (ORR), while graphitic nitrogen yields selectivity for the 2-electron ORR pathway [1].<br/>In this study, we investigate in detail the nitrogen doping of graphene through post-synthesis methods by heat treatments under an inert atmosphere, using graphene oxide as a precursor. Understanding the nitrogen incorporation process by heat treatments represents a key tool to optimize the Nitrogen species proportion incorporated and aiding on its selectivity as electrocatalyst. We monitored the physicochemical and structural properties of the obtained materials using a range of techniques including X-ray photoelectron spectroscopy (XPS), thermogravimetric analysis (TGA), Raman spectroscopy, X-ray diffraction (XRD), optical microscopy, and atomic force microscopy (AFM). This was analyzed together with DFT calculations to elucidate and propose a N doping process mechanism.<br/>The results indicate that the doping process can be tailored to optimize the Nitrogen species proportion and therefore the materials performance for specific electrocatalytic applications. It is emphasized the potential of nitrogen-doped graphene for applications requiring electrocatalytic activity and stability. This makes nitrogen-doped graphene a highly attractive material for the next generation of Non-Platinum Group electrocatalysts.<br/>[1] Fernandez Escamilla, H. N., Guerrero Sanchez, J., Contreras, E., Ruiz Marizcal, J. M., Alonso Nunez, G., Contreras, O. E., ... & Takeuchi, N. (2021). Understanding the selectivity of the oxygen reduction reaction at the atomistic level on nitrogen doped graphitic carbon materials. Advanced Energy Materials, 11(3), 2002459.<br/>Acknowledgements:<br/>We thank financial support from the DGAPA-UNAM through PAPIIT project IN111223 and IN105722 and UI/UNAM System Joint Research Collaboration program project. Calculations were performed in the DGTIC-UNAM Supercomputing Center projects LANCADUNAM-DGTIC-051 and LANCAD-UNAM-DGTIC-382. We thank Francisco Ruíz, Eduardo Murillo, David Dominguez, Eloísa Aparicio, Israel Gradilla, Jesús Díaz and Jaime Mendoza for technical support and all the AG&P groupmates for fruitful discussions.