Characterization of Complex Interactions at the Gas–Solid Interface with in Situ Spectroscopy: The Case of Nitrogen-Functionalized Carbon

Interactions at the gas–solid interface drive physicochemical processes in many energy and environmental applications; however, the challenges associated with characterization and development of these dynamic interactions in complex systems limit progress in developing effective materials. Therefore, structure–property–performance correlations greatly depend on the development of advanced techniques and analysis methods for the investigation of gas–solid interactions. In this work, adsorption behavior of O2 and humidified O2 on nitrogen-functionalized carbon (N–C) materials was investigated to provide a better understanding of the role of nitrogen species in the oxygen reduction reaction (ORR). N–C materials were produced by solvothermal synthesis and N-ion implantation, resulting in a set of materials with varied nitrogen amount and speciation in carbon matrices with different morphologies. Adsorption behavior of the N–C samples was characterized by in situ diffuse reflectance infrared Fourier-transform spectroscopy (DRIFTS) and ambient pressure X-ray photoelectron spectroscopy (AP-XPS) experiments. A new analysis method for the interpretation of AP-XPS data was developed, allowing both the determination of overall adsorption behavior of each N–C material and identification of which nitrogen species were responsible for adsorption. The complementary information provided by in situ DRIFTS and AP-XPS indicates that O2 adsorption primarily takes place on either electron-rich nitrogen species like pyridine, hydrogenated nitrogen species, or graphitic nitrogen. Adsorption of O2 and H2O occurs competitively on solvothermally prepared N–Cs, whereas adsorption of H2O and O2 occurs at different sites on N-ion implanted N–Cs, highlighting the importance of tuning the composition of N–C materials to promote the most efficient ORR pathway.