Enhancement in Oxygen Reduction Reaction Activity of Nitrogen-Doped Carbon Nanostructures in Acidic Media through Chloride-Ion Exposure

Nitrogen-doped carbon nanostructures (CNx) are promising cathode materials as catalysts for the oxygen reduction reaction (ORR) in polymer electrolyte membrane (PEM) fuel cells. Incorporation of chlorine into CNx catalysts using a facile methodology can lead to a significant improvement in the ORR activity in acidic media, as confirmed by electrochemical half-cell measurements. The chlorine-containing CNx catalyst (CNx-Cl) is synthesized by soaking CNx powder in 0.3 M HCl. The analysis of near-edge X-ray absorption fine structure spectra collected in the C K-edge region and Fourier-transform infrared spectra confirm the formation of C-Cl bonds in CNx-Cl. X-ray photoelectron spectroscopy (XPS) results reveal the presence of three distinct chlorine species in the CNx-Cl sample: (i) organic chlorine (C-Cl), (ii) anionic chloride in the positively charged environment of a pyridinium ring (N+Cl-), and (iii) physisorbed ionic chloride. Results from temperature-programmed desorption studies under inert atmosphere corroborate the conclusions from XPS depth profiling analysis. The improvement in ORR activity after exposure of the CNx catalyst to chloride anions can be attributed to the creation of C-Cl functionalities as additional active sites. The difference in the electronegativity of C and Cl atoms results in a net positive charge on adjacent carbon sites, leading to the side-on adsorption of oxygen molecules and breakage of the O-O bond during ORR.