The difference between positions of pyrrolic and pyridinic N is also about 1 eV. Using C 1s as internal calibration standard may result in as large as 1 eV difference in calibration shift as compared to using Au 4f. 21–24 Moreover, the majority of XPS studies use internal carbon for charge calibration of the spectra, while C 1s may have maximum at various binding energies (BEs), from 284 to 285.6 eV, depending on relative amounts of carbides, graphitic, aliphatic and oxygenated types of carbon present.
The assignment of peaks and nitrogen coordination are not straightforward due to overlapping peaks that appear within a narrow energy window of 2.5 eV and the “full-width-half-maximum” (fwhm) for individual species is on the order of 1.2–1.5 eV. Identification of the chemical species from XPS spectra generally involves peak deconvolution through curve-fitting. XPS which heavily relies on use of reference spectra in accurate identification of species cannot address this issue directly as no reference compounds with Me–N 2 moieties are available. Me–N 4 centers and their specific role still remain unresolved. 1,3,5,6,9,13,15–20 Even though there is an agreement that Me–N x may serve as one of the possible active sites in ORR, the distribution of Me–N 2 vs. 1,2,5,9–15 X-ray Photoelectron Spectroscopy ( XPS) has been the main surface analysis method for determining the chemical environment and coordination of nitrogen and transition metal (Me) in the electrocatalyst. 1–8 The continued interest in pyrolyzed macrocycles ( porphyrins or phthalocyanines) or other nitrogen-containing electrocatalysts is largely motivated by the need to find cost-effective and efficient material solutions for replacing Pt in polymer electrolyte membrane ( PEM) fuel cells. The active site/sites of non-Platinum Group Metal (non-PGM) oxygen reduction reaction (ORR) electrocatalysts have been contentious for the past 50 years. This supports the understanding of such catalysts as vacancy-and-substitution defects in a graphene-like matrix. Our results validate the use of independent DFT predicted BE shifts for defect identification and constraining ambient pressure XPS observations for Me–N x moieties in pyrolyzed carbon based ORR electrocatalysts.