“…Besides direct observation through electron microscopy, spectroscopic methods such as X-ray photoelectron spectroscopy (XPS) are also used to analyze the binding conditions and valence states of elements. 86,87 The N 1s peak detected by XPS is always deconvoluted as pyridinic N, graphitic N, pyrrolic N, oxidized N as well as metal–N (Co–N x /Fe–N x ) to resolve the coordinated condition, 85,88,89 in which the metal–N ratio is generally used to evaluate the content of Fe/Co active sites qualitatively. Nevertheless, it is worth noting that XPS analysis cannot resolve the atomic dispersion and coordination environment of metal sites by itself and thus usually has to be combined with other techniques like X-ray absorption spectroscopy (XAS).…”