fig2
Figure 2. Hard XAS/XES for electrocatalysts characterization. (A and C) representative energy level diagrams depict the origin of the spectral features. The XAS involves the excitation of core-level electron to the unoccupied states or into the continuum, serving as a probe of the unoccupied orbitals of chemical species and local atomic structure. In contrast, the XES monitors the decay of core holes (fluorescent X-rays associated with electron from higher-lying orbitals decay to fill the core hole) and, therefore, probes the occupied valence states in an atom-specific projection; (B) in-situ Co K-edge XANES study on CoNC and CoNOC during oxygen reduction reaction operation. Comparative study reveals the stable and rigid in-plane embedded Co1Nx moiety and the flexible edge-hosted Co1N2 sites. The observed changes in the XANES spectra are primarily dictated by potential-driven structure evolution, whereas the adsorption of oxygenated intermediates exerts only a negligible effect. These figures are quoted with permission from Hu et al[27]; (D) comparison of in-situ Kβ mainline XES spectra recorded on DW21 catalyst in N2-saturated 0.5 M H2SO4 at OCV and 0.2 V vs. RHE, along with corresponding fit results. The inset shows a magnified view of the Kβ’ region; (E) the time course of applied bias in the in situ XES measurements, whereby each potential hold lasted 10 min; (F) Average spin states at each corresponding potential. These figures in