Fig. 15.

a In-situ XANES spectra of Cu K-edge for Cu–N–C SAC catalyst in the cathode (top) and anode (bottom) at different potentials; The selecting XANES spectra at b 0.82 V, c 0.50 V, d 0.10 V, with theoretical Cu–N₄, Cu–N₃, Cu–N₂–OH as comparison, respectively, also with each model in each right bottom inset; e At U = 0 V, the Gibbs Free Energy plots of each ORR steps on Cu–N₄ and Cu–N₃ sites [138]; Reused with approval; Copyright 2021 American Chemical Society. f In-situ Mössbauer spectroscopy of the ⁵⁷Fe–N–C at various potentials and g the matching relative content of D1, D2, D3, D4 for FeN₄; The AIMD simulations for the average bond distance in FeN₄C₈ * 2OH at h 0 K, vacuum, i 298 K, electrolyte, electric field of 0.4 V Å⁻¹ [143]. Reused with approval; Copyright 2022 Elsevier