Figure 15. The in-situ/operando ⁵⁷Fe Mössbauer spectra of NiFe_0.2-O_xH_y collected at (A) the open circuit voltage, (B) 1.37 V, (C) 1.42 V, (D) 1.47 V, and (E) 1.57 V (vs. RHE). (F) Ex-situ ⁵⁷Fe Mössbauer spectrum of NiFe_0.2-O_xH_y collected after OER. The unit of ⁵⁷Fe Mössbauer parameter of isomer shift (δ) is mm·s^-1 relative to standard α-Fe foil. (G) The current-time curves at different applied potentials obtained during the in-situ/operando measurements. (H) Cyclic voltammogram without i_R correction of NiFe_0.2-O_xH_y recorded during the in-situ/operando measurements. (I) The content of Fe⁴⁺ and corresponding electric current determined at different applied potentials^[66]. Copyright 2021. ELSEVIER B.V. Reproduced with permission. (color on line)

.