Fig. 3: In situ X-ray absorption spectroscopy in presence of alkali metal cations.

In situ XAS was performed at the Ni and Fe K-edges of the Ni₆₅Fe₃₅(OOH) catalyst in purified 0.1 M XOH electrolytes (X = Li⁺, Na⁺, K⁺, Rb⁺, Cs⁺). a Ni XANES at 1.1 V (solid curves) and at 1.48 V (dashed curves). The inset shows an enlargement of the pre-edge region and the arrow indicates the direction of change upon oxidizing potential. b Fe XANES at 1.1 V (solid curves) and at 1.48 V (dashed curves). The inset shows an enlargement of the pre-edge region with the direction of change indicated by the arrow. c The k³-weighted FT-EXAFS at the Ni K-edges. d The k³-weighted FT-EXAFS at the Fe K-edges. e Ni and Fe K-edge positions vs. the applied potential. The colored areas on top indicate the different regions (non-catalytic, oxidative “OX”, and “OER” catalytic). f Trend-plots of the Ni–O and Fe–O coordination distances at 1.48 V vs. the alkali metal cation (X) in the XOH electrolyte. Oxidation states are indicated in e and f with following colors: +2 (yellow), +3 (green), and +4 (purple). Error bars show the standard error from the overall number of measurements.