Figure 4. Quantitative analysis of in situ structural characterization of Co₃O₄ films.

Structural coherence length of Co₃O₄ (a), shift of the Co X-ray absorption K-edge (b), change in EXAFS coordination number of di- and mono-μ-oxo bridges between Co ions (c) with respect to the as-prepared state. The state at +1.62 V corresponds to the catalytically active oxygen evolution state (cf. Fig. 2a and Supplementary Fig. 12). Electrode potentials in this particular experiment are not corrected for Ohmic losses but we note that Ohmic correction decreased the electrode potential at +1.62 V by less than ∼1 mV in the quasi-stationary experiment. The structural coherence length was calculated using the Scherrer equation from the integral breadth of the Co₃O₄(511) and (440) reflections65. A shape factor k of 0.89 was used. The error of the structural coherence. length was calculated from the estimated standard deviation of the fitted integral breadth. The likely error of the shift of the Co K edge was estimated to be ±0.1 eV. The coordination numbers were determined by simulations of k³-weighted EXAFS spectra. The error ranges of the EXAFS fit parameters were estimated from the covariance matrix of the fit and represent the 68% confidence intervals. Further details on data analysis are given in the caption of Supplementary Tables 1 and 2.