Fig. 5: Size-dependent reversible adaptation of Co oxidation state and Co–O bond contraction to the OER. | Nature Energy

Fig. 5: Size-dependent reversible adaptation of Co oxidation state and Co–O bond contraction to the OER.

From: Size effects and active state formation of cobalt oxide nanoparticles during the oxygen evolution reaction

Fig. 5

a, Average Co oxidation state of the 1 nm (blue), 2 nm (brown), 4 nm (yellow), 6 nm (magenta) and 9 nm (green) CoOx(OH)y NPs calculated by the integral method28 from the operando XANES data shown in Fig. 3 at selected states. b, Reversible change in the charge δ (blue, left axis) and Co–O bond length (red, right axis; Supplementary Table 9) determined by EXAFS fitting during OER as compared with the state after OER. The size dependency of the changes in Co oxidation state and Co–O bond can be described by a parabola, with an extrapolation to 0.287 nm, indicated by the vertical dashed purple line, resembling the outermost surface layer with the thickness of a CoO6 building block. Error bars of the Co oxidation result from the accuracy of the linear calibration (Supplementary Fig. 11). The depicted error bars of the Co–O contraction reflect the standard errors calculated from the EXAFS fitting results. The NP heights reported are the mean of the normal distribution and the error the standard deviation.

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