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. 2019 Nov 15;10:5208. doi: 10.1038/s41467-019-13061-0

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© The Author(s) 2019

Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. The images or other third party material in this article are included in the article’s Creative Commons license, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons license and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this license, visit http://creativecommons.org/licenses/by/4.0/.

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Fig. 2 — Accumulated (+) and (++) species for each electrocatalyst. a Normalised ΔO.D. spectra (to 1 at peak ΔO.D.) for the first (0/+), precatalytic, oxidation of FeOOH (orange), FeOOHNiOOH (green) and Ni(Fe)OOH (grey), obtained from the difference between the spectra at open circuit potential (OCP) after and before activation. b Normalised ΔO.D. spectra for the (+/++) oxidation wave correlated with water oxidation for FeOOH (orange), FeOOHNiOOH (green) and Ni(Fe)OOH (grey), obtained by subtracting spectra at OCP after activation from the spectra at 1 mA cm⁻² of current density. c Concentrations of oxidised MOOH(+) states generated in the activation process (black) and of MOOH(++) states generated at a current density of 1 mA cm⁻² (red) for each electrocatalyst. Inset: schematic representation of the corresponding densities of states (DOS) for these electrocatalysts. d Accumulated concentrations of MOOH(++) states vs. the applied overpotential (empty diamonds, solid lines). For both graphs FeOOH (orange), FeOOHNiOOH (green) and Ni(Fe)OOH (grey)