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Funder: National Research Foundation Singapore, Perovskite oxides based on earth-abundant transition metals have been extensively explored as promising oxygen evolution reaction (OER) catalysts in alkaline media. The (electro)chemically induced transformation of their initially crystalline surface into an amorphous state has been reported for a few highly active perovskite catalysts. However, little knowledge is available to distinguish the contribution of the amorphized surface from that of the remaining bulk toward the OER. In this work, we utilize the promoting effects of two types of Fe modification, i.e., bulk Fe dopant and Fe ions absorbed from the electrolyte, on the OER activity of SrCoO3-δ model perovskite to identify the active phase. Transmission electron microscopy and X-ray photoelectron spectroscopy confirmed the surface amorphization of SrCoO3-δ as well as SrCo0.8Fe0.2O3-δ after potential cycling in Fe-free KOH solution. By further cycling in Fe-spiked electrolyte, Fe was incorporated into the amorphized surface of SrCoO3-δ (SrCoO3-δ + Fe3+), yielding approximately sixfold increase in activity. Despite the difference in remaining perovskites, SrCoO3-δ + Fe3+ and SrCo0.8Fe0.2O3-δ exhibited remarkably similar activity. These results reflect that the in situ developed surface species are directly responsible for the measured OER activity, whereas the remaining bulk phases have little impact.

Additional file 1. File containing the following additional information discussed in the main text: growth curves, chlorophyll a and pH profiles, sample raw confocal images, sample processed confocal image, cell length histograms, theoretical iron concentration profile, correlations between ([Fe3+]b − [Fe3+]s) and morphological parameters and iron concentration, and a comparison of Coulter counter volume measurements and rod-shaped model evaluation of volume.

A good understanding of the correlation between electronic properties and catalytic performance is vital to the rational design of active oxygen evolution reaction (OER) catalysts. Here, a volcano-shaped correlation between the OER activity and the Cr substitution amount was found for spinel oxides ZnFe2-xCrxO4 (x = 0-2), in which Zn2+ resides in tetrahedral (Td) sites while Fe3+ and Cr3+ occupy octahedral (Oh) sites. Such a relationship probably is because Cr substitution tunes the eg occupancy of Oh-site transition metals (TMOh) via the Oh-Oh superexchange effect. Density functional calculations further revealed the hybridization degree between the TMOh 3d and the O 2p states, and a volcano-shaped trend was also found in the variation of TMOh 3d-O 2p hybridization with the amount of Cr substitution. The good correlation between the OER activity and the hybridization highlights the important role of metal-oxygen hybridization in determining the OER activity of these spinel oxides.