Active Phase on SrCo<sub>1–<i>x</i></sub>Fe<sub><i>x</i></sub>O<sub>3−δ</sub> (0 ≤ <i>x</i> ≤ 0.5) Perovskite for Water Oxidation: Reconstructed Surface versus Remaining Bulk
Haiyan Li, Yubo Chen, Jingjie Ge, Xianhu Liu, Adrian C. Fisher, Matthew Sherburne, Joel W. Ager, Zhichuan J. Xu
Abstract
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.