In-situ structure and catalytic mechanism of NiFe and CoFe layered double hydroxides during oxygen evolution
Nature Communications 11(1): May 2020 

Dionigi, Fabio; Zeng, Zhenhua; Sinev, Ilya; Merzdorf, Thomas; Deshpande, Siddharth; Lopez, Miguel Bernal; Kunze, Sebastian; Zegkinoglou, Ioannis; Sarodnik, Hannes; Fan, Dingxin; Bergmann, Arno; Drnec, Jakub; de Araujo, Jorge Ferreira; Gliech, Manuel; Teschner, Detre; Zhu, Jing; Li, Wei-Xue; Greeley, Jeffrey; Cuenya, Beatriz Roldan; Strasser, Peter


NiFe and CoFe (MFe) layered double hydroxides (LDHs) are among the most active electrocatalysts for the alkaline oxygen evolution reaction (OER). Herein, we combine electrochemical measurements, operando X-ray scattering and absorption spectroscopy, and density functional theory (DFT) calculations to elucidate the catalytically active phase, reaction center and the OER mechanism. We provide the first direct atomic-scale evidence that, under applied anodic potentials, MFe LDHs oxidize from as-prepared α-phases to activated γ-phases. The OER-active γ-phases are characterized by about 8% contraction of the lattice spacing and switching of the intercalated ions. DFT calculations reveal that the OER proceeds via a Mars van Krevelen mechanism. The flexible electronic structure of the surface Fe sites, and their synergy with nearest-neighbor M sites through formation of O-bridged Fe-M reaction centers, stabilize OER intermediates that are unfavorable on pure M-M centers and single Fe sites, fundamentally accounting for the high catalytic activity of MFe LDHs. 

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