Structural Transformations in Ni (Oxy)Hydroxide Host Structures Under Operating Conditions for Oxygen Evolution Electrocatalysts

The incorporation of Fe into the brucite-like Ni(OH) 2 host structure increases significantly the catalytic activity for the oxygen evolution reaction (OER). 1 The resulting hydrotalcite-like crystalline structure is known as NiFe layered double hydroxide (LDH). However, even the most active NiFe LDH catalyst still shows a considerable overpotential for the OER. 2 Understanding the nature of the catalytic active sites and the catalytic mechanism in NiFe LDHs are key challenges to develop better OER electrocatalysts, as well as the fundamental understanding of the role of the Ni-based host structure. In this contribution, atomic-scale details of the catalytic active phase will be presented, showing that NiFe LDHs are oxidized under applied anodic potentials from as-prepared α-phases to activated γ-phases. 3 The interlayer and in-plane lattice parameters of the OER-active γ-phase were obtained by performing wide angle X-ray scattering (WAXS) measurements on NiFe LDH nanoplatelets during operating electrochemical conditions and were characterized by a contraction of about 8%. Operando WAXS was then performed for other selected catalysts belonging to the transition metal LDH family of materials. Structural similarities of the catalytically active phases will be highlighted. In addition, the structural transformations of β-Ni(OH) 2 will be discussed, as potential host structure for novel multi-element catalysts. Also for Ni(OH) 2 , the γ-phase is found to be the catalytically active phase. However, the structural transformations are more complex, involving multiple limiting phases. Their structural stability is presented and the discussion supported by density functional theory calculations. References L. Trotochaud, S. L. Young, J. K. Ranney and S. W. Boettcher, Journal of the American Chemical Society, 2014, 136: 6744-6753. F. Dionigi and P. Strasser, Advanced Energy Materials, 2016, 6 1600621. F. Dionigi, Z. Zeng, I. Sinev, T. Merzdorf, S. Deshpande, M. B. Lopez, S. Kunze, I. Zegkinoglou, H. Sarodnik, D. Fan, A. Bergmann, J. Drnec, J. Ferreira de Araujo, M. Gliech, D. Teschner, J. Zhu, W.-X. Li, J. Greeley, B. Roldan Cuenya and P. Strasser, Nat Commun, 2020, 11 2522.