The incorporation of Fe dramatically increases the catalytic activity of Ni hydroxides. This enhancement is so significant that NiFe (oxy)hydroxides are among the most active electrocatalysts for the oxygen evolution reaction (OER) in alkaline electrolytes. However, even the most active NiFe (oxy)hydroxide catalyst still shows a considerable overpotential for the OER. Understanding the nature of the catalytic active sites and the catalytic mechanism are key challenges to develop better OER electrocatalysts. In this contribution, atomic-scale details of the catalytic active phase will be presented, showing that NiFe layered double hydroxides (LDHs), the crystalline phase of NiFe oxyhydroxides, are oxidized under applied anodic potentials from as-prepared α-phases to activated γ-phases. 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, as well as for ß-Ni(OH)2. Structural similarities of the catalytically active phases will be highlighted. Finally, the results of our theoretical reactivity analysis will be presented and an OER volcano for Ni-based (oxy)hyxdroxide catalysts proposed.

Catalytic active phase of Ni-based (oxy)hydroxide electrocatalysts for the oxygen evolution reaction

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