NiOOH Exfoliation-free Nickel Octahedra as Highly Active and Durable Electrocatalysts toward the Oxygen Evolution Reaction in an Alkaline Electrolyte.

A layered β-NiOOH crystal with under-coordinated facets is an active and economically viable non-noble catalyst for the oxygen evolution reaction (OER) in alkaline electrolytes. However, it is extremely difficult to enclose the β-NiOOH crystal with under-coordinated facets due to its inevitable crystal transformation to γ-NiOOH, resulting in the exfoliation of the catalytic surfaces. Herein, we demonstrate {111}-faceted Ni octahedra as the parent substrates which surfaces are easily transformed to catalytically active β-NiOOH during the alkaline OER. Electron microscopic measurements demonstrate that the horizontally stacked β-NiOOH on the surfaces of Ni octahedra has resistance to further oxidation to γ-NiOOH. By contrast, significant crystal transformation and thus the exfoliation of the γ-NiOOH sheets can be observed on the surfaces of Ni cubes and rhombic dodecahedra (RDs). Electrocatalytic measurements exhibit that the β-NiOOH formed on Ni octahedra performs highly enhanced OER durability compared to the Ni cubes, Ni RDs, and the state-of-the-art Ir/C catalysts.