Mosaic Texture and Double c‑Axis Periodicity of β‑NiOOH: Insights from First-Principles and Genetic Algorithm Calculations

Fe-doped NiO_x has recently emerged as a promising anode material for the oxygen evolution reaction, but the origin of the high activity is still unclear, due largely to the structural uncertainty of the active phase of NiO_x. Here, we report a theoretical study of the structure of β-NiOOH, one of the active components of NiO_x. Using a genetic algorithm search of crystal structures combined with dispersion-corrected hybrid density functional theory calculations, we identify two groups of favorable structures: (i) layered structures with alternate Ni­(OH)₂ and NiO₂ layers, consistent with the doubling of the c axis observed in high resolution transmission electron microscopy (TEM) measurements, and (ii) tunnel structures isostructural with MnO₂ polymorphs, which can provide a rationale for the mosaic textures observed in TEM. Analysis of the Ni ions oxidation state further indicates a disproportionation of half of the Ni³⁺ cations to Ni²⁺/Ni⁴⁺ pairs. Hybrid density functionals are found essential for a correct description of the electronic structure of β-NiOOH.