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

Abstract

Fe-doped NiOx 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 NiOx. Here, we report a theoretical study of the structure of β-NiOOH, one of the active components of NiOx. 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 Ni(3+) cations to Ni(2+)/Ni(4+) pairs. Hybrid density functionals are found essential for a correct description of the electronic structure of β-NiOOH.