Structural and Electronic Features of β-Ni(OH)₂ and β-NiOOH from First Principles

NiO_x, long studied for its use in nickel-based secondary batteries, has been the subject of much recent interest due to its efficacy as an oxygen evolution catalyst. Despite extensive study over more than a century, however, many outstanding questions remain surrounding both the structure and the activity of NiO_x. Further compounding this ambiguity is the recent finding that much of the previous experimental work on NiO_x may have been influenced by incidental doping. Here, we report a computational study of the two simplest members of the NiO_x family: β-Ni(OH)₂ and β-NiOOH. Using DFT+U calculations, we first identify a β-NiOOH structure with a staggered arrangement of intercalated protons that is more consistent with experimental crystal structures of β-NiOOH than previously proposed geometries. Next, by conducting a thorough study of various initial spin configurations of this β-NiOOH structure, we found that a low-spin d⁷ Ni³⁺ configuration is always favored, which suggests a Jahn-Teller distortion, rather than disproportionation, explains the different Ni-O bond distances found in experiment. G₀W₀ calculations performed on β-Ni(OH)₂ and β-NiOOH reveal electronic structures consistent with previous experimental results. Lastly, calculations of various low-index surface energies of both β-Ni(OH)₂ and β-NiOOH demonstrate that the (001) surface is the most thermodynamically stable surface, in keeping with numerous experimental results but in contrast to recent computational models.