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

NiOx, 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 NiOx. Further compounding this ambiguity is the recent finding that much of the previous experimental work on NiOx may have been influenced by incidental doping. Here, we report a computational study of the two simplest members of the NiOx 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.