Revealing the Conducting Character of the β‑NiOOH Catalyst through Defect Chemistry

Nickel oxy-hydroxide (β-NiOOH) is one of the best used inorganic catalysts for water oxidation reaction and energy conversion. Despite the broad interest, establishing what type of vacancy defects is created and therefore whether this material is n-type or p-type has not yet been confirmed. In this article, we examine the defect chemistry of this ternary compound β-NiOOH using density functional theory with three different approaches: PBE + U as well as the nonlocal screened-exchange hybrid functionals PBE0 and HSE06. We have used thermodynamic laws with various materials as secondary materials to analytically determine the constraints that yield effective chemical potentials for calculating the vacancy formation energies at varying reservoir conditions. In addition, we have also examined the effect of the Fe doping on the vacancy formation energy of β-NiOOH. Neutral and charged vacancy formation energy results have shown that the vacancy formation is energetically most favorable for the O and OH vacancies. Therefore, we conclude that β-NiOOH is n-type under relevant reservoir conditions. These results based on defect chemistry analysis reveal a basic conductivity property for a most widely used material.