Thermodynamic evaluation of trace-amount transition-metal-ion doping in NiOOH films

In aqueous solution, NiOOH has a strong tendency to spontaneously incorporate various transition-metal ions. These impurities dramatically affect the properties of NiOOH and may have been present unknowingly in NiOOH systems throughout the past several decades. A better understanding of this tendency of NiOOH to spontaneously incorporate possible contaminants therefore is of great interest. In this work, we calculated the free energies of adsorption and incorporation of Fe³⁺, Co³⁺, and Mn⁴⁺ dopants in β-NiOOH using ab initio thermodynamics. We define the aqueous phase energies for these dopants using both calculated solid phase bulk energies and their corresponding experimental solubilities. We then demonstrate that only a weak driving force exists for adsorption of Fe³⁺ and Co³⁺ complexes on the surface of β-NiOOH and no tendency at all for the adsorption of Mn⁴⁺ aquo and hydroxo complexes. We predict a strong thermodynamic driving force for bulk incorporation of Fe³⁺ and Co³⁺, even at trace concentrations (ppb to ppm levels) under ambient and basic aqueous conditions, but no such tendency for Mn⁴⁺. These results provide a thermodynamic explanation for the scavenging and doping behavior of NiOOH observed experimentally and highlight the importance of careful control of the experimental conditions when studying this material.