The Influence of Ni(II) and Co(II) Adsorptions in the Anomalous Behavior of Co-Ni Alloys: Density Functional Theory and Experimental Studies

The anomalous behavior arising during plating of Co-Ni alloys has been extensively investigated, whence different qualitative proposals have been suggested to describe it, although most of them have been limited to capture underlying atomic interactions between substrate and electroactive species. This study undertakes a different approach to account such phenomenon based on density functional theory (DFT) calculations supported on experimental data. Alloys formed experimentally consistently present an anomalous behavior, except at the most cathodic current. XRD, XPS and voltammetry confirm the formation of solid solutions over alloy compositions from 40 to 90 wt% Co. SEM reveals that alloy morphology strongly depends on applied current density, which likewise affects Co content as output parameter. The effects of CoSO₄ and NiSO₄ (ion pairs) adsorptions on the anomalous behavior are explained using DFT. More favorable adsorption free energies of CoSO₄ are obtained on multiple alloy surfaces (different arrangements of 50-50 wt% Co-Ni) and pure metals in comparison with NiSO₄. Additionally, rich Co sites (substrate) enhance CoSO₄ adsorption, while the specific solvation of the cation significantly contributes to the adsorption strength of the ion pairs, indicating that Co(II) reduction energetically possesses a definite advantage in alloy formation. These theoretical findings provide strong evidence to explain the anomalous behavior of Co-Ni alloys through the NiSO₄ and CoSO₄ competitive adsorptions.