z Electro, Physical & Theoretical Chemistry The Influence of Ni(II) and Co(II) Adsorptions in the Anomalous Behavior of Co-Ni Alloys: Density Functional Theory and Experimental Studies Jorge Vazquez-Arenas + ,* [a] Guadalupe Ramos-Sanchez + ,* [a] Rene H. Lara, [b] Issis Romero- Ibarra, [c] M. Eng. Francisco Almazan, [a] and Luis Lartundo-Rojas [d] 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) calcu- lations supported on experimental data. Alloys formed exper- imentally consistently present an anomalous behavior, except at the most cathodic current. XRD, XPS and voltammetry confirm the formation of solid solutions over alloy composi- tions 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 CoSO 4 and NiSO 4 (ion pairs) adsorptions on the anomalous behavior are explained using DFT. More favorable adsorption free energies of CoSO 4 are obtained on multiple alloy surfaces (different arrangements of 50-50 wt% Co-Ni) and pure metals in comparison with NiSO 4 . Additionally, rich Co sites (substrate) enhance CoSO 4 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 NiSO 4 and CoSO 4 competitive adsorptions. 1. Introduction Plating of Co–Ni alloys has been the motivation of multiple studies due to their excellent magnetic properties, hardness, light weight, wear, versatility, abrasion and corrosion resistance, and particularly its prominent catalytic activity. [1] This method is attractive due to its low cost, flexibility (e.g., deposition as single layer or multi-layer coatings on planar and non-planar substrates), efficiency, ease of high volume production, and composition modulation. [1d,2] As occurs with other iron-group alloys (e.g. Fe–Ni, Fe–Co), the co-deposition is classified as anomalous since the mole fraction of the less noble component (i.e. Co) produced in the alloy becomes higher than the [Co 2 + ]/([Co 2 + ] + [Ni 2 + ]) ratio in the solution. [3] This behavior has spurred a considerable amount of research, stemming different proposals to account for it: faster adsorption rate of the less noble component (e.g. cobalt hydroxide Co(OH) + ads ) in comparison with the more noble species (e.g. Ni(OH) + ads ), [4] enhanced surface coverage of the less noble Me(I) ads intermediate, [5] formation of intermediate adsorbed species (i.e. NiCo(III) ads ) which increase the cobalt content in the alloy and release Ni(II) species, [6] differences in electronic structures and labilities of Co(II) and Ni(II) species on the basis of ligand field theory, [7] and faster charge-transfer of single Co(II) reduction in comparison with Ni(II) reduction. [8] Additionally, it has been consistently reported that anomalous deposition arises regardless of the electrolyte composition and the presence of additives in the bath, [4b,7,9] although alloy composition varies. Based on these findings, there are strong evidences to consider that the rate-controlling step describing this anomaly needs to be evaluated at the atomistic scale, since it presumably relies on differences in electronic structures for Co and Ni atoms during reduction, rather than pH or solvent effects. Likewise, experimental and modelling investigations developed at the micro- and macroscopic scales have not been able to comprehensively determine (i.e. isolating) and quantify such process. This has resulted from multiple limitations to evaluate kinetics of electrochemical reactions (e.g. surface concentrations, inclusion of material properties), and the [a] Dr. J. Vazquez-Arenas, + Dr. G. Ramos-Sanchez, + M. E. F. Almazan Departamento de Química Universidad Autónoma Metropolitana Iztapalapa, C.P. 09340 MØxico, D.F., MØxico E-mail: jgva@xanum.uam.mx jorge_gva@hotmail.com gramossa@conacyt.mx [b] Dr. R. H. Lara Facultad de Ciencias Químicas Universidad Juµrez del Estado de Durango, Av. Veterinaria S/N Circuito Universitario, 34120, Durango, Dgo., MØxico [c] Dr. I. Romero-Ibarra Unidad Profesional Interdisciplinaria en Ingeniería y Tecnologías Avanza- das- Instituto PolitØcnico Nacional. Av. IPN No. 2580 Gustavo A. Madero, C.P. 07340, Ciudad de MØxico [d] Dr. L. Lartundo-Rojas Instituto PolitØcnico Nacional Centro de Nanociencias y Micro y Nanotecnologías UPALM, Zacatenco MØxico-D.F. 07738, MØxico [ + ] CONACYT Research Fellow Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/slct.201601957 Full Papers DOI: 10.1002/slct.201601957 1826 ChemistrySelect 2017, 2, 1826 – 1834  2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim