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DFT analysis of interfacial processus occuring in the first steps of electrodeposition of nickel from chloride melt
Gwénola Lemaire, Pascal Hébant and Gérard S. Picard
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Laboratoire d'Electrochimie et de Chimie Analytique |
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Abstract
Because of their high electrical conductivity, molten salts are industrially used as electrolytes for obtaining most of reactive metals. Understanding the nature of solute substrate interactions is crucial in order to optimize the electrolytic production of metals at the industrial scale. In this work, we have studied the cathodic reduction of Ni(II) - dissolved in the LiCl-KCl eutectic melt - at a nickel cathode.
The structure of LiCl-KCl melts has been previously determined and applying DFT calculations performed with DMol and DGauss, we were able to determine (i) the nature of the Ni(II) solvated species (ii) the corresponding equilibrium chloride exchange reactions as well as (iii) the cation influence on these species and equilibria .
Geometrical considerations and orbital shapes of the Ni(II) entities allowed us to find the most likely species to approach the cathode : KNiCl3. The nickel cathode was simply modelized as a monoatomic planar surface composed of 36 Ni atoms. The interaction between the cathode and KNiCl3 complex has been then investigated. Electronic density contour maps show the role played by chlorine atoms in the Ni adsorption process.
Introduction
Molten salts are often used as reaction media in extractive metallurgy and as electrolytes for electrowinning and electrorefining of very reactive light and transition or refractory metals [1]. Some of them are obtained at the liquid state (e.g. lithium) while others are obtained at the solid state (e.g. nickel). The latter case is called electrodeposition.
The main steps of electrodeposition are described in Fig. 1. The electroactive species which is solvated in melt has to be adsorbed at the cathode surface before reduction occurs. Then the ad-atom can diffuse on the surface to reach growth centers.
Knowing the fundamental aspects of electrodeposition is crucial in order to be able to optimize industrial processes. Many experimental works have been done [2] and many mathematical model of electrodeposition have been proposed [3]. As experimental techniques do not allow the whole understanding of the reduction process, molecular modeling, then, appears to be a very useful tool in order to investigate how the metal is solvated in melt and what is the influence of the adsorbtion step on reaction mechanism and surface growth.
In this work, we focused on nickel electrodeposition from eutectic LiCl-KCl melt which is the most used molten salt in industrial processes [4]. In the first part of this work, we have studied bulk complexes of nickel, then we studied the influence of melt cation on these complexes and we determined the interaction between a modelized nickel cathode and an adsorbed complex.
Computational details
Calculations were made with DMol 2.36 & 95.0 [5, 6] from Biosym Technologies/Molecular Simulations Inc. on a Silicon Graphics Iris Indigo R4400. This program uses the DFT technique [7, 8] to solve the Schrödinger equation. We have performed all calculations with the DNP basis set which is very efficient [9] . We used the UHF level. In addition, we have included the Becke-Lee-Yang-Parr gradient correction (BLYP) for the correlation-exchange term [10, 11] . We also used DGauss software [12, 13] of Cray Research Inc./Oxford Molecular on a Cray C98 computer. The gaussian 6-31G* basis was used with the Becke-Lee-Yang-Parr (BLYP) gradient correction. For isolated species, vibrational frequencies were calculated by finite difference to check wether systems are stable or not, as a local minimum cannot present negative frequencies due to the localy parabolic shape of the Born Oppenheimer surface.
Results and Discussion
A spectroscopic study of nickel(II) in eutectic LiCl-KCl for temperature between 364 and 540°C has shown that two different complexes were present in melt [14]. They have been identified to be NiCl42- of tetrahedrical and distorted tetragonal symetries [15, 16]. The influence of melt composition and temperature have also been investigated , the distorted tetragonal form is shown to result of Li+ influence on nickel complexes while K+ cations seems to promote the tetrahedrical symmetry [17-19].
Nickel Complexes
Using DMol, we have calculated the different Ni(II) chlorocomplexes. Results (see Table I and Fig. 2) show that complexes with more than 4 chlorine atoms are not stable.