Keywords: Cyclic voltammetry; Scanning electron micrography; Anodic polarization; Cathodic ..... mation yielded by interpretation of the electrochemical.
Journal of Electroanalytical Chemistry 442 Ž1998. 113–123
Electrochemical oxidation of nickel in alkaline solutions: a voltammetric study and modelling A. Seghiouer, J. Chevalet ) , A. Barhoun 1, F. Lantelme Laboratoire d’Electrochimie 2 , Case 51 UniÕersite´ Pierre et Marie Curie, 4 place Jussieu, 75252 Paris Cedex 05, France Received 27 May 1997; received in revised form 1 October 1997
Abstract The reaction mechanisms of the oxidation of nickel in alkaline solutions has been studied by cyclic voltammetry and scanning electron micrography. It is shown that the formation of a layer of nickel hydroxide depends on the surface treatment of the nickel electrode. Even on a freshly polished electrode a hydroxide deposit was hardly obtained from an anodic polarization; cathodic prepolarization was required to obtain formation of nickel hydroxide. The quantity of nickel hydroxide electrochemically generated is related to the potential value and the duration of the cathodic treatment. The set of experimental data issued from this voltammetric study is used for comparison with the computed values obtained by digital simulation. The general multiple electrode reaction model used for these calculations is presented here. It involves the formation of nickel hydroxide according to a nucleation and growth mechanism, and redox reactions such as nickel hydroxide| nickel oxyhydroxide, hydrogen and oxygen evolution. A reasonable fit is obtained between experimental and computed responses by introducing the values of the thermodynamic and kinetic parameters found in the literature or calculated for this purpose. q 1998 Elsevier Science S.A. Keywords: Cyclic voltammetry; Scanning electron micrography; Anodic polarization; Cathodic prepolarization
1. Introduction Nickel is involved in many electrochemical reactions of applied interest. There is a considerable body of literature dealing with nickel deposition in acidic media w1,2x to form protective coatings such as amorphous phosphorus–nickel alloys w3x. Apart from nickel deposition studies, one fundamental and practical interest of nickel in electrochemistry is due to its surface oxidation properties. Nickel behavior, nickel oxide formation, the nature, structure and properties of the surface layers and passivation phenomena have been widely investigated for a long time. It seems interesting to recall that Faraday was the first to observe this passivation of nickel. More recently Žsince 1960. many articles have been devoted to the passivation phenomena and to the study of the composition of the oxidation films Žsee Ref.
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Corresponding author. Present address: Departement de Chimie, Faculte´ des Sciences, B.P. ´ 2121, Tetouan, Maroc. 2 Unite´ de Recherche Associee ´ No. 430, CNRS. 1
0022-0728r98r$19.00q 1998 Elsevier Science S.A. All rights reserved. PII S 0 0 2 2 - 0 7 2 8 Ž 9 7 . 0 0 4 9 8 - 1
w4x for a review on these papers.. Among many research papers, MacDougall and Cohen w5x, MacDougall w6x, MacDougall and Graham w7,8x, MacDougall w9x and MacDougall et al. w10x have produced extensive analyses of these subjects, mainly in acidic and neutral media. They have reached some conclusive point on the nature of the oxidation film of NiO which results in direct surface oxidation of nickel w9x. In alkaline media, a number of articles, such as those of a Makrides w11x, and more recently Hummel et al. w4x and Smith et al. w12x described the anodic behavior of nickel and production of NiO and NiŽOH. 2 . Their description and their conclusion on the presence of NiŽOH. 2 in the film was, however, controversial with the results of MacDougall. Other studies by Okuyama and Haruyama w13x stated that the film was made of NiO and Ni 3 O4 . It seems now, after the work by Visscher and Barendrecht w14x and de Souza et al. w15x using coupled ellipsometry, that a first layer of NiO is covered after the positive scan with a thick film of b NiŽOH. 2 which is the base material for the practical application to batteries. In alkaline media, it is not possible to obtain a solution
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containing nickel ions since the solubility product of nickel hydroxide is very low. However, this compound plays an important role in the field of energy conversion since the redox couple NiŽOH. 2rNiOOH is widely used as a positive electrode for rechargeable batteries w16x. The electrode substrate is made of a sheet of nickel and mechanically coated with NiŽOH. 2 . A thermal treatment is required to obtain a good contact between the active material and the nickel substrate w17,18x. The aim of the present paper is to follow by cyclic voltammetry the direct electrochemical preparation of nickel hydroxide at the surface of a nickel electrode in an alkaline solution and to determine the electrochemical properties of the hydroxide layer. As expected, multiple reactions, including electrocrystallization steps occur during the cycling of the electrode and since no theory is available to describe quantitatively the voltammograms, a model is developed based on digital simulation using the finite difference method.
Fig. 2. Effect of the polarization time at a negative potential. Voltammograms for Ni oxidation in 1 M KOH. Temperature: 258C. Nickel electrode. Area: 0.196 cm2 . Sweep rate: 50 mV sy1 . Potential window: lower limit, Ec sy1.0 V; upper limit, Ea sy0.1 V. Polarization time at Ec before potential sweep. Curve a: 10 s. Curve b: 30 s. Curve c: 60 s.
2. Experimental The experiments were carried out in 1 M KOH electrolyte in a conventional three-electrode cell. The working electrode was a disk electrode with interchangeable bolt holders. The apparent surface area of the nickel disk ŽJohnson Matthey. was about 0.2 cm2 . It was polished with 1200 SiC paper and 0.05 m m alumina powder and cleaned by supersonic sound. The counter and reference electrodes were a nickel foil and a mercury oxide electrode ŽHg
