II. Structural, morphological, chemical and electrochemical analysis of nickel covered by electrochemically deposited Co 3 O 4 in molten Li 2 CO 3 –Na 2 CO 3 at 650 °C L. Mendoza a , A. Ringuede ´ a , M. Cassir a, * , A. Galtayries b, * ,1 a Ecole Nationale Supe ´rieure de Chimie de Paris, Laboratoire dÕElectrochimie et de Chimie Analytique (UMR 7575 du CNRS), 11 rue Pierre et Marie Curie, 75231 Paris Cedex 05, France b Ecole Nationale Supe ´rieure de Chimie de Paris, Laboratoire de Physico-Chimie des Surfaces (UMR 7045 du CNRS), 11 rue Pierre et Marie Curie, 75231 Paris Cedex 05, France Received 3 May 2004; received in revised form 24 September 2004; accepted 30 September 2004 Available online 2 December 2004 Abstract The behaviour of dense nickel substrates coated by electrochemically deposited Co 3 O 4 was analysed in Li 2 CO 3 –Na 2 CO 3 under the usual molten carbonate fuel cell conditions. The cobalt oxide deposit reacts rapidly yielding LiCoO 2 , detected by X-ray diffrac- tion (XRD) and X-ray photoelectron spectroscopy (XPS). Nevertheless, after 1 h of immersion in the carbonate melt, XPS shows that, in the outermost layers (<10 nm), LiCoO 2 is no longer detected and has probably gone from the substrate by mechanical dis- integration or chemical transformation in the molten carbonate. The progressive loss of cobalt from the sample is proved by energy dispersive spectroscopy (EDS) and confirmed by all the other techniques (XRD, SEM, XPS) including the open-circuit-potential and cyclic voltammetry. However, the remaining cobalt detected by EDS, even after 48 h of immersion in the melt, could be pro- gressively transformed into a mixed cobalt-nickel compound, which diffuses in the bulk. A significant decrease in the nickel solubility can be noted when using the coated substrate. Ó 2004 Elsevier B.V. All rights reserved. Keywords: LiCoO 2 ; Co 3 O 4 ; Nickel cathode; Molten carbonate; Fuel cell; X-ray diffraction; Scanning electron microscopy; X-ray photoelectron spectroscopy; Electrochemistry 1. Introduction The well-known chemical dissolution of the usual molten carbonate fuel cell (MCFC) nickel cathode can be significantly decreased, replacing this electrode by LiCoO 2 or covering its surface by a film of cobalt oxide, i.e., Co 3 O 4 [1–5]. The use of a very thin layer of this oxide is of particular interest because it allows combining the conductive and electrocatalytic proper- ties of the nickel cathode with the protective role of lithium cobaltite. Even though no thermochemical data can be found in the literature, most authors agree that LiCoO 2 is the stable form of cobalt oxide under the MCFC conditions. Very few electrochemical techniques have been dedi- cated to the formation of cobalt oxide layers on Ni sub- strates for MCFC application. The formation of Co 3 O 4 by electrochemical potentiostatic deposition, a cheap and low temperature technique, has been used to pro- duce a Co 3 O 4 coating on nickel or nickel oxide sub- strates; a thorough description was given in a previous 0022-0728/$ - see front matter Ó 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jelechem.2004.09.033 * Corresponding authors. Tel.: +33 1 55426387; fax: +33 1 42276750. E-mail addresses: michel-cassir@enscp.jussieu.fr (M. Cassir), anouk-galtayries@enscp.jussieu.fr (A. Galtayries). 1 Tel.: +33 1 44276737; fax: +33 1 46340753 www.elsevier.com/locate/jelechem Journal of Electroanalytical Chemistry 576 (2005) 147–160 Journal of Electroanalytical Chemistry