Selective Formation of Ce-Ni Hydrogen Storage Alloys By Electrodeposition in LiCl-KCl-CeCl3 Melts Using Ni As Cathode

Abstract

Cerium-nickel based hydrogen storage alloys are one of the important new rare earth-based hydrogen storage alloys due to their high discharge capacities, easy activation and good hydrogen pressure. Cerium-nickel based alloy compound, CeNi5, is usually used as the hydrogen storage alloy in Ni/MH battery, due to their favorable electrochemical activation, improved discharge and charge kinetic properties. CeNi2 and CeNi3, are also used as hydrogen storage alloys. The electrochemical way in molten salts is more economic for the metallurgical industry, because it has simpler in operation and lower in energy consumption than many existing industrial technologies. However, the electrochemical formation of Ce-Ni hydrogen storage alloys by selective electrodeposition has not been reported on a Ni cathode in LiCl-KCl molten salts. In this study, the electrochemical behavior of Ce(III) ions was explored in molten LiCl-KCl salts using nickel as working electrode to produce the Ce-Ni hydrogen storage alloys. Cyclic voltammetric and square wave voltammetric results indicated that the electrochemical reduction of Ce (III) appeared at a less negative value than that recorded on a W electrode because of the formation of Ce-Ni alloy compounds by the electro-deposition of Ce on a Ni electrode. The nucleation mechanism of Ce deposited on a Ni electrode based on the Scharifker-Hill non-dimensional theoretical model was found to be the progressive nucleation. The change of electrode surface morphology with the deposited time was observed by SEM, which clearly indicated that the nucleation and growth of Ce follows the mechanisms of dendritic growth. Based on the studies of the above electrochemical behavior, Ce-Ni alloys were prepared using potentiostatic electrolysis, five kinds of Ce-Ni intermetallic compounds, CeNi5, Ce2Ni7, CeNi3, CeNi2 and CeNi were formed under our experiment conditions in LiCl-KCl-CeCl3 melts. XRD and SEM-EDS analysis results showed that the hydrogen storage alloys of CeNi5, CeNi3 and CeNi2 could be selectively prepared using potentiostatic electrolysis by controlling applied potential. Furthermore, The thermodynamic properties of Ce-Ni compounds, such as Gibbs free energy(ΔG), enthalpy(ΔH) and entropy(ΔS) of formation for each Ce-Ni alloy compound as well as activity and partial molar Gibbs energy of Ce in Ce-Ni alloy were computed employing open circuit chronopotentiometry measurement. Therefore, this work presents a promising method for the preparation of the Cerium-nickel based hydrogen storage alloys. Keywords: Electrochemical behaviour; Electrochemical formation; Ce-Ni intermetallic compounds; Thermodynamic properties References [1] Y. Zhang, H.Y. Yin, S.D. Zhang, D.Y. Tang, Z.W. Yuan, T.H.Yan, W.F. Zheng, D.H. Wang,Preparation of CeNi2 intermetallic compound by direct electroreduction of solid CeO2-2NiO in molten LiCl,J. Rare Earth.3(9)(2012) 923-927.[2] B.J. Zhao, L. Wang, L. Dai, G.H. Cui, H.Z. Zhou, R.V. Kumar, Direct electrolytic preparation of cerium/nickel hydrogen storage alloy powder in molten salt,J. Alloys Compd. 468 (2009) 379-385.[3] B.J. Zhao, X.G. Lu, Q.D.Zhong, C.H. Li, S.L. Chen, Direct electrochemical preparation of CeNi5 and LaxCe1-xNi5 alloys from mixed oxides by SOM process,Electrochim. Acta 55 (2010) 2996-3001.Fig.1 Square wave voltammogram for Ni electrode in LiCl-KCl-CeCl3 melts at 873 K. Pulse height: 0.1 V; potential step: 0.002 V; frequency: 20 Hz. Figure 1