“Soft Chemistry” synthesis of superfine powder alloys AB for Ni-MH batteries

Journal of Chemistry, Vol. 42 (2), P.241-249, 2004 “Soft Chemistry” synthesis of superfine powder alloys AB5 for Ni-MH batteries Received 25-12-2003 Ngo Quoc Quyen, Nguyen Quynh Anh, Phan Thi Binh Lab. for Appl. Electrochemistry, Vietnamese Academy of Science and Technology Summary The oxide reduction diffusion (ORD) procedure has recently been applied in synthesizing hydrogen storage materials AB5 for Ni-MH batteries. Starting from metal hydroxides and La oxalat precursor, superfine powder alloys LaNi5, LaNi4.5Co0.5 and LaNi3.87Mn1.13 were obtained by this “soft-chemistry” route. Chemical composition, structure and morphology of alloy phases were examined by different analysis techniques such as AAS, EPMA, X-ray and TEM. The H2-absorption and desorption behavior of crystalline products was determined by Sieverts’ method. Electrochemical properties of alloy samples were characterized by CV, EIS and Battery Test method. I - Introduction Soft-Chemistry synthesis of superfine powder alloys AB5 is based on the reduction of oxides by calciothermic reaction, which was carried out by R. E. Cech [1] many years ago, however there are only a few reports [2 - 5] dealing with this synthesis route for hydrogen storage electrode materials although nickel-metal hydrid batteries (Ni-MH) have especially been directed towards practical use recently. This procedure can, in case of known LaNi5, be represented by: La O +10 NiO+ 13 Ca 1300 K 2 LaNi Argon + 13CaO (1) The formation mechanism of LaNi5, according to T. Tanabe and Z. Asaki [6], includes two stages: • The reduction of La2O3 and NiO by Ca: La2O3 Ca La (2a) NiO Ca Ni (2b) • The simultaneous diffusion of the just-formed rare earth and transition metal (Ni) in molten calcium leads to initial formation of CaNi5 (3), following the substitution of Ca by La to form the more thermodyna-mically stable alloy LaNi5 (4): Ca + 5 Ni CaNi5 (3) La + CaNi5 LaNi5 + Ca (4) Single phase crystals of LaNi5 growth in the CaO – Ca slurry as micron-size loose particle of angular shape, whose hexagonal structures are closely related to that of CaCu5. Particle can be easily recovered after washing in weak acidic solution. The purpose of our work is on the ORD-route to produce some non-stochiometric phases of 241 well definite composition, such as LaNi4.5Co0.5, LaNi3.87Mn1.13, used for Ni-MH batteries. II - experimental procedure Schematic drawing of the ORD procedure is shown in figure 1 and includes two main stages: - The preparation of precussors. - The calciothermic synthesis. One of the advantages of the ORD method is the ability of using metal oxides as starting materials. In the synthesis procedure used here, Nitrates of Ni, Co, Mn, … Fine sols of hydroxide of Ni, Co, Mn, … however, superfine powder mixture of transition metal and rare earth oxides were prepared first of all by sol-gel process. The composition of constituent oxides can be tailored by varying the concentration of metal ion in the starting salt solution. The preparation conditions to the formation of superfine precussors are very important for the following ORD synthesis. It is known that, employment of superfine precussors in the ORD-reaction can significantly reduce the reaction tempera-ture and reaction time which relate to the short diffusion length and large diffusion coefficients of the small particle size. Nitrate of La Oxalate of La Microware Heating Mixture of oxides Calcining Complex oxides of spinel phase ORD-Reaction with Ca (T=1300K, Argon) Fine Powder AB5 LaNi4.5Co0.5 Test LaNi3.87Mn1.13 Figure 1: Flow chart of the synthesis procedure • Stoichiometry by EPMA, ASS • Structure and morphology analysis by X-ray and SEM • H2-absorption/-desorption isotherms (by Sieverts method) • Electrochemical characterization by CV, EIS and Modelling • Battery tester 242 Among many others, some main conditions are summarized as followed: - The mixture of fine hydroxide sol of transition metal (Ni, Co, Mn... and oxalate of La was first converted into oxides by microware decomposition and then into complex oxides of spinel phase by intensive calcining (at 800oC for 2 h). - The main ORD-reaction with excess calcium was carried out in the stainless steel reactor (Fig. 2) at ~1000OC for ~4 h under purified argon. After quenching to room temperature the black fine crystalline powder of AB5 was recovered by thorough washing with dilute acetic acid up to complete eliminating of Ca(OH)2 by-product. The chemical composition of alloy samples was determined by AAS and EPMA. Phase structure and morphology were examined by X-ray diffractometry (Siemens D-5000) and TEM (EM-125K). The behavior of the hydrogen absorption as well as desorption of obtained alloy powder was determined by Sieverts’ methode. The electrochemical proper-ties of samples were measured by Cyclic Voltammetry and Electrochemical Impedance Spectroscopy (Zahner-IM6). Some storage characteristics were estimated by Battery-Tester method (ZSW-Basytec). In this work, we mainly described the research results on compounds LaNi5 and LaNi4.5Co0.5. Figure 2: Reactor of ORD processing 1, 2, 6 Electrical Furnace 3 Stainless steel crucible 4 Reactor - Chamber 5, 10 Thermocouple and Thermocontrol unit 7 Cooling top cap 8 Argon flux 9 Outgas 243 III - results and discussion 1. Structure and morphology analysis Figures 3a, 3b and 3c represent the X-ray patterns of some obtained AB5 compounds. Despite these rather rough growth conditions of the ORD procedure, one always observes a remarkable crystal quality with5 sharp X-ray diffraction line. All the samples were pure phase and their X-ray patterns were refined in the CaCu5 – type hexagonal structure of LaNi5. Figures 4 represents particle morphology of a LaNi4.5Co0.5 alloy observed by TEM with selected area of electron diffraction. In general the particles of the AB5 alloys, formed during the ORD process, consists a mixture of crystalline (~70%) and amorphous phase (~30%) and are narrowly distributed with a typical size of a few micrometers. 30 35 40 45 50 55 60 65 70 75 Figure 3a: X-ray pattern of LaNi5 30 35 40 45 50 55 60 65 Figure 3b: X-ray patte of LaNi3.87Mn1.13 244 25 30 35 40 45 50 55 60 70 75 Figure 3c: X-ray pattern of LaNi4.5Co0.5 Figure 4: Particle morphology of crystalline LaNi4.5Co0.5 observed by TEM 245 ... - tailieumienphi.vn