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

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.
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“Soft Chemistry” synthesis of superfine powder alloys AB for Ni-MH batteriesJournal 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 Ca La2O3 La (2a) Soft-Chemistry synthesis of superfine Capowder alloys AB5 is based on the reduction of NiO Ni (2b)oxides by calciothermic reaction, which was • The simultaneous diffusion of the just-carried out by R. E. Cech [1] many years ago, formed rare earth and transition metal (Ni)however there are only a few reports [2 - 5] in molten calcium leads to initial formationdealing with this synthesis route for hydrogen of CaNi5 (3), following the substitution ofstorage electrode materials although nickel- Ca by La to form the more thermodyna-metal hydrid batteries (Ni-MH) have especially mically stable alloy LaNi5 (4):been directed towards practical use recently. Ca + 5 Ni CaNi5 (3) This procedure can, in case of knownLaNi5, be represented by: La + CaNi5 LaNi5 + Ca (4) 1300 K Single phase crystals of LaNi5 growth in theLa2O3 + 10 NiO + 13 Ca 2 LaNi5 CaO – Ca slurry as micron-size loose particle of Argon + 13CaO (1) angular shape, whose hexagonal structures are closely related to that of CaCu5. Particle can be The formation mechanism of LaNi5, easily recovered after washing in weak acidicaccording to T. Tanabe and Z. Asaki [6], solution.includes two stages: The purpose of our work is on the ORD-route • The reduction of La2O3 and NiO by Ca: to produce some non-stochiometric phases of 241well definite composition, such as LaNi4.5Co0.5, however, superfine powder mixture ofLaNi3.87Mn1.13, used for Ni-MH batteries. transition metal and rare earth oxides were prepared first of all by sol-gel process. The II - experimental procedure composition of constituent oxides can be tailored by varying the concentration of metal Schematic drawing of the ORD procedure ion in the starting salt solution. The preparationis shown in figure 1 and includes two main conditions to the formation of superfinestages: precussors are very important for the following - The preparation of precussors. ORD synthesis. It is known that, employment of superfine precussors in the ORD-reaction - The calciothermic synthesis. can significantly reduce the reaction tempera- One of the advantages of the ORD method ture and reaction time which relate to the shortis the ability of using metal oxides as starting diffusion length and large diffusion coeffic ...