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

Journal of Chemistry, Vol. 42 (2), P. 241 - 249, 2004<br /> <br /> <br /> <br /> “Soft Chemistry” synthesis of superfine powder<br /> alloys AB5 for Ni-MH batteries<br /> Received 25-12-2003<br /> Ngo Quoc Quyen, Nguyen Quynh Anh, Phan Thi Binh<br /> Lab. for Appl. Electrochemistry, Vietnamese Academy of Science and Technology<br /> <br /> <br /> Summary<br /> The oxide reduction diffusion (ORD) procedure has recently been applied in synthesizing<br /> hydrogen storage materials AB5 for Ni-MH batteries. Starting from metal hydroxides and La<br /> oxalat precursor, superfine powder alloys LaNi5, LaNi4.5Co0.5 and LaNi3.87Mn1.13 were<br /> obtained by this “soft-chemistry” route.<br /> Chemical composition, structure and morphology of alloy phases were examined by<br /> different analysis techniques such as AAS, EPMA, X-ray and TEM. The H2-absorption and<br /> desorption behavior of crystalline products was determined by Sieverts’ method.<br /> Electrochemical properties of alloy samples were characterized by CV, EIS and Battery<br /> Test method.<br /> <br /> <br /> I - Introduction Ca<br /> La2O3 La (2a)<br /> Soft-Chemistry synthesis of superfine Ca<br /> powder alloys AB5 is based on the reduction of NiO Ni (2b)<br /> oxides by calciothermic reaction, which was<br /> • The simultaneous diffusion of the just-<br /> carried out by R. E. Cech [1] many years ago,<br /> formed rare earth and transition metal (Ni)<br /> however there are only a few reports [2 - 5] in molten calcium leads to initial formation<br /> dealing with this synthesis route for hydrogen of CaNi5 (3), following the substitution of<br /> storage electrode materials although nickel- Ca by La to form the more thermodyna-<br /> metal hydrid batteries (Ni-MH) have especially mically stable alloy LaNi5 (4):<br /> been directed towards practical use recently.<br /> Ca + 5 Ni CaNi5 (3)<br /> This procedure can, in case of known<br /> LaNi5, be represented by: La + CaNi5 LaNi5 + Ca (4)<br /> 1300 K Single phase crystals of LaNi5 growth in the<br /> La2O3 + 10 NiO + 13 Ca 2 LaNi5 CaO – Ca slurry as micron-size loose particle of<br /> Argon + 13CaO (1) angular shape, whose hexagonal structures are<br /> closely related to that of CaCu5. Particle can be<br /> The formation mechanism of LaNi5, easily recovered after washing in weak acidic<br /> according to T. Tanabe and Z. Asaki [6], solution.<br /> includes two stages:<br /> The purpose of our work is on the ORD-route<br /> • The reduction of La2O3 and NiO by Ca: to produce some non-stochiometric phases of<br /> <br /> 241<br /> well definite composition, such as LaNi4.5Co0.5, however, superfine powder mixture of<br /> LaNi3.87Mn1.13, used for Ni-MH batteries. transition metal and rare earth oxides were<br /> prepared first of all by sol-gel process. The<br /> II - experimental procedure composition of constituent oxides can be<br /> tailored by varying the concentration of metal<br /> Schematic drawing of the ORD procedure ion in the starting salt solution. The preparation<br /> is shown in figure 1 and includes two main conditions to the formation of superfine<br /> stages: precussors are very important for the following<br /> - The preparation of precussors. ORD synthesis. It is known that, employment<br /> of superfine precussors in the ORD-reaction<br /> - The calciothermic synthesis. can significantly reduce the reaction tempera-<br /> One of the advantages of the ORD method ture and reaction time which relate to the short<br /> is the ability of using metal oxides as starting diffusion length and large diffusion coefficients<br /> materials. In the synthesis procedure used here, of the small particle size.<br /> <br /> <br /> Nitrates of Nitrate of<br /> Ni, Co, Mn, … La<br /> Preparation of precursors<br /> <br /> <br /> <br /> <br /> Fine sols of<br /> Oxalate of<br /> hydroxide of<br /> La<br /> Ni, Co, Mn, …<br /> <br /> Microware<br /> Heating<br /> <br /> Mixture of<br /> oxides<br /> <br /> Calcining<br /> <br /> Complex oxides<br /> of spinel phase<br /> <br /> ORD-Reaction with Ca • Stoichiometry by<br /> Calciothermic<br /> <br /> Synthesizing<br /> <br /> <br /> <br /> <br /> (T=1300K, Argon) EPMA, ASS<br /> • Structure and<br /> Fine Powder AB5 morphology analysis<br /> LaNi5 by X-ray and SEM<br /> Test<br /> LaNi4.5Co0.5 • H2-absorption/-<br /> LaNi3.87Mn1.13 desorption isotherms<br /> (by Sieverts method)<br /> • Electrochemical<br /> characterization by<br /> CV, EIS and<br /> Modelling<br /> • Battery tester<br /> Figure 1: Flow chart of the synthesis procedure<br /> <br /> 242<br />  Among many others, some main conditions The chemical composition of alloy samples<br /> are summarized as followed: was determined by AAS and EPMA. Phase<br /> - The mixture of fine hydroxide sol of structure and morphology were examined by<br /> transition metal (Ni, Co, Mn... and oxalate of La X-ray diffractometry (Siemens D-5000) and<br /> was first converted into oxides by microware TEM (EM-125K). The behavior of the<br /> decomposition and then into complex oxides of hydrogen absorption as well as desorption of<br /> spinel phase by intensive calcining (at 800oC for obtained alloy powder was determined by<br /> 2 h). Sieverts’ methode. The electrochemical proper-<br /> - The main ORD-reaction with excess calcium ties of samples were measured by Cyclic<br /> was carried out in the stainless steel reactor (Fig. 2) Voltammetry and Electrochemical Impedance<br /> at ~1000OC for ~4 h under purified argon. After Spectroscopy (Zahner-IM6). Some storage<br /> quenching to room temperature the black fine characteristics were estimated by Battery-<br /> crystalline powder of AB5 was recovered by Tester method (ZSW-Basytec). In this work,<br /> thorough washing with dilute acetic acid up to we mainly described the research results on<br /> complete eliminating of Ca(OH)2 by-product. compounds LaNi5 and LaNi4.5Co0.5.<br /> <br /> <br /> <br /> <br /> Figure 2: Reactor of ORD processing<br /> 1, 2, 6 Electrical Furnace<br /> 3 Stainless steel crucible<br /> 4 Reactor - Chamber<br /> 5, 10 Thermocouple and Thermocontrol unit<br /> 7 Cooling top cap<br /> 8 Argon flux<br /> 9 Outgas<br /> <br /> <br /> 243<br />  III - results and discussion CaCu5 – type hexagonal structure of LaNi5.<br /> 1. Structure and morphology analysis Figures 4 represents particle morphology of<br /> a LaNi4.5Co0.5 alloy observed by TEM with<br /> Figures 3a, 3b and 3c represent the X-ray selected area of electron diffraction.<br /> patterns of some obtained AB5 compounds.<br /> Despite these rather rough growth conditions of In general the particles of the AB5 alloys,<br /> the Figures 3a, 3b andone<br /> ORD procedure, 3c represent the X-raya<br /> always observes formed during the ORD process, consists a<br /> patterns of some obtained AB compounds.<br /> remarkable crystal quality with sharp X-ray<br /> 5 mixture of crystalline (~70%) and amorphous<br /> Despite these<br /> diffraction rather<br /> line. rough<br /> All the growthwere<br /> samples conditions of<br /> pure phase phase (~30%) and are narrowly distributed<br /> and their X-ray patterns were refined in the with a typical size of a few micrometers.<br /> <br /> <br /> <br /> <br /> 30 35 40 45 50 55 60 65 70 75<br /> Figure 3a: X-ray pattern of LaNi5<br /> <br /> <br /> <br /> <br /> 30 35 40 45 50 55 60 65<br /> Figure 3b: X-ray patte of LaNi3.87Mn1.13<br /> <br /> 244<br /> 25 30 35 40 45 50 55 60 70 75<br /> Figure 3c: X-ray pattern of LaNi4.5Co0.5<br /> <br /> <br /> <br /> <br /> Figure 4: Particle morphology of crystalline LaNi4.5Co0.5<br /> observed by TEM<br /> 245<br />  The crystallite appear in angular shapes and, Details of the experimental apparatus used<br /> in many cases, the rectangular- or hexagonal- in this study are described in a previous article<br /> shaped crystals are identified (as in Fig. 4). [7].<br /> The amorphous phase can be crystallized Figure 5 shows the change in the absorption<br /> after annealing, but it is not necessary for using properties in LaNi5 resulting from the partial<br /> as electrode materials in field of the battery replacement of Ni by Co in the form<br /> technology. LaNi4.5Co0.5.<br /> The relationship between the concentration<br /> 2. Hydrogen absorption behavior of the<br /> of hydrogen loading in the -phase of AB5 (N<br /> obtained AB5 at 30oC [H]<br /> [AB ]<br /> ) and the equilibrium hydrogen pressure<br /> It was found at room temperature that the 5<br /> AB5 compounds can be reversibly absorbed up (PH ) can be represented by linear Sieverts’<br /> 2<br /> to six atoms of hydrogen per formula unit at equation (5):<br /> equilibrium hydrogen pressure. Therefore, the N = KS . pH 1/2 + K0 (5)<br /> hydrogen absorption properties of obtained alloy 2<br /> powders such as LaNi5 and LaNi4.5Co0.5 were The calculated Sieverts’ parameter KS, K0<br /> measured by means of the Sieverts’ method. are represented in table 1<br /> <br /> pH 2 [atm]<br /> pH 2 [atm]<br /> <br /> 5<br /> <br /> 4.5<br /> <br /> 4<br /> LaNi5<br /> 3.5<br /> LaNi4.5Co0.5<br /> 3<br /> <br /> 2.5<br /> <br /> 2<br /> <br /> 1.5<br /> <br /> 1<br /> <br /> 0.5<br /> <br /> 0<br /> 0 0.0005 0.001 0.0015 0.002 0.0025<br /> N =N=[H]<br /> [H] / /[AB<br /> [AB5]5 ]<br /> Figure 5<br /> Table 1: Sieverts’ parameter KS, K0 and PH -range at 30oC<br /> 2<br /> <br /> AB5 1/21/2 pH - range, atm Storage capacity*,<br /> N = KS . pH + K0 2<br /> 2 mAh/g<br /> <br /> 1÷4<br /> N =N0=.0507<br /> LaNi5 0.0507 p H 2 0.0006 ~ 80<br /> pH 2<br /> 0.0006<br /> LaNi4.5Co0.5 N = 0.0023 p H 2 0.0005 0÷1 ~ 320<br /> <br /> * estimated by battery-tester method at 30oC and 1 atm.<br /> <br /> 246<br />  The effect of partical cobalt substitution for carried out.<br /> nickel shows clairly in the H2-absorption Gohr model is suitable for electrode materials<br /> behavior. Sieverts’ constante KS indicating the having porosity, roughness distribution as well<br /> plateau slope decreases remarkably so that the as polycrystallinity, particle-size effects such as<br /> H2-absorption shifts to direction of the high hydrogen insertion AB5-electrode [8]<br /> hydrogen concentration even by lowering PH -<br /> 2 Figure 6 shows typical Nyquist impedance<br /> range in the vicinity of internal gas pressure of spectra of electrode material LaNi4.5Co0.5 at<br /> battery ~1 atm. The comparison measurements different potentials in the whole frequency range<br /> of initial storage capacity determined by battery- (103 to 10-3Hz). The Nyquist plot in the vicinity<br /> tester method are also given in table 1. The high of equilibrium potential (-1.0 V vs Hg/HgO)<br /> storage capacity of LaNi4.5Co0.5 in comparison consist of two distinct semicircles, whereas<br /> with LaNi5 again results from this fact. the plots in the discharge range (-0.8 V to -0.4 V<br /> 3. EIS measurements and modeling based on vs Hg/HgO) consist of only a depressed<br /> semicircle and a diffusional region, which is<br /> Zahner-IM6 Messtechnik<br /> described not by a 45o line but by a line at<br /> The performance of the MH-electrode is increasing angle in depend on applied potential.<br /> mainly controlled by kinetics of the charge They show a restricted diffusion behavior.<br /> transfer on the surface as well as by the mass Curves fitting of Nyquist plots were made by the<br /> transfer of hydrogen within the bulk of the complex non-linear least square method to<br /> storage alloys. In order to obtain more insight determine the electrochemical components of<br /> into functioning of MH-electrode, modelling by equivalent circuit (Fig. 7) for the MH-electrode<br /> EIS method according to Gohrs’ concept was containing the finite-length diffusion response.<br /> 50<br /> <br /> <br /> <br /> 40<br /> Imagynary Part (Ohm)<br /> <br /> <br /> <br /> <br /> 30<br /> <br /> <br /> <br /> 20<br /> <br /> <br /> <br /> 10<br /> <br /> <br /> <br /> 0<br /> <br /> <br /> <br /> -10<br /> 0 10 20 30 40 50 60 70<br /> Real Part (Ohm)<br /> <br /> Figure 6: Nyquist diagram of electrode material LaNi4.5Co0.5<br /> at different potentials (vs. Hg/HgO)<br /> <br /> 247<br />  1 : R1 5 : Cdl<br /> 2 : C2 6 : Cin<br /> 3 : C3 7 : Rct<br /> 4 : R4 8 : Re<br /> <br /> <br /> <br /> <br /> Figure 7: Equivalent circuit for MH-electrode, according to Gohrs’ model<br /> Table 2 shows in details of the so-called of resistances Rct and Rin and the ratio of<br /> stack interface impedance Z( ), which may be capacitances Cdl and Cin. The limiting shapes of<br /> expressed in terms of the impedance element Figure 6 were obtained as a consequence Cdl <<br /> such as ZT (electrode top interface consisting Cin (and probably Rct Rin when low - frequency<br /> of R1 and C1), ZP (pore ground interface impedance date were carried out in extended<br /> consisting of C3 and R4) and ZR (a modified frequency range to < 10-3Hz in order to<br /> Randles circuit consisting of double layer separate the semi-infinite diffusion process (the<br /> capacitance Cdl, charge transfer resistance Rct, Warburg impedance) from the finite-length<br /> Cin insertion capacitance and Re electrolyt diffusion effect) according to J. S. Chen [9].<br /> resistance). However, this is not discussed further, as not<br /> The shape of the impedance diagram with enough data are available to characterize the<br /> restricted diffusion behavior depends on the ratio phenomenon at present.<br /> <br /> Table 2: Parameters values obtained by equivalent-circuit analysis according to Gohrs’ model<br /> Potential ZT ZP ZR<br /> [V vs Hg/ Remark<br /> HgO] R1[ ] C2 [mF] C3 [µF] R4 [ ] Cdl [µF] Cin [mF] Rct [ ] Re [ ]<br /> <br /> -1.0 48.8 918.6 6.97 1.74 51.1 1.36 11.33 6.95 H2-evolution<br /> reaction<br /> -0.8 108.7 5.42 4.74 4.95 12.81 1.85 49.4 8.75 H2 oxydation<br /> reaction<br /> -0.6 75.5 8.18 5.68 3.25 12.63 2.07 38.0 9.23<br /> -0.4 73.9 11.06 6.43 2.91 11.69 3.51 37.8 9.31 End of<br /> decharge<br /> <br /> At potential in the vicinity of equilibrium 96500 As, Rct = 11.33 , A = 0.054 cm2, then i0<br /> potential -1.0V vs. Hg/HgO, the exchange of obtained LaNi4.5Co0.5 powder is 42.7 mAcm-2.<br /> current density i0 is expressed by:<br /> In addition, the high reaction resistance Rct<br /> 1 RT in the range of the discharge process (at –0.8 ÷<br /> i0 = (6)<br /> A FR ct -0.4V vs Hg/HgO) results from depth discharge<br /> where R is 8.314 J mol-1 K-1, T = 303 K, F = (DOD) dependence.<br /> <br /> 248<br />  IV - conclusion References<br /> This study has shown that the ORD method 1. R. E. Cech. J. Met., Vol. 26, P. 32 (1974).<br /> seems to be a very attractive way to produce the 2. Z. Li, K. Yasuda, et al.. J. Alloys & Comp.,<br /> AB5 compounds for Ni-MH batteries because of Vol. 193, P. 26 - 28 (1993).<br /> its simple procedure (using inexpensive oxide<br /> precursors to avoid starting from high cost pure 3. D. Y. Kim, M. Ohtsuka, et al.. Metall.<br /> metals; low expense for equipment; under mild Review of MMIJ, Vol. 10, P. 2 - 45 (1993).<br /> synthesizing conditions in comparison with 4. Ng« Quèc QuyÒn, NguyÔn TiÕn TXi vX nnk.<br /> traditional arc-melting or HF methods and “Nghiªn cøu chÕ t¹o hîp chÊt liªn kim lo¹i<br /> finely crystalline powder as endproduct without hä AB5 tÝch tr÷ hidro øng dông cho nguån<br /> use of mechanical milling). ®iÖn hãa”, §Ò tXi cÊp TT KHTN&CNQG<br /> The properties of ORD powder can be 1999 - 2001).<br /> easily controlled by varying the composition of 5. N. Q. Quyen, N. Q. Anh. “Soft Chemistry<br /> metal ions and synthesizing conditions based synthesis of superfine powder alloys for<br /> on the sol-gel chemistry. Results of different metal hydrid batteries”, P. 51, Chemical<br /> analysis techniques show that no significant Nanotechnology Talks IV – Frankfurt a.<br /> differences of properties of obtained ORD Main/D (2003).<br /> powder were found in comparison with the<br /> same products of other metallurgy methods. 6. T. Tanabe, Z. Asaki. Metall. and Materials<br /> Finally, EIS – modelling according to Gohrs’ Trans., Vol. 29B, P. 331 - 338 (1998).<br /> concept allows to obtain interesting insight into 7. Ng« Quèc QuyÒn, NguyÔn ThÞ Quúnh Anh,<br /> functioning of the MH-electrode. NguyÔn TiÕn TXi, Vò Duy HiÓn, Ph¹m V¨n<br /> L©m. T¹p chÝ Hãa häc, T. 41, sè 2, Tr. 11 -<br /> Acknowledgement: This work was supported 15 (2003).<br /> by a Grant-in-Aid for Basic Research No.<br /> 5.31.301 from the Ministry of Science and 8. H. Gohr. Dechema Monographie der GDCh<br /> Technology of Vietnam. The authors wish also to – Fachgruppe Angewandte Electrochimie,<br /> express their thank to Humboldt fellowship and Munich (1980).<br /> BMF of Germany for the important support of 9. J. S. Chen. J. of Electroanal. Chem., Vol.<br /> research equipments in the course of this work. 406, P. 1 (1996).<br /> <br /> <br /> <br /> <br /> 249<br />