On the synthesis, characterization and hydrogenation behaviour of Fe1 − xTi1 + yNix (x = 0.2, y = 0.3) hydrogen storage material

Vol. 22,No. 8, pp. 805808, 1997 0 1997InternationalAssociationfor HydrogenEnergy ElsevierScienceLtd All rightsreserved.Printedin GreatBritain 0360-3199/97 $17.00+0.00

Inr. J. Hydrogen Energy,

Pergamon PII: SO360-3199(%)002lS

ON THE SYNTHESIS, CHARACTERIZATION AND HYDROGENATION BEHAVIOUR OF Fe, -xTil +yNix (x = 0.2, y = 0.3) HYDROGEN STORAGE MATERIAL B. K. SINGH, A. K. SINGH and 0. N. SRIVASTAVA Physics Department, Banaras Hindu University, Varanasi-221005, India

hydrogen absorption characteristics of TiFe partially substituted with small amount of Ni for Fe and corresponding to Fe0.8Ti,.3Ni,,2has been synthesised through RF melting. The XRD and TEM characterization of Fe,,Ti,,,Ni,,z revealed the occurrence of curious structural phases;namely, a superlattice with “a” periodicity of 5.93 a (which is double that of the 2.98 A FeTi,.r phase) and a modulated phase having a modulation period of 4 times along [ 1IO] as compared to dllo of the superlattice phase. The Fe,,Ti, 3Ni0.2phase has been found to have faster hydrogen desorption kinetics (3040 cc/min) which is about 2-3 times higher than that of the parent phase FeTi, 3.It has been suggested that the faster desorption kinetics are due to the Ni substitution leading to the material FeO.sTil,Ni, 2. This material is biphasic in nature and embodies built-in interfaces and has faster crack propagation on hydrogenation. 0 1997International Association for Hydrogen Energy

Abstract-The

INTRODUCTION In view of the depletion and pollution aspects of fossil fuels (e.g. petroleum) and the ozone depletion trends of conventional refrigerants, the chlorofluorcarbons, hydrogen is believed to be a potential future fuel (e.g. a replacement for petroleum) and air conditioning medium (hydride air conditioners) [l]. Both the above modes of use of hydrogen require storage; solid state storage in the form of reversible hydrides is thought to be one of the most effective storage systems. The two earliest prototypes, namely LaNi, and FeTi, are still the most efficient and widely used hydrogen storage materials. Of these,the former gets activated easily even at room temperature, whereas the latter (FeTi) is rather more subtle; it does not get activated unless heated up to 450°C. Furthermore, the reversible transfer capacity of hydrogen for the material FeTi decreases significantly after repeated hydrogenation and dehydrogeneration in the presenceof other gas forming impurities in hydrogen [2]. Several processeshave been employed for facilitating activation, including synthesising the alloys with substitutions through transition elements like Mm, Zr, Sn, Mn etc. in place of Fe, forming Ti-rich phases corresponding to FeTi, +X [3]. These are aimed at making the material amenable to near room-temperature hydrogenation. In addition to activation, the storage capacities of the various phases in the Fe-Ti system are rather curious. For example, where the storage capacity of FeTi activated at

-500°C is reported to be about 1.75 wt% [4] for FeTi(Mn), it has also been reported to be - 1.60 wt% [5]. One of the most common applications of FeTi, +Xhas been in the use of hydride/hydrogen driven vehicular transport [3], where exhaust gasescontinuously heat the hydride bed to moderate temperature (- 1OO’C).In view of the significance of the FeTi systemin hydrogen storage, there has been an ongoing effort to develop better hydrogen storage materials; one of which is Fe-Ti-Ni. When Ni is substituted, the stability of the hydride phase increases so much that equilibrium partial pressure of hydrogen is below 1 atm at room temperature. The storage capacity is reduced, since Ni appears to suppress formation of the Y phase, activation is promoted drastically and the resistanceto the effect of impure elemental gasesis enhanced [6-g]. Therefore, FeTi containing Ni is useful in the high temperature range of lOO-200°C. In this paper, we have carried out investigations on the synthesis, characterization and hydrogenation behaviour of Fe,-,Ti,+,Ni, (x = 0.2 and y = 0.3). The actual material, as optimized by taking several feasible concentrations of Fe, Ti and Ni, corresponded to Fe,, Ti,,,Ni,,,. It may be pointed out that in this work, a somewhat new approach relates to the substitution of Fe by 20 at.% Ni. The development of a substituted FeTi with 20% nickel (for Fe) has not been studied previously. A new result observed is in regard to the observation of hydrogen storage capacity of - 1.08 wt% which is the highest for the material TiFeNi.

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Pure iron powder (purity 99.9 wt%) titanium and nickel (purity 99.9 wt%) was used as a starting material. The compound of Fe, -xTi, +yNix (x = 0.2 and y = 0.3) was taken in the right stoichiometric proportions, pressed into a pellet measuring (1 x 0.5 cm) and melted employing an R.F. induction furnace (maximum power 12 kw) under an argon atmosphere in a previously outgassed graphite crucible which was kept inside the silica tube. The as-prepared alloy ingot was melted repeatedly (5-6 times) to achieve homogeneity. The agglomerate produced in this way was removed from the graphite crucible, crushed and a powder of -0.1-1.0 mm was selected for hydrogenation experiments. Before the experiment, the compound was analysed by X-ray diffraction characterization employing a Philips X-ray diffractometer (PW- 1710) using Cuk, radiation. The hydrogen absorption and desorption behaviours were investigated using a Sievert’s type apparatus fabricated in our laboratory and utilized earlier for hydrogenation studies of other hydrogen storage materials [9]. A known quantity of fine alloy powder was placed in a reactor and was evacuated up to 10m4torr. About 40 kg/cm2of hydrogen was introduced from a high pressure gas cylinder. Since the as-synthesized alloy does not absorb hydrogen at room temperature, this was suitably activated. Several activation modes, including activation at high temperatures ( 10&500°C) was attempted for the as-synthesized and powdered forms of Fe, -,Ti, +,Nix alloy phases.It was found that the most favourable activation processcorresponded to that where the alloy was heated to 400 f 5°C in approximately 40 kg/cm2hydrogen pressure. In order to activate the alloy, the reactor was heated at temperature of 400f 5°C for 3-5 h continuously and then cooled to room temperature over 10 h duration. The hydrogendesorption characteristics of the alloy were monitored at room temperature; it was found that desorption did not take place at room temperature. Several desorption runs at different temperatures (200, 300 and 400°C) were monitored. Finally, the hydrogen desorption characteristics of the activated sample were then evaluated at 400°C by monitoring the pressure-composition-isotherm (P-C-I curve) through volumetric method.

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Fig. 1. Desorptionkinetics(hydrogenconcentrationvs time) at 400°Cfor FeosTilZNi,,,alloy. Fe,-,Ti,+,Ni, (x = 0.2 and y = 0.3) alloy is shown in Fig. 2. The . maximum storage capacity obtained for Feo.sTOb.2, corresponds to - 1.08 wt% at 4OO“C.In order to unravel the curious hydrogenation behaviour of Fe,,.8Ti,,3Ni0.2, structural (XRD) and microstructural (TEM) characterization of the as-synthesizedand hydrogenated sampleswere carried out. Figure 3 shows the representative XRD pattern of the as-synthesized Fe0,8Ti,3Ni,2 alloy. The analysis of the XRD pattern revealed that the as-synthesized sample is multiphasic. A close look at XRD patterns reveals that the prominent peaks are explicable based on a cubic system with lattice parameter a = 5.93 A. Since the parent phase corresponding to FeTi,,, has a lattice parameter of 2.98 A, this observed phase appears to be a superlattice originating from the parent phase and having a lattice parameter which is nearly double of the parameter of the initial parent phase. In order to obtain further insight regarding the existence and formation of the superlattice phases, the structural/microstructural

RESULTS AND DISCUSSION The hydrogen desorption kinetics (hydrogen concentration vs time) for the optimized hydrogen storage alloy Fe,,8Ti,.3Ni,,2was measuredat 400°C under pressure 20 kg/cm’. Figure 1 shows the rate of desorption of Fe0,8Ti,,3Ni,2material; a comparison between Fig. 1 with the known desorption characteristics of FeTi reveals that the initial hydriding rate is much faster but the total absorption capacity is lesser than the native FeTi,.,. Desorption of hydrogen to about 80% of the saturation Hydrogen content (wt%) value is accomplished within 3 min. However, the total time required for complete hydrogenation has beenfound Fig. 2. Pressurecompositionisothermof Fe,,Ti, zNi, Zalloy at 400°C. to be about 7 min. The representative P-C-I curve of

CHARACTERISTICS

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Fig. 3. XRD pattern of the as-synthesizedFe,-,Ti,+,Ni,

(x = 0.2,~ = 0.3) alloy.

investigations of the Fe,,Ti,,,Ni,, were carried out employing the technique of transmission electron microscopy. Figure 4 shows a typical selected area electron diffraction pattern in [Oil] orientation of the assynthesized Fe0.8Ti,.3Ni0.2 phase. This was explicable based on the XRD deduced “a” parameter of 5.93 A, thus it conforms the formation of the superlattice phase on substitution of Ni for Fe. Further explorations revealed the occurrence of yet another type of curious phase. A representative SAD for this phase is shown in Fig. 5, which brings out the [OOl] reciprocal lattice net. The indexing of the pattern clearly revealed the occurrence of a modulated phase with modulation periodicity of four times along the (110). Several diffraction patterns compatible with the above modulated phase were

Fig. 5. Selected area electron diffraction pattern of the as-synthesized FeosTi, jNi,, phase depicting the modulation periodicity of four times along the [l lo].

Fig. 4. Selected area electron diffraction pattern in [Ol l] orientation of the as-synthesizedFe, 8Ti, ,Ni,,, phase.

obtained. The occurrence of this modulated phase suggests ordering of Ni atoms on the Fe sublattice along (110) direction in such a way that equivalent Ni atoms et ordered along (110) direction with a period of 15.74 i which is four times the di10 of the superlattice phase. It is interesting to note that the replacement of part of Fe with Ni brings about curious structural transitions with (a) the formation of a superlattice phase with a lattice periodicity double of the parent FeTi,,, phase and (b) the occurrence of a modulated phase with a periodicity of 4 times along (110) as compared to the (d, ,,J of the initial superlattice phase. The origin of these secondary

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phasesseemto be inherent in the ordering of the Ni atoms on the Fe sublattice. The presenceof the modulated phase would turn the material into a biphasic system. The inherent interface original/modulated, materials would make the initial hydrogenation-dehydrogenation and the crack propagation on continued hydrogenation embodying lattice expansion more amenable. Thus, faster hydrogenation-dehydrogenation kinetics in the biphasic material are expected as the result. This is in keeping with the observed results, (see Fig. 1.) where the faster dehydrogenation kinetics have been found to be present for Fe0.8Ti,,3Ni,,2which has been found to be biphasic having the initial (superlattice) and the modulated phases. CONCLUSIONS The hydrogen storage material Fe,,8Ti1.3Ni02obtained by substitution of Fe by Ni exhibits faster kinetics (by about 30 to 40%) as compared to the native material FeTi,.,. This material is multiphasic in nature, embodying the superlattice phase with a = 5.93 A (double the original FeTi,.3 phase) and also a modulated phase with modulation periodicity along (110) of four times than that of the superlattice periodicity di,,,. This multiphasic character is thought to result in the faster kinetics by providing additional channel for hydrogen diffusion

through interface; also providing a fresh surface through cracking on volume expansion results in hydrogenation. Acknowledgements-The authors are grateful to Professor A. R. Verma, Dr K. La1 for helpful discussions and Professor H. P. Gautam for encouragement. We are thankful to Mr Arvind Kumar Singh of our laboratory for prolonged discussions and several useful suggestions. B. K. Singh is grateful to C.S.I.R., New Delhi for financial support.

REFERENCES 1. Dantzer, P. and Meunier, F., Material Science Forum, 1988, 31, 1-18. 2. Sandrock, G. D. and Goodeb, P. D., Journal of Less Common Metals, 1980, 73, 161-168. 3. Lee, S. M. and Perng, T. P., Journal of Alloys and Compounds, 1991,177, 107-118. 4. Reilly, J. J. and Wiswall, R. H., Journal of Inorganic Chemistry, 1974, 13,218-222. 5. Hansen, M., Constitution of Binary Alloys, 2nd edn. McGrawHill, New York, 1958, p. 723. 6. Huston, E. L. and Sandrock, G. D., Journal of Less Common Metals, 1980,74,43543. 7. Eisenberg, F. G. and Goodell, P. D., JournalofLess Common Metals, 1983,89, 55-62. 8. Bershadsky, E. and Zeitschrift, M. Ron, fur physikalische Chemic Bd. 1993,179, 125-131. 9. Ramakrishna, K., Singh, S. K., Singh, A. K. and Srivastava, 0. N. In Progress in Hydrogen Energy, Vol. 7, ed. R. P. Dahiya, Reidel, Boston, MA, 1987, p. 81.