- Email: [email protected]
Structural and thermodynamic properties of metallic hydrides used for energy storage☆
Journal of Physics and Chemistry of Solids 65 (2004) 517–522 www.elsevier.com/locate/jpcs
Structural and thermodynamic properties of metallic hydrides used for energy storageq M. Latroche* Laboratoire de Chimie Me´tallurgique des Terres Rares, CNRS, 2-8 rue Henri Dunant, 94320 Thiais Cedex, France Accepted 4 August 2003
Abstract Nowadays, energetic needs are mainly covered by fossil energies leading to pollutant emissions mostly responsible for global warming. Among the different possible solutions for the greenhouse effect reduction, hydrogen has been proposed for energy transportation. Indeed, H2 can be seen as a clean and efficient energy carrier. However, beside the difficulties related to hydrogen production, efficient high capacity storage is still to be developed. Hydrogen can be stored as a compressed gas, in liquefied tanks or absorbed in solids. Many metals and alloys are able to store large amounts of hydrogen. This latter solution is of interest in terms of safety, global yield and long time storage. However, to be suitable for applications, such compounds must present high capacity, good reversibility, fast reactivity and sustainability. In this paper, we will review on the structural and thermodynamic properties of metallic hydrides. Their solid – gas hydrogenation behaviour and the related absorption –desorption isotherm curves are examined as a useful criterion for the selection of suitable materials for applications. The storage performances obtained with these alloys are reported and some solutions to common problems such as corrosion, passivation, decrepitation, poor kinetic and short cycle life are discussed. q 2003 Elsevier Ltd. All rights reserved. Keywords: A. Alloys; A. Intermetallic compounds; B. Chemical synthesis; D. Thermodynamic properties
1. Introduction Hydrogen absorption properties of intermetallic compounds have been deeply investigated as they are potential candidates for energy storage. Nowadays, the most successful application for these materials is found in socalled nickel-metal hydride (Ni-MH) batteries. Negative electrodes based on LaNi5-type compounds replace widely cadmium ones in portable devices (mobile phones, portable tools, laptop computers) with increased capacity and decreased toxicity. However, as shortage of fossil energies is expected and because fossil fuels lead to pollutant emissions mainly responsible for global warming, hydrogen is foreseen to be a possible energy carrier for the future. Metal hydrides can be used as hydrogen storage materials for supplying fuel cells. However, to be suitable for gas storage, such compounds must present high capacity, good reversibility, fast reactivity and sustainability. In this paper, the thermodynamic and structural properties of metallic * Tel.: þ33-1-49-78-12-10; fax: þ 33-1-49-78-12-03. E-mail address: [email protected] (M. Latroche). q Plenary Lecture 0022-3697/$ - see front matter q 2003 Elsevier Ltd. All rights reserved. doi:10.1016/j.jpcs.2003.08.037
hydrides will be reviewed. Other aspects like decrepitation, activation, kinetic and cycle lives will be described and common solutions to these problems like chemical substitution effects will be presented.
2. Metal forming hydrides Many metals and alloys are able to react spontaneously with hydrogen. These materials, either a defined compound or a solid solution, are designed as metallic hydrides [1,2]. Excepting the peculiar case of palladium, pure metals may be classified into two categories according to their reactivity towards hydrogen. Alkali metals, alkaline earths, early transition metals such as zirconium, titanium, magnesium or the rare-earths metals (R-type elements) form highly stable hydrides at room temperature whereas late transition metals such as chromium, iron or nickel or some p-elements (M-type elements) do not form stable hydrides at room temperature. If intermetallic compounds RMn exist in the R – M phase diagram, they generally exhibit intermediate hydrogenation thermodynamic properties between those of
518
M. Latroche / Journal of Physics and Chemistry of Solids 65 (2004) 517–522
the two elements R and M leading in many cases to reversible behaviour near atmospheric pressure and room temperature. RMn binary compounds are usually classified by families according to their stoichiometry n. For practical reasons, the most studied families for hydrogen storage correspond today to values equal to n ¼ 1=2; 1, 2 and 5. Depending on the value of n, various storage capacities are obtained. Some typical data for these different families are given in Fig. 1. Weight capacities (i.e. the mass of hydrogen per mass of metallic compounds) are usually hindered by the use of heavy metals like rare-earths leading to maximum values below 1.5 wt%. However, significant improvement is obtained by using light elements like magnesium for which capacities close to 3.6 wt% can be achieved. Alloy composition is less determinant as concern the volumic storage capacity. The hydrogen density stored in a compound like LaNi5 (140 g dm23) is twice bigger than that of liquid hydrogen (140 g dm23) with the advantages to be easily handle under 1.7 bar of equilibrium pressure at room temperature. Most of the binary compounds can be modified by chemical substitution on the metallic sublattices. Indeed pseudo-binary compounds R 12xRx0 (M 12yMy0 )n can be formed for a wide range of elements and compositions. In many cases, such substitutions do not affect the basic crystallographic structure of the binary compound but induce important changes in the geometric and electronic properties that are often used to tune the thermodynamic properties of the hydrides.
3. Properties of metallic hydrides 3.1. Reversible storage capacity Hydrogen absorption in intermetallic compounds is characterized by a mean hydrogen capacity of about 1.1 H/M (hydrogen atom per metal atom). However, this value can vary in very large proportions depending on the compounds and the largest reported value reaches 4.5 H/M for BaReH9 [3]. The stability of the formed hydride is determined by its hydrogen equilibrium pressure at a given temperature. Thus, thermodynamic properties of hydrides are usually described in pressure-composition isothermal (PCI) curves. A typical set of PCI curves for an ideal intermetallic compound able to reversibly absorb hydrogen is represented in Fig. 2. Three domains can be observed. At low H content (x , amax in H/M), a solid solution singlephase domain occurs, the so-called a-phase, where hydrogen absorption/desorption can be described by the reaction MHx þ
dx H Y MHxþdx 2 2
ð1Þ
For amax , x , bmin ; a two-phase domain appears. The saturated a-phase, with composition x ¼ amax ; transforms into the b-phase, with composition x ¼ bmin : It corresponds to the plateau pressure that extends in composition as long as the following equilibrium reaction takes place MHx ðamax Þ þ
y2x H2 $ MHy ðbmin Þ 2
ð2Þ
The formation and decomposition of the hydride occur on the plateau and is usually characterized by a hysteresis phenomenon. The hydrogen equilibrium pressure during absorption is higher than that of desorption. This is
Fig. 1. Comparison between mass (top) and volume (bottom) capacities for various RMn-type compounds (n ¼ 1=2; 1, 2 and 5). The weight capacity is strongly linked to the molar mass of the compounds. Using light elements like Mg leads to significant improvement in the storage capacity. Composition is less significant for the volume capacity and most compounds exhibit hydrogen densities twice larger than that of liquid hydrogen.
Fig. 2. Typical pressure-composition isotherm (PCI) curves for an ideal metal-hydrogen system (from Ref. [1]). Left part shows the evolution of the logarithm of the hydrogen equilibrium pressure as a function of the capacity for various temperatures. Below Tc, a plateau pressure is observed corresponding to the progressive transformation of the a-phase into the b one. The right curve shows the linear evolution of lnðPÞ as a function of the inverse of the temperature ð1=TÞ: This behaviour is described according to the Van’t Hoff equation.
M. Latroche / Journal of Physics and Chemistry of Solids 65 (2004) 517–522
commonly attributed to the extra energy necessary to overcome the constraints related to the lattice expansion. For x . bmin ; again a solid solution single-phase domain occurs, corresponding to the b-phase, and which can be also described according to Eq. (1). 3.2. Equilibrium pressure adjustment The fine tuning of equilibrium pressure for a given binary alloy is often necessary to match the application needs. Solid state chemistry allows by chemical substitution to modify the thermodynamic properties of metallic hydrides [4]. As an example, the consequences of substituting lanthanum by Yttrium in the pseudo-binary system La12xYxNi5 is shown in Fig. 3. Both compounds LaNi5 and YNi5 crystallise in the same hexagonal CaCu5-type structure (P6=mmm space group) and La (or Y) occupies the crystallographic site 1a. As the atomic radius of Yttrium is smaller than that of La, increasing the substitution rate leads to a linear reduction of the cell volume according to the well-known Vegard’s law. Similarly, the corresponding hydrides show a linear relation between the logarithm of the equilibrium pressure and the cell volume. This property allows a fine adjustment of the equilibrium pressure by controlling the substitution rate x. This feature is generally observed for most of the metallic hydrides and it has been widely used for pressure modifications in many applications. 3.3. Structural properties Hydrogen absorption induces a large expansion of the unit cell volume of the metallic compounds. This volume
519
increase is closely related to the capacity and ranges ˚ 3 per H atom [5]. The discrete volume between 2 and 3 A expansion DV=V observed between the two phases a and b on the equilibrium plateau can vary from 8 to 30%. Hydrogen dissolves within the metallic structure as a proton (and not as molecular hydrogen). In most cases, the parent structure of the hydride is preserved and hydrogen absorption induces isotropic cell volume expansion. However, some compounds (like CeNi3 [6]) exhibit highly anisotropic behaviour. In some other cases, structural transformation occurs as for b.c.c. (well-known-cubic) compounds that change to f.c.c. (faced-centred-cubic) structure when hydrided [7 –9]. Hydrogen atoms occupy predominantly tetrahedral and octahedral interstices in the metal atom lattice, are quite mobile at room temperature, and are usually randomly distributed over several different sites, which are connected to form three-dimensional diffusion paths through the structures. The number of available interstices in the structures generally greatly exceed the number of hydrogen atoms absorbed. The interstices, which are preferentially occupied, are large enough [10] and have great electronic affinity with hydrogen as the surrounding atoms are mainly hydride forming elements (Zr, Ti, rare-earth…). As a matter of fact, the filling of all available interstices in the metal lattice is never observed. Two parameters limit the maximum hydrogen capacity. The first one is electronic and was proposed by Switendick [11]. Charge transfer between the hydrogen atom and the transition metal creates electric charge on the hydrogen atom that causes repulsive interaction. This imposes a minimum internuclear distance ˚ between two neighbouring hydrogen atoms. of about 2.1 A
Fig. 3. Evolution as a function of the substitution rate x of the cell volume V of the intermetallic compounds (left) and the equilibrium pressure P (absorption and desorption) of the hydrides (right) for the pseudo-binary system La12xYxNi5. The linear behaviour observed for the two parameters V and P allows any adjustments of the equilibrium pressure by controlling the substitution rate x.
520
M. Latroche / Journal of Physics and Chemistry of Solids 65 (2004) 517–522
to particle size below 10 mm. One of the advantage of this phenomenon is the improvement of the reaction rate with cycling due to the increasing active surface area but it also activates corrosion by the renewal of the fresh surfaces [13]. 3.5. Activation and kinetic
Fig. 4. SEM photograph of a LaNi5-type compound after electrochemical cycling. Heavy cracking of the original grain is observed and is attributed to embrittlement induced by the volume expansion observed during hydrogen cycling.
The second parameter is based on geometric considerations and was proposed by Westlake [5,12]. If one considers the radius of the sphere tangent to the closest metallic atoms of the site assuming the atoms are hard spheres, the minimum ˚ radius of such an interstitial site must be larger than 0.40 A to be filled by one hydrogen atom. Considering these two criteria, only 20% of the available sites are found occupied in a metallic hydrides like LaNi5H6.6. 3.4. Decrepitation As a consequence of the large volume increase upon hydrogenation, intermetallic compounds break into small particles in a process called decrepitation. This phenomenon is related to embrittlement, cracking, and reduction of the grain particle size. Typical grain morphology is shown in Fig. 4 for a LaNi5-type material observed by SEM after electrochemical cycling. Cracking of the original grain leads
Starting from a virgin alloy, hydrogenation reaction can be described following three steps. The first one consists in a so-called incubation period. The pressure remains constant without any hydrogen absorption. This phenomenon is frequently attributed to passivation effect related to existence of metal oxide layers at the grain surface. Following this early stage, absorption occurs and the reaction rate can be followed by measuring the pressure variations as a function of time. Finally, equilibrium is reached and the pressure remains constant for a given temperature. A typical example of this behaviour is given in Fig. 5a showing the hydrogen absorption curve for a virgin alloy ZrMn2 for which the three steps can be clearly identified: incubation ð0 # t # 3:4 hÞ; absorption ð3:4 , t # 7:5 hÞ; equilibrium ðt . 7:5 hÞ: Composition modifications of the binary compound can be easily achieved. ZrMn2 adopts the hexagonal C14-type Laves phase structure and the manganese sublattice can be substituted by nickel in a wide range of concentration. Indeed, the compound ZrMn1.7Ni0.3 can be prepared keeping the same C14 structure with only a small cell volume reduction. Following the absorption of this pseudobinary compound in the same experimental conditions than for ZrMn2, significant improvements are observed (Fig. 5b). The incubation time is reduced to 220 s and absorption occurs in less than 1 h for the same hydrogen capacity for both samples. As bulk diffusion is not expected to be significantly modified by the nickel substitution, such improvement is attributed to the segregation of small particles of nickel at the grain boundaries that plays a catalytic role for the hydrogen molecule dissociation.
Fig. 5. Evolution of the pressure and the capacity as a function of time for the pristine alloys ZrMn2 (a) and ZrMn1.7Ni0.3 (b). Hydrogen loading occurs following three different steps: incubation, absorption and equilibrium. The nickel substituted pseudo-binary phase exhibits much shorter incubation time and faster absorption that the binary compound.
M. Latroche / Journal of Physics and Chemistry of Solids 65 (2004) 517–522
Such effect is also commonly observed in other systems. Analysis of the surface composition after activation by solid –gas process has been performed by Siegmann et al. [14] on LaNi5 by means of X-ray photoemission, Auger spectroscopy and magnetic susceptibility measurements. They found nickel surface enrichment associated with rareearth oxide (or hydroxide). They established that nickel remains metallic, forming clusters on the surface containing about 6000 atoms highly catalytic towards hydrogen molecule dissociation. 3.6. Cycle life Oxidation is the main factor leading to storage capacity decay on cycling. This process, which is rather limited in solid –gas reaction, can be very pronounced in electrochemical medium. Willems [15,16] has shown that electrochemical capacity decay is related to the transformation of the intermetallic compound into rare-earth hydroxide and metallic 3d elements. The nature of the corrosion products have been characterised by Maurel et al. [13] using X-ray diffraction, scanning and transmission electron microscopies. They have shown that electrochemical cycling leads to the formation at the surface of the grain of rare-earth hydroxide needles covering a continuous nanocrystalline corrosion scale composed of metallic (Ni, Co) solid solution mixed with oxide (Ni, Co)O solid solution. Some authors [17,18] have tempted to link the loss of capacity to the so-called discrete volume expansion (i.e. the volume change between the saturated amax and undersaturated bmin phases). Due to the important strains generated during the transition between these two phases, larger decrepitation occurs leading to the formation of small particles and therefore increased surface corrosion. Notten et al. [19,20] have shown that better cycle lives can be achieved with so-called over stoichiometric La(Ni,M)5þx (M ¼ Mn, Cu) samples for which the discrete volume expansion is reduced. Finally, alloy composition plays a determinant role in cycle life. Bowman et al. [21] have shown that thousands of cycles can be performed in solid – gas reaction with tin substituted LaNi5 materials without significant capacity decay. Similarly, in electrochemical process, lanthanum replacement by mischmetal lead to substantial increase of the electrode cycle life due to the presence of cerium [22,23]. Cobalt substitution to nickel has also been shown to significantly affect the resistance to corrosion, especially when combined in so-called threesubstituted compounds like MmNi3.55Mn0.4Al0.3Co0.75 [24], which is widely used nowadays as negative electrode material in commercial Ni-MH batteries.
4. Conclusions LaNi5-type alloys have shown their ability to store hydrogen conveniently either by solid–gas or electrochemical
521
processes. They are now used world-wide as negative electrodes in Ni-MH batteries. However, they remain too heavy to be of practical use for the future mobile applications. Although presenting larger hydrogen absorption capacities than RM5-type compounds, RM2 Laves phases are difficult to prepare. Obtaining single-phase material remains challenging and good knowledge of phase diagrams is still necessary to assess the phase limits of the Laves phases. Moreover, the RM2 compounds suffer from poor activation, surface degradation and corrosion. Among RM compounds, TiNi-based alloys possess also a complex metallurgy due to the easy formation of secondary phases and the difficulty to control the temperature of the martensitic transformation. However, TiNi-based alloys present considerable interest, especially concerning the high storage capacities of Ti–Ni–Zr martensitic alloys. Mg-based compounds present the most interesting capacities due to the very light molar weight of Mg. However, the use of magnesium is still limited by difficult metallurgy and poor kinetic observed with this element. Low temperature synthesis techniques, such as ball milling, can be used to alloy light elements like Mg with other d-metals forming new metastable phases. Using such routes, metallic compounds with enhanced properties towards hydrogen absorption could be prepared in order to produce efficient energy storage materials for the future.
References [1] L. Schlapbach, Hydrogen in Intermetallic Compounds I, vol. 63, Springer, Berlin, 1988, p. 350. [2] L. Schlapbach, Hydrogen in Intermetallic Compounds II, vol. 67, Springer, Berlin, 1992, p. 328. [3] N.T. Stetson, K. Yvon, P. Fischer, On the structure of the complex hydride BaReH9, Inorg. Chem. 33 (1994) 4598–4599. [4] J.-C. Achard, A. Percheron-Gue´gan, H. Diaz, F. Briaucourt, F. Demany, Rare earth ternary hydrides. Hydrogen storage applications, In: 2nd International Congress on Hydrogen in Metals, Paris, 1977. [5] D.G. Westlake, Site occupancies and stoichiometries in hydrides of intermetallic compounds: geometric considerations, J. Less Common Met. 90 (1983) 251–273. [6] V.A. Yartys, O. Isnard, A.B. Riabov, L.G. Akselrud, Unusual effects on hydrogenation: anomalous expension and volume contraction, J. Alloys Compd 356–357 (2003) 109–113. [7] Y. Nakamura, E. Akiba, New hydride phase with a deformed FCC structure in the Ti–V–Mn solid solution-hydrogen system, J. Alloys Compd 311 (2000) 317–321. [8] Y. Nakamura, K.I. Oikawa, T. Kamiyama, E. Akiba, Crystal structure of two hydrides formed from a Ti –V–Mn BCC solid solution alloy studied by time-of-flight neutron powder diffraction—a NaCl structure and a CaF2 structure, J. Alloys Compd 316 (2001) 284–289. [9] Y. Nakamura, E. Akiba, Hydriding properties and crystal structure of NaCl-type mono-hydrides formed from Ti – V – Mn BCC solid solutions, J. Alloys Compd 345 (2002) 175–182. [10] C.E. Lundin, F.E. Lynch, C.B. Magee, A correlation between the interstitial hole sizes in intermetallic compounds and the thermodynamic properties of the hydrides formed from those compounds, J. LessCommon Met. 56 (1977) 19–37.
522
M. Latroche / Journal of Physics and Chemistry of Solids 65 (2004) 517–522
[11] A.C. Switendick, Band structure calculations for metal hydrogen systems, Z. Phys. Chem. Neue Folge. 117 (1979) 89 –112. [12] D.G. Westlake, Stoichiometries and interstitial site occupation in the hydrides of ZrNi and other isostructural intermetallic compounds, J. Less Common Met. 75 (1980) 177 –185. [13] F. Maurel, B. Knosp, M. Backhaus-Ricoult, Characterization of corrosion products of AB5-type hydrogen storage alloys for nickel-metal hydride batteries, J. Electrochem. Soc. 147 (2000) 78 –86. [14] H.C. Siegmann, L. Schlapbach, C.R. Brundle, Self-restoring of the active surface in the hydrogen sponge LaNi5, Phys. Rev. Lett. 40 (1978) 972. [15] J.J.G. Willems, Metal hydride electrodes stability of LaNi5-related compounds, Philips J. Res. 39 (1984) 1–94. [16] J.J.G. Willems, Investigation of a new type of rechargeable battery, the nickel-hydride cell, Philips Tech. Rev. 43 (1986) 22–34. [17] P.H.L. Notten, R.E.F. Einerhand, J.L.C. Daams, On the nature of the electrochemical cycling stability of non-stoichiometric LaNi5-based hydride-forming compounds. Part I. Crystallography and electrochemistry, J. Alloys Compd 210 (1994) 221– 232. [18] P.H.L. Notten, J.L.C. Daams, R.E.F. Einerhand, On the nature of the electrochemical cycling stability of non-stoichiometric LaNi5-based
[19]
[20]
[21]
[22]
[23]
[24]
hydride-forming compounds. Part II. In situ X-ray diffractometry, J. Alloys Compd 210 (1994) 233 –241. P.H.L. Notten, M. Latroche, A. Percheron-Gue´gan, The influence of Mn on the crystallography and electrochemistry of non-stoichiometric AB5-type hydride forming compounds, J. Electrochem. Soc. 146 (1999) 3181–3189. M. Latroche, P.H.L. Notten, A. Percheron-Gue´gan, In situ neutron diffraction study of solid gas desorption of non-stoichiometric AB5 type hydrides, J. Alloys Compd 253–254 (1997) 295–297. R.C. Bowman Jr., C.H. Luo, C.C. Ahn, C.K. Witham, B. Fultz, The effect of tin on the degradation of LaNi52ySny metal hydride during thermal cycling, J. Alloys Compd 217 (1995) 185– 192. G.D. Adzic, J.R. Johnson, J.J. Reilly, J. McBreen, S. Mukerjee, Cerium content and cycle life of multicomponent AB5 hydride electrodes, J. Electrochem. Soc. 142 (1995) 3429–3433. J.J. Reilly, G.D. Adzic, J.R. Johnson, T. Vogt, S. Mukerjee, J. McBreen, The correlation between composition and electrochemical properties of metal hydride electrodes, J. Alloys Compd 293 –295 (1999) 569 –582. H. Ogawa, M. Ikoma, H. Kawano, I. Matsumoto, Metal hydride electrode for high energy density sealed nickel-metal hydride battery, J. Power Sources 12 (1988) 393–410.
Structural and thermodynamic properties of metallic hydrides used for energy storageq M. Latroche* Laboratoire de Chimie Me´tallurgique des Terres Rares, CNRS, 2-8 rue Henri Dunant, 94320 Thiais Cedex, France Accepted 4 August 2003
Abstract Nowadays, energetic needs are mainly covered by fossil energies leading to pollutant emissions mostly responsible for global warming. Among the different possible solutions for the greenhouse effect reduction, hydrogen has been proposed for energy transportation. Indeed, H2 can be seen as a clean and efficient energy carrier. However, beside the difficulties related to hydrogen production, efficient high capacity storage is still to be developed. Hydrogen can be stored as a compressed gas, in liquefied tanks or absorbed in solids. Many metals and alloys are able to store large amounts of hydrogen. This latter solution is of interest in terms of safety, global yield and long time storage. However, to be suitable for applications, such compounds must present high capacity, good reversibility, fast reactivity and sustainability. In this paper, we will review on the structural and thermodynamic properties of metallic hydrides. Their solid – gas hydrogenation behaviour and the related absorption –desorption isotherm curves are examined as a useful criterion for the selection of suitable materials for applications. The storage performances obtained with these alloys are reported and some solutions to common problems such as corrosion, passivation, decrepitation, poor kinetic and short cycle life are discussed. q 2003 Elsevier Ltd. All rights reserved. Keywords: A. Alloys; A. Intermetallic compounds; B. Chemical synthesis; D. Thermodynamic properties
1. Introduction Hydrogen absorption properties of intermetallic compounds have been deeply investigated as they are potential candidates for energy storage. Nowadays, the most successful application for these materials is found in socalled nickel-metal hydride (Ni-MH) batteries. Negative electrodes based on LaNi5-type compounds replace widely cadmium ones in portable devices (mobile phones, portable tools, laptop computers) with increased capacity and decreased toxicity. However, as shortage of fossil energies is expected and because fossil fuels lead to pollutant emissions mainly responsible for global warming, hydrogen is foreseen to be a possible energy carrier for the future. Metal hydrides can be used as hydrogen storage materials for supplying fuel cells. However, to be suitable for gas storage, such compounds must present high capacity, good reversibility, fast reactivity and sustainability. In this paper, the thermodynamic and structural properties of metallic * Tel.: þ33-1-49-78-12-10; fax: þ 33-1-49-78-12-03. E-mail address: [email protected] (M. Latroche). q Plenary Lecture 0022-3697/$ - see front matter q 2003 Elsevier Ltd. All rights reserved. doi:10.1016/j.jpcs.2003.08.037
hydrides will be reviewed. Other aspects like decrepitation, activation, kinetic and cycle lives will be described and common solutions to these problems like chemical substitution effects will be presented.
2. Metal forming hydrides Many metals and alloys are able to react spontaneously with hydrogen. These materials, either a defined compound or a solid solution, are designed as metallic hydrides [1,2]. Excepting the peculiar case of palladium, pure metals may be classified into two categories according to their reactivity towards hydrogen. Alkali metals, alkaline earths, early transition metals such as zirconium, titanium, magnesium or the rare-earths metals (R-type elements) form highly stable hydrides at room temperature whereas late transition metals such as chromium, iron or nickel or some p-elements (M-type elements) do not form stable hydrides at room temperature. If intermetallic compounds RMn exist in the R – M phase diagram, they generally exhibit intermediate hydrogenation thermodynamic properties between those of
518
M. Latroche / Journal of Physics and Chemistry of Solids 65 (2004) 517–522
the two elements R and M leading in many cases to reversible behaviour near atmospheric pressure and room temperature. RMn binary compounds are usually classified by families according to their stoichiometry n. For practical reasons, the most studied families for hydrogen storage correspond today to values equal to n ¼ 1=2; 1, 2 and 5. Depending on the value of n, various storage capacities are obtained. Some typical data for these different families are given in Fig. 1. Weight capacities (i.e. the mass of hydrogen per mass of metallic compounds) are usually hindered by the use of heavy metals like rare-earths leading to maximum values below 1.5 wt%. However, significant improvement is obtained by using light elements like magnesium for which capacities close to 3.6 wt% can be achieved. Alloy composition is less determinant as concern the volumic storage capacity. The hydrogen density stored in a compound like LaNi5 (140 g dm23) is twice bigger than that of liquid hydrogen (140 g dm23) with the advantages to be easily handle under 1.7 bar of equilibrium pressure at room temperature. Most of the binary compounds can be modified by chemical substitution on the metallic sublattices. Indeed pseudo-binary compounds R 12xRx0 (M 12yMy0 )n can be formed for a wide range of elements and compositions. In many cases, such substitutions do not affect the basic crystallographic structure of the binary compound but induce important changes in the geometric and electronic properties that are often used to tune the thermodynamic properties of the hydrides.
3. Properties of metallic hydrides 3.1. Reversible storage capacity Hydrogen absorption in intermetallic compounds is characterized by a mean hydrogen capacity of about 1.1 H/M (hydrogen atom per metal atom). However, this value can vary in very large proportions depending on the compounds and the largest reported value reaches 4.5 H/M for BaReH9 [3]. The stability of the formed hydride is determined by its hydrogen equilibrium pressure at a given temperature. Thus, thermodynamic properties of hydrides are usually described in pressure-composition isothermal (PCI) curves. A typical set of PCI curves for an ideal intermetallic compound able to reversibly absorb hydrogen is represented in Fig. 2. Three domains can be observed. At low H content (x , amax in H/M), a solid solution singlephase domain occurs, the so-called a-phase, where hydrogen absorption/desorption can be described by the reaction MHx þ
dx H Y MHxþdx 2 2
ð1Þ
For amax , x , bmin ; a two-phase domain appears. The saturated a-phase, with composition x ¼ amax ; transforms into the b-phase, with composition x ¼ bmin : It corresponds to the plateau pressure that extends in composition as long as the following equilibrium reaction takes place MHx ðamax Þ þ
y2x H2 $ MHy ðbmin Þ 2
ð2Þ
The formation and decomposition of the hydride occur on the plateau and is usually characterized by a hysteresis phenomenon. The hydrogen equilibrium pressure during absorption is higher than that of desorption. This is
Fig. 1. Comparison between mass (top) and volume (bottom) capacities for various RMn-type compounds (n ¼ 1=2; 1, 2 and 5). The weight capacity is strongly linked to the molar mass of the compounds. Using light elements like Mg leads to significant improvement in the storage capacity. Composition is less significant for the volume capacity and most compounds exhibit hydrogen densities twice larger than that of liquid hydrogen.
Fig. 2. Typical pressure-composition isotherm (PCI) curves for an ideal metal-hydrogen system (from Ref. [1]). Left part shows the evolution of the logarithm of the hydrogen equilibrium pressure as a function of the capacity for various temperatures. Below Tc, a plateau pressure is observed corresponding to the progressive transformation of the a-phase into the b one. The right curve shows the linear evolution of lnðPÞ as a function of the inverse of the temperature ð1=TÞ: This behaviour is described according to the Van’t Hoff equation.
M. Latroche / Journal of Physics and Chemistry of Solids 65 (2004) 517–522
commonly attributed to the extra energy necessary to overcome the constraints related to the lattice expansion. For x . bmin ; again a solid solution single-phase domain occurs, corresponding to the b-phase, and which can be also described according to Eq. (1). 3.2. Equilibrium pressure adjustment The fine tuning of equilibrium pressure for a given binary alloy is often necessary to match the application needs. Solid state chemistry allows by chemical substitution to modify the thermodynamic properties of metallic hydrides [4]. As an example, the consequences of substituting lanthanum by Yttrium in the pseudo-binary system La12xYxNi5 is shown in Fig. 3. Both compounds LaNi5 and YNi5 crystallise in the same hexagonal CaCu5-type structure (P6=mmm space group) and La (or Y) occupies the crystallographic site 1a. As the atomic radius of Yttrium is smaller than that of La, increasing the substitution rate leads to a linear reduction of the cell volume according to the well-known Vegard’s law. Similarly, the corresponding hydrides show a linear relation between the logarithm of the equilibrium pressure and the cell volume. This property allows a fine adjustment of the equilibrium pressure by controlling the substitution rate x. This feature is generally observed for most of the metallic hydrides and it has been widely used for pressure modifications in many applications. 3.3. Structural properties Hydrogen absorption induces a large expansion of the unit cell volume of the metallic compounds. This volume
519
increase is closely related to the capacity and ranges ˚ 3 per H atom [5]. The discrete volume between 2 and 3 A expansion DV=V observed between the two phases a and b on the equilibrium plateau can vary from 8 to 30%. Hydrogen dissolves within the metallic structure as a proton (and not as molecular hydrogen). In most cases, the parent structure of the hydride is preserved and hydrogen absorption induces isotropic cell volume expansion. However, some compounds (like CeNi3 [6]) exhibit highly anisotropic behaviour. In some other cases, structural transformation occurs as for b.c.c. (well-known-cubic) compounds that change to f.c.c. (faced-centred-cubic) structure when hydrided [7 –9]. Hydrogen atoms occupy predominantly tetrahedral and octahedral interstices in the metal atom lattice, are quite mobile at room temperature, and are usually randomly distributed over several different sites, which are connected to form three-dimensional diffusion paths through the structures. The number of available interstices in the structures generally greatly exceed the number of hydrogen atoms absorbed. The interstices, which are preferentially occupied, are large enough [10] and have great electronic affinity with hydrogen as the surrounding atoms are mainly hydride forming elements (Zr, Ti, rare-earth…). As a matter of fact, the filling of all available interstices in the metal lattice is never observed. Two parameters limit the maximum hydrogen capacity. The first one is electronic and was proposed by Switendick [11]. Charge transfer between the hydrogen atom and the transition metal creates electric charge on the hydrogen atom that causes repulsive interaction. This imposes a minimum internuclear distance ˚ between two neighbouring hydrogen atoms. of about 2.1 A
Fig. 3. Evolution as a function of the substitution rate x of the cell volume V of the intermetallic compounds (left) and the equilibrium pressure P (absorption and desorption) of the hydrides (right) for the pseudo-binary system La12xYxNi5. The linear behaviour observed for the two parameters V and P allows any adjustments of the equilibrium pressure by controlling the substitution rate x.
520
M. Latroche / Journal of Physics and Chemistry of Solids 65 (2004) 517–522
to particle size below 10 mm. One of the advantage of this phenomenon is the improvement of the reaction rate with cycling due to the increasing active surface area but it also activates corrosion by the renewal of the fresh surfaces [13]. 3.5. Activation and kinetic
Fig. 4. SEM photograph of a LaNi5-type compound after electrochemical cycling. Heavy cracking of the original grain is observed and is attributed to embrittlement induced by the volume expansion observed during hydrogen cycling.
The second parameter is based on geometric considerations and was proposed by Westlake [5,12]. If one considers the radius of the sphere tangent to the closest metallic atoms of the site assuming the atoms are hard spheres, the minimum ˚ radius of such an interstitial site must be larger than 0.40 A to be filled by one hydrogen atom. Considering these two criteria, only 20% of the available sites are found occupied in a metallic hydrides like LaNi5H6.6. 3.4. Decrepitation As a consequence of the large volume increase upon hydrogenation, intermetallic compounds break into small particles in a process called decrepitation. This phenomenon is related to embrittlement, cracking, and reduction of the grain particle size. Typical grain morphology is shown in Fig. 4 for a LaNi5-type material observed by SEM after electrochemical cycling. Cracking of the original grain leads
Starting from a virgin alloy, hydrogenation reaction can be described following three steps. The first one consists in a so-called incubation period. The pressure remains constant without any hydrogen absorption. This phenomenon is frequently attributed to passivation effect related to existence of metal oxide layers at the grain surface. Following this early stage, absorption occurs and the reaction rate can be followed by measuring the pressure variations as a function of time. Finally, equilibrium is reached and the pressure remains constant for a given temperature. A typical example of this behaviour is given in Fig. 5a showing the hydrogen absorption curve for a virgin alloy ZrMn2 for which the three steps can be clearly identified: incubation ð0 # t # 3:4 hÞ; absorption ð3:4 , t # 7:5 hÞ; equilibrium ðt . 7:5 hÞ: Composition modifications of the binary compound can be easily achieved. ZrMn2 adopts the hexagonal C14-type Laves phase structure and the manganese sublattice can be substituted by nickel in a wide range of concentration. Indeed, the compound ZrMn1.7Ni0.3 can be prepared keeping the same C14 structure with only a small cell volume reduction. Following the absorption of this pseudobinary compound in the same experimental conditions than for ZrMn2, significant improvements are observed (Fig. 5b). The incubation time is reduced to 220 s and absorption occurs in less than 1 h for the same hydrogen capacity for both samples. As bulk diffusion is not expected to be significantly modified by the nickel substitution, such improvement is attributed to the segregation of small particles of nickel at the grain boundaries that plays a catalytic role for the hydrogen molecule dissociation.
Fig. 5. Evolution of the pressure and the capacity as a function of time for the pristine alloys ZrMn2 (a) and ZrMn1.7Ni0.3 (b). Hydrogen loading occurs following three different steps: incubation, absorption and equilibrium. The nickel substituted pseudo-binary phase exhibits much shorter incubation time and faster absorption that the binary compound.
M. Latroche / Journal of Physics and Chemistry of Solids 65 (2004) 517–522
Such effect is also commonly observed in other systems. Analysis of the surface composition after activation by solid –gas process has been performed by Siegmann et al. [14] on LaNi5 by means of X-ray photoemission, Auger spectroscopy and magnetic susceptibility measurements. They found nickel surface enrichment associated with rareearth oxide (or hydroxide). They established that nickel remains metallic, forming clusters on the surface containing about 6000 atoms highly catalytic towards hydrogen molecule dissociation. 3.6. Cycle life Oxidation is the main factor leading to storage capacity decay on cycling. This process, which is rather limited in solid –gas reaction, can be very pronounced in electrochemical medium. Willems [15,16] has shown that electrochemical capacity decay is related to the transformation of the intermetallic compound into rare-earth hydroxide and metallic 3d elements. The nature of the corrosion products have been characterised by Maurel et al. [13] using X-ray diffraction, scanning and transmission electron microscopies. They have shown that electrochemical cycling leads to the formation at the surface of the grain of rare-earth hydroxide needles covering a continuous nanocrystalline corrosion scale composed of metallic (Ni, Co) solid solution mixed with oxide (Ni, Co)O solid solution. Some authors [17,18] have tempted to link the loss of capacity to the so-called discrete volume expansion (i.e. the volume change between the saturated amax and undersaturated bmin phases). Due to the important strains generated during the transition between these two phases, larger decrepitation occurs leading to the formation of small particles and therefore increased surface corrosion. Notten et al. [19,20] have shown that better cycle lives can be achieved with so-called over stoichiometric La(Ni,M)5þx (M ¼ Mn, Cu) samples for which the discrete volume expansion is reduced. Finally, alloy composition plays a determinant role in cycle life. Bowman et al. [21] have shown that thousands of cycles can be performed in solid – gas reaction with tin substituted LaNi5 materials without significant capacity decay. Similarly, in electrochemical process, lanthanum replacement by mischmetal lead to substantial increase of the electrode cycle life due to the presence of cerium [22,23]. Cobalt substitution to nickel has also been shown to significantly affect the resistance to corrosion, especially when combined in so-called threesubstituted compounds like MmNi3.55Mn0.4Al0.3Co0.75 [24], which is widely used nowadays as negative electrode material in commercial Ni-MH batteries.
4. Conclusions LaNi5-type alloys have shown their ability to store hydrogen conveniently either by solid–gas or electrochemical
521
processes. They are now used world-wide as negative electrodes in Ni-MH batteries. However, they remain too heavy to be of practical use for the future mobile applications. Although presenting larger hydrogen absorption capacities than RM5-type compounds, RM2 Laves phases are difficult to prepare. Obtaining single-phase material remains challenging and good knowledge of phase diagrams is still necessary to assess the phase limits of the Laves phases. Moreover, the RM2 compounds suffer from poor activation, surface degradation and corrosion. Among RM compounds, TiNi-based alloys possess also a complex metallurgy due to the easy formation of secondary phases and the difficulty to control the temperature of the martensitic transformation. However, TiNi-based alloys present considerable interest, especially concerning the high storage capacities of Ti–Ni–Zr martensitic alloys. Mg-based compounds present the most interesting capacities due to the very light molar weight of Mg. However, the use of magnesium is still limited by difficult metallurgy and poor kinetic observed with this element. Low temperature synthesis techniques, such as ball milling, can be used to alloy light elements like Mg with other d-metals forming new metastable phases. Using such routes, metallic compounds with enhanced properties towards hydrogen absorption could be prepared in order to produce efficient energy storage materials for the future.
References [1] L. Schlapbach, Hydrogen in Intermetallic Compounds I, vol. 63, Springer, Berlin, 1988, p. 350. [2] L. Schlapbach, Hydrogen in Intermetallic Compounds II, vol. 67, Springer, Berlin, 1992, p. 328. [3] N.T. Stetson, K. Yvon, P. Fischer, On the structure of the complex hydride BaReH9, Inorg. Chem. 33 (1994) 4598–4599. [4] J.-C. Achard, A. Percheron-Gue´gan, H. Diaz, F. Briaucourt, F. Demany, Rare earth ternary hydrides. Hydrogen storage applications, In: 2nd International Congress on Hydrogen in Metals, Paris, 1977. [5] D.G. Westlake, Site occupancies and stoichiometries in hydrides of intermetallic compounds: geometric considerations, J. Less Common Met. 90 (1983) 251–273. [6] V.A. Yartys, O. Isnard, A.B. Riabov, L.G. Akselrud, Unusual effects on hydrogenation: anomalous expension and volume contraction, J. Alloys Compd 356–357 (2003) 109–113. [7] Y. Nakamura, E. Akiba, New hydride phase with a deformed FCC structure in the Ti–V–Mn solid solution-hydrogen system, J. Alloys Compd 311 (2000) 317–321. [8] Y. Nakamura, K.I. Oikawa, T. Kamiyama, E. Akiba, Crystal structure of two hydrides formed from a Ti –V–Mn BCC solid solution alloy studied by time-of-flight neutron powder diffraction—a NaCl structure and a CaF2 structure, J. Alloys Compd 316 (2001) 284–289. [9] Y. Nakamura, E. Akiba, Hydriding properties and crystal structure of NaCl-type mono-hydrides formed from Ti – V – Mn BCC solid solutions, J. Alloys Compd 345 (2002) 175–182. [10] C.E. Lundin, F.E. Lynch, C.B. Magee, A correlation between the interstitial hole sizes in intermetallic compounds and the thermodynamic properties of the hydrides formed from those compounds, J. LessCommon Met. 56 (1977) 19–37.
522
M. Latroche / Journal of Physics and Chemistry of Solids 65 (2004) 517–522
[11] A.C. Switendick, Band structure calculations for metal hydrogen systems, Z. Phys. Chem. Neue Folge. 117 (1979) 89 –112. [12] D.G. Westlake, Stoichiometries and interstitial site occupation in the hydrides of ZrNi and other isostructural intermetallic compounds, J. Less Common Met. 75 (1980) 177 –185. [13] F. Maurel, B. Knosp, M. Backhaus-Ricoult, Characterization of corrosion products of AB5-type hydrogen storage alloys for nickel-metal hydride batteries, J. Electrochem. Soc. 147 (2000) 78 –86. [14] H.C. Siegmann, L. Schlapbach, C.R. Brundle, Self-restoring of the active surface in the hydrogen sponge LaNi5, Phys. Rev. Lett. 40 (1978) 972. [15] J.J.G. Willems, Metal hydride electrodes stability of LaNi5-related compounds, Philips J. Res. 39 (1984) 1–94. [16] J.J.G. Willems, Investigation of a new type of rechargeable battery, the nickel-hydride cell, Philips Tech. Rev. 43 (1986) 22–34. [17] P.H.L. Notten, R.E.F. Einerhand, J.L.C. Daams, On the nature of the electrochemical cycling stability of non-stoichiometric LaNi5-based hydride-forming compounds. Part I. Crystallography and electrochemistry, J. Alloys Compd 210 (1994) 221– 232. [18] P.H.L. Notten, J.L.C. Daams, R.E.F. Einerhand, On the nature of the electrochemical cycling stability of non-stoichiometric LaNi5-based
[19]
[20]
[21]
[22]
[23]
[24]
hydride-forming compounds. Part II. In situ X-ray diffractometry, J. Alloys Compd 210 (1994) 233 –241. P.H.L. Notten, M. Latroche, A. Percheron-Gue´gan, The influence of Mn on the crystallography and electrochemistry of non-stoichiometric AB5-type hydride forming compounds, J. Electrochem. Soc. 146 (1999) 3181–3189. M. Latroche, P.H.L. Notten, A. Percheron-Gue´gan, In situ neutron diffraction study of solid gas desorption of non-stoichiometric AB5 type hydrides, J. Alloys Compd 253–254 (1997) 295–297. R.C. Bowman Jr., C.H. Luo, C.C. Ahn, C.K. Witham, B. Fultz, The effect of tin on the degradation of LaNi52ySny metal hydride during thermal cycling, J. Alloys Compd 217 (1995) 185– 192. G.D. Adzic, J.R. Johnson, J.J. Reilly, J. McBreen, S. Mukerjee, Cerium content and cycle life of multicomponent AB5 hydride electrodes, J. Electrochem. Soc. 142 (1995) 3429–3433. J.J. Reilly, G.D. Adzic, J.R. Johnson, T. Vogt, S. Mukerjee, J. McBreen, The correlation between composition and electrochemical properties of metal hydride electrodes, J. Alloys Compd 293 –295 (1999) 569 –582. H. Ogawa, M. Ikoma, H. Kawano, I. Matsumoto, Metal hydride electrode for high energy density sealed nickel-metal hydride battery, J. Power Sources 12 (1988) 393–410.