Ternary LaNi4.75M0.25 hydrogen storage alloys: Surface segregation, hydrogen sorption and thermodynamic stability

international journal of hydrogen energy xxx (xxxx) xxx

Available online at www.sciencedirect.com

ScienceDirect journal homepage: www.elsevier.com/locate/he

Ternary LaNi4.75M0.25 hydrogen storage alloys: Surface segregation, hydrogen sorption and thermodynamic stability Z. Łodziana a,*, A. De˛bski b, G. Cios c, A. Budziak a Institute of Nuclear Physics, Polish Academy of Sciences, ul. Radzikowskiego 152, 31-342, Krako
w, Poland Institute of Metallurgy and Materials Science PAS, 25 Reymonta St., 30-059 Krakow, Poland c AGH University of Science and Technology, Academic Centre for Materials and Nanotechnology, Al. A. Mickiewicza 30, 30-059, Krak
ow, Poland a

b

article info

abstract

Article history:

Modification of compounds like LaNi5 toward ternary compositions change alloy hydrogen

Received 18 September 2018

storage properties and influence resistance to hydrogen contamination. Below thermo-

Received in revised form

dynamic properties of ternary alloys LaNi4.75M0.25 are investigated with ab initio methods

9 November 2018

and synthesized in order to select the composition with hydrogen sorption properties not

Accepted 13 November 2018

worse than LaNi5. The specific volume change, surface segregation energy and change of

Available online xxx

the hydride formation enthalpy are calculated for 34 elements (M: Ag, Al, Au, B, Bi, Ca, Cd, Cu, Cr, Fe, Ga, Ge, In, Ir, K, Mg, Mn, Mo, Nb, Pb, Pd, Pt, Rh, Ru, Sb, Sn, Ti, V, W, Y, Zn, Zr)

Keywords:

substituting Ni. Five ternary compounds are synthesized and analyzed with respect to

AB5 based alloys

crystal structure and hydrogen sorption properties. Compounds like LaNi4.75Ag0.25. and

Metal hydrides

LaNi4.75Pb0.25 show favorable stability and H2 sorption thermodynamics. The substituting

Ternary alloys

elements segregating toward the surface are expected to be catalytically active for

Hydrogen sorption

hydrogen contamination gasses.

Electronic structure

© 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

X-ray diffraction

Introduction Among intermetallic alloys these with AB5 composition (A e lanthanide or alkaline earth metal; B e transition metal or pblock element) have a prominent place. An important representative of this group: LaNi5 is known for hydrogen storage capability [1], hydrogen compression [2] and it has found its place in application in nickel metal hydride (Ni-MH) batteries [3e5]. LaNi5 forms hydrides (LaNi5H6.7) at relatively low H2 pressure (over a dozen bar) and ambient temperatures [1];

there is ~25% lattice expansion related to formation of the hydride phase [1]. Moreover, this alloy can be easily activated since it does do not form strong protective layers thus it is characterized by a good tolerance to the low oxygen content or water vapor contaminations of the primary hydrogen stream [6]. On the other hand, hydrogen contaminations like CO, CO2, CH4 strongly affect reversibility of hydrogen storage properties of this material [7]. Modification of this compound toward a better tolerance of CO or CO2 would allow hydrogen storage from the sources that have traces of these gasses. Such modification might be achieved via formation of ternary

* Corresponding author. E-mail address: [email protected] (Z. Łodziana). https://doi.org/10.1016/j.ijhydene.2018.11.104 0360-3199/© 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article as: Łodziana Z et al., Ternary LaNi4.75M0.25 hydrogen storage alloys: Surface segregation, hydrogen sorption and thermodynamic stability, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2018.11.104

2

international journal of hydrogen energy xxx (xxxx) xxx

compound, where the third metal is substituting Ni. The ternary compound must be stable against hydrogen absorption/desorption cycling and the additional metal can be catalytically active for CO evolution. LaNi5 may be combined with various elements partially substituting A or B sub-lattice and forming stable ternary alloys. With such substitution the enthalpy of hydrogenation can be tuned as well as the long term stability can be improved [8]. LaNi5-xCox is one of the important examples used in alkaline batteries [3]. The process of hydrogenation is characterized by a flat plateau in the pressure composition isotherm (PCI, often referred to pressure-composition-temperature, pcT experiments), shown in Fig. 1. In the ideal system the plateau line is horizontal and its position is determined by the thermodynamic properties of the system. Within these conditions the Van't Hoff construction can be done and thermodynamic properties like enthalpy of formations DH and entropy DS are determined. In a real system the plateau is not purely horizontal (see Fig. 1) due to departure from the equilibrium conditions (finite time of the experiment) but sloped or multiple plateaus can be also observed. The horizontal black lines, in the middle region of Fig. 1 show an ideal plateau corresponding to the equilibrium pressure of hydrogen as a function of c. Red points and line corresponds to absorption isotherm measured for LaNi5. The horizontal lines for different temperatures T (T1
alloy. Than H2 dissociates into individual H atoms that diffuse into the bulk. This process is related to the phase a in Fig. 1. In the region of the plateau (coexisting a and b phases) the hydrogen is distributed in the metal in the random manner and the hydride b-phase is formed. The process is related to hydrogen diffusion in the metal lattice and the crystal lattice expansion. It stops when the ordered metal hydride b-phase is formed and the chemical potential of hydrogen at the surface equates with this in the hydride phase. A sloped plateau of the PCI is an indication of the perturbation of this process, the reasons can be related to inhomogeneity of the alloy composition, that is known for LaNi5 being subject to a large number of hydrogenation/ dehydrogenation cycles [9]. In that case fractions of the sample have slightly different composition and they release/ absorb hydrogen at different thermodynamic conditions that effectively can lead to a sloped plateau. For the ternary alloys, like LaNi5-xMx the slopes or multiple plateau can exist due to the presence of separate phases that are in the equilibrium but they have different hydride formation enthalpy [7]. Formation of such separate phases is related to diffusion of heavy metal atoms that is a slow process compared to hydrogen diffusion in metals; thus it can be observed after large number of hydrogenation cycles or it can be effectively shadowed by non-equilibrium conditions, where only the equilibrium of the hydrogen chemical potential is reached [10]. Another reason for sloping plateau can be related to deterioration of the hydrogen absorption at the surface and H2 dissociation [6]. This can occur often for contaminated hydrogen gas. For example, oxygen would lead to formation of La2O3 or La(OH)3 that would cause the surface segregation of Ni, preventing hydrogen diffusion into the bulk [8]. The contamination with CO or CO2 could lead to formation of nickel carbonyl at the surface that effectively decrease number of active Ni sites for the first hydrogenations step [8]. Alloying of LaNi5 with additional elements would affect all of the properties pointed out above, therefore detailed analysis of the variety of ternary compositions would serve as a guideline for synthesis of LaNi5-xMx alloys that are optimized for stability of hydrogen

Fig. 1 e The schematic plot of hydrogen absorption in metal (pcT curves). The Peq - gas pressure; c - hydrogen concentration in the metal; a e metal phase with a constant hydrogen "solution", the b region is a metal hydride and two-phase coexistence is in the region (a þ b). Further explanations are in the main text. Please cite this article as: Łodziana Z et al., Ternary LaNi4.75M0.25 hydrogen storage alloys: Surface segregation, hydrogen sorption and thermodynamic stability, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2018.11.104

international journal of hydrogen energy xxx (xxxx) xxx

contamination resistance. Ab initio calculations allow separation of surface and bulk effects, provide insight into electronic and thermodynamic properties; below we use this method in analysis of 34 ternary compositions. Already in the early days of research on LaNi5 significant efforts were put into its modification to achieve thermodynamic destabilization of ternary hydride [11], lowering hydrogen sorption pressure [11] or improving the long term cycling properties [12]. This was done by substitution of La with metals like Mg, Ca, Sr, Ba [13] or Ni with Al, Co, Fe, Cr, Ge, Si, Sn, Pd, Mn, B, Se [13]. The search for appropriate substitution was often directed by the empirical models, like Hume-Rothery rules (similarity of atomic radii, electronegativity, and crystal structure) [14], Miedema's model (based on the charge density and the work function of metals) [15] or so called Pettifor structure maps (with chemical scale c introduced to order the elements in Mendeleev Table) [16]. Despite simplicity of such models variety of LaNi5 alloy modifications were proposed and thoroughly investigated. In particular compounds with the small amounts of ternary element and stoichiometry LaNi5xMx (M ¼ Al, Ga, In, Si, Ge, Sn) for x ¼ 0e0.5 were investigated [13] mainly in relation to improvement of the alkaline batteries. The volume change of the unit cell was correlated with the hydrogen equilibrium pressure [17]. Hydrogen sorption properties of AB5 compounds with the variety of modifications are collected in the Metal Hydride Properties Database [18]. More comprehensive discussion of LaNi5-xMx alloys is presented in the result section below. Here, we point out that one common feature of all hydrogen sorption experiments is the purity of H2 that is usually larger than 99.99%. Once hydrogen with this purity is available a thermodynamic characteristics of an alloy determines properties for practical storage of this gas. In reality H2 purification is an expensive procedure especially when hydrogen is obtained in small quantities and sources like fermentation reactors [19]. Hydrogen from such sources usually contains CO, CO2, CH4 and other gases. They are present even after initial purification. Such impurities of hydrogen may directly influence kinetic performance and storage capacity of LaNi5 therefore finding a ternary composition that improves resistance to CO or CO2 is desired. The first step toward this direction is identification of a possible ternary compositions that are thermodynamically stable, do not deteriorate hydrogen capacity of LaNi5 and possibly have beneficial catalytic effect for the conversion of CO, CO2, O2 into volatile molecules at temperatures and pressures encountered at hydrogenation conditions. In this paper we report the extensive ab initio search for such ternary alloy with the composition LaNi4.75M0.25 and substitution elements M: Ag, Al, Au, B, Bi, Ca, Cd, Cu, Cr, Fe, Ga, Ge, In, Ir, K, Mg, Mn, Mo, Nb, Pb, Pd, Pt, Rh, Ru, Sb, Sn, Ti, V, W, Y, Zn, and Zr. The volume expansion upon substitution, the change of hydride formation enthalpy and the surface segregation of the substituted element are taken as relevant descriptors of ternary alloy. Our predictions are verified by the synthesis of five ternary alloys that are predicted to possess both superior and inferior hydrogen storage properties compared to pure LiNi5. Alloy with a nominal stoichiometry LaNi4.75Ag0.25 is a promising candidate for COx resistant composition.

3

Methods The last decade brought new dimension to the materials science once a large scale computational databases of materials properties became available [20]. These databases contain calculated properties of inorganic compounds that are incorporated in structural data collections like ICSD as well as modification of these compounds optimized for specific applications [20]. Recent search of selected computational datasets reveal that there are 2893 entries for La-Ni ternary compounds in the AFLOWLIB.org database [21] (among them for La-Ni-Pb there are 51 entries, 54 entries for La-Ni-Al or 63 entries for La-Ni-Sn), However, the records are related to the compositions like LaNi4M1 or similar with fraction of ternary element x > 1. The Materials Project Database [20] has 72 entries for La-Ni ternary compositions including oxides. In comparison the ICSD database has 453 entries for ternary La-Ni-X. For the detailed analysis of the LaNi5-xMx ternary compositions the direct usage of existing databases has a limited use. For the obvious reasons ternary compositions with x in the range 0 < x < 1 are not present. They retain the symmetry of the parent LaNi5 phase and the combinatorial problems related to calculations of for example configurational entropy prevents a general large scale screening at present. Below we report extensive calculations of the thermodynamic properties of such compositions for 34 elements substituting Ni in the nominal stoichiometry LaNi4.75M0.25. The purpose of this search is to rationalize methods based on Density Functional Theory (DFT) for this specific applications that is design thermodynamically stable ternary LaNi4.75M0.25 compound that has hydrogens sorption properties not worse than LaNi5. A variety of properties including structural, elastic, electronic and thermodynamic properties were already calculated for LaNi5 [22e26] its hydrides, like LaNi5H7 [26,27]. Ternary compositions LaNi5-xMx were also considered on theoretical grounds [28e30]. In the present work we report computer screening and synthesis of new intermetallic compositions and detailed analysis of hydrogen absorption properties toward identification of the substitution of nickel that could be beneficial for the cyclic absorption of hydrogen contaminated by CO or CO2.

Theoretical In the present work density functional theory (DFT) implemented in the plane wave basis set [31,32] and projectoraugmented wave (PAW) potentials [33,34] was used. The exchange correlation functional was represented by generalized gradient approximation (GGA) [35]. The plane wave energy cutoff was set to 500 eV and the k-point sampling was set to 6  4  4 in the supercell with 24 metal atoms. The ground state electronic density was determined by iterative diagonalization of the KohnSham Hamiltonian, and Gaussian smearing of 0.01 eV. All calculations were spin polarized and for transition metals: V, Cr, Mn, Fe, Co, Ni, Cu the GGA þ U method was applied, with effective on-site Coulomb interaction [36] Ueff ¼ U e J ¼ 4.0 eV. Further details are given in the supplementary data.

Please cite this article as: Łodziana Z et al., Ternary LaNi4.75M0.25 hydrogen storage alloys: Surface segregation, hydrogen sorption and thermodynamic stability, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2018.11.104

4

international journal of hydrogen energy xxx (xxxx) xxx

Sequence of optimizations consisting of conjugated gradient (CG) relaxation of the internal atomic positions with fixed unit cell followed by relaxation of the supercell volume was done for each composition. These steps were repeated until the volume change of the supercell was lower than 0.1%. At final step the ground state energy was calculated for that volume with CG method. We considered 34 substitution elements plus pure LaNi5 that results in over 140 independent optimized structures, including LaNi5H7 hydrides and several hundreds of independent calculations for the surface properties. The surface calculations were performed in the slab gepffiffiffi pffiffiffi ometry with 12  A of vacuum, and 6 layers slab with 2 2  2 surface cell of the (0001) facet. Two bottom layers were frozen. In the present approach we use supercells with 24 atoms for the bulk calculations. For hydrides supercells with the same number of metal atoms were used. We focus on the low substitution limit, replacing one Ni atom in the supercell that gives the composition LaNi4.75M0.25. For substitution atoms M, a broad selection of metals was considered. This group consists of: Ag, Al, Au, B, Bi, Ca, Cd, Cu, Cr, Fe, Ga, Ge, In, Ir, K, Mg, Mn, Mo, Nb, Pb, Pd, Pt, Rh, Ru, Sb, Sn, Ti, V, W, Y, Zn, and Zr. Such selection include alkali and alkaline earth metals, transition metals with various filling of d-states, boron, carbon or nitrogen group elements. We assume each structure remains in the hexagonal symmetry with the space group lowered appropriately due to substitution of Ni. Similarity of all structures assures that vibrational properties are alike for all compositions, thus only the electronic contribution to the ground state energy and the lattice expansion due to Ni substitution are considered. Some of the considered elements are known to substitute La sublattice, however within proposed methodology only Ni lattice sites are substituted.

Experimental The host materials LaNi5 and LaNi5-xMx were prepared from the high purity elements (99.5%e99.9%, Sigma Aldrich) using the standard arc melting technique (the Mini Arc Melting System MAM-1) under argon atmosphere (purity 99.999%). After annealing for minimum 24 h and temperature above 600 K, the samples were examined by X-ray diffraction (XRD) in order to determine the structure and the phase homogeneity. The XRD measurements were carried out on the X'Pert A and l2 PRO diffractometer with Cu-Ka radiation (l1 ¼ 1.5406  ¼ 1.5444  A), with graphite monochromator and strip detector (X'Celerator). To preclude any extra diffraction lines, the samples were placed onto a “zero-background” silicon plate. The experiments were performed at the room temperature. The Rietveld and Le Bail methods [37,38] implemented in FULLPROF software [39] were used to analyze the X-ray diffraction (XRD) patterns. All parameters were refined by the least-squares method [40]. To determine additional phases the PDF-2 database supplied by the International Centre for Diffraction Data (ICDD) was used. The pressure and composition isotherms were measured on the high accuracy IMI HTP analyzer (Hiden Isochema) with Sievert method [10]. The high purity H2 gas (99.9999%) was used. The typical mass of the samples for pcT measurements was 700e900 mg. The pcT measurements were performed after activation process, which consisted of 1 h heating at

473 K, followed by outgassing, and 3-fold cycling of H2 sorption/desorption at room temperature, under 4 MPa hydrogen pressure. The sorption isotherms were collected at 313 K with adsorption (15 min/step) and desorption (40 min/step) times for each of the sample. After pcT studies the structure of the samples was examined with the XRD. Analysis of chemical composition of the samples was performed by use of X-ray energy dispersive spectroscopy (EDAX Genesis) on FEI Quanta 3D FEG Scanning Electron Microscope equipped with Trident energy dispersive X-ray spectrometer. Chemical characterization of the samples was conducted at voltages of 15 kV, electron beam currents of 11 nA, working distance of 10.0 mm in secondary electrons (SE). ZAF correction (atomic number, absorption and fluoresce) was applied. X-ray spectra were recorded from the representative areas located on the samples.

Results The literature provide an extensive information about LaNi5xMx alloys in terms of absorption/desorption thermodynamic properties at different temperatures and for various ternary substitutions of Ni by the metal M. These data are often non-systematic and due to a different experimental temperatures, combination of absorption and desorption it is difficult to compare published results directly. Therefore before we proceed to the presentation of our results a brief overview of data published for hydrogen ab-/desorption in LaNi5 based alloys is presented. We have extracted equilibrium pressure for desorption and absorption separately dividing data according to the experimental temperature into four groups: (293e298) K, 303 K, 308 K, 313 K. In Fig. 2 the equilibrium hydrogen pressure for temperature range of (293e298) K and for absorption Fig. 2(a) and desorption Fig. 2(b) are shown as a function of the concentration of element M for 0 < x < 1 and extensive selection of ternary compositions. Similar data for temperatures 303 K, 308 K, and 313 K are shown in Supplementary Fig. S1-1 e Fig. S1-3 in the Supplementary Information. The most widely described are the alloys with M ¼ Al [40,41,47e49] and Sn [52,62e66]. Fairly consistent hydrogen absorption properties of LaNi5-xMx emerge from this data (see, Fig. 2): with increasing concentration x of a substituted element the equilibrium pressure decreases. This is traditionally related to the change of the equilibrium volume; sophisticated models of this dependency were proposed [17]. Even for such well-studied elements like Al and Sn some inconsistencies in the literature exist, that can be seen when comparing Fig. 2 and Supplementary Fig.S1. This problem is known to be due to sophisticated experimental techniques, it was critically reviewed by Broom and Webb [67]. Besides Al and Sn most of the substituted elements presented in Fig. 2 are not broadly covered in the literature and contradictory reports prevent from definitive conclusions about the influence of substitution on the hydrogen sorption properties. While elements like In, Ge seem to have qualitatively similar effect on the equilibrium pressure like Al or Sn, some of the transition metals appear not to lower this pressure with

Please cite this article as: Łodziana Z et al., Ternary LaNi4.75M0.25 hydrogen storage alloys: Surface segregation, hydrogen sorption and thermodynamic stability, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2018.11.104

international journal of hydrogen energy xxx (xxxx) xxx

5

element M; DV is defined as the ratio of the LaNi4.75M0.25 unit cell volume and the unit cell volume of LaNi5 with the same number of metal atoms. The volume is expanded upon substitution when DV > 1; (ii) the change of the metal hydride formation enthalpy (DE), the definition is presented below. The hydride is thermodynamically more stable than the LaNi5 for DE > 0; (iii) surface segregation energy (EX), this is the energy difference between Ni substitution at the first surface layer and the subsequent layers of the (0001) surface of LaNi5. When EX > 0 substituted elements will segregate toward the surface. The formation enthalpy of a hydride (DE) is defined for the reactions (all considerations are normalized to one formula unit): 7 LaNi5 þ H2 /LaNi5 H7 2 7 LaNi4:75 M0:25 þ H2 /LaNi4:75 M0:25 H7 2

Fig. 2 e The effect of alloying addition on the value of the equilibrium pressure in LaNi5-xMx compounds, where M e metal. Data collected for the temperature region (293e298) K, (a) absorption and (b) desorption. On the basis: Ni [41e46]; Al [40,41,47e49]; Au [50] B [51,52]; C [13]; Co [53,54] Cr [40,55]; Cu [40]; Ga [56]; Ge [57]; In Ref. [56]; Ir [50]; Mn [40,58]; Pd [13,59]; Pt [60]; Rh [50]; Si [61,62]; Sn [52,62e66]; Zn [46].

increasing concentration (Au, Pd, Co, Cu) the others like Fe do decrease peq with concentration increasing from x ¼ 0.25 to 1.0, or any definitive conclusion can be drawn for Mn. Analysis (Supplementary Fig. S1-1 e Fig. S1-3) also indicates that for the compositional phase diagram of LaNi5-xAgx for x > 0.05 the intermetallic is not formed [68], that stands in conflict with the reports of absorption isotherms for LaNi4Ag [11] or compositional analysis [69]. The results presented below shed some light into such discrepancies.

Assumptions of the model calculations The problem of fractional substitution in LaNi5-xMx compounds is a complex one from the computational perspective. In principle it requires analysis of variety of configurations of the substituted element in the lattice for several molar fractions x of substituted element M. Here, we limit calculations to only one composition with x ¼ 0.25 that makes the problem tractable. In order to identify compositions that are interesting for practical synthesis we define three parameters/descriptors that allow comparison between compounds with LaNi5-xMx composition: (i) change of the unit cell volume, that is equivalent to the specific volume change, (DV) upon Ni substitution by new

With DE1 ¼ E0 ðLaNi5 H7 Þ  E0 ðLaNi5 Þ  72E0 ðH2 Þ, for the LaNi5 compound; is DE2 ¼ E0 ðLaNi4:75 M0:25 H7 Þ  E0 ðLaNi4:75 M0:25 Þ  72E0 ðH2 Þ enthalpy for the composition with Ni substituted by ternary element. The difference of these enthalpies DE ¼ DE2  DE1 ¼ E0 ðLaNi4:75 M0:25 H7 Þ  E0 ðLaNi4:75 M0:25 Þ  ðE0 ðLaNi5 H7 Þ  E0 ðLaNi5 Þ Þ is used here as a simple measure of the stability of hydrated new composition related to the stability of LaNi5 H7 . It requires calculations of the ground state energy for intermetallics and their hydrides. Thermodynamic stability of the ternary alloys with respect to segregation to LiNi5 plus substituted metal M or variety of binary compositions is not considered here. Thus only relative energies are compared. This reduces possible errors of the approximations, however results rely on the assumption that segregation to binary phases does not occur. We focus on the hydride phase LaNi5H7 that is rarely obtained experimentally, however it defines the thermodynamic limit of the hydride stability. Further we assume hexagonal structure for both: LaNi5 (space group: P6/mmm) [70] and LaNi5H7 (space group P63mc or P31c) compounds [71] as well that for substituted compositions the atomic configurations related to these symmetries are preserved. In the optimization procedure the symmetry is not imposed on the internal degrees of freedom and the change of the unit cell shape is not allowed, only the equilibrium volume is relaxed. Wasz et al. [72] reported linear increase of a and c lattice parameters of LaNi5xSnx with the same slope in the range of 0  x  0.44 that justifies the present approach. Such assumptions impose simplifications, however their simplicity allows for effective screening procedure.

Stability parameters Within LiNi5 structure (symmetry P6/mmm) there are two lattice sites for Ni located at 2c Wyckoff position, they share the ab-plane with La, there are three La nearest neighbors 2.899  A apart from Ni. The second site is 3g Wyckoff position placed within ab-plane in between La layers, there are four La nearest neighbors separated by 3.205  A from Ni. Further details of the optimized structures used for the present calculations are shown in Supplementary Table ST1.

Please cite this article as: Łodziana Z et al., Ternary LaNi4.75M0.25 hydrogen storage alloys: Surface segregation, hydrogen sorption and thermodynamic stability, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2018.11.104

6

international journal of hydrogen energy xxx (xxxx) xxx

For each LaNi5-xMx composition we have performed full optimization for substituted atom M located at 2c or 3g site. Similar calculations were performed for each hydride. In agreement with previous measurements [72] and calculations [28,29] substitution of Ni at 3g site energetically favorable for all substitution metals but B and Cu. These two elements have slight preference for 2c lattice sites occupation. This is along the experimental evidence for boron located within La plane in LaNi4B [73] and partial distribution of copper within 2c and 3g sites in La(Ni1-zCuz)x [74]. The details for the energy difference between 2c and 3g lattice site substitution are presented in Supplementary Fig. S2. The DV is shown in Fig. 3. The volume of the unit cell is lower than for the stoichiometric LaNi5 only for boron and silicon that is in agreement with previous studies of LaNi4B [73] and LaNi4.5Si0.5 [60]. For other elements the volume changes are up to 4%, with exception of Bi, Ca, K, Pb, Rh, Y, and Zr. Where possible the present predictions are compared with the experimental data in Fig. 3. There is a good agreement for all elements. When compared to the previous calculations of the substitution LaNi5-xMx (x ¼ 0.5) for Al, Co, Fe, Mn [28]; the volume change calculated here is smaller due to lower fraction of substituted metal in the present spin polarized calculations. Test calculations without spin polarization agree with equilibrium volume ordering reported in Ref. 8. The volume change for LaNi4.75Al0.25 agrees well with DV ¼ 0.9% calculated in Ref. [29]. The good agreement between calculated volume change DV and the experimental data assures the validity of the present approach. The large lattice expansion for alkaline earth metals, Y or Zr require clarification: it the present methodology that assumes only Ni substitution, while alkaline earth metal elements might prefer substitution at the La sublattice [13].

The second parameter of interest: the change of the hydride formation enthalpy DE allows identification of interesting compositions to be synthesized and carefully studied for hydrogen sorption properties. The DE close to zero means that the thermodynamic stability of a new hydride is not significantly altered with respect to LiNi5H7. This is important once the cycling stability is crucial for hydrogen storage, the large hydride formation enthalpy alteration could lead to the phase segregation and local inhomogeneity of the composition. The calculated DE extends toward positive and negative values that means the substitution of Ni can either stabilize or destabilize the hydride phase depending on a substituting element, see Fig. 4. For the elements like Ag, B, Co, Fe, Ga, Pd, Sb, W or Zn the DE is within the range ± 0.05 eV (~4.8 kJ/mol), marked with gray shade in Fig. 4. Metals like Al or Sn known as having a beneficial effect on the kinetic and thermodynamic hydrogen sorption properties of LaNi5 are marginally destabilizing the calculated ground state formation enthalpy of the hydrides. Very large destabilizing effect of Ca, K, Y and Zr is due to their substitution at the Ni sublattice. In Fig. 5 the segregation energy Ex per formula unit is presented. Only the low energy (0001) surface facet is considered. The energy was calculated for LaNi4.75M0.25 stoichiometry. For the slab geometry with 12 formula units it requires distribution of 3 independent substituted atoms in the surface region. Several possible configurations were considered and substituted atoms were placed up to third sub-surface layer, details are presented in Supplementary Fig. S3 of the Supplementary Information. Only the lowest energy configuration for each element in the subsurface distribution is considered for calculation of Ex. With exception of: Al, Co, Cr, Fe, Ga, Ir, Mn, Mo, Nb, Ru, Si, Ti, W, V, and Zr substituted elements segregate toward the surface. Incorporation of

Fig. 3 e The volume per formula unit of the substituted LaNi4.75M0.25 versus the volume of LaNi5. Experimental data are marked with black, green and red bars [42,61,75e78]. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article). Please cite this article as: Łodziana Z et al., Ternary LaNi4.75M0.25 hydrogen storage alloys: Surface segregation, hydrogen sorption and thermodynamic stability, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2018.11.104

international journal of hydrogen energy xxx (xxxx) xxx

7

Fig. 4 e The stability of the substituted hydrides with respect to the stability of LaNiH7, per formula unit of hydride; for definition of DE see text. The gray region marks the stability with the deviation not larger than ±0.05 eV per formula unit. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article).

Fig. 5 e The surface segregation energy Ex, see text. The light blue lines indicate compositions where surface segregation is not favorable and the stable bulk alloys are formed. The presented energy is related to the most stable sub-surface configuration. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article).

substituted atoms in the subsurface layers require energy of the range 0.1e0.3 eV/f. unit (9.6e29.0 kJ/mol). The elements not segregating toward the surface would alloy and should remain stable in the bulk upon cycling

hydrogen ab/desorption. This is well known for LaNi5-xAlx, however, interestingly for LaNi5-xSnx the present calculations indicate Sn segregation toward the surface. The segregation energy is small (Ex ~0.05 eV, Fig. 5) that prevents from and

Please cite this article as: Łodziana Z et al., Ternary LaNi4.75M0.25 hydrogen storage alloys: Surface segregation, hydrogen sorption and thermodynamic stability, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2018.11.104

8

international journal of hydrogen energy xxx (xxxx) xxx

definitive conclusions, as the kinetic effects might be decisive. However, Sn surface enrichment can be expected upon cycling. The presence of Sn within the surface layers might enhance kinetics of the hydrogen diffusion. In general, all substituted elements located at surface layer shall alter the activity of an alloy with respect to the dissociation/activation of adsorbed gas molecules. Also the surface substitution would influence the hydrogen diffusion toward the bulk LaNi5. This process will be faster for the compositions that form less stable hydrides than LaNi5, more stable hydrides might form passivation layer and slow down the kinetics. In order to verify these predictions four elements were chosen for experimental synthesis and measurements of absorption isotherms.

Selection of new LaNi5 compositions Based on three calculated parameters the selection rules for new composition that has a potential to be catalytically active are following: (1) the volume change (DV) of the intermetallic compound shall be small to avoid phase segregation; (2) the change of the hydride formation enthalpy (DE) shall be small compared to the pure LaNi5 in order to prevent from segregation of the hydride phases and reaching the equilibrium conditions; (3) the doped atoms shall segregate toward the surface, as we expect that only modification of the surface composition is active toward the contamination gasses, like CO2 or CO. Based on the analysis presented in Figs. 3e5 the following ternary compositions were synthesized: LaNi4.75Ag0.25, as silver appears to be the element that fulfills all criteria above. The volume change of the LaNi4.75Ag0.25 is ~2%, the hydride (LaNi4.75Ag0.25H7) formation enthalpy decreases by less than 0.5 eV/f. unit and there is significant thermodynamic driving force for the surface segregation. LaNi4.75Bi0.25 e Bi shows a low hydride formation enthalpy change, it strongly modifies the intermetallic structure as well as it has large surface segregation energy. Combination of these parameters shall have negative influence on the hydrogen capacity and might lead to the segregation into binary alloys. LaNi4.75Fe0.25 e Fe weakly affects the structure as well as thermodynamic of the hydride, it does not segregate toward the surface e thus we do not expect any catalytic activity of this element. The formation of LaNi4.75Fe0.25 shall be neutral for hydrogen capacity. LaNi4.75Pb0.25 e Pb introduced into LaNi5 induces structural modifications and alters the hydride formation enthalpy in a similar range as Sn. LaNi5-xSnx is known to retain advantageous influence on the equilibrium pressure of H2. Pb has large surface segregation energy. Thus, if the ternary alloy LaNi4.75Pb0.25 is formed it shall affect hydrogen sorption/ desorption in a similar manner as Sn. LaNi4.75Si0.25 e Si is the element that not cause the lattice expansion, shall not segregate toward the surface and LaNi4.75Si0.25H7 shall be more stable than LaNi5H7. Before we proceed to the discussion of the results the synthesis, structural characterization and hydrogen sorption properties of these compounds is presented.

XRD and pcT measurements The reference alloy LaNi5 and alloys with a nominal initial stoichiometry of LaNi4.75M0.25, where M ¼ Ag, Fe, Bi, Pb, and Si were prepared by a repetitive melting in an argon shield. The bulk material, after annealing, was crushed in the air and crushed/pulverized with an agate mortar in ethyl alcohol. The structure of the powder samples was examined with X-ray diffraction at RT and the results are presented in Fig. 6(a). This structural analysis indicates that for all samples, the reflections corresponding to the hexagonal structure, described with the space group of P63/mmm are present. The lattice parameters, enthalpy of H2 desorption, XRD refinement patterns for all synthesized samples are presented in the Supplementary data (Table ST2, Fig. S7-1 e Fig. S7-7). Lattice parameters for LaNi5 correspond very well to the literature [79,80], and they agree well with the present calculations, Supplementary Table ST1. The lattice parameters of the ternary alloys with Fe and Si agree very well with the previous literature reports extrapolated to x ¼ 0.25 [81,82] and they also fit quite well to the present calculations (see Fig. 3). In the sample with bismuth a strong additional lines corresponding to Bi-La binary alloy (Fm-3m) were observed (dots in Fig. 6(a)) [83] and the lattice parameters for the hexagonal phase are very close to these for LaNi5. For alloys with Ag and Pb the measured volume change is much smaller than expected from the calculations. For LaNi4.75Ag0.25 nominal composition the determined lattice

Fig. 6 e The XRD patterns for LaNi5 and LaNi5-xMx alloys (with nominally x ¼ 0.25), where M ¼ Bi, Ag, Si, Pb, Fe; (a) before hydrogenation, (b) after hydrogenation (and pcT studies). All the samples reveal the hexagonal structure. Circles correspond to the BiLa compound (the strongest lines). More in text.

Please cite this article as: Łodziana Z et al., Ternary LaNi4.75M0.25 hydrogen storage alloys: Surface segregation, hydrogen sorption and thermodynamic stability, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2018.11.104

international journal of hydrogen energy xxx (xxxx) xxx

parameters of the hexagonal structure (P6/mmm) perfectly match the structure of LaNi4.95Ag0.05 [68]. In fact it was suggested [68] that the solubility limit of Ag in LaNi5-xAgx is x ¼ 0.05 and it is not possible to obtain LaNi5-xAgx, with x > 0.05. As the present calculations do not tackle the stability of individual ternary alloys we assume that the very low solubility of Ag in Ni is responsible for discrepancy of the lattice parameters, and Ag segregates toward the surface according to calculations. The sample with Pb has the volume very close to the pure LaNi5 alloy and we observe no indication of any other binary phases nor elemental Pb. We conclude that Pb is distributed randomly at the surface of the crystallites as the surface segregation energy is large for this element. In fact in the phase diagram for La-Ni-Pb only three stable phases with compositions La4Ni3Pb4, La5NiPb3, and La12Ni6Pb were reported [88] but we do not observe any of these structures in the X-ray diffraction pattern. XRD results for samples after pcT studies are presented in Fig. 6(b). In general, it can be concluded that the samples retain the initial structure. Even in the case of a sample doped with bismuth, the admixture of the BiLa phase was preserved. For each sample, after hydrogenation, the diffraction lines are broader and diffraction lines are reduced due to the hydrogen decrepitation (HD) [84]. Hydrogen decrepitation is caused by the volume expansion which for highly saturated LaNi5 belongs to the largest among intermetallic compounds (more than 25%) [1,85] Hydrogen introduced into LaNi5 causes the expansion of the unit cell and straining. When the strain exceeds a critical threshold for cracking spalling occurs. Fresh surfaces formed by the spalling cause fast hydrogen absorption which results in further straining. This process leads to pulverization of the starting material [86] which consequently manifests itself in the broadening of the diffraction lines. The desorption curves obtained at 313 K in pcT measurements for samples with an initial nominal stoichiometry of

9

LaNi4.75M0.25 with M: Ni, Fe, Si, Bi and M: Ni, Pb, Ag are presented in Fig. 7(a) and Fig. 7(b) respectively. The compositions of alloys shown in Fig. 7(a) have sorption properties inferior compared to LaNi5. There is no distinct plateau pressure for any of the Bi, Si or Fe substitutions and the desorption curves are sloped without multiple plateau features. The hydrogen saturation is lower compared to LaNi5. It was noted that the sample with Fe substitution is more difficult to desorb hydrogen compared to the other samples (hydrogen pressure does not reach even 0.2 MPa). This suggest a slower kinetics of the desorption reaction in this alloy, thus the hydrogen desorption was not completed within the time of the experiment (40 min) as shown in Fig. 7(a). In contrast to data presented in Fig. 7(a), clear single plateau are observed for alloys with Ni substituted with Pb and Ag (see, Fig. 7(b)). In each case, the hydrogen saturation is similar; c.a. 1.4 wt% at hydrogen pressure ~0.4 MPa. Also, the reversible storage capacity of ~1.2 wt %, which represents an usable application area [10] for LaNi5 is observed for alloys with Ag and Pb. The pcT measurements were repeated three times in order to confirm full reversibility of hydrogen cycling. No significant changes in H2 absorption/desorption were observed after third cycle. The hydrogen pressures corresponding to the desorption plateau centers are estimated at 0.300 MPa (Pb), 0.285 MPa (Ni) and 0.250 MPa (Ag). The admixture of Pb introduces minor increase of the equilibrium hydrogen pressure with respect to the LaNi5, while this pressure is reduced by more than 14% by Ag substitution. In these two compounds (LaNi4.75Ag0.25, LaNi4.75Pb0.25) a single horizontal plateaus are observed. For comparative purposes, the LaNi5 and nominal LaNi4.75Ag0.25 alloy pcT curves are depicted in Supplementary data in Fig. S4. Plateau centers for absorption is at 0.380 MPa (Ni) and at 0.340 MPa (Ag). This means that the alloy with Ag absorbs/desorbs at lower hydrogen pressures than LaNi5, while maintaining the same saturation and

Fig. 7 e PCI (pressure e composition isotherms), H2 desorption for LaNi5-xMx M: Ni, Fe, Si, Bi (a) and M ¼ Ni, Pb, Ag (b) alloys at T ¼ 313 K. The nominal stoichiometry was x ¼ 0.25. Please cite this article as: Łodziana Z et al., Ternary LaNi4.75M0.25 hydrogen storage alloys: Surface segregation, hydrogen sorption and thermodynamic stability, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2018.11.104

10

international journal of hydrogen energy xxx (xxxx) xxx

reversible storage capacity. The adsorption/desorption parameters of the reference LaNi5 match very well the literature data [87]. Comprehensive visualization of all measured PCI curves is presented in Fig. S4-2 to Fig. S4-6 of the supplementary data.

Discussion Above we have analyzed the influence of Ni substitution by a broad range of elements in LaNi5 compound. DFT calculations were performed in order to asses change of the specific volume, formation enthalpy of hydrides, and surface segregation energy for the compositions LaNi4.75M0.25 with elements M: Ag, Al, Au, B, Bi, Ca, Cd, Cu, Cr, Fe, Ga, Ge, In, Ir, K, Mg, Mn, Mo, Nb, Pb, Pd, Pt, Rh, Ru, Sb, Sn, Ti, V, W, Y, Zn, and Zr. With exception of B and Si, in agreement with previous studies of LaNi4B [73] and LaNi4.5Si0.5 [60], the volume change is below 4% for the majority of substituted elements. Larger volume expansion is observed for Bi, Ca, K, Rh, Y, and Zr. This is related to preference of these elements to locate at La lattice site, not at Ni site as considered in the present approach. The surface segregation energy for all substituted elements is in the range ± 0.2 eV (20 kJ/mol) and the majority of elements have affinity to segregation toward the surface. Elements: Al, Co, Cr, Fe, Ga, Ir, Mn, Mo, Nb, Ru, Si, Ti, W, V, and Zr are more stable in the bulk substitution. The change of the hydride formation enthalpy due to Ni substitution varies from hydride stabilization up to 0.12 eV/f. unit (11.6 kJ/mol) to destabilization by 0.55 eV/f. unit (53 kJ/mol). For elements like Ag, B, Co, Fe, Ga, Pd, Sb, W or Zn the change of the hydride formation enthalpy is within the range ±0.05 eV (~4.8 kJ/mol). Based on theoretical predictions five of these compositions (with M ¼ Bi, Fe, Si, Pb, and Ag) were synthesized and analyzed with respect to structure and hydrogen absorption/desorption properties. The LaNi4.75Ag0.25 composition shows H2 absorption/desorption properties clearly favorable when compared to LaNi5. Also for LaNi4.75Pb0.25 hydrogen cycling is not worse than in the initial composition. These two elements, according to the present calculations, shall segregate toward the surface as well as modify thermodynamic stability of the hydride and induce volume expansion of ternary alloy with respect to LaNi5. Pb has stronger tendency or surface segregation, moreover no solubility of Pb component in the binary compound LayNix was reported [88]. Only three ternary compositions are known: La4Ni3Pb4, La5NiPb3, and La12Ni6Pb, we do not observe any of these phases. The phase diagram of La-Ni-Ag suggests that LaNi5xAgx for x > 0.05 the intermetallic shall not exist [68], that stands in conflict with the reports of absorption isotherms for LaNi4Ag [11] or the compositional analysis of the same composition [69]. Since this composition has the most interesting properties among the synthesized samples we have determined the composition of the sample after pcT experiments with nominal stoichiometry LaNi4.75Ag0.25, using SEM analysis. Excluding oxygen, the average abundance of elements in five different areas of the sample was as follows: Ag -> 3.0 (5) At%, La -> 17.5 (7) At%, and Ni -> 79.5 (9) At%, which translates into relative stoichiometry of La100Ni457(25)Ag171(20). This results indicate the imbalance of

the nominal stoichiometry, on the one hand, but it confirms the presence of elements in the sample. The mass loss of the post-melted sample was ~2% due to evaporation. Calculations suggest that Ag is located in the vicinity of the crystallite surface. The elemental analysis was also performed for Bi doped sample. It indicates the tendency to segregate into separate phases that is also visible on XRD. In fact calculations predict large surface segregation and structure modification for Bi added to LaNi5. The pcT analysis for Bi as well as for Fe and Si indicate that for these elements degeneration of hydrogen interaction with these ternary alloys is observed - both as lack of distinct plateau pressure and slow kinetics. Calculations indicate that Fe and Si prefer to form bulk phases, thus slow kinetics can be explained as a perturbation in the hydrogen diffusion in the bulk material preventing prompt formation of the hydride phase. The pcT curve for LaNi4.75Bi0.25 is far from ideal what is directly related to the phase segregation. The present calculations provide variety of properties for ternary LaNi4.75M0.25 alloys. Calculated change of the alloys specific volume (DV) with composition can be related to atomic radii of Ni and substituted elements. In Supplementary Fig. S5-1 the (DV)1/3 against atomic radii according to Pauling [89], Slater [90] and Rahm et al. [91] are presented. For atomic radii according to Pauling and Slater fairly obvious linear dependence of the volume change with increasing radius of substituted element is observed. This indicates that in the low substitution limit the volume occupied by substituted atoms is decisive for the increase/ decrease of the specific volume of LaNi4.75M0.25 alloy. This correlation does not exclude other factors, like electronic effects, structural modification to be present at higher concentrations of doped element. Experimentally observed the volume alternation indicates deviation for the linear relation between volume and amount of doped elements above x ¼ 0.5 for LaNi4.75M0.25 [72]. The surface segregation could be one of the reasons. Since a large surface energy differences can result in segregation of elements in an alloy an attempt to relate the surface energies for elements calculated by Skriver and Rosengaard [92] and surface segregation calculated here was undertaken. Taking the lowest surface energy for each element [92] one can compare this value with surface energy of Ni. Once the surface energy of the substituted element is lower than this for Ni this might be a driving force for segregation. Such comparison gives no clear evidence that the segregation of elements is related to the surface energy of elements, see Supplementary Fig. S5-2. The substitution of Ni in LaNi5 is also related to the change of the electronic structure, especially alternation of the dstates below the Fermi level. This is directly related to the catalytic activity of the surface [93]. The ternary compositions with preference to the surface segregation shall possess properties that affect adsorption or evolution of gasses like CO or CO2. In Supplementary Fig. S6 the density of states related to the substituted elements are shown for Ag, Pb, Fe, Bi, Si and LaNi5. Ag and Fe have localized states at the energy e 5 eV below the Fermi level. These states would interact with the bonding orbitals of adsorbed molecules. Since substitution of Ni with Fe does not have preference for surface segregation the catalytic activity of this metal is not expected to have any

Please cite this article as: Łodziana Z et al., Ternary LaNi4.75M0.25 hydrogen storage alloys: Surface segregation, hydrogen sorption and thermodynamic stability, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2018.11.104

international journal of hydrogen energy xxx (xxxx) xxx

prominent effect on adsorbed molecules. On the contrary, surface segregation of Ag points to expected activity of this element for evolution of adsorbed CO or CO2. For other elements in the series (Supplementary Fig. S6) weak states at lower energies are present, they are not relevant for the alternation of the catalytic properties of the surface.

Conclusion The influence of Ni substitution in LaNi5 by a broad range of elements was investigated with ab initio methods, synthesized and analyzed toward hydrogen sorption/desorption. We show that the volume expansion is correlated with the atomic radii of substituting elements and it falls below 4% for the majority of elements M in the composition LaNi4.75M0.25. Elements like Al, Co, Cr, Fe, Ga, Ir, Mn, Mo, Nb, Ru, Si, Ti, W, V, and Zr are more stable in the bulk while the majority of investigated substitutions shall segregate toward the surface. The segregation energy for all substituted elements is in the range ± 0.2 eV (20 kJ/mol). The change of the hydride formation enthalpy due to Ni substitution varies from stabilization by 0.12 eV/f. unit (11.6 kJ/mol) to destabilization by 0.55 eV/f. unit (53 kJ/mol). For elements like Ag, B, Co, Fe, Ga, Pd, Sb, W or Zn the change of the hydride formation enthalpy is within the range ± 0.05 eV (~4.8 kJ/mol). The substituted elements can be catalytically active, and the ternary compounds with thermodynamic properties similar to LaNi5 where substituted element is segregating toward the surface are promising candidates for catalytically active ternary alloys. Based on the calculations five ternary compounds are synthesized and analyzed with respect to crystal structure and hydrogen sorption properties. The LaNi4.75Bi0.25, LaNi4.75Fe0.25, LaNi4.75Si0.25, confirm theoretical predictions as an examples of LaNi5 modification not beneficial for hydrogen storage. Compounds like LaNi4.75Ag0.25. and LaNi4.75Pb0.25 show favorable stability and H2 sorption thermodynamics.

[2]

[3]

[4]

[5]

[6]

[7]

[8]

[9]

[10]

Acknowledgements This work was financed in the frame of projects: BIOSTRATEG2/297310/13/NCBR/2016 by NCBiR and the research program 04-4-1121-2015/2020 (Poland-JINR Dubna), CPU allocation at PL-Grid is kindly acknowledged. The authors
for the microstructure characteracknowledge Dr. A. Sypien ization (SEM).

[11]

[12]

[13]

Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.ijhydene.2018.11.104.

references

[14]

[15]

[16] [1] Van Vucht JHN, Kuijpers FA, Bruning HCAM. Reversible room-temperature absorption of large quantities of

11

hydrogen by intermetallic compounds. Philips Res Rep 1970;25:133e40. Lototskyy MV, Yartys VA, Pollet BG, Bowman RC. Metal hydride hydrogen compressors: a review. Int J Hydrogen Energy 2014;239:5818e51. Kim J-K, Park I-S, Kim K J, Gawlik K. A hydrogen-compression system using porous metal hydride pellets of LaNi5-xAlx. Int J Hydrogen Energy 2008;33:870-77. https://doi.org/10.1016/j.ijhydene.2007.10.027, https://doi. org/10.1016/j.ijhydene.2014.01.158. Ouyang L, Huang J, Wang H, Liu J, Zhu M. Progress of hydrogen storage alloys for Ni-MH rechargeable power batteries in electric vehicles: a review. Mater Chem Phys 2017;200:164e78. https://doi.org/10.1016/j.matchemphys.2017.07.002. Jurczyk M, Smardz L, Smardz K, Nowak M, Jankowska E. Nanocrystalline LaNi5-type electrode materials for Ni-MHx batteries. J Solid State Chem 2003;171:30e7. https://doi.org/ 10.1016/S0022-4596(02)00142-1. Cuevas F, Joubert J-M, Latroche M, Percheron-Guegan A. Intermetallic compounds as negative electrodes of Ni/MH batteries. Appl Phys A 2001;72:225e38. https://doi.org/10. 1007/s003390100775. Sandrock GD, Goodell PD. Surface poisoning of LaNi5, FeTi and (Fe,Mn)Ti by O2, CO and H2O. J Less Common Met 1980;73:161e8. https://doi.org/10.1016/0022-5088(80)90355-0. Sandrock GD, Goodell PD. Cyclic life of metal hydride with impure hydrogen: overview and engineering considerations. J Less-Common Met 1984;104:159e73. Yang FS, Chen XY, Wu Z, Wang SM, Wang GX, Zhang ZX, Wang YQ. Experimental studies on the poisoning properties of a low-plateau hydrogen storage alloy LaNi4.3Al0.7 against CO impurities, Int J Hydrogen Energy 2017; 42: 16225 -16234. Modibane KD, Williams M, Lototskyy M, Davids MW, Klochko Y, Pollet BG. Poisoning-tolerant metal hydride materials and their application for hydrogen separation from CO2/CO containing gas mixtures hydrogen energy Int J Hydrogen Energy 2013; 38: 9800-9810. Ivey DG, Northwood DO. Storing energy in metal hydrides: a review of the physical metallurgy. J Mater Sci 1983;18:321e47. https://doi.org/10.1007/BF00560621. Park Ch-N, Luo S, Flanagan TB. Analysis of sloping plateaux in alloys and intermetallic hydrides: I. Diagnostic features. J Alloys Compd 2004;384:203e7. https://doi.org/10.1016/j. jallcom.2004.04.101. Broom DP. Hydrogen storage materials the characterisation of their storage properties. Springer e Verlag: London Ltd; 2011. Zu¨ttel A, Borgschulte A, Schlapbach L, Hydrogen as a future energy Carrier. Wiley-VCH Verlag GmbH & Co KGaA. Weinheim; 2008. Van Mal HH, Buschow KHJ, Miedema AR. Hydrogen absorption in LaNi5 and related compounds: Experimental observations and their explanation. J Less-Common Met 1974;35:65e76. https://doi.org/10.1016/0022-5088(74)90146-5. Borzone EM, Blanco MV, Baruj A, Meyer GO. Stability of LaNi5xSnx cycled in hydrogen. Int J Hydrogen Energy 2014;39:8791e6. https://doi.org/10.1016/j.ijhydene.2013.12.031. Goodell PD, Sandrock GD, Huston EL. Microstructure and hydriding studies of AB5 hydrogen storage compounds. Rept. SAND79-7095. Albuquerque NM: Sandia N. L.; 1980. p. 1e17. 87185. Hume-Rothery W, Mabbott GW, Channel Evans KM. The freezing points, melting points, and solid solubility limits of the alloys of silver and copper with the elements of the b sub-groups. Philos Trans R Soc Lond Ser A 1934;233:1e97. De Boer FR, Boom R, Matten WCM, Miedema AR, Niessen AK. Cohesion in metals. Transition metal alloys. 1989. NorthHolland, Amsterdam. Pettifor DG. The structures of binary compounds. I. Phenomenological structure maps. J Phys C Solid State Phys 1986;19:285e313.

Please cite this article as: Łodziana Z et al., Ternary LaNi4.75M0.25 hydrogen storage alloys: Surface segregation, hydrogen sorption and thermodynamic stability, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2018.11.104

12

international journal of hydrogen energy xxx (xxxx) xxx

[17] Mendelsohn M, Gruen D, Dwight A. Group 3A and 4A substituted AB5 hydrides. Inorg Chem 1979;18:3343e5. https://doi.org/10.1021/ic50202a013. [18] US Department of Energy, http://hydrogenmaterialssearch. govtools.us/search.aspx. [19] Chojnacka A, Błaszczyk MK, Szcze˛sny MK, Nowak K, _
ska M, Tomczyk-Zak Sumin K, et al. Comparative analysis of hydrogen-producing bacterial biofilms and granular sludge formed in continuous cultures of fermentative bacteria. Biores Technol 2011;102:10057e64. https://doi.org/10.1016/j. biortech.2011.08.063. [20] Jain DA, Ong SP, Hautier G, Chen W, Richards WD, Dacek S, et al. Commentary: the Materials Project: a materials genome approach to accelerating materials innovation. APL Mater 2013;011002:1e11. https://doi.org/10.1063/1.4812323. [21] Taylor RH, Rose F, Toher C, Levy O, Yang K, Nardelli MB, et al. A RESTful API for exchanging materials data in the AFLOWLIB.org consortium. Comput Mater Sci 2014;93:178e92. [22] Malik SK, Arlinghaus FJ, Wallace WE. Calculation of the spinpolarized energy-band structure of LaNi5 and GdNi5. Phys Rev B 1982;25:6488e91. [23] Sluiter M, Takahashi M, Kawazoe Y. Theoretical study of phase stability in LaNi5-LaCo5 alloys. J Alloys Compd 1997;248:90e7. [24] Gupta M. Electronic structure and stability of hydrides of intermetallic compounds. J Alloys Compd 1999;293e295:190e201. [25] Yang JB, Tai CY, Marasinghe GK, Waddill GD, Pringle OA, James WJ. Electronic structures and magnetism of LaNi5exFex compounds. J Appl Phys 2001;89:7311e3. [26] Gupta M. Electronic properties of LaNi5 and LaNi5H7. J LessCommon Met 1987;130:219e28. [27] Hector Jr LG, Herbst JF, Capehart TW. Electronic structure calculations for LaNi5 and LaNi5H7: energetics and elastic properties. J Alloys Compd 2003;353:74e85. [28] Zhang C, Liu Y, Zhao X, Yan M, Gao T. First-principles study of the crystal structures and electronic properties of LaNi4.5M0.5 (M¼Al, Mn, Fe, Co. Compd Mater Sci 2013;69:520e6. [29] Zhang RJ, Wang YM, Lu MQ, Xu DS, Yang K. Firstprinciples study on the crystal, electronic structure and stability of LaNi5xAlx (x ¼0, 0.25, 0.5, 0.75 and 1). Acta Mater 2005;53:3445e52. https://doi.org/10.1016/j.actamat. 2005.04.005. [30] Herbst JF, Hector Jr LG. La(TM)5 hydrides (TM ¼ Fe, Co, Ni): theoretical perspectives. J Alloys Compd 2007;446e447:188e94. https://doi.org/10.1016/j.jallcom.2006. 12.003. [31] Kresse G, Furthmuller J. Efficient iterative schemes for ab initio total-energy calculations using a plane-wave basis set. Phys Rev B 1996;54:11169e86. [32] Kresse G, Furthmuller J. Efficiency of ab-initio total energy calculations for metals and semiconductors using a planewave basis set. Comput Mater Sci 1996;6:15e50. [33] Blochl PE. Projector augmented-wave method. Phys Rev B 1994;50:17953e79. [34] Kresse G, Joubert D. From ultrasoft pseudopotentials to the projector augmented-wave metho. Phys Rev B 1999;59:1758e75. [35] Perdew JP, Burke K, Ernzerhof M. Generalized gradient approximation made simple. Phys Rev Lett 1996;77:3865e8. [36] Dudarev SL, Botton GA, Savrasov SY, Humphreys CJ, Sutton J. Electron-energy-loss spectra and the structural stability of nickel oxide: an LSDAþU study. Phys Rev B 1998;57:1505e9. [37] Rietveld HM. Line profiles of neutron powder-diffraction peaks for structure refinement. Acta Crystallogr

[38]

[39] [40] [41]

[42]

[43]

[44]

[45]

[46]

[47]

[48]

[49]

[50]

[51]

[52]

[53]

1967;22:151e2. Rietveld HM. A profile refinement method for nuclear and magnetic structures. J Appl Crystallogr 1969;2:65-71. https://doi.org/10.1107/S0021889869006558, https://doi.org/10.1107/S0365110X67000234. Le Bail A, Duroy H, Fourquet JL. Ab-initio structure determination of LiSbWO6 by X-ray powder diffraction. Mat. Res Bull 1988;23:447e52. Rodrigues-Carvajal J. FullProf.2k ILL. 2011. http://www.ill.eu/ sites/fullprof/php/programs.html. Pawley GS. Unit-cell refinement from powder diffraction scans. J Appl Crystallogr 1981;14:357e61. Sakai T, Oguro K, Miyamura H, Kuriyama N, Kato A, Ishikawa H. Some factors affecting the cycle lives of LaNi5based alloy electrodes of hydrogen batteries. J Less-Common Met 1990;161:193e202. Mordkovich VZ. Equilibria in CexLa1xNi5yAly-H2 systems at subcritical and supercritical parameters. J Alloy Compd 1992;187:9-15. https://doi.org/10.1016/0925-8388(92)90515-B, https://doi.org/ 10.1016/0022-5088(90)90027-H. Lakner JF, Uribe FS, Steward SA. Hydrogen and deuterium sorption by selected rare earth intermetallic compounds at pressures up to 1500 atm. J Less-Common Met 1980;72:87e105. https://doi.org/10.1016/0022-5088(80)90255-6. Shilov AL, Kist ME, Kuznetsov NT. The system LaNi5H2. J Less-Common Met 1988;144:23e30. https://doi.org/10.1016/ 0022-5088(88)90345-1. Van Vucht JHN, Kuljpers FA, Bruning HCAM. Reversible room-temperature absorption of large quantities of hydrogen by intermetallic compounds. Philips Res Refits 1970;25:133e40. Kisi EH, Mac E, Gray A, Kennedy SJ. A neutron diffraction investigation of the LaNi5D phase diagram. J Alloy Compd 1994;216:123e9. https://doi.org/10.1016/0925-8388(94)910537.
ska-Kiełbik B, Iwasieczko W, Drulis H, Pavlyuk VV, _ zy _ n Rozd Bala H. Hydrogenation equilibria characteristics of LaNi5xZnx intermetallics. J Alloy Compd 2000;298:237e43. https://doi.org/10.1016/S0925-8388(99)00616-7. Verbetsky VA, Shilov AL, Kuznetsov NT. Hydrogen interaction with LaNi5þ/-xMnyAlz. J Inorg Chem-USSR 1989;34:2403e6. Liu J, Li K, Cheng H, Yan K , Wang Yu, Liu Yi, Jin H, Zheng Z. New insights into the hydrogen storage performance degradation and Al functioning mechanism of LaNi5-xAlx alloys, Int J Hydrogen Energy Liu, 2017; 42: 24904-24914.
gan A, Chabre Y, Bouet J, Latroche M, Percheron-Gue Pannetier J, Ressouche E. Intrinsic behaviour analysis of substituted LaNi5-type electrodes by means of in-situ neutron diffraction. J Alloy Compd 1995;231:537e45. https:// doi.org/10.1016/0925-8388(95)01723-2. Huston EL, Sandrock GD. Engineering properties of metal hydrides. J Less-Common Met 1980;74:435e43. https://doi. org/10.1016/0022-5088(80)90182-4. Prigent J, Joubert J-M, Gupta M. Modification of the hydrogenation properties of LaNi5 upon Ni substitution by Rh, Ir, Pt or Au. J Alloys Compd 2012;511:95e100. https://doi. org/10.1016/j.jallcom.2011.08.094. Spada F, Oesterreicher H. Hydrogen absorption in La3Ni13B2. J Less-Common Met 1983;90:L1e4. https://doi.org/10.1016/ 0022-5088(83)90128-5. Mendelsohn MH, Gruen DM. The Effect of group III a and IV a element substitutions (M) on the hydrogen dissociation Pressures of LaNi5-xMx hydrides. In: McCarthy GJ, Rhyne JJ, Silber HB, editors. The rare earths in modern science and technology, vol. 2. Plenum Press; 1980. p. 593e8. Percheron-Guegan A, Latroche M, Achard JC, Chabre Y, Bouet J. Hydrogen and metal hydride batteries. In: Bennett PD, Sakai T, editors. The electrochemical society, vols. 94e27; 1994. p. 196e218.

Please cite this article as: Łodziana Z et al., Ternary LaNi4.75M0.25 hydrogen storage alloys: Surface segregation, hydrogen sorption and thermodynamic stability, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2018.11.104

international journal of hydrogen energy xxx (xxxx) xxx


gan A, Boure
e-Vigneron F. [54] Latroche M, Percheron-Gue Investigation of the crystallographic structures of LaNi4CoD4.4 and LaNi3.55Mn0.4Al0.3Co0.75Dx (x ¼2.0 and 4.6 D/ f.u.) by neutron powder diffraction. J Alloy Compd 1998;265:209e14. https://doi.org/10.1016/S0925-8388(97) 00426-X. [55] Misawa T, Nomachi AP, Yajima S, Sugawara H. Effect of Fe and Cr Substitution on the hydride Formation in LaNi5. J Japan Inst Metals 1979;43:104e10. [56] Mendelsohn MH, Gruen DM, Dwight AE. The effect on hydrogen decomposition pressures of group IIIA and IVA element substitutions for Ni in LaNi5 alloys. Mater Res Bull 1978;13:1221e4. https://doi.org/10.1016/0025-5408(78)90212-X. [57] Witham C, Bowman Jr RC, Fultz B. Gas-phase H2 absorption and microstructural properties of LaNi5-x Gex alloys. J Alloys Compd 1997;253e254:574e8. https://doi.org/10.1016/S09258388(96)02931-3. [58] Lundin CE, Lynch FE. Modification of hydriding Properties of AB5 type hexagonal Alloys through manganese substitution. In: Veziroglu TN, editor. Proc int conf on alternate energy sources. Univ of Miami; 1978. p. 3803e20. [59] Prigent J, Joubert J-M, Gupta M. Investigation of modification of hydrogenation and structural properties of LaNi5 intermetallic compound induced by substitution of Ni by Pd. J Solid State Chem 2011;184:123e33. https://doi.org/10.1016/j. jssc.2010.10.037. [60] Joubert J-M, Charton J, Percheron-Gue
gan A. Investigation of structural and hydrogen absorption properties in the LaNi5xPtxeH2 system. J Solid State Chem 2003;173:379e86. https:// doi.org/10.1016/S0022-4596(03)00116-6. [61] Meli F, Zuettel A, Schlapbach L. Surface and bulk properties of LaNi5-xSix alloys from the viewpoint of battery applications. J Alloy Compd 1992;190:17e24. https://doi.org/ 10.1016/0925-8388(92)90167-8. [62] Ting J, Pecharsky VK, Anderson IE, Whitam C, Bowman Jr RC, Fultz B. Hydrogen in semiconductors and metals. In: Nickel NH, Jackson WB, Bowman RC, Leisure RG, editors. Materials research society symp proc, vol. 513; 1998. p. 305e10. [63] Mendelsohn M, Gruen D, Dwight A. Group 3A and 4A substituted AB5 hydrides. Inorg Chem 1979;18:3343e5. https://doi.org/10.1021/ic50202a013. [64] Cantrell JS, Beiter TA, Bowman Jr RC. Crystal structure and hydriding behavior of LaNi5ySny. J Alloy Compd 1994;207e208:372e6. https://doi.org/10.1016/0925-8388(94) 90242-9. [65] Bowman Jr RC, Lindensmith CA, Luo S, Flanagan TB, Voght T. Degradation behavior of LaNi5-xSnxHz (x ¼ 0.20 - 0.25) at elevated temperatures. J Alloys Compd 2002;330e332:271e5. https://doi.org/10.1016/S0925-8388(01)01521-3. [66] Joubert J-M, Latroche M, Cerny R, Bowman Jr RC, PercheronGuegan A, Yvon K. Crystallographic study of LaNi5-xSnx (0.2<¼x<¼0.5) compounds and their hydrides. J Alloys Compd 1999;293e295:124e9. https://doi.org/10.1016/S09258388(99)00311-4. [67] Broom DP, Webb CJ. Pitfalls in the characterisation of the hydrogen sorption properties of materials. Int J Hydrogen Energy 2017;42:29320e43. https://doi.org/10.1016/j.ijhydene. 2017.10.028. [68] Prigent J, Joubert J-M. The phase diagrams of the ternary systems LaeNieM (M ¼ Re, Ru, Os, Rh, Ir, Pd, Ag, Au) in the La-poor region. Intermetallics 2011;19:295e301. https://doi. org/10.1016/j.intermet.2010.10.016. [69] Zu¨chner H, Kintrup J, Dobrileit R, Untiedt I. Chemical structure and bonding characteristics of metal hydrogen systems studied by the surface analytical techniques SIMS and XPS. J Alloys Compd 1999;293e295:202e12. [70] Kisi EH, Buckley CE, Gray EM. The hydrogen activation of LaNi5. J Alloys Compd 1992;185:369e84. dos Santos DS,

[71]

[72] [73]

[74]

[75] [76]

[77]

[78]

[79]

[80]

[81]

[82]

[83]

[84]

[85]

[86]

[87]

[88]

13

Bououdina M, Fruchart D. Structural and hydrogenation properties of an 80wt%TiCr1.1V0.9e20wt%LaNi5 composite material , Int J Hydrogen Energy 2003; 28: 1237 e 1241, https://doi.org/10.1016/0925-8388(92)90484-Q. Nakamura H, Nguyen-Manh D, Pettifor DG. Electronic structure and energetics of LaNi5, a-La2Ni10H and bLa2Ni10H14. J Alloys Compd 1998;281:81e91. https://doi.org/ 10.1016/S0925-8388(98)00794-4. Wasz ML, Desch PB, Schwarz RB. The effect of tin alloying on the structure of LaNi5. Phil Mag 1996;74:15e22. Filinchuk Y, Yvon K. Boron-induced hydrogen localization in the novel metal hydride LaNi3BH(x) (x ¼ 2.5 - 3.0). Inorg Chem 2005;44:4398e406. Latroche M, Joubert JM, Percheron-Guegan A, Notten PHL. Crystal structure of nonstoichiometric copper-substituted La(Ni1-zCuz)x compounds studied by neutron and synchrotron anomalous powder diffraction. J Solid State Chem 1999;146:313e21. Mendelsohn M, Gruen D, Dwight A. Group 3A and 4A substituted AB5 hydrides. Inorg Chem 1979;18:3343e5. Witham RCC, Bowman Jr RC, Fultz B. Gas-phase H absorption and microstructural properties of LaNi5-xGex alloys. J Alloys Compd 1997;253e254:574e8.  Interaction of hydrogen with the ina Z. Dra sner A, Blaz LaNi4.9In0.1, LaNi4.8In0.2 and LaNi4.8 alloys and their Nd analogues. J Alloys Compd 2006;420:2137. https://doi.org/10. 1016/j.jallcom.2005.11.003. Pandey SK, Srivastava A, Srivastava ON. Improvement in hydrogen storage capacity in LaNi5 through substitution of Ni by Fe. Int J Hydrogen Energy 2007;32:2461e5. 
gan A, Joubert J-M, Latroche M, Cerny R, Percheron-Gue Yvon K. Hydrogen cycling induced degradation in LaNi5 type materials. J Alloy Compd 2002;330e332:208e14. https:// doi.org/10.1016/S0925-8388(01)01640-1.
gan A, Achard JC. Thermodynamic Diaz H, Percheron-Gue and structural properties of LaNi5yAly compounds and their related hydrides. Int J Hydrogen Energy 1979;4:445e54. https://doi.org/10.1016/0360-3199(79)90104-6. Crivello J-C, Gupta M. Electronic properties of LaNi4.75Sn0.25, LaNi4.5M0.5 (M¼Si, Ge, Sn), LaNi4.5Sn0.5H5. J Alloys Compd 2003;356e357:151e5. https://doi.org/10.1016/S0925-8388(02) 01224-0. Nakamura Y, Oguro K, Uehara I, Akiba E. X-ray diffraction peak broadening and lattice strain in LaNi-based alloys. J Alloys Compd 2000;298:138e45. Yoshihara K, Taylor JB, Calvert LD, Despault JG. Rare-earth bismuthides. J Less-Common Met 1975;41:329e37. https:// doi.org/10.1016/0022-5088(75)90038-7. Uchida H, Uchida H, Huang YC. Effect of the pulverization of LaNi5 on the hydrogen absorption rate and the X -ray diffraction patterns. J Less-Common Met 1984;101:459e68. Takeshita T. Some applications of hydrogenation e decomposition e desorption - recombination (HDDR) and hydrogen - decrepitation (HD) in metals processing. J Alloys Compd 1995;231:51e9. Semboshi S, Masahashi N, Hanada S. Degradation of hydrogen absorbing capacity in cyclically hydrogenated TiMn2. Acta Mater 2001;49:927e35. An XH, Gu QF, Zhang JY, Chen SL, Yu XB, Li Q. Experimental investigation and thermodynamic reassessment of LaeNi and LaNi5 e H systems. Calphad 2013;40:48e55. https://doi. org/10.1016/j.calphad.2012.12.002. Gulay LD. Investigation of the phase diagrams of the systems ReNiePb (R¼Y, La, Ce, Dy) and crystal structures of the compounds RNiPb (R¼La, Ce, Pr) and RNiPb (R¼Y, La, Ce, Pr, Tb, Dy, Ho, Er, Tm, Lu). J Alloys Compd 2005;392:165e72.

Please cite this article as: Łodziana Z et al., Ternary LaNi4.75M0.25 hydrogen storage alloys: Surface segregation, hydrogen sorption and thermodynamic stability, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2018.11.104

14

international journal of hydrogen energy xxx (xxxx) xxx

[89] Pauling L. Atomic radii and interatomic distances in metals. J Am Chem Soc 1947;69:542e53. https://doi.org/10.1021/ ja01195a024. [90] Slater JC. Atomic radii in crystals. J Chem Phys 1964;41:3199e204. https://doi.org/10.1063/1.1725697. [91] Rahm M, Hoffmann R, Ashcroft NW. Atomic and ionic radii of elements 1e96. Chem Eur J 2016;22:14625e32. https://doi. org/10.1002/chem.201602949.

[92] Skriver HL, Rosengaard NM. Surface energy and work function of elemental metals. Phys Rev B 1992;46:7157e68. https://doi.org/10.1103/PhysRevB.46.7157. [93] Nørskov JK, Bligaard T, Rossmeisl J, Christensen CH. Towards the computational design of solid catalysts. Nat Chem 2009;1:37e46.

Please cite this article as: Łodziana Z et al., Ternary LaNi4.75M0.25 hydrogen storage alloys: Surface segregation, hydrogen sorption and thermodynamic stability, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2018.11.104