Hydrogen storage of C14-CruFevMnwTixVyZrz alloys

Accepted Manuscript Hydrogen storage of C14-CruFevMnwTixVyZrz alloys

Swe-Kai Chen, Po-Han Lee, Hui Lee, Hsen-Tsun Su PII:

S0254-0584(17)30624-7

DOI:

10.1016/j.matchemphys.2017.08.008

Reference:

MAC 19906

To appear in:

Materials Chemistry and Physics

Received Date:

01 May 2017

Revised Date:

02 August 2017

Accepted Date:

08 August 2017

Please cite this article as: Swe-Kai Chen, Po-Han Lee, Hui Lee, Hsen-Tsun Su, Hydrogen storage of C14-CruFevMnwTixVyZrz alloys, Materials Chemistry and Physics (2017), doi: 10.1016/j. matchemphys.2017.08.008

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ACCEPTED MANUSCRIPT Hydrogen storage of C14-CruFevMnwTixVyZrz alloys Swe-Kai Chen1, Po-Han Lee2, Hui Lee3, Hsen-Tsun Su3 1. Center for Nanotechnology, Materials Science, and Microsystems, National Tsing Hua University, Hsinchu 30013, Taiwan, ROC 2. Affiliated Senior High School of National Taiwan Normal University, Taipei 10658, Taiwan, ROC 3. Department of Materials Science and Engineering, National Tsing Hua University, 101 Kuang Fu Road Sec. 2, Hsinchu 30013, Taiwan, ROC Abstract The hydrogen storing behavior of C14 Laves-CruFeMnTiVZr, -CrFevMnTiVZr, CrFeMnwTiVZr, -CrFeMnTixVZr, -CrFeMnTiVyZr, and -CrFeMnTiVZrz (0 ≤ u, v, w, x, y, z ≤ 2) alloys that are designated as C14 Laves-CruFevMnwTixVyZrz alloys are investigated in this study via results of pressure-composition-isotherms (PCIs) and hydrogen absorption kinetics of the alloys. XRD patterns show that alloy structure remains as C14 Laves after hydrogen absorption suggesting that the pulverization effect is less and this benefits cycle lives of alloy. Study of change in lattice parameters before and after absorption and desorption of hydrogen, i.e., the change in volume, illustrates the pulverization effect. Maximal hydrogen absorption in atomic percent is closely related to the hydride formation enthalpy shown in a (Hatom/Matom)Hhydride curve. A composition trace map for hydrogen absorption, which benefits alloy design, is also shown in this study. Keywords: C14 Laves-CruFevMnwTixVyZrz; Pressure-composition-isotherms (PCIs); Kinetics; (Hatom/Matom)-Hhydride curve; Composition trace map 1. Introduction For the past decade, physical and chemical properties of HEAs have been extensively studied for their better properties as compared to conventional alloys. HEAs are of simple microstructure of multielement solid solution [1, 2] with capability of forming nanoprecipitate(s) [3, 4], high thermal stability [5], superior compressive mechanical properties [6], high hardness [7], good anti-corrosion [7], and with thermoelectricity [8]. There is a comprehensive study on the effect of configurational entropy on enthalpy in HEAs [9].

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ACCEPTED MANUSCRIPT Climate change due to the emission of CO2 gas [10, 11] has tried to reduce by green energy technologies such as hydrogen energy, solar energy, wind and bio power, earth heat, tide and ocean thermal conversion. Among these technologies, hydrogen energy is an effective and convenient one. Hydrogen is stored as high-pressure gas, liquefied, carbohydrides, and metal hydrides [12]. Investigations such as ThH2 [13], HfH [14], rare earth hydrides MH2 (M = La, Ce, Pr, Nd and Sm) [15], metallic hydrogen [16], LaNi5 [17], FeTi [18], Ti-Fe-V-Mn [19], LaNi4.7A10.3 [20], Mg2Ni, FeTi and LaNi5 [21], MgH2 [22], Ti0.95Zr0.05Cr1.2Mn0.8 [23], point defects in NbCr2 [24], TiVMn [25], Mg2Ni [26], TiVFe [27], organic hydrogen [28], metal and alloy hydrides [29] have been extensively explored. Recently metal and alloy hydrides have been considered as a superior way for hydrogen storage due to its characteristics for safety, low cost, and no toxic emission as well as the high amount of storage and good absorption and desorption properties. For example, LaNi5 has been a good commercial storage material [17]. Most recently, the elemental multiplication of the metal hydride system has gradually become a developing trend. Kinetics for H absorption and desorption as well as cycle life has also attracted investigators [20, 30]. HEAs have met most requirements of H storage. Chen et al. have proposed CoFeMnTixVyZrz [31], others, such as TiVFeZr [32], ZrTiVCrFeNi [33], TiZrNiMoV [34], and TiVZrNbHf [35] have also proposed. Alloys in this kind of studies are closely related with Zr. Chen proposed a composition trace map, as shown in Fig. 1, to illustrate the multiply of elements to systematically design a lot of hydrogen storing high-entropy alloys in 2007 [36]. The alloys were added from ternary TiVFe to 7-element alloys via different routes in which alloys show capable of hydrogen absorption and desorption being given a designation as shown in Fig. 1. A rule seems to appear that Zr is important in this kind of component multiplication of hydrogen alloys. The result of alloy 6E-2 (C14-CoFeMnTixVyZrz) has appeared in [36]. In this paper, we show another C14 H alloy, CrFeMnTiVZr (6E-3), where Cr substitutes Co in 6E-2, to check the effect of composition on the H storage property. 2. Experimental 2.1. Preparation of alloys and X-ray diffraction (XRD) of alloys As-cast CrurFeMnTiVZr, CrFevMnTiVZr, CrFeMnwTixVZr, CrFeMnTixVZr, CrFeMnTiVyZr, and CrFeMnTiVZrz (designated individually as Cru, Fev, Mnw, Tix, Vy and Zrz, where 0 ≤ u, v, w, x, y, z ≤ 2) were prepared in a water-cooled Cu mould in a vacuum arc remelter. Details of melting can be seen in [31]. Tables 1 & 2 show 2

ACCEPTED MANUSCRIPT data of related alloys. The deviation in composition between the designed and samples is within 2%. Alloy structures were examined in XRD (Rigaku ME510FM2, using a Cu target of Kα1 of 1.54056 Å, operated at 30 kV and 20 mA) and SEM/EDS (JEOL JSM840A with an Oxford EDS system) was used to check the microstructure of the alloys. Lattice parameters obtained from XRD were used to check the volume expansion of lattice as shown in Equation (1),

𝜀=

∆𝑉 𝑉0

=

𝑎2𝑐 ‒ 𝑎02𝑐0 𝑎02𝑐0

,

(1)

where ∆V, V0, a, c, a0, and c0 are, respectively, volume change before and after H absorption, volume before absorption, and lattice parameters after (a and c) and before (a0 and c0) H absorption. 2.2. PCI and kinetics tests Sieverts’ type H absorption and desorption tester was used to test pressurecomposition-isotherms (PCIs). As-cast samples were crushed into powders to the size of less than100-mesh for PCI and kinetics tests. Before tests, sample powders were activated at 400oC for 2 h. H-absorption kinetics were tested at 25oC, 80oC, and 150oC. LaNi5H6 was taken as a calibration reference in PCI tests. Kinetic tests were recorded with a starting applied pressure of 9.73 atm H2 for 103 s at a constant temperature. The wt% H absorption was recorded as a function of time. The time to reach an amount of 90% of maximum H absorption is defined as t0.9 as a parameter to compare the rate of H absorption. 3. Results and discussion 3.1. XRD results Lattice structure and lattice parameters before (as-cast and crushed) and after (as-Habsorbed and -desorbed) hydrogen absorption were tested by XRD. Figure 2a shows a unit cell of C14 Laves structure. XRD curve for Cr1 (Mn1) in Fig. 2b has demonstrated that the alloy has C14 Laves before and after hydrogenization. Figure 2c shows XRD patterns in detail that are of C14 Laves for as-cast and as-Habsorbed CrFeMnTiVZr. XRD patterns in Figs. 2b and 2d-2ε show only one C14Laves phase before and after hydrogenization, like that in Figure 2c, except for Fig. 2β. Since only before-hydrogenization XRD data for Zr0 is able to be obtained, there 3

ACCEPTED MANUSCRIPT are no data for the after-hydrogenization Zr0. Except for the Laves phase, there are 3 alloys with an extra HCP phase existing in Fe0, V0, and Zr2 before and after hydrogenization and BCC phase existing in Cr1, Fe1.5, Fe2, Ti0, Ti0.5, V1.5 and Zr0.5. V2.0 has the maximum amount of FCC precipitation (see Fig. 2α). Why the BCC phase after hydrogenization (Fig. 2α) changes to the FCC is still unknown. Since there are two possible ways to absorb hydrogen for the BCC phase, one is keeping the BCC solid solution and the other changing to FCC hydride. Therefore it is reasonable for existing a change of BCC solution to FCC hydride in this case. In Fig. 2c, a BCC weak pattern ascribed to a less amount of precipitate is seen in the patterns. Peak shifts in patterns are possibly due to retained hydrogen and lattice distortion after Hdesorption. From the analyses of volume change after H-absorption plastic deformation is not the major factor of peak shifts. The retained H is the major cause for peak shifts. This is the same as that mentioned in [31]. 3.2. Crystal structure and microstructure of alloys Cru, Fev, Mnw, Tix, Vy, and Zrz (CruFevMnwTixVyZrz) Alloys investigated in this study belong principally to AB2 C14 Laves with little amount of precipitates, especially for off-stoichemetric compositions. This conforms to that the more the nonstoichiometric, the more amount the precipitates. Since Zr and Ti are A-type H-absorbing elements, the other four elements Cr, Fe, Mn, and V are Btype H-desorbing elements. As alloys deviate their C14 stoichiometric compositions, A-type and B-type atoms will do their anti-site substitution and make their lattice distorted. Subsequently, this make the diffraction peak broadened. Figures 3a to 3g are, respectively, XRD patterns for as-cast and after H-absorbed samples of Cru before and after PCI tests (3a), Fev after PCI tests (3b), …, Cr2 after homogenization (3g), The alloy hydrides show two kinds of peak shifts, one to the right for lattice shrinkage, the other to the left for lattice expansion. Volume changes of alloy hydrides are shown in Fig. 4, and related data for them are listed in Table 4. Volume changes of hydrides for alloys Cru, Fev, and Mnw decrease as values of u, v, and w getting larger, while those of alloys Tix and Zrz increase as values of x and z getting larger. For Vy alloy, the volume increases till y value exceeds 1.0. This is due to the size of Ti, V, and Zr atoms being larger than the size of Cr, Fe, and Mn atoms. The maxima of volume expansion in this study is less than 20%, less than LaNi5, therefore the pulverization resistance of these C14 alloys is better than that of LaNi5. It is noted that as-homogenized Cr2.0 is also C14 with weaker peak intensity than its as-cast state (see Fig. 3g).

4

ACCEPTED MANUSCRIPT 3.3 Kinetics in Cru, Fev, Mnw, Tix, Vy, and Zrz alloys Figure 5 shows H absorption kinetics of Cru, Fev, Mnw, Tix, Vy, and Zrz. For convenience kinetic curves of all alloys at 25oC are compared, and those of u, v, w, x, y, and z = 0.5 at 5oC, 25oC, and 80oC are also compared. It is concluded that the higher the temperature, the higher the reaction rate or the shorter the t0.9. The kinetic parameter (t0.9) increases with increasing u, v, and w, but decreases with increasing z, while no regular trend with x and y for t0.9. The decreasing t0.9 with z is ascribed to the atomic size of Zr is the largest among different atoms giving an easiest way for H diffusion. All alloys but Zr0 (Zr-free) can absorb H. Since the enthalpy for Zr hydride is the highest (Table 6), the Zr-free alloy cannot absorb H is understandable. All samples except the Zr can be activated at 400oC and 40 atm within 3 cycles of activation. It is noted that at 25oC t0.9 decreases with increasing volume of unit cell illustrating the larger the space of interstitial site, the shorter the t0.9. In this study, except Zr0, all alloys have superior t0.9 at 25oC. For cpmparison, data for t0.9 are listed in the 3rd column of Table 5 and Curves of t0.9 vs. ratio x are illustrated in Figs. 5m-5o. The t0.9s for Cru, Mnw and Fev approximately become larger as the ratio x increases at various temperatures (i.e., 5oC, 25oC and 80oC) except for Fe2.0 at 80oC. This is due to the fact that Cr, Mn and Fe are the hydrogen releasing B elements in AB2 compounds. 3.4. PCI measurements for Cru, Fev, Mnw, Tix, Vy, and Zrz. Results of PCI measurements for Cru, Fev, Mnw, Tix, Vy, and Zrz are shown in Figs. 6a-6f, respectively. The tested pressure ranged from 10-2 atm to 22.5 atm for H absorption, down low to 0.2 atm for H desorption. 3.4.1. Maximum H-absorption, (H/M)max Table 5 lists values, at 5oC, 25oC, and 80oC, in wt% of (H/M)max, H/Mf.u., and (H/M)eff, where H/Mf.u., and (H/M)eff means, respectively, H storage per unit formula of hydride, and effective hydrogen absorption, which is defined as the maximum H absorbed minus the retained H after PCI test. The desortion to absorption ratio (DAR) in Table 5 is defined as (H/M)eff/(H/M)max. PCI results manifest that as temperature increases (H/M)max decreases, DAR increases, and the maximum equilibrium pressure increases. The DAR increases as temperature increases is because the H desorption reaction is a heat absorption (endothermic) reaction. The H absorption isotherms of 5

ACCEPTED MANUSCRIPT alloys appear to shift to the upper left as the temperature rises illustrating that as the temperature rises the maximum H absorption falls and the plateau pressure raises. Kinetic curves show that both t0.9 and plateau pressure fall as lattice parameters increase. Since the atomic sizes for Zr, Ti, and V are respectively 1.60 Å, 1.47 Å, and 1.34 Å and the averaged radius calculated for stoichiometric CrFeMnTiVZr is 1.37 Å, adding more Zr and Ti thus increases the interstitial space of the lattice of alloys, making both t0.9 and the plateau pressure decreasing with increasing z in Zrz and x in Tix. Tables 5 indicates that (H/M)max increases with x and z in Tix and Zrz, while it decreases with u, v, and w in Cru, Fev, and Mnw. The values of (H/M)max at 25oC are 2.09 wt％, and 2.17 wt％, respectively, for Ti2 and Zr2. The value of (H/M)max at 5oC for Fe-free (Fe0) is 2.23 wt%. The effective H desorption of Fe1.5 is notably 1.14 wt% at 80oC. From these facts, it indicates that in AB2 alloys although the A type elements can improve the H absorption, the B type elements can help to release the H in desortion. The essential factor depends on the formation enthalpy of hydride of alloys. The formation enthalpy of V hydride is near that of the equimolar alloys (Table 6), therefore the content of V in the multielement alloy do not significantly change the value of (H/M)max. Very often there are no obvious plateau pressures in some PCI curves where a midpoint pressure, e.g., between 0.3 atm and maximum pressure, is taken as the plateau pressure. Comparing Figs. 5m, 5n 5o with Figs. 6g, 6h and 6i, an approximate trend for t0.9 exists in Zrz because Zr is the highly hydrogen absorbing element in compouunds. Therefore, it is obvious for t0.9 relating to the plateau pressure in Zrz. However, viewing that the t0.9 is obtained from kinetics, while plateau pressure belongs to thermodynamics, the link between t0.9 and plateau pressure is not clear for other elements. Figure 7 shows (H/M)max and H desorption rate (%) as a function of element content at 25oC for alloys. As discussed above, elements like Zr, Ti, and V with the higher negative value of formation enthalpy of hydride, which are categorized as A elements, promote H absorption, and conversely, those like Cr, Mn, and Fe, as B elements, promote H desorption. 3.5. Thermodynamic analyses The calculated formation enthalpy of hydride (∆Hcal) is made by using the averaged formation enthalpy method described in Equation (2), ∆Hcal = [(xiHi)( Hatom/Matom)max]/(Hatom/Matom)cal,

6

(2)

ACCEPTED MANUSCRIPT where xi, ∆Hi, (Hatom/Matom)max, and (Hatom/Matom)cal are, respectively, molar fraction of each i element, formation enthalpy of hydride of i element, and (H/M)max and (H/M)cal. (H/M)cal can be regarded as the coordinate number of H in alloys. In this approximation of calculation, the formation enthalpy of alloys is neglected because the value, in most cases, is smaller than the value of hydride. For example, in LaNi5 the hydrides are LaH2 and NiH and (H/M)max is calculated as 1.39 wt% (see [31]). Micrographs of Cr0.5, Fe0.5, Mn0.5, Ti0.5, V0.5, and Zr0.5 are shown in Fig. 8. As-cast samples show typical dendritic-interdendritic structure. EDS mapping shows that white dendritic phase (BEI) is the primary phase rich in Zr, while the gray interdendritic phase is rich in Ti. As the amount of B elements increases, BCC precipitation rich in V occurs in V-containing alloys, while the amount of A elements increases HCP precipitation of Zr/Ti-rich phase appears. Figure 9 shows a plot for ∆Hcal in kJ/mole vs. (H/M)max in atomic ratio. For comparison, data in [31] are also shown in this figure. It is noted that the data points in this study site in the same area of AB2 alloys. From this result, one can see the close relation of (H/M)max with the hydride formation enthalpy of these high-entropy alloys. 3.6. DSC analyses Samples for DSC analyses up to 460oC were taken from that of PCI tests. Figure 10 shows DSC curves of Cru, Fev, Mnw, Tix, Vy, and Zrz. There are some obvious reactions for alloys of u, v, w, x, y, and z in intervals of (0,1), (0,1), (0,1), (1,1.5), (0,2), and (1,2), respectively, while u, v, and w are in the interval of (1,2), and x is in the interval of (1.5,2), there are no heat absorption peaks. No obvious reaction peaks occur for higher values of u, v, w, and x may be due to (1) there is no retained H in the samples after PCI tests, or (2) the retained H might dissolve until some temperatures above the DSC test temperature (i.e., 460oC). Since the alloys show no reactions at temperatures lower than 460oC, and their hybrid decomposition is an endothermic reaction, therefore endothermic reactions in Fig. 10 are ascribed to H desorption. Experimental results for DSC show that as u and v in Cru and Mnw increase the endothermic reaction temperature increases. Therefore one can use the adjustment of u and v to control the H desorption temperature. In this study the use of Cr to replace Co like that in the case of [31] has the better effect on the H absorption (Fig. 11), i.e., alloy system of 6E-3 (Fig. 1) is better than alloy system of 6E-2. Figure 12 shows the alloy phase diagrams of Cov and Mnt in 7

ACCEPTED MANUSCRIPT CovFeMntTiVZr and Cru and Mnw in CruFeMnwTiVZr. The stoichiometric gray regions for the latter two (Cru and Mnw) are wider than Mnt, but narrower than Cov. 4. Conclusion From XRD patterns alloys in CruFevMnwTixVyZrz high-entropy alloy system, i.e., high-entropy alloys Cru, Fev, Mnw, Tix, Vy, and Zrz, are of C14 Laves AB2 before H absorption and after PCI tests. Since the stabilization of C14-Laves phase is important and is the base for designing an alloy having a long cycle life in absorbing and desorbing hydrogen, alloys and their hydrides in this study are with the same C14 would make one able to design long cycle-life alloys. It shows that Ti and Zr are Habsorbing A elements, while the other four elements are H-desorbing B elements in AB2. Ti and Zr raise (H/M)max, while Cr, Fe, and Mn decrease (H/M)max. Although V is a B element in AB2, the amount of V in alloy has no significant effect on (H/M)max. (H/M)max(Fe0, 5oC) equals to 2.23 wt％, while (H/M)max(Zr2, 5oC) equals to 2.17 wt ％. (H/M)eff(Fe1.5, 80oC) is 1.14 wt％ similar to (H/M)eff(Zr0.5, 5oC) which is also 1.14 wt％. The major controlling factor for (H/M)max of high-entropy alloys is still hydride formation enthalpy, rather than entropy for formation alloys themselves, like the cases in the conventional alloys. An alloy generation map for multiplying alloys from four elements to seven elements starting from TiVFe is shown in this study, which illustrating that Zr is the most important element for H absorption for these kinds of high-entropy alloys. Both the plateau pressure in PCIs and t0.9 depend on lattice parameters of the alloy lattice. The larger the lattice parameters, the bigger the interstitial space and the lower plateau pressure and the shorter the t0.9. In this study the use of Cr to replace Co like that in the case of [31] has the better effect on the H absorption, i.e., alloy system of 6E-3 is better than alloy system of 6E2. Acknowledgement SKC would like to thank the National Science and Technology Department of the ROC, Taiwan for financial support of this study under the contract of NSC96-2221E007-066-MY3. References [1] Y.F. Kao, T.J. Chen, S.K. Chen, J.W. Yeh. Microstructure and mechanical 8

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ACCEPTED MANUSCRIPT hydrogen storage, J. Alloys Compd. 288 (1999) 217-225. [23] G. Sandrock, A panoramic overview of hydrogen storage alloys from a gas reaction point of view, J. Alloys Compd. 293–295 (1999) 877-888. [24] J.H. Zhu, L.M. Pike, C.T. Liu, P.K. Liaw, Point defects in binary Laves phase alloys, Acta Mater. 47 (1999) 2003-2018. [25] 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. [26] M.V. Simicic, M. Zdujic, R. Dimitrijevic, Lj Nikolic-Bujanovic, N.H. Popovic. Hydrogen absorption and electrochemical properties of Mg2Ni-type alloys synthesized by mechanical alloying. J. Power Sources 158 (2006) 730-734. [27] X.B.Yu, Z.X.Yang, S.L. Feng, Z.Wu, N.X. Xu, Influence of Fe addition on hydrogen storage characteristics of Ti–V-based alloy, Int. J. Hydrogen Energy 31 (2006) 1176-1181. [28] N.L. Rosi, J. Eckert, M. Eddaoudi, D.T. Vodak, J, Kim, M. O'Keeffe, O.M. Yaghi, Hydrogen storage in microporous metal-organic frameworks, Science 300 (2003) 1127-1129. [29] Y. Osumi, Hydrogen-storage alloy properties and applications. Tokyo: Agnetechnical Center; 1993 [in Japanese]. [30] K.C. Chou, Q. Li, Q. Lin, L.J. Jiang, K.D. Xu, Kinetics of absorption and desorption of hydrogen in alloy powder, Int. J. Hydrogen Energy 30 (2005) 301-309. [31] Y.F. Kao, S.K. Chen, J.H. Sheu, J.T. Lin, W.E. Lin, J.W. Yeh, S.J. Lin, T.H. Liou, C.W. Wang, Hydrogen storage properties of multi-principal-component CoFeMnTixVyZrz alloys, Int. J. Hydrogen Energy 35 (2010) 9046-9059. [32] Z. Hang, X. Xiao, D. Tan, Z. He, W. Li, S. Li, C. Chen, L. Chen, Microstructure and hydrogen storage properties of Ti10V84-xFe6Zrx (x = 1–8) alloys, Int. J. Hydrogen Energy 35 (2010) 3080-3086. [33] I. Kunce, M. Polanski, J. Bystrzycki, Structure and hydrogen storage properties 11

ACCEPTED MANUSCRIPT of a high entropy ZrTiVCrFeNi alloy synthesized using laser engineered net shaping (LENS), Int. J. Hydrogen Energy 38 (2013) 12180-12189. [34] I. Kunce, M. Polanski, J. Bystrzycki, Microstructure and hydrogen storage properties of a TiZrNbMoV high entropy alloy synthesized using laser engineered net shaping (LENS), Int. J. Hydrogen Energy 39 (2014) 9904-9910. [35] M. Sahlberg, D. Karlsson, C. Zlotea, U. Jansson, Superior hydrogen storage in high entropy alloys, Scientific Reports 6 (2016) 36770-36775. [36] J.T. Lin, Design and study on the hydrogen-storage 4- to 7-element high-entropy alloys based on composition of TiVFe, MS thesis, Department of Materials Science Engineering, National Tsing Hua University (2007). Lin’s adviser was Swe-Kai Chen who developed the the composition trace map and Lin collected this figure in his thesis, as shown in Fig. 1. Figure captions: Fig. 1. A composition trace map to illustrate the multiply of elements to systematically design a lot of hydrogen storing high-entropy alloys in 2007 [36]. Fig. 2. (a) C14 laves structure. (b) Cr1(Mn1) before and after PCI tests (c) XRD patterns that are of C14 Laves for as-cast and as-H-absorbed CrFeMnTiVZr, Cr1(Mn1) (d) Cr0 before and after PCI tests (e) Cr0.5 before and after PCI tests (f) Cr0.75 before and after PCI tests (g) Cr1.25 before and after PCI tests (h) Cr1.5 before and after PCI tests (i) Cr2 before and after PCI tests (j) Fe0 before and after PCI tests (k) Fe0.5 before and after PCI tests (l) Fe1.5 before and after PCI tests (m) Fe2 before and after PCI tests (n) Mn0 before and after PCI tests (o) Mn 0.5 before and after PCI tests (p) Mn 0.75 before and after PCI tests (q) Mn 1.25 before and after PCI tests (r) Mn1.5 before and after PCI tests (s) Mn2 before and after PCI tests (t) Ti0 before and after PCI tests (u) Ti0.5 before and after PCI tests (v) Ti1.5 before and after PCI tests (w) Ti2 before and after PCI tests (x) V0 before and after PCI tests (y) V0.5 before and after PCI tests (z) V1.5 before and after PCI tests (α) V2 before and after PCI tests (β) Zr0 before PCI tests (γ) Zr0.5 before and after PCI tests (δ) Zr1.5 before and after PCI tests (ε) Zr2 before and after PCI tests Fig. 3. XRD patterns for as-cast and after H-absorbed samples of Cru before and after PCI tests (a), Fev after PCI tests (b), Mnw after PCI tests (c), Tix after PCI tests (d), Vx after PCI tests (e), Zrz after PCI tests (f), Cr2 with the process of homogenization, 12

ACCEPTED MANUSCRIPT before and after PCI tests (g). Fig. 4. Volume changes of alloy hydrides vs. composition ratio. Fig. 5. H absorption kinetic of Cru at 25oC (a), Cr0.5 at 5oC, 25oC, and 80oC (b), Fev at 25oC (c), Fe0.5 at 5oC, 25oC, and 80oC (d), Mnw at 25oC (e), Mn0.5 at 5oC, 25oC, and 80oC (f), Tix at 25oC (g), Ti0.5 at 5oC, 25oC, and 80oC (h), Vx at 25oC (i), V0.5 at 5oC, 25oC, and 80oC (j), Zrz at 25oC (k) and Zr0.5 at 5oC, 25oC, and 80oC (l), t0.9 vs ratio x at 5oC (m), t0.9 vs ratio x at 25oC (n), t0.9 vs ratio x at 80oC (o) Fig. 6. PCI measurements in (a) CruFeMnTiVZr (b) CrFevMnTiVZr (c) CrFeMnwTiVZr (d) CrFeMnTixVZr (e) CrFeMnTiVyZr (f) CrFeMnTiVZrz (g) plateau pressure (Ppla) vs ratio x at 5oC, (h ) plateau pressure (Ppla) vs ratio x at 25oC, (i) plateau pressure (Ppla) vs ratio x at 80oC . Fig. 7. (H/M)max (a) and H desorption rate (%) (b) as a function of element content at 25oC for alloys. Fig. 8. Micrographs of (a) Cr0.5, (b) Fe0.5, (c) Mn0.5, (d) Ti0.5, (e) V0.5 and (f) Zr0.5. Fig. 9. The plot of ∆Hcal vs. (H/M)max in atomic ratio for CruFevMnwTixVyZrz alloys and related data from [31]. Fig. 10. Experimental results for DSC in (a) CruFeMnTiVZr, (b) CrFevMnTiVZr, (c) CrFeMnwTiVZr, (d) CrFeMnTixVZr, (e) CrFeMnTiVyZr, and (f) CrFeMnTiVZrz. Fig. 11. Comparison of H absorption for Cov and Mnt in CovFeMntTiVZr and Cru and Mnw in CruFeMnwTiVZr. Fig. 12. Alloy phase diagrams of Cov and Mnt in CovFeMntTiVZr and Cru and Mnw in CruFeMnwTiVZr List of tables: Table 1. Designations and densities of alloys in this study. Table 2. The values of atomic weight, structure, atomic radius (Å), e/a, melting point (oC) and density (g/cm3) for elements. 13

ACCEPTED MANUSCRIPT Table 3. Calculated hydrogen storage in LaNi5H6. Table 4. The values of lattice parameters (a and c, Å), c/a, unit cell volume (Å3), V, and change in volume (∆V/V, %), for alloys. Table 5. The PCI data for alloys. Table 6. Calculated values of hydride formation enthalpy (kJ/mole alloy) at 25oC for CruFevMnwTixVyZrz. Table 7. The formation enthalpy (ΔH) of hydrides for elements. Table 8. The EDS composition (at. %) for phases of alloys. Table 1. Designations and densities of alloys in this study. Alloys Designation

Density (g/cm3)

FeMnTiVZr

Cr0

6.50

Cr0.5FeMnTiVZr

Cr0.5

6.60

Cr0.75FeMnTiVZr

Cr0.75

6.53

CrFeMnTiVZr

Cr1 (Mn1, 6E-3)

6.55

Cr1.25FeMnTiVZr

Cr1.25

6.57

Cr1.5FeMnTiVZr

Cr1.5

6.44

Cr2FeMnTiVZr

Cr2

6.43

CrMnTiVZr

Fe0

6.13

CrFe0.5MnTiVZr

Fe0.5

6.37

CrFe1.5MnTiVZr

Fe1.5

6.45

CrFe2.0MnTiVZr

Fe2.0

6.79

CrFeTiVZr

Mn0

6.45

CrFeMn0.5TiVZr

Mn0.5

6.40

CrFeMn0.75TiVZr

Mn0.75

6.39

CrFeMn1.25TiVZr

Mn1.25

6.52

CrFeMn1.5TiVZr

Mn1.5

6.49

CrFeMn2TiVZr

Mn2

6.66

CrFeMnVZr

Ti0

6.91

CrFeMnTi0.5VZr

Ti0.5

6.47

CrFeMnTi1.5VZr

Ti1.5

6.33

CrFeMnTi2.0VZr

Ti2.0

6.59

14

ACCEPTED MANUSCRIPT CrFeMnTiZr

V0

6.68

CrFeMnTiV0.5Zr

V0.5

6.59

CrFeMnTiV1.5Zr

V1.5

6.50

CrFeMnTiV2.0Zr

V2.0

6.52

CrFeMnTiV

Zr0

6.62

CrFeMnTiVZr0.5

Zr0.5

6.48

CrFeMnTiVZr1.5

Zr1.5

6.01

CrFeMnTiVZr2.0

Zr2.0

6.64

Cr16.5Fe7.8Mn11.8Ti13V50Zr

BCC

6.24

Table 2. The values of atomic weight, structure, atomic radius (Å), e/a, melting point (oC) and density (g/cm3) for elements. Elements Atomic Structure weight

Atomic radius

e/a

Melting point

Density

Ti

47.88

HCP

1.47

4

1668

4.51

V

50.94

BCC

1.34

5

1905

6.10

Cr

51.99

BCC

1.28

6

1903

7.19

Mn

54.94

BCC

1.27

7

1244

7.43

Fe

55.85

BCC

1.26

8

1535

7.86

Zr

91.22

HCP

1.60

4

1850

6.49

Co

58.93

HCP

1.25

9

1492

8.90

Table 3. Calculated hydrogen storage in LaNi5H6. Hydrogen capacity Calculation for LaNi5H6 (1) H to alloy in wt % 6 g/432.36 g = 1.39 (wt %) (2) H to hydride in wt % 6 g/438.36 g = 1.37 (wt %) (3) H to 1 mole LaNi5 ratio 6 moles/1 mole = 6 (H/LaNi5) (4)

H to alloy atomic ratio

6 moles/6 moles = 1 (H/M)

Table 4. The values of lattice parameters (a and c, Å), c/a, unit cell volume (Å3), V, and change in volume (∆V/V, %), for alloys. Alloys Cr0 Cr0.5

State

a

c

c/a

V

∆V/V

BH

5.020

8.211

1.635

179.2

15.79

AH

5.271

8.621

1.635

207.5

BH

4.995

8.167

1.634

176.5

AH

5.236

8.507

1.624

202.0

15

14.42

ACCEPTED MANUSCRIPT Cr0.75 Cr1.0 Cr1.25 Cr1.5 Cr2.0 Fe0 Fe0.5 Fe1.0 Fe1.5 Fe2.0 Mn0

Mn0.5 Mn0.75 Mn1.0 Mn1.25 Mn1.5 Mn2.0

BH

4.987

8.162

1.636

175.9

AH

5.086

8.323

1.636

186.5

BH

4.982

8.132

1.632

174.8

AH

5.017

8.208

1.636

179.0

BH

4.977

8.128

1.633

174.4

AH

4.995

8.167

1.634

176.5

BH

4.974

8.113

1.631

173.9

AH

4.985

8.201

1.645

176.5

BH

4.969

8.083

1.626

172.9

AH

4.971

8.138

1.637

174.2

BH

5.056

8.195

1.621

181.4

AH

5.358

8.766

1.636

217.9

BH

5.028

8.193

1.629

179.4

AH

5.290

8.649

1.635

209.6

BH

4.920

8.090

1.644

169.6

AH

4.986

8.152

1.635

175.5

BH

4.915

8.069

1.642

168.8

AH

4.968

8.151

1.641

174.2

BH

4.909

8.068

1.644

168.4

AH

4.940

8.094

1.639

171.0

BH

5.033

8.233

1.636

180.7

AH

5.215

8.480

1.626

199.7

BH

5.009

8.204

1.677

178.3

AH

5.120

8.402

1.641

190.8

BH

4.985

8.147

1.634

175.3

AH

5.061

8.293

1.638

184.0

BH

4.982

8.132

1.632

174.8

AH

5.017

8.209

1.636

178.9

BH

4.964

8.160

1.644

174.1

AH

4.985

8.174

1.640

175.9

BH

4.959

8.143

1.642

173.4

AH

4.966

8.122

1.635

173.6

BH

4.948

8.136

1.644

172.5

16

6.04 2.37 1.24 1.52 0.77 20.13 16.85 3.49 3.19 1.60 10.55

7.00 4.92 2.37 1.03 0.12 0.13

ACCEPTED MANUSCRIPT

Ti0 Ti0.5 Ti1.0 Ti1.5 Ti2.0 V0 V0.5 V1.0 V1.5 V2.0 Zr0 Zr0.5 Zr1.0 Zr1.5 Zr2.0

AH

4.971

8.138

1.637

174.2

BH

4.990

8.186

1.640

176.6

AH

5.012

8.215

1.639

178.7

BH

4.976

8.156

1.639

174.9

AH

5.012

8.217

1.640

178.8

BH

4.920

8.090

1.644

169.6

AH

4.986

8.152

1.635

175.5

BH

4.986

8.164

1.637

175.8

AH

5.117

8.414

1.644

190.8

BH

5.004

8.177

1.634

177.3

AH

5.233

8.548

1.634

202.7

BH

4.956

8.133

1.641

173.0

AH

5.002

8.218

1.643

178.1

BH

4.977

8.158

1.639

175.0

AH

5.047

8.359

1.656

184.4

BH

4.920

8.090

1.644

169.6

AH

4.986

8.152

1.635

175.5

BH

4.974

8.149

1.638

174.6

AH

5.096

8.357

1.640

187.9

BH

4.993

8.168

1.636

176.3

AH

5.249

8.587

1.636

204.9

BH

4.830

7.897

1.635

159.5

AH

N/A

N/A

N/A

N/A

BH

4.919

8.055

1.638

168.8

AH

4.923

8.066

1.639

169.3

BH

4.920

8.090

1.644

169.6

AH

4.986

8.152

1.635

175.5

BH

5.030

8.232

1.637

180.3

AH

5.320

8.696

1.635

213.2

BH

5.071

8.294

1.636

184.7

AH

5.394

8.798

1.631

221.7

Table 5. The PCI data for alloys. T (oC) Alloys t0.9 (s)

(H/M)max 17

(H/M)eff

1.22 2.22 3.49 8.55 14.33 2.96 5.38 3.49 7.63 16.21 N/A 0.30 3.49 18.22 20.02

Desorption

ACCEPTED MANUSCRIPT

Cr0

Cr0.5

Cr0.75

Cr1 (Mn1)

Cr1.25

Cr1.5

Cr2

Fe0

Fe0.5

Fe1.0

Fe1.5

Fe2.0

Mn0

5 25 80 5 25 80 5 25 80 5 25 80 5 25 80 5 25 80 5 25 80 5 25 80 5 25 80 5 25 80 5 25 80 5 25 80 5

27 23 21 33 33 30 40 37 32 44 40 36 47 43 39 50 47 45 57 49 47 23 23 19 23 27 29 34 50 67 67 57 95 135 84 61 25

(wt%)

(H/Mf.u.)

(wt%)

2.05 1.88 1.53 1.87 1.71 1.32 1.83 1.60 1.27 1.76 1.5 1.12 1.65 1.47 1.15 1.61 1.47 1.21 1.43 1.23 0.80 2.23 1.92 1.54 2.05 1.93 1.57 1.80 1.72 1.18 1.60 1.43 1.17 1.18 1.18 0.60 1.98

6.16 5.66 4.60 6.11 5.58 4.25 5.98 5.44 4.32 6.20 5.29 3.95 6.04 5.38 4.21 6.10 5.57 4.58 5.79 5.26 3.97 6.62 5.70 4.57 6.66 6.27 5.10 6.35 6.07 4.16 6.09 5.44 4.45 4.82 4.82 2.45 5.90

0.48 0.55 0.69 0.37 0.38 0.43 0.37 0.48 0.64 0.59 0.65 0.84 0.70 0.65 0.72 0.69 1.00 1.00 0.51 0.76 0.74 0.36 0.31 0.27 0.37 0.26 0.47 0.39 0.37 0.61 0.76 0.94 1.14 0.90 1.09 0.60 0.55

18

ratio (%) 23.41 29.25 45.09 19.79 22.22 23.30 19.47 30.00 50.39 33.52 43,33 75.00 42.42 44.21 62.21 42.86 68.00 83.47 35.66 61.78 92.50 16.14 16.15 17.53 18.05 13.47 29.94 21.67 21.51 51.69 47.50 65.73 97.44 76.27 92.37 100.00 27.78

ACCEPTED MANUSCRIPT

Mn0.5

Mn0.75

Mn1 (Cr1)

Mn1.25

Mn1.75

Mn2.0

Ti0

Ti0.5

Ti1.0

Ti1.5

Ti2.0

V0

25 80 5 25 80 5 25 80 5 25 80 5 25 80 5 25 80 5 25 80 5 25 80 5 25 80 5 25 80 5 25 80 5 25 80 5 25 80

23 21 38 35 33 41 37 37 44 40 36 47 42 38 43 47 41 58 48 46 53 19 74 33 36 40 34 50 67 20 28 54 25 25 24 21 23 32

1.78 1.42 1.90 1.71 1.24 1.83 1.61 1.11 1.76 1.5 1.12 1.72 1.47 1.18 1.68 1.27 1.04 1.61 1.20 0.80 1.11 1.19 0.71 1.43 1.25 0.97 1.80 1.72 1.18 1.91 1.70 1.35 2.09 1.85 1.45 1.71 1.54 1.18 19

5.30 4.23 6.18 5.56 4.03 6.21 5.46 3.76 6.20 5.29 3.95 6.30 5.39 4.33 6.39 4.83 3.96 6.56 4.89 3.26 3.38 3.63 2.17 4.70 4.11 3.19 6.35 6.07 4.16 7.20 6.40 5.09 8.37 7.41 5.81 5.16 4.65 3.56

0.42 0.40 0.47 0.48 0.46 0.4 0.38 0.6 0.59 0.65 0.84 0.73 0.58 0.9 0.73 0.76 0.81 0.73 0.9 0.78 0.33 0.33 0.51 0.27 0.57 0.62 0.39 0.37 0.61 0.42 0.27 0.42 0.19 0.36 0.34 0.29 0.24 0.77

23.60 28.17 27.74 28.07 38.10 21.86 23.60 54.05 33.52 43,33 75.00 42.44 39.00 76.27 43.45 59.84 77.88 45.34 75 97.5 29.73 27.73 71.83 18.88 45.60 63.92 21.67 21.51 51.69 21.99 15.88 31.11 9.09 19.46 23.45 16.96 15.58 65.25

ACCEPTED MANUSCRIPT V0.5

V1.0

V1.5

V2.0

Zr0.5

Zr1.0

Zr1.5

Zr2.0

5 25 80 5 25 80 5 25 80 5 25 80 5 25 80 5 25 80 5 25 80 5 25 80

30 36 45 34 50 67 36 43 39 35 33 27 140 52 44 34 50 67 20 30 24 17 18 153

1.96 1.74 1.40 1.80 1.72 1.18 1.90 1.68 1.21 1.91 1.85 1.06 1.14 0.83 0.55 1.80 1.72 1.18 2.01 1.80 1.33 2.17 1.82 1.66

6.42 5.70 4.58 6.35 6.07 4.16 7.19 6.36 4.58 7.71 7.47 4.28 3.50 2.55 1.69 6.35 6.07 4.16 8.01 7.17 5.30 9.64 8.08 7.37

0.27 0.41 0.76 0.39 0.37 0.61 0.43 0.54 0.57 0.60 0.54 0.54 1.13 0.83 0.55 0.39 0.37 0.61 0.26 0.41 0.29 0.24 0.23 0.28

13.78 23.56 54.29 21.67 21.51 51.69 22.63 32.14 47.11 31.41 29.19 50.94 99.12 100.00 100.00 21.67 21.51 51.69 12.94 22.78 21.80 11.06 12.64 16.87

Table 6. Calculated values of hydride formation enthalpy (kJ/mole alloy) at 25oC for CruFevMnwTixVyZrz. Alloys ∆Hcal Alloys ∆Hcal Cr0

-46.50

Mn0

-42.44

Cr0.5

-39.72

Mn0.5

-39.09

Cr0.75

-36.21

Mn0.75

-36.12

Cr1 (Mn1)

-31.83

Mn1 (Cr1)

-31.83

Cr1.25

-31.56

Mn1.25

-31.81

Cr1.5

-30.78

Mn1.5

-27.01

Cr2

-25.95

Mn2

-24.71

Fe0

-51.21

Ti0

-21.94

Fe0.5

-54.28

Ti0.5

-25.63

20

ACCEPTED MANUSCRIPT Fe1.0

-37.87

Ti1.0

-37.87

Fe1.5

-28.98

Ti1.5

-37.76

Fe2.0

-21.88

Ti2.0

-44.03

V0

-42.46

Zr0

N/A

V0.5

-41.02

Zr0.5

-14.81

V1.0

-37.87

Zr1.0

-37.87

V1.5

-35.08

Zr1.5

-46.17

V2.0

-36.59

Zr2.0

-52.48

Table 7. The formation enthalpy (ΔH) of hydrides for elements. Hydride ΔH (kJ/mole alloy) ΔH (kJ/mole H2) TiH2

-125.4

-125.4

VH2

-40.2

-40.2

CrH

-8.4

-16.8

MnH

-16.7

33.4

FeH

16.7

33.4

ZrH2

-163.0

-163.0

CoH

16.7

33.4

Table 8. The EDS composition (at. %) for phases of alloys. Cr0.5 Ti V Cr Mn Designed 18.2 18.2 9.0 18.2 Exp. 18.0 16.8 8.8 18.8 White phase 15.1 17.2 9.4 19.0

Fe 18.2 17.9 17.5

Zr 18.2 19.8 21.9

Gray phase Fe0.5 Designed Exp. Dendrite Interdendrite Mn0.5 Designed Exp. White phase Gray phase

16.9 Fe 9.1 8.6 9.2 8.6 Fe 18.2 17.3 18.8 13.7

16.5 Zr 18.2 19.3 23.9 14.5 Zr 18.2 19.9 24.5 13.6

22.3 Ti 18.2 19.1 12.9 25.4 Ti 18.2 18.0 12.16 36.79

19.4 V 18.2 17.7 15.1 20.8 V 18.2 18.1 15.5 18.8

7.9 Cr 18.2 17.4 20.5 14.4 Cr 18.2 18.1 19.7 10.5 21

17.1 Mn 18.2 17.9 18.3 16.3 Mn 9.0 8.6 9.3 6.5

ACCEPTED MANUSCRIPT Ti0.5 Designed Exp. Dendrite Interdendrite Precipitate V0.5 Designed Exp. Dendrite Interdendrite Precipitate Zr0.5 Designed Exp. Dendrite Interdendrite Precipitate

Ti 9.1 10.6 8.2 15.0 8.8 Ti 18.2 18.4 12.6 18.8 18.2 Ti 18.2 18.6 17.4 21.3 10.9

V 18.2 18.0 14.1 15.8 35.1 V 9.1 9.4 8.0 9.5 9.1 V 18.2 18.4 13.8 18.0 37.7

Cr 18.2 17.9 17.8 14.9 23.6 Cr 18.2 17.6 20.4 17.7 18.2 Cr 18.2 18.1 18.0 15.9 23.9

22

Mn 18.2 17.8 17.4 19.2 16.3 Mn 18.2 17.2 17.8 17.5 18.2 Mn 18.2 17.7 17.5 18.6 15.9

Fe 18.2 16.9 17.9 19.4 12.6 Fe 18.2 18.4 17.8 18.2 18.2 Fe 18.2 17.3 18.5 17.8 11.0

Zr 18.2 18.9 24.7 15.6 3.6 Zr 18.2 19.0 23.4 18.3 18.2 Zr 36.4 9.9 14.8 8.5 0.5

ACCEPTED MANUSCRIPT 1) CruFevMnwTixVyZrz is better than CouFevMnwTixVyZrz and LaNi5 for 25oC-H storage. 2) C14 Laves remains before H absorption and after PCI tests. 3) Less pulverization.