Properties of La0.2Y0.8Ni5−xMnx alloys for high-pressure hydrogen compressor

international journal of hydrogen energy 35 (2010) 8262–8267

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Properties of La0.2Y0.8Ni5LxMnx alloys for high-pressure hydrogen compressor G. Luo a,b, J.P. Chen c, S.L. Li a,b, W. Chen a,b, X.B. Han a,b, D.M. Chen a,*, K. Yang a a

Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, China Graduate School of Chinese Academy of Sciences, Beijing 100049, China c School of Mechanical Engineering, Shanghai Jiao Tong University, Shanghai 200240, China b

article info

abstract

Article history:

Hydrogen absorption/desorption properties of La0.2Y0.8Ni5xMnx (x ¼ 0.2, 0.3, 0.4) alloys for

Received 11 September 2009

high-pressure hydrogen compression application were investigated systematically. The

Received in revised form

Pressure–Composition isotherms and absorption kinetics were measured at 293, 303 and

3 November 2009

313 K by the volumetric method. XRD analyses showed that all the investigated alloys

Accepted 4 December 2009

presented CaCu5 type hexagonal structure and the unit cell volume increased in both a and

Available online 19 January 2010

c lattice axes with Mn substitution. Hydrogen absorption/desorption measurements revealed that Mn could lower the plateau pressure effectively, improve the hydrogen

Keywords:

storage capacity and absorption kinetics but slightly increase the slope of the pressure

Hydrogen storage

plateau and hysteresis. The study results suggest that La0.2Y0.8Ni4.8Mn0.2 is suitable for the

Metal hydride

high-pressure stage compression of the hydrogen compressor and the other two alloys,

Hydrogen compressor

La0.2Y0.8Ni4.7Mn0.3 and La0.2Y0.8Ni4.6Mn0.4, for the preliminary stage.

Pulverization

1.

ª 2009 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.

Introduction

With the development of global industries and population, the energy crisis and environmental pollution from the conventional fossil fuels are getting more and more serious. Nowadays, a great deal of effort is made on finding renewable energies to solve these problems. Hydrogen energy is one of the ideal solutions for it is clean, light, powerful, abundant and versatile [1]. According to Ball et al. [2], transportation today is responsible for about 18% of the primary energy consumption and 17% of the global CO2 emissions. Therefore, hydrogen fuel cell vehicles are drawing more and more attention. However, owing to some technical bottlenecks, the high-pressure cylinder seems to be the realistic hydrogen supply style for the near-term fuel cell vehicles [3]. Compared to the conventional mechanical compressor, the hydrogen

compressor based on reversible metal hydride presents many merits, such as the one of being able to export hydrogen over a wide range of pressures with high purity, free of mechanical movement or friction, low energy consumption, noiseless operation and easy maintenance [4–7]. To be suitable for a compressor, the hydride should have large reversible hydrogen storage capacity, flat pressure plateau, little hysteresis factor and high compression ratio. Meanwhile, admirable reaction kinetics and pulverization resistance are also needed. Considering that the working principle of the metal hydride compressor is similar to that of the hydride heat pump based on van’t Hoff relation, therefore, in this paper, a series of AB5 type alloys, La0.2Y0.8Ni5xMnx (x ¼ 0.2, 0.3, 0.4), were chosen as the experimental materials because this kind of alloys could be used in the heat pumps [8–10]. The hydrogen

* Corresponding author. Tel.: þ86 24 2397 1641; fax: þ86 24 2389 1320. E-mail address: [email protected] (D.M. Chen). 0360-3199/$ – see front matter ª 2009 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2009.12.020

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absorption/desorption isotherms, absorption kinetics and particle size distribution after hydrogen absorption/desorption cycling were measured in order to investigate the effect of manganese (Mn), a substitution element, on the thermodynamics, kinetics and pulverization resistance of the alloys.

2.

Table 1 – Crystal parameters of La0.2Y0.8Ni5LxMnx alloys. Alloys

a/nm

c/nm

V  103/nm3

Rs  10/nm

La0.2Y0.8Ni4.8Mn0.2 La0.2Y0.8Ni4.7Mn0.3 La0.2Y0.8Ni4.6Mn0.4

0.4920 0.4932 0.4938

0.3978 0.3989 0.3997

83.4096 84.0496 84.4049

0.3145 0.3153 0.3157

Experimental details

The La0.2Y0.8Ni5xMnx (x ¼ 0.2, 0.3, 0.4) alloys were prepared by arc melting the mixture of pure elements La, Y, Ni and Mn with purities of 99%, 99%, 99.5%, 99.7%, respectively, under argon atmosphere. The alloy ingots were remelted up and down for several times in order to diminish the segregation. Then the as-cast ingots were vacuum annealed under argon atmosphere at 1323 K for 6 h to obtain a single-phase structure. The hydrogen absorption/desorption Pressure–Composition isotherms at 293, 303 and 313 K were obtained by means of conventional volumetric method in a Sieverts-type apparatus [11] using the commercially available hydrogen with a purity of 99.999%. The sample was fully activated for ten times before the absorption/desorption PC isotherms and absorption kinetic curves were measured. The hysteresis factor, Hf, was calculated as ln (Pa/Pd), where Pa and Pd are the absorption and desorption equilibrium pressures, respectively. The hydrogen concentration at the endpoint of the absorption plateau was treated as the hydrogen uptake capacity. The slope of the pressure plateau, Sf, was defined as (dln P)/d(H/M ), where P is the equilibrium pressure and H/M is the hydrogen content expressed in an atomic ratio. Absorption kinetics experiments were conducted on the condition that, when the hydrogenation reaction ended, the equilibrium pressure was 1 MPa higher than that of the absorption plateau endpoint corresponding to the PC isotherms at the same temperature, because the same pressure driving force coming from the same pressure difference could be ensured by this way. t0.9 was denoted as the time taken by a sample to absorb 90% of its maximum hydrogen capacity. The more detailed experimental procedure was the same as that previously described [12].

Fig. 1 – XRD patterns of La0.2Y0.8Ni5LxMnx alloys.

The crystal structures were determined by X-ray diffraction (XRD) with Cu Ka radiation. The lattice parameters were calculated using a least square refinement technique [13]. Powder morphologies of the alloys after hydrogen absorption/ desorption were observed on a JSM 6301F scanning electron microscopy (SEM). Particle size distributions of the alloys after 16 absorption/desorption cycles were analyzed by a Malvern Laser-diffraction particle size analyzer.

3.

Results and discussion

3.1.

Crystal structure

The XRD patterns of the annealed La0.2Y0.8Ni5xMnx (x ¼ 0.2, 0.3, 0.4) alloys are shown in Fig. 1. It is obvious that all the alloys presented a homogenous single phase with CaCu5 type hexagonal structure, which belongs to the typical AB5 type intermetallic compounds. According to Bragg’s relation, the lattice parameter, a and c, and crystal cell volume, V, were calculated and are listed in Table 1. It can be seen that all the lattice parameters and cell volumes increased with increasing Mn content. This may be explained by the fact that the atomic radius of Mn (0.179 nm) is larger than that of Ni (0.160 nm) and the partial replacement of Ni by Mn results in an expansion of the cell volume if there is no crystal structure change.

3.2.

PC isotherms and thermodynamic properties

3.2.1.

PC isotherms

The absorption/desorption PC isotherms of the alloys at 303 K are shown in Fig. 2. There were two single-phase regions, solid

Fig. 2 – Pressure–Composition isotherms of La0.2Y0.8Ni5Lx Mnx alloys at 303 K.

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Table 2 – PC isotherms characteristics of La0.2Y0.8Ni5LxMnx alloys at 303 K. Alloys

La0.2Y0.8Ni4.8Mn0.2 La0.2Y0.8Ni4.7Mn0.3 La0.2Y0.8Ni4.6Mn0.4

P/MPa

H/M

Absorption

Desorption

5.178 2.068 1.106

3.580 1.415 0.671

Absorption

Desorption

0.062 0.090 0.087

0.104 0.121 0.072

0.58 0.62 0.73

solution a and hydride b, and one dual-phase coexisting region, a þ b, in all the investigated hydrides. The measured equilibrium pressure, slope and hysteresis factors are listed in Table 2. It can be clearly seen that the equilibrium pressure (Pa and Pd) dramatically decreased while the hydrogen capacity (H/M ) increased steadily with increasing Mn content. However, it seems that the change of Mn content had no remarkable effect on the slope or hysteresis of the pressure plateau, which is important for hydrogen compression. It is generally accepted that the lower the equilibrium pressure, the more stable the hydride is. Lundin et al. [14,15] proposed an approach to predict the stability of the hexagonal AB5Hx and cubic ABHx intermetallic hydrides. They argued that for the same class of metal hydrides, the equilibrium pressure is proportional to the tetrahedral interstitial holes size in their intermetallic compounds. During the hydrogen absorption, the hydrogen atoms prefer to occupy the tetrahedral interstitial holes for a low system energy level. The larger the radius of the hole is, the easier is for the hydrogen atom to occupy it, correspondingly, the lower the equilibrium pressure and the more stable the hydrides are. Generally speaking, when the unit cell expands, especially in both a and c directions, the interstitial hole size increases, and the equilibrium pressure decreases. The radii Rs of the tetrahedral interstitial holes for the three investigated alloys were calculated according to the formulas of the literature [15] and are listed in Table 1. It is found that the equilibrium pressure decreased monotonously with the increases of V and Rs, which is consistent with the above theory. The hydrogen uptake capacity of a metal hydride is not only dependent on the geometry factor, the amount and the size of the interstitial holes, but also on the electron factor [14,15]. According to the simulation results of Zhao et al. [16], the alloy with a lower electron concentration, (e/a)2/3, and a smaller average electronegativity difference, DX2, will possess a higher hydrogen content. The (e/a)2/3 and DX2 were calculated and are listed in Table 3. It can be deduced from Tables 1 and 3 that the change trend of the electron concentration (e/a)2/3, the unit cell volume and the radius of the

Hf

Sf

0.37 0.38 0.50

Table 3 – Electron factors of La0.2Y0.8Ni5LxMnx alloys. Alloys

(e/a)2/3

DX2

La0.2Y0.8Ni4.8Mn0.2 La0.2Y0.8Ni4.7Mn0.3 La0.2Y0.8Ni4.6Mn0.4

4.1819 4.1656 4.1492

0.0722 0.0731 0.0735

tetrahedral interstitial hole have a positive effect on the hydrogen storage capacity while that of the average electronegativity difference DX2 has a negative effect, and the ‘net’ result is that the hydrogen uptake capacity increased with increasing Mn content in the present study. Hysteresis phenomena in the hydrogen absorption and desorption of the hydrides originates from the irreversible transformation strains during the formation and decomposition of the hydride [17–20]. From Table 2, it can be seen that with increasing Mn content in the alloys, the hysteresis factor also increased although it was low and changed lightly. This is because the higher hydrogen content will result in the higher irreversible strain in the lattice.

3.2.2.

Thermodynamic properties

The enthalpies and entropies of hydrogenation/dehydrogenation of the alloys were calculated based on the following van’t Hoff equation lnðP=P0 Þ ¼ DH=RT  DS=R; where P is the equilibrium pressure of metal–hydrogen system, P0 is the standard pressure, T is the reaction temperature and R is the universal gas constant. The calculated thermodynamic parameters for hydriding and dehydriding are listed in Table 4. The steady increase in the absolute reaction enthalpy indicated that, with the increase of Mn content, the hydrides come to be more stable. Furthermore, the thermodynamic parameters in Table 4 can be used to predict the application potential of the alloys in

Table 4 – Thermodynamics of La0.2Y0.8Ni5LxMnx alloys. Alloys

La0.2Y0.8Ni4.8Mn0.2 La0.2Y0.8Ni4.7Mn0.3 La0.2Y0.8Ni4.6Mn0.4

DS/J/(mol K)

DH/kJ/mol Absorption

Desorption

Absorption

Desorption

20.01 24.36 25.40

22.98 26.07 27.10

98.86 105.40 103.73

105.41 110.47 105.25

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hydrogen compression. If the heating source is a 363 K hot water, with the increase of Mn content in the alloys the hydrogen pressure gets the values of 16 MPa, 11 MPa and 4 MPa, respectively. If a 405 K oil is used as the heating media, the theoretical output pressure of the hydride with Mn content x ¼ 0.2 can exceed 35 MPa. Therefore, for a two-stage hydrogen compressor, the above alloy can be used as a working material for the high-pressure stage compression, and the other two alloys for the preliminary stage.

3.3.

Pulverization resistance

Although the alloy volumes alternated between expanding and contracting during hydrogen absorption/desorption cycling, they still showed good pulverization resistance. Fig. 3 shows the particle size distribution of the alloys after 16 absorption/desorption cycles by using a Malvern Laserdiffraction particle size analyzer. The horizontal axis denotes the particle diameter; the left axis is the mean particle volume ratio, which is for the parabolic curve, and the right axis represents the total particle volume ratio. Obviously, accumulating the mean particle volume ratio makes the total ratio. The results showed that the alloy particle sizes covered a range from less than 1 mm to more than 100 mm. The mean particle size for the three alloys reduced from 55.8 to 39.3, and 25.6 mm respectively with increasing Mn content, which can be explained by the fact that the hydrogen uptake capacity increased with increasing Mn content, followed by an increase of unit cell expansion on hydrogenation. Fig. 4 shows the SEM images of the above alloys, which revealed a consistency between the SEM and laser-diffraction data.

3.4.

Hydrogenation kinetics

The whole reaction kinetic curves at 303 K, i.e. from the solid solution a, to the dual-phase region (a þ b), and finally to the hydride b, were measured and are shown in Fig. 5. The hydrogen absorption kinetics of the investigated alloys was fast. The elapsed times for the alloys to reach 90% of the maximum hydrogen capacity were 210, 188, and 90 s with Mn

Fig. 4 – SEM images of La0.2Y0.8Ni5LxMnx alloys (a) x [ 0.2; (b) x [ 0.3; (c) x [ 0.4.

Fig. 3 – Particle size distributions of La0.2Y0.8Ni5LxMnx alloys after 16 cycles.

content x ¼ 0.2, 0.3, and 0.4, respectively. The alloy with the highest Mn content exhibited the fastest kinetics. This could be attributed to the following reason: the alloys with higher Mn content tended to pulverize and generate more fresh surfaces, which improved the reaction rate. In this study the kinetics was found sluggish when the reaction reached the dual-phase region. Comparatively, the kinetics was very fast in the single-phase regions. It has been shown in the PC isotherms that the hydrogen storage capacity of the alloy mainly depends on the width of the platform, i.e. the phase transformation process. Therefore, it is important

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4.

Conclusions

The crystal structures, hydrogen absorption/desorption properties and pulverization resistance of La0.2Y0.8Ni5xMnx (x ¼ 0.2, 0.3, 0.4) alloys were investigated systematically. Conclusions could be drawn as the following:

Fig. 5 – Hydrogen absorption kinetics of La0.2Y0.8Ni5LxMnx alloys at 303 K.

to analyze the kinetic behavior of this phase transformation region. Johnson–Mehl–Avrami (JMA) model is a classical model to analyze the hydriding kinetics, which comprises both diffusion of hydrogen atoms, and nucleation and growth of hydride [21–24]. It can be expressed by the time-dependent reacted fraction as f ¼ 1  expðktn Þ;

(1)

where f is the hydriding fraction shown in Fig. 6, k is the reaction constant, and n is the reaction order. Eq. (1) can be transformed into lnð1  f Þ ¼ ktn ;

(2)

Relationships of ln(1  f ) and t at 303 K are shown in Fig. 6. In the (a þ b) region, all the original plots agreed well with the linear fit, i.e. the reaction order n ¼ 1. From the slopes of the fit lines, the reaction constants were calculated to be 0.01054, 0.01137 and 0.02564, respectively with the increase of Mn content, which is also consistent with the kinetics of the three alloys.

Fig. 6 – Lln (1 L f ) vs. ln t for La0.2Y0.8Ni5LxMnx alloys at 303 K.

(1) All the alloys showed a homogeneous single phase with CaCu5 type hexagonal structure, and with the increase of Mn content the unit cell increased in both a and c lattice axes. (2) With increasing Mn content, the plateau pressure dramatically decreased, the hydrogen uptake capacity increased, and the pressure plateau slope and hysteresis were affected slightly. It was shown that Mn is a favorable candidate for adjusting the equilibrium pressure of the alloys used for hydrogen compression. (3) The absorption kinetics was improved with increasing Mn content, which could be attributed to the increase of the surface area for hydrogenation. (4) La0.2Y0.8Ni4.8Mn0.2 alloy should be suitable for the highpressure stage compression of the hydrogen compressor, and the other two alloys, La0.2Y0.8Ni4.7Mn0.3 and La0.2Y0.8Ni4.6Mn0.4, for the preliminary stage.

Acknowledgments This work was financially supported by the National Natural Science Foundation of China (NO. 50776094), High-Tech Research and Development Project of China (NO. 2006A A05Z135), and National Basic Research Program of China (NO. 2010CB631305).

references

[1] Jain IP. Hydrogen the fuel for 21st century. Int J Hydrogen Energy 2009;34:7368–78. [2] Ball M, Wietschel M. The future of hydrogen – opportunities and challenges. Int J Hydrogen Energy 2009;34(2):615–27. [3] Wang XH, Bei YY, Song XC, Fang GH, Li SQ, Chen CP, et al. Investigation on high-pressure metal hydride hydrogen compressors. Int J Hydrogen Energy 2007;32(16):4011–5. [4] Ivanovsky AI, Kolosov VI, Lotosky MV, Solovey VV, Shmal’ko YuF, Kennedy LA. Metal hydride thermosorption compressors with improved dynamic characteristics. Int J Hydrogen Energy 1996;21:1053–5. [5] Shmal’ko YuF, Ivanovsky AI, Lotosky MV, Kolosov VI, Volosnikov DV. Sample pilot plant of industrial metal– hydride compressor. Int J Hydrogen Energy 1999;24:645–8. [6] Shmal’ko YuF, Ivanovsky AI, Lotosky MV, Karnatsevich LV, Milenko YuYa. Cryo-hydride high-pressure hydrogen compressor. Int J Hydrogen Energy 1999;24:649–50. [7] Muthukumar P, Maiya AP, Murthy SS. Performance tests on a thermally operated hydrogen compressor. Int J Hydrogen Energy 2008;33(1):463–9. [8] Nakamura H, Nakamura Y, Fujitani S, Yonezu I. A method for designing a hydrogen absorbing LaNi5xyMnxAly alloy for a chemical refrigeration system. J Alloys Compd 1997;252(1–2): 83–7.

international journal of hydrogen energy 35 (2010) 8262–8267

[9] Du P. Materials research of metal hydride air-conditioner systems. Ph.D. dissertation. Institute of Metal Research, Chinese Academy of Sciences, China; 2005. [10] Qin F. Research on the pulverization, expansion and heat transfer characteristics for a new type of rare earth-based hydrogen absorbing alloys and their applications in heat driven air-conditioning system. Ph.D. dissertation. Shanghai Jiaotong University, China; 2007. [11] Cheng HH, Deng XX, Li SL, Chen W, Chen DM, Yang K. Design of PC based high pressure hydrogen absorption/desorption apparatus. Int J Hydrogen Energy 2007;32:3046–53. [12] Li SL, Cheng HH, Deng XX, Chen W, Chen DM, Yang K. Investigation of hydrogen absorption/desorption properties of ZrMn0.85xFe1þx alloys. J Alloys Compd 2008;460:186–90. [13] Cheng HH, Chen DM, Yang K. Application of MATLAB in precise determination of lattice constants. J Syst Simul 2005; 17(z2):98–100. 104. [14] Lundin CE. Hydrogen storage properties and characteristics of rare earth compounds. J Phys Colloq 1979;40(C5):286–91. [15] Lundin CE, Lynch FE, Magee CB. A correlation between the interstitial hole sizes in intermetallic compounds and the thermodynamic properties of the hydrides formed from those compounds. J Less-Common Met 1977;56(1):19–37. [16] Zhao S, Lin Q, Chen N, Ma L, Ye W. The calculation and prediction for hydrogen storage properties of LaNi5xMx alloys. Acta Metall Sin 1999;35(1):65–9.

8267

[17] Balasubramaniam R. Hysteresis in metal–hydrogen systems. J Alloys Compd 1997;253–254:203–6. [18] Baranowski B, Debowska L. Kinetic and thermodynamic hysteresis in transition metal–hydrogen systems. J Alloys Compd 2007;440(1–2):L1–2. [19] Buckley CE, Gray EMA, Kisi EH. Stability of the hydrogen absorption and desorption plateaux in LaNi5–H Part 1: hysteresis dynamics and location of the equilibrium isotherm. J Alloys Compd 1994;215(1–2):195–9. } m MT, Klyamkin SN, Lund PD. Effect of [20] Hagstr o substitution on hysteresis in some high-pressure AB2 and AB5 metal hydrides. J Alloys Compd 1999;293–295: 67–73. [21] Muthukuma P, Satheesh A, Linder M, Mertz R, Groll M. Studies on hydriding kinetics of some La-based metal hydride alloys. Int J Hydrogen Energy 2009;34:7253–69. [22] Srinivas G, Sankaranarayanan V, Ramaprabhu S. Kinetics of hydrogen absorption in Ho1xMmxCo2 alloys. J Alloys Compd 2008;448(1–2):159–65. [23] Mani N, Ramaprabhu S. Effect of substitutional elements on hydrogen absorption properties in Mm-based AB5 alloys. J Alloys Compd 2004;363(1–2):275–91. [24] Sivakuma R, Ramaprabhu S, Rama Rao KVS, Anton H, Schmidt PC. Kinetics of hydrogen absorption and thermodynamics of dissolved hydrogen in Tb1xZrxFe3 system. Int J Hydrogen Energy 2000;25(5):463–72.