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The effect of V, VCl3 and VC catalysts on the MgH2 hydrogen sorption properties
Journal of Alloys and Compounds 586 (2014) S101–S104
Contents lists available at SciVerse ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
The effect of V, VCl3 and VC catalysts on the MgH2 hydrogen sorption properties M.O.T. da Conceição, M.C. Brum, D.S. dos Santos ⇑ PEMM-COPPE/Federal University of Rio de Janeiro, P.O. Box 68505, 21941972 Rio de Janeiro, Brazil
a r t i c l e
i n f o
Article history: Available online 29 December 2012 Keywords: Composite materials Energy storage materials Hydrogen absorbing materials Metal hydrides Mechanical alloying
a b s t r a c t MgH2 based composites processed by mechanical alloying with the addition of 5 wt.% of V, VC and VCl3 were evaluated for their absorption/desorption hydrogen capacities at 300 and 350 °C. These composites were investigated by the following techniques: X-ray diffraction, XRD, Pressure–Composition Isotherms, PCI and Differential Scanning Calorimetry, DSC. It was observed that the addition of V, VC and VCl3 compounds improves hydrogen absorption and desorption kinetics in comparison to the MgH2. The MgH2-5 wt.% VCl3 composite showed the faster absorption kinetics which absorbed 6.0 wt.% of H2 in 7.5 min at 350 °C. The desorption activation energy of the composite containing VCl3 (47 kJ mol1) is smaller than VC (63 kJ mol1). This indicates that this catalyst is more effective to be used in hydrogen storage system. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction Over the past 40 years metal hydrides have been thoroughly investigated mainly in search of the development of high performance alloys with fast and elevated hydrogen absorption and desorption capacity that can be controlled. These are the requirements needed for enabling its use in hydrogen storage systems for stationary or mobile energy applications [1]. The design of intelligent reservoirs [2] makes Mg a promising candidate for use in storage systems. Mg can absorb high amounts of hydrogen and for this reason it was often considered one of the most promising candidates for this purpose. However, it presents slow kinetics and only releases H2 at high temperature. Adding elements to an extent sufficient to promote the formation of intermetallic compounds such as Mg–Ni, Mg–Fe, Mg–Co, among others was initially a great motivation to several researchers [3]. Later researches have pointed to the use of transition elements in low amount to avoid the intermetallic compounds formation, such as light alloys. These additions contribute to decrease the temperature and pressure needed to break the hydrogen–metal bond. Nowadays, transition elements, intermetallic and organic compounds [4–7] are widely used as catalysts, especially combined with non-equilibrium thermodynamics processes in order to promote an energy excess, via the formation of defects in the metallic crystal structure and providing the refinement of the microstructure. Vanadium has been widely used among the transition metals since it presents one of the best catalytic properties [8–11]. However it is a high cost material and easily oxidates, so alternatives to its use are widely investigated. For these reasons, the present ⇑ Corresponding author. Tel.: +55 21 25628502. E-mail address: [email protected] (D.S. dos Santos). 0925-8388/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2012.12.131
study aims to comparatively evaluate the hydrogen absorption/ desorption kinetics in the MgH2 samples with addition of V, VCl3 and VC produced by mechanical alloying. 2. Materials and methods Commercial MgH2 powder with a purity of 99.4%, purchased by Sigma–Aldrich, was pre-milled in a hermetically closed steel vial with tungsten carbide balls under H2 atmosphere at 2 bar for 24 h using a Fritsch planetary mill with a rotation speed of 300 rpm. After that, the MgH2 was ball milled for 20 min more with 5 wt.% of V, VCl3 and VC catalysts. After milling, the samples structural properties were examined using X-ray diffraction measurements (Shimadzu XRD-6000, Cu Ka). Differential scanning calorimetric (DSC) measurements were performed using a DSC Sensys Evo (Setaram) under argon atmosphere at different heating rates equal to 10, 15, 20 and 25 K min1. The hydrogen sorption kinetics tests and pressure–composition isotherms (PCIs) were measured by an automatic Sievert’s type apparatus designed by PCT-Pro 2000. The hydrogen absorption and desorption measurements were conducted at 1 MPa and 0.01 MPa of hydrogen pressure, respectively, at 300 and 350 °C. The samples were handled in a glove box under argon atmosphere.
3. Results and discussion Fig. 1 shows the XRD patterns of MgH2 with catalysts and pure MgH2 after being ball milled. The peaks related to b-MgH2 and c-MgH2 due to the initial grinding of MgH2 are observed in all diffractograms. The metastable c-MgH2 phase is generated after mechanical milling due to the energy conditions created during processing. The sample containing 5 wt.% of V presents the VH0.81 peaks besides the Mg hydride peaks as reported by Liang et al. [12]. The X-ray pattern of MgH2 + 5 wt.% VC composite (Fig. 1) presents the peaks related to VC which indicates that the catalyst does not decompose with the processing performed. Even after hydrogen absorption and desorption cycles, the peaks related to the VC are identified which demonstrates the high stability of this
M.O.T. da Conceição et al. / Journal of Alloys and Compounds 586 (2014) S101–S104
V
Intensity (a.u.)
VC MgO Mg
β-MgH2
(a) 6
γ-MgH2
MgH2+5wt.% V MgH2+5wt.% VC MgH2+5wt.% VCl3
Hydrogen Capacity (wt.%)
S102
5
MgH2+5wt.% VCl3 4
MgH2+5wt.% VC MgH2+5wt.% V
3
2
1
300 °C 1 MPa
MgH2 milled 20
30
40
50
60
70
0
80
0
2θ (degree)
composite at high temperature in the presence of MgH2. The MgH2 + 5 wt.% VCl3 composite X-ray pattern presents peaks related to Mg hydride (b-MgH2 and c-MgH2) and no reflection related to VCl3 was identified, this occurs even after hydrogenation. This result contrasts with those observed by Malka et al. and Ma et al. [6,13] that observed traces of MgCl2 formation after hydrogenation. The difference between the literature and the results obtained in the present study can be explained due to the lower VCl3 concentration used in the latter. Besides this, the amount of V present in VCl3 is smaller so the catalyst costs can be reduced. Fig. 2a shows the absorption kinetics curves at 300 °C for MgH2 samples with different catalysts under 1 MPa of H2. MgH2 + 5 wt.% VCl3 and MgH2 + 5 wt.% VC samples presented a similar kinetics at the beginning (until 5 min). However the sample containing VCl3 reached the capacity of 6.0 wt.% of H2 in 7.5 min while the sample with VC attained a capacity of 5.3 wt.% of H2 in the same time interval. Despite the fact that the MgH2 + 5 wt.% V sample achieved the capacity of 6.0 wt.% of H2 it presented a slower kinetics compared to the other ones. This result obtained with VCl3 showed an increase of 1 wt.% of H2 capacity comparing to the one obtained with the same amount of another catalyst studied by Da Conceição et al. [4]. The desorption results at 300 °C (Fig. 2b) show that the best desorption kinetics was obtained for the MgH2 + 5 wt.% VC composite. The desorption rate obtained at this temperature for the VC addition is 1.0 102 wt.% s1 that is greater than the rates obtained for the VCl3 and V addition, 6.5 103 wt.% s1 and 3.1 102 wt.% s1, respectively. Fig. 3 shows the hydrogen absorption and desorption kinetic curves at 350 °C for the different catalysts added to MgH2. It is possible to notice that the absorption kinetics becomes slower than the ones obtained before but the addition of VCl3 still exhibits the best kinetics. In Fig. 3b, it can be seen that the VCl3 addition presents the fastest desorption kinetics. The desorption rate obtained at 350 °C for the VCl3 addition is 2.7 102 wt.% s1, overcoming even the desorption rate of the sample containing VC (2.2 102 wt.% s1). Fig. 4 shows the PCI curves for MgH2 with different V-based catalysts at 300 °C. The MgH2 milled with V and VCl3 exhibited a maximum hydrogen capacity of 6.3 wt.%. The hysteresis between hydrogen absorption and desorption of MgH2 + 5 wt.% VCl3 was smaller than MgH2 + 5 wt.% V and this can be attributed to the decreased mechanical stress between the metal and the hydride phases [14,15].
20
30
Time (min)
(b) 0
MgH2+5wt.% VCl3 -1
Hydrogen Capacity (wt.%)
Fig. 1. XRD patterns of MgH2 + 5 wt.% V, MgH2 + 5 wt.% VC, MgH2 + 5 wt.% VCl3 and MgH2 after milled.
10
MgH2+5wt.% VC MgH2+5wt.% V
-2
-3
-4
-5
-6
300 °C 0.01 MPa
-7 0
20
40
Time (min) Fig. 2. Comparison of the sorption kinetics of MgH2 containing 5 wt.% of V, VCl3 and VC: (a) absorption and (b) desorption, both at 300 °C.
According to Shahi et al. [10] VCl3 presents the best kinetics results when vanadium and V based compounds (VCl3 and V2O5) are added to Mg(NH2)2LiH. They attributed this finding to the fact that this compound has unoccupied orbital and increased tendency to interact electronically with the electron pair from N-atoms of other compounds. This ability overcomes pure V and V2O5 and may explain the decrease in the binding energy of hydrogen with the Mg thereby facilitating the hydrogen absorption and desorption kinetics. Fig. 5 shows the thermograms obtained by DSC for the MgH2 mixed with different vanadium-based catalysts samples. For comparison purposes the typical peak value for the as-received MgH2 sample is around 440 °C [16]. Regarding the addition of catalysts to MgH2, the lowest peak temperature obtained was 375 °C for MgH2 + 5 wt.% VCl3 and for the other samples it was 387 °C (MgH2 + 5 wt.% VC) and 400 °C (MgH2 + 5 wt.% V). These values express the desorption temperatures of hydrogen and a lower peak temperature in a thermogram cannot be directly correlated with a fastest desorption kinetics, because DSC peaks do not always correspond to a higher reaction rate, since the exothermic and endothermic peaks can overlap and consequently shift the peak value [17].
S103
M.O.T. da Conceição et al. / Journal of Alloys and Compounds 586 (2014) S101–S104 6
6
5
MgH2+5wt.% VC - absorption MgH2+5wt.% VCl3 - absorption MgH2+5wt.% V - absorption MgH2+5wt.% VC - desorption
5
MgH2+5wt.% VCl3 MgH2+5wt.% V
4
MgH2+5wt.% VCl3 - desorption
4
MgH2+5wt.% VC
Pressure (bar)
Hydrogen Capacity (wt.%)
(a) 7
3
MgH2+5wt.% V - desorption 3
2
2
1 1
350 °C 1 MPa
300 °C 0
0 0
10
20
0
30
1
2
Time (min)
(b) 0
4
5
6
Fig. 4. PCI curves of MgH2 + 5 wt.% x (x = V, VCl3 and VC) at 300 °C.
MgH2+5wt.% VCl3
dride decomposition was calculated by using Kissinger’s equation [18]
MgH2+5wt.% VC
-1
MgH2+5wt.% V
Hydrogen Capacity (wt.%)
3
Hydrogen Capacity (wt.%)
dðln b=T 2p Þ E ¼ R dð1=T p Þ
-2
-3
-4
-5
350 °C 0.01 MPa -6 0
10
20
30
Time (min) Fig. 3. Comparison of the sorption kinetics of MgH2 containing 5 wt.% of V, VCl3 and VC: (a) absorption and (b) desorption both at 350 °C.
Based on the results obtained from the non-isothermal DSC tests, with different heating rates, the activation energy for the hy-
ð1Þ
where b is the heating rate, Tp is peak temperature, E is activation energy and R is the gas constant. The activation energy for the hydride decomposition was obtained from the slope of the fitted line plot as shown in the inset (Fig. 5) (ln b=T 2p vs 1000/Tp). The values obtained were 47 and 63 kJ mol1 for MgH2 + 5 wt.% VCl3 and MgH2 + 5 wt.% VC, respectively. These values are lower than the one obtained for the pure MgH2 (120 kJ mol1) and the MgH2 with other chloride and some fluorides additives as MgH2 + 7 wt.% TiCl3 (97 kJ mol1), MgH2 + 7 wt.% NbF5 (83 kJ mol1) and MgH2 + 7 wt.% ZrF4 (77 kJ mol1) [14,19]. Although several articles report the effects of vanadium and its compounds as promising catalysts for hydrogen absorption and desorption in MgH2 [8–10,12], it is difficult to correlate their results with those obtained in the present work. This is mainly because of the different processing conditions (mechanical alloying, cold rolling, ECAP), amount of catalysts and tests parameters (temperature and pressure). The best conditions concerning these process parameters still have to be investigated.
0 -13.8
MgH2+5wt.% VCl3 MgH2+5wt.% VC -14.0
-2000
Ea = 63 ± 3 kJ/mol 2
ln( β/Tp)
Heat flow (mW/g)
-14.2
-4000
-14.4
-14.6
Ea = 47.5 ± 6 kJ/mol
-14.8 0.140 0.142 0.144 0.146 0.148 0.150 0.152 0.154 0.156 0.158 0.160
1/Tp*1000
-6000
-8000
MgH2+5wt.% VCl3 Exo
MgH2+5wt.% VC MgH2+5wt.% V
-10000 100
150
200
250
300
350
400
450
500
Temperature (°C) Fig. 5. DSC curves of MgH2 + 5 wt.% x (x = V, VCl3 and VC) after milling (heating rate: 10 K min1) and the inset shows the Kissinger plot for Ea calculation.
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4. Conclusions The hydrogen sorption kinetics of MgH2 + 5 wt.% x (x = V, VCl3 and VC) composites was analyzed at 300 and 350 °C. The sample containing VC showed fast desorption curves; however the high stability of VC can contribute for the slow absorption kinetics after 5 min. The addition of VCl3 compound acts as a better catalyst compared to the others. This catalyst promotes the reduction of the hydrogen desorption temperature and hydrogen desorption activation energy as shown in the DSC experiments. This composite attained the capacity of 6.0 wt.% of H2 in 7.5 min which is considered to be fast. This indicates a high enhancement of the hydrogen storage with only 5 wt.% of VCl3 which has less vanadium than pure vanadium in the same amount, and this can reduce the final costs and for these reasons mentioned above the use of this type of compound was shown to be promising. Acknowledgments The authors thank FINEP, CNPq and CAPES, for the financial support to this research. References [1] L. Schlapbach, A. Züttel, Nature 414 (2001) 353–358.
[2] A. Chaise, P. Rango, Ph. Marty, D. Fruchart, Int. J. Hydrogen Energy 35 (2010) 6311–6322. [3] L. Schlapbach, Hydrogen in Intermetallic Compounds II, Series: Topics in Applied Physics, first ed., Springer-Verlag, Germany, 1988. pp. 97–195. [4] M.O.T. da Conceição, M.C. Brum, D.S. dos Santos, M. Dias, J. Alloys Compd. 550 (2013) 179–184. [5] M.C. Brum, M.O.T. da Conceição, C.S. Guimarães, D.S. dos Santos, M. Dias, Int. J. Mater. Res. 103 (2012) 1144–1146 (formerly Z. Metallkd). [6] I.E. Malka, T. Czujko, J. Bystrzycki, Int. J. Hydrogen Energy 35 (2010) 1706– 1712. [7] J.F. Fernandez, J.R. Ares, F. Cuevas, J. Bodega, F. Leardini, C. Sanchez, Intermetallics 18 (2010) 233–241. [8] G. Liang, J. Huot, S. Boily, R. Schulz, J. Alloys Compd. 305 (2000) 239–245. [9] W. Oelerich, T. Klassen, R. Bormann, J. Alloys Compd. 322 (2001) L5–L9. [10] R.R. Shahi, T.P. Yadav, M.A. Shaz, O.N. Srivastva, Int. J. Hydrogen Energy 35 (2010) 238–246. [11] J. Charbonnier, P. de Rango, D. Fruchart, S. Miraglia, L. Pontonnier, S. Rivoirard, N. Skryabina, P. Vulliet, J. Alloys Compd. 383 (2004) 205–208. [12] G. Liang, J. Huot, S. Boily, A. Van Neste, R. Schulz, J. Alloys Compd. 291 (1999) 295–299. [13] L.-P. Ma, X.-D. Kang, H.-B. Dai, Y. Liang, Z.-Z. Fang, P.-J. Wang, P. Wang, H.-M. Cheng, Acta Mater. 57 (2009) 2250–2258. [14] T. Spassov, V. Rangelova, P. Solsona, M.D. Baro, D. Zander, U. Koster, J. Alloys Compd. 398 (2005) 139–144. [15] Y. Yoo, M. Tuck, R. Kondakindi, C.-Y. Seo, Z. Dehouche, K. Belkacemi, J. Alloys Compd. 446–447 (2007) 84–89. [16] R.A. Varin, T. Czujko, Z.S. Wronski, first ed., Springer, New York, 2008. pp. 90– 103. [17] M. Polanski, J. Bystrzycki, J. Alloys Compd. 486 (2009) 697–701. [18] H.E. Kissinger, Anal. Chem. 29 (1957) 1702–1706. [19] I.E. Malka, M. Pisarek, T. Czujko, J. Bystrzycki, Int. J. Hydrogen Energy 36 (2011) 12909–12917.
Contents lists available at SciVerse ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
The effect of V, VCl3 and VC catalysts on the MgH2 hydrogen sorption properties M.O.T. da Conceição, M.C. Brum, D.S. dos Santos ⇑ PEMM-COPPE/Federal University of Rio de Janeiro, P.O. Box 68505, 21941972 Rio de Janeiro, Brazil
a r t i c l e
i n f o
Article history: Available online 29 December 2012 Keywords: Composite materials Energy storage materials Hydrogen absorbing materials Metal hydrides Mechanical alloying
a b s t r a c t MgH2 based composites processed by mechanical alloying with the addition of 5 wt.% of V, VC and VCl3 were evaluated for their absorption/desorption hydrogen capacities at 300 and 350 °C. These composites were investigated by the following techniques: X-ray diffraction, XRD, Pressure–Composition Isotherms, PCI and Differential Scanning Calorimetry, DSC. It was observed that the addition of V, VC and VCl3 compounds improves hydrogen absorption and desorption kinetics in comparison to the MgH2. The MgH2-5 wt.% VCl3 composite showed the faster absorption kinetics which absorbed 6.0 wt.% of H2 in 7.5 min at 350 °C. The desorption activation energy of the composite containing VCl3 (47 kJ mol1) is smaller than VC (63 kJ mol1). This indicates that this catalyst is more effective to be used in hydrogen storage system. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction Over the past 40 years metal hydrides have been thoroughly investigated mainly in search of the development of high performance alloys with fast and elevated hydrogen absorption and desorption capacity that can be controlled. These are the requirements needed for enabling its use in hydrogen storage systems for stationary or mobile energy applications [1]. The design of intelligent reservoirs [2] makes Mg a promising candidate for use in storage systems. Mg can absorb high amounts of hydrogen and for this reason it was often considered one of the most promising candidates for this purpose. However, it presents slow kinetics and only releases H2 at high temperature. Adding elements to an extent sufficient to promote the formation of intermetallic compounds such as Mg–Ni, Mg–Fe, Mg–Co, among others was initially a great motivation to several researchers [3]. Later researches have pointed to the use of transition elements in low amount to avoid the intermetallic compounds formation, such as light alloys. These additions contribute to decrease the temperature and pressure needed to break the hydrogen–metal bond. Nowadays, transition elements, intermetallic and organic compounds [4–7] are widely used as catalysts, especially combined with non-equilibrium thermodynamics processes in order to promote an energy excess, via the formation of defects in the metallic crystal structure and providing the refinement of the microstructure. Vanadium has been widely used among the transition metals since it presents one of the best catalytic properties [8–11]. However it is a high cost material and easily oxidates, so alternatives to its use are widely investigated. For these reasons, the present ⇑ Corresponding author. Tel.: +55 21 25628502. E-mail address: [email protected] (D.S. dos Santos). 0925-8388/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2012.12.131
study aims to comparatively evaluate the hydrogen absorption/ desorption kinetics in the MgH2 samples with addition of V, VCl3 and VC produced by mechanical alloying. 2. Materials and methods Commercial MgH2 powder with a purity of 99.4%, purchased by Sigma–Aldrich, was pre-milled in a hermetically closed steel vial with tungsten carbide balls under H2 atmosphere at 2 bar for 24 h using a Fritsch planetary mill with a rotation speed of 300 rpm. After that, the MgH2 was ball milled for 20 min more with 5 wt.% of V, VCl3 and VC catalysts. After milling, the samples structural properties were examined using X-ray diffraction measurements (Shimadzu XRD-6000, Cu Ka). Differential scanning calorimetric (DSC) measurements were performed using a DSC Sensys Evo (Setaram) under argon atmosphere at different heating rates equal to 10, 15, 20 and 25 K min1. The hydrogen sorption kinetics tests and pressure–composition isotherms (PCIs) were measured by an automatic Sievert’s type apparatus designed by PCT-Pro 2000. The hydrogen absorption and desorption measurements were conducted at 1 MPa and 0.01 MPa of hydrogen pressure, respectively, at 300 and 350 °C. The samples were handled in a glove box under argon atmosphere.
3. Results and discussion Fig. 1 shows the XRD patterns of MgH2 with catalysts and pure MgH2 after being ball milled. The peaks related to b-MgH2 and c-MgH2 due to the initial grinding of MgH2 are observed in all diffractograms. The metastable c-MgH2 phase is generated after mechanical milling due to the energy conditions created during processing. The sample containing 5 wt.% of V presents the VH0.81 peaks besides the Mg hydride peaks as reported by Liang et al. [12]. The X-ray pattern of MgH2 + 5 wt.% VC composite (Fig. 1) presents the peaks related to VC which indicates that the catalyst does not decompose with the processing performed. Even after hydrogen absorption and desorption cycles, the peaks related to the VC are identified which demonstrates the high stability of this
M.O.T. da Conceição et al. / Journal of Alloys and Compounds 586 (2014) S101–S104
V
Intensity (a.u.)
VC MgO Mg
β-MgH2
(a) 6
γ-MgH2
MgH2+5wt.% V MgH2+5wt.% VC MgH2+5wt.% VCl3
Hydrogen Capacity (wt.%)
S102
5
MgH2+5wt.% VCl3 4
MgH2+5wt.% VC MgH2+5wt.% V
3
2
1
300 °C 1 MPa
MgH2 milled 20
30
40
50
60
70
0
80
0
2θ (degree)
composite at high temperature in the presence of MgH2. The MgH2 + 5 wt.% VCl3 composite X-ray pattern presents peaks related to Mg hydride (b-MgH2 and c-MgH2) and no reflection related to VCl3 was identified, this occurs even after hydrogenation. This result contrasts with those observed by Malka et al. and Ma et al. [6,13] that observed traces of MgCl2 formation after hydrogenation. The difference between the literature and the results obtained in the present study can be explained due to the lower VCl3 concentration used in the latter. Besides this, the amount of V present in VCl3 is smaller so the catalyst costs can be reduced. Fig. 2a shows the absorption kinetics curves at 300 °C for MgH2 samples with different catalysts under 1 MPa of H2. MgH2 + 5 wt.% VCl3 and MgH2 + 5 wt.% VC samples presented a similar kinetics at the beginning (until 5 min). However the sample containing VCl3 reached the capacity of 6.0 wt.% of H2 in 7.5 min while the sample with VC attained a capacity of 5.3 wt.% of H2 in the same time interval. Despite the fact that the MgH2 + 5 wt.% V sample achieved the capacity of 6.0 wt.% of H2 it presented a slower kinetics compared to the other ones. This result obtained with VCl3 showed an increase of 1 wt.% of H2 capacity comparing to the one obtained with the same amount of another catalyst studied by Da Conceição et al. [4]. The desorption results at 300 °C (Fig. 2b) show that the best desorption kinetics was obtained for the MgH2 + 5 wt.% VC composite. The desorption rate obtained at this temperature for the VC addition is 1.0 102 wt.% s1 that is greater than the rates obtained for the VCl3 and V addition, 6.5 103 wt.% s1 and 3.1 102 wt.% s1, respectively. Fig. 3 shows the hydrogen absorption and desorption kinetic curves at 350 °C for the different catalysts added to MgH2. It is possible to notice that the absorption kinetics becomes slower than the ones obtained before but the addition of VCl3 still exhibits the best kinetics. In Fig. 3b, it can be seen that the VCl3 addition presents the fastest desorption kinetics. The desorption rate obtained at 350 °C for the VCl3 addition is 2.7 102 wt.% s1, overcoming even the desorption rate of the sample containing VC (2.2 102 wt.% s1). Fig. 4 shows the PCI curves for MgH2 with different V-based catalysts at 300 °C. The MgH2 milled with V and VCl3 exhibited a maximum hydrogen capacity of 6.3 wt.%. The hysteresis between hydrogen absorption and desorption of MgH2 + 5 wt.% VCl3 was smaller than MgH2 + 5 wt.% V and this can be attributed to the decreased mechanical stress between the metal and the hydride phases [14,15].
20
30
Time (min)
(b) 0
MgH2+5wt.% VCl3 -1
Hydrogen Capacity (wt.%)
Fig. 1. XRD patterns of MgH2 + 5 wt.% V, MgH2 + 5 wt.% VC, MgH2 + 5 wt.% VCl3 and MgH2 after milled.
10
MgH2+5wt.% VC MgH2+5wt.% V
-2
-3
-4
-5
-6
300 °C 0.01 MPa
-7 0
20
40
Time (min) Fig. 2. Comparison of the sorption kinetics of MgH2 containing 5 wt.% of V, VCl3 and VC: (a) absorption and (b) desorption, both at 300 °C.
According to Shahi et al. [10] VCl3 presents the best kinetics results when vanadium and V based compounds (VCl3 and V2O5) are added to Mg(NH2)2LiH. They attributed this finding to the fact that this compound has unoccupied orbital and increased tendency to interact electronically with the electron pair from N-atoms of other compounds. This ability overcomes pure V and V2O5 and may explain the decrease in the binding energy of hydrogen with the Mg thereby facilitating the hydrogen absorption and desorption kinetics. Fig. 5 shows the thermograms obtained by DSC for the MgH2 mixed with different vanadium-based catalysts samples. For comparison purposes the typical peak value for the as-received MgH2 sample is around 440 °C [16]. Regarding the addition of catalysts to MgH2, the lowest peak temperature obtained was 375 °C for MgH2 + 5 wt.% VCl3 and for the other samples it was 387 °C (MgH2 + 5 wt.% VC) and 400 °C (MgH2 + 5 wt.% V). These values express the desorption temperatures of hydrogen and a lower peak temperature in a thermogram cannot be directly correlated with a fastest desorption kinetics, because DSC peaks do not always correspond to a higher reaction rate, since the exothermic and endothermic peaks can overlap and consequently shift the peak value [17].
S103
M.O.T. da Conceição et al. / Journal of Alloys and Compounds 586 (2014) S101–S104 6
6
5
MgH2+5wt.% VC - absorption MgH2+5wt.% VCl3 - absorption MgH2+5wt.% V - absorption MgH2+5wt.% VC - desorption
5
MgH2+5wt.% VCl3 MgH2+5wt.% V
4
MgH2+5wt.% VCl3 - desorption
4
MgH2+5wt.% VC
Pressure (bar)
Hydrogen Capacity (wt.%)
(a) 7
3
MgH2+5wt.% V - desorption 3
2
2
1 1
350 °C 1 MPa
300 °C 0
0 0
10
20
0
30
1
2
Time (min)
(b) 0
4
5
6
Fig. 4. PCI curves of MgH2 + 5 wt.% x (x = V, VCl3 and VC) at 300 °C.
MgH2+5wt.% VCl3
dride decomposition was calculated by using Kissinger’s equation [18]
MgH2+5wt.% VC
-1
MgH2+5wt.% V
Hydrogen Capacity (wt.%)
3
Hydrogen Capacity (wt.%)
dðln b=T 2p Þ E ¼ R dð1=T p Þ
-2
-3
-4
-5
350 °C 0.01 MPa -6 0
10
20
30
Time (min) Fig. 3. Comparison of the sorption kinetics of MgH2 containing 5 wt.% of V, VCl3 and VC: (a) absorption and (b) desorption both at 350 °C.
Based on the results obtained from the non-isothermal DSC tests, with different heating rates, the activation energy for the hy-
ð1Þ
where b is the heating rate, Tp is peak temperature, E is activation energy and R is the gas constant. The activation energy for the hydride decomposition was obtained from the slope of the fitted line plot as shown in the inset (Fig. 5) (ln b=T 2p vs 1000/Tp). The values obtained were 47 and 63 kJ mol1 for MgH2 + 5 wt.% VCl3 and MgH2 + 5 wt.% VC, respectively. These values are lower than the one obtained for the pure MgH2 (120 kJ mol1) and the MgH2 with other chloride and some fluorides additives as MgH2 + 7 wt.% TiCl3 (97 kJ mol1), MgH2 + 7 wt.% NbF5 (83 kJ mol1) and MgH2 + 7 wt.% ZrF4 (77 kJ mol1) [14,19]. Although several articles report the effects of vanadium and its compounds as promising catalysts for hydrogen absorption and desorption in MgH2 [8–10,12], it is difficult to correlate their results with those obtained in the present work. This is mainly because of the different processing conditions (mechanical alloying, cold rolling, ECAP), amount of catalysts and tests parameters (temperature and pressure). The best conditions concerning these process parameters still have to be investigated.
0 -13.8
MgH2+5wt.% VCl3 MgH2+5wt.% VC -14.0
-2000
Ea = 63 ± 3 kJ/mol 2
ln( β/Tp)
Heat flow (mW/g)
-14.2
-4000
-14.4
-14.6
Ea = 47.5 ± 6 kJ/mol
-14.8 0.140 0.142 0.144 0.146 0.148 0.150 0.152 0.154 0.156 0.158 0.160
1/Tp*1000
-6000
-8000
MgH2+5wt.% VCl3 Exo
MgH2+5wt.% VC MgH2+5wt.% V
-10000 100
150
200
250
300
350
400
450
500
Temperature (°C) Fig. 5. DSC curves of MgH2 + 5 wt.% x (x = V, VCl3 and VC) after milling (heating rate: 10 K min1) and the inset shows the Kissinger plot for Ea calculation.
S104
M.O.T. da Conceição et al. / Journal of Alloys and Compounds 586 (2014) S101–S104
4. Conclusions The hydrogen sorption kinetics of MgH2 + 5 wt.% x (x = V, VCl3 and VC) composites was analyzed at 300 and 350 °C. The sample containing VC showed fast desorption curves; however the high stability of VC can contribute for the slow absorption kinetics after 5 min. The addition of VCl3 compound acts as a better catalyst compared to the others. This catalyst promotes the reduction of the hydrogen desorption temperature and hydrogen desorption activation energy as shown in the DSC experiments. This composite attained the capacity of 6.0 wt.% of H2 in 7.5 min which is considered to be fast. This indicates a high enhancement of the hydrogen storage with only 5 wt.% of VCl3 which has less vanadium than pure vanadium in the same amount, and this can reduce the final costs and for these reasons mentioned above the use of this type of compound was shown to be promising. Acknowledgments The authors thank FINEP, CNPq and CAPES, for the financial support to this research. References [1] L. Schlapbach, A. Züttel, Nature 414 (2001) 353–358.
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