Comparative study on the dehydrogenation properties of TiCl4-doped LiAlH4 using different doping techniques

Comparative study on the dehydrogenation properties of TiCl4-doped LiAlH4 using different doping techniques

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Comparative study on the dehydrogenation properties of TiCl4-doped LiAlH4 using different doping techniques Jie Fu a, Lars Ro¨ntzsch b,*, Thomas Schmidt b, Marcus Tegel b, Thomas Weißga¨rber b, Bernd Kieback a,b a

Technische Universita¨t Dresden, Institute for Materials Science, Helmholtzstr. 7, 01069 Dresden, Germany Fraunhofer Institute for Manufacturing Technology and Advanced Materials (IFAM), Branch Lab Dresden, Winterbergstr. 28, 01277 Dresden, Germany b

article info

abstract

Article history:

Lithium aluminum hydride (LiAlH4) is an attractive hydrogen storage material because of

Received 11 April 2012

its comparatively high gravimetric hydrogen storage capacity. In this study, titanium

Received in revised form

tetrachloride (TiCl4), which is liquid at room temperature, was chosen as dopant because of

30 May 2012

its high catalytic efficiency regarding the dehydrogenation of LiAlH4. Three low-energy

Accepted 1 June 2012

doping methods (additive dispersion via ball milling at low rotation speed, magnetic stir-

Available online 20 July 2012

ring and magnetic stirring in ethyl ether) with different TiCl4 concentrations were compared in order to obtain optimum dehydrogenation properties of LiAlH4. At 80  C,

Keywords:

TiCl4-doped LiAlH4 can release up to 6.5 wt.%-H2, which opens the way to use of exhaust

Hydrogen storage material

heat of PEM fuel cells to trigger the hydrogen release from LiAlH4.

Lithium aluminum hydride

Copyright ª 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights

Dehydrogenation

reserved.

Doping methods Titanium tetrachloride

1.

Introduction

With regard to hydrogen as carbon-free energy carrier, which can be produced from renewable energies, metal hydrides have been investigated intensively as hydrogen storage materials in recent years [1]. Among the different candidate materials, lithium aluminum hydride (LiAlH4) is particularly attractive because of its high hydrogen storage capacity (volumetric: 95 g-H2 l1; gravimetric: 10.6 wt.% H2) in combination with rather low decomposition temperatures (dehydrogenation onset temperature below 100  C under argon atmosphere after doping [2]). It is generally accepted that pristine LiAlH4 decomposes in three steps expressed by reactions (1.1)e(1.3) [3,4]:

3LiAlH4 / Li3AlH6 þ 2Al þ 3H2 (150e175  C, 5.31 wt.% H2 of (1.1) LiAlH4) Li3AlH6 / 3LiH þ Al þ 3/2H2 (180e220  C, 2.66 wt.% H2 of (1.2) LiAlH4)

3LiH þ 3Al / 3LiAl þ 3/2H2 (350e400  C, 2.66 wt.% H2 of (1.3) LiAlH4) Although the reversible reaction from Li3AlH6 to LiAlH4 is thought to be endergonic [5], LiAlH4 can serve as single-use

* Corresponding author. Tel.: þ49 351 2537 411; fax: þ49 351 2537 399. E-mail address: [email protected] (L. Ro¨ntzsch). 0360-3199/$ e see front matter Copyright ª 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ijhydene.2012.06.009

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hydrogen storage material for various special applications, for example, portable hydrogen fuel cell systems. The dehydrogenation onset temperatures of LiAlH4 and its intermediately formed hydrides during dehydrogenation shown in reactions (1.1)e(1.3) can be greatly decreased after doping LiAlH4 with small quantities of catalytically active compounds (5 mol%). In addition, the dehydrogenation kinetics of LiAlH4 can be improved by the addition of suchlike dopants as shown in the literature [2,4,6e14]. Among frequently studied dopants, titanium tetrachloride (TiCl4) exhibits prominent catalytic effects. Its high catalytic activity has been attributed to the in situ formation of microcrystalline Al3Ti [13]. However, similar to the problem we have met and resolved in the process of doping various transition metal chloride powders into LiAlH4 [14], the addition of TiCl4 may also cause considerable dehydrogenation of LiAlH4 already during the doping process which greatly decreases the amount of hydrogen release for later use [8,13]. In this study, TiCl4 was chosen as dopant in view of its prominent effect on reducing the hydrogen release temperature and improving the dehydrogenation kinetics of LiAlH4. Three different preparation methods and five concentrations of TiCl4, which is a liquid at room temperature, were compared in order to optimize the LiAlH4 doping process with regard to the maximum amount of hydrogen release in combination with fast dehydrogenation kinetics. The dehydrogenation of TiCl4-doped LiAlH4 was examined under a H2 pressure of 1 bar at 80  C, which is the operating temperature of proton exchange membrane (PEM) fuel cells [15], aiming at applications where the exhaust heat of the fuel cell is used to trigger the dehydrogenation of the hydrogen storage material.

2.

Experimental

2.1.

Materials and pre-milling

Lithium aluminum hydride powder (99% purity) was purchased from Alfa-Aesar. Titanium (IV) chloride (99.9% purity) was purchased from SigmaeAldrich. Prior to doping, as-received LiAlH4 was milled for 30 min in a Fritsch P6 using a steel bowl and 10 mm steel balls with a ball-to-powder weight ratio of 20:1 with a rotation speed of 300 rpm. The powder preparation was done in a glovebox (MBraun) under argon to prevent unwanted oxidation (<2 ppm O2, <3 ppm H2O).

Table 1 e Sample preparation conditions. ID

Status of LiAlH4

S1 S2 S3 S4 S5 S6 S7 S8 S9 S10

As-received Pre-milled

100:2.0 Pre-milled

100:1.0 100:0.5 100:0.3 100:0.1 100:0

e e Ball milling Magnetic stirring Magnetic stirring in Et2O and then evacuating

2.2.2.

LiAlH4 doped with TiCl4 by magnetic stirring

Pre-milled LiAlH4 and 2 mol% TiCl4 were loaded into a flask under argon atmosphere. The mixture was stirred for 20 min by a magnetic stirrer with a speed of 1000 rpm at 0  C.

2.2.3. LiAlH4 doped with TiCl4 by magnetic stirring in ethyl ether (Et2O) The whole experimental process was performed at 0  C using an iceewater mixture. 1 g pre-milled LiAlH4, 2 ml Et2O and x mol% TiCl4 with x ¼ {0; 0.1; 0.3; 0.5; 1.0; 2.0} were added into a flask under argon atmosphere. The mixture was stirred for 10 min by a magnetic stirrer with a speed of 1000 rpm and then evacuated for 90 min to evaporate Et2O.

2.3.

Phase analysis

X-ray diffraction (XRD) was performed on a Bruker D8 Advance diffractometer using Cu-Ka1 radiation at a tube voltage of 40 kV and a tube current of 40 mA. The scanning range of the diffraction angle (2q) was from 10 to 100 . All samples were covered with a Kapton foil to prevent any unwanted oxidation during analysis. Rietveld refinement of the X-ray diffraction patterns was performed using the TOPAS software (version 4.2). The fundamental parameter approach was used as reflection profiles. Preferred orientations of the crystallites were described using spherical harmonics functions.

Microscopy

Doping procedure

For comparison, pre-milled LiAlH4 was doped with TiCl4 using three different methods. In total, ten samples with various preparation conditions have been investigated (cf. Table 1).

The morphology of the powder has been analyzed in an EVO 50 ZEISS scanning electron microscope (SEM) using a backscattered electron (BSE) detector.

2.5. 2.2.1.

100:0 100:0

Doping method

well-known decomposition of LiAlH4 during ball milling as described in various literature reports [8,13].

2.4. 2.2.

Molar ratio LiAlH4:TiCl4

Dehydrogenation kinetics

LiAlH4 doped with TiCl4 by ball milling

Pre-milled LiAlH4 and 2 mol% TiCl4 were loaded into a steel milling bowl with 10 mm steel balls (20:1 ball-to-powder weight ratio) under argon atmosphere and were ball-milled for 15 min with a rotation speed of 100 rpm at room temperature. The low rotation speed was chosen due to the

Thermogravimetric measurements to determine the dehydrogenation kinetics were carried out in a magnetic suspension balance (Rubotherm) with a precision of 10 mg at a H2 back pressure of 1 bar (99.9999% purity). The respective sample mass was about 250 mg. The applied heating rate was 1 K/min.

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(f)

Table 2 e Phase percentage of LiAlH4 doped with TiCl4 in Et2O.

2 7

ID Sample composition Percentage of main phases (wt.%)

Intensity (lg (Counts))

(e)

Al

(d)

Raw Corrected Raw Corrected (c) S1 S2 S9 S8 S7 S6 S5

(b) (a)

20

30

2

40

As-received LiAlH4 Pre-milled LiAlH4 LiAlH4 þ 0.1 mol% TiCl4 LiAlH4 þ 0.3 mol% TiCl4 LiAlH4 þ 0.5 mol% TiCl4 LiAlH4 þ 1.0 mol% TiCl4 LiAlH4 þ 2.0 mol% TiCl4

0.1 0.2 0.1 0.2 0.8 1.1 4.5

e 0.2 0.1 0.2 0.8 1.1 4.6

0.7 2.3 2.6 3.0 3.6 5.6 9.7

e 0 0.3 0.7 1.3 3.4 7.6

50

(degree)

Fig. 1 e X-ray diffractograms of (a) as-received LiAlH4 (S1), (b) pre-milled LiAlH4 (S2), (c) pre-milled LiAlH4 stirred in Et2O (S10), (d) 2 mol% TiCl4-doped LiAlH4 prepared by ball milling (S3), (e) 2 mol% TiCl4-doped LiAlH4 prepared by magnetic stirring (S4), (f) 2 mol% TiCl4-doped LiAlH4 prepared by magnetic stirring in Et2O (S5).

3.

LiCl

Results and discussion

Fig. 1 presents the X-ray diffractograms of undoped and freshly 2 mol% TiCl4-doped LiAlH4 prepared by the different doping methods applied in this study. Accordingly, asreceived LiAlH4 contains LiCl as impurity (Fig. 1a). After premilling of LiAlH4 for 30 min (Fig. 1b) or stirring pre-milled LiAlH4 in Et2O and then evacuating (Fig. 1c), no decomposition of LiAlH4 can be detected. In contrast, weak aluminum peaks have been found in all 2 mol% TiCl4-doped LiAlH4 samples (Fig. 1def), thus indicating a partial reduction of TiCl4 to Ti or TieAl phases most likely according to reaction (2)

during the three different doping processes. Because of the particle size detection limit of XRD [16], neither TiaAl1a nor Ti phase have been found in the X-ray diffractograms due to the potential formation of nanoparticles and their relatively small molar proportion. Unlike the complete transformation of LiAlH4 reported by Balema et al. [13], Li3AlH6 has not been found in any X-ray diffractogram of the samples investigated. Thus, the decomposition of LiAlH4, according to reaction (1.1), has been prevented obviously because of the low energy input during all three doping processes applied. Interestingly, LiAl2H7 has been detected in the pre-milled LiAlH4 stirred in Et2O without TiCl4 (Fig. 1c).

TiCl4 þ 4LiAlH4 / 4LiCl þ (1  b)∕aTiaAl1a þ bTi þ (5a þ b  1  ab)∕aAl þ 8H2

(2)

Fig. 2 displays the X-ray diffractograms of LiAlH4 doped with different concentrations of TiCl4 in Et2O. Weak aluminum peaks can be observed when the concentration of TiCl4 increases over 0.5 mol%. The crystalline phase compositions of doped LiAlH4 were determined by quantitative

(f)

LiAlH4 85.8%

LiCl 9.7% Al 4.5%

Intensity (Counts)

Intensity (lg (Counts))

(e) (d)

(c)

(b)

(a)

20

20

30

2

40

50

(degree)

Fig. 2 e X-ray diffractograms of (a) pre-milled LiAlH4 (S2), (b) 0.1 mol% TiCl4-doped LiAlH4 (S9), (c) 0.3 mol% TiCl4-doped LiAlH4 (S8), (d) 0.5 mol% TiCl4-doped LiAlH4 (S7), (e) 1.0 mol% TiCl4-doped LiAlH4 (S6), (f) 2.0 mol% TiCl4-doped LiAlH4 (S5). The doping was conducted in Et2O (cf. subsection 2.2.3).

30

40

2θ (degree)

Fig. 3 e X-ray diffractograms of 2.0 mol% TiCl4-doped LiAlH4 (S5) and Rietveld refinement (blue curve: experimental XRD pattern, red curve: calculated XRD pattern, gray curve: difference). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

50

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Fig. 4 e SEM micrographs in the BSE mode of (a) as-received LiAlH4, (b) pre-milled LiAlH4, (c) 2 mol% TiCl4-doped LiAlH4 prepared by ball milling, (d) 2 mol% TiCl4-doped LiAlH4 prepared by stirring and (e) 2 mol% TiCl4-doped LiAlH4 prepared by magnetic stirring in Et2O and vacuuming.

Therefore, the particle size is not considered as a crucial factor causing any significant difference of the dehydrogenation properties of the differently doped LiAlH4 samples. Unlike solid dopants, liquid TiCl4 is expected to distribute much more homogeneously during the preparation process and to react further into Ti or TieAl phases. However, it was not possible to detect those ultra-dispersed products in the SEM micrographs due to the uniform distribution of TiCl4 and hence the relative low concentration of Ti in the samples.

0

90

70 Temperature

-2

S1 S10 S4

60

S2 S3 S5

50 40

-4

30

Temperature (°C)

80

Hydrogen content (wt.%)

Rietveld analyses. Table 2 lists the crystalline weight percentage of LiCl and Al of all samples. Fig. 3 shows an exemplary XRD pattern considering the samples composed of only three observed phases: LiAlH4, LiCl and Al. The results indicate that the LiCl percentage in LiAlH4 (S1) increases after pre-milling (S2), which is induced by the reaction between LiAlH4 and impurities. Therefore both the LiCl and Al fractions in the TiCl4-doped LiAlH4 samples (S5eS9) were corrected by the amount of LiCl formed during pre-milling. It was found that the amount of LiCl increases nearly linearly with increasing TiCl4 concentration, which is consistent with the transformation of the additive according to reaction (2). Also the determined crystalline Al percentage in TiCl4-doped LiAlH4 increases with increasing TiCl4 concentration, but not linearly. Neither elemental Ti nor intermetallic TieAl compounds were observed in any X-ray diffraction pattern, which implies that these phases are either X-ray amorphous and/or an Alrich TiaAl1a alloy is formed. Moreover the corrected crystalline phase percentages of Al are considerably lower than the maximum expected values. This indicates that at least some TiaAl1ea is X-ray amorphous, which also explains the nonlinear increase of the observed Al phase with increasing TiCl4 concentration. The morphology of the powders has been analyzed by SEM micrographs in BSE mode (Fig. 4). The results reveal that asreceived LiAlH4 consists of irregularly shaped powder particles with a broad size distribution ranging from a few micrometers up to several 10 mm (Fig. 4a). For comparison, the powder particle size of pre-milled or TiCl4-doped LiAlH4 is similar and in the range of several to 10 mm (Fig. 4bee).

20 10 0

-6 0

100

200

300

400

500

Time (min)

Fig. 5 e Dehydrogenation characteristics of undoped and 2 mol% TiCl4-doped LiAlH4: as-received LiAlH4 (S1), premilled LiAlH4 (S2), pre-milled LiAlH4 stirred in Et2O (S10), 2 mol% TiCl4-doped LiAlH4 prepared by ball milling (S3), 2 mol% TiCl4-doped LiAlH4 prepared by stirring (S4), 2 mol % TiCl4-doped LiAlH4 prepared by stirring in Et2O (S5).

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Table 3 e Dehydrogenation properties of pre-milled LiAlH4 doped with TiCl4 (cf. Figs. 5 and 6). Percentage of the maximum hydrogen capacity considering (1.1) and (1.2)

After 590 min at 80  C

After additional 80 min at 240  C

After 590 min at 80  C

After additional 80 min at 240  C

0.19 0.12 1.45 3.55 5.35 5.67 6.50 6.58 6.02 0.22

e 7.60 6.03 6.22 6.79 7.25 7.53 7.70 7.56 7.57

2.4% 1.5% 19.7% 48.2% 72.6% 74.5% 83.9% 84.3% 76.6% 2.8%

e 96.3% 81.9% 84.5% 92.2% 95.2% 97.2% 98.7% 96.2% 96.0%

90

0

80 -1 S10

70

-2

S9 S8

60

-3

S7 S6 S5

-4

50 40

Temperature

30

Temperature (°C)

The dehydrogenation properties of undoped and 2 mol% TiCl4-doped LiAlH4 have been investigated in the temperature range from room temperature to constant 80  C (Fig. 5). The amounts of released hydrogen of all samples at 80  C after 590 min and the total hydrogen amounts, derived from the mass loss after additional 80 min at 240  C are listed in Table 3. The dehydrogenation curves demonstrate that undoped LiAlH4 (S1) does release merely a small amount of hydrogen at 80  C (0.19 wt.% within 590 min). In contrast, all 2 mol% TiCl4doped LiAlH4 samples prepared by the three different methods (S3, S4, S5) exhibit a considerable mass loss, which clearly indicates dehydrogenation. Among all 2 mol% TiCl4doped LiAlH4, the sample prepared in Et2O (S5) releases the highest amount of hydrogen and exhibits the best dehydrogenation kinetics at 80  C, while the sample prepared by ball milling (S3) shows the worst one. These results can be attributed to the distribution of TiCl4 in LiAlH4 during the preparing process. The addition of Et2O as liquid solvent improves the distribution of liquid TiCl4 during stirring and leads to a very homogeneous mixing of doped LiAlH4. Unfortunately, the distribution of Ti species in LiAlH4 cannot be seen in the SEM micrographs as discussed above (Fig. 4). According to the amount of hydrogen release at 80  C and 240  C given in Table 3, all three samples doped with 2 mol% TiCl4 prepared with different methods (S3, S4, S5) release more than 6 wt.% hydrogen at 240  C, which is significantly higher than the hydrogen release reported in [8,13]. Furthermore, reaction (1.2) has obviously already started at 80  C after doping LiAlH4 with TiCl4 since the theoretical maximal amount of hydrogen release for reaction (1.1) is only 5.26 wt.%. Since the TiCl4-doped LiAlH4 prepared in Et2O shows the best dehydrogenation properties, the effect of dopant concentration on the dehydrogenation of TiCl4-doped LiAlH4 prepared in Et2O has been investigated (Fig. 6). The results clearly demonstrate that 0.5 mol% TiCl4-doped LiAlH4 exhibits the best dehydrogenation performance. With regard to dehydrogenation kinetics, it releases about 4.6 wt.%-H2 during the first 100 min. After 590 min it has released about 6.5 wt.%-H2 which is the best result of the samples under investigation (together with the 0.3 mol% TiCl4doped LiAlH4). When the concentration of TiCl4 is above 0.5 mol %, the overall amount of hydrogen release is reduced due to the decreasing weight percentage of LiAlH4.

Hydrogen content (wt.%)

S1 S2 S3 S4 S5 S6 S7 S8 S9 S10

Experimental amount of hydrogen release (wt.%)

-5 20 -6

10 0

-7 0

100

200

300

400

500

Time (min)

Fig. 6 e Dehydrogenation characteristics of TiCl4-doped LiAlH4 prepared in Et2O: undoped LiAlH4 (S10), 0.1 mol% TiCl4-doped LiAlH4 (S9), 0.3 mol% TiCl4-doped LiAlH4 (S8), 0.5 mol% TiCl4-doped LiAlH4 (S7), 1.0 mol% TiCl4-doped LiAlH4 (S6), 2.0 mol% TiCl4-doped LiAlH4 (S5).

(b)

Intensity (lg (Counts))

ID

3

6

(a)

20

30

40

50

2

60

70

80

(degree)

Fig. 7 e X-ray diffractograms of 2 mol% TiCl4-doped LiAlH4 prepared in Et2O after dehydrogenation: (a) at 80  C and (b) at 240  C.

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Fig. 7 presents the X-ray diffractograms of LiAlH4 doped with 2 mol% TiCl4 prepared in Et2O after dehydrogenation at 80  C and 240  C, respectively. The results indicate that the majority phase, LiAlH4, in the TiCl4-doped sample has been transformed completely already at 80  C. However, some residual Li3AlH6 is still detected. Thus, the complete decomposition of the intermediately formed Li3AlH6 obviously requires higher temperatures than 80  C. A temperature treatment for 80 min at 240  C is sufficient for a complete dehydrogenation of Li3AlH6. As a result of this comparative study, LiAlH4 doped with TiCl4 in Et2O exhibits the best dehydrogenation kinetics. After evacuating at low temperature, the doped hydride powder particles are separated from Et2O keeping a high hydrogen capacity: Upto 6.5 wt.% hydrogen can be released at 1 bar hydrogen pressure and 80  C, which is the operating temperature of PEM fuel cells. These comprehensive dehydrogenation properties render suchlike TiCl4-doped LiAlH4 as outstanding single-use hydrogen storage material of PEM fuel cells systems. However, we reported that effective transition metal chloride dopants, such as ZrCl4 or TiCl3, may trigger the dehydrogenation of LiAlH4 powder at room temperature [14] over several weeks. In this regard, TiCl4 may have similar effects which may lead to a loss of hydrogen during long-term storage of doped LiAlH4. Therefore, investigations are under way to study the long-term storage properties of TiCl4-doped LiAlH4 at different storage conditions including low-temperature storage and storage under higher hydrogen pressures.

4.

Conclusion

1. Doping pre-milled LiAlH4 powder with liquid TiCl4 can significantly reduce the dehydrogenation onset temperature and improve the dehydrogenation kinetics. 2. Among three low-energy preparation methods ball milling, magnetic stirring and magnetic stirring in Et2O the latter one exhibits the best dehydrogenation properties. 3. Among five LiAlH4 samples doped with different concentration of TiCl4 prepared by magnetic stirring in Et2O, 0.5 mol% TiCl4-doped LiAlH4 shows the best dehydrogenation properties. 4. At 80  C, TiCl4-doped LiAlH4 can release up to 6.5 wt.%-H2, which opens the way to use of exhaust heat of PEM fuel cells to trigger the hydrogen release from LiAlH4.

Acknowledgement Fu Jie thanks the China Scholarship Council for financial support. Further, support from the Fraunhofer Attract

program is gratefully acknowledged. The authors thank M. Eckardt, S. Kalinichenka, T. Himmelreich and V. Pacheco for experimental assistance.

references

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