Dehydrogenation kinetics, reversibility, and reaction mechanisms of reversible hydrogen storage material based on nanoconfined MgH2−NaAlH4

Journal of Physics and Chemistry of Solids 87 (2015) 16–22

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Dehydrogenation kinetics, reversibility, and reaction mechanisms of reversible hydrogen storage material based on nanoconfined MgH2
NaAlH4 Praphatsorn Plerdsranoy, Sukanya Meethom, Rapee Utke n School of Chemistry, Institute of Science, Suranaree University of Technology, Nakhon Ratchasima 30000, Thailand

art ic l e i nf o

a b s t r a c t

Article history: Received 2 April 2015 Received in revised form 10 June 2015 Accepted 24 July 2015 Available online 28 July 2015

Studies of dehydrogenation kinetics, reversibility, and reaction mechanisms during de/rehydrogenation of nanoconfined MgH2
NaAlH4 into carbon aerogel scaffold (CAS) for reversible hydrogen storage material are for the first time proposed. Two different MgH2:NaAlH4 molar ratios (1:1 and 2:1) of hydride composite are melt infiltrated into CAS under 1:1 (CAS:hydride composite) weight ratio. Successful nanoconfinement is confirmed by N2 adsorption
desorption. Multiple-step dehydrogenation of milled samples is reduced to two-step reaction due to nanoconfinement. Peak temperatures corresponding to main dehydrogenation of nanoconfined samples significantly reduce as compared with those of milled samples, i.e., ΔT ¼up to 50 and 34 °C for nanoconfined sample with 1:1 and 2:1 (MgH2:NaAlH4) molar ratios, respectively. Hydrogen content released (the 1st cycle) and reproduced (the 2nd, 3rd, and 4th cycles) of nanoconfined samples enhance up to 80% and 68% with respect to theoretical hydrogen storage capacity, respectively, while those of milled samples are 71% and 38%, respectively. Remarkable hydrogen content reproduced after nanoconfinement is due to the fact that metallic Al obtained after dehydrogenation (T ¼ 300 °C under vacuum) of nanoconfined samples prefer to react with MgH2 and produces Al12Mg17, favorable for reversibility of MgH2
NaAlH4 system, whereas that of milled samples stays in the form of unreacted Al under the same temperature and pressure condition. & 2015 Elsevier Ltd. All rights reserved.

Keywords: Intermetallic compounds Microporous materials Nanostructures Differential scanning calorimetry (DSC) X-ray diffraction

1. Introduction Magnesium hydride (MgH2) has great potential for hydrogen storage applications due to not only its high hydrogen storage capacity and density of 7.6 wt% and 110 g/L, respectively, but also low cost and abundance of magnesium, favorable for large scale storage materials [1,2]. However, MgH2 is thermally stable (ΔH¼
74 kJ/ mol), leading to high dehydrogenation temperature (i.e., more than 300 °C for p (H2)¼1 bar) [3]. Moreover, its slow hydrogen sorption kinetics hinder practical uses as hydrogen storage material. Several strategies based on (i) reduction of particle size to nanoscale by mechanical ball-milling [4,5], (ii) catalytic doping, such as transition metals (V, Ti, Mn, Fe, and Ni) [6–8], metal oxides (Nb2O5 and Cr2O3) [9, 10], and metal halides (FeF3, NaF, NaCl, MgF, and CrCl3) [11–14], and (iii) reactive hydride composites (RHCs) [15–22] have been investigated to improve hydrogen storage properties of MgH2. On the basis of RHCs, by compositing with complex hydrides of LiAlH4, Li3AlH6, and LiBH4, destabilization of MgH2 was obtained due to the n

Corresponding author. E-mail address: [email protected] (R. Utke).

http://dx.doi.org/10.1016/j.jpcs.2015.07.018 0022-3697/& 2015 Elsevier Ltd. All rights reserved.

formation of some intermediate phases (Li0.92Mg4.08, Al12Mg17, and MgB2) [17,18, 21]. Ismail et al. [23] reported that dehydrogenation temperature (Tdes) and activation energy (EA) of MgH2 were reduced significantly (i.e., ΔTdes ¼155 °C and ΔEA ¼ 20 kJ/mol H2) after compositing with NaAlH4 under 4:1 (MgH2:NaAlH4) molar ratio. Furthermore, the effects of MgH2:NaAlH4 mole ratios (1:2, 1:1, and 2:1) and catalysts (TiO2, Nb2O5, single wall carbon nanotube, and graphene nano sheets) were studied [24–26]. It was found that different MgH2:NaAlH4 mole ratios profoundly impacted on reaction pathway, for example, dehydrogenation of 1:1 and 1:2 (MgH2:NaAlH4) mole ratios was three-step reaction, while that of 2:1 performed four steps. Onset dehydrogenation temperature of MgH2
NaAlH4 composites under all mole ratios was  150 °C, considerably lower than that of pristine MgH2 (ΔT up to 200 °C). Moreover, by catalytic doping further reduction of onset dehydrogenation temperature ( 50 °C) was accomplished, however, multiple-step dehydrogenation were still observed and only about 50% of total hydrogen storage capacity released at temperature up to 300 °C [24–26]. Furthermore, hydrogen storage properties of MgH2 can be improved by nanoconfinement into porous carbon aerogel scaffolds (CAS), where particle size of hydride was constrained in nanoscale

P. Plerdsranoy et al. / Journal of Physics and Chemistry of Solids 87 (2015) 16–22

through cycling. The latter resulted in increase of surface area and short diffusion distance for hydrogen exchange reaction [27–30]. Nanoconfined MgH2 into CAS (10 nm pore diameter) with 15– 17 wt% loading released hydrogen at the rate of 1.09 wt%/h at 252 °C, which was approximately ten times faster than ball-milled MgH2 (0.13 wt%/h) [28]. Recently, nanoconfined 2LiBH4
MgH2 prepared by direct melt infiltration were found to release hydrogen at significantly lower temperature as compared with bulk material, for example, onset dehydrogenation temperature of nanoconfined 2LiBH4
MgH2 was at about 290 °C, while that of bulk sample required up to 350 °C [31,32]. Besides, approximately ten times faster dehydrogenation kinetics of 2LiBH4-MgH2 composite was achieved after nanoconfinement [32]. On the basis of melt infiltration of 2LiBH4
MgH2 composite into CAS, molten LiBH4 not only confined into CAS by wetting its porous structure, but also functioned as a carrier to transport MgH2 particles into CAS. It was reported that particle size reduction of MgH2 by simply premilling prior to milling with LiBH4 and melt infiltration into CAS provided effective nanoconfinement of both LiBH4 and MgH2. This led to fast dehydrogenation kinetics, suppression of toxic diborane (B2H6) gas, no stable [B12H12]2
phase formed during cycling, and the mildest dehydrogenation condition (T¼ 320 °C, p(H2)¼ 3
4 bar) as compared with other modified 2LiBH4
MgH2 systems [33]. In the present work, we would like to extend our studies to nanoconfinement of MgH2
NaAlH4 composite prepared by direct melt infiltration at 185 °C (melting point of NaAlH4) under 110 bar H2. Prior to mixing with NaAlH4 and melt infiltration into CAS, MgH2 was premilled for 5 h to reduce particle size and to improve nanoconfinement. Two different molar ratios of MgH2:NaAlH4 (1:1 and 2:1) were melt infiltrated into CAS under the weight ratio of CAS:hydride composite of 1:1. Successful nanoconfinement of composite hydrides into porous structure of CAS was confirmed by N2 adsorption-desorption experiments. Dehydrogenation temperatures and profiles of milled and nanoconfined samples were studied by simultaneous differential scanning calorimetry (DSC)
thermogravimetry (TG)-mass spectroscopy (MS) technique. Dehydrogenation kinetics and reversibility were confirmed by titration measurements and reaction mechanisms during de/rehydrogenation were assured by ex situ powder X-ray diffraction (PXD).

2. Experimental details 2.1. Sample preparation Carbon aerogel scaffold (CAS) obtained from carbonization of resorcinol-formaldehyde polymer aerogel was synthesized according to the procedures previously reported [32,34]. The mixture of 31.0400 g resorcinol (99%, Sigma-Aldrich), 42.90 mL deionized water, 42.60 mL formaldehyde in water (37 wt% stabilized by 10
15 wt% methanol, QRëC), and 0.1180 g anhydrous Na2CO3 (99.999%, Aldrich) was continuous stirred till homogeneity. The mixture sealed in a polyethylene bottle was aged at room temperature for 24 h, 50 °C for 24 h, and 90 °C for 72 h, and cooled to room temperature to obtain the polymer aerogel. The aerogel was cut into small pieces, soaked in an acetone bath three times within 2 days, and dried at room temperature for several days in the fume hood. The dried aerogel was carbonized in a tubular furnace at constant temperature of 950 °C (10 °C/min) for 5 h under CO2 flow (60 mL/min). The furnace was turned off and the sample was cooled down naturally to room temperature. The gel obtained was further treated at 500 °C under vacuum for 6 h to obtain a carbon aerogel scaffold, denoted as CAS. MgH2 (95%, Acros) was packed in a sealed vial (8004 Tungsten carbide vial set, SPEX SamplePrep, USA) under an argon

17

atmosphere in a glove box and milled by using a SPEX SamplePrep s 8000D DUAL Mixer/Mill . Ball-to-powder weight ratio and milling time were 10:1 and 5 h, respectively. Milled MgH2 powder was physically mixed in the mortar with NaAlH4 (Z93%, hydrogen storage grade, Sigma–Aldrich) under MgH2: NaAlH4 mole ratios of 1:1 and 2:1, denoted as MgH2
NaAlH4 and 2MgH2
NaAlH4, respectively. The mixtures of MgH2
NaAlH4 and 2MgH2
NaAlH4 were further ground with CAS under the weight ratio of 1:1 (CAS: hydride composite) to obtain CAS
MgH2
NaAlH4 and CAS-2MgH2
NaAlH4, respectively. Nanoconfinement was carried out by heating the powder samples of CAS-MgH2
NaAlH4 and CAS-2MgH2
NaAlH4 to 185 °C (5 °C/min) under 110 bar H2, dwelling at 185 °C for 45 min, and cooling to room temperature to achieve nanoconfined samples of MgH2
NaAlH4 and 2MgH2
NaAlH4 in CAS, denoted as nano MgH2
NaAlH4 and nano 2MgH2
NaAlH4, respectively. The mixtures of MgH2 and NaAlH4 under the molar ratios of 1:1 and 2:1 (MgH2: NaAlH4) were milled for 30 min under ball-to-powder weight ratio of 15:1. 2.2. Characterizations Texture parameters of CAS and nanoconfined samples based on specific surface area, pore size, and pore volume were characterized by N2 adsorption
desorption measurements at 77 K using a BELsorp-mini II, Japan. Prior to the measurements, a known amount of sample was degassed at 350 °C (CAS) and at room temperature (nanoconfined samples) under vacuum for 24 h. All samples were studied with a full adsorption and desorption isotherm in the pressure range of 0–1 (p/p0) at liquid nitrogen temperature with nitrogen gas as an adsorbent. The measurement was programed to continuously change the pressure ratio to 1 for adsorption, and to 0 for desorption. Data was analyzed by the t-plot method [35–36], the Brunner Emmet Teller (BET) method [37], and the Barret Joyner Halenda (BJH) method [38]. The highest point of the isotherm measurements (where p/p0∼1) was used to calculate the total volume of the sample. Differential scanning calorimetry (DSC) and thermogravimetry (TG) of milled and nanoconfined samples during dehydrogenation were carried out simultaneously by a Netzsch STA 449F3 Jupiter. The powder sample of 5
10 mg was heated from room temperature to 450 °C with a heating rate of 5 °C/min under an N2 flow of 50 mL/min. The relative composition of hydrogen (H2) gas released during dehydrogenation was continuously detected by a Netzsch QMS 403C mass spectrometer (MS). De/rehydrogenation kinetics of milled and nanoconfined samples was studied by using a laboratory scale setup of a Sievert-type apparatus previously reported [39]. The powder sample of  60– 80 mg was loaded in a stainless steel sample holder (316SS, Swagelok) under argon atmosphere in the glove box and transferred to the apparatus. Two K-type thermocouples (
250 to 1300 °C, SL heater) were attached to the sample holder and the furnace to measure the temperature of the system. Pressure transducers (C206, Cole Parmer) in the pressure range of 0
500 psig and 0
3000 psig were used to measure the pressure changes due to hydrogen desorption and absorption, respectively. Thermocouples and pressure transducers were connected to an AI 210I module convertor data logger (Wisco), measuring and transferring (every 1 s) the pressure and temperature changes of the sample to the computer for further evaluation. Dehydrogenation was done by heating the sample from room temperature to 300 °C (3.6 °C/min) under vacuum. For rehydrogenation, the dehydrogenated powder sample was pressurized under 95 bar H2 (purity ¼ 99.999%) at 300 °C for 8 h. Once the pressure reading was constant over a period of time, the amount of hydrogen released was calculated by the pressure change (ΔP) and the following equations:

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P. Plerdsranoy et al. / Journal of Physics and Chemistry of Solids 87 (2015) 16–22

(ΔP) V = nRT M a s s s p e c tr o s c o p ic in t e n s i ty (a .u .)

Desorbed H2 (wt.%)

Mass spectroscopic intensity (a.u.)

100

III

II

Desorbed H2 (wt.%)

I

H2 desorbed (wt. %) = [(n × 2.0158)/sample weight] × 100

98 6.52 wt.%

D S C (m W /m g ) E x o

where P, V, and T are hydrogen pressure (atm), volume of the system (L), and temperature (K), respectively, n is the number of hydrogen moles (mol), and R is gas constant (0.0821 L atm K
1 mol
1). Ex situ powder X-ray diffraction (PXD) of milled and nanoconfined samples were performed by using a Bruker D2 PHASER with Cu Kα radiation (λ ¼0.15406 nm). The powder sample was packed in an airtight sample holder under argon atmosphere in the glove box. The diffraction patterns were collected in the 2θ range of 10–80° with the scanning step of 0.01 °/s.

96 94

256 C 369 C

186 C

92 90

384 C

88 H

100

200

86 400

300

Temperature ( C) 3. Results and discussion

3NaAlH4 → Na3AlH6 + 2Al + 3H2

(1)

Na3AlH6 → 3NaH + Al + 3/2H2

(2)

For the second step releasing 2.10 wt% H2 (300
375 °C), not only dehydrogenation of MgH2 is accomplished, but also the formation of Al12Mg17 (Eqs. (3) and (4)) [24]. In this case, Al acts as a destabilizing agent for MgH2
NaAlH4 system by reacting with MgH2 to form Al12Mg17. Afterwards, the last step hints at the decomposition of NaH (375
400 °C) and desorbs 1.07 wt% H2 (Eq. (5) (Fig. 1(A)) [24]. Table 1 Texture parameters of CAS and nanoconfined samples. Samples

SBET (m2 /g)

CAS 1175 Nano MgH2
NaAlH4 201 Nano 2MgH2
NaAlH4 379

I

Vmeso (mL/g)

Dmax (nm)

Vtot (mL/ g)

0.44 0.07 0.05

1.73 0.53 0.84

7.9 12.0 10.5

2.31 0.60 1.00

100

IV

III

6.40 wt.%

96 94 250 C

364 C

92

185 C

379 C 90

H2

100

200

88 400

300

Temperature ( C) TG

DSC

MS

Fig. 1. Simultaneous DSC
TG-MS of milled MgH2
NaAlH4 (A) and milled 2MgH2
NaAlH4 (B).

MgH2 → Mg + H2

(3)

17MgH2 + 12Al → Al12 Mg17 + 17H2

(4)

NaH → Na + 1/2H2

(5)

In the case of milled 2MgH2
NaAlH4 (Fig. 1(B)), four-step reaction is detected in the temperature range of 160
400 °C together with 6.40 wt% H2, corresponding to the previous studies [24–26]. First step (160
275 °C) releases 2.50 wt% H2, corresponding to dehydrogenation of NaAlH4 and Na3AlH6 (Eqs. (1) and (2)). In the range of 292
340 °C, besides the dehydrogenation of MgH2 and the formation of Al12Mg17 as in case of milled MgH2
NaAlH4 (Eqs. (3) and (4)), NaMgH3 is produced from the reaction between NaH and MgH2 (Eq. (6)) [24]. This step desorbs 1.60 wt% H2 (Fig. 1(B)). It was found that higher concentration of MgH2 favored the formation of NaMgH3 due to large amount of interfaces between the reacting phases (NaAlH4 and MgH2 in this case) [24].

NaH + MgH2 → NaMgH3

Vmicro (mL/g)

II

98

D S C (m W /m g ) E x o

Successful nanoconfinement of MgH2
NaAlH4 composite hydrides in CAS was characterized by N2 adsorption
desorption measurements. From Table 1, CAS used in this work shows surface area, pore size, and total pore volume of 1175 m2/g, 7.9 nm, and 2.31 mL/g, respectively. For nano MgH2
NaAlH4 and nano 2MgH2
NaAlH4, surface area and total pore volume decrease to the range of 201
379 m2/g and 0.60
1.00 mL/g, respectively, suggesting successful nanoconfinement of hydride material in CAS (Table 1). However, it could be also possible that some of hydride particles (or phases) are on the surface due to pore blocking during melt infiltration. Thus, further results based on de/rehydrogenation temperatures and pathways are used to confirm whether this hydride composite is confined in CAS. In addition, from Table 1, it should be noted that by increasing MgH2 content in the hydride composite, deficient nanoconfinement is observed as revealed as higher surface area and pore volume of nano 2MgH2
NaAlH4 with respect to nano MgH2
NaAlH4 (Table 1). Preliminary results based on dehydrogenation of milled samples were studied by simultaneous DSC
TG-MS technique. From Fig. 1(A), milled MgH2
NaAlH4 reveals approximately three-step dehydrogenation in the temperature range of 160
400 °C together with total hydrogen content released of 6.52 wt%, approaching to the previous work [24,25]. The first step relating to exothermic and endothermic peaks in the range of 160
280 °C and 3.35 wt% H2 refers to dehydrogenation of NaAlH4 and Na3AlH6 (Eqs. (1) and (2)) [24].

(6)

Afterwards, the third-step reaction (340
371 °C) leading to the dehydrogenation of NaMgH3 gives 1.50 wt% H2 (Eq. (7)). For the last step, decomposition of NaH (Eq. (5)) desorbing 0.80 wt% H2 is accomplished in the range of 374
400 °C.

NaMgH3 → NaH + Mg + H2

(7)

H2

nano MgH -NaAlH

milled MgH -NaAlH

Temperature

300

3.5

250

3.0 2.5

200

2.0 150

1.5

Temperature ( C)

358 C

19

4.0

Desorbed H (wt.%)

216 C

Mass spectroscopic intensity (a.u.)

DSC (mW/mg) Exo

P. Plerdsranoy et al. / Journal of Physics and Chemistry of Solids 87 (2015) 16–22

100

1.0 0.5

50

0.0

H2

100

200

300

400

Temperature ( C)

DSC

MS

Fig. 2. Simultaneous DSC
MS of nano MgH2
NaAlH4 (A) and nano 2MgH2
NaAlH4 (B).

Dehydrogenation profiles of nanoconfined samples were also determined by simultaneous DSC
MS technique. From Fig. 2 (A) and (B), approximately two-step dehydrogenation is observed from both nano MgH2
NaAlH4 (at 160
260 and 325
400 °C, respectively) and nano 2MgH2
NaAlH4 (at 180
250 and 275
400 °C, respectively). The peak temperatures relating to the first and last step dehydrogenations of nano MgH2
NaAlH4 are at 216 and 358 °C, respectively, while those of nano 2MgH2
NaAlH4 are at 206 and 345 °C, respectively. It should be noted that not only multiple-step dehydrogenation (three and four steps for milled MgH2
NaAlH4 and milled 2MgH2
NaAlH4, respectively) reduces to two steps after melt infiltration, but also reduction of dehydrogenation temperature is considerably achieved. Regarding the peak temperatures of the first and last step dehydrogenations, milled MgH2
NaAlH4 reveals at 256 and 384 °C, respectively, while those of milled 2MgH2
NaAlH4 are 250 and 379 °C, respectively (Fig. 1). Therefore, it should be remarked that after melt infiltration in CAS dehydrogenation temperatures of MgH2
NaAlH4 reduce up to ΔT¼50 and 40 °C for the first and last steps, respectively, while those of 2MgH2
NaAlH4 are ΔT¼ 34 and 21 °C, respectively. Dehydrogenation kinetics and hydrogen reproducibility of milled and nanoconfined samples were determined by laboratory scale test station of Sievert-type apparatus. Dehydrogenation and rehydrogenation of milled and nanoconfined samples were carried out at the same temperature of ∼300 °C under vacuum and 95 bar H2, respectively.

4

6

8

10

12

14

Fig. 3. Dehydrogenation kinetics and reversibility of milled and nano MgH2
NaAlH4.

With respect to weight ratio of CAS:hydride composite (1:1) and hydrogen content released from milled MgH2
NaAlH4 and 2MgH2
NaAlH4 of 6.52 and 6.40 wt%, respectively (TG thermograms in Fig. 1), theoretical hydrogen storage capacities of nano MgH2
NaAlH4 and nano 2MgH2
NaAlH4 are 3.26 and 3.20 wt% H2, respectively. From Fig. 3, milled MgH2
NaAlH4 desorbs 3.58 and 1.03 wt% H2 after 6 h (55% and 16% with respect to theoretical H2 capacity, respectively) during the 1st and 2nd dehydrogenations, respectively. In the case of nano MgH2
NaAlH4, 1.97 wt% H2 (60% with respect to theoretical H2 capacity) is detected after 6 h during the 1st dehydrogenation, while those of the 2nd, 3rd, and 4th cycles are comparable of ∼1.83 wt% H2 (56% with respect to theoretical H2 capacity) (Fig. 3). For 2MgH2
NaAlH4 system, milled sample gives 4.52 and 2.41 wt% H2 (71% and 38% with respect to theoretical H2 capacity, respectively) during the 1st and 2nd dehydrogenations, respectively (Fig. 4). Nano 2MgH2
NaAlH4 releases 2.55 wt% H2 (80% with respect to theoretical H2 capacity) during the 1st dehydrogenation, while those of the 2nd, 3rd, and 4th cycles are comparable of 2.17 wt% H2 (68% with respect to theoretical H2 capacity) (Fig. 4). It should be remarked that after melt infiltration not only the enhancement of hydrogen content released from MgH2
NaAlH4 and 2MgH2
NaAlH4 systems during the 1st dehydrogenation (up to 5% and 9%, respectively) is obtained, but also significant amount of hydrogen reproduced up to 40% and 30%, respectively, in the further (2nd, 3rd, and 4th) cycles is accomplished. Regarding the previous milled 2MgH -NaAlH

nano 2MgH -NaAlH Temperature

5

300 250

4

200

3

150

2

100

Temperature ( C)

345 C

2

Time (h)

Desorbed H (wt.%)

206 C

Mass spectroscopic intensity (a.u.)

DSC (mW/mg) Exo

0

1 50 0 0

2

4

6 8 Time (h)

10

12

14

Fig. 4. Dehydrogenation kinetics and reversibility of milled and nano 2MgH2
NaAlH4.

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P. Plerdsranoy et al. / Journal of Physics and Chemistry of Solids 87 (2015) 16–22

MgH2 NaAlH4 Na3AlH6 NaMgH3 Al Al12Mg17

(d)

MgH2 NaAlH4 Na3AlH6 NaMgH3 Al Al12Mg17 MgO (d) sample holder

MgO

sample holder

(c) I n t e n s i t y (a . u . )

In te n sity (a.u .)

(c)

(b)

(a)

20

(b)

(a)

30

40

50

60

70

80

2

20

30

40

50

60

70

80

2

Fig. 5. Ex situ PXD spectra of milled MgH2
NaAlH4 (a) and as-prepared (b), dehydrogenated (T ¼300 °C under vacuum) (c), and rehydrogenated (T ¼ 300 °C under 95 bar H2) (d) powder samples of nano MgH2
NaAlH4.

Fig. 6. Ex situ PXD spectra of milled 2MgH2
NaAlH4 (a) and as-prepared (b), dehydrogenated (T ¼ 300 °C under vacuum) (c), and rehydrogenated (T ¼300 °C under 95 bar H2) (d) powder samples of nano 2MgH2
NaAlH4.

works [32,40,41], significant improvement of de/rehydrogenation performance, for example, reduction of dehydrogenation temperature, faster kinetics, and alteration of reaction pathways from several steps to few steps, were achieved due to nanoconfinement, whereas these properties could not be obviously obtained from catalytic effects of carbon bulk. Based on DSC
MS and dehydrogenation kinetic results of nanoconfined samples in this work, where significant reduction of dehydrogenation temperature, and alteration of reaction pathway from multiple-step dehydrogenation to two steps, and faster kinetics were observed (Figs. 2–5), nanoconfinement of hydride composite in CAS can be confirmed. However, according to some crystalline phases (e.g., Al) found in nanoconfined samples, revealed as sharp PXD peaks in Figs. 5 and 6, it refers that some of reactive materials are expected also to be on the surface of CAS. Furthermore, the reaction mechanisms during hydrogen desorption and absorption of nanoconfined samples is investigated by ex situ powder X-ray diffraction. From Fig. 5(a), milled MgH2
NaAlH4 reveals clear and sharp signals of MgH2 and NaAlH4, suggesting no reaction between MgH2 and NaAlH4 during milling. For nano MgH2
NaAlH4, diffraction patterns of MgH2, Na3AlH6, and Al are mainly observed together with slight amount of NaAlH4, suggesting dehydrogenation of NaAlH4 (Eq. (1)) during nanoconfinement (Fig. 5(b)). This leads to the deficient content of hydrogen released during the 1st dehydrogenation of nano MgH2
NaAlH4 (Fig. 3). Besides, it should be mentioned that the broader characteristic peaks of some phases (e.g., MgH2) in nano MgH2
NaAlH4 as compared with those of milled sample hint at amorphous state due to nanoconfinement in CAS [42,43], in agreement with N2

adsorption
desorption results (Table 1). After dehydrogenation, Fig. 5(c) shows the characteristic peaks of NaMgH3, Al12Mg17, Al, and MgO. These confirm the dehydrogenation of Na3AlH6 to produce NaH and Al (Eq. (2)) and Al further reacts with MgH2 to release hydrogen and form Al12Mg17 (Eq. (4)). Moreover, dehydrogenation of MgH2 (Eq. (3)) and formation of NaMgH3 (Eq. (6)) are accomplished. For MgO, it could be due to the reaction of Mg (from dehydrogenation of MgH2) or MgH2 with oxygen during the experiments. Regarding the 1st dehydrogenation of milled MgH2
NaAlH4, where 55% of theoretical hydrogen capacity release (Fig. 3), only decompositions of NaAlH4 and Na3AlH6 (Eqs. (1) and (2)) are included. Interestingly, due to nanoconfinement into CAS further steps of dehydrogenation; that is, dehydrogenation of MgH2 and formations of Al12Mg17 and NaMgH3 (Eqs. (3), (4), and (6)), are obtained from MgH2
NaAlH4 hydride composite under the same temperature, pressure, and time condition. The latter is in agreement with the reduction of dehydrogenation temperature of MgH2
NaAlH4 system after nanoconfinement (simultaneous thermal analysis-MS results in Figs. 1 and 2). Therefore, superior hydrogen content desorbed from nano MgH2
NaAlH4 to milled sample is achieved (Fig. 3). Afterwards, rehydrogenated powder sample of nano MgH2
NaAlH4 exhibits the diffraction patterns of Na3AlH6, MgH2, Al, and MgO (Fig. 5(d)). The formation of MgH2 can be achieved from both hydrogenation of Mg (Mg þ H2 - MgH2) and probably reverse reaction of Eq. (6) (i.e., NaMgH3-NaHþMgH2). In the case of Na3AlH6, it is obtained from rehydrogenation of NaH and Al (3NaH þ Al þ 3/2H2 - Na3AlH6). Recoveries of MgH2 and Na3AlH6 confirm the reversibility of this system. Due to

P. Plerdsranoy et al. / Journal of Physics and Chemistry of Solids 87 (2015) 16–22

irreversibility of some Al after rehydrogenation, deficient amount of hydrogen released in the 2nd cycle is observed (Fig. 3). However, reproducibility of hydrogen in the 3rd and 4th cycles is comparable to that of the 2nd one, hinting at no more losses of Al
H-containing phases to metallic Al after the 2nd dehydrogenation. In addition, effective reversibility (in the 2nd cycle) of nano MgH2
NaAlH4 as compared with milled sample can be due to the fact that some of metallic Al from dehydrogenation of nano MgH2
NaAlH4 (at 300 °C under vacuum) reacts with MgH2 to produce Al12Mg17, whereas that of milled sample stays unreactive during dehydrogenation under the same temperature and pressure condition. For 2MgH2
NaAlH4 system, milled sample shows sharp and clear diffraction patterns of NaAlH4 and MgH2 (Fig. 6(a)) as similar as milled sample MgH2
NaAlH4, hinting at no reaction between hydride materials during milling. After nanoconfinement by melt infiltration, broad diffraction peaks of hydride materials (MgH2 and Na3AlH6) as well as the signals of Al and MgO are detected from nano 2MgH2
NaAlH4 (Fig. 6(b)), suggesting the decomposition of NaAlH4 to Na3AlH6 and Al (Eq. (1)) and amorphization of hydrides due to nanoconfinement as in case of nano MgH2
NaAlH4. Partial dehydrogenation during nanoconfinement results in inferior hydrogen content desorbed to theoretical hydrogen storage capacity during the 1st cycle (Fig. 4). Afterwards, dehydrogenated powder of nano 2MgH2
NaAlH4 exhibits the signals of NaMgH3, Al12Mg17, and Al, as well as slight amount of MgH2 (Fig. 6(c)). The phases formed during dehydrogenation refer to the decompositions of Na3AlH6 and MgH2 (Eqs. (2) and (3), respectively) together with the formations of Al12Mg17 and NaMgH3 (Eq. (4) and (6), respectively). Although these reaction mechanisms during dehydrogenation of both nanoconfined samples under different MgH2:NaAlH4 molar ratios (1:1 and 2:1) are comparable, it should be remarked that the relative content of Al12Mg17 with respect to Al after dehydrogenation of nano 2MgH2
NaAlH4 is significantly higher than that of nano MgH2
NaAlH4 (Figs. 5(c) and 6(c)). On the basis of Eq. (4), therefore, nano 2MgH2
NaAlH4 releases greater content of hydrogen than nano MgH2
NaAlH4, corresponding to dehydrogenation kinetics during the 1st cycles (Figs. 3 and 4). Furthermore, rehydrogenated powder of nano 2MgH2
NaAlH4 reveals the characteristic peaks of MgH2, Na3AlH6, Al, and MgO (Fig. 6(d)), hinting at its reversibility via the similar pathway as nano MgH2
NaAlH4. As previously identified that Al12Mg17 was responsible for thermodynamic improvement of MgH2
NaAlH4 composite (independently of MgH2:NaAlH4 molar ratio) [23,24,44], the higher the content of Al12Mg17 formed during dehydrogenation, the more the effectiveness of reversibility. The latter is in agreement with the fact that the relative contents of Na3AlH6 and MgH2 with respect to Al recovered after rehydrogenation of nano 2MgH2
NaAlH4 (Fig. 6(d)) are remarkably higher than those of nano MgH2
NaAlH4 (Fig. 5 (d)). Besides, nano 2MgH2
NaAlH4 shows superior contents of hydrogen reproduced in the 2nd, 3rd, and 4th cycles (up to 68%) to those of nano MgH2
NaAlH4 (56%) (Figs. 3 and 4). In conclusion, reduction of dehydrogenation temperatures and improvements of kinetics and reversibility of MgH2
NaAlH4 systems are obtained after nanoconfinement. However, although nanoconfinement of nano MgH2
NaAlH4 is more effective than that of nano 2MgH2
NaAlH4 due to inferior amount of MgH2 as discussed in N2 adsorption
desorption results (Table 1), its hydrogen contents desorbed and reproduced are deficient. Therefore, it seems that the enhancement of MgH2 content significantly influences the performance of MgH2
NaAlH4 hydride composite over effectiveness of nanoconfinement. 4. Conclusions MgH2
NaAlH4 hydride composites with different molar ratios of 1:1 and 2:1 (MgH2:NaAlH4) were nanoconfined into carbon

21

aerogel scaffold (CAS) under 1:1 (CAS:hydride composite) weight ratio by melt infiltration to obtain nano MgH2
NaAlH4 and nano 2MgH2
NaAlH4, respectively. Successful nanoconfinement of hydride composites was confirmed by N2 adsorption
desorption. It was found that the higher MgH2 content, the less the effectiveness of nanoconfinement. Multiple-step dehydrogenation of milled samples (three and four steps for milled MgH2
NaAlH4 and milled 2MgH2
NaAlH4, respectively) reduced to two-step reaction after nanoconfinement. In addition, peak temperatures corresponding to the first and the last step dehydrogenations of nanoconfined samples decreased significantly as compared with those of milled samples (ΔT ¼50 and 40 °C, respectively, for nano MgH2
NaAlH4 and ΔT ¼34 and 21 °C, respectively, for nano 2MgH2
NaAlH4). Regarding titration measurements, hydrogen content released during the 1st cycle of hydride composites enhanced up to 9% due to nanoconfinement. Interestingly, hydrogen reproduced in the 2nd, 3rd, and 4th cycles of nanoconfined samples were reserved in the range of 56
68%, while those of milled samples were only 16
38%. This was due to the fact that under the same temperature and pressure condition metallic Al obtained from dehydrogenation of nanoconfined samples favored to react with MgH2 and produced Al12Mg17, whereas that of milled samples seemed to stay unreacted. Considering only nanoconfined samples, although nanoconfinement of nano MgH2
NaAlH4 was more effective than nano 2MgH2
NaAlH4, the higher content of MgH2 in nano 2MgH2
NaAlH4 provided greater amount of Al12Mg17 after dehydrogenation, resulting in superior reversibility.

Acknowledgments The authors would like to acknowledge “Grants for the Promotion of Patent-Oriented Research and International Publications”, Suranaree University of Technology (SUT) (grant number of 1/2558) for financial supports.

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