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Article

The Improvement of Dehydriding the Kinetics of NaMgH3 Hydride via Doping with Carbon Nanomaterials

by
Zhong-Min Wang
*,
Song Tao
,
Jia-Jun Li
,
Jian-Qiu Deng
,
Huaiying Zhou
and
Qingrong Yao
School of Material Science and Engineering, Guilin University of Electronic Technology, Guilin 541004, China
*
Author to whom correspondence should be addressed.
Metals 2017, 7(1), 9; https://doi.org/10.3390/met7010009
Submission received: 21 October 2016 / Revised: 14 December 2016 / Accepted: 23 December 2016 / Published: 30 December 2016
(This article belongs to the Special Issue Metal Hydrides)
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Abstract

:
NaMgH3 perovskite hydride and NaMgH3–carbon nanomaterials (NH-CM) composites were prepared via the reactive ball-milling method. To investigate the catalytic effect of CM on the dehydriding kinetic properties of NaMgH3 hydride, multiwall carbon nanotubes (MWCNTs) and graphene oxide (GO) were used as catalytic additives. It was found that dehydriding temperatures and activation energies (ΔE1 and ΔE2) for two dehydrogenation steps of NaMgH3 hydride can be greatly reduced with a 5 wt. % CM addition. The NH–2.5M–2.5G composite presents better dehydriding kinetics, a lower dehydriding temperature, and a higher hydrogen-desorbed amount (3.64 wt. %, 638 K). ΔE1 and ΔE2 can be reduced by about 67 kJ/mol and 30 kJ/mol, respectively. The results suggest that the combination of MWCNTs and GO is a better catalyst as compared to MWCNTs or GO alone.

1. Introduction

Perovskite-type hydride, NaMgH3, as a potential candidate for on-board applications, has received considerable attention for its high gravimetric and volumetric hydrogen densities (6 wt. % and 88 kg/m3, respectively), as well as its reversible hydriding and dehydriding reactions [1,2,3,4,5,6,7]. NaMgH3 is characterized by an orthorhombic perovskite structure comprising [MgH6] octahedra and [NaH12] cubo-octahedra, which is analogous to GdFeO3-type perovskite (space group Pnma). This space group is typical for low-tolerance-factor oxide perovskites, where the singly charged Na cation occupies eight-fold coordinated voids [8]. Sheppard et al. [9] reported the desorption enthalpy and entropy are 86.6 ± 1.0 kJ/(mol·H2) and 132.2 ± 1.3 kJ/(mol·H2) for the decomposition of NaMgH3 into NaH and Mg, respectively, indicating that NaMgH3 is thermodynamically more stable than magnesium hydride (MgH2). One of the most promising approaches to improve its poor dehydrogenation property is incorporating with other metals or complex hydrides (e.g., Na1 − xLixMgH3 [10,11,12], NaMgH3-g-C3N4 [13], and NaMgH3-MgH2 [14,15]).
The addition of small amounts of catalytic material (transition metals, carbon materials, etc.) to MgH2 has successfully reduced the time taken to absorb or desorb hydrogen [16,17,18,19,20,21,22]. Recently, carbon nanomaterials have been investigated as catalytic materials for enhancing the uptake and release of hydrogen from MgH2 due to its lightweight nature. Alsabawi et al. [23] investigated the catalytic effect of up to 10 wt. % carbon buckyballs (C60) on the kinetics of hydrogen desorption and the subsequent absorption of MgH2. They found that a 1–2 wt. % C60 additive with 2 h of milling time are the optimum conditions for the best desorption kinetics. Imamura et al. reported the enhanced kinetics of a ball-milled Mg–graphite composite with organic additives (tetrahydrofuran, cyclohexane, or benzene), and Raman characterization indicated that the organic additives allowed the graphite to shear along planes rather than grind into small particles, as it did without organic additives [24,25]. Thiangviriya et al. showed that the improvement of dehydrogenation kinetics of the 2LiBH4–MgH2 composite by doping with activated carbon nanofibers is due to the increase in the hydrogen diffusion pathway and thermal conductivity [26]. Kadri et al. demonstrated that the presence of both a V-based catalyst and carbon nanotubes reduces the enthalpy and entropy of MgH2, and partially destroyed CNTs are better at enhancing the hydrogen sorption performance [27]. Other forms of carbon, such as amorphous carbon [28,29], carbon black [28,30], activated carbons [31,32], and carbon nanotubes [28,33,34,35,36,37,38,39], have also been studied for their catalytic effect on the magnesium hydrogen system. NaMgH3 can be synthesized via reactive mechanochemical means. Indeed, mechanochemical approaches provide not only less energy intensive routes to the hydrides, but also ensure that particles sizes are minimized, improving the dehydrogenation kinetics of the ternary hydrides compared to those prepared at high temperature [10,40].
In this work, multiwall carbon nanotubes (MWCNTs) and graphite (G) were used as catalytic additives. The reactive ball-milling method was employed to prepare NaMgH3 perovskite hydride and NaMgH3-carbon nanomaterials (NH-CM) composites. The effects of different additives on the structure, thermal stability, and dehydriding kinetic properties of NaMgH3 hydride were investigated.

2. Materials and Methods

All sample handlings were undertaken in an argon atmosphere glovebox (Mbraun, Labstar, 1 ppm H2O, 1 ppm O2). NaMgH3 hydride was prepared by reactive milling of stoichiometric mixtures of MgH2 (>98% purity, Alfa-Aesar, Ward Hill, MA, USA) and NaH (99.9% purity, Sigma-Aldrich, Billerica, MA, USA). The mixed powders were milled with stainless steel balls, with a ball-to-powder weight ratio of 80:1, under a H2 atmosphere (0.8 MPa) for 45 h at 320 rpm using a QM-3SP2 planetary mill at ambient temperature. Multiwall carbon nanotubes (MWCNTs, tube length: 0.5–2 μm, tube diameter: 30–50 nm, >95% purity, Chengdu Organic Chemistry Co., Ltd., Chengdu, China) and graphene oxide (GO, <10 sheets, >95% purity, Chengdu Organic Chemistry Co., Ltd., Chengdu, China) were introduced to NaMgH3 hydride as catalysts. NH-CM composites were prepared by reactive milling the mixture of MgH2, NaH, and catalytic additives under the same milling conditions. The composites of NaMgH3 with different loading of carbon nanomaterials were labeled as follows: NH–5M (NaMgH3 + 5 wt. % MWCNT composite), NH–5G (NaMgH3 + 5 wt. % GO composite), and NH–2.5M–2.5G (NaMgH3 + 2.5 wt. % MWCNTs + 2.5 wt. % G composite).
X-ray diffraction (XRD) samples were prepared in a glove box. To avoid exposure to air during the measurement, the sample was spread uniformly on the sample holder and covered with Scotch tape. XRD analysis was performed on Empyrean PIXcel 3D (PANalytical B.V., Almelo, The Netherlands) (Cu Kα radiation) with a scanning speed of 5°/min. The mean crystallite size was determined by the Scherer formula (D = kλ/(β_(hkl)cosθ)), where D is the crystallite size, k is the shape factor (0.89), λ is the wavelength of the Cu Kα radiation (0.154056 nm), β_hkl is the FWHM (full width of peak at half maximum), and θ is the diffraction angle. The evaluations of the hydrogen-desorbed amount and the dehydriding kinetic properties of the samples were carried out in an automatic Sievert-type apparatus (PCTpro2000, Setaram Co., Caluire, France). Thermal properties of the NaMgH3 and NH-CM composites were investigated by differential scanning calorimetry (DSC, NETZSCH STA 449F3, Selb, Germany) at different ramping rates (5, 10, and 15 K/min) under a continuous argon flow (20 mL/min) from 298 K to 725 K. The sample was loaded into an alumina crucible in the glove box. The crucible was then placed in a sealed glass bottle in order to prevent oxidation during transportation from the glove box to the TGA/DSC apparatus. An empty alumina crucible was used as a reference.

3. Results and Discussion

3.1. Structure Characterization of As-Prepared NaMgH3 Hydride and NH-CM Composites

Figure 1 shows the XRD patterns of as-prepared NaMgH3 hydride and NH-CM composites (NH–5M, NH–5G, and NH–2.5M–2.5G). The red long string is the standard line of NaMgH3 phase. As can been seen, typical peaks of NaMgH3 phase can be observed in all samples, which indicate an orthorhombic perovskite structure, similar to those of the GdFeO3 type perovskite (space group Pnma) [41,42]. The sharp peaks are more prominent for NaMgH3 hydride without a catalyst addition. A slight peak shift to a lower angle also is observed in these NH-CM composites. The calculated crystallite size are 15.8 nm for the as-synthesized NaMgH3 sample, 14.9 nm for the NH–5M sample, 14.0 nm for the NH–5G sample, and 13.4 nm for the NH–2.5M–2.5G sample, respectively. The results indicate that the addition of 5 wt. % catalyst will reduce the crystallite size, which may improve dehydriding kinetics. The peaks of the catalysts are absent, probably because only a small amount of CM were added to the composites, and these additives were appeared in the form of an amorphous phase after ball-milling [11]. The existence of MgO can be attributed to the slight oxidization of samples in the handling process [43].

3.2. Thermal Stabilities of NaMgH3 Hydride and NH-CM Composites

Figure 2 presents the DSC curves of NaMgH3 hydride and three NH-CM composites at different heating rates (5, 10, and 20 K/min) under a continuous argon flow from 298 K to 750 K, where Tx and Tp represent the start and peak temperature of dehydrogenation, respectively. From the DSC curves, there are two endothermic peaks for all four samples, which are related to two decomposition steps of NaMgH3 hydride. The decomposition steps can be expressed as follows [2]:
NaMgH3 → NaH + Mg + H2↑
NaH → Na + ½H2↑
In comparison with NaMgH3 hydride without a catalyst addition, an obvious decrease of dehydriding temperature is observed in all composites. At a heating rate of 5 K/min, NH–2.5M–2.5G has the largest dehydriding temperatures reduction with temperature difference values of ΔTx1, ΔTp1, ΔTx2, and ΔTp2 are 52.2 K, 51.1 K, 44.1 K, and 20.4 K, respectively. The next sample with the biggest reduction temperature differences was NH–5G, followed by NH–5M.
In order to estimate the dehydriding activation energy (ΔE) of NaMgH3 hydride, the Kissinger plot is used for non-isothermal DSC analysis and can be expressed in the form as follows [44]:
ln (T2/υ) = ΔE/RT + ln (ΔE/Ru0)
where T is the peak temperature, R is the gas constant (8.3145 J/(K·mol)), and υ and u0 are the heating rate and frequency factor, respectively.
In Figure 3, both datasets at Tx and Tp show a good linear relation between ln (T2/υ) and (1000/T) with a slope of ΔE/R. The calculated ΔE values are listed in Table 1. An obvious decrease of ΔE for both dehydrogenation steps of NaMgH3 hydride is observed for those samples milled with the CM additive. The calculated ΔE1 (the first decomposition step) and ΔE2 (the second decomposition step) are 113.8 kJ/mol and 126.6 kJ/mol for the NH–2.5M–2.5G sample, 139.8 kJ/mol and 147.6 kJ/mol for the NH–5G sample, and 146.4 kJ/mol and 153.9 kJ/mol for the NH–5M sample, respectively. In comparison with NaMgH3 hydride without a CM addition (ΔE1 = 180.3 kJ/mol, ΔE2 = 156.2 kJ/mol), the NH–2.5M–2.5G sample has the highest reduction of activation energy, ΔE, where the deviation values of ΔE1 and ΔE2 are about 67 kJ/mol and 30 kJ/mol, respectively. The results indicate that the activation energy (ΔE) for the dehydrogenation steps of the NaMgH3 hydride can be greatly decreased by milling with a 5 wt. % CM addition, especially in the case of the NH–2.5M–2.5G composite. These observations correspond well with the DSC results, such that the dehydriding temperatures are lowered by the activation energy.

3.3. Dehydriding Kinetic Properties of NaMgH3 Hydride and NH-CM Composites

The isothermal dehydriding properties of the four samples at different temperatures (593 K, 613 K, and 638 K) are shown in Figure 4, Figure 5 and Figure 6, respectively. In comparison with NaMgH3 hydride, all NH-CM composites present better dehydriding kinetic properties. The hydrogen-desorbed amount increases with the increase in temperature. Among these three NH-CM composites, the NH–2.5M–2.5G composite has the best catalytic effect in improving the dehydriding kinetic properties of the NaMgH3 hydride, where 90% of the maximum theoretical capacity (3.64 wt. % hydrogen) is released within 20 min at 638 K. Table 2 shows the maximum amount of hydrogen desorbed from the NaMgH3 hydride and the NH-CM composites at different temperatures. These results agree well with that observed in Figure 2 and Figure 3, indicating that dehydriding kinetics and dehydriding temperatures can be effectively reduced by a combined catalytic addition of MWCNTs and GO.
To illustrate the decomposition mechanism of the NH-CM hydride composites, the XRD patterns of the NaMgH3 + 2.5 G + 2.5 M sample after dehydriding at different temperatures are shown in Figure 7. With the increase in temperature from 593 K to 638 K, peaks of the NaMgH3 phase become weakened, and peaks of the NaH phase and Mg phase become strengthened, such results agree well with our previous work for pristine NaMgH3 hydride reported in [12]. In another word, the decomposition of NaMgH3 is a two-step reaction: NaMgH3 → NaH + Mg + H2 → Na + Mg + 3/2H2. The dopping with NM cannot change the decomposition process of NaMgH3, but contribute to its dehydriding kinetics.
One of the possible reasons for the enhancement of dehydriding kinetics of NaMgH3 hydride is because of the ball milling process with CM additives, which creates more defects, a refined grain size, and a distorted crystal structure. Such structural features and observed improvements have also been reported in the case of the reactive ball milling of magnesium hydride with carbon additives in hydrogen gas [10,11,41,42,43]. A synergetic effect may exist in the NH–2.5M–2.5G composite, where the presence of MWCNTs may hinder the restacking of GO; hence, improving the dehydriding kinetics. Bhatnagar et al. reported this synergetic effect in MgH2–NaAlH4 composite with the addition of 1.5 wt. % of graphene nanosheets and 0.5 wt. % of single wall carbon nanotube [45]. Further work is still needed to illustrate its possible mechanism.

4. Conclusions

NaMgH3 perovskite hydride and NH-CM composites were prepared via the reactive ball-milling method under a H2 atmosphere. MWCNTs and GO were used as a catalyst to improve the dehydriding kinetic properties of NaMgH3 hydride. Dehydriding temperature and activation energy (ΔE) for two dehydrogenation steps of NaMgH3 hydride can be greatly reduced with a 5 wt. % CM addition, especially the composite with a combined addition of 2.5 wt. % MWCNTs + 2.5 wt. % GO (NH–2.5M–2.5G). In comparison with NaMgH3 hydride, the ΔE1 and ΔE2 of the NH–2.5M–2.5G composite are reduced by about 67 kJ/mol and 30 kJ/mol, respectively. The maximum amount of hydrogen desorbed is 3.64 wt. % at 638 K, and about 90% of the maximum amount was released within 20 min. This can be attributed to the synergetic effect between MWCNTs and GO, indicating that the combination of MWCNTs and GO is a better catalyst as compared to MWCNTs or GO alone.

Acknowledgments

This work was financially supported by the National Natural Science Foundation of China (51261003, 51471055) and the Guangxi Natural Science Foundation (2016GXNSFGA380001).

Author Contributions

Wang and Tao conceived and designed the study. Tao and Li performed the experiments. Deng provided the DSC analyses. Yao contributed significantly to XRD refinement and analysis; Wang wrote the manuscript. Zhou helped perform the analysis with constructive discussions. All authors read and approved the manuscript.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. The XRD patterns of as-prepared NaMgH3 hydride and NH-CM composites.
Figure 1. The XRD patterns of as-prepared NaMgH3 hydride and NH-CM composites.
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Figure 2. DSC curves of NaMgH3 hydride and NH-CM composites at different heating rates. (a) NH–2.5M–2.5G; (b) NH–5G; (c) NH–5M; (d) NaMgH3 hydride.
Figure 2. DSC curves of NaMgH3 hydride and NH-CM composites at different heating rates. (a) NH–2.5M–2.5G; (b) NH–5G; (c) NH–5M; (d) NaMgH3 hydride.
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Figure 3. The relationship curves between ln(Tp2/υ) and (1000/Tp) for NaMgH3 hydride and composites fitted by Kissinger method. (A) NH–2.5M–2.5G; (B) NH–5G; (C) NH–5M; (D) NaMgH3 hydride (a) for the first decomposition step of NaMgH3 hydride and (b) for the second decomposition step of NaMgH3 hydride.
Figure 3. The relationship curves between ln(Tp2/υ) and (1000/Tp) for NaMgH3 hydride and composites fitted by Kissinger method. (A) NH–2.5M–2.5G; (B) NH–5G; (C) NH–5M; (D) NaMgH3 hydride (a) for the first decomposition step of NaMgH3 hydride and (b) for the second decomposition step of NaMgH3 hydride.
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Figure 4. Dehydriding kinetic curves of the NaMgH3 hydride and the NH-CM composites at 593 K.
Figure 4. Dehydriding kinetic curves of the NaMgH3 hydride and the NH-CM composites at 593 K.
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Figure 5. Dehydriding kinetic curves of the NaMgH3 hydride and the NH-CM composites at 613 K.
Figure 5. Dehydriding kinetic curves of the NaMgH3 hydride and the NH-CM composites at 613 K.
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Figure 6. Dehydriding kinetic curves of the NaMgH3 hydride and the NH-CM composites at 638 K.
Figure 6. Dehydriding kinetic curves of the NaMgH3 hydride and the NH-CM composites at 638 K.
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Figure 7. XRD patterns of NaMgH3 + 2.5 G + 2.5 M hydride composite after dehydriding at different temperatures (593 K, 613 K, and 638 K).
Figure 7. XRD patterns of NaMgH3 + 2.5 G + 2.5 M hydride composite after dehydriding at different temperatures (593 K, 613 K, and 638 K).
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Table
Table 1. Calculated value of ΔE of the decomposition of the NaMgH3 hydride.
Table 1. Calculated value of ΔE of the decomposition of the NaMgH3 hydride.
Sample1st Step ΔE1 (kJ/mol)2nd Step ΔE2 (kJ/mol)
NH–2.5M–2.5G113.8126.6
NH–5G139.8147.6
NH–5M146.4153.9
NaMgH3 hydride180.3156.2
Table
Table 2. The maximum amount of hydrogen desorbed from NaMgH3 hydride and NH-CM composites at different temperatures.
Table 2. The maximum amount of hydrogen desorbed from NaMgH3 hydride and NH-CM composites at different temperatures.
Sample/Temperature593 K (wt. %)613 K (wt. %)638 K (wt. %)
NaMgH31.132.033.42
NH–5M1.362.463.40
NH–5G1.212.493.64
NH–2.5G–2.5M1.462.563.64

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Wang, Z.-M.; Tao, S.; Li, J.-J.; Deng, J.-Q.; Zhou, H.; Yao, Q. The Improvement of Dehydriding the Kinetics of NaMgH3 Hydride via Doping with Carbon Nanomaterials. Metals 2017, 7, 9. https://doi.org/10.3390/met7010009

AMA Style

Wang Z-M, Tao S, Li J-J, Deng J-Q, Zhou H, Yao Q. The Improvement of Dehydriding the Kinetics of NaMgH3 Hydride via Doping with Carbon Nanomaterials. Metals. 2017; 7(1):9. https://doi.org/10.3390/met7010009

Chicago/Turabian Style

Wang, Zhong-Min, Song Tao, Jia-Jun Li, Jian-Qiu Deng, Huaiying Zhou, and Qingrong Yao. 2017. "The Improvement of Dehydriding the Kinetics of NaMgH3 Hydride via Doping with Carbon Nanomaterials" Metals 7, no. 1: 9. https://doi.org/10.3390/met7010009

APA Style

Wang, Z. -M., Tao, S., Li, J. -J., Deng, J. -Q., Zhou, H., & Yao, Q. (2017). The Improvement of Dehydriding the Kinetics of NaMgH3 Hydride via Doping with Carbon Nanomaterials. Metals, 7(1), 9. https://doi.org/10.3390/met7010009

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