The Improvement of Dehydriding the Kinetics of NaMgH 3 Hydride via Doping with Carbon Nanomaterials

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 (∆E1 and ∆E2) 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). ∆E1 and ∆E2 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.

The addition of small amounts of catalytic material (transition metals, carbon materials, etc.) to MgH 2 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 MgH 2 due to its lightweight nature.Alsabawi et al. [23] investigated the catalytic effect of up to 10 wt.% carbon buckyballs (C 60 ) on the kinetics of hydrogen desorption and the subsequent absorption of MgH 2 .They found that a 1-2 wt.% C 60 additive with 2 h of milling time are the optimum conditions for the best desorption kinetics.Imamura et al. reported the Metals 2017, 7, 9; doi:10.3390/met7010009www.mdpi.com/journal/metalsenhanced 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 2LiBH 4 -MgH 2 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 MgH 2 , 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.NaMgH 3 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 NaMgH 3 perovskite hydride and NaMgH 3 -carbon nanomaterials (NH-CM) composites.The effects of different additives on the structure, thermal stability, and dehydriding kinetic properties of NaMgH 3 hydride were investigated.
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 NaMgH 3 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.

Structure Characterization of As-Prepared NaMgH 3 Hydride and NH-CM Composites
Figure 1 shows the XRD patterns of as-prepared NaMgH 3 hydride and NH-CM composites (NH-5M, NH-5G, and NH-2.5M-2.5G).The red long string is the standard line of NaMgH 3 phase.As can been seen, typical peaks of NaMgH 3 phase can be observed in all samples, which indicate an orthorhombic perovskite structure, similar to those of the GdFeO 3 type perovskite (space group Pnma) [41,42].The sharp peaks are more prominent for NaMgH 3 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 NaMgH 3 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-2.5Gsample, 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].

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-2.5Gsample, 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].

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]: 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-2.5Ghas the largest dehydriding temperatures reduction with temperature difference values of ΔTx1, ΔTp1, ΔTx2, and ΔTp2 are 52.2K, 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]:

Thermal Stabilities of NaMgH 3 Hydride and NH-CM Composites
Figure 2 presents the DSC curves of NaMgH 3 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 T x and T p 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 NaMgH 3 hydride.The decomposition steps can be expressed as follows [2]: In comparison with NaMgH 3 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-2.5Ghas the largest dehydriding temperatures reduction with temperature difference values of ∆T x1 , ∆T p1 , ∆T x2 , and ∆T p2 are 52.2K, 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 (∆E) of NaMgH 3 hydride, the Kissinger plot is used for non-isothermal DSC analysis and can be expressed in the form as follows [44]: where T is the peak temperature, R is the gas constant (8.3145J/(K•mol)), and υ and u 0 are the heating rate and frequency factor, respectively.
Metals 2017, 7, 9 4 of 10 where T is the peak temperature, R is the gas constant (8.3145J/(K•mol)), and υ and u0 are the heating rate and frequency factor, respectively.In Figure 3, both datasets at T x and T p show a good linear relation between ln (T 2 /υ) and (1000/T) with a slope of ∆E/R.The calculated ∆E values are listed in Table 1.An obvious decrease of ∆E for both dehydrogenation steps of NaMgH 3 hydride is observed for those samples milled with the CM additive.The calculated ∆E 1 (the first decomposition step) and ∆E 2 (the second decomposition step) are 113.8kJ/mol and 126.6 kJ/mol for the NH-2.5M-2.5Gsample, 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 NaMgH 3 hydride without a CM addition (∆E 1 = 180.3kJ/mol, ∆E 2 = 156.2kJ/mol), the NH-2.5M-2.5Gsample has the highest reduction of activation energy, ∆E, where the deviation values of ∆E 1 and ∆E 2 are about 67 kJ/mol and 30 kJ/mol, respectively.The results indicate that the activation energy (∆E) for the dehydrogenation steps of the NaMgH 3 hydride can be greatly decreased by milling with a 5 wt.% CM addition, especially in the case of the NH-2.5M-2.5Gcomposite.These observations correspond well with the DSC results, such that the dehydriding temperatures are lowered by the activation energy.

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 Figures 4-6, 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-2.5Gcomposite 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 Figures 2 and 3, indicating that dehydriding kinetics and dehydriding temperatures can be effectively reduced by a combined catalytic addition of MWCNTs and GO.

Dehydriding Kinetic Properties of NaMgH 3 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 Figures 4-6, respectively.In comparison with NaMgH 3 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-2.5Gcomposite has the best catalytic effect in improving the dehydriding kinetic properties of the NaMgH 3 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 NaMgH 3 hydride and the NH-CM composites at different temperatures.These results agree well with that observed in Figures 2 and 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-2.5Gcomposite, 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 To illustrate the decomposition mechanism of the NH-CM hydride composites, the XRD patterns of the NaMgH 3 + 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 NaMgH 3 phase become weakened, and peaks of the NaH phase and Mg phase become strengthened, such results agree well with our previous work for pristine NaMgH 3 hydride reported in [12].In another word, the decomposition of NaMgH 3 is a two-step reaction: NaMgH The dopping with NM cannot change the decomposition process of NaMgH 3 , but contribute to its dehydriding kinetics.One of the possible reasons for the enhancement of dehydriding kinetics of NaMgH 3 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-2.5Gcomposite, where the presence of MWCNTs may hinder the restacking of GO; hence, improving the dehydriding kinetics.Bhatnagar et al. reported this synergetic effect in MgH 2 -NaAlH 4 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.

Conclusions
NaMgH 3 perovskite hydride and NH-CM composites were prepared via the reactive ball-milling method under a H 2 atmosphere.MWCNTs and GO were used as a catalyst to improve the dehydriding kinetic properties of NaMgH 3 hydride.Dehydriding temperature and activation energy (∆E) for two dehydrogenation steps of NaMgH 3 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 NaMgH 3 hydride, the ∆E 1 and ∆E 2 of the NH-2.5M-2.5Gcomposite 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.

Figure 1 .
Figure 1.The XRD patterns of as-prepared NaMgH3 hydride and NH-CM composites.

Figure 1 .
Figure 1.The XRD patterns of as-prepared NaMgH 3 hydride and NH-CM composites.

Figure 2 .
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.In Figure3, both datasets at Tx and Tp show a good linear relation between ln (T 2 /υ) and (1000/T) with a slope of ΔΕ/R.The calculated ΔE values are listed in Table1.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 113.8kJ/mol and 126.6 kJ/mol for the NH-2.5M-2.5Gsample, 139.8 kJ/mol and 147.6 kJ/mol for

Figure 4 .
Figure 4. Dehydriding kinetic curves of the NaMgH3 hydride and the NH-CM composites at 593 K.

Figure 5 .
Figure 5. Dehydriding kinetic curves of the NaMgH3 hydride and the NH-CM composites at 613 K.

Figure 4 .
Figure 4. Dehydriding kinetic curves of the NaMgH 3 hydride and the NH-CM composites at 593 K.

Figure 4 .
Figure 4. Dehydriding kinetic curves of the NaMgH3 hydride and the NH-CM composites at 593 K.

Figure 5 .
Figure 5. Dehydriding kinetic curves of the NaMgH3 hydride and the NH-CM composites at 613 K.Figure 5. Dehydriding kinetic curves of the NaMgH 3 hydride and the NH-CM composites at 613 K.

Figure 5 .Figure 6 .
Figure 5. Dehydriding kinetic curves of the NaMgH3 hydride and the NH-CM composites at 613 K.Figure 5. Dehydriding kinetic curves of the NaMgH 3 hydride and the NH-CM composites at 613 K.

Figure 6 .
Figure 6.Dehydriding kinetic curves of the NaMgH 3 hydride and the NH-CM composites at 638 K.

Table 1 .
Calculated value of ΔE of the decomposition of the NaMgH3 hydride.

Table 1 .
Calculated value of ∆E of the decomposition of the NaMgH 3 hydride.

Table 2 .
The maximum amount of hydrogen desorbed from NaMgH 3 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.

Table 2 .
The maximum amount of hydrogen desorbed from NaMgH3 hydride and NH-CM composites at different temperatures.