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graphene
Journal of Power Sources 371 (2017) 112–118
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High hydrogen desorption properties of Mg-based nanocomposite at moderate temperatures: The effects of multiple catalysts in situ formed by adding nickel sulfides/graphene
T
Xiubo Xie, Ming Chen, Peng Liu, Jiaxiang Shang, Tong Liu∗ Key Laboratory of Aerospace Materials and Performance (Ministry of Education), School of Materials Science and Engineering, Beihang University, No.37 Xueyuan Road, Beijing, 100191, China
H I G H L I G H T S
G RA P H I C A L AB S T R A C T
Sulfides/Graphene was pro• Nickel and graduced by co-reducing Ni 2+
• • • •
phene oxide. Mg-5wt%NiS/rGO nanocomposite is prepared by ball milling Mg and NiS/ rGO. It shows best desorption properties compared with samples catalyzed by NiS and rGO. Nickel sulfides in situ change to MgS, Mg2Ni and Ni multiple catalysts. MgS, Mg2Ni, Ni and rGO synergistically catalyze the hydrogen desorption of Mg.
A R T I C L E I N F O
A B S T R A C T
Keywords: Magnesium Hydrogen storage Multiple-phase catalysts Catalytic effects Nanocomposite
Nickel sulfides decorated reduced graphene oxide (rGO) has been produced by co-reducing Ni2+ and graphene oxide (GO), and is subsequently ball milled with Mg nanoparticles (NPs) produced by hydrogen plasma metal reaction (HPMR). The nickel sulfides of about 800 nm completely in situ change to MgS, Mg2Ni and Ni multiple catalysts after first hydrogenation/dehydrogenation process at 673 K. The Mg-5wt%NiS/rGO nanocomposite shows the highest hydrogen desorption kinetics and capacity properties, and the catalytic effect order of the additives is NiS/rGO, NiS and rGO. At 573 K, the Mg-NiS/rGO nanocomposite can quickly desorb 3.7 wt% H2 in 10 min and 4.5 wt% H2 in 60 min. The apparent hydrogen absorption and desorption activation energies of the Mg-5wt%NiS/rGO nanocomposite are decreased to 44.47 and 63.02 kJ mol−1, smaller than those of the Mg-5wt %rGO and Mg-5wt%NiS samples. The best hydrogen desorption properties of the Mg-5wt%NiS/rGO nanocomposite can be explained by the synergistic catalytic effects of the highly dispersed MgS, Mg2Ni and Ni catalysts on the rGO sheets, and the more nucleation sites between the catalysts, rGO sheets and Mg matrix.
1. Introduction Magnesium (Mg) as one of the most attractive solid-state hydrogen storage materials used in applications of on-board hydrogen storage, heat storage of portable and stationary energy storage has been widely ∗
studied over the past decades for its high theoretical weight storage capacity (7.6 wt%) and relatively better cycling properties [1–7]. Nowadays, the main research has focused on improving the hydrogen absorption and desorption kinetics and decreasing the thermodynamic stability of Mg-H bond that leads to the operation temperature above
Corresponding author. E-mail address: [email protected] (T. Liu).
http://dx.doi.org/10.1016/j.jpowsour.2017.10.054 Received 12 September 2017; Received in revised form 15 October 2017; Accepted 16 October 2017 Available online 21 October 2017 0378-7753/ © 2017 Elsevier B.V. All rights reserved.
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milled with Mg NPs prepared by hydrogen plasma-metal reaction (HPMR) approach. The nickel sulfides completely changed to MgS, Mg2Ni and Ni catalysts during the first hydrogenation and dehydrogenation process. The Mg-NiS/rGO nanocomposite showed enhanced sorption properties with best hydrogen release kinetics and highest desorption capacity compared with Mg-NiS, Mg-rGO nanocomposite and pure ball milled Mg NPs. The catalytic mechanism of the Mg-NiS/rGO nanocomposite was further studied in detail in this work.
673 K [8,9]. Various attempts have been tried to address the limitations, such as reducing the particle size of Mg to nanoscale to reduce the distance of the hydrogen diffusion [10–12], alloying with metal elements [13,14] or constructing Mg-X substitutional solid solution [15,16] to change the desorption enthalpy of Mg-based material, and the most common way, introducing different catalysts to catalyze the hydrogen dissociation and Mg/MgH2 nucleation during hydrogenation/ dehydrogenation cycle process [17,18], have been used to enhance the hydrogen storage properties. Mechanical milling is a traditional way to disperse different catalysts of 3d-transition metals [3,18–21] and their oxides [22,23], fluorides [24,25], and sulfides [26,27] into Mg/MgH2 system. Liang et al. studied the MgH2 system ball milled with different metals and found that the Mg-Ni powder showed best absorption and desorption kinetics [19]. Xie et al. confirmed that the Mg-10 wt%Ni composite can release 6.1 wt% H2 in 10 min at 523 K under an initial pressure of 0.01 bar [20]. Chen and coworkers ball milled Mg with 4 at% Ni nanofibers and found that the composite can quickly release 4.5 wt% H2 within 60 min at 523 K [21]. Therefore, the additive of Ni based catalysts into Mg/MgH2 system can effectively enhance its hydrogen storage properties. However, some of the catalysts NPs tend to be agglomerate and grow to large particles during ball milling process, leading to their decreased catalytic activity [28]. RGO is reported to be an effective catalyst and it can be served as a media to disperse catalysts uniformly in the Mg/MgH2 system [28,29]. Bhatnagar et al. reported that the graphene sheets can prevent the Fe3O4 particles from agglomerating and the MgH2:Fe3O4@GS composite can absorb 6.2 wt% hydrogen in 2.5 min and the onset desorption temperature was 535 K, better than that of the MgH2:Fe3O4 of 571 K [29]. Wang and coworkers found that the MgH2-5wt%(Ni-CeOx)/GNS composite can release 6.5 wt % H2 within 10 min at 573 K at an initial hydrogen pressure of 5 KPa, superior to the desorbed H2 amount of 0.16 wt% for MgH2 sample under the same condition [30]. Furthermore, very recent studies showed that in situ formed multiple catalysts especially the transition metal sulfides can significantly improve the hydrogen storage properties of Mg [27,31–34]. Unfortunately, the desorption temperature of these Mg-based materials was still high, and the desorption kinetics needed further improvement. Zhang et al. found that the additive of Fe3S4 can in situ change to MgS and Fe multiple catalytic phases in the composite when ball milled with MgH2. Compared with the Mg-MgS and Mg-Fe composites with one catalyst, the Mg-Fe3S4 composite with multiple catalysts showed better hydrogen absorption rates. However, the Mg-Fe3S4 composite showed still low desorption kinetics, which can only release 4.0 wt% H2 within 20 min at 623 K [27]. Jia et al. studied the Mg-MoS2 system and found that the in situ formed MgS catalyst can decrease the onset desorption temperature of the Mg/MgH2 system, however, its hydrogen release content was only 0.2 wt% H2 at 573 K [28]. In our previous work [35], we studied the Mg-5wt%NiS nanocomposite and found that the NiS in situ decomposed into Ni, MgS and Mg2Ni multiple-phases after first hydrogenation/dehydrogenation cycle process. The desorption activation energy of the nanocomposite decreased to 64.71 kJ mol−1, and it can quickly uptake 3.5 wt % H2 within 10 min at 423 K. But its desorption temperature was still high and the hydrogen release kinetics needed further improvement. Finding an efficient catalyst to enhance the desorption kinetics of the Mg-based hydrogen material still remains a challenge. Nickel sulfides can react with Mg during the hydrogen absorption and desorption process and in situ form multiple catalysts. Moreover, the uniformly dispersed nickel sulfides on the rGO sheets could provide more nucleation sites for Mg/MgH2 and prevent the catalysts from aggregation, leading to the enhanced desorption properties of the Mg material. In this work, we synthesized NiS/rGO composite by solvothermal reaction method, the Ni2+ was reduced to nickel sulfides in situ decorating on rGO sheets. The nickel sulfides particles of about 800 nm were in spherical shape. The composite was subsequently ball
2. Experimental section 2.1. Preparation of NiS/rGO composite The CH4N2S (Aladdin, > 99.9%) and Na3C6H5O7·3H2O (Aladdin, > 99.9%) were used as received. The GO was fabricated in our laboratory by modified Hummers' method [36]. The NiS/rGO composite was produced by solvothermal reaction method, which was described previously [35]. The Na3C6H5O7·3H2O and CH4N2S were successively dissolved in 30 mL of distilled water containing 1 mmol NiCl2·6H2O by electromagnetically stirring. Distilled water of 10 mL containing 10 mg GO under ultrasonic treatment for 1 h was added to the above-mentioned solution, and the pH value was tuned to 12 by adding ammonia (28 wt%) dropwise. The black solution was shifted into a 50 mL sealed Teflon-lined autoclave and maintained at 453 K for 24 h. The as-made product was dried by vacuum pumping at 323 K for two days after washed with distilled water and absolute alcohol several times. Based on the content of the reactant of NiCl2·6H2O, the yield of the NiS was calculated to be 90.7 mg. Therefore, the ratio of NiS to rGO in the as-synthesized NiS/rGO composite is 90.1:9.9. The pure rGO sheets were also prepared by the same process mentioned above. 2.2. Preparation of Mg-NiS/rGO nanocomposite The dried NiS/rGO powder was mechanically milled under H2 atmosphere (99.999%) with the Mg NPs (weight ratio, 5:95) prepared by HPMR approach [37], the ball to powder ratio was 20:1, the rotation speed and ball milling time were 220 rpm and 2 h. Brittle MgH2 NPs at a weight ratio of 9:1 (Mg/MgH2) were added to avoid agglomeration of the Mg NPs. The preparation and collection of the ball milled materials were performed in a glove-box filled with purified argon (99.999%) and a drying agent to prevent the milled material from undergoing hydroxide and/or oxidation formation, we refer the sample as Mg-NiS/rGO, hereafter. The pure Mg and Mg-rGO composites were also made by the same method for comparison. 2.3. Characterization The hydrogen absorption and desorption kinetics properties of the composites were measured by Sieverts-type apparatus with reactor chamber volume of 60 ml. The experimental dosage of the samples is 100 mg and they should be activated before mesurement by one hydrogen absorption and desorption cycle under hydrogen pressures of 4 MPa and 100 Pa at 673 K. Then the Sieverts-type apparatus was maintained at 673 K and pumped to 10−3 Pa for more than 60 min to ensure that the complete hydrogen release in the material, the composites were heated up to the tested temperatures to measure the corresponding hydrogenation (initial pressure, 4 MPa) and dehydrogenation (initial pressure, 100 Pa) kinetic curves. The thermodynamic property of the Mg-NiS/rGO nanocomposite was investigated by conventional pressure-volume-temperature equipment. For the pressurecomposition (P-C) isotherm curves at a certain temperature, we suggest that the measurement reaches equilibrium if the change of hydrogen pressure is no more than 20 Pa/s. The phase changes of the Mg-NiS/rGO composite during the hydrogenation/dehydrogenation process were measured by an X-ray diffraction (XRD) measurement with monochromatic Cu Kα radiation 113
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Fig. 1. XRD patterns of the rGO (a), and the nickel sulfides decorated rGO (b).
(Rigaku X-ray diffractometer). Field emission scanning electronic microscopes (SEM, JEM7500, 15 kV) and transmission electron microscopy (TEM, JEOL-JSM-2100) were applied to analyze the morphology and microstructures of the samples. The energy dispersive spectroscopy (EDS) results were obtained to study the elemental distributions. The Raman spectrum was measured to observe the characteristic peaks of rGO by using a Lab RAM HR800 Raman spectrometer.
Fig. 3. XRD patterns of the Mg-NiS/rGO nanocomposite: (a) as-prepared, (b) after hydrogenation under 4 MPa at 673 K, (c) after dehydrogenation under 100 Pa at 673 K.
particles are in spherical shape with an average diameter of 800 nm, decorating uniformly on the rGO sheets. The Raman spectra of the NiS/ rGO composite is displayed in Fig. 2c, and the characteristic D, G and 2D band peaks at about 1350, 1590 and 2675 cm−1 imply the existence of the rGO sheets. Generally, the D band represents the structure defects and disordered structures of the sp2 domains. The G band becomes active in perfect graphite caused by the sp2-hybridized carbon. Moreover, the 2D band is usually used to investigate the layering in the graphene [9]. These results suggest that the GO has been co-reduced during the formation process of nickel sulfides particles, agree well with the XRD result in Fig. 1b. Fig. 3a displays the XRD pattern of the ball milled Mg-NiS/rGO nanocomposite. The pattern is mainly composed of Mg, MgH2, β-NiS and α-NiS phases, suggesting that there are no phase changes during the ball milling process, which is consistent with the Mg-NiS nanocomposite system in our previous work [35]. After hydrogenation process at 673 K, the phases in the composite change to MgH2, Mg2NiH4, MgS and Ni. The disappearance of the diffraction peaks of β-NiS and α-NiS phases suggests their complete reaction with Mg during hydrogen absorption process. We suggest the nickel sulfides particles in the rGO sheets can not fully contact with the inside of Mg, leading to the formation of Ni in the composite [35], see the diffraction peak at 44.1° in Fig. 3b. The MgH2 and Mg2NiH4 recover to Mg and Mg2Ni, while the Ni and MgS remain unchanged after dehydrogenation at 673 K. Given that
3. Results and discussion 3.1. Particle features Fig. 1a shows the XRD pattern of the as-made rGO sheets. After reduction treatment by thiourea, the broad diffraction peak centering at about 26° can be attributed to the (002) crystal plane of pure rGO, see Fig. 1a, similar to that of the Wang's report [38], indicating that the complete reduction of GO (XRD pattern in Fig. S1 shows no diffraction peak at 26°). It is interesting that the α-NiS (JCPDS: 65–5762, space group: P63/mmc), β-NiS (JCPDS: 12–0041, space group: R3m) and a little Ni3S4 (JCPDS: 47–1739, space group: Fd-3m) phase come into being once the GO and Ni2+ are co-reduced from solution, see Fig. 1b. We suggest that the GO obstructed contact between the citrate, S anions and Ni2+ to some extent [35,39,40], leading to the formation of different nickel sulfides, which can completely react with Mg in the activation process. These nickel sulfides disperse uniformly on rGO sheets and could give more catalytic sites for the Mg/MgH2 system. Fig. 2a shows the TEM image of rGO sheets with typical 2D crumpled sheet-like structure prepared by reducing GO sheets in the thiourea solution [30]. It can be seen from Fig. 2b that the nickel sulfides
Fig. 2. (a) TEM image of the as-prepared rGO, (b) SEM image and (c) Roman spectra of the nickel sulfides decorated rGO sheets.
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Fig. 4. SEM image of the Mg-NiS/rGO nanocomposite (a), element maps of Mg (b), C (c), Ni (d) and S (e).
After the first hydrogen absorption and desorption process, the in situ formed Mg2Ni, Ni and MgS catalysts steadily catalyze the hydrogen sorption of the Mg-NiS/rGO nanocomposite.
the preparation and collection of the Mg-NiS/rGO nanocomposite and the packing of the sample to the Sieverts-type test apparatus are under Ar and/or H2 atmospheres, which can prevent the composite from oxidization. The MgO phase centering at 42.9° in Fig. 3b and c are mainly attributed to the NPs taken out of the measurement chamber without being passivated sufficiently [33]. The morphologies of the Mg-NiS/rGO nanocomposite before and after hydrogen absorption and desorption process were observed by SEM and the results were shown in Figs. S2a and b. It can be seen from Fig. S2a that the particles are spherical and the particle size is about 500 nm, similar to that of pure Mg NPs [37]. The particle size is almost unchanged after hydrogenation and dehydrogenation cycles, see Fig. S2b, indicating the good size stability of the composite. The SEM images of the Mg-NiS and Mg-rGO nanocomposite are also shown in Figs. S3a–b. It can be seen that their morphologies are similar to that of the Mg-NiS/rGO nanocomposite, suggesting that the morphology is not the factor to cause the differences in sorption properties of these nanocomposites. To determine the uniformity of the elements distribution in the composite, Fig. 4a–e shows the SEM images of the Mg-NiS/rGO nanocomposite and the corresponding element maps of the Mg, C, Ni and S. It can be seen that the C, Ni and S elements distribute uniformly on the surface of Mg particles, implying that the NiS/rGO composite catalyst has been dispersed homogeneously in the Mg/MgH2 system. The uniformly distributed catalysts can be treated as diffusion channels and enhance the hydrogen diffusion at Mg/MgH2 surfaces [41]. Based on the analysis above, the phase changes of the Mg-NiS/rGO nanocomposite during the first hydrogen absorption (equation (1)) and desorption (equation (2)) processes can be summarized as following:
3.2. Hydrogen storage properties and catalytic mechanism To study the catalytic activation of the in situ formed Mg2Ni, Ni, MgS and rGO catalysts on the Mg NPs, we compared the desorption kinetics properties of the Mg-NiS/rGO nanocomposite with the pure Mg and samples catalyzed with NiS and rGO catalysts, and the kinetics curves tested at desorption temperature of 573 K are shown in Fig. 5a–d. It can be seen that the catalytic effect order of the three samples is NiS/rGO, NiS and rGO. The Mg-NiS/rGO nanocomposite can
Mg + αNiS + βNiS + Ni3 S4 + H2 → MgH2 + Mg2 NiH4 + MgS + Ni (1)
MgH2 + Mg2 NiH4 + MgS + Ni → Mg + Mg2 Ni + MgS + Ni + H2
Fig. 5. Isothermal dehydrogenation kinetics curves of (a) pure Mg (Ref. [42]), (b) MgrGO, (c) Mg-NiS (Ref. [35]) and (d) Mg-NiS/rGO nanocomposites at 573 K.
(2) 115
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temperature [27], suggesting that the MgS, Mg2Ni, Ni and rGO catalysts further improve the hydrogenation kinetics of Mg/MgH2. Similarly, within hydrogen absorption time of 60 min, the hydrogen absorption capacities of the Mg-NiS/rGO nanocomposite reach to 5.2, 5.2, 5.5, 6.2 at 523, 573, 623 and 673 K, respectively, showing better absorption performances than that of the Mg-NiS [35], Mg-rGO (see Fig. S4a) and ball milled Mg [42] samples. Fig. 6b shows the dehydrogenation kinetics curves of Mg-NiS/rGO nanocomposite at different temperatures. It shows the best desorption properties among the four samples, see Fig. 6b, reference [35] and Fig. S4b. The composite can quickly release 5.3 wt% H2 within 5 min, and reach its saturated hydrogen release capacity of 5.6 wt% within 10 min at 673 K. And it can desorb 5.1 wt% H2 in 5 min at 623 K. Even at desorption temperature 523 K, the MgNiS/rGO nanocomposite can release 1.0 wt% H2 in 30 min and 2.2 wt% H2 in 60 min. The other three samples catalyzed without or with single catalysts NiS and rGO are rarely release hydrogen at such low temperature, indicating the good catalytic effects of the MgS, Mg2Ni, Ni and rGO catalysts on desorption properties of Mg-NiS/rGO nanocomposite. To further determine the hydrogen absorption and desorption activation energies (Ea) of the composites, conventional Johnson-MehlAvrami-Kolmogorov (JMAK) model and the Arrhenius theory are employed here and the data are plotted in Figs. S5a–b, Figs. S6a–b and Fig. 6c–d according to following linear equations [45].
release 3.7 wt% H2 in just 10 min and the hydrogen desorption capacity is as high as 4.5 wt% H2 within 60 min, see Fig. 5d. This is superior to that of Mg-Nb, Mg-Ti, Mg-V and Mg-14 wt% Ni-6wt% NbF5 composites [33,43–45]. The MgH2 particles catalyzed by Ni2P NPs supported on rGO sheets can only release less than 3.0 wt% H2 within 10 min at the same desorption temperature [46]. Moreover, the Mg-CeOx/rGO composite can only release 3.0 wt% H2 within 30 min at 573 K [30]. We suggest that the various in situ formed Mg2Ni, Ni and MgS catalysts decorated on rGO sheets in the Mg-NiS/rGO nanocomposite can disperse more uniformly in the Mg/MgH2 system, and the rGO can restrain the agglomeration of the catalysts and Mg NPs during the hydrogen absorption and desorption process. Although its operating temperature is still high for the on-board hydrogen storage application, the Mg-NiS/ rGO nanocomposite is very promising in heat storage system that can be connected to parabolic trough solar power plant (operating temperature of 573–773 K). The solar power can be collected to heat MgH2 decompose into Mg and H2 in daytime, and at night or in cloudy days Mg absorbs H2 to form MgH2, which can continuously convert heat (about 0.9 kWh kg MgH2) into electricity [6,7]. To further study the catalytic effects of the MgS, Mg2Ni, Ni and rGO catalysts on the hydrogen storage properties of the Mg-NiS/rGO nanocomposite system, we measured its hydrogen absorption and desorption kinetics curves at different temperatures and the results are displayed in Fig. 6a and b. The hydrogen absorption capacities decrease with the decrease of the temperatures. The Mg-NiS/rGO nanocomposite can uptake 0.5 wt% H2 within 60 min at room temperature. At 373 K, it can quickly absorb 1.7 wt% H2 in 5 min and 3.6 wt% H2 in 60 min, better than that of the Mg-NiS sample [35]. This is also superior to that of the Mg-based material containing Ni, V and Ti catalyst [3,14,37]. Zou et al. found that the nanostructured Mg-Ni composite can uptake less than 1.0 wt% H2 even at 423 K [47]. The composite can quickly absorb 3.6 wt% H2 in 5 min when the absorption temperature increases to 423 K, better than those of the Mg-NiS [35] and Mg-rGO composites (see Fig. S4a). Moreover, the MgH2-16.7 wt% MoS2 composite can only uptake about 2.6 wt% H2 within 10 min at 423 K [26]. In addition, the Mg-NiS/rGO nanocomposite can uptake 4.2 wt% H2 in 5 min and 5.0 wt % H2 in 60 min at 473 K. The MgH2-20 wt% MgS and MgH2-20 wt% Fe3S4 composites can absorb 3.3 and 3.7 wt% H2 in 10 min at the same
ln[−ln(1 − α )] = η ln t + η ln k
(3)
k = A exp(Ea/ RT )
(4)
in which α means the fraction of the hydrogenated Mg at time t, k is a constant of reaction rate, and η stands for the reaction order, A is the temperature-independent coefficient, R is a constant of 8.314 J mol−1 K−1, and T is the absolute temperature. Based on the data of the hydrogen absorption and desorption kinetics curves at different temperatures in Fig. 6a–b, the k values at different temperatures can be calculated from the slope η and intercept ηlnk in the ln[-ln(1-α)] versus ln t fitting lines, see Figs. S5a–b. Generally, the rate-limiting process, growth dimensionality, and nucleation behavior of the hydrides can affect the reaction order η in equation (3), which represents different controlling mechanisms for the rates [48]. The η values at Fig. 6. Hydrogen absorption curves under 4 MPa hydrogen pressure (a), desorption curves under 100 Pa (b), plots of ln k vs 1000/T for the hydrogenation (c) and dehydrogenation (d) of the Mg-NiS/rGO nanocomposite.
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Fig. 7. Schematic diagram of the catalytic mechanism of Ni, Mg2Ni, MgS and rGO catalysts during the hydrogenation/dehydrogenation processes of the Mg-NiS/rGO nanocomposite.
Fig. 8. P–C isotherm curves at 623, 648, and 673 K (a) and Van't Hoff plots (b) for the Mg-NiS/rGO nanocomposite.
indicating that the transmission of H atoms into Mg particles can not be suppressed by rGO sheets. Moreover, the expansion-stain-absorption mechanism [50] caused by the phase transformation between the Mg2Ni and Mg2NiH4 can enhance the diffusion rate of the H atoms. With the steadily diffusion of the H atoms, the defects on the surface of Mg particles formed during the ball milling process can provide more nucleation sites for MgH2, and then MgH2 nucleus grows up and the Mg completely transforms into MgH2. For the dehydrogenation process, the Mg2NiH4 dehydrogenate into Mg2Ni before MgH2 [51] and can served as“hydrogen pump” together with Ni and MgS catalysts that continuously transfer the H atoms from the MgH2 particles. The Mg quickly nucleates at the defects on the surface of the MgH2 particles and transforms gradually into Mg. The thermodynamic properties of the Mg-NiS/rGO nanocomposite were measured based on the data from P-C isotherm curves (Fig. 8a) of the hydrogenation and dehydrogenation processes at different temperatures. The hydrogen equilibrium pressures of the absorption plateaus at 673, 648, and 623 K are 1.98, 1.19, 0.72 MPa, respectively. The hydrogen equilibrium pressures of the desorption plateaus at 673, 648, and 623 K are 1.62, 0.97, and 0.57 MPa, respectively. Then, the ln P versus 1000/T plots (Van't Hoff plots) for both hydrogenation and dehydrogenation are shown in Fig. 8b. The determined enthalpy for the hydrogenation process of the composite is −71.0 kJ mol−1, similar to that of the MgH2 (−75 kJ mol−1) in other work [45]. And the determined enthalpy for the dehydrogenation process of the composite is 72.5 kJ mol−1. This suggests that the hydrogenation/dehydrogenation thermodynamics of the Mg-NiS/rGO nanocomposite can not be modified apparently by adding MgS, Mg2Ni, Ni, and rGO catalysts. Thus, the enhanced hydrogen sorption kinetics and capacity are mainly caused by the catalytic effects of the multiple-phase catalysts MgS, Mg2Ni, Ni and rGO.
different temperatures of about 0.5 in Fig. S5a suggest that the hydrogen absorption process of the Mg-NiS/rGO nanocomposite is mainly determined by diffusion rate and belongs to one-dimensional growth. Similarly, the reaction order η of about 1 in Fig. S5b implies that the dehydrogenation at different temperatures is controlled by either diffusion-rate limited transformation of two-dimensional growth or interface-controlled transformation with one-dimensional growth. Fig. 6c–d displays the plots of lnk versus 1000/T for the hydrogenation and dehydrogenation processes. The calculated hydrogenation Ea value of the Mg-NiS/rGO nanocomposite is 44.5 kJ mol−1, much smaller than that of the Mg-NiS, Mg-rGO and ball milled Mg NPs of 45.5 [35], 76.5, (Fig. S6c) 81.7 [35] KJ mol−1. This is also smaller than that of the Mg5wt% Ni and Mg-Nb nanocomposites of 88.9 and 70.9 kJ mol−1 [3,45]. The Ea value of the Mg-NiS/rGO nanocomposite for dehydrogenation is calculated to be 63.6 kJ mol−1, which is significantly lower than that of Mg-based materials catalyzed by NiS [35] and rGO (see Fig. S6d) catalysts, as well as the ball milled Mg NPs [42]. This is also much smaller than that of the MgH2-MoS2 composite of 87.19 kJ mol−1 [26], and the Fe3O4/rGO catalyzed MgH2 of 90.53 kJ mol−1 [29]. The smallest desoption activation energy leads to the fastest hydrogen release kinetics and best hydrogen release content among the Mg-NiS/rGO, MgNiS, Mg-rGO and ball milled Mg samples. Fig. 7 shows the catalytic mechanism of the Ni, Mg2Ni, MgS and rGO multiple catalysts during the hydrogenation/dehydrogenation cycle processes of the Mg-NiS/rGO nanocomposite. After activation process (first hydrogenation and dehydrogenation process at 673 K), the NiS and Ni3S4 compounds decorating on the rGO sheets change to Mg2Ni, Ni, and MgS catalysts. The improved hydrogen desorption performances of the Mg-NiS/rGO nanocomposite can be summarized as follows. The rGO sheets can uniformly disperse the catalysts and thus provide better catalytic activation by providing more hydrogen diffusion channels on the surface of the Mg/MgH2 [41]. During the hydrogenation process, the catalysts can continuously spill over H atoms into Mg surface after the hydrogen molecules dissociated into atoms by the Ni, Mg2Ni, and MgS catalysts. Du reported that the rGO sheets can promote the atomic hydrogen diffuse into the sub-layer quickly [49],
4. Conclusions Nickel sulfides decorated rGO sheets have been produced by coreducing Ni2+ and GO from thiourea solution. Then they are ball milled 117
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with Mg NPs to catalyze the hydrogen sorption of the Mg/MgH2 system. The nickel sulfides of about 800 nm completely in situ change to MgS, Mg2Ni and Ni catalysts after first hydrogenation/dehydrogenation process. Among the NiS/rGO, NiS and rGO catalysts, the NiS/rGO composite shows the best catalytic effect on Mg/MgH2 system. The MgNiS/rGO nanocomposite can desorb 3.7 wt% H2 in 10 min and 4.5 wt% H2 in 60 min at 573 K, while the Mg-rGO composite only desorbs 2.2 wt % H2 in 60 min. The apparent hydrogen absorption and desorption activation energies of the Mg-NiS/rGO nanocomposite are reduced to 44.47 and 63.02 kJ mol−1, smaller than those of the Mg-rGO (76.5 and 119.8 kJ mol−1), and Mg-NiS (45.45 and 64.71 kJ mol−1) samples. The best hydrogen sorption properties of the Mg-NiS/rGO nanocomposite can be explained by the synergistic catalytic effects of the highly dispersed Mg2Ni, Ni, and MgS catalysts on the rGO, and the more nucleation sites between the catalysts, rGO sheets and Mg matrix. Acknowledgements The authors acknowledge the support of this work by the Joint Fund of the National Natural Science Foundation of China and Baosteel Group Corporation (No. U1560106), the Aeronautical Science Foundation of China (No. 2016ZF51050), the Scientific Research Foundation for the Returned Overseas Chinese Scholars (State Education Ministry). Appendix A. Supplementary data Supplementary data related to this article can be found at http://dx. doi.org/10.1016/j.jpowsour.2017.10.054. References [1] Q.Y. Zhang, Y. Wang, L. Zang, X.Y. Chang, L.F. Jiao, H.T. Yuan, Y.J. Wang, J. Alloys Compd. 703 (2017) 381–388. [2] J.G. Zhang, Y.F. Zhu, H.J. Lin, Y.N. Liu, Y. Zhang, S.Y. Li, Z.L. Ma, L.Q. Li, Adv. Mater 29 (2017) 1700760. [3] H. Fu, W.S. Wu, Y. Dou, B.Z. Liu, H.N. Li, Q.M. Peng, J. Power Sources 320 (2016) 212–221. [4] X.L. Ding, Y.T. Li, F. Fang, D.L. Sun, Q.G. Zhang, J. Mater. Chem. A 5 (2017) 5067–5076. [5] P.E. Jongh, P. Adelhelm, ChemSusChem 3 (2010) 1332–1348. [6] H.Y. Shao, G.B. Xin, J. Zheng, X.G. Li, E. Akib, Nano Energy 1 (2012) 590–601. [7] B. Bogdanović, A. Ritter, B. Spliethoff, Angew. Chem. Int. Ed. 29 (1990) 223. [8] M. Jangir, A. Jain, S. Yamaguchi, T. Ichikawa, C. Lal, I.P. Jain, Int. J. Hydrogen Energy 41 (2016) 14178–14183. [9] Y. Wang, L. Li, C.H. An, Y.J. Wang, C. Chen, L.F. Jiao, H.T. Yuan, Nanoscale 6 (2014) 6684. [10] E.S. Cho, A.M. Ruminski, S. Aloni, Y.S. Liu, J.H. Guo, J.J. Urban, Nat. Commun. 7 (2016) 10804. [11] M. Calizzi, D. Chericoni, L.H. Jepsen, T.R. Jensen, L. Pasquini, Int. J. Hydrogen Energy 41 (2016) 14447–14454. [12] N.S. Norberg, T.S. Arthur, S.J. Fredrick, A.L. Prieto, J. Am. Chem. Soc. 133 (2011) 10679–10681. [13] A. Ebrahimi-Purkani, S.F. Kashani-Bozorg, J. Alloys Compd. 456 (2008) 211–215. [14] H.Y. Shao, K. Asano, H. Enoki, E. Akiba, J. Alloys Compd. 477 (2009) 301–306.
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High hydrogen desorption properties of Mg-based nanocomposite at moderate temperatures: The effects of multiple catalysts in situ formed by adding nickel sulfides/graphene
T
Xiubo Xie, Ming Chen, Peng Liu, Jiaxiang Shang, Tong Liu∗ Key Laboratory of Aerospace Materials and Performance (Ministry of Education), School of Materials Science and Engineering, Beihang University, No.37 Xueyuan Road, Beijing, 100191, China
H I G H L I G H T S
G RA P H I C A L AB S T R A C T
Sulfides/Graphene was pro• Nickel and graduced by co-reducing Ni 2+
• • • •
phene oxide. Mg-5wt%NiS/rGO nanocomposite is prepared by ball milling Mg and NiS/ rGO. It shows best desorption properties compared with samples catalyzed by NiS and rGO. Nickel sulfides in situ change to MgS, Mg2Ni and Ni multiple catalysts. MgS, Mg2Ni, Ni and rGO synergistically catalyze the hydrogen desorption of Mg.
A R T I C L E I N F O
A B S T R A C T
Keywords: Magnesium Hydrogen storage Multiple-phase catalysts Catalytic effects Nanocomposite
Nickel sulfides decorated reduced graphene oxide (rGO) has been produced by co-reducing Ni2+ and graphene oxide (GO), and is subsequently ball milled with Mg nanoparticles (NPs) produced by hydrogen plasma metal reaction (HPMR). The nickel sulfides of about 800 nm completely in situ change to MgS, Mg2Ni and Ni multiple catalysts after first hydrogenation/dehydrogenation process at 673 K. The Mg-5wt%NiS/rGO nanocomposite shows the highest hydrogen desorption kinetics and capacity properties, and the catalytic effect order of the additives is NiS/rGO, NiS and rGO. At 573 K, the Mg-NiS/rGO nanocomposite can quickly desorb 3.7 wt% H2 in 10 min and 4.5 wt% H2 in 60 min. The apparent hydrogen absorption and desorption activation energies of the Mg-5wt%NiS/rGO nanocomposite are decreased to 44.47 and 63.02 kJ mol−1, smaller than those of the Mg-5wt %rGO and Mg-5wt%NiS samples. The best hydrogen desorption properties of the Mg-5wt%NiS/rGO nanocomposite can be explained by the synergistic catalytic effects of the highly dispersed MgS, Mg2Ni and Ni catalysts on the rGO sheets, and the more nucleation sites between the catalysts, rGO sheets and Mg matrix.
1. Introduction Magnesium (Mg) as one of the most attractive solid-state hydrogen storage materials used in applications of on-board hydrogen storage, heat storage of portable and stationary energy storage has been widely ∗
studied over the past decades for its high theoretical weight storage capacity (7.6 wt%) and relatively better cycling properties [1–7]. Nowadays, the main research has focused on improving the hydrogen absorption and desorption kinetics and decreasing the thermodynamic stability of Mg-H bond that leads to the operation temperature above
Corresponding author. E-mail address: [email protected] (T. Liu).
http://dx.doi.org/10.1016/j.jpowsour.2017.10.054 Received 12 September 2017; Received in revised form 15 October 2017; Accepted 16 October 2017 Available online 21 October 2017 0378-7753/ © 2017 Elsevier B.V. All rights reserved.
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milled with Mg NPs prepared by hydrogen plasma-metal reaction (HPMR) approach. The nickel sulfides completely changed to MgS, Mg2Ni and Ni catalysts during the first hydrogenation and dehydrogenation process. The Mg-NiS/rGO nanocomposite showed enhanced sorption properties with best hydrogen release kinetics and highest desorption capacity compared with Mg-NiS, Mg-rGO nanocomposite and pure ball milled Mg NPs. The catalytic mechanism of the Mg-NiS/rGO nanocomposite was further studied in detail in this work.
673 K [8,9]. Various attempts have been tried to address the limitations, such as reducing the particle size of Mg to nanoscale to reduce the distance of the hydrogen diffusion [10–12], alloying with metal elements [13,14] or constructing Mg-X substitutional solid solution [15,16] to change the desorption enthalpy of Mg-based material, and the most common way, introducing different catalysts to catalyze the hydrogen dissociation and Mg/MgH2 nucleation during hydrogenation/ dehydrogenation cycle process [17,18], have been used to enhance the hydrogen storage properties. Mechanical milling is a traditional way to disperse different catalysts of 3d-transition metals [3,18–21] and their oxides [22,23], fluorides [24,25], and sulfides [26,27] into Mg/MgH2 system. Liang et al. studied the MgH2 system ball milled with different metals and found that the Mg-Ni powder showed best absorption and desorption kinetics [19]. Xie et al. confirmed that the Mg-10 wt%Ni composite can release 6.1 wt% H2 in 10 min at 523 K under an initial pressure of 0.01 bar [20]. Chen and coworkers ball milled Mg with 4 at% Ni nanofibers and found that the composite can quickly release 4.5 wt% H2 within 60 min at 523 K [21]. Therefore, the additive of Ni based catalysts into Mg/MgH2 system can effectively enhance its hydrogen storage properties. However, some of the catalysts NPs tend to be agglomerate and grow to large particles during ball milling process, leading to their decreased catalytic activity [28]. RGO is reported to be an effective catalyst and it can be served as a media to disperse catalysts uniformly in the Mg/MgH2 system [28,29]. Bhatnagar et al. reported that the graphene sheets can prevent the Fe3O4 particles from agglomerating and the MgH2:Fe3O4@GS composite can absorb 6.2 wt% hydrogen in 2.5 min and the onset desorption temperature was 535 K, better than that of the MgH2:Fe3O4 of 571 K [29]. Wang and coworkers found that the MgH2-5wt%(Ni-CeOx)/GNS composite can release 6.5 wt % H2 within 10 min at 573 K at an initial hydrogen pressure of 5 KPa, superior to the desorbed H2 amount of 0.16 wt% for MgH2 sample under the same condition [30]. Furthermore, very recent studies showed that in situ formed multiple catalysts especially the transition metal sulfides can significantly improve the hydrogen storage properties of Mg [27,31–34]. Unfortunately, the desorption temperature of these Mg-based materials was still high, and the desorption kinetics needed further improvement. Zhang et al. found that the additive of Fe3S4 can in situ change to MgS and Fe multiple catalytic phases in the composite when ball milled with MgH2. Compared with the Mg-MgS and Mg-Fe composites with one catalyst, the Mg-Fe3S4 composite with multiple catalysts showed better hydrogen absorption rates. However, the Mg-Fe3S4 composite showed still low desorption kinetics, which can only release 4.0 wt% H2 within 20 min at 623 K [27]. Jia et al. studied the Mg-MoS2 system and found that the in situ formed MgS catalyst can decrease the onset desorption temperature of the Mg/MgH2 system, however, its hydrogen release content was only 0.2 wt% H2 at 573 K [28]. In our previous work [35], we studied the Mg-5wt%NiS nanocomposite and found that the NiS in situ decomposed into Ni, MgS and Mg2Ni multiple-phases after first hydrogenation/dehydrogenation cycle process. The desorption activation energy of the nanocomposite decreased to 64.71 kJ mol−1, and it can quickly uptake 3.5 wt % H2 within 10 min at 423 K. But its desorption temperature was still high and the hydrogen release kinetics needed further improvement. Finding an efficient catalyst to enhance the desorption kinetics of the Mg-based hydrogen material still remains a challenge. Nickel sulfides can react with Mg during the hydrogen absorption and desorption process and in situ form multiple catalysts. Moreover, the uniformly dispersed nickel sulfides on the rGO sheets could provide more nucleation sites for Mg/MgH2 and prevent the catalysts from aggregation, leading to the enhanced desorption properties of the Mg material. In this work, we synthesized NiS/rGO composite by solvothermal reaction method, the Ni2+ was reduced to nickel sulfides in situ decorating on rGO sheets. The nickel sulfides particles of about 800 nm were in spherical shape. The composite was subsequently ball
2. Experimental section 2.1. Preparation of NiS/rGO composite The CH4N2S (Aladdin, > 99.9%) and Na3C6H5O7·3H2O (Aladdin, > 99.9%) were used as received. The GO was fabricated in our laboratory by modified Hummers' method [36]. The NiS/rGO composite was produced by solvothermal reaction method, which was described previously [35]. The Na3C6H5O7·3H2O and CH4N2S were successively dissolved in 30 mL of distilled water containing 1 mmol NiCl2·6H2O by electromagnetically stirring. Distilled water of 10 mL containing 10 mg GO under ultrasonic treatment for 1 h was added to the above-mentioned solution, and the pH value was tuned to 12 by adding ammonia (28 wt%) dropwise. The black solution was shifted into a 50 mL sealed Teflon-lined autoclave and maintained at 453 K for 24 h. The as-made product was dried by vacuum pumping at 323 K for two days after washed with distilled water and absolute alcohol several times. Based on the content of the reactant of NiCl2·6H2O, the yield of the NiS was calculated to be 90.7 mg. Therefore, the ratio of NiS to rGO in the as-synthesized NiS/rGO composite is 90.1:9.9. The pure rGO sheets were also prepared by the same process mentioned above. 2.2. Preparation of Mg-NiS/rGO nanocomposite The dried NiS/rGO powder was mechanically milled under H2 atmosphere (99.999%) with the Mg NPs (weight ratio, 5:95) prepared by HPMR approach [37], the ball to powder ratio was 20:1, the rotation speed and ball milling time were 220 rpm and 2 h. Brittle MgH2 NPs at a weight ratio of 9:1 (Mg/MgH2) were added to avoid agglomeration of the Mg NPs. The preparation and collection of the ball milled materials were performed in a glove-box filled with purified argon (99.999%) and a drying agent to prevent the milled material from undergoing hydroxide and/or oxidation formation, we refer the sample as Mg-NiS/rGO, hereafter. The pure Mg and Mg-rGO composites were also made by the same method for comparison. 2.3. Characterization The hydrogen absorption and desorption kinetics properties of the composites were measured by Sieverts-type apparatus with reactor chamber volume of 60 ml. The experimental dosage of the samples is 100 mg and they should be activated before mesurement by one hydrogen absorption and desorption cycle under hydrogen pressures of 4 MPa and 100 Pa at 673 K. Then the Sieverts-type apparatus was maintained at 673 K and pumped to 10−3 Pa for more than 60 min to ensure that the complete hydrogen release in the material, the composites were heated up to the tested temperatures to measure the corresponding hydrogenation (initial pressure, 4 MPa) and dehydrogenation (initial pressure, 100 Pa) kinetic curves. The thermodynamic property of the Mg-NiS/rGO nanocomposite was investigated by conventional pressure-volume-temperature equipment. For the pressurecomposition (P-C) isotherm curves at a certain temperature, we suggest that the measurement reaches equilibrium if the change of hydrogen pressure is no more than 20 Pa/s. The phase changes of the Mg-NiS/rGO composite during the hydrogenation/dehydrogenation process were measured by an X-ray diffraction (XRD) measurement with monochromatic Cu Kα radiation 113
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Fig. 1. XRD patterns of the rGO (a), and the nickel sulfides decorated rGO (b).
(Rigaku X-ray diffractometer). Field emission scanning electronic microscopes (SEM, JEM7500, 15 kV) and transmission electron microscopy (TEM, JEOL-JSM-2100) were applied to analyze the morphology and microstructures of the samples. The energy dispersive spectroscopy (EDS) results were obtained to study the elemental distributions. The Raman spectrum was measured to observe the characteristic peaks of rGO by using a Lab RAM HR800 Raman spectrometer.
Fig. 3. XRD patterns of the Mg-NiS/rGO nanocomposite: (a) as-prepared, (b) after hydrogenation under 4 MPa at 673 K, (c) after dehydrogenation under 100 Pa at 673 K.
particles are in spherical shape with an average diameter of 800 nm, decorating uniformly on the rGO sheets. The Raman spectra of the NiS/ rGO composite is displayed in Fig. 2c, and the characteristic D, G and 2D band peaks at about 1350, 1590 and 2675 cm−1 imply the existence of the rGO sheets. Generally, the D band represents the structure defects and disordered structures of the sp2 domains. The G band becomes active in perfect graphite caused by the sp2-hybridized carbon. Moreover, the 2D band is usually used to investigate the layering in the graphene [9]. These results suggest that the GO has been co-reduced during the formation process of nickel sulfides particles, agree well with the XRD result in Fig. 1b. Fig. 3a displays the XRD pattern of the ball milled Mg-NiS/rGO nanocomposite. The pattern is mainly composed of Mg, MgH2, β-NiS and α-NiS phases, suggesting that there are no phase changes during the ball milling process, which is consistent with the Mg-NiS nanocomposite system in our previous work [35]. After hydrogenation process at 673 K, the phases in the composite change to MgH2, Mg2NiH4, MgS and Ni. The disappearance of the diffraction peaks of β-NiS and α-NiS phases suggests their complete reaction with Mg during hydrogen absorption process. We suggest the nickel sulfides particles in the rGO sheets can not fully contact with the inside of Mg, leading to the formation of Ni in the composite [35], see the diffraction peak at 44.1° in Fig. 3b. The MgH2 and Mg2NiH4 recover to Mg and Mg2Ni, while the Ni and MgS remain unchanged after dehydrogenation at 673 K. Given that
3. Results and discussion 3.1. Particle features Fig. 1a shows the XRD pattern of the as-made rGO sheets. After reduction treatment by thiourea, the broad diffraction peak centering at about 26° can be attributed to the (002) crystal plane of pure rGO, see Fig. 1a, similar to that of the Wang's report [38], indicating that the complete reduction of GO (XRD pattern in Fig. S1 shows no diffraction peak at 26°). It is interesting that the α-NiS (JCPDS: 65–5762, space group: P63/mmc), β-NiS (JCPDS: 12–0041, space group: R3m) and a little Ni3S4 (JCPDS: 47–1739, space group: Fd-3m) phase come into being once the GO and Ni2+ are co-reduced from solution, see Fig. 1b. We suggest that the GO obstructed contact between the citrate, S anions and Ni2+ to some extent [35,39,40], leading to the formation of different nickel sulfides, which can completely react with Mg in the activation process. These nickel sulfides disperse uniformly on rGO sheets and could give more catalytic sites for the Mg/MgH2 system. Fig. 2a shows the TEM image of rGO sheets with typical 2D crumpled sheet-like structure prepared by reducing GO sheets in the thiourea solution [30]. It can be seen from Fig. 2b that the nickel sulfides
Fig. 2. (a) TEM image of the as-prepared rGO, (b) SEM image and (c) Roman spectra of the nickel sulfides decorated rGO sheets.
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Fig. 4. SEM image of the Mg-NiS/rGO nanocomposite (a), element maps of Mg (b), C (c), Ni (d) and S (e).
After the first hydrogen absorption and desorption process, the in situ formed Mg2Ni, Ni and MgS catalysts steadily catalyze the hydrogen sorption of the Mg-NiS/rGO nanocomposite.
the preparation and collection of the Mg-NiS/rGO nanocomposite and the packing of the sample to the Sieverts-type test apparatus are under Ar and/or H2 atmospheres, which can prevent the composite from oxidization. The MgO phase centering at 42.9° in Fig. 3b and c are mainly attributed to the NPs taken out of the measurement chamber without being passivated sufficiently [33]. The morphologies of the Mg-NiS/rGO nanocomposite before and after hydrogen absorption and desorption process were observed by SEM and the results were shown in Figs. S2a and b. It can be seen from Fig. S2a that the particles are spherical and the particle size is about 500 nm, similar to that of pure Mg NPs [37]. The particle size is almost unchanged after hydrogenation and dehydrogenation cycles, see Fig. S2b, indicating the good size stability of the composite. The SEM images of the Mg-NiS and Mg-rGO nanocomposite are also shown in Figs. S3a–b. It can be seen that their morphologies are similar to that of the Mg-NiS/rGO nanocomposite, suggesting that the morphology is not the factor to cause the differences in sorption properties of these nanocomposites. To determine the uniformity of the elements distribution in the composite, Fig. 4a–e shows the SEM images of the Mg-NiS/rGO nanocomposite and the corresponding element maps of the Mg, C, Ni and S. It can be seen that the C, Ni and S elements distribute uniformly on the surface of Mg particles, implying that the NiS/rGO composite catalyst has been dispersed homogeneously in the Mg/MgH2 system. The uniformly distributed catalysts can be treated as diffusion channels and enhance the hydrogen diffusion at Mg/MgH2 surfaces [41]. Based on the analysis above, the phase changes of the Mg-NiS/rGO nanocomposite during the first hydrogen absorption (equation (1)) and desorption (equation (2)) processes can be summarized as following:
3.2. Hydrogen storage properties and catalytic mechanism To study the catalytic activation of the in situ formed Mg2Ni, Ni, MgS and rGO catalysts on the Mg NPs, we compared the desorption kinetics properties of the Mg-NiS/rGO nanocomposite with the pure Mg and samples catalyzed with NiS and rGO catalysts, and the kinetics curves tested at desorption temperature of 573 K are shown in Fig. 5a–d. It can be seen that the catalytic effect order of the three samples is NiS/rGO, NiS and rGO. The Mg-NiS/rGO nanocomposite can
Mg + αNiS + βNiS + Ni3 S4 + H2 → MgH2 + Mg2 NiH4 + MgS + Ni (1)
MgH2 + Mg2 NiH4 + MgS + Ni → Mg + Mg2 Ni + MgS + Ni + H2
Fig. 5. Isothermal dehydrogenation kinetics curves of (a) pure Mg (Ref. [42]), (b) MgrGO, (c) Mg-NiS (Ref. [35]) and (d) Mg-NiS/rGO nanocomposites at 573 K.
(2) 115
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temperature [27], suggesting that the MgS, Mg2Ni, Ni and rGO catalysts further improve the hydrogenation kinetics of Mg/MgH2. Similarly, within hydrogen absorption time of 60 min, the hydrogen absorption capacities of the Mg-NiS/rGO nanocomposite reach to 5.2, 5.2, 5.5, 6.2 at 523, 573, 623 and 673 K, respectively, showing better absorption performances than that of the Mg-NiS [35], Mg-rGO (see Fig. S4a) and ball milled Mg [42] samples. Fig. 6b shows the dehydrogenation kinetics curves of Mg-NiS/rGO nanocomposite at different temperatures. It shows the best desorption properties among the four samples, see Fig. 6b, reference [35] and Fig. S4b. The composite can quickly release 5.3 wt% H2 within 5 min, and reach its saturated hydrogen release capacity of 5.6 wt% within 10 min at 673 K. And it can desorb 5.1 wt% H2 in 5 min at 623 K. Even at desorption temperature 523 K, the MgNiS/rGO nanocomposite can release 1.0 wt% H2 in 30 min and 2.2 wt% H2 in 60 min. The other three samples catalyzed without or with single catalysts NiS and rGO are rarely release hydrogen at such low temperature, indicating the good catalytic effects of the MgS, Mg2Ni, Ni and rGO catalysts on desorption properties of Mg-NiS/rGO nanocomposite. To further determine the hydrogen absorption and desorption activation energies (Ea) of the composites, conventional Johnson-MehlAvrami-Kolmogorov (JMAK) model and the Arrhenius theory are employed here and the data are plotted in Figs. S5a–b, Figs. S6a–b and Fig. 6c–d according to following linear equations [45].
release 3.7 wt% H2 in just 10 min and the hydrogen desorption capacity is as high as 4.5 wt% H2 within 60 min, see Fig. 5d. This is superior to that of Mg-Nb, Mg-Ti, Mg-V and Mg-14 wt% Ni-6wt% NbF5 composites [33,43–45]. The MgH2 particles catalyzed by Ni2P NPs supported on rGO sheets can only release less than 3.0 wt% H2 within 10 min at the same desorption temperature [46]. Moreover, the Mg-CeOx/rGO composite can only release 3.0 wt% H2 within 30 min at 573 K [30]. We suggest that the various in situ formed Mg2Ni, Ni and MgS catalysts decorated on rGO sheets in the Mg-NiS/rGO nanocomposite can disperse more uniformly in the Mg/MgH2 system, and the rGO can restrain the agglomeration of the catalysts and Mg NPs during the hydrogen absorption and desorption process. Although its operating temperature is still high for the on-board hydrogen storage application, the Mg-NiS/ rGO nanocomposite is very promising in heat storage system that can be connected to parabolic trough solar power plant (operating temperature of 573–773 K). The solar power can be collected to heat MgH2 decompose into Mg and H2 in daytime, and at night or in cloudy days Mg absorbs H2 to form MgH2, which can continuously convert heat (about 0.9 kWh kg MgH2) into electricity [6,7]. To further study the catalytic effects of the MgS, Mg2Ni, Ni and rGO catalysts on the hydrogen storage properties of the Mg-NiS/rGO nanocomposite system, we measured its hydrogen absorption and desorption kinetics curves at different temperatures and the results are displayed in Fig. 6a and b. The hydrogen absorption capacities decrease with the decrease of the temperatures. The Mg-NiS/rGO nanocomposite can uptake 0.5 wt% H2 within 60 min at room temperature. At 373 K, it can quickly absorb 1.7 wt% H2 in 5 min and 3.6 wt% H2 in 60 min, better than that of the Mg-NiS sample [35]. This is also superior to that of the Mg-based material containing Ni, V and Ti catalyst [3,14,37]. Zou et al. found that the nanostructured Mg-Ni composite can uptake less than 1.0 wt% H2 even at 423 K [47]. The composite can quickly absorb 3.6 wt% H2 in 5 min when the absorption temperature increases to 423 K, better than those of the Mg-NiS [35] and Mg-rGO composites (see Fig. S4a). Moreover, the MgH2-16.7 wt% MoS2 composite can only uptake about 2.6 wt% H2 within 10 min at 423 K [26]. In addition, the Mg-NiS/rGO nanocomposite can uptake 4.2 wt% H2 in 5 min and 5.0 wt % H2 in 60 min at 473 K. The MgH2-20 wt% MgS and MgH2-20 wt% Fe3S4 composites can absorb 3.3 and 3.7 wt% H2 in 10 min at the same
ln[−ln(1 − α )] = η ln t + η ln k
(3)
k = A exp(Ea/ RT )
(4)
in which α means the fraction of the hydrogenated Mg at time t, k is a constant of reaction rate, and η stands for the reaction order, A is the temperature-independent coefficient, R is a constant of 8.314 J mol−1 K−1, and T is the absolute temperature. Based on the data of the hydrogen absorption and desorption kinetics curves at different temperatures in Fig. 6a–b, the k values at different temperatures can be calculated from the slope η and intercept ηlnk in the ln[-ln(1-α)] versus ln t fitting lines, see Figs. S5a–b. Generally, the rate-limiting process, growth dimensionality, and nucleation behavior of the hydrides can affect the reaction order η in equation (3), which represents different controlling mechanisms for the rates [48]. The η values at Fig. 6. Hydrogen absorption curves under 4 MPa hydrogen pressure (a), desorption curves under 100 Pa (b), plots of ln k vs 1000/T for the hydrogenation (c) and dehydrogenation (d) of the Mg-NiS/rGO nanocomposite.
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Fig. 7. Schematic diagram of the catalytic mechanism of Ni, Mg2Ni, MgS and rGO catalysts during the hydrogenation/dehydrogenation processes of the Mg-NiS/rGO nanocomposite.
Fig. 8. P–C isotherm curves at 623, 648, and 673 K (a) and Van't Hoff plots (b) for the Mg-NiS/rGO nanocomposite.
indicating that the transmission of H atoms into Mg particles can not be suppressed by rGO sheets. Moreover, the expansion-stain-absorption mechanism [50] caused by the phase transformation between the Mg2Ni and Mg2NiH4 can enhance the diffusion rate of the H atoms. With the steadily diffusion of the H atoms, the defects on the surface of Mg particles formed during the ball milling process can provide more nucleation sites for MgH2, and then MgH2 nucleus grows up and the Mg completely transforms into MgH2. For the dehydrogenation process, the Mg2NiH4 dehydrogenate into Mg2Ni before MgH2 [51] and can served as“hydrogen pump” together with Ni and MgS catalysts that continuously transfer the H atoms from the MgH2 particles. The Mg quickly nucleates at the defects on the surface of the MgH2 particles and transforms gradually into Mg. The thermodynamic properties of the Mg-NiS/rGO nanocomposite were measured based on the data from P-C isotherm curves (Fig. 8a) of the hydrogenation and dehydrogenation processes at different temperatures. The hydrogen equilibrium pressures of the absorption plateaus at 673, 648, and 623 K are 1.98, 1.19, 0.72 MPa, respectively. The hydrogen equilibrium pressures of the desorption plateaus at 673, 648, and 623 K are 1.62, 0.97, and 0.57 MPa, respectively. Then, the ln P versus 1000/T plots (Van't Hoff plots) for both hydrogenation and dehydrogenation are shown in Fig. 8b. The determined enthalpy for the hydrogenation process of the composite is −71.0 kJ mol−1, similar to that of the MgH2 (−75 kJ mol−1) in other work [45]. And the determined enthalpy for the dehydrogenation process of the composite is 72.5 kJ mol−1. This suggests that the hydrogenation/dehydrogenation thermodynamics of the Mg-NiS/rGO nanocomposite can not be modified apparently by adding MgS, Mg2Ni, Ni, and rGO catalysts. Thus, the enhanced hydrogen sorption kinetics and capacity are mainly caused by the catalytic effects of the multiple-phase catalysts MgS, Mg2Ni, Ni and rGO.
different temperatures of about 0.5 in Fig. S5a suggest that the hydrogen absorption process of the Mg-NiS/rGO nanocomposite is mainly determined by diffusion rate and belongs to one-dimensional growth. Similarly, the reaction order η of about 1 in Fig. S5b implies that the dehydrogenation at different temperatures is controlled by either diffusion-rate limited transformation of two-dimensional growth or interface-controlled transformation with one-dimensional growth. Fig. 6c–d displays the plots of lnk versus 1000/T for the hydrogenation and dehydrogenation processes. The calculated hydrogenation Ea value of the Mg-NiS/rGO nanocomposite is 44.5 kJ mol−1, much smaller than that of the Mg-NiS, Mg-rGO and ball milled Mg NPs of 45.5 [35], 76.5, (Fig. S6c) 81.7 [35] KJ mol−1. This is also smaller than that of the Mg5wt% Ni and Mg-Nb nanocomposites of 88.9 and 70.9 kJ mol−1 [3,45]. The Ea value of the Mg-NiS/rGO nanocomposite for dehydrogenation is calculated to be 63.6 kJ mol−1, which is significantly lower than that of Mg-based materials catalyzed by NiS [35] and rGO (see Fig. S6d) catalysts, as well as the ball milled Mg NPs [42]. This is also much smaller than that of the MgH2-MoS2 composite of 87.19 kJ mol−1 [26], and the Fe3O4/rGO catalyzed MgH2 of 90.53 kJ mol−1 [29]. The smallest desoption activation energy leads to the fastest hydrogen release kinetics and best hydrogen release content among the Mg-NiS/rGO, MgNiS, Mg-rGO and ball milled Mg samples. Fig. 7 shows the catalytic mechanism of the Ni, Mg2Ni, MgS and rGO multiple catalysts during the hydrogenation/dehydrogenation cycle processes of the Mg-NiS/rGO nanocomposite. After activation process (first hydrogenation and dehydrogenation process at 673 K), the NiS and Ni3S4 compounds decorating on the rGO sheets change to Mg2Ni, Ni, and MgS catalysts. The improved hydrogen desorption performances of the Mg-NiS/rGO nanocomposite can be summarized as follows. The rGO sheets can uniformly disperse the catalysts and thus provide better catalytic activation by providing more hydrogen diffusion channels on the surface of the Mg/MgH2 [41]. During the hydrogenation process, the catalysts can continuously spill over H atoms into Mg surface after the hydrogen molecules dissociated into atoms by the Ni, Mg2Ni, and MgS catalysts. Du reported that the rGO sheets can promote the atomic hydrogen diffuse into the sub-layer quickly [49],
4. Conclusions Nickel sulfides decorated rGO sheets have been produced by coreducing Ni2+ and GO from thiourea solution. Then they are ball milled 117
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