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Magnesium Supported on Nickel Nanobelts for Hydrogen Storage: Coupling Nanosizing and Catalysis Yahui Sun, Tianyuan Ma, and Kondo-Francois Aguey-Zinsou ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.8b00033 • Publication Date (Web): 14 Feb 2018 Downloaded from http://pubs.acs.org on February 22, 2018

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ACS Applied Nano Materials

Magnesium Supported on Nickel Nanobelts for Hydrogen Storage: Coupling Nanosizing and Catalysis Yahui Sun†, Tianyuan Ma and Kondo-Francois Aguey-Zinsou†,* †

MERLin, School of Chemical Engineering, The University of New South Wales, Sydney NSW 2052, Australia, E-mail: [email protected]

Abstract Magnesium nanoparticles with the mean particle size of ∼18 nm were synthesized through the thermal decomposition of di-n-butylmagnesium using Ni nanobelts as a catalyst and support with the aim of improving the hydrogen storage properties of magnesium by coupling nanozing and catalysis. Full hydrogenation of the magnesium nanoparticles was achieved at a low temperature of 100 °C and hydrogen release occurred at around 230 °C. The material showed fast hydrogen absorption with good structural stability during cycling. Hydrogen desorption occurred in 200 min at 250 °C. This enhanced hydrogen storage properties were assigned to a lower activation energy (69.2 ± 2.5 kJ mol-1 H2), and the remarkably low enthalpy of hydrogenation (34.4 ± 5.4 kJ mol-1 H2) although partially compensated by a reduced entropy of 76. 9 ± 5.4 J K-1 mol-1 H2. The improvement of both kinetics and thermodynamics is believed to result from the coupling effects of nanosizing and catalysis.

Keywords: Magnesium, Hydrogen storage, Nanosize, Catalysis, Interface

1. Introduction Magnesium (Mg) is regarded as a promising hydrogen storage material due to its high theoretical storage capacity (7.6 mass%) and good reversibility.1-2 However, the main 1 ACS Paragon Plus Environment

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obstacles preventing the practical use of Mg as a hydrogen storage medium remain (i) the high temperature required to reversibly store hydrogen (> 300 °C) and (ii) the slow hydrogen storage kinetics. Catalysis at the surface of Mg particles is an effective option to improve the hydrogenation kinetics, and the effect of various catalysts, oxides,12-14 metal halides9, 15 and alloys

6, 16

3-9

including metals,3, 10-11 metal

has been reported. In particular, nickel (Ni) has

been one of the most widely used catalysts for the purpose of improving the kinetics of hydrogen storage materials.3, 17-18 Besides its catalytic effect, Ni has also been reported to lead to the destabilization of the Mg/MgH2 system through the formation of the Mg2Ni phase.7, 19 However, the formation of this alloy leads to a significant reduction of the storage capacity of Mg (from 7.6 to 3.6 mass%) and the resulting reduction in enthalpy (from 75 to 67 kJ mol-1 H2 20) is not sufficient to enable room temperature hydrogen uptake and release. Typically, Mg2Ni needs temperatures in excess of 250 °C to effectively cycle hydrogen.19 Besides catalysis, recently, nanosizing has emerged as an alternative approach with the potential of leading to improvements in both kinetics and thermodynamics due to particle size effects at the nanoscale.21-27 This is based on the assumptions that: (i) highly divided Mg leads to a larger surface area and thus more hydrogen dissociation sites, (ii) hydrogen diffusion paths are shorter in small particles, which in terms enhances kinetics, and (iii) higher surface energy of particles at the nanoscale can potentially lead to altered Mg/MgH2 thermodynamics assuming that the surface energy of the hydride is larger than that of the metal.21 Indeed, several reports have shown a decrease in the enthalpy as low as 44.5-66 kJ mol-1 H2,28-31 and a significant decrease in the hydrogen release temperature (from >300 °C to 253 °C) for Mg nanoparticles with a size below 4 nm.27-28 For such small particle sizes, the main synthetic approach remains through the use of a carbon scaffolds, 27-29, 32-34 to minimize the agglomeration of ultrasmall Mg particles. Hence, through melt filtration of Mg or hydrogenolysis of di-n-butylmagnesium, Mg/MgH2 nanoparticles with the size ranging from

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1.3 to 20 nm have been reported and these nanoparticles remained stable upon several hydrogen uptake and release cycles.27-29, 33-34 However, it should be noted that despites such small particle sizes, the temperature at which hydrogen can be released is limited by the observed enthalpy/entropy compensation effect, i.e. the significant reduction of enthalpy is also accompanied by a decrease in entropy limiting the release of hydrogen at low temperatures.2 Hydrogen storage kinetics are also relatively slow in comparison to catalyzed ball milled Mg.35 For example, for Mg nanoparticles confined in carbon aerogel with a size of 5-20nm, 67 min is required for 75% hydrogen absorption at 300 °C.34 In comparison, for MgH2 ball milled with 5 at% Ni, the absorption of 75% hydrogen takes only 17 min at 300 °C.8 Hence even at the nanoscale, catalysis still needs to be coupled with thermodynamic destabilization to improve the overall hydrogen storage properties of Mg. Attempts of coupling catalysis and nanoconfinement have been reported in the form of P-doping,27 or Ni and Cu decoration36 of the carbon supports. It was found that both Ni and Cu decoration of carbon aerogels lead to faster hydrogen desorption kinetics for nanoconfined MgH2.36 Comparison of various doping elements on nanosized Mg also revealed superior hydrogen storage kinetics upon Ni doping.3,

18, 37-38

Additional

drawbacks of current porous carbon materials is associated with their chemical nature and low surface tension, which leads to difficulties of wetting and thus inefficient loading of Mg.39-40 Wettability of the carbon support material can be altered by applying a wetting layer of Ni or Cu; however the overall storage capacity of the nanoconfined Mg still remains limited (i.e. < 1 mass % H2). 36 In the current investigation, we attempted to directly combine the catalytic effect of Ni and nanosizing by using Ni nanobelts as support material with better wettability for the stabilization of Mg nanoparticles. Previous work showed the possibility to use a Ni foil to support Mg particles and directly catalyze the hydrogen release at the Ni/Mg interface.41



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However, the use of a Ni foil as a support significantly hampered the overall hydrogen storage capacity which was < 0.1 mass %. In comparison, Ni nanobelts have the potential to offer a larger surface area and thus an improved hydrogen storage capacity. Through the thermal decomposition of di-n-butylmagnesium, Mg nanoparticles were effectively deposited at the surface of Ni nanobelts. The physical properties, hydrogen storage behavior, as well as the cycling properties and stability of this composite material are reported.

2. Experimental All operations including syntheses, material handling and preparation for characterization were carried out under an inert atmosphere in an argon-filled LC-Technology glove box (< 1 ppm O2 and H2O). 2.1. Materials 1.0 M di-n-butylmagnesium (MgBu2) in heptane, Nickel (II) chloride hexahydrate (NiCl2·6H2O, ≥97%) , Sodium tartrate dibasic dehydrate (Na2C4H4O6·2H2O, ≥99%), sodium dodecyl benzenesulfonate (SDBS, technical grade), Sodium hypophosphite monohydrate (NaH2PO2·H2O, ≥99%) were purchased from Sigma-Aldrich. Sodium hydroxide pellet (NaOH, 97%) was purchased from Ajax-Finechem. All the chemicals were used as received. Tetrahydrofuran (THF) was purchased as HPLC grade from Fisher Scientific and dried by a LC Technology SP-1 solvent purification system. 2.2. Synthesis of Ni nanobelts Ni nanobelts were synthesized through hydrothermal method according to the literature.42 NiCl2·6H2O (50 mM), Na2C4H4O6·2H2O (0.75 M), NaOH (5 M) and sodium dodecyl benzenesulfonate (SDBS) (15 mM) were first dissolve in 50 mL milli-Q water. Then NaH2PO2·H2O (0.4 M) was added to the solution. After stirring, the mixture was transferred to an autoclave and the autoclave was heated at 110 °C for 24 h. The Ni nanobelts were then 4 ACS Paragon Plus Environment

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collected by filtration and washed with milli-Q water and absolute ethanol 3 times (Scheme 1). The resulting black powder was finally dried in a vacuum oven at 60 °C. 2.3. Synthesis of Mg nanoparticles supported on Ni nanobelts In a typical synthesis, MgBu2 (2 mL, 1M in heptane) was added dropwise onto 100 mg of Ni nanobelts. The mixture was stirred for 1 h before drying overnight under vacuum on a Schlenk line. The dried mixture was then placed in a quartz crucible. The latter was slid in a quartz tube closed with a vacuum valve. The quartz tube was placed under vacuum in a tube furnace and the decomposition of MgBu2 was carried out at 300 °C for 1 h (Scheme 1). The amount of supported Mg was varied by changing the volume of MgBu2 solution (1, 2, 3 mL) impregnated onto the 100 mg Ni nanobelts. The corresponding theoretical Mg loading are 19, 32 and 42 mass%, respectively. The resulting materials are denoted according to their theoretical Mg mass fraction, i.e. 19% Mg-Ni, 32% Mg-Ni and 42% Mg-Ni, respectively. For reference, MgBu2 was also decomposed at the same temperature and the resulting Mg is denoted pristine-Mg. 2.4 Characterization The morphology, elemental mapping, Selected Area Electron Diffraction (SAED) was determined by Transmission Electron Microscopy (TEM) using a Philips CM200 operated at 200 kV. For TEM analysis, the materials were dispersed in THF, sonicated for a few minutes and then dropped onto a carbon coated copper grid. The crystalline nature of the materials was determined by X-Ray Diffraction (XRD) using a Philips X’pert Multipurpose XRD system operated at 40 mA and 45 kV with a monochromated Cu Ka radiation (λ=1.541Å) - step size = 0.01, 0.02 or 0.05, time per step = 10 or 20 s/step. The materials were protected against oxidation from air by a Kapton foil. Thermogravimetric Analysis (TGA) and Differential Scanning Calorimetry (DSC) in conjunction with Mass Spectrometry (MS) were conducted at 10 °C min-1 using alumina 5 ACS Paragon Plus Environment


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crucibles and under an argon flow of 20 mL min-1 using a Mettler Toledo TGA/DSC 1 coupled with an Omnistar MS. Masses between m/e = 2 and 100 were followed. The surface area of the Ni-nanobelts was determined by BET measurement which was performed using a Micromeritics TriStar 3000 Analyzer from Micrometrics Instrument Corporation. The material was degassed at 150 °C for 3 h prior to the measurement, and the measurement was performed by adsorption of N2 at -196 °C. Hydrogen uptake of the Mg-Ni belt samples was carried out with a homemade Sievert apparatus at 100°C with 6 MPa hydrogen pressure. Hydrogen cycling properties, including kinetics and Pressure Composition Temperature (PCT) isotherms were determined by using a high-pressure magnetic balance of 1 µg resolution equipped with capability for simultaneous density measurements (Rubotherm). Around 70 mg of material was used, with a hydrogen pressure of 6 MPa for absorption and 0.01 MPa for desorption. Hydrogen uptake and release were determined from the weight changes. For an accurate determination of the amount of hydrogen stored, a blank measurement with the empty sample holder was performed to determine the mass and volume of the sample holder. Further measurements were performed under a helium atmosphere with the material fully desorbed to determine the density of the material and corresponding parameters for buoyancy corrections. The hydrogen absorption and desorption were carried out at 150 and 250 °C. The PCT for absorption were used to determine the van’t Hoff plot. To obtain the PCT isotherms and avoid any contribution from the unsupported MgH2 particles, the material was first fully hydrogenated at 150 °C and then dehydrogenated at 250 °C to release hydrogen from the supported Mg nanoparticles while keeping the unsupported MgH2 particles fully hydrogenated (Figure S6). Absorption PCT measurements were then carried out at higher temperatures of 225, 250 and 275°C to fasten kinetics and thus easily reach the equilibrium conditions.


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3. Results and discussion 3.1. Physical properties of the Ni nanobelts and Mg nanoparticles supported on these nanobelts As seen on Figure 1a,b, TEM analysis revealed that Ni nanobelts with the thickness of ~15 nm, a width of ∼500 nm and a length of several µm were successfully synthesized through the hydrothermal route reported in the literature.42 However, XRD showed that besides the cubic Ni phase (01-070-0989), a hexagonal Ni(OH)2 phase (00-014-0117) also coexisted as a result of the synthetic process (Figure 1c). In order to avoid any potential reaction between the Ni(OH)2 phase with the MgBu2 precursor during the Mg loading process, the assynthesized Ni nanobelts were reduced under hydrogen flow at 300 °C for 2 h (Scheme 1). Upon this treatment, the morphology of the Ni nanobelts did not significantly change (Figure 1c), and XRD analysis further confirmed that the nanobelts were fully reduced to the cubic Ni phase (01-070-0989) (Figure 1d). The Ni nanobelts after reduction in hydrogen flow did not show any mass loss by TGA analysis which corroborated their full reduction (Figure S1). The surface area of the Ni nanobelts as determined by BET was 38 m2 g-1. After the loading of Mg through the thermal decomposition of MgBu2 at the surface of the Ni nanobelts (materials denoted Mg-Ni, Scheme 1), the structure of Ni nanobelts evolved with highly dispersed Mg nanoparticles (∼ 18 nm) at their surface (Figure 2a,b). Elemental mapping further confirmed the dispersion of Mg at the Ni nanobelts surface (Figure 2d-f). The corresponding SAED (Figure 2c) also proved the crystalline nature of the Mg and Ni particles observed in agreement with XRD analysis of Mg-Ni (Figure 3). Hence, Mg nanoparticles were successfully deposited at the surface of the Ni nanobelts. It is noteworthy that in addition to the supported Mg nanoparticles, unsupported MgH2 particles (∼ 50 nm) were also observed besides the Ni nanobelts (Figure S2). And this reflected the difficulty of controlling the deposition of Mg at specific sites only. Furthermore, compared to the 7 ACS Paragon Plus Environment


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decomposition of MgBu2 alone (pristine-Mg) under the same conditions leading to only the β-MgH2 phase (Figure 3), the decomposition of MgBu2 on the Ni nanobelts led to the formation of hexagonal Mg and/or β-MgH2 phase depending on the level of MgBu2 loading (Figure S4). The formation of hexagonal Mg phase was interpreted as a particle size effect coupled with the catalytic effect of Ni nanobelts. Indeed, MgBu2 decomposes following the reaction paths:43 (C4H9)2Mg → 2C4H8 + β-MgH2

120-280 °C

(1)

β-MgH2 → Mg + H2

280-400 °C

(2)

Hence, at a synthesis temperature of 300 °C, hexagonal Mg and/or β-MgH2 can be formed. Since small MgH2 particles (< 50 nm) have the potential to release hydrogen at lower temperatures,2,

28

it is most likely that at the synthesis temperature of 300 °C, these Ni

nanobelts supported nanoparticles will instantaneously release hydrogen to yield Mg nanoparticles. The catalytic effect of the Ni support may also facilitate this.37 In contrast, large MgH2 particles (Fig S5) obtained from the decomposition of MgBu2 require a temperature in excess of 300 °C to fully release hydrogen (Figure 4 and Figure S6), thus they will remain hydrogenated. Accordingly, during the thermal decomposition of MgBu2, Mg or MgH2 particles were formed and these corresponded to the supported and unsupported particles, respectively. This effect can also been observed by comparing the hydrogen desorption profile of the Mg-Ni material as-synthesized and after hydrogen absorption at 100 °C. As shown Fig 4, the as-synthesized material released hydrogen after 300 °C (410°C). However once hydrogenated, in addition to the high temperature peak, hydrogen was released from 100 °C with a peak at 235 °C corresponding to the desorption of hydrogen from the Ni supported MgH2 particles.


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3.2. Effect of MgBu2 loading In order to increase the amount of Mg nanoparticles supported at the surface of the Ni nanobelts and optimize the hydrogen storage capacity, an attempt was made by varying the amount of MgBu2 decomposed on the Ni nanobelts and the morphology of the resulting materials are summarized in Fig S3. In addition to Mg nanoparticles at the surface of the Ni nanobelts, unsupported β-MgH2 particles were observed for all the materials upon varying the loading of MgBu2 from 1 to 3 mL (Figure S3c, f and i). Correspondingly, the amount of hydrogen released at high temperatures, i.e. > 300 °C, was found to increase with increasing MgBu2 loading (Figure S7a and b), and this was interpreted as the result of an increasing amount of unsupported MgH2 particles. Further analysis of the particle size distribution of the supported Mg nanoparticles as function of the Mg loading revealed a decrease in the mean particle size for higher MgBu2 loadings; and this was concomitant with a reduction in the temperature for hydrogen release (Figure 5a). It can thus be concluded from this result that smaller Mg particles lead to lower hydrogen release temperatures in agreement with the hypothesis above and previous reports.44-45 Such a reduction in Mg particle size for increasing MgBu2 loadings may be interpreted in the context of the nucleation and growth theory,46 and the decreasing amount of available surface for the stabilization of Mg nuclei. As the surface of the Ni nanobelts gets quickly saturated by the newly formed Mg nuclei resulting from the thermal decomposition of MgBu2, and the excess of nuclei contributes to the formation of unsupported MgH2 particles, there are less Mg nuclei at the surface of the Ni nanobelts available for the growth of larger Mg nanoparticles. Hence, it resulted in the formation of smaller supported Mg particles at high MgBu2 loadings. To further determine the impact of the MgBu2 loading on the hydrogen storage capacity of the Mg-Ni materials at low temperatures, an hydrogenation was carried out at 100 °C under a hydrogen pressure of 6 MPa, and the materials were characterized by TGA/DSC/MS for their 9 ACS Paragon Plus Environment


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hydrogen desorption properties (Figure S7c and d). These results are further summarized in Figure 5b as function of the MgBu2 loading and the hydrogen content at low and high temperatures, i.e. below or higher than 300 °C. From these results, it can be seen that the amount of hydrogen released at low temperatures increases for higher Mg loading (Figure 5b); and this indicates a larger amount of Mg nanoparticles supported at the surface of the Ni nanobelts. However, when considering the ratio of low temperature hydrogen content (related to supported Mg particles on Ni) to high temperature hydrogen content (related to unsupported Mg particles) as determined by MS (Figure S7c), the optimum Mg loading is found to occur for 32%Mg-Ni (Figure 5b). This corresponds to a hydrogen storage capacity of 0.67 mass % below 300 °C, i.e. a hydrogen capacity of 0.67 mass % for the Mg nanoparticles supported on the Ni nanobelts. The difficulty to further increase the portion of supported Mg nanoparticles is due to the limited surface area of the Ni nanobelts. Assuming that the surface of the Ni nanobelts (38 m2 g-1) can be fully covered by Mg nanoparticles of 18 nm, the theoretical loading limit of Mg is 78 mg per 100 mg Ni nanobelts (i.e. 44 mass% of Mg), which corresponds to a theoretical hydrogen storage capacity of 3.3 mass%. The deviation from this ideal assumption is due to the difficulty to fully cover the surface of Ni nanobelts. Nonetheless, considering the loading efficiency per surface area, the Ni nanobelts reported here show a higher loading efficiency (3.07 mg m-2) compared to that of carbon materials ( 300 °C)) of the amount of hydrogen released and the Mg content of the Mg/Ni nanobelts materials. The ratios were determined from the MS analysis (Figure S7d). The Mg content corresponds to the theoretical amount of Mg resulting from the decomposition of varied MgBu2 loadings on the Ni nanobelts.


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Figure 6

Figure 6. Absorption and desorption kinetics of 32% Mg-Ni at 150 and 250°C.



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Figure 7

Figure 7.

(a) PCT curves of the 32% Mg-Ni at 225, 250 and 275°C and (b) the

corresponding van’t Hoff plot.


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