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Low temperature synthesis of LaNi5 nanoparticles for hydrogen storage
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 5 ) 1 e9
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Low temperature synthesis of LaNi5 nanoparticles for hydrogen storage Wei Liu, Kondo-Francois Aguey-Zinsou* MERLin Group, School of Chemical Engineering, The University of New South Wales, Sydney, NSW 2052, Australia
article info
abstract
Article history:
LaNi5 nanoparticles as small as 170 and 250 nm were prepared by combustion and pre-
Received 29 August 2015
cipitation methods followed by calcium hydride reduction, respectively. The formation of
Received in revised form
such small particles was due to the effective reduction of the nanoscale lanthanum/nickel
29 October 2015
oxide precursors at low temperatures (600 C). LaNi5 nanoparticles were found to be stable
Accepted 29 October 2015
and hydrogen cycling did not change their morphology but led to a significant decrease in
Available online xxx
crystallite size reflecting the formation of lattice defects. The reduction of particle size enhanced hydrogen kinetics with full desorption occurring in a few minutes. Particle size
Keywords:
effects on thermodynamics were also investigated by measuring the equilibrium plateau
Hydrogen storage
pressure (Peq) of the LaNi5/H2 reaction at various temperatures. Nanosized LaNi5 showed a
Intermetallic
relatively large hysteresis between Peq for hydrogen absorption and desorption because of
Nanoparticles
lattice defects. However, no significant changes in enthalpy and entropy were observed for
LaNi5
these nanoparticles.
Synthesis
Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Introduction LaNi5 is one of the most investigated intermetallic compounds due to its ability to reversibly store 1.4 mass % of hydrogen at room temperature by electrochemical means or hydrogen pressure. Owing to its low hydrogen storage capacity, applications have mainly been restricted to Ni-metal hydride batteries [1,2]; however LaNi5 remains a very interesting hydrogen storage material. LaNi5 displays magnetic properties [3,4]. LaNi5 has also shown interesting catalytic hydrogenation properties notably for the Sabatier reaction, i.e. the conversion of carbon dioxide into methane [5e7]. In this context, the ability to synthesise nanoparticles of LaNi5 is of prime interest, as increased surface area would lead to higher catalytic reaction rates. Understanding the evolution of the
hydrogen storage properties of LaNi5 at the nanoscale is also highly interesting with respect to particle size effects as a mean to control and tune the hydrogen storage properties of hydrides of much higher storage capacity such as magnesium, lithium and aluminium [8,9]. However, to date LaNi5 is mainly synthesised by high temperature melting methods which are inadequate for the synthesis of nanosized particles. Additional grinding and activation procedures to enable the reversible storage of hydrogen lead to a pulverisation of the alloy but this generally results in large particles in the range of 10e20 mm [10]. Mechanical milling also produces particles in the range of 20e100 mm, with some particles in the range of 100 nm [11]; however this is at the detriment of the hydrogen storage properties of LaNi5. Indeed, the nanostructuration and amorphisation resulting from mechanical milling has been found to be detrimental to the cyclability and hydrogen
* Corresponding author. Tel.: þ61 (0) 2 938 57970; fax: þ61 (0) 2 938 55966. E-mail address: [email protected] (K.-F. Aguey-Zinsou). http://dx.doi.org/10.1016/j.ijhydene.2015.10.128 0360-3199/Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Liu W, Aguey-Zinsou K-F, Low temperature synthesis of LaNi5 nanoparticles for hydrogen storage, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/j.ijhydene.2015.10.128
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storage capacity of LaNi5 [12e15]. Lui et al. reported the gasphase synthesis of 28e58 nm LaNi5 particles, however their hydrogen storage properties were not characterised and gasphase techniques often ensue low yields. An alternative to gas-phase evaporation is through the reduction of lanthanum/nickel oxides nano-precursors synthesised by wet chemistry approaches [31]. Nickel oxides can be readily reduced under hydrogen flow at 350 C [16,17]; however lanthanum oxide requires temperatures in excess of 2000 C. According to previous reports this can be overcome by using calcium hydride (CaH2) as a reducing agent [18]. Hence, Xiao et al. reported the synthesis of 40 mm long rod-shaped LaNi5 upon reduction of a mixed lanthanum nickel oxide phase with calcium hydride [19]. Yasuda et al., also reported large particles of ~30 mm upon the reduction of lanthanum oxide and nickel powder with calcium at 1500 C under a high hydrogen pressure of 1 MPa [32]. Burlakova and Shilkin reported 200 nm LaNi5 nanoparticles prepared by precipitation method and then CaH2 reduction at 1000 C; however no evidences were shown to support this claim [20]. Herein, we report on much softer approaches for the synthesis of LaNi5 nanoparticles. Starting from previous findings, lanthanum nickel oxide phases were synthesised by combustion and precipitation methods and then fully reduced to LaNi5 with CaH2 under hydrogen flow at 600 C only. The physical as well as hydrogen storage properties of the LaNi5 nanoparticles synthesised by both combustion and precipitation are reported.
Material and methods Materials Lanthanum nitrate hexahydrate (La(NO3)3$6H2O), lanthanum chloride heptahydrate (LaCl3$7H2O), nickel nitrate hexahynickel chloride hexahydrate drate (Ni(NO3)2$6H2O), (NiCl2$6H2O), lithium chloride (LiCl), ammonia chloride (NH4Cl) and lanthanum-nickel alloy (LaNi5) were purchased from SigmaeAldrich. Glycine and calcium hydride (CaH2) were from Merck. Sodium hydroxide (NaOH) and sodium carbonate (Na2CO3) were obtained from Univar. All chemicals were used as received.
Synthesis approach LaNi5 nanomaterials were synthesised by two approaches: 1) combustionereduction and 2) precipitationereduction method. Commercial LaNi5 was activated by cycling the alloy at 80 C under 4 MPa hydrogen pressure for absorption and 0.02 MPa for desorption. After five cycles, the commercial alloy was fully activated and pulverised into a fine powder. The activated material was denoted “commercial LaNi5”.
Combustionereduction synthesis Firstly, LaNi5 nanoparticles were synthesised using a two-step modified combustionereduction previously reported [19]. In the first step mixed lanthanum nickel oxide phases were formed by combustion (1), then these phases were reduced to LaNi5 with CaH2 (2). In a typical synthesis, 0.433 g (1 mmol)
La(NO3)3.6H2O, 1.454 g (5 mmol) Ni(NO3)2$6H2O and 0.976 g (7.2 mmol) glycine were dissolved in 10 mL MilliQ water under magnetic stirring. The amounts used corresponded to stoichiometric conditions according to reaction (1), i.e. 4 ¼ 1. 65 4C2 H5 NO2 9 65 1 9 130 4CO2 þ ð4 1ÞO2 0 La2 NiO4 þ NiO þ 4 2 2 9 325 13 4 H2 O þ ð54 þ 9ÞN2 þ 36 þ 18 18
LaðNO3 Þ3 ,6H2 O þ 5NiðNO3 Þ2 ,6H2 O þ
La2 NiO4 þ 9NiO þ 13CaH2 02LaNi5 þ 13CaO þ 13H2
(1)
(2)
The solution was then transferred into an alumina crucible and evaporated at 100 C in an oven to obtain a transparent gel. The combustion was then ignited by increasing the temperature at a rate of 10 C min1 to 500 C and the product was calcined at this temperature for 2 h (reaction 1). The black powder obtained was then finely mixed with CaH2 and LiCl in a mass ratio of 1:2:0.9 in an argon-filled glove box and transferred into a tube furnace. The furnace was then heated from room temperature to 600 C at 5 C min1 and kept at 600 C for 5 h. At this temperature, the material was reduced by reductionediffusion process under a H2 flow of 10 mL min1. At the end of the reduction process (reaction (2)), CaO was removed by suspending the resulting powder in an absolute ethanol solution saturated with ammonia chloride (0.11 M). Following reaction (3), this led to the conversion of insoluble calcium oxide (CaO) into soluble calcium chloride (CaCl2). This was performed in an argon filled glove box to avoid any oxidation of the alloy. CaO þ 2NH4 Cl0CaCl2 þ2NH3 þH2 O
(3)
The suspension was stirred for 6 h, and the material was separated by centrifugation, washed three times with absolute ethanol and finally dried on a Schlenk line overnight to lead LaNi5 nanoparticles.
Precipitationereduction synthesis For comparison purposes, LaNi5 nanoparticles were also prepared by a modified precipitationereduction method reported by Burlakova and supposedly leading to 220 nm LaNi5 particles [21]. At first, mixed lanthanum nickel carbonate phases were formed by precipitation (4), then these phases were reduced in a two-step process to LaNi5 with CaH2 (5 and 6). 2LaCl3 þ 10NiCl2 þ 8Na2 CO3 þ 10NaOH þ 54H2 O0La2 Ni10 ðCO3 Þ8 ðOHÞ10 ,54H2 O þ 26NaCl
(4)
La2 Ni10 ðCO3 Þ8 ðOHÞ10 ,54H2 O þ 15H2 0La2 O3 þ 10Ni þ 8CO2 þ 69H2 O La2 O3 þ 10Ni þ 3CaH2 02LaNi5 þ 3CaO þ 3H2
(5) (6)
In a typical synthesis, 0.186 g (0.5 mmol) LaCl3.6H2O and 0.594 g (2.5 mmol) NiCl2.6H2O were dissolved in a mixture of 10 mL MilliQ water/5 mL acetone and magnetically stirred for 30 min. Then 9.2 mL of a 0.5 M Na2CO3 was added dropwise into the previous mixture until pH ¼ 10 was reached. 2.2 mL of 1.2 M NaOH solution was then slowly added to that solution to
Please cite this article in press as: Liu W, Aguey-Zinsou K-F, Low temperature synthesis of LaNi5 nanoparticles for hydrogen storage, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/j.ijhydene.2015.10.128
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increase the pH to 12. With increasing pH precipitation of yellowish La2Ni10(CO3)8(OH)10$54H2O gradually occurred (reaction (4)). The solution/precipitate mixture was finally allowed to age overnight and then dried in a vacuum oven at 40 C overnight. Reduction was then carried out in two steps. In the first step, La2Ni10(CO3)8(OH)10$54H2O was reduced into metallic nickel and intermediate lanthanum oxides (reaction (5)) at 750 C for 5 h under a flow of hydrogen of 10 mL min1 and a heating ramp of 10 C min1 from room temperature. The material obtained was then further reduced with CaH2 following the procedure described above for the combustion method (reaction (6)). It should be noted that the temperature for this second reduction was 600 C, which is much lower than the 1000 C required in Burlakova's method [21].
Characterisation The size, morphology were determined by Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM). Elemental composition as evaluated by Energy Dispersive X-ray Spectroscopy (EDS). SEM analysis was performed on a FEI Nova NanoSEM 230 FESEM equipped with a Bruker Silicon Drift EDS system at an acceleration voltage of 10.0 kV and a working distance of 5e10 mm. SEM samples were dispersed on a carbon tape on a SEM stub. TEM analysis was done using a Tecnai G2 microscope (FEI) operated at 200 kV. The materials were dispersed in THF, dropped onto a carbon coated copper grid and dried before transfer to the microscope. Particle size distribution was determined from the TEM images. More than 100 particles were manually measured to determine a particle size distribution from the TEM images. The crystalline nature of the materials synthesised 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 (l ¼ 1.541 A) e step size ¼ 0.01, 0.02 or 0.05, time per step ¼ 10 or 20 s/step. The LaNi5 nanoparticles were protected against oxidation from air by a Kapton foil. Hydrogen absorption and desorption kinetics as well as the PressureeComposition Isotherms (PCI) were measured on an automatic Sieverts apparatus (Advanced Materials Corporation). For the kinetic measurements, a hydrogen pressure of 1 MPa was used for absorption and pressures of 0.05 MPa for desorption. Prior PCI measurements the materials were cycled 5 times at room temperature. The PCI measurements were then performed with a step size of 0.02 MPa at 3 different temperatures.
Results and discussion Physical properties Fig. 1 shows the crystalline nature of the materials along the synthesis steps. The combustion method effectively led to the formation of a mixed lanthanum nickel oxide phase (La2NiO4) as per previous reports [19,22]. However, an additional nickel oxide (NiO) phase was detected by XRD and this is inherent to the combustion process which may lead to different oxide phases (Fig. 1a) [23]. In the case of the
Fig. 1 e XRD of (a) the materials obtained along the various synthetic steps for the combustion method, (b) the materials obtained along the various synthetic steps for the precipitation method and (c) the cycled LaNi5 nanoparticles by combustion method and precipitation method.
precipitation method, the material resulting directly from the precipitation was found to be amorphous (Fig. 1b). After a first reduction under hydrogen flow at 750 C, the formation of La2O3 and Ni phases was clearly detected in agreement with previous reports [21]. Further characterisation of these materials by TEM revealed relatively large agglomerates of particles ranging from 15 to 75 nm for the combustion method (Fig. 2a). Similar particle sizes have been reported for LaNiO3 and LaxNiyOz obtained by combustion methods [19,22]. However, the nanoparticles obtained through the precipitation method were much larger and particle size much broader with sizes ranging from 15 to 255 nm (Fig. 2b and c). After the reduction with CaH2, XRD analysis
Please cite this article in press as: Liu W, Aguey-Zinsou K-F, Low temperature synthesis of LaNi5 nanoparticles for hydrogen storage, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/j.ijhydene.2015.10.128
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Fig. 2 e Typical bright field TEM images and size distributions of the precursors obtained from (a) combustion, (b) precipitation and (c) first reduction of (b) under hydrogen at 750 C.
confirmed the effective conversion of the oxide phases previously formed into LaNi5 (Fig. 1a and b). After the removal of CaO no additional phase was detected, and this was used to confirm the effectiveness of the approach in leading to a “pure” LaNi5. Additional analysis of the XRD patterns using the Scherrer equation revealed crystallite sizes of 52 ± 3 nm and 34 ± 3 nm for LaNi5 obtained by combustion and precipitation methods, respectively (Table 1). The nanosize nature of the LaNi5 particles was confirmed by SEM (Fig. 3) and TEM (Fig. 4) analysis. As shown Fig. 3a and b, both methods led to relatively small particles with homogenous lanthanum and nickel distribution as evidenced by
EDS elemental mapping. This was further confirmed by TEM analysis showing nanoparticles with a particle size distribution centred around 170 and 250 nm for the combustion and precipitation method, respectively (Fig. 4a and b). Hence, both approaches led to a significant increase in particle sizes during the CaH2 reduction process. However, these LaNi5 particles remained much smaller than the micron sized particles of commercial LaNi5 produced by conventional melting approaches (Figs. 3c and 4e and f). LaNi5 is conventionally produced by separate extraction and refining of La and Ni, followed by several energy intensive steps including melting, casting, annealing and grinding. The proposed route provides
Please cite this article in press as: Liu W, Aguey-Zinsou K-F, Low temperature synthesis of LaNi5 nanoparticles for hydrogen storage, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/j.ijhydene.2015.10.128
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Table 1 e The plateau pressures, hysteresis factor (Hf ¼ ln (Pabs/Pdes)), enthalpy and entropy of absorption and desorption of LaNi5 obtained from the combustion and precipitation methods, and commercial LaNi5. Material
Crystallite size (±3 nm)
As-synthesised
After cycling
Commercial
e
e
Combustion
52
21
Precipitation
34
25
PCI temp. ( C)
26 50 80 26 50 80 26 50 80
a simpler approach and thus akin the recently reported electro-reduction method [24], could be more-cost effective.
Hydrogen storage properties The hydrogen sorption kinetics and storage capacity of these materials were determined with a Sieverts instrument. As shown Fig. 5, the hydrogen absorption/desorption kinetics of the materials were fast and LaNi5 produced by combustion
Plateau pressure (bar) Abs
des
2.50 5.67 16.09 3.82 8.83 22.29 3.74 9.55 25.02
1.47 4.54 13.15 1.06 3.26 10.28 1.34 4.15 12.78
Hf
0.53 0.22 0.20 1.28 1.00 0.77 1.03 0.83 0.67
Enthalpy (DH) (kJ mol1 H2)
Entropy (DS) (J K1 mol1 H2)
Abs
des
Abs
des
30 ± 2
36 ± 1
109 ± 5
122 ± 4
28 ± 1
37 ± 1
107 ± 1
124 ± 1
31 ± 1
38 ± 1
114 ± 1
125 ± 2
method displayed the fastest hydrogen kinetics. It took less than 5 min to carry out full hydrogen absorption or desorption. Furthermore, the hydrogen kinetics of the LaNi5 prepared by combustion and precipitation methods were found to be significantly faster than that of commercial LaNi5, and this was attributed to their nanosize regime. Hence, the smaller particles of LaNi5 obtained by combustion displayed the fastest hydrogen kinetics. However, the storage capacity of LaNi5 obtained by combustion was the lowest, i.e. 1 mass% H2 (Fig. 5).
Fig. 3 e Typical SEM images of (a) as-synthesised LaNi5 by combustion method, (b) as-synthesised LaNi5 by precipitation, (c) commercial LaNi5, and associated elemental mapping of (a) and (b). Please cite this article in press as: Liu W, Aguey-Zinsou K-F, Low temperature synthesis of LaNi5 nanoparticles for hydrogen storage, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/j.ijhydene.2015.10.128
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Fig. 4 e Typical bright field TEM images and size distributions of (a) as-synthesised LaNi5 produced by combustion method (b) as-synthesised LaNi5 produced by precipitation method, (c) cycled LaNi5 produced by combustion method, (d) cycled LaNi5 produced by precipitation method, and associated EDS spectra of (a) and (b). Typical TEM images of commercial LaNi5 are included in (e and f) for comparison.
Further characterisation by XRD of materials after hydrogen cycling confirmed the stability of the LaNi5 phase synthesised (Fig. 1c). However, determination of the crystallite size from these XRD patterns revealed a broadening of the LaNi5 peak and a reduction of the crystallite size in both materials to around 20 nm (Table 1). Such a broadening has previously been reported for bulk LaNi5 as a result of hydrogen cycling and the associated pulverisation and generation of lattice defects leading to an amorphisation and thus degradation of the hydrogen storage properties and capacity of LaNi5 [13,25,26]. Indeed, characterisation by TEM of the materials after hydrogen cycling revealed a few dislocations and within the nanoparticles small crystalline domains surrounded by lattice defects (Fig. 6). However, hydrogen cycling did not affect the morphology and particle size distribution of the LaNi5 nanoparticles (Fig. 4c and d). The reduction in
crystallite sizes was thus attributed to lattice defects introduced into nanosized LaNi5 during hydrogen cycling. It can thus be expected that, akin bulk LaNi5, the hydrogen sorption properties of nanosized LaNi5 would degrade upon extensive hydrogen cycling. This result may also explain the lower storage capacity observed for LaNi5 material obtained by combustion as compared to LaNi5 formed by precipitation, since the decrease of the crystallite size upon hydrogen cycling is much larger for the combusted materials, i.e. from 52 to 21 ± 3 nm (Table 1). It is noteworthy that this behaviour is different to that of the MgeH system, since crystalline growth is usually observed for nanocrystalline magnesium upon hydrogen cycling [27,28]. Potential evolutions of the enthalpy (DH) and entropy (DS) associated with the hydrogen absorption/desorption process were also determined by PCI measurements and associated
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lattice defects as previously discussed (Fig. 6). As summarised Table 1, DH and DS were found to be relatively similar for all materials and close to that of commercial LaNi5. Accordingly for such relatively large LaNi5 nanoparticles, no change in thermodynamic properties was observed, which is in agreement with previous reports suggesting thermodynamic shifts for particle sizes below 20 nm [29,30].
Conclusions
Fig. 5 e Hydrogen absorption (abs) and desorption (des) kinetics for commercial LaNi5 and LaNi5 obtained by combustion and precipitation method. Kinetics were measured at 26 C and under 1 MPa hydrogen pressure for absorption and 0.05 MPa for desorption.
Van't Hoff plots (Fig. 7 and 8). As shown Fig. 7, all the materials showed a single plateau pressure with some relatively large hysteresis between the absorption and desorption for the nanosized LaNi5 which was attributed to the generation of
In this paper, we report the synthesis of LaNi5 nanoparticles prepared by combustionereduction and precipitationereduction method, respectively. The small particle size of the lanthanum/nickel oxide precursors (15e255 nm) and the use of lithium chloride as a melting agent were essential for the low temperature (600 C) reduction of the precursors and for obtaining nanosized LaNi5 (37e450 nm). These LaNi5 nanoparticles were very stable upon hydrogen cycling and their morphology remained unchanged. However upon hydrogen cycling, the LaNi5 nanoparticles showed a large hysteresis in their plateau pressure and XRD peak broadening which was attributed to the formation of lattice defects. Extensive hydrogen cycling could lead to a
Fig. 6 e Typical bright field TEM images of the materials after hydrogen cycling a)-b) LaNi5 produced by combustion method, c)-d) LaNi5 produced by precipitation method. The arrows indicate dislocations and the circles nanocrystalline domains surrounded by lattice defects. Please cite this article in press as: Liu W, Aguey-Zinsou K-F, Low temperature synthesis of LaNi5 nanoparticles for hydrogen storage, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/j.ijhydene.2015.10.128
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Fig. 8 e Van't Hoff plots as determined from the PCI measurements for commercial LaNi5 and LaNi5 produced by combustion and precipitation method.
Acknowledgements The authors gratefully acknowledge the financial support by the UNSW Internal Research Grant program. We appreciate the use of instruments in the Mark Wainwright Analytical Centre at UNSW.
references
Fig. 7 e PCI curves of LaNi5 a) commercial, b) prepared by combustion, and c) prepared by precipitation method.
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Low temperature synthesis of LaNi5 nanoparticles for hydrogen storage Wei Liu, Kondo-Francois Aguey-Zinsou* MERLin Group, School of Chemical Engineering, The University of New South Wales, Sydney, NSW 2052, Australia
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abstract
Article history:
LaNi5 nanoparticles as small as 170 and 250 nm were prepared by combustion and pre-
Received 29 August 2015
cipitation methods followed by calcium hydride reduction, respectively. The formation of
Received in revised form
such small particles was due to the effective reduction of the nanoscale lanthanum/nickel
29 October 2015
oxide precursors at low temperatures (600 C). LaNi5 nanoparticles were found to be stable
Accepted 29 October 2015
and hydrogen cycling did not change their morphology but led to a significant decrease in
Available online xxx
crystallite size reflecting the formation of lattice defects. The reduction of particle size enhanced hydrogen kinetics with full desorption occurring in a few minutes. Particle size
Keywords:
effects on thermodynamics were also investigated by measuring the equilibrium plateau
Hydrogen storage
pressure (Peq) of the LaNi5/H2 reaction at various temperatures. Nanosized LaNi5 showed a
Intermetallic
relatively large hysteresis between Peq for hydrogen absorption and desorption because of
Nanoparticles
lattice defects. However, no significant changes in enthalpy and entropy were observed for
LaNi5
these nanoparticles.
Synthesis
Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Introduction LaNi5 is one of the most investigated intermetallic compounds due to its ability to reversibly store 1.4 mass % of hydrogen at room temperature by electrochemical means or hydrogen pressure. Owing to its low hydrogen storage capacity, applications have mainly been restricted to Ni-metal hydride batteries [1,2]; however LaNi5 remains a very interesting hydrogen storage material. LaNi5 displays magnetic properties [3,4]. LaNi5 has also shown interesting catalytic hydrogenation properties notably for the Sabatier reaction, i.e. the conversion of carbon dioxide into methane [5e7]. In this context, the ability to synthesise nanoparticles of LaNi5 is of prime interest, as increased surface area would lead to higher catalytic reaction rates. Understanding the evolution of the
hydrogen storage properties of LaNi5 at the nanoscale is also highly interesting with respect to particle size effects as a mean to control and tune the hydrogen storage properties of hydrides of much higher storage capacity such as magnesium, lithium and aluminium [8,9]. However, to date LaNi5 is mainly synthesised by high temperature melting methods which are inadequate for the synthesis of nanosized particles. Additional grinding and activation procedures to enable the reversible storage of hydrogen lead to a pulverisation of the alloy but this generally results in large particles in the range of 10e20 mm [10]. Mechanical milling also produces particles in the range of 20e100 mm, with some particles in the range of 100 nm [11]; however this is at the detriment of the hydrogen storage properties of LaNi5. Indeed, the nanostructuration and amorphisation resulting from mechanical milling has been found to be detrimental to the cyclability and hydrogen
* Corresponding author. Tel.: þ61 (0) 2 938 57970; fax: þ61 (0) 2 938 55966. E-mail address: [email protected] (K.-F. Aguey-Zinsou). http://dx.doi.org/10.1016/j.ijhydene.2015.10.128 0360-3199/Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Liu W, Aguey-Zinsou K-F, Low temperature synthesis of LaNi5 nanoparticles for hydrogen storage, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/j.ijhydene.2015.10.128
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storage capacity of LaNi5 [12e15]. Lui et al. reported the gasphase synthesis of 28e58 nm LaNi5 particles, however their hydrogen storage properties were not characterised and gasphase techniques often ensue low yields. An alternative to gas-phase evaporation is through the reduction of lanthanum/nickel oxides nano-precursors synthesised by wet chemistry approaches [31]. Nickel oxides can be readily reduced under hydrogen flow at 350 C [16,17]; however lanthanum oxide requires temperatures in excess of 2000 C. According to previous reports this can be overcome by using calcium hydride (CaH2) as a reducing agent [18]. Hence, Xiao et al. reported the synthesis of 40 mm long rod-shaped LaNi5 upon reduction of a mixed lanthanum nickel oxide phase with calcium hydride [19]. Yasuda et al., also reported large particles of ~30 mm upon the reduction of lanthanum oxide and nickel powder with calcium at 1500 C under a high hydrogen pressure of 1 MPa [32]. Burlakova and Shilkin reported 200 nm LaNi5 nanoparticles prepared by precipitation method and then CaH2 reduction at 1000 C; however no evidences were shown to support this claim [20]. Herein, we report on much softer approaches for the synthesis of LaNi5 nanoparticles. Starting from previous findings, lanthanum nickel oxide phases were synthesised by combustion and precipitation methods and then fully reduced to LaNi5 with CaH2 under hydrogen flow at 600 C only. The physical as well as hydrogen storage properties of the LaNi5 nanoparticles synthesised by both combustion and precipitation are reported.
Material and methods Materials Lanthanum nitrate hexahydrate (La(NO3)3$6H2O), lanthanum chloride heptahydrate (LaCl3$7H2O), nickel nitrate hexahynickel chloride hexahydrate drate (Ni(NO3)2$6H2O), (NiCl2$6H2O), lithium chloride (LiCl), ammonia chloride (NH4Cl) and lanthanum-nickel alloy (LaNi5) were purchased from SigmaeAldrich. Glycine and calcium hydride (CaH2) were from Merck. Sodium hydroxide (NaOH) and sodium carbonate (Na2CO3) were obtained from Univar. All chemicals were used as received.
Synthesis approach LaNi5 nanomaterials were synthesised by two approaches: 1) combustionereduction and 2) precipitationereduction method. Commercial LaNi5 was activated by cycling the alloy at 80 C under 4 MPa hydrogen pressure for absorption and 0.02 MPa for desorption. After five cycles, the commercial alloy was fully activated and pulverised into a fine powder. The activated material was denoted “commercial LaNi5”.
Combustionereduction synthesis Firstly, LaNi5 nanoparticles were synthesised using a two-step modified combustionereduction previously reported [19]. In the first step mixed lanthanum nickel oxide phases were formed by combustion (1), then these phases were reduced to LaNi5 with CaH2 (2). In a typical synthesis, 0.433 g (1 mmol)
La(NO3)3.6H2O, 1.454 g (5 mmol) Ni(NO3)2$6H2O and 0.976 g (7.2 mmol) glycine were dissolved in 10 mL MilliQ water under magnetic stirring. The amounts used corresponded to stoichiometric conditions according to reaction (1), i.e. 4 ¼ 1. 65 4C2 H5 NO2 9 65 1 9 130 4CO2 þ ð4 1ÞO2 0 La2 NiO4 þ NiO þ 4 2 2 9 325 13 4 H2 O þ ð54 þ 9ÞN2 þ 36 þ 18 18
LaðNO3 Þ3 ,6H2 O þ 5NiðNO3 Þ2 ,6H2 O þ
La2 NiO4 þ 9NiO þ 13CaH2 02LaNi5 þ 13CaO þ 13H2
(1)
(2)
The solution was then transferred into an alumina crucible and evaporated at 100 C in an oven to obtain a transparent gel. The combustion was then ignited by increasing the temperature at a rate of 10 C min1 to 500 C and the product was calcined at this temperature for 2 h (reaction 1). The black powder obtained was then finely mixed with CaH2 and LiCl in a mass ratio of 1:2:0.9 in an argon-filled glove box and transferred into a tube furnace. The furnace was then heated from room temperature to 600 C at 5 C min1 and kept at 600 C for 5 h. At this temperature, the material was reduced by reductionediffusion process under a H2 flow of 10 mL min1. At the end of the reduction process (reaction (2)), CaO was removed by suspending the resulting powder in an absolute ethanol solution saturated with ammonia chloride (0.11 M). Following reaction (3), this led to the conversion of insoluble calcium oxide (CaO) into soluble calcium chloride (CaCl2). This was performed in an argon filled glove box to avoid any oxidation of the alloy. CaO þ 2NH4 Cl0CaCl2 þ2NH3 þH2 O
(3)
The suspension was stirred for 6 h, and the material was separated by centrifugation, washed three times with absolute ethanol and finally dried on a Schlenk line overnight to lead LaNi5 nanoparticles.
Precipitationereduction synthesis For comparison purposes, LaNi5 nanoparticles were also prepared by a modified precipitationereduction method reported by Burlakova and supposedly leading to 220 nm LaNi5 particles [21]. At first, mixed lanthanum nickel carbonate phases were formed by precipitation (4), then these phases were reduced in a two-step process to LaNi5 with CaH2 (5 and 6). 2LaCl3 þ 10NiCl2 þ 8Na2 CO3 þ 10NaOH þ 54H2 O0La2 Ni10 ðCO3 Þ8 ðOHÞ10 ,54H2 O þ 26NaCl
(4)
La2 Ni10 ðCO3 Þ8 ðOHÞ10 ,54H2 O þ 15H2 0La2 O3 þ 10Ni þ 8CO2 þ 69H2 O La2 O3 þ 10Ni þ 3CaH2 02LaNi5 þ 3CaO þ 3H2
(5) (6)
In a typical synthesis, 0.186 g (0.5 mmol) LaCl3.6H2O and 0.594 g (2.5 mmol) NiCl2.6H2O were dissolved in a mixture of 10 mL MilliQ water/5 mL acetone and magnetically stirred for 30 min. Then 9.2 mL of a 0.5 M Na2CO3 was added dropwise into the previous mixture until pH ¼ 10 was reached. 2.2 mL of 1.2 M NaOH solution was then slowly added to that solution to
Please cite this article in press as: Liu W, Aguey-Zinsou K-F, Low temperature synthesis of LaNi5 nanoparticles for hydrogen storage, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/j.ijhydene.2015.10.128
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increase the pH to 12. With increasing pH precipitation of yellowish La2Ni10(CO3)8(OH)10$54H2O gradually occurred (reaction (4)). The solution/precipitate mixture was finally allowed to age overnight and then dried in a vacuum oven at 40 C overnight. Reduction was then carried out in two steps. In the first step, La2Ni10(CO3)8(OH)10$54H2O was reduced into metallic nickel and intermediate lanthanum oxides (reaction (5)) at 750 C for 5 h under a flow of hydrogen of 10 mL min1 and a heating ramp of 10 C min1 from room temperature. The material obtained was then further reduced with CaH2 following the procedure described above for the combustion method (reaction (6)). It should be noted that the temperature for this second reduction was 600 C, which is much lower than the 1000 C required in Burlakova's method [21].
Characterisation The size, morphology were determined by Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM). Elemental composition as evaluated by Energy Dispersive X-ray Spectroscopy (EDS). SEM analysis was performed on a FEI Nova NanoSEM 230 FESEM equipped with a Bruker Silicon Drift EDS system at an acceleration voltage of 10.0 kV and a working distance of 5e10 mm. SEM samples were dispersed on a carbon tape on a SEM stub. TEM analysis was done using a Tecnai G2 microscope (FEI) operated at 200 kV. The materials were dispersed in THF, dropped onto a carbon coated copper grid and dried before transfer to the microscope. Particle size distribution was determined from the TEM images. More than 100 particles were manually measured to determine a particle size distribution from the TEM images. The crystalline nature of the materials synthesised 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 (l ¼ 1.541 A) e step size ¼ 0.01, 0.02 or 0.05, time per step ¼ 10 or 20 s/step. The LaNi5 nanoparticles were protected against oxidation from air by a Kapton foil. Hydrogen absorption and desorption kinetics as well as the PressureeComposition Isotherms (PCI) were measured on an automatic Sieverts apparatus (Advanced Materials Corporation). For the kinetic measurements, a hydrogen pressure of 1 MPa was used for absorption and pressures of 0.05 MPa for desorption. Prior PCI measurements the materials were cycled 5 times at room temperature. The PCI measurements were then performed with a step size of 0.02 MPa at 3 different temperatures.
Results and discussion Physical properties Fig. 1 shows the crystalline nature of the materials along the synthesis steps. The combustion method effectively led to the formation of a mixed lanthanum nickel oxide phase (La2NiO4) as per previous reports [19,22]. However, an additional nickel oxide (NiO) phase was detected by XRD and this is inherent to the combustion process which may lead to different oxide phases (Fig. 1a) [23]. In the case of the
Fig. 1 e XRD of (a) the materials obtained along the various synthetic steps for the combustion method, (b) the materials obtained along the various synthetic steps for the precipitation method and (c) the cycled LaNi5 nanoparticles by combustion method and precipitation method.
precipitation method, the material resulting directly from the precipitation was found to be amorphous (Fig. 1b). After a first reduction under hydrogen flow at 750 C, the formation of La2O3 and Ni phases was clearly detected in agreement with previous reports [21]. Further characterisation of these materials by TEM revealed relatively large agglomerates of particles ranging from 15 to 75 nm for the combustion method (Fig. 2a). Similar particle sizes have been reported for LaNiO3 and LaxNiyOz obtained by combustion methods [19,22]. However, the nanoparticles obtained through the precipitation method were much larger and particle size much broader with sizes ranging from 15 to 255 nm (Fig. 2b and c). After the reduction with CaH2, XRD analysis
Please cite this article in press as: Liu W, Aguey-Zinsou K-F, Low temperature synthesis of LaNi5 nanoparticles for hydrogen storage, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/j.ijhydene.2015.10.128
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Fig. 2 e Typical bright field TEM images and size distributions of the precursors obtained from (a) combustion, (b) precipitation and (c) first reduction of (b) under hydrogen at 750 C.
confirmed the effective conversion of the oxide phases previously formed into LaNi5 (Fig. 1a and b). After the removal of CaO no additional phase was detected, and this was used to confirm the effectiveness of the approach in leading to a “pure” LaNi5. Additional analysis of the XRD patterns using the Scherrer equation revealed crystallite sizes of 52 ± 3 nm and 34 ± 3 nm for LaNi5 obtained by combustion and precipitation methods, respectively (Table 1). The nanosize nature of the LaNi5 particles was confirmed by SEM (Fig. 3) and TEM (Fig. 4) analysis. As shown Fig. 3a and b, both methods led to relatively small particles with homogenous lanthanum and nickel distribution as evidenced by
EDS elemental mapping. This was further confirmed by TEM analysis showing nanoparticles with a particle size distribution centred around 170 and 250 nm for the combustion and precipitation method, respectively (Fig. 4a and b). Hence, both approaches led to a significant increase in particle sizes during the CaH2 reduction process. However, these LaNi5 particles remained much smaller than the micron sized particles of commercial LaNi5 produced by conventional melting approaches (Figs. 3c and 4e and f). LaNi5 is conventionally produced by separate extraction and refining of La and Ni, followed by several energy intensive steps including melting, casting, annealing and grinding. The proposed route provides
Please cite this article in press as: Liu W, Aguey-Zinsou K-F, Low temperature synthesis of LaNi5 nanoparticles for hydrogen storage, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/j.ijhydene.2015.10.128
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Table 1 e The plateau pressures, hysteresis factor (Hf ¼ ln (Pabs/Pdes)), enthalpy and entropy of absorption and desorption of LaNi5 obtained from the combustion and precipitation methods, and commercial LaNi5. Material
Crystallite size (±3 nm)
As-synthesised
After cycling
Commercial
e
e
Combustion
52
21
Precipitation
34
25
PCI temp. ( C)
26 50 80 26 50 80 26 50 80
a simpler approach and thus akin the recently reported electro-reduction method [24], could be more-cost effective.
Hydrogen storage properties The hydrogen sorption kinetics and storage capacity of these materials were determined with a Sieverts instrument. As shown Fig. 5, the hydrogen absorption/desorption kinetics of the materials were fast and LaNi5 produced by combustion
Plateau pressure (bar) Abs
des
2.50 5.67 16.09 3.82 8.83 22.29 3.74 9.55 25.02
1.47 4.54 13.15 1.06 3.26 10.28 1.34 4.15 12.78
Hf
0.53 0.22 0.20 1.28 1.00 0.77 1.03 0.83 0.67
Enthalpy (DH) (kJ mol1 H2)
Entropy (DS) (J K1 mol1 H2)
Abs
des
Abs
des
30 ± 2
36 ± 1
109 ± 5
122 ± 4
28 ± 1
37 ± 1
107 ± 1
124 ± 1
31 ± 1
38 ± 1
114 ± 1
125 ± 2
method displayed the fastest hydrogen kinetics. It took less than 5 min to carry out full hydrogen absorption or desorption. Furthermore, the hydrogen kinetics of the LaNi5 prepared by combustion and precipitation methods were found to be significantly faster than that of commercial LaNi5, and this was attributed to their nanosize regime. Hence, the smaller particles of LaNi5 obtained by combustion displayed the fastest hydrogen kinetics. However, the storage capacity of LaNi5 obtained by combustion was the lowest, i.e. 1 mass% H2 (Fig. 5).
Fig. 3 e Typical SEM images of (a) as-synthesised LaNi5 by combustion method, (b) as-synthesised LaNi5 by precipitation, (c) commercial LaNi5, and associated elemental mapping of (a) and (b). Please cite this article in press as: Liu W, Aguey-Zinsou K-F, Low temperature synthesis of LaNi5 nanoparticles for hydrogen storage, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/j.ijhydene.2015.10.128
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Fig. 4 e Typical bright field TEM images and size distributions of (a) as-synthesised LaNi5 produced by combustion method (b) as-synthesised LaNi5 produced by precipitation method, (c) cycled LaNi5 produced by combustion method, (d) cycled LaNi5 produced by precipitation method, and associated EDS spectra of (a) and (b). Typical TEM images of commercial LaNi5 are included in (e and f) for comparison.
Further characterisation by XRD of materials after hydrogen cycling confirmed the stability of the LaNi5 phase synthesised (Fig. 1c). However, determination of the crystallite size from these XRD patterns revealed a broadening of the LaNi5 peak and a reduction of the crystallite size in both materials to around 20 nm (Table 1). Such a broadening has previously been reported for bulk LaNi5 as a result of hydrogen cycling and the associated pulverisation and generation of lattice defects leading to an amorphisation and thus degradation of the hydrogen storage properties and capacity of LaNi5 [13,25,26]. Indeed, characterisation by TEM of the materials after hydrogen cycling revealed a few dislocations and within the nanoparticles small crystalline domains surrounded by lattice defects (Fig. 6). However, hydrogen cycling did not affect the morphology and particle size distribution of the LaNi5 nanoparticles (Fig. 4c and d). The reduction in
crystallite sizes was thus attributed to lattice defects introduced into nanosized LaNi5 during hydrogen cycling. It can thus be expected that, akin bulk LaNi5, the hydrogen sorption properties of nanosized LaNi5 would degrade upon extensive hydrogen cycling. This result may also explain the lower storage capacity observed for LaNi5 material obtained by combustion as compared to LaNi5 formed by precipitation, since the decrease of the crystallite size upon hydrogen cycling is much larger for the combusted materials, i.e. from 52 to 21 ± 3 nm (Table 1). It is noteworthy that this behaviour is different to that of the MgeH system, since crystalline growth is usually observed for nanocrystalline magnesium upon hydrogen cycling [27,28]. Potential evolutions of the enthalpy (DH) and entropy (DS) associated with the hydrogen absorption/desorption process were also determined by PCI measurements and associated
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lattice defects as previously discussed (Fig. 6). As summarised Table 1, DH and DS were found to be relatively similar for all materials and close to that of commercial LaNi5. Accordingly for such relatively large LaNi5 nanoparticles, no change in thermodynamic properties was observed, which is in agreement with previous reports suggesting thermodynamic shifts for particle sizes below 20 nm [29,30].
Conclusions
Fig. 5 e Hydrogen absorption (abs) and desorption (des) kinetics for commercial LaNi5 and LaNi5 obtained by combustion and precipitation method. Kinetics were measured at 26 C and under 1 MPa hydrogen pressure for absorption and 0.05 MPa for desorption.
Van't Hoff plots (Fig. 7 and 8). As shown Fig. 7, all the materials showed a single plateau pressure with some relatively large hysteresis between the absorption and desorption for the nanosized LaNi5 which was attributed to the generation of
In this paper, we report the synthesis of LaNi5 nanoparticles prepared by combustionereduction and precipitationereduction method, respectively. The small particle size of the lanthanum/nickel oxide precursors (15e255 nm) and the use of lithium chloride as a melting agent were essential for the low temperature (600 C) reduction of the precursors and for obtaining nanosized LaNi5 (37e450 nm). These LaNi5 nanoparticles were very stable upon hydrogen cycling and their morphology remained unchanged. However upon hydrogen cycling, the LaNi5 nanoparticles showed a large hysteresis in their plateau pressure and XRD peak broadening which was attributed to the formation of lattice defects. Extensive hydrogen cycling could lead to a
Fig. 6 e Typical bright field TEM images of the materials after hydrogen cycling a)-b) LaNi5 produced by combustion method, c)-d) LaNi5 produced by precipitation method. The arrows indicate dislocations and the circles nanocrystalline domains surrounded by lattice defects. Please cite this article in press as: Liu W, Aguey-Zinsou K-F, Low temperature synthesis of LaNi5 nanoparticles for hydrogen storage, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/j.ijhydene.2015.10.128
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Fig. 8 e Van't Hoff plots as determined from the PCI measurements for commercial LaNi5 and LaNi5 produced by combustion and precipitation method.
Acknowledgements The authors gratefully acknowledge the financial support by the UNSW Internal Research Grant program. We appreciate the use of instruments in the Mark Wainwright Analytical Centre at UNSW.
references
Fig. 7 e PCI curves of LaNi5 a) commercial, b) prepared by combustion, and c) prepared by precipitation method.
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