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Novel process for preparation of metal-polymer composite membranes for hydrogen separation
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Novel process for preparation of metal-polymer composite membranes for hydrogen separation D.V. Strugova a, M. Yu Zadorozhnyy a, E.A. Berdonosova b, M. Yu Yablokova b, P.A. Konik b, M.V. Zheleznyi a, D.V. Semenov a, G.S. Milovzorov a, Mahesh Padaki a,c, S.D. Kaloshkin a, V. Yu Zadorozhnyy a, S.N. Klyamkin a,b,* a
National University of Science and Technology «MISIS», Moscow, 119049, Russia Department of Chemistry, M.V. Lomonosov Moscow State University, 119991, Moscow, Russia c Centre for Nano and Material Sciences, Jain University, Jain Global Campus, Kanakapura, Ramanagaram, Bangalore 562112, India b
article info
abstract
Article history:
In the present work, a new preparation method for metal-polymer composite materials for
Received 10 January 2018
hydrogen separation which consist of hydride-forming intermetallic compound LaNi5 and
Received in revised form
polyethylene was developed. According to this technique, the mechanical activation of the
6 April 2018
initial powder mixtures was employed to provide good interface between the phases. A
Accepted 23 April 2018
series of composite membranes with various filler concentrations was synthesized and
Available online xxx
characterized by X-ray diffraction, scanning electron microscopy and differential scanning calorimetry. The gas transport properties of the obtained materials in relation to H2, O2, N2,
Keywords:
CO2 and CH4 were tested. The results indicate that the addition of the hydride-forming
Hydrogen separation membranes
intermetallic compound to the barrier polymer leads to significantly improved selectivity
High-energy ball milling
with respect to hydrogen. The proposed method can be considered as a promising
Intermetallic compound/polymer
approach to producing of high performance composite membranes for hydrogen
composite materials
separation.
Hydrogen storage
© 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Gas transport properties
Introduction Hydrogen is used as an ecologically clean and efficient energy carrier [1,2]. Therefore, the increasing demand for highquality hydrogen fuel is the driving force for developing efficient purification technology. The membrane gas separation has attracted considerable attention due to several advantages not found in other traditional methods [3,4]. Relatively
new and rapidly developing, the membrane technology is environment-friendly, less energy-consuming and more economically efficient compared to other gas separation methods. Moreover, it can be easily combined with other techniques to increase efficiency and economy of the separation processes [5]. Regarding hydrogen separation, palladium-based dense metallic membranes are intensely explored because of their unique selectivity [6e10]. However, palladium undergoes a
* Corresponding author. Department of Chemistry, M.V. Lomonosov Moscow State University, 119991, Moscow, Russia. E-mail address: [email protected] (S.N. Klyamkin). https://doi.org/10.1016/j.ijhydene.2018.04.183 0360-3199/© 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Strugova DV, et al., Novel process for preparation of metal-polymer composite membranes for hydrogen separation, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.04.183
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phase transformation at temperatures below 300 C and at hydrogen pressure above 2 MPa at this temperature. This phenomenon is called hydrogen-induced embrittlement. Moreover, the poisoning of the surface with impurities, especially sulfur compounds, can lead to the significant reduction of permeability. And last, but not the least important, is the expensiveness and economical inefficiency of palladium. The aforementioned problems can be solved using the polymer membrane technology. Currently there is rapid development in the field of gas separation with the use of polymer membranes which account for up to 80% of the global membrane market [11]. The polymer technology is simple and economically efficient, and the variety and wide possibility of functionalization make these materials applicable for different separation processes. Polysulfone (PSf), cellulose acetate (CA), polyethylene (PE), polyimides, and aromatic polyamides (aramids) [12e15] are now widely used in hydrogen separation because of their high mechanical and chemical stability. However, there is a fundamental trade-off between the permeability and the selectivity of polymer membranes [16,17]. To overcome this constraint, the use of co-polymers combining properties of different polymer chains is considered [18,19]. Such materials possess improved permeability and selectivity in hydrogen separation processes. Another way is the modification of polymers by highly selective inorganic fillers such as zeolites [12], porous titanosilicates [20], silica [21,22], carbon nanomaterials [23] or metalorganic frameworks [24] with formation of composite mixed matrix membranes (MMM). A combination of two different components in the material allows developing more efficient membranes which successfully merge technological simplicity of polymers with high selectivity of inorganic substances. The addition of inorganic nanoparticles can also improve physical and mechanical properties of the resulting modified composite membranes. Regarding hydrogen separation, hydride-forming intermetallic compounds (IMCs) can be considered as promising fillers in composite membrane materials due to their high and selective affinity towards hydrogen [25]. For example, the alloy of lanthanum with nickel, LaNi5, reversibly absorbs large amount of hydrogen at room temperature [26,27]. The hydrogen-saturated hydride phase, LaNi5H6, easily decomposes at slight heating or pressure decrease forming the initial intermetallic compound and gaseous hydrogen. Meanwhile, new technological approaches are required in order to create membranes containing hydride-forming IMCs. Conventional techniques for membrane preparation such as diffusion-induced (DIPS), temperature-induced (TIPS) or vapor-induced (VIPS) phase separation lead to unwanted agglomeration of inorganic particles and formation of defect polymer matrix e filler interface. One of potential solutions of this problem is the use of mechanical activation [28,29]. This method is based on strong deformation of processed solid materials that significantly influences the microstructure of the crystal lattice. The mechanical activation is actively employed in the chemistry of
hydrides for their purposeful modification, introduction and homogeneous distribution of alloying additives, formation of non-equilibrium polymetallic compositions and, ultimately, to achieve high reactivity. Internal energy accumulated in the metal hydride materials during high energy ball milling promotes steady bonding between the particle at the subsequent pressing that makes it possible to obtain bulk samples with improved durability in hydrogen sorption-desorption cycles [30,31]. In a series of works [32e34], the use of a ball milling technology for production of metal-polymer composites based on LaNi5-type hydride-forming intermetallic compound and acrylonitrile-butadiene-styrene copolymer was reported. The authors showed that the materials obtained in this way reversibly reacted with hydrogen without any fragmentation. The fixed composite beds obtained in such a manner demonstrated the enhanced thermal conductivity and were successfully employed in hydrogen storage and heat pump systems [34]. An important practical feature of ball milled composites was recently discovered for TiFe/polytetrafluoroethylene system [35]: the polymer coating kept the hydrogen storage ability of the metal hydride while prevented its surface from oxidation. Regardless the above mentioned promising experimental data, the application of mechanical activation processes to membrane preparation has not been yet reported. Thus, the main purpose of this work is to develop a novel method for creating composite membrane materials based on hydride-forming intermetallic compound LaNi5 and polyethylene binder. These membranes combine the high reaction activity with respect to hydrogen, good hydrogen permeability and hydrogen/gas selectivity, and are capable of maintaining the integrity and mechanical strength during repeated hydrogenation-dehydrogenation cycles.
Materials and methods The materials used in this study were low-pressure polyethylene (PE) and hydride-forming intermetallic compound LaNi5. Polyethylene powder was a commercial polymer (Russian State Standard GOST 16338-85, molecular weight ¼ 80000e300000, density ¼ 0.95 g/cm3). LaNi5 was produced from pure metals by conventional arc melting.
Preliminary activation of the intermetallic compound Before the preparation of the composites, LaNi5 ingots were subjected to three hydrogenation-dehydrogenation cycles in order to transform them into fine powder having the fresh and active (not oxidized) surface. This activation procedure was carried out in a home-made Sieverts-type apparatus equipped with a vacuum system and a metal-hydride source of hydrogen (99.9999%). The as-cast alloy was first degassed in vacuum of 102 Torr, and then exposed to hydrogen at 3 MPa and room temperature until the complete hydrogenation occurs. The subsequent dehydrogenation was performed by hydrogen release under dynamic vacuum. The procedure was repeated three times.
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100% crystalline polymer, xm e mass fraction of the filler in the sample.
Membrane preparation In order to study the influence of the filler concentration on properties of the membranes composites containing 0, 10, 50, and 70 wt percent (% wt.) of LaNi5 were prepared. The dehydrogenated intermetallic compound was mixed with the corresponding amount of polyethylene powder and mechanically activated in a high energy water-cooled planetary ball mill AGO-2S. Steel reactors with 4 mm balls were used for processing. The mass ratio of the balls and the processed material was maintained as 10:1. The powder mixtures were treated under argon atmosphere with rotation speed of 840 min1 for 3 min to ensure complete coating of the IMC particles by the polymer, as demonstrated previously in Ref. [35]. Thermo-pressing of the obtained mixtures was performed on air in a vulcanizing hydraulic press APVM-904. The mechanically activated composites with specified PE/LaNi5 ratio were evenly spread on a metal plate, pressed under 1.75 MPa, exposed at 140 C for 40 min, and then cooled to room temperature. To form the dense film membranes with uniform thickness of 150e200 mm, the obtained material was subjected to rolling on laboratory rolls (UB-6175) at 90 C.
Characterization The LaNi5 particle size was determined by size grading method on a laser analyzer Fritsch Analysette-22 Nanotech. The microstructure of the samples was studied by scanning electron microscopy (SEM) using JEOL JSM-6610LV with accelerating voltage of 20 kV. The cross-sections of the membrane films were prepared by “freeze fracturing” at liquid nitrogen in order to get clear images. The surfaces of the samples were sputter-coated with a thin metal layer of platinum in order to produce a conductive surface necessary for scanning electron microscopy. The XRD analysis of the materials was performed using a DRON diffractometer with monochromatic CuKa radiation within the 2q range of 10e120 . Crystal structure and phase composition at different production stages were measured by step scan mode (step of 0.1, collection time of 5 s). The average size of coherent-scattering regions and lattice strains were calculated by a generalized Rietveld method using a RIGAKU pdxl-2 program [36,37]. Annealed and recrystallized silicon powder was used as a reference. The thermal properties of the composites was analyzed by differential scanning calorimetry (NETZSCH DSC 204 F1 Phoenix) within the temperature range from 20 to 180 С with heating rate of 10 С/min under argon flow. According to [38], the DSC method makes possible to evaluate the degree of crystallinity of a polymer from the magnitude of the thermal effect corresponding to its melting process. This parameter is defined as the ratio of the thermal effects of the experimental sample under consideration and the fully crystallized polymer. For composite materials, the mass fraction of a polymer must be taken into account: I¼
DН $100% ð1 xm Þ$Н 100
3
(1)
where I is the defined degree of crystallinity, DH e the melting heat of the experimental sample, H100 e the melting heat of
Gas permeation measurements The gas permeation experiments were conducted using automatic GKSS barometric equipment. It includes a Pfeiffer Vacuum station, a MKS Baratron pressure sensor, and Labview software. The measurements were carried out for O2, N2, CO2, CH4, and H2 at (25 ± 1) С. The gases were supplied at 1 atm. The integral registration mode was used for measurements. The gas permeability of flat membranes was defined as the flux of gas penetrating through the membrane for a given pressure difference: P¼
J$l Dp$A
(2)
where P is the permeability of the membrane, J e the gas flux penetrating through the membrane, Dp e the pressure difference at the opposite sides of the membrane, l e the thickness of the membrane and A e the area of the membrane. Ideal selectivity of the membranes, a, was calculated as the ratio of individual permeabilities related to two different gases.
Results and discussion LaNi5 powder subjected to the hydrogenating activation procedure consists of cracked particles of irregular shape (Fig. 1a) with average particle size of 15e25 mm (Fig. 1b) that correlates with previously reported data [25]. The PE/LaNi5 composite produced by high-energy ball milling is shown in Fig. 2a. As estimated from the SEM images using the method of random secants, the mechanical activation process leads to reduction of the metallic particle size to approximately 10e15 mm. Thickness of the LaNi5 containing composite membranes becomes higher in comparison with plain PE membranes. It ranges from ~80 (0% wt. LaNi5) to ~200 mm (50%; 70% wt. LaNi5). With the increase of the filler concentration, the polymer has less space to spread. Nevertheless, according to cross-section images, the structure of the composite material remains dense, rigid and stiff (Fig. 2b and c). The X-ray diffraction data of the materials at different stages of processing are presented in Fig. 3. As it can be seen, the mechanical activation of the mixtures and the membrane forming do not alter the phase composition: the characteristic peaks of both PE and LaNi5 [39,40] are observed in XRD patterns. The calculated structure data including unit cell parameters, the crystallite size (coherent scattering domain), and the lattice strain (Table 1) correlate with the previously reported ones [40,41] that confirms the aforementioned conclusion. The method of differential scanning calorimetry (DSC) was employed to analyze the thermal properties of the materials under consideration. The obtained results revealed that the PE/LaNi5 metal-polymer composites and membranes are stable up to 120 С: there are no visible thermal effects associated
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Fig. 1 e SEM image (a) and size distribution (b) of LaNi5 powder after hydrogenation-dehydrogenation processing.
Fig. 2 e SEM images: PE þ 50% wt. LaNi5 composite (a) and cross-sections of the metal-polymer membranes PE þ 10% wt. LaNi5 (b), PE þ 50% wt. LaNi5 (c).
Fig. 3 e XRD patterns of the initial substances, their mixture and the composite membrane: PE powder (a); LaNi5 powder after three hydrogenation-dehydrogenation cycles (b); mechanically activated PE þ 50% wt. LaNi5 composite (c); PE þ 50% wt. LaNi5 membrane (d).
with chemical transformations of the components in this temperature range (Fig. 4). For the composite membrane, a clear peak related to PE melting slightly shifts to lower temperatures. This change can be explained by thermal pressing and rolling processes, which affect the degree of crystallinity of polyethylene (Table 2). This parameter was calculated using the Equation (1) taking into account the value of the melting heat of 100% crystalline polyethylene (H100) e 286.4 J/g [42]. Table 3 presents the experimental data on the gas separation properties of the composite PE/LaNi5 membranes with 0; 10; 50 and 70% wt. of the intermetallic filler. We emphasize a drastic increase of hydrogen permeability of the membranes (3 orders of magnitude!) with the addition of LaNi5 while permeabilities related to other gases change less significantly. This fact can be attributed to the peculiar selective activity of this hydride-forming IMC with respect to hydrogen. The proposed producing mode provides good interfacing between PE and LaNi5 that allows the forming of dense structure without defect voids where all gases could easily pass. We have to emphasize the role of mechanical activation of the initial mixture. Behavior of the membranes reported here was compared with those produced from the mixtures of the same composition and by the same procedure except the stage of mechanical activation. In the latter case the membranes demonstrated very low selectivity (i.e. high defectiveness) regardless of the filler concentration. Since mechanical treatment is well known as a strong promoter of the interphase interaction, we suggest this stage is decisive in achieving good conjunction between the polymer matrix and the metallic filler.
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Table 1 e The unit cell parameters, the crystallite size and the lattice strains of LaNi5, PE and composite materials. Phase name PE PE [41] LaNi5 (after hydrogenating activation) LaNi5 [40] PE þ 50% wt. LaNi5 composite PE LaNi5 PE PE þ 50% wt. LaNi5 membrane LaNi5
a, nm
b,nm
c, nm
Crystallite size, nm
Lattice strain, %
0.745 0.742 0.504 0.501 0.742 0.504 0.742 0.504
0.462 0.494 0.504 0.501 0.494 0.504 0.495 0.504
0.256 0.254 0.402 0.398 0.255 0.402 0.258 0.403
e e 40 e e 42 e 36
e e 0.1 e e 0.1 e 0.1
Fig. 4 e DSC data on the PE powder (a), the plain PE membrane (b), the PEþ50% LaNi5 composite (c) and the PEþ50% LaNi5 membrane (d).
Table 2 e Experimental data on the melting heats and degree of crystallinity of polyethylene for the plain PE powder and membrane and the composite PE/LaNi5 mixture and membrane containing 50% wt. LaNi5. Samples
PE powder PE membrane PEþ50% wt. LaNi5 composite PEþ50% wt. LaNi5 membrane
Melting heat DH, J/g
Degree of crystallinity of polyethylene I, %
216.1 206.7 109.1 101.6
75 72 76 71
The data presented in Table 3 clearly indicate that the optimal concentration of the filler in the PE/LaNi5 membranes is 10 % wt. At this concentration, the best ratio of permeability and selectivity of hydrogen is achieved. For the membranes containing 30, 50 and 70% wt. LaNi5, permeability for all gases increases and selectivity deteriorates. Most likely, higher filler concentrations lead to the formation of defects at the metal/ polymer interface that reduce the selectivity of the membrane materials. Analysis of the available published data shows that the composites under consideration, as expected, are inferior in hydrogen selectivity compared to dense palladium-based metal membranes which have the record values a(H2/N2)
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Table 3 e Gas permeability and ideal selectivity values of plain PE and composite PE/LaNi5 membranes. Samples
Permeability P, Barrer (a)
LaNi5% wt. H2
PE [43] PE PE/LaNi5 PE/LaNi5 PE/LaNi5 PE/LaNi5
0 0 10 30 50 70
P
J (b )
1.7 1.84 1320 1508 1720 2156
e 7.1 2700 3200 2600 3300
Ideal selectivity a
O2
N2
CO2
СH4
0.4e0.67 0.34 16 19 31 404
0.17 0.18 20 27 86 431
3.8e5.7 2.22 10 18 43 458
e 0.44 7 8 150 636
H2/O2
H2/N2
H2/CO2
H2/СH4
3.2 5.4 83 79 56 5.3
10 10.2 66 55 20 5.0
0.3 0.8 132 83 40 4.7
e 4.2 188 167 11.5 3.4
The best composition is marked by bold. a The maximum standard deviation of the measurements is 2%. b Permeance, 1015 mol/m2 s Pa.
of up to 10000 [7,8]. We should emphasize here a spectacular difference in the operating temperatures: 400e500 C for palladium membranes instead of room temperature in our work. The expensiveness of the materials must be taken into account, too. Meanwhile, the PE/LaNi5 membranes are very competitive with regard to polymeric ones and polymer-based composites. They noticeably outperform recently reported advanced materials specially designed for € ger's base containing polymers gas separation, namely, Tro [18], poly-(benzoxazole-co-imides) [19], 6FDA-Durene/ZIF-71 MMM [24], both in terms of hydrogen permeability and selectivity.
Conclusions 1. The preparation technology of composite film membrane consisting of hydride-forming IMC LaNi5 and hydrogen permeable polymer PE was developed. The proposed technology is based on the mechanical activation treatment method which helps to achieve the ultra-dispersed activated state of the metal component and its optimal interface with the polymer matrix. 2. The permeability of the obtained membrane composite materials related to H2, O2, N2, CO2, CH4 was determined. It was found that the membrane composite materials with 10% LaNi5 have maximum hydrogen selectivity (188 for H2/ CH4, 132 for H2/CO2, 66 for H2/N2 and 83 for H2/O2). 3. The obtained results indicate that the addition of a hydride-forming intermetallic compound to a polymer matrix is a promising approach to increasing the hydrogen separation performance of membrane materials.
Acknowledgements This work was partly supported by the Ministry of Education and Science of the Russian Federation within the framework of the Increase Competitiveness Program of MISiS (project No. K3-2017-016), by Russian Foundation for Basic Researches, project no. 18-52-53027, and by Russian Science Foundation, project No. 17-73-20272.
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Novel process for preparation of metal-polymer composite membranes for hydrogen separation D.V. Strugova a, M. Yu Zadorozhnyy a, E.A. Berdonosova b, M. Yu Yablokova b, P.A. Konik b, M.V. Zheleznyi a, D.V. Semenov a, G.S. Milovzorov a, Mahesh Padaki a,c, S.D. Kaloshkin a, V. Yu Zadorozhnyy a, S.N. Klyamkin a,b,* a
National University of Science and Technology «MISIS», Moscow, 119049, Russia Department of Chemistry, M.V. Lomonosov Moscow State University, 119991, Moscow, Russia c Centre for Nano and Material Sciences, Jain University, Jain Global Campus, Kanakapura, Ramanagaram, Bangalore 562112, India b
article info
abstract
Article history:
In the present work, a new preparation method for metal-polymer composite materials for
Received 10 January 2018
hydrogen separation which consist of hydride-forming intermetallic compound LaNi5 and
Received in revised form
polyethylene was developed. According to this technique, the mechanical activation of the
6 April 2018
initial powder mixtures was employed to provide good interface between the phases. A
Accepted 23 April 2018
series of composite membranes with various filler concentrations was synthesized and
Available online xxx
characterized by X-ray diffraction, scanning electron microscopy and differential scanning calorimetry. The gas transport properties of the obtained materials in relation to H2, O2, N2,
Keywords:
CO2 and CH4 were tested. The results indicate that the addition of the hydride-forming
Hydrogen separation membranes
intermetallic compound to the barrier polymer leads to significantly improved selectivity
High-energy ball milling
with respect to hydrogen. The proposed method can be considered as a promising
Intermetallic compound/polymer
approach to producing of high performance composite membranes for hydrogen
composite materials
separation.
Hydrogen storage
© 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Gas transport properties
Introduction Hydrogen is used as an ecologically clean and efficient energy carrier [1,2]. Therefore, the increasing demand for highquality hydrogen fuel is the driving force for developing efficient purification technology. The membrane gas separation has attracted considerable attention due to several advantages not found in other traditional methods [3,4]. Relatively
new and rapidly developing, the membrane technology is environment-friendly, less energy-consuming and more economically efficient compared to other gas separation methods. Moreover, it can be easily combined with other techniques to increase efficiency and economy of the separation processes [5]. Regarding hydrogen separation, palladium-based dense metallic membranes are intensely explored because of their unique selectivity [6e10]. However, palladium undergoes a
* Corresponding author. Department of Chemistry, M.V. Lomonosov Moscow State University, 119991, Moscow, Russia. E-mail address: [email protected] (S.N. Klyamkin). https://doi.org/10.1016/j.ijhydene.2018.04.183 0360-3199/© 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Strugova DV, et al., Novel process for preparation of metal-polymer composite membranes for hydrogen separation, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.04.183
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phase transformation at temperatures below 300 C and at hydrogen pressure above 2 MPa at this temperature. This phenomenon is called hydrogen-induced embrittlement. Moreover, the poisoning of the surface with impurities, especially sulfur compounds, can lead to the significant reduction of permeability. And last, but not the least important, is the expensiveness and economical inefficiency of palladium. The aforementioned problems can be solved using the polymer membrane technology. Currently there is rapid development in the field of gas separation with the use of polymer membranes which account for up to 80% of the global membrane market [11]. The polymer technology is simple and economically efficient, and the variety and wide possibility of functionalization make these materials applicable for different separation processes. Polysulfone (PSf), cellulose acetate (CA), polyethylene (PE), polyimides, and aromatic polyamides (aramids) [12e15] are now widely used in hydrogen separation because of their high mechanical and chemical stability. However, there is a fundamental trade-off between the permeability and the selectivity of polymer membranes [16,17]. To overcome this constraint, the use of co-polymers combining properties of different polymer chains is considered [18,19]. Such materials possess improved permeability and selectivity in hydrogen separation processes. Another way is the modification of polymers by highly selective inorganic fillers such as zeolites [12], porous titanosilicates [20], silica [21,22], carbon nanomaterials [23] or metalorganic frameworks [24] with formation of composite mixed matrix membranes (MMM). A combination of two different components in the material allows developing more efficient membranes which successfully merge technological simplicity of polymers with high selectivity of inorganic substances. The addition of inorganic nanoparticles can also improve physical and mechanical properties of the resulting modified composite membranes. Regarding hydrogen separation, hydride-forming intermetallic compounds (IMCs) can be considered as promising fillers in composite membrane materials due to their high and selective affinity towards hydrogen [25]. For example, the alloy of lanthanum with nickel, LaNi5, reversibly absorbs large amount of hydrogen at room temperature [26,27]. The hydrogen-saturated hydride phase, LaNi5H6, easily decomposes at slight heating or pressure decrease forming the initial intermetallic compound and gaseous hydrogen. Meanwhile, new technological approaches are required in order to create membranes containing hydride-forming IMCs. Conventional techniques for membrane preparation such as diffusion-induced (DIPS), temperature-induced (TIPS) or vapor-induced (VIPS) phase separation lead to unwanted agglomeration of inorganic particles and formation of defect polymer matrix e filler interface. One of potential solutions of this problem is the use of mechanical activation [28,29]. This method is based on strong deformation of processed solid materials that significantly influences the microstructure of the crystal lattice. The mechanical activation is actively employed in the chemistry of
hydrides for their purposeful modification, introduction and homogeneous distribution of alloying additives, formation of non-equilibrium polymetallic compositions and, ultimately, to achieve high reactivity. Internal energy accumulated in the metal hydride materials during high energy ball milling promotes steady bonding between the particle at the subsequent pressing that makes it possible to obtain bulk samples with improved durability in hydrogen sorption-desorption cycles [30,31]. In a series of works [32e34], the use of a ball milling technology for production of metal-polymer composites based on LaNi5-type hydride-forming intermetallic compound and acrylonitrile-butadiene-styrene copolymer was reported. The authors showed that the materials obtained in this way reversibly reacted with hydrogen without any fragmentation. The fixed composite beds obtained in such a manner demonstrated the enhanced thermal conductivity and were successfully employed in hydrogen storage and heat pump systems [34]. An important practical feature of ball milled composites was recently discovered for TiFe/polytetrafluoroethylene system [35]: the polymer coating kept the hydrogen storage ability of the metal hydride while prevented its surface from oxidation. Regardless the above mentioned promising experimental data, the application of mechanical activation processes to membrane preparation has not been yet reported. Thus, the main purpose of this work is to develop a novel method for creating composite membrane materials based on hydride-forming intermetallic compound LaNi5 and polyethylene binder. These membranes combine the high reaction activity with respect to hydrogen, good hydrogen permeability and hydrogen/gas selectivity, and are capable of maintaining the integrity and mechanical strength during repeated hydrogenation-dehydrogenation cycles.
Materials and methods The materials used in this study were low-pressure polyethylene (PE) and hydride-forming intermetallic compound LaNi5. Polyethylene powder was a commercial polymer (Russian State Standard GOST 16338-85, molecular weight ¼ 80000e300000, density ¼ 0.95 g/cm3). LaNi5 was produced from pure metals by conventional arc melting.
Preliminary activation of the intermetallic compound Before the preparation of the composites, LaNi5 ingots were subjected to three hydrogenation-dehydrogenation cycles in order to transform them into fine powder having the fresh and active (not oxidized) surface. This activation procedure was carried out in a home-made Sieverts-type apparatus equipped with a vacuum system and a metal-hydride source of hydrogen (99.9999%). The as-cast alloy was first degassed in vacuum of 102 Torr, and then exposed to hydrogen at 3 MPa and room temperature until the complete hydrogenation occurs. The subsequent dehydrogenation was performed by hydrogen release under dynamic vacuum. The procedure was repeated three times.
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100% crystalline polymer, xm e mass fraction of the filler in the sample.
Membrane preparation In order to study the influence of the filler concentration on properties of the membranes composites containing 0, 10, 50, and 70 wt percent (% wt.) of LaNi5 were prepared. The dehydrogenated intermetallic compound was mixed with the corresponding amount of polyethylene powder and mechanically activated in a high energy water-cooled planetary ball mill AGO-2S. Steel reactors with 4 mm balls were used for processing. The mass ratio of the balls and the processed material was maintained as 10:1. The powder mixtures were treated under argon atmosphere with rotation speed of 840 min1 for 3 min to ensure complete coating of the IMC particles by the polymer, as demonstrated previously in Ref. [35]. Thermo-pressing of the obtained mixtures was performed on air in a vulcanizing hydraulic press APVM-904. The mechanically activated composites with specified PE/LaNi5 ratio were evenly spread on a metal plate, pressed under 1.75 MPa, exposed at 140 C for 40 min, and then cooled to room temperature. To form the dense film membranes with uniform thickness of 150e200 mm, the obtained material was subjected to rolling on laboratory rolls (UB-6175) at 90 C.
Characterization The LaNi5 particle size was determined by size grading method on a laser analyzer Fritsch Analysette-22 Nanotech. The microstructure of the samples was studied by scanning electron microscopy (SEM) using JEOL JSM-6610LV with accelerating voltage of 20 kV. The cross-sections of the membrane films were prepared by “freeze fracturing” at liquid nitrogen in order to get clear images. The surfaces of the samples were sputter-coated with a thin metal layer of platinum in order to produce a conductive surface necessary for scanning electron microscopy. The XRD analysis of the materials was performed using a DRON diffractometer with monochromatic CuKa radiation within the 2q range of 10e120 . Crystal structure and phase composition at different production stages were measured by step scan mode (step of 0.1, collection time of 5 s). The average size of coherent-scattering regions and lattice strains were calculated by a generalized Rietveld method using a RIGAKU pdxl-2 program [36,37]. Annealed and recrystallized silicon powder was used as a reference. The thermal properties of the composites was analyzed by differential scanning calorimetry (NETZSCH DSC 204 F1 Phoenix) within the temperature range from 20 to 180 С with heating rate of 10 С/min under argon flow. According to [38], the DSC method makes possible to evaluate the degree of crystallinity of a polymer from the magnitude of the thermal effect corresponding to its melting process. This parameter is defined as the ratio of the thermal effects of the experimental sample under consideration and the fully crystallized polymer. For composite materials, the mass fraction of a polymer must be taken into account: I¼
DН $100% ð1 xm Þ$Н 100
3
(1)
where I is the defined degree of crystallinity, DH e the melting heat of the experimental sample, H100 e the melting heat of
Gas permeation measurements The gas permeation experiments were conducted using automatic GKSS barometric equipment. It includes a Pfeiffer Vacuum station, a MKS Baratron pressure sensor, and Labview software. The measurements were carried out for O2, N2, CO2, CH4, and H2 at (25 ± 1) С. The gases were supplied at 1 atm. The integral registration mode was used for measurements. The gas permeability of flat membranes was defined as the flux of gas penetrating through the membrane for a given pressure difference: P¼
J$l Dp$A
(2)
where P is the permeability of the membrane, J e the gas flux penetrating through the membrane, Dp e the pressure difference at the opposite sides of the membrane, l e the thickness of the membrane and A e the area of the membrane. Ideal selectivity of the membranes, a, was calculated as the ratio of individual permeabilities related to two different gases.
Results and discussion LaNi5 powder subjected to the hydrogenating activation procedure consists of cracked particles of irregular shape (Fig. 1a) with average particle size of 15e25 mm (Fig. 1b) that correlates with previously reported data [25]. The PE/LaNi5 composite produced by high-energy ball milling is shown in Fig. 2a. As estimated from the SEM images using the method of random secants, the mechanical activation process leads to reduction of the metallic particle size to approximately 10e15 mm. Thickness of the LaNi5 containing composite membranes becomes higher in comparison with plain PE membranes. It ranges from ~80 (0% wt. LaNi5) to ~200 mm (50%; 70% wt. LaNi5). With the increase of the filler concentration, the polymer has less space to spread. Nevertheless, according to cross-section images, the structure of the composite material remains dense, rigid and stiff (Fig. 2b and c). The X-ray diffraction data of the materials at different stages of processing are presented in Fig. 3. As it can be seen, the mechanical activation of the mixtures and the membrane forming do not alter the phase composition: the characteristic peaks of both PE and LaNi5 [39,40] are observed in XRD patterns. The calculated structure data including unit cell parameters, the crystallite size (coherent scattering domain), and the lattice strain (Table 1) correlate with the previously reported ones [40,41] that confirms the aforementioned conclusion. The method of differential scanning calorimetry (DSC) was employed to analyze the thermal properties of the materials under consideration. The obtained results revealed that the PE/LaNi5 metal-polymer composites and membranes are stable up to 120 С: there are no visible thermal effects associated
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Fig. 1 e SEM image (a) and size distribution (b) of LaNi5 powder after hydrogenation-dehydrogenation processing.
Fig. 2 e SEM images: PE þ 50% wt. LaNi5 composite (a) and cross-sections of the metal-polymer membranes PE þ 10% wt. LaNi5 (b), PE þ 50% wt. LaNi5 (c).
Fig. 3 e XRD patterns of the initial substances, their mixture and the composite membrane: PE powder (a); LaNi5 powder after three hydrogenation-dehydrogenation cycles (b); mechanically activated PE þ 50% wt. LaNi5 composite (c); PE þ 50% wt. LaNi5 membrane (d).
with chemical transformations of the components in this temperature range (Fig. 4). For the composite membrane, a clear peak related to PE melting slightly shifts to lower temperatures. This change can be explained by thermal pressing and rolling processes, which affect the degree of crystallinity of polyethylene (Table 2). This parameter was calculated using the Equation (1) taking into account the value of the melting heat of 100% crystalline polyethylene (H100) e 286.4 J/g [42]. Table 3 presents the experimental data on the gas separation properties of the composite PE/LaNi5 membranes with 0; 10; 50 and 70% wt. of the intermetallic filler. We emphasize a drastic increase of hydrogen permeability of the membranes (3 orders of magnitude!) with the addition of LaNi5 while permeabilities related to other gases change less significantly. This fact can be attributed to the peculiar selective activity of this hydride-forming IMC with respect to hydrogen. The proposed producing mode provides good interfacing between PE and LaNi5 that allows the forming of dense structure without defect voids where all gases could easily pass. We have to emphasize the role of mechanical activation of the initial mixture. Behavior of the membranes reported here was compared with those produced from the mixtures of the same composition and by the same procedure except the stage of mechanical activation. In the latter case the membranes demonstrated very low selectivity (i.e. high defectiveness) regardless of the filler concentration. Since mechanical treatment is well known as a strong promoter of the interphase interaction, we suggest this stage is decisive in achieving good conjunction between the polymer matrix and the metallic filler.
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Table 1 e The unit cell parameters, the crystallite size and the lattice strains of LaNi5, PE and composite materials. Phase name PE PE [41] LaNi5 (after hydrogenating activation) LaNi5 [40] PE þ 50% wt. LaNi5 composite PE LaNi5 PE PE þ 50% wt. LaNi5 membrane LaNi5
a, nm
b,nm
c, nm
Crystallite size, nm
Lattice strain, %
0.745 0.742 0.504 0.501 0.742 0.504 0.742 0.504
0.462 0.494 0.504 0.501 0.494 0.504 0.495 0.504
0.256 0.254 0.402 0.398 0.255 0.402 0.258 0.403
e e 40 e e 42 e 36
e e 0.1 e e 0.1 e 0.1
Fig. 4 e DSC data on the PE powder (a), the plain PE membrane (b), the PEþ50% LaNi5 composite (c) and the PEþ50% LaNi5 membrane (d).
Table 2 e Experimental data on the melting heats and degree of crystallinity of polyethylene for the plain PE powder and membrane and the composite PE/LaNi5 mixture and membrane containing 50% wt. LaNi5. Samples
PE powder PE membrane PEþ50% wt. LaNi5 composite PEþ50% wt. LaNi5 membrane
Melting heat DH, J/g
Degree of crystallinity of polyethylene I, %
216.1 206.7 109.1 101.6
75 72 76 71
The data presented in Table 3 clearly indicate that the optimal concentration of the filler in the PE/LaNi5 membranes is 10 % wt. At this concentration, the best ratio of permeability and selectivity of hydrogen is achieved. For the membranes containing 30, 50 and 70% wt. LaNi5, permeability for all gases increases and selectivity deteriorates. Most likely, higher filler concentrations lead to the formation of defects at the metal/ polymer interface that reduce the selectivity of the membrane materials. Analysis of the available published data shows that the composites under consideration, as expected, are inferior in hydrogen selectivity compared to dense palladium-based metal membranes which have the record values a(H2/N2)
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Table 3 e Gas permeability and ideal selectivity values of plain PE and composite PE/LaNi5 membranes. Samples
Permeability P, Barrer (a)
LaNi5% wt. H2
PE [43] PE PE/LaNi5 PE/LaNi5 PE/LaNi5 PE/LaNi5
0 0 10 30 50 70
P
J (b )
1.7 1.84 1320 1508 1720 2156
e 7.1 2700 3200 2600 3300
Ideal selectivity a
O2
N2
CO2
СH4
0.4e0.67 0.34 16 19 31 404
0.17 0.18 20 27 86 431
3.8e5.7 2.22 10 18 43 458
e 0.44 7 8 150 636
H2/O2
H2/N2
H2/CO2
H2/СH4
3.2 5.4 83 79 56 5.3
10 10.2 66 55 20 5.0
0.3 0.8 132 83 40 4.7
e 4.2 188 167 11.5 3.4
The best composition is marked by bold. a The maximum standard deviation of the measurements is 2%. b Permeance, 1015 mol/m2 s Pa.
of up to 10000 [7,8]. We should emphasize here a spectacular difference in the operating temperatures: 400e500 C for palladium membranes instead of room temperature in our work. The expensiveness of the materials must be taken into account, too. Meanwhile, the PE/LaNi5 membranes are very competitive with regard to polymeric ones and polymer-based composites. They noticeably outperform recently reported advanced materials specially designed for € ger's base containing polymers gas separation, namely, Tro [18], poly-(benzoxazole-co-imides) [19], 6FDA-Durene/ZIF-71 MMM [24], both in terms of hydrogen permeability and selectivity.
Conclusions 1. The preparation technology of composite film membrane consisting of hydride-forming IMC LaNi5 and hydrogen permeable polymer PE was developed. The proposed technology is based on the mechanical activation treatment method which helps to achieve the ultra-dispersed activated state of the metal component and its optimal interface with the polymer matrix. 2. The permeability of the obtained membrane composite materials related to H2, O2, N2, CO2, CH4 was determined. It was found that the membrane composite materials with 10% LaNi5 have maximum hydrogen selectivity (188 for H2/ CH4, 132 for H2/CO2, 66 for H2/N2 and 83 for H2/O2). 3. The obtained results indicate that the addition of a hydride-forming intermetallic compound to a polymer matrix is a promising approach to increasing the hydrogen separation performance of membrane materials.
Acknowledgements This work was partly supported by the Ministry of Education and Science of the Russian Federation within the framework of the Increase Competitiveness Program of MISiS (project No. K3-2017-016), by Russian Foundation for Basic Researches, project no. 18-52-53027, and by Russian Science Foundation, project No. 17-73-20272.
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