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Development of polymer nanocomposites with sodium alanate for hydrogen storage
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Development of polymer nanocomposites with sodium alanate for hydrogen storage Cesar Augusto Gonc¸alves Beatrice a,*, Bruna Rodrigues Moreira a, Amanda Dantas de Oliveira b, Fabio Roberto Passador c, Gabriel Rodrigues de Almeida Neto a, Daniel Rodrigo Leiva a, Luiz Antonio Pessan a a
Department of Materials Engineering, Graduate Program in Materials Science and Engineering, Federal University ~o Carlos, Rodovia Washington Luiz, km 235, 13565-905, Sa ~o Carlos, SP, Brazil of Sa b Centre of Technology Development, Federal University of Pelotas, Rua Gomes Carneiro, 1, 96010-610, Pelotas, RS, Brazil c ~o Paulo, Avenida Cesare Monsueto Giulio Lattes, 1201, Institute of Science and Technology, Federal University of Sa ~o Jose dos Campos, SP, Brazil 12247-014, Sa
highlights Polymer nanocomposites were successfully prepared by solvent-based techniques. PEIS/NaAlH4 (70/30 wt%) can store 1.1 wt% of H2 after 12 h at 120 C and 32 bar. Pani microspheres with NaAlH4 dispersed into them were obtained after spray drying. 50 wt% of NaAlH4 yielded a 67% higher H2 sorption capacity than neat Pani after 6 h in the same conditions.
article info
abstract
Article history:
The development of materials based on polymer nanocomposites for hydrogen storage with
Received 16 March 2019
lower temperature of desorption might contribute to the consolidation of the use of
Received in revised form
hydrogen as a sustainable energy. The purpose of this work was to develop hybrid porous
10 May 2019
materials consisting of polyaniline or sulfonated polyetherimide as polymer matrices and a
Accepted 24 June 2019
potential hydride for hydrogen storage e sodium alanate. Multiwall carbon nanotubes and
Available online xxx
titanium dioxide were also added in order to improve the hydrogen absorption capacity of the sodium alanate. The nanocomposites were prepared via solution mixing and analyzed by
Keywords:
differential scanning calorimetry, thermogravimetric analysis, transmission and scanning
Storage
electron microscopy and kinetic of hydrogen sorption. The nanoparticles had some influence
Nanocomposite
on the polymers structures, modifying its thermal properties, such as glass transition tem-
Polyaniline
perature and the onset temperature of degradation. Microscope analyses showed that not all
Polyetherimide
the particles were always well dispersed and distributed through the matrices. However,
Sodium alanate
kinetics of hydrogen sorption tests indicated a significant amount of hydrogen (up to 1.2 wt %) in the nanocomposites after 12 h at relatively low temperature (120 C) and 32 bar. © 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
* Corresponding author. E-mail address: [email protected] (C.A.G. Beatrice). https://doi.org/10.1016/j.ijhydene.2019.06.169 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article as: Beatrice CAG et al., Development of polymer nanocomposites with sodium alanate for hydrogen storage, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.06.169
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Introduction In an age where there is a growing demand for energy and fossil fuels are still the major energy source is alarming. To explore sustainable clean energy sources and green technologies to fulfill the world's energy demands is one of the main challenges for this century [1,2]. One possible replacement for fossil fuels is to use hydrogen as an energy carrier, once it is widely available and non-toxic. Its compatibility with fuel cells is an attracting property. The hydrogen fuel cells are more efficient than the ones with common fuels, as gasoline or diesel. However, a safe, economical and efficient storage method is still required for the progress of the hydrogen economy [3]. The ideal hydrogen storage material must present low sorption/desorption temperatures, fast kinetics, high gravimetric and volumetric hydrogen densities and good reversibility [4]. In addition to the traditional storage methods as pressurized gas and liquefaction, hydrogen can be stored by chemisorption and physisorption in solid materials. Light metal hydrides are among the materials that store hydrogen by chemisorption, in other words, they chemically bind to hydrogen. They can reach high gravimetric hydrogen density, decent thermodynamics, low reactivity (high safety), lower storage pressure, but there is still work needed to be done to improve its kinetics [5]. Complex hydrides as sodium aluminum hydrides, also known as sodium alanate (NaAlH4), have high gravimetric capacity, with a theoretical hydrogen content of 7.4 wt% in a three-step reaction (Eqs. (1)e(3)) [6]. But the last one occurs above 400 C, therefore, the feasible hydrogen content is obtained by the first two reactions, which occur below 225 C [6] and yield a hydrogen capacity of 5.6 wt%. The relatively high desorption temperature and limited reversibility limit its application [7]. It was found that doping with transition metals compounds may decrease the reactions temperature, being able to release hydrogen at reduced temperature, increase the reactions kinetic and improve the reversibility [8]. TiO2 has been shown a good doping agent for NaAlH4 [9e13], Rafi-ud-din et al. [9] reported a reduction of the first decomposition step by 50 C and increased the dehydriding rate by 11e12 fold. Other materials, such as carbon nanomaterials [14e17], were explored as dopant agent. T ¼ 180e190 C 3NaAlH4 4 Na3AlH6 þ 2Al þ 3H2
(1)
T ¼ 190e225 C Na3AlH6 4 3NaH þ Al þ H2
(2)
T 400 C
(3)
NaH 4 Na þ ½ H2
Sodium alanate is extremely reactive towards water and oxygen, requiring extra care during manipulation [18]. The confinement of hydride particles within a polymeric matrix may overcome this issue and protect the hydride during the sorption/desorption cycles [19e23]. The polymer will act as a selective barrier, allowing hydrogen molecule, which has a small kinetic diameter, to diffuse between the polymeric chains, whilst oxygen, which is many times larger, will face
difficulty to reach the hydride. Furthermore, the incorporation of the hydrides into polymeric matrices maintains stable the particles dimension, inhibiting any pulverization and agglomeration during cycles. However, the polymer phase not only performs the protective role, it also has an intrinsic potential to store hydrogen. Porous polymers have been attracting increasing interest for applications for hydrogen storage due to their high surface area, low cost and thermal stability [5]. However, the studied porous materials have not attained the desired properties for hydrogen storage applications [24]. In most cases, the ability to store hydrogen is related to their intrinsic porosity, but few polymers with unique interaction with hydrogen have been found [25]. According to the literature, the hydrogen storage capacity may be increased by the chemical modification of a polymer matrix [26]. Polyaniline (Pani) and sulfonated polymers are among the potential materials for hydrogen storage. Pani is an intrinsic conducting polymer that has been explored alone or as a nanocomposite component for hydrogen storage. Early work reported 6 wt% storage at room temperature and 9 MPa for HCl-treated Pani [27]. However, this result was controversial and was not reproducible [28]. The mechanism behind Pani sorption properties was claimed to be related to its charge delocalization in the backbone chain that might create active sites with potential to interact with hydrogen [29]. Many other works have explored Pani storage ability, whether as electrospun fibers [30], nanofibers [31], acid-treated [32], loaded with LiBH4 [33], AB3 alloy [34], vanadium pentoxide [35], aluminum [36], multiwall carbon nanotubes (MWCNT) [36] and tin oxide [36]. Sulfonated polymers as sulfonated polyetheretherketone (PEEKS) [37e40] and sulfonated polyetherimide (PEIS) [41,42] are promising materials for hydrogen storage systems due to their high thermal stability and chemical resistance. The sulfonation process boosts the polymer protonic conductivity by introducing polar groups in its structure through electrophilic aromatic substitution, and it also increases the porosity in the meso range (20e500 A) [37]. The incorporation of particles, such as manganese oxide [37e39] and hexagonal boron nitride [40], into PEEKS promoted higher hydrogen storage capacity. Pedicini et al. [38] reached ~3 wt% of hydrogen absorption at 50 C and 40 bar for a system of PEEKS and MnO2 (80 wt%) and it displayed good reversibility. Hexagonal boron nitride addition increased PEEKS hydrogen desorption capacity by nearly 350%, as obtained by TGA measurements [40]. Therefore, the incorporation of hydrides into polymers can be an alternative to obtain materials with excellent hydrogen storage properties. Combining their hydrogen sorption abilities, superior protection against oxidation, hydrides morphology stability and the low density of the polymers, allows them to be further explored for mobile hydrogen storage applications. In this study, two systems of nanocomposites for hydrogen storage were prepared using solvent-based techniques (solution precipitation and spray drying). The systems were based on sulfonated polyetherimide (PEIS) and Pani, as polymer matrices, filled with NaAlH4 and TiO2 or MWCNT.
Please cite this article as: Beatrice CAG et al., Development of polymer nanocomposites with sodium alanate for hydrogen storage, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.06.169
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was terminated after 6 h by pouring it into methanol. Pani precipitated as a dark green powder, it was filtered, washed multiple times with methanol and dried under vacuum at 75 C for 72 h.
Experimental Materials Polyetherimide (PEI) was sulfonated in order to be used as matrix of some nanocomposites. The PEI, grade Ultem™ 1000, was supplied by Sabic Innovative Plastics, with a density of 1.28 g/cm3 and melt flow index (MFI) of 9.0 g/10 min (6.6 kg/ 337 C). The sulfonation was performed using a sulfonating agent (acetyl sulfate) made of distilled N-Methyl-2-pyrrolidinone (NMP), acetic anhydride (C4H6O3) and sulfuric acid (H2SO4) e all commercial grade purchased from Sigma-Aldrich e in a volume ratio of 20:2:1. Polyaniline (Pani) was synthetized to be used as polymer matrix of some nanocomposites. Aniline was distilled under vacuum and stored in a refrigerator. Ammonium peroxydisulfate (APS) and dodecylbenzenesulfonic acid (DBSA) were used without purification. Table 1 contains all reagents used in the synthesis of polyaniline and description of their main function, molar mass, chemical formula and supplier. Multiwall carbon nanotubes (MWCNT) were purchased from Nanostructured and Amorphous Materials Inc., with 95% of purity, average diameter of 8 nm and length ranging from 10 to 30 mm. Sodium alanate (NaAlH4, 54.0 g/mol) was purchased from Sigma-Aldrich. Nanoparticles of titanium dioxide (TiO2, 79.9 g/mol), used as a dopant agent for NaAlH4, were synthetized using a solution of titanium trichloride (TiCl3, 15% in HCl and 154.2 g/mol), hydrochloric acid (HCl, 36.5 g/mol) and sodium hydroxide (NaOH, 40.0 g/mol), all supplied by Sigma-Aldrich.
TiO2 synthesis The nanoparticles synthesis was based on by Cassaignon et al. [45] procedure. A TiCl3 solution (15% in HCl) was added to deionized water under intense stirring to obtain a [Ti3þ] ¼ 0.15 mol/L. The synthesis was performed in a noncontrolled atmosphere. The pH of the blue-violet solution was regulated between 4.5 and 5.5 by a NaOH solution at ambient temperature. This solution was heated at 60 C for 24 h. The obtained solid material was centrifuged with distilled water in order to remove the formed salts. This procedure was repeated until all salt was removed. The obtained TiO2 was dried under vacuum at 85 C.
Preparation of PEIS/NaAlH4 nanocomposites Firstly, 0.25 g PEIS was solubilized in 5 mL of NMP at 40 C for 30 min and then MWCNT (20 wt%) was added without NaAlH4 in order to obtain a better dispersion. Afterwards, a fixed content of NaAlH4 (30 wt%) was added in the solution. This dispersion was stirred for 30 min at 40 C, precipitated with acetone and sonicated for 10 min. Lastly, the precipitate was filtered, collected and heated at 80 C for 48 h under vacuum. Additionally, a mixture without the incorporation of MWCNT was prepared to understand its effect on the nanocomposites’ properties. These nanocomposites will be referred as PEIS/ NaAlH4 and PEIS/MWCNT/NaAlH4.
Preparation of Pani/NaAlH4 nanocomposites Methods
A small round bottom flask equipped with a magnetic stirrer was charged with 6 g of PEI dissolved in 40 mL of NMP at 80 C. The acetyl sulfate was added to this solution gradually within 1 h and after that the reaction continued for another 1 h. Afterwards, the reaction product (PEIS) was precipitated by the addition of ethanol (C2H6O), washed three times with the same solvent, filtered and dried under vacuum at 80 C for 72 h.
A dispersion was prepared with Pani, NaAlH4 (50 wt%) and with or without the incorporation of TiO2 (2 mol % in terms of NaAlH4 weight). Initially, 10 g of the nanocomposite was mixed with 350 mL of toluene and maintained under intense stirring and nitrogen atmosphere for 24 h at room temperature. Toluene was evaporated using a Bu¨chi B-190 spray dryer in order to encapsulate NaAlH4 and TiO2 (when present in the dispersion) in Pani. The spray parameters were nitrogen flow of 473 L/h, feed temperature of 115 C and equilibrium temperature of 100 C. These nanocomposites will be referred as Pani/NaAlH4 and Pani/TiO2/NaAlH4.
Pani polymerization
Evaluation of the PEI sulfonation
Aniline was emulsion polymerized using an adapted methodology by Barra et al. [43,44]. The synthesis was carried out by dissolving 0.075 mol of DBSA and 0.051 mol of aniline in 1000 mL of toluene and maintained at 0 C. A solution of 0.051 mol of APS and 100 mL of deionized water was added slowly over 10 min to avoid overheating. The polymerization
Infrared spectroscopic (FTIR) studies were done in a Thermo Scientifics spectrophotometer, model 6700, pressing the samples with KBr into 2 mm discs. To determine the degree of sulfonation (DS), elemental analysis was performed. The content of sulfonic acid, in other words, the degree of sulfonation, may be calculated using Eq. (4).
PEI sulfonation
Table 1 e Reagents used in the synthesis of polyaniline. Material Aniline Ammonium peroxydisulfate Dodecylbenzene-sulfonic acid Methanol Toluene
Function
Molar mass (g/mol)
Formula
Supplier
Monomer Initiator Proton supplier Precipitating agent Solvent
93.1 228.2 326.5 32.0 92.1
C6H5NH2 (NH4)2S2O8 C12H25C6H4SO3H CH4O C6H5CH3
Sigma-Aldrich Sigma-Aldrich ~ o Carlos Quı´mica Sa Sigma-Aldrich Sigma-Aldrich
Please cite this article as: Beatrice CAG et al., Development of polymer nanocomposites with sodium alanate for hydrogen storage, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.06.169
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DS ¼
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598 100:S 32 100 80:S
(4)
Where S is the sulfur concentration (wt%) in the polymer, and 598, 32 and 80 represents the molecular weight of PEI repeating unit, sulfur and sulfonic group, respectively.
Thermal characterization The thermal properties were obtained in a differential scanning calorimeter (DSC Q2000 e TA Instruments) from 25 to 250 C at a heating rate of 10 C/min under nitrogen atmosphere and also in a thermogravimetric analyzer (TGA Q50 e TA Instruments) from 25 to 800 C at a heating rate of 20 C/ min under nitrogen atmosphere.
Morphological and structural characterization Morphological analyses of the nanocomposites were carried out on a transmission electron microscope (TEM), Magelan, model 400L, and on a scanning electron microscope (SEM), Philips, FEG XL 30, both operating with a voltage of 25 kV. Wide angle x-ray scattering (WAXS) of the TiO2 and NaAlH4 nanoparticles was done in order to evaluate the stability of their structures in a Siemens diffractometer, model D5005, with CuKa radiation (l ¼ 1.54056 Å), operating at 40 kV and 80 mA. Samples were scanned between 20 and 80 at a rate of 1 /min. Measurements were recorded at each 0.02 .
Hydrogen sorption kinetics Hydrogen sorption kinetics were collected in a Sievert-type volumetric device, built and designed in the Laboratory of Hydrogen in Metallic Materials (DEMa/LHM/UFSCar). The analyses were carried out at temperature of 120 C and hydrogen pressure of 32 bar.
Results and discussion Characterization of the synthetized titanium dioxide Fig. 1 shows the WAXS pattern of the synthetized TiO2 and commercial anatase TiO2 and TEM images of the synthetized nanoparticles. It is noticed a great similarity between the diffractograms of synthetized TiO2 and commercial anatase TiO2, presenting broad and weak peaks, characteristics of nanoparticles. The synthetized nanoparticles have needlelike shape with length of nearly 150 nm and diameter of about 10 nm.
PEIS/NaAlH4 nanocomposites Fig. 2 presents FTIR spectrum of PEIS, where it is observed PEIS main characteristic bands. The bands in the region of 1777e1720 cm1 are characteristic of carbonyl vibrations, present in the imide group. The band at 1650 cm1 is attributed to stretching of imide carbonyl and the band at 1360 cm1 corresponds to stretching of CeN(CeNeC). At 1250 cm1 is observed the characteristic band of CeOeC asymmetric stretching. The band at 860 cm1 is attributed to angular deformation of the tetra-substituted aromatic ring and the band at 735 cm1 is attributed to angular deformation of four
adjacent hydrogens (ortho-substituted aromatic rings). After PEI sulfonation, it was observed a broad band at 1684 cm1, which may have appeared due to intermolecular bond between hydrogen of sulfonic group and carbonyl groups of PEI. The bands at 1173 cm1 e 1090 cm1 are attributed to symmetric stretching of sulfonic groups and the band at 779 cm1 corresponds to SeO bond of the sulfonic group. The degree of sulfonation (DS) resulted from the PEI chemical modification can be calculated from sulfur weight fraction. The sulfur weight fraction as obtained by the PEIS elemental analysis is 0.9897%, which yields a DS of 19%. Fig. 3 shows TEM images for the sample containing 20 wt% of MWCNT and 30 wt% of NaAlH4. The NaAlH4 particles have a spherical shape of different sizes with diameter ranging from 100 to 500 nm. It can be observed the presence of large agglomerates of MWCNT and a highly heterogeneous morphology. The carbon nanotubes have strong tendency to aggregate and form micron-sized agglomerates that limit their uniform dispersion. The dispersion challenge for MWCNTs is rather different than for other conventional fillers, such as carbon fibers, due to MWCNT nanometer scale diameter, high aspect ratio (>1000) and extremely large surface area. As can be observed in Fig. 3, the MWCNT physically interacts with the NaAlH4 particles. In some regions it is observed nanotubes covering NaAlH4 particles. In other regions of the micrographs, the NaAlH4 acts as a barrier, penetrating between the agglomerates of MWCNTs and separating them. This can contribute positively to the sorption of hydrogen, since reducing the size of agglomerates, the surface area increases and hence might be beneficial to enhance hydrogen storage kinetics. Fig. 4 shows SEM images of PEIS/MWCNT/NaAlH4 nanocomposites. In these micrographs, large agglomerates of NaAlH4 are observed. It is likely that the procedure used for mixing and dispersing the nanoparticles was not able to break the agglomerates, even though ultrasonic treatments were employed. It is expected that the hydrogen sorption characteristics in this nanocomposite will be hindered since a better dispersion of the particles would increase the contact area between the particles and hydrogen. Fig. 5 shows the DSC curves for the PEIS systems. Neat PEI presents a glass transition temperature (Tg) of 216 C. It was observed that the sulfonation decreased the polymer Tg to 190 C; this may be a result of the incorporation of sulfonic groups that increased the polymer free volume facilitating the motion of the polymeric chains. An additional endothermic peak was observed after the incorporation of NaAlH4, which is related to its melting point. The nanocomposite with MWCNT (PEIS/MWCNT/NaAlH4) presented weaker NaAlH4 melting peaks. Fig. 6 presents the TGA thermograms for the PEIS systems. PEI presents a high thermal stability, with a single weight loss event at around 520 C attributed to PEI chain decomposition. On the other hand, PEIS presented weight loss events in two steps. The first one at 190 C is related to the degradation of sulfonic groups, while the second one is related to the decomposition of PEI backbone. Regarding the nanocomposites, the first mass loss event is a combination of the degradation of PEIS sulfonic groups and NaAlH4 first-step
Please cite this article as: Beatrice CAG et al., Development of polymer nanocomposites with sodium alanate for hydrogen storage, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.06.169
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Fig. 1 e (a) WAXS pattern of synthetized nanoparticles and commercial anatase TiO2 and (b) TEM micrograph of synthetized nanoparticles.
Fig. 2 e Infrared spectroscopy (FTIR) of sulfonated polyetherimide (PEIS). The bands related to the introduced sulfonic groups are identified in red. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
decomposition, which releases hydrogen at nearly 180 C. The PEIS/MWCNT/NaAlH4 nanocomposite presents higher residue weight at 750 C, which is expected since MWCNT is stable in high temperatures. Fig. 7 shows the curves of hydrogen sorption kinetics of the PEIS systems. The hydrogen absorption profiles for the nanocomposites were similar. Initially, the hydrogen sorption increased sharply, followed by a slower increase, and in the final period of the analysis the hydrogen sorption rate was even smaller. At 6 h, the PEIS/NaAlH4 absorbed 0.5 wt% of H2, while PEIS/MWCNT/NaAlH4 reached 0.9 wt% of H2. However, at 12 h of the experiment, the difference of hydrogen capacity between them decreased, absorbing 1.1 and 1.2 wt% of H2 for PEIS/NaAlH4 and PEIS/MWCNT/NaAlH4, respectively. The introduction of MWCNT improved the sorption kinetics, especially in the first part of the experiment. This may be due to its hollow structure, which offers an easier path for the transport of hydrogen to the surface of NaAlH4, accelerating the hydrogenation process. For a longer period of hydrogenation, the difference of hydrogen capacity between the nanocomposites tends to decrease, since the hydrogen absorption capacity of NaAlH4 in the experimental conditions was reached. It is observed that neat PEIS also presents
Fig. 3 e TEM micrographs of the PEIS/MWCNT/NaAlH4 nanocomposite. Please cite this article as: Beatrice CAG et al., Development of polymer nanocomposites with sodium alanate for hydrogen storage, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.06.169
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Fig. 4 e SEM micrographs of the PEIS/MWCNT/NaAlH4 nanocomposite.
Fig. 5 e DSC curves of PEI, PEIS and PEIS nanocomposites.
hydrogen sorption properties, reaching a capacity of 0.47% at 12 h.
Fig. 6 e TGA curves of PEI, PEIS and PEIS nanocomposites.
Fig. 7 e Hydrogen sorption for PEIS, PEIS/NaAlH4 and PEIS/ MWCNT/NaAlH4. Measurements at 120 C and 32 bar.
Pani/NaAlH4 nanocomposites The successful doping of Pani was previously confirmed by Moreira et al. [44] via UVeVis spectrophotometry and FTIR analyses. Fig. 8 shows the morphologies of the samples. NaAlH4 have a needle-like shape. The nanocomposites SEM micrographs showed microspheres of Pani with NaAlH4 dispersed in them. NaAlH4 particles uniformly dispersed in the polymer matrix can increase the hydrogen storage performance due to nanoconfinement effect [47]. Fig. 9 presents the DSC curves for Pani and the nanocomposites. Pani's glass transition temperature (Tg) was around 108 C, as observed by the endothermic inflection. The endothermic peak is related to removal of adsorbed water and is presented at nearly 130 C. Regarding the nanocomposites, an addition endothermic peak is present. It is related to NaAlH4 melting (~180 C) that remains nearly constant regardless of the composition. The incorporation of NaAlH4 did not change Pani Tg. In regards to Pani/TiO2/NaAlH4, even though the loading of TiO2 was low, the incorporation shifted Pani Tg to lower temperatures. This indicates that TiO2
Please cite this article as: Beatrice CAG et al., Development of polymer nanocomposites with sodium alanate for hydrogen storage, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.06.169
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Fig. 8 e SEM micrographs of Pani/TiO2/NaAlH4.
Fig. 9 e DSC thermograms of Pani, Pani/NaAlH4 and Pani/ TiO2/NaAlH4. The curves are vertically offset for clarity. addition facilitated the polymer chain motions, presumably by increasing the polymer free volume. The TGA curves of Pani-ADBS and the nanocomposites are presented in Fig. 10. The samples displayed similar behavior,
Fig. 10 e TGA curves for Pani and the nanocomposites.
with three major weight loss events. The first one near 100 C is related to the removal of adsorbed water. The second event in the range of 200e320 C may be related to the release of dopant and/or degradation of Pani. The last one that starts at ~400 C is attributed to the decomposition of Pani [46]. The incorporation of TiO2 did not change the nanocomposite mass loss behavior, which was expected due to the small TiO2 content (2 mol % in terms of NaAlH4 weight). Disregarding the first weight loss event, the nanocomposites presented less intense weight loss during the second and third events than neat Pani, due to the contribution of the filler thermal stability. Indeed, the nanocomposites displayed higher residue weight at 750 C. Fig. 11 shows the hydrogen sorption kinetic curves for the samples. They presented a continuous increase of hydrogen capacity during the experiment. In the final step of the analysis, the hydrogenation rate is significantly lower, almost reaching a plateau. Neat Pani-ABDS reached 0.9 wt% at 12 h. For shorter times (4 h), it was observed that the composite Pani-ABDS/TiO2/NaAlH4 absorbed 0.5 wt% of H2, higher than 0.3 wt% for the Pani-ABDS at the same conditions. This result indicates the potential of the proposed composites as hydrogen storage materials.
Fig. 11 e Hydrogen sorption kinetics curves for Pani-ADBS and the composites Pani-ADBS/TiO2/NaAlH4 obtained by spray drying. Measurements at 120 C and 32 bar.
Please cite this article as: Beatrice CAG et al., Development of polymer nanocomposites with sodium alanate for hydrogen storage, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.06.169
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Fig. 12 e Diffractogram pattern of NaAlH4 stored in a desiccator under vacuum for more than 6 months (NaAlH4 vacuum > 6 m), 24 h exposed NaAlH4 (NaAlH4 e 24 h) and just opened NaAlH4 (NaAlH4 - Fresh).
Characterization of NaAlH4 for hydrogen storage Overall, the hydrogen absorption capacity of the systems ranged from 0.9 to 1.2 wt% after 12 h of sorption. The capacities are appreciable and close to some established hydridesforming alloys as LaNi5 (1.5 wt%), hydralloy (1.6 wt%) and TiFe (1.9 wt%). However, they are far from the theoretical NaAlH4 (5.6 wt%). A reason for this behavior may be due to decomposition of NaAlH4 during storage. It was shown that it decomposed into NaAlCO3(OH)2 and Na2CO3 during storage for more than 6 months at a desiccator under vacuum (Fig. 12). Shorter time exposure to ambient atmosphere does not seem so harmful. The sample exposed to ambient atmosphere for 24 h maintained NaAlH4 pattern but with less intense peaks.
and the polymer matrices and nanofillers chosen in this work have a great potential to obtain new materials with ability to store larger amounts of hydrogen. Furthermore, the importance of the polymer phase is not limited to improve hydrogen sorption capacity. It is expected to protect the fillers from oxygen and moisture, taking in account that sodium alanate is very sensitive to them. Also, since sodium alanate dehydrogenation comprises more than one decomposition step, it is desired that the decomposition products are alongside each other to allow the reverse reaction (hydrogenation). The polymer phase may confine them in their original position, facilitating the reaction.
Acknowledgment Conclusion Polymer nanocomposites were successfully prepared by solvent-based techniques: nanocomposites of PEIS with NaAlH4 and MWCNT (solution precipitation) and Pani with NaAlH4 doped with TiO2 (spray dryer). These nanocomposites were obtained in order to produce new materials to reach hydrogen storage performance for mobile applications. The thermal, morphological and hydrogenation properties of the nanocomposites were investigated. For PEIS nanocomposites, transmission microscopy showed that the MWCNT tend to be poorly dispersed in the polymer matrix with a highly aggregated morphology. SEM analyses showed the presence of large agglomerates of NaAlH4. It was shown that PEIS/NaAlH4 can store 1.1 wt% of H2 at 120 C and 32 bar. The incorporation of MWCNT has pronounced effect on the hydrogen absorption only for shorter times, presumably due to its hollow structure that creates an easier path for hydrogen transport. Regarding Pani nanocomposites, the SEM micrographs showed the formation of Pani microspheres with NaAlH4 dispersed into them. Pani/TiO2/NaAlH4 nanocomposites yielded a hydrogen sorption capacity of 0.5 wt% of H2 after 6 h at 120 C and 32 bar. It presented a increase of 67% comparing to neat Pani. After 12 h, this nanocomposite reached 0.95 wt% of H2. Obtaining polymeric materials for hydrogen storage is important for the scientific and technological development
This work was supported by CNPq (Brazilian Counsel of Technological and Scientific Development; processes 132947/ ~ o Paulo 2011-0, 159187/2014-1 and 140455/2018-3), FAPESP (Sa State Research Foundation; processes 2012/08040-9 and 2013/ 23586-0) and CAPES (Coordination for the Improvement of Higher Education Personnel; finance code 001). The authors would like to thank the Laboratory of Structural Character~o Carlos (LCE/DEMa/ ization of the Federal University of Sa UFSCar) for the scanning and transmission electron microscopies and for the wide angle x-ray scattering facilities.
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Development of polymer nanocomposites with sodium alanate for hydrogen storage Cesar Augusto Gonc¸alves Beatrice a,*, Bruna Rodrigues Moreira a, Amanda Dantas de Oliveira b, Fabio Roberto Passador c, Gabriel Rodrigues de Almeida Neto a, Daniel Rodrigo Leiva a, Luiz Antonio Pessan a a
Department of Materials Engineering, Graduate Program in Materials Science and Engineering, Federal University ~o Carlos, Rodovia Washington Luiz, km 235, 13565-905, Sa ~o Carlos, SP, Brazil of Sa b Centre of Technology Development, Federal University of Pelotas, Rua Gomes Carneiro, 1, 96010-610, Pelotas, RS, Brazil c ~o Paulo, Avenida Cesare Monsueto Giulio Lattes, 1201, Institute of Science and Technology, Federal University of Sa ~o Jose dos Campos, SP, Brazil 12247-014, Sa
highlights Polymer nanocomposites were successfully prepared by solvent-based techniques. PEIS/NaAlH4 (70/30 wt%) can store 1.1 wt% of H2 after 12 h at 120 C and 32 bar. Pani microspheres with NaAlH4 dispersed into them were obtained after spray drying. 50 wt% of NaAlH4 yielded a 67% higher H2 sorption capacity than neat Pani after 6 h in the same conditions.
article info
abstract
Article history:
The development of materials based on polymer nanocomposites for hydrogen storage with
Received 16 March 2019
lower temperature of desorption might contribute to the consolidation of the use of
Received in revised form
hydrogen as a sustainable energy. The purpose of this work was to develop hybrid porous
10 May 2019
materials consisting of polyaniline or sulfonated polyetherimide as polymer matrices and a
Accepted 24 June 2019
potential hydride for hydrogen storage e sodium alanate. Multiwall carbon nanotubes and
Available online xxx
titanium dioxide were also added in order to improve the hydrogen absorption capacity of the sodium alanate. The nanocomposites were prepared via solution mixing and analyzed by
Keywords:
differential scanning calorimetry, thermogravimetric analysis, transmission and scanning
Storage
electron microscopy and kinetic of hydrogen sorption. The nanoparticles had some influence
Nanocomposite
on the polymers structures, modifying its thermal properties, such as glass transition tem-
Polyaniline
perature and the onset temperature of degradation. Microscope analyses showed that not all
Polyetherimide
the particles were always well dispersed and distributed through the matrices. However,
Sodium alanate
kinetics of hydrogen sorption tests indicated a significant amount of hydrogen (up to 1.2 wt %) in the nanocomposites after 12 h at relatively low temperature (120 C) and 32 bar. © 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
* Corresponding author. E-mail address: [email protected] (C.A.G. Beatrice). https://doi.org/10.1016/j.ijhydene.2019.06.169 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article as: Beatrice CAG et al., Development of polymer nanocomposites with sodium alanate for hydrogen storage, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.06.169
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Introduction In an age where there is a growing demand for energy and fossil fuels are still the major energy source is alarming. To explore sustainable clean energy sources and green technologies to fulfill the world's energy demands is one of the main challenges for this century [1,2]. One possible replacement for fossil fuels is to use hydrogen as an energy carrier, once it is widely available and non-toxic. Its compatibility with fuel cells is an attracting property. The hydrogen fuel cells are more efficient than the ones with common fuels, as gasoline or diesel. However, a safe, economical and efficient storage method is still required for the progress of the hydrogen economy [3]. The ideal hydrogen storage material must present low sorption/desorption temperatures, fast kinetics, high gravimetric and volumetric hydrogen densities and good reversibility [4]. In addition to the traditional storage methods as pressurized gas and liquefaction, hydrogen can be stored by chemisorption and physisorption in solid materials. Light metal hydrides are among the materials that store hydrogen by chemisorption, in other words, they chemically bind to hydrogen. They can reach high gravimetric hydrogen density, decent thermodynamics, low reactivity (high safety), lower storage pressure, but there is still work needed to be done to improve its kinetics [5]. Complex hydrides as sodium aluminum hydrides, also known as sodium alanate (NaAlH4), have high gravimetric capacity, with a theoretical hydrogen content of 7.4 wt% in a three-step reaction (Eqs. (1)e(3)) [6]. But the last one occurs above 400 C, therefore, the feasible hydrogen content is obtained by the first two reactions, which occur below 225 C [6] and yield a hydrogen capacity of 5.6 wt%. The relatively high desorption temperature and limited reversibility limit its application [7]. It was found that doping with transition metals compounds may decrease the reactions temperature, being able to release hydrogen at reduced temperature, increase the reactions kinetic and improve the reversibility [8]. TiO2 has been shown a good doping agent for NaAlH4 [9e13], Rafi-ud-din et al. [9] reported a reduction of the first decomposition step by 50 C and increased the dehydriding rate by 11e12 fold. Other materials, such as carbon nanomaterials [14e17], were explored as dopant agent. T ¼ 180e190 C 3NaAlH4 4 Na3AlH6 þ 2Al þ 3H2
(1)
T ¼ 190e225 C Na3AlH6 4 3NaH þ Al þ H2
(2)
T 400 C
(3)
NaH 4 Na þ ½ H2
Sodium alanate is extremely reactive towards water and oxygen, requiring extra care during manipulation [18]. The confinement of hydride particles within a polymeric matrix may overcome this issue and protect the hydride during the sorption/desorption cycles [19e23]. The polymer will act as a selective barrier, allowing hydrogen molecule, which has a small kinetic diameter, to diffuse between the polymeric chains, whilst oxygen, which is many times larger, will face
difficulty to reach the hydride. Furthermore, the incorporation of the hydrides into polymeric matrices maintains stable the particles dimension, inhibiting any pulverization and agglomeration during cycles. However, the polymer phase not only performs the protective role, it also has an intrinsic potential to store hydrogen. Porous polymers have been attracting increasing interest for applications for hydrogen storage due to their high surface area, low cost and thermal stability [5]. However, the studied porous materials have not attained the desired properties for hydrogen storage applications [24]. In most cases, the ability to store hydrogen is related to their intrinsic porosity, but few polymers with unique interaction with hydrogen have been found [25]. According to the literature, the hydrogen storage capacity may be increased by the chemical modification of a polymer matrix [26]. Polyaniline (Pani) and sulfonated polymers are among the potential materials for hydrogen storage. Pani is an intrinsic conducting polymer that has been explored alone or as a nanocomposite component for hydrogen storage. Early work reported 6 wt% storage at room temperature and 9 MPa for HCl-treated Pani [27]. However, this result was controversial and was not reproducible [28]. The mechanism behind Pani sorption properties was claimed to be related to its charge delocalization in the backbone chain that might create active sites with potential to interact with hydrogen [29]. Many other works have explored Pani storage ability, whether as electrospun fibers [30], nanofibers [31], acid-treated [32], loaded with LiBH4 [33], AB3 alloy [34], vanadium pentoxide [35], aluminum [36], multiwall carbon nanotubes (MWCNT) [36] and tin oxide [36]. Sulfonated polymers as sulfonated polyetheretherketone (PEEKS) [37e40] and sulfonated polyetherimide (PEIS) [41,42] are promising materials for hydrogen storage systems due to their high thermal stability and chemical resistance. The sulfonation process boosts the polymer protonic conductivity by introducing polar groups in its structure through electrophilic aromatic substitution, and it also increases the porosity in the meso range (20e500 A) [37]. The incorporation of particles, such as manganese oxide [37e39] and hexagonal boron nitride [40], into PEEKS promoted higher hydrogen storage capacity. Pedicini et al. [38] reached ~3 wt% of hydrogen absorption at 50 C and 40 bar for a system of PEEKS and MnO2 (80 wt%) and it displayed good reversibility. Hexagonal boron nitride addition increased PEEKS hydrogen desorption capacity by nearly 350%, as obtained by TGA measurements [40]. Therefore, the incorporation of hydrides into polymers can be an alternative to obtain materials with excellent hydrogen storage properties. Combining their hydrogen sorption abilities, superior protection against oxidation, hydrides morphology stability and the low density of the polymers, allows them to be further explored for mobile hydrogen storage applications. In this study, two systems of nanocomposites for hydrogen storage were prepared using solvent-based techniques (solution precipitation and spray drying). The systems were based on sulfonated polyetherimide (PEIS) and Pani, as polymer matrices, filled with NaAlH4 and TiO2 or MWCNT.
Please cite this article as: Beatrice CAG et al., Development of polymer nanocomposites with sodium alanate for hydrogen storage, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.06.169
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was terminated after 6 h by pouring it into methanol. Pani precipitated as a dark green powder, it was filtered, washed multiple times with methanol and dried under vacuum at 75 C for 72 h.
Experimental Materials Polyetherimide (PEI) was sulfonated in order to be used as matrix of some nanocomposites. The PEI, grade Ultem™ 1000, was supplied by Sabic Innovative Plastics, with a density of 1.28 g/cm3 and melt flow index (MFI) of 9.0 g/10 min (6.6 kg/ 337 C). The sulfonation was performed using a sulfonating agent (acetyl sulfate) made of distilled N-Methyl-2-pyrrolidinone (NMP), acetic anhydride (C4H6O3) and sulfuric acid (H2SO4) e all commercial grade purchased from Sigma-Aldrich e in a volume ratio of 20:2:1. Polyaniline (Pani) was synthetized to be used as polymer matrix of some nanocomposites. Aniline was distilled under vacuum and stored in a refrigerator. Ammonium peroxydisulfate (APS) and dodecylbenzenesulfonic acid (DBSA) were used without purification. Table 1 contains all reagents used in the synthesis of polyaniline and description of their main function, molar mass, chemical formula and supplier. Multiwall carbon nanotubes (MWCNT) were purchased from Nanostructured and Amorphous Materials Inc., with 95% of purity, average diameter of 8 nm and length ranging from 10 to 30 mm. Sodium alanate (NaAlH4, 54.0 g/mol) was purchased from Sigma-Aldrich. Nanoparticles of titanium dioxide (TiO2, 79.9 g/mol), used as a dopant agent for NaAlH4, were synthetized using a solution of titanium trichloride (TiCl3, 15% in HCl and 154.2 g/mol), hydrochloric acid (HCl, 36.5 g/mol) and sodium hydroxide (NaOH, 40.0 g/mol), all supplied by Sigma-Aldrich.
TiO2 synthesis The nanoparticles synthesis was based on by Cassaignon et al. [45] procedure. A TiCl3 solution (15% in HCl) was added to deionized water under intense stirring to obtain a [Ti3þ] ¼ 0.15 mol/L. The synthesis was performed in a noncontrolled atmosphere. The pH of the blue-violet solution was regulated between 4.5 and 5.5 by a NaOH solution at ambient temperature. This solution was heated at 60 C for 24 h. The obtained solid material was centrifuged with distilled water in order to remove the formed salts. This procedure was repeated until all salt was removed. The obtained TiO2 was dried under vacuum at 85 C.
Preparation of PEIS/NaAlH4 nanocomposites Firstly, 0.25 g PEIS was solubilized in 5 mL of NMP at 40 C for 30 min and then MWCNT (20 wt%) was added without NaAlH4 in order to obtain a better dispersion. Afterwards, a fixed content of NaAlH4 (30 wt%) was added in the solution. This dispersion was stirred for 30 min at 40 C, precipitated with acetone and sonicated for 10 min. Lastly, the precipitate was filtered, collected and heated at 80 C for 48 h under vacuum. Additionally, a mixture without the incorporation of MWCNT was prepared to understand its effect on the nanocomposites’ properties. These nanocomposites will be referred as PEIS/ NaAlH4 and PEIS/MWCNT/NaAlH4.
Preparation of Pani/NaAlH4 nanocomposites Methods
A small round bottom flask equipped with a magnetic stirrer was charged with 6 g of PEI dissolved in 40 mL of NMP at 80 C. The acetyl sulfate was added to this solution gradually within 1 h and after that the reaction continued for another 1 h. Afterwards, the reaction product (PEIS) was precipitated by the addition of ethanol (C2H6O), washed three times with the same solvent, filtered and dried under vacuum at 80 C for 72 h.
A dispersion was prepared with Pani, NaAlH4 (50 wt%) and with or without the incorporation of TiO2 (2 mol % in terms of NaAlH4 weight). Initially, 10 g of the nanocomposite was mixed with 350 mL of toluene and maintained under intense stirring and nitrogen atmosphere for 24 h at room temperature. Toluene was evaporated using a Bu¨chi B-190 spray dryer in order to encapsulate NaAlH4 and TiO2 (when present in the dispersion) in Pani. The spray parameters were nitrogen flow of 473 L/h, feed temperature of 115 C and equilibrium temperature of 100 C. These nanocomposites will be referred as Pani/NaAlH4 and Pani/TiO2/NaAlH4.
Pani polymerization
Evaluation of the PEI sulfonation
Aniline was emulsion polymerized using an adapted methodology by Barra et al. [43,44]. The synthesis was carried out by dissolving 0.075 mol of DBSA and 0.051 mol of aniline in 1000 mL of toluene and maintained at 0 C. A solution of 0.051 mol of APS and 100 mL of deionized water was added slowly over 10 min to avoid overheating. The polymerization
Infrared spectroscopic (FTIR) studies were done in a Thermo Scientifics spectrophotometer, model 6700, pressing the samples with KBr into 2 mm discs. To determine the degree of sulfonation (DS), elemental analysis was performed. The content of sulfonic acid, in other words, the degree of sulfonation, may be calculated using Eq. (4).
PEI sulfonation
Table 1 e Reagents used in the synthesis of polyaniline. Material Aniline Ammonium peroxydisulfate Dodecylbenzene-sulfonic acid Methanol Toluene
Function
Molar mass (g/mol)
Formula
Supplier
Monomer Initiator Proton supplier Precipitating agent Solvent
93.1 228.2 326.5 32.0 92.1
C6H5NH2 (NH4)2S2O8 C12H25C6H4SO3H CH4O C6H5CH3
Sigma-Aldrich Sigma-Aldrich ~ o Carlos Quı´mica Sa Sigma-Aldrich Sigma-Aldrich
Please cite this article as: Beatrice CAG et al., Development of polymer nanocomposites with sodium alanate for hydrogen storage, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.06.169
4
DS ¼
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598 100:S 32 100 80:S
(4)
Where S is the sulfur concentration (wt%) in the polymer, and 598, 32 and 80 represents the molecular weight of PEI repeating unit, sulfur and sulfonic group, respectively.
Thermal characterization The thermal properties were obtained in a differential scanning calorimeter (DSC Q2000 e TA Instruments) from 25 to 250 C at a heating rate of 10 C/min under nitrogen atmosphere and also in a thermogravimetric analyzer (TGA Q50 e TA Instruments) from 25 to 800 C at a heating rate of 20 C/ min under nitrogen atmosphere.
Morphological and structural characterization Morphological analyses of the nanocomposites were carried out on a transmission electron microscope (TEM), Magelan, model 400L, and on a scanning electron microscope (SEM), Philips, FEG XL 30, both operating with a voltage of 25 kV. Wide angle x-ray scattering (WAXS) of the TiO2 and NaAlH4 nanoparticles was done in order to evaluate the stability of their structures in a Siemens diffractometer, model D5005, with CuKa radiation (l ¼ 1.54056 Å), operating at 40 kV and 80 mA. Samples were scanned between 20 and 80 at a rate of 1 /min. Measurements were recorded at each 0.02 .
Hydrogen sorption kinetics Hydrogen sorption kinetics were collected in a Sievert-type volumetric device, built and designed in the Laboratory of Hydrogen in Metallic Materials (DEMa/LHM/UFSCar). The analyses were carried out at temperature of 120 C and hydrogen pressure of 32 bar.
Results and discussion Characterization of the synthetized titanium dioxide Fig. 1 shows the WAXS pattern of the synthetized TiO2 and commercial anatase TiO2 and TEM images of the synthetized nanoparticles. It is noticed a great similarity between the diffractograms of synthetized TiO2 and commercial anatase TiO2, presenting broad and weak peaks, characteristics of nanoparticles. The synthetized nanoparticles have needlelike shape with length of nearly 150 nm and diameter of about 10 nm.
PEIS/NaAlH4 nanocomposites Fig. 2 presents FTIR spectrum of PEIS, where it is observed PEIS main characteristic bands. The bands in the region of 1777e1720 cm1 are characteristic of carbonyl vibrations, present in the imide group. The band at 1650 cm1 is attributed to stretching of imide carbonyl and the band at 1360 cm1 corresponds to stretching of CeN(CeNeC). At 1250 cm1 is observed the characteristic band of CeOeC asymmetric stretching. The band at 860 cm1 is attributed to angular deformation of the tetra-substituted aromatic ring and the band at 735 cm1 is attributed to angular deformation of four
adjacent hydrogens (ortho-substituted aromatic rings). After PEI sulfonation, it was observed a broad band at 1684 cm1, which may have appeared due to intermolecular bond between hydrogen of sulfonic group and carbonyl groups of PEI. The bands at 1173 cm1 e 1090 cm1 are attributed to symmetric stretching of sulfonic groups and the band at 779 cm1 corresponds to SeO bond of the sulfonic group. The degree of sulfonation (DS) resulted from the PEI chemical modification can be calculated from sulfur weight fraction. The sulfur weight fraction as obtained by the PEIS elemental analysis is 0.9897%, which yields a DS of 19%. Fig. 3 shows TEM images for the sample containing 20 wt% of MWCNT and 30 wt% of NaAlH4. The NaAlH4 particles have a spherical shape of different sizes with diameter ranging from 100 to 500 nm. It can be observed the presence of large agglomerates of MWCNT and a highly heterogeneous morphology. The carbon nanotubes have strong tendency to aggregate and form micron-sized agglomerates that limit their uniform dispersion. The dispersion challenge for MWCNTs is rather different than for other conventional fillers, such as carbon fibers, due to MWCNT nanometer scale diameter, high aspect ratio (>1000) and extremely large surface area. As can be observed in Fig. 3, the MWCNT physically interacts with the NaAlH4 particles. In some regions it is observed nanotubes covering NaAlH4 particles. In other regions of the micrographs, the NaAlH4 acts as a barrier, penetrating between the agglomerates of MWCNTs and separating them. This can contribute positively to the sorption of hydrogen, since reducing the size of agglomerates, the surface area increases and hence might be beneficial to enhance hydrogen storage kinetics. Fig. 4 shows SEM images of PEIS/MWCNT/NaAlH4 nanocomposites. In these micrographs, large agglomerates of NaAlH4 are observed. It is likely that the procedure used for mixing and dispersing the nanoparticles was not able to break the agglomerates, even though ultrasonic treatments were employed. It is expected that the hydrogen sorption characteristics in this nanocomposite will be hindered since a better dispersion of the particles would increase the contact area between the particles and hydrogen. Fig. 5 shows the DSC curves for the PEIS systems. Neat PEI presents a glass transition temperature (Tg) of 216 C. It was observed that the sulfonation decreased the polymer Tg to 190 C; this may be a result of the incorporation of sulfonic groups that increased the polymer free volume facilitating the motion of the polymeric chains. An additional endothermic peak was observed after the incorporation of NaAlH4, which is related to its melting point. The nanocomposite with MWCNT (PEIS/MWCNT/NaAlH4) presented weaker NaAlH4 melting peaks. Fig. 6 presents the TGA thermograms for the PEIS systems. PEI presents a high thermal stability, with a single weight loss event at around 520 C attributed to PEI chain decomposition. On the other hand, PEIS presented weight loss events in two steps. The first one at 190 C is related to the degradation of sulfonic groups, while the second one is related to the decomposition of PEI backbone. Regarding the nanocomposites, the first mass loss event is a combination of the degradation of PEIS sulfonic groups and NaAlH4 first-step
Please cite this article as: Beatrice CAG et al., Development of polymer nanocomposites with sodium alanate for hydrogen storage, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.06.169
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Fig. 1 e (a) WAXS pattern of synthetized nanoparticles and commercial anatase TiO2 and (b) TEM micrograph of synthetized nanoparticles.
Fig. 2 e Infrared spectroscopy (FTIR) of sulfonated polyetherimide (PEIS). The bands related to the introduced sulfonic groups are identified in red. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
decomposition, which releases hydrogen at nearly 180 C. The PEIS/MWCNT/NaAlH4 nanocomposite presents higher residue weight at 750 C, which is expected since MWCNT is stable in high temperatures. Fig. 7 shows the curves of hydrogen sorption kinetics of the PEIS systems. The hydrogen absorption profiles for the nanocomposites were similar. Initially, the hydrogen sorption increased sharply, followed by a slower increase, and in the final period of the analysis the hydrogen sorption rate was even smaller. At 6 h, the PEIS/NaAlH4 absorbed 0.5 wt% of H2, while PEIS/MWCNT/NaAlH4 reached 0.9 wt% of H2. However, at 12 h of the experiment, the difference of hydrogen capacity between them decreased, absorbing 1.1 and 1.2 wt% of H2 for PEIS/NaAlH4 and PEIS/MWCNT/NaAlH4, respectively. The introduction of MWCNT improved the sorption kinetics, especially in the first part of the experiment. This may be due to its hollow structure, which offers an easier path for the transport of hydrogen to the surface of NaAlH4, accelerating the hydrogenation process. For a longer period of hydrogenation, the difference of hydrogen capacity between the nanocomposites tends to decrease, since the hydrogen absorption capacity of NaAlH4 in the experimental conditions was reached. It is observed that neat PEIS also presents
Fig. 3 e TEM micrographs of the PEIS/MWCNT/NaAlH4 nanocomposite. Please cite this article as: Beatrice CAG et al., Development of polymer nanocomposites with sodium alanate for hydrogen storage, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.06.169
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Fig. 4 e SEM micrographs of the PEIS/MWCNT/NaAlH4 nanocomposite.
Fig. 5 e DSC curves of PEI, PEIS and PEIS nanocomposites.
hydrogen sorption properties, reaching a capacity of 0.47% at 12 h.
Fig. 6 e TGA curves of PEI, PEIS and PEIS nanocomposites.
Fig. 7 e Hydrogen sorption for PEIS, PEIS/NaAlH4 and PEIS/ MWCNT/NaAlH4. Measurements at 120 C and 32 bar.
Pani/NaAlH4 nanocomposites The successful doping of Pani was previously confirmed by Moreira et al. [44] via UVeVis spectrophotometry and FTIR analyses. Fig. 8 shows the morphologies of the samples. NaAlH4 have a needle-like shape. The nanocomposites SEM micrographs showed microspheres of Pani with NaAlH4 dispersed in them. NaAlH4 particles uniformly dispersed in the polymer matrix can increase the hydrogen storage performance due to nanoconfinement effect [47]. Fig. 9 presents the DSC curves for Pani and the nanocomposites. Pani's glass transition temperature (Tg) was around 108 C, as observed by the endothermic inflection. The endothermic peak is related to removal of adsorbed water and is presented at nearly 130 C. Regarding the nanocomposites, an addition endothermic peak is present. It is related to NaAlH4 melting (~180 C) that remains nearly constant regardless of the composition. The incorporation of NaAlH4 did not change Pani Tg. In regards to Pani/TiO2/NaAlH4, even though the loading of TiO2 was low, the incorporation shifted Pani Tg to lower temperatures. This indicates that TiO2
Please cite this article as: Beatrice CAG et al., Development of polymer nanocomposites with sodium alanate for hydrogen storage, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.06.169
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Fig. 8 e SEM micrographs of Pani/TiO2/NaAlH4.
Fig. 9 e DSC thermograms of Pani, Pani/NaAlH4 and Pani/ TiO2/NaAlH4. The curves are vertically offset for clarity. addition facilitated the polymer chain motions, presumably by increasing the polymer free volume. The TGA curves of Pani-ADBS and the nanocomposites are presented in Fig. 10. The samples displayed similar behavior,
Fig. 10 e TGA curves for Pani and the nanocomposites.
with three major weight loss events. The first one near 100 C is related to the removal of adsorbed water. The second event in the range of 200e320 C may be related to the release of dopant and/or degradation of Pani. The last one that starts at ~400 C is attributed to the decomposition of Pani [46]. The incorporation of TiO2 did not change the nanocomposite mass loss behavior, which was expected due to the small TiO2 content (2 mol % in terms of NaAlH4 weight). Disregarding the first weight loss event, the nanocomposites presented less intense weight loss during the second and third events than neat Pani, due to the contribution of the filler thermal stability. Indeed, the nanocomposites displayed higher residue weight at 750 C. Fig. 11 shows the hydrogen sorption kinetic curves for the samples. They presented a continuous increase of hydrogen capacity during the experiment. In the final step of the analysis, the hydrogenation rate is significantly lower, almost reaching a plateau. Neat Pani-ABDS reached 0.9 wt% at 12 h. For shorter times (4 h), it was observed that the composite Pani-ABDS/TiO2/NaAlH4 absorbed 0.5 wt% of H2, higher than 0.3 wt% for the Pani-ABDS at the same conditions. This result indicates the potential of the proposed composites as hydrogen storage materials.
Fig. 11 e Hydrogen sorption kinetics curves for Pani-ADBS and the composites Pani-ADBS/TiO2/NaAlH4 obtained by spray drying. Measurements at 120 C and 32 bar.
Please cite this article as: Beatrice CAG et al., Development of polymer nanocomposites with sodium alanate for hydrogen storage, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.06.169
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Fig. 12 e Diffractogram pattern of NaAlH4 stored in a desiccator under vacuum for more than 6 months (NaAlH4 vacuum > 6 m), 24 h exposed NaAlH4 (NaAlH4 e 24 h) and just opened NaAlH4 (NaAlH4 - Fresh).
Characterization of NaAlH4 for hydrogen storage Overall, the hydrogen absorption capacity of the systems ranged from 0.9 to 1.2 wt% after 12 h of sorption. The capacities are appreciable and close to some established hydridesforming alloys as LaNi5 (1.5 wt%), hydralloy (1.6 wt%) and TiFe (1.9 wt%). However, they are far from the theoretical NaAlH4 (5.6 wt%). A reason for this behavior may be due to decomposition of NaAlH4 during storage. It was shown that it decomposed into NaAlCO3(OH)2 and Na2CO3 during storage for more than 6 months at a desiccator under vacuum (Fig. 12). Shorter time exposure to ambient atmosphere does not seem so harmful. The sample exposed to ambient atmosphere for 24 h maintained NaAlH4 pattern but with less intense peaks.
and the polymer matrices and nanofillers chosen in this work have a great potential to obtain new materials with ability to store larger amounts of hydrogen. Furthermore, the importance of the polymer phase is not limited to improve hydrogen sorption capacity. It is expected to protect the fillers from oxygen and moisture, taking in account that sodium alanate is very sensitive to them. Also, since sodium alanate dehydrogenation comprises more than one decomposition step, it is desired that the decomposition products are alongside each other to allow the reverse reaction (hydrogenation). The polymer phase may confine them in their original position, facilitating the reaction.
Acknowledgment Conclusion Polymer nanocomposites were successfully prepared by solvent-based techniques: nanocomposites of PEIS with NaAlH4 and MWCNT (solution precipitation) and Pani with NaAlH4 doped with TiO2 (spray dryer). These nanocomposites were obtained in order to produce new materials to reach hydrogen storage performance for mobile applications. The thermal, morphological and hydrogenation properties of the nanocomposites were investigated. For PEIS nanocomposites, transmission microscopy showed that the MWCNT tend to be poorly dispersed in the polymer matrix with a highly aggregated morphology. SEM analyses showed the presence of large agglomerates of NaAlH4. It was shown that PEIS/NaAlH4 can store 1.1 wt% of H2 at 120 C and 32 bar. The incorporation of MWCNT has pronounced effect on the hydrogen absorption only for shorter times, presumably due to its hollow structure that creates an easier path for hydrogen transport. Regarding Pani nanocomposites, the SEM micrographs showed the formation of Pani microspheres with NaAlH4 dispersed into them. Pani/TiO2/NaAlH4 nanocomposites yielded a hydrogen sorption capacity of 0.5 wt% of H2 after 6 h at 120 C and 32 bar. It presented a increase of 67% comparing to neat Pani. After 12 h, this nanocomposite reached 0.95 wt% of H2. Obtaining polymeric materials for hydrogen storage is important for the scientific and technological development
This work was supported by CNPq (Brazilian Counsel of Technological and Scientific Development; processes 132947/ ~ o Paulo 2011-0, 159187/2014-1 and 140455/2018-3), FAPESP (Sa State Research Foundation; processes 2012/08040-9 and 2013/ 23586-0) and CAPES (Coordination for the Improvement of Higher Education Personnel; finance code 001). The authors would like to thank the Laboratory of Structural Character~o Carlos (LCE/DEMa/ ization of the Federal University of Sa UFSCar) for the scanning and transmission electron microscopies and for the wide angle x-ray scattering facilities.
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Please cite this article as: Beatrice CAG et al., Development of polymer nanocomposites with sodium alanate for hydrogen storage, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.06.169