Development of polymer nanocomposites with sodium alanate for hydrogen storage
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), (2), (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 [9], [10], [11], [12], [13], Rafi-ud-din et al. [9] reported a reduction of the first decomposition step by 50 °C and increased the dehydriding rate by 11–12 fold. Other materials, such as carbon nanomaterials [14], [15], [16], [17], were explored as dopant agent.T = 180–190 °C 3NaAlH4 ↔ Na3AlH6 + 2Al + 3H2T = 190–225 °C Na3AlH6 ↔ 3NaH + Al + H2T ≥ 400 °C NaH ↔ Na + 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 [19], [20], [21], [22], [23]. 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) [37], [38], [39], [40] 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 (20–500 Å) [37]. The incorporation of particles, such as manganese oxide [37], [38], [39] 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.
Section snippets
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) – all commercial grade purchased from Sigma-Aldrich – in a volume ratio of
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 needle-like 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
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
Acknowledgment
This work was supported by CNPq (Brazilian Counsel of Technological and Scientific Development; processes 132947/2011-0, 159187/2014-1 and 140455/2018-3), FAPESP (São Paulo 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 Characterization of the Federal University of São Carlos (LCE/DEMa/UFSCar) for the scanning and
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