Sodium borohydride and propylene glycol, an effective combination for the generation of 2.3 wt% of hydrogen
Introduction
The 21st Conference of the Parties of the United Nations Framework Convention on Climate Change through the Paris Climate Agreement is a turning-point in the worldwide awareness of global warming and its urgent mitigation [1]. For many years now fossil fuels have been identified as one of the major sources of the most abundant greenhouse gas, i.e. carbon dioxide. A lever for mitigating global warming is to develop sustainable and/or renewable energies as substitutes of fossil fuels, and several solutions have evolved in the past decades [2]. One of them is hydrogen [3].
Hydrogen (as a molecule) is a great opportunity in the field of sustainable and/or renewable energies but, because of some of its properties, the development of the so-called near-future hydrogen economy faces scientific and technical challenges. The state of the art shows three main challenges. (i) Hydrogen (as an atom) is abundant on earth, and de facto is found combined to other elements like carbon and oxygen. The molecule, which is a gas under normal conditions, has to be produced from sources like methane and water [4]. (ii) The gaseous state raises storage and distribution issues in terms of safety and energy density. Hence, classical (compression and cryogenics) and new (based on liquid or solid materials) storage solutions have been developed [5]. (iii) Hydrogen is mainly viewed as an energy carrier, which means that the chemical energy of the HH bond (D = 436 kJ mol−1) is expected to be converted into electrical energy. The convertor is the fuel cell technology [6].
In the recent years the field of hydrogen storage has been decidedly dynamic resulting in the development of an impressive number of (old and new) materials that fall into two categories. The first category includes porous materials (e.g. metal organic frameworks, carbonaceous hosts, polymers with intrinsic porosity) that are able to reversibly store H2 at sub-zero temperatures. This is named physical H2 storage [7]. The second category regards chemical H storage, that is, materials where H is bound to a heteroatom like boron, magnesium, aluminum or a transition metal. They attractively offer high gravimetric/volumetric H storage capacities, but for most them storage is only partially reversible or even irreversible [8]. One example of chemical H storage material is sodium borohydride NaBH4.
Sodium borohydride is an old material, discovered in the 1940s and which offers 10.8 wt% of hydridic hydrogens Hδ− capable of spontaneously reacting with protic hydrogens Hδ+ coming from sources like water or alcohols (ROH) [9]. In these conditions, both NaBH4 and water (or an alcohol) are H carriers and H2 sources. Hydrolysis of NaBH4 has been studied extensively over the past 15 years, with a special focus on the catalytic materials [10]. Research is nowadays much more applied than in the past. There are experimental prototypes [11], [12], [13] and even commercialized devices [14], [15], [16]. Of important note is that the weak development of the catalytic hydrolysis of NaBH4 is directly related to the regeneration inefficiency of the spent hydrolysis fuel [14], [16], [17], [5], [8].
Alcoholysis of NaBH4 has been also explored, but in a lesser extent. Several alcohols have been examined so far. With methanol CH3OH, the conversion of NaBH4 is complete [18], and the reaction is more efficient than hydrolysis [19], [20]. Improved methanolysis kinetics takes place in the presence of (perchloric or boric) acids [21], metal catalysts (e.g. PtLiCoO2, CoCl2, NiCl2, Ni2P/SiO2) [22], [23], [24] and poly (ethylene imine) microgels [25]. Ethanol C2H5OH also reacts with NaBH4 but the kinetics of ethanolysis is 70 times slower than for methanolysis [18]. Accelerated reaction was reported in the presence of catalysts (acids and metal chlorides) [26] as well as for catalyzed ethanol-water mixtures [27]. Two other alcohols, i.e. isopropanol CH3CH(OH)CH3 and tert-butanol CH3C(CH3)(OH)CH3, were found to be rather inert towards NaBH4 [18]. A last example of reactive alcohol is ethylene glycol HOCH2CH2OH, which can be used pure [28] or mixed with water [29].
At first sight, hydrolysis of NaBH4 may be seen as being superior to alcoholysis for two reasons: abundance of water and higher gravimetric hydrogen storage capacities (Fig. 1). However, a further comparison highlights clear benefits of using alcohols instead of water: i.e. potential for utilization in cold conditions (low freezing point), easy handling and efficient regeneration options for the spent fuels, and lower vapor pressure than water for the heavier alcohols [18], [19], [20], [21], [22], [23], [24], [25], [26], [27], [28], [29]. In other words, there is a real interest in investigating various Hδ+ carriers in order to provide some options for the release of H2 from NaBH4.
Propylene glycol HOCH2CH(OH)CH3 (C3H8O2) appeals to alcoholysis of NaBH4. An advantage of this diol is that it is quite safe. It is for instance much less toxic than methanol. It is besides comparable to water in terms of density (1.036 vs. 1 g cm−3). Otherwise, the theoretical gravimetric hydrogen storage capacity of the couple NaBH44C3H8O2 (Fig. 1) is adequate (even relatively high) when it is compared to the capacities reported so far in the field of hydrogen storage [5], [6], [7], [8]. Other attractive features have been identified through the systematic work presented hereafter, where the underinvestigated propylene glycol has been considered as the sole source of Hδ+. The primary objectives of the work were thus as follows: (i) showing the potential of the couple NaBH44C3H8O2 for chemical hydrogen storage via the catalyst-free alcoholysis of the borohydride; (ii) optimizing the gravimetric hydrogen storage capacity at 25 °C; (iii) collecting data in order to better understand the features of the alcoholysis reaction; and (iv) putting the advantages of the diol forward.
Section snippets
Experimental
Sodium borohydride NaBH4 (Sigma–Aldrich) and propylene glycol HOCH2CH(OH)CH3 (C3H6(OH)2; Sigma–Aldrich) were used as received. The former chemical was opened, handled and stored in an argon filled glove box (MBraun M200B, O2 < 0.1 ppm, H2O < 0.1 ppm).
The alcoholysis reaction (i.e. H2 evolution experiment) was performed as follows. In the glove box, NaBH4 and a small magnetic rod were transferred in a Schlenk tube that was sealed with a septum. Out of the glove box, the tube was immersed in an
Results
The reactivity of propylene glycol (denoted PG) with sodium borohydride (denoted SB) was verified in a beaker in the presence of an excess of the diol. A visual inspection showed rapid and significant H2 evolution, at least in comparison to hydrolysis. The determination of the solubility of SB in PG was not possible because of alcoholysis.
The alcoholysis of SB (i.e. H2 evolution) was studied for different amounts of PG at 25 °C. The mole ratio PG/SB (denoted also x) was varied based on the
Discussion
With PG as source of protic hydrogens Hδ+, it is possible to generate H2 by alcoholysis of SB. In fact, PG has a number of advantages over water (W). This is discussed hereafter on the basis of Table 1.
The conversion of SB is total (i.e. 100% of H2 released) when at least 4 equivalents of PG are reacted with one equivalent of the borohydride. This is better than the hydrolysis of SB. Indeed hydrolysis is never complete at similar conditions because the kinetics is drastically reduced with the
Conclusion
The reaction (i.e. alcoholysis or glycolysis) between one equivalent of SB and four equivalents of PG results in the evolution of four equivalents of H2 at temperatures starting from 25 °C. The conversion of SB is total (i.e. 100% of H2 released). The H2 generation rates are acceptable (e.g. superior to 1, 3, 14 and 28 L (H2) h−1 at 25, 35, 45 and 55 °C respectively) and high enough for energy systems requiring safe production of H2 (e.g. stationary systems conceived for continuous H2
Acknowledgments
This work was supported by the French government via the: (i) Institut Carnot Chimie Balard, Montpellier, France (project n°. 408-15AC1) and (ii) Agence Nationale de la Recherche, Paris, France (project MobiDiC) (ANR-16-CE05-0009).
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