Silane-based hydrogen storage materials for fuel cell application: Hydrogen release via methanolysis and regeneration by hydride reduction from organosilanes

Abstract

A series of cyclic- and linear organosilanes, 1–5, was prepared and examined as potential hydrogen storage materials. When a stoichiometric amount of methanol was added to a mixture of cyclic organosilane, (CH₂SiH₂)₃ (1) or (CH₂SiH₂CHSiH₃)₂ (2), and 5 mol% NaOMe, rapid hydrogen release was observed at room temperature within 10–15 s. The hydrogen storage capacities of compounds 1 and 2 were estimated to be 3.70 and 4.04 wt.-% H₂, respectively. However, to ensure the complete methanolysis from organosilanes including methanol evaporation at exothermic dehydrogenation condition, two equivs of methanol were used. The resulting methoxysilanes, (CH₂Si(OMe)₂)₃ (6) and (CH₂Si(OMe)₂CHSi(OMe)₃)₂ (7), were regenerated to the starting organosilanes in high yields by LiAlH₄ reduction. Linear organosilanes, SiH₃CH₂SiH₂CH₂SiH₃ (3), SiH₃CH₂CH(SiH₃)₂ (4), and SiH₃CH₂CH(SiH₃)CH₂SiH₃ (5) also showed fast hydrogen release kinetics at room temperature with hydrogen storage capacities of 4.26, 4.55, and 4.27 wt.% H₂, respectively; the corresponding methoxysilanes were successfully regenerated by LiAlH₄. Compound 1 was further tested as hydrogen source for fuel cell operation.

Introduction

Considerable effort has been made to use hydrogen as a source of clean energy due to the growing concern regarding both the shortage of fossil fuels and the concomitant generation of green-house gases [1]. Challenges facing the implementation of a “hydrogen economy” lie in the ability to store enough fuel for portable power [2], [3], [4], [5], [6], particularly in a liquid form.

Currently available hydrogen storage candidates include pressurized or liquefied hydrogen [7], [8], metal organic frameworks [9], and chemical hydrides [10], [11], [12]. Each has advantages and disadvantages, depending on the purpose and the application conditions. For vehicular and portable use of fuel-cell systems, a great deal of interest has focused recently on synthesizing chemical hydrides having fast kinetics and examining their stability under a wide range of operating conditions [13], [14], [15], [16]. In particular, the hydrolysis [17], [18], [19], [20], [21], [22], [23], [24], [25], [26] and alcoholysis [27], [28], [29], [30] of borohydrides and ammonia borane have been examined intensively over the past decades for hydrogen generation. These systems may be serious hydrogen storage contenders. However, a major disadvantage is the lack of energy-efficient methods to reintroduce hydrogen to the spent fuel once it has been released. In addition, since both of these materials are solids that are soluble only in a relatively polar coordinating solvent, the hydrogen weight percent is lower when they are applied in fuel cells.

In the search for alternate hydrogen storage materials derived from other sources than the boron atom, we have focused on organosilanes. Properly modified organosilanes can be stabilized and yet reactive. Alcoholysis of organosilanes in the presence of a catalyst to produce hydrogen and the corresponding siloxanes is a well-known chemical reaction. However, the use of silane as a potential hydrogen source has not been studied carefully, since most organosilanes have a low boiling point owing to their small molecular weight, which is drawback in terms of safety, stability, hazards and so on. Hence, to implement organosilane as a hydrogen storage material, complete conversion in ambient conditions and selective reactions with fast dehydrogenation are needed. Among the many options for harnessing hydrogen from organosilanes, milder catalytic methanolysis appears to be desirable over hydrolysis, even given the unfavorable thermodynamics for regenerating the spent fuel owing to the strong Si–O bond. This avenue offers several advantages because (1) methanolysis leads to quantitative production of molecular hydrogen, (2) the hydrogen yield is proportional to the silane/methanol stoichiometry, (3) Si–OMe can be regenerated to Si–H using metal hydrides, (4) hydrogen release can be controlled since methanol is not a good nucleophile for attacking silanes in the absence of a catalyst, and (5) most importantly, a stoichiometric amount of methanol is sufficient to dissolve the organosilanes and allow further dehydrogenation. Recently, Ison et al. reported the production of hydrogen under ambient conditions from the catalytic hydrolytic oxidation of organosilanes [31], as shown in eq. (1).

$${\text{R}}_{\text{n}}{\text{SiH}}_{4-\text{n}}+{\text{R}}^{\prime }\text{OH}\stackrel{\text{catalyst}}{\to }{\text{R}}_{\text{n}}\text{Si}{\left({\text{OR}}^{\prime }\right)}_{4-\text{n}}+4-{\text{n}\text{H}}_2$$

They reported hydrogen production from hydrolytic oxidation of organosilane derivatives using a cationic oxorhenium catalyst but a slow reaction time (1 h). More recently, Brunel reported methanolysis of various organosilane derivatives as a new methodology for storage and production of pure hydrogen [32]. Among them, tetrasilylmethane was introduced as an excellent organosilane hydrogen carrier derivative with a gravimetric weight percent of up to 17.8. However, most reported organosilanes have relatively low boiling points, and some are reactive in methanol media without a catalyst. Although tetrasilylmethane has a high gravimetric weight percent, it cannot be used in fuel cells, since boiling and melting points are 85 and 5 °C, respectively. Therefore, new hydrogen-rich organosilanes must be developed that are suitable for the conditions in fuel cells [33].

This paper introduces several organosilanes and evaluates their utility as hydrogen storage materials. These organosilanes consist of cyclic- or linear structures with SiH₃ units attached to a Si–C backbone. Cyclic- (1–2) and linear (3–5) organosilanes were prepared directly from the corresponding chlorosilanes [34], [35] by lithium aluminum hydride (LiAlH₄) reduction. By using organosilanes, 1–5, not only can dehydrogenation be facilitated with the aid of catalytic amounts of sodium methoxide via methanolysis, but also the spent fuels can be regenerated by LiAlH₄ reduction. Furthermore, cyclic silane 1 is also applied as a hydrogen source for a fuel cell.