NaBH4 GEOPOLYMER COMPOSITES

Lars Schomborg, Claus H. Rüscher, J. Christian Buhl, Florian Kiesel

Institut für Mineralogie, Leibniz Universität Hannover
Hannover, Germany

ABSTRACT

NaBH4 salt can easily be solved in sodium aluminate and silicate solutions. Brought together reveal recrystallization of NaBH4 crystals enclosed and protected in a geopolymer type matrix without any loss of the NaBH4 input. A fully controllable, catalyst free hydrogen release is achieved by the addition of acidic solution at room temperature. The composite can be optimized releasing up to 80 % of the amount obtained for pure NaBH4 in the reaction with H2O, i.e. a total content of 10 wt% or 2 l H2 per gram BH4-containing gel can be received.

INTRODUCTION

Geopolymers are inorganic polymers possessing a X-ray amorphous three dimensional network. It consists of SiO4 and AlO4 tetrahedral type units that are linked by sharing oxygen atoms which has been described by the general chemical formula: Mn[-(SiO2)z-AlO2-]n·wH2O, M = Na+, K+; n = degree of polymerization; z = number of silicate units; and w = number of water molecules1, 2. Usually waterglass solution is used together with metakaolin or slag-waste products, rice husk ash, etc.. It has been shown that high mechanical strength is achieved with the formation and protection of long silicate chains (more than 30 Si-O units) crosslinked via sialate (Si-O-Al) or siloxo Si-O-Si bondings, depending on the source material used3, 4, 5. It has also been shown that geopolymers possessing significant strength could also be prepared using sodium aluminate and sodium silicate (waterglass) solutions6. A new approach is the enclosure and handling of NaBH4 in such type of geopolymer matrix as a hydrogen storage material.

Sodium boronhydride (NaBH4) contains 10 wt% hydrogen, respectively 1.2 l of hydrogen per gram NaBH4 (density 1.07 g/cm3). The release of hydrogen is doubled in the reaction with water:

However, there is some risk of self ignition and uncontrollable loss of hydrogen in handling with the pure salt. It has been reported that a saver and easy handling of hydrogen release could be achieved for NaBH4 grown in a geopolymer matrix7, 8, 9. Here further details on the preparation, tests of hydrogen release and stability checks will be described.

EXPERIMENTAL

NaBH4-containing aluminosilicate gels are synthesized following the route described earlier8, 9: Sodium-aluminate solution with added NaBH4-salt (solution I) and a sodium-silicate solution, also including NaBH4-salt (solution II) were prepared in a first step. Typically up to 400 mg NaAlO2 (Riedel de Haen 13404) and up to 850 mg NaBH4-salt (Merck 806 373) were dissolved in 1.5 ml water to prepare solution I. Solution II was prepared using up to 600 mg Na2SiO3 (Fluka 307815) beside the same amount of water and NaBH4 as in the case of solution I. Mixing of these solutions causes immediate gel precipitation. The obtained NaBH4-bearing aluminosilicate gels were subsequently dried under open conditions in a drying cabinet. Two further series were synthesized as described in more detail here. One series was dried at 80°C in air for 4 h and one dried under vacuum conditions (10-2 bar) at room temperature for 3 h with reactant compositions as given in Tab. 1. The NaBH4 to solid ratio was fixed to R = 0.5 and the Si/Al ratio set at 0.5, 1.0, 1.5 and 2.0 within each series. The composition of the sample revealing highest hydrogen release properties (see below) are also given in Tab. 1.

Table 1 Drying method, amount of solids used, NaBH4/solid ratio (R) and molar ratio Si/Al of series of samples Gel_V, Gel_80 and sample S1.

XRD was carried out on as received samples directly after drying at 80°C and vacuum dried on a Bruker D8 powder diffractometer (CuKa radiation, 2 Theta range 5° to 80°, step width of 0.02° and measuring time of 2 sec/step). The obtained data were evaluated with STOE WinXpow and Bruker TOPAS software. Thermogravimetric/Differential Thermo Analyses (TG/DTA) were also carried out of the two series of the as received samples between 20°C and 375°C using a Setaram 1650 equipment. Heating/cooling runs were carried out under He flow (20 ml/minute) with a rate of 5°C per minute including a 30 minute delay time at 375°C. Before heating up, the sample has been evacuated up to 10-5 mbar for about 15 minutes. FTIR spectra (Bruker Vertex 80v) were taken systematically using the KBr method of the as received samples, after the TG experiment, and after 3 and 9 month of holding the as received samples in closed glass tubes.

The hydrogen content of the NaBH4 geopolymer composites aged for 9 month were measured using the same procedure as described earlier8, 9. Briefly, for a complete release of the hydrogen from the NaBH4-geopolymer composite, the diluted acid (1M HCl) were added in a surplus. The added volume of acid was varied between 5 and 8 ml and was subtracted afterwards from the shown volume at the gas syringe (Fig. 1) to register only the amount of released gas. The diluted acid was injected with an injection needle trough the plug, so the apparatus remains gastight. For every sample different masses in a wide range from 10 to 80 mg were investigated to get more reliable results from linear regression. With the linear regression an amount of released hydrogen per 100 mg sample was calculated. Due to the vertical assembling of the gas syringe, the additional weight of the plunger had to be comprised. Its influence on the measured volumes was calculated and added to the results. The error bars, shown in the hydrogen release related figures (see below) were calculated and contain the influence of the volume error of the added acid and the measured gas volume in the gas syringe, the reading error and the error from linear regression, using the RGP-function of Microsoft Excel. The released gas volume contains hydrogen, which was checked by the hydrogen-oxygen-reaction. Additionally the released gas was checked with a gas detector, showing the released gas contains no CO2 (detection limit < 1000 ppm).

Figure 1 Glass ware apparatus for gas release (using a 100 ml bulb)

RESULTS

XRD and IR Absorption

The X-ray powder pattern of series of samples received by vacuum drying and at 80°C are shown in Fig. 2 a-d and Fig. 3 a-d, respectively. The vacuum dried samples show pronounced very broad peaks in the range 20 to 40 °2Theta with maxima around 28-32 °2Theta related mainly to the formation of short range ordered polysiloxo and polysialate units. The somewhat broader diffraction peaks, which appear in the pattern of Si/Al ratio 1, 1.5 and 2 are related to some type of nanocrystalline or not well crystallized sodalites (marked by A in Fig. 2, 3). In all cases but one with Si/Al = 1 the presence of NaBH4 crystals can be identified as marked by B. For better comparison the X-ray powder pattern typically obtained for the NaBH4-salt is shown in Fig. 1 a (Peaks consistent with ICSD 165707). Further peaks could be identified as sodium carbonate (C) the sample holder...