Controlled hydrogen generation by reaction of aluminum/sodium ...

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international journal of hydrogen energy 37 (2012) 5811e5816
metallic Al using well-established technologies [8,9]. But the
development of Al/H 2 O system as practical hydrogen source
requires a solution to the long-standing problem of surface
passivation of Al. Currently, there are a number of approaches
under investigation, which can be divided into two categories:
one is addition of hydroxides [10,11], metal oxides [12,13] or
selected salts [14,15] to disrupt the passivation layer; the other
is alloying Al with low melting point metals to inhibit the
formation of a coherent passivation layer [16e19]. In our study
of the Al/H 2 O reaction system, we mainly focused on the alkali
approach because the alkali-promoted system always
outperforms other systems in terms of HG kinetics. But
meanwhile, the use of alkali promoter causes corrosion of the
system apparatus. From a practical point of view, novel
technologies that enable minimization of alkali concentration
without compromising of HG performance are highly
desirable.
Quite recently, our study found that a combined usage of
NaOH and Na 2 SnO 3 can dramatically improve the HG
kinetics of the Al/H 2 Osystem[20]. Furthermore, the addition
of a small amount of Na 2 SnO 3 causes a remarkable decrease
of NaOH concentration that is required for achieving favorable
HG performance. This finding provides a simple but
effective method for simultaneously addressing the reaction
kinetics and alkali corrosion problems of the Al/H 2 O system.
Stimulated by this finding, we further investigated a new
system, which is composed of Al/NaOH/Na 2 SnO 3 solid
mixture and H 2 O. In comparison with the traditional Al/H 2 O
system using aqueous NaOH solution, the new system
exhibits a series of advantages on HG performance, manipuility
and adaptability, which makes it promising for
portable hydrogen source applications. We herein show the
study results.
2. Experimental
2.1. Preparation of solid fuel and sample
characterization
Al powder (99% purity), NaOH (98% purity) and sodium stannate
trihydrate (Na 2 SnO 3 $3H 2 O, 99% purity) were purchased
from Sinopharm Chemical Reagent Corporation. In the
present study, a series of Al powder samples with different
particle sizes were employed and characterized by a Mastersizer
2000 particle size analyzer. Anhydrous sodium stannate
(Na 2 SnO 3 ) was prepared by calcining Na 2 SnO 3 $3H 2 O at 350 C
for 1 h.
The Al/NaOH/Na 2 SnO 3 powder mixtures in varied mass
ratios were hand-milled using a glass mortar and pestle or
mechanically milled by a Fritsch 7 planetary mill at 400 rpm
for 1 h. After the ball-milling, the solid fuel mixture was
pressed into a tablet by a pellet machine at 10 MPa for 10 min.
The diameter and height of the resulting tablet were 15 and
3 mm, respectively.
The solid residue of the Al/H 2 O reaction was analyzed by
powder X-ray diffraction (XRD, Rigaku D/max-2500, Cu Ka
radiation). In preparation of the XRD samples, the solid
residue was dried at 80 C under a dynamic vacuum condition
for 48 h.
2.2. Hydrogen generation performance testing
The experimental setup used for measurement of HG properties
has been described in a previous paper [21]. The reaction
was carried out in a 250 ml three-neck flask, wherein the solid
fuel was preloaded. The water was fed into contact with the
solid fuel using a pressure-equalizing dropping funnel. Typically,
the feeding rate of water was controlled at around
10 g min 1 . The generated hydrogen gas passed through
a trap/heat exchanger to cool to room temperature followed
by contacting with a silica drier to remove the moisture in the
gas stream. The HG rate was measured using an online mass
flow meter (Sevenstar Huachang, MFM D07e7BM, accuracy
within 2%) that was equipped with a computer. The HG
volume was calculated by integrating the measured HG rate
over time. In the whole testing process, no attempt was made
to control the temperature of the reaction system. The reaction
temperature was monitored using a thermocouple
embedded in the solid fuel and recorded using an online
recorder. Each experiment was repeated twice. The determined
relative error was no more than 5%.
3. Results and discussion
3.1. Development of a new Al-based hydrogen
generation system
The newly developed HG system is composed of Al/NaOH/
Na 2 SnO 3 solid mixture and water. In comparison with the
previous systems that use aqueous solution of promoting
additives, the present system using water as a reaction
controlling agent is clearly of increased practicability. As
shown in Fig. 1, feeding water into contact with the Al/NaOH/
Na 2 SnO 3 solid fuel rapidly initiates vigorous H 2 release. The
system can complete a 100% fuel conversion within 3 min
with a maximum HG rate of 1740 ml min 1 . In contrast, the
systems containing the same mass of individual NaOH or
Fig. 1 e A comparison of the HG rate (top) and yield
(bottom) among the systems: (a) 1 g of Al powder/0.2 g of
NaOH/0.1 g of Na 2 SnO 3 /5 g of H 2 O; (b) 1 g of Al powder/0.3 g
of NaOH/5 g of H 2 O; (c) 1 g of Al powder/0.3 g of Na 2 SnO 3 /5 g
of H 2 O.