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

5814
international journal of hydrogen energy 37 (2012) 5811e5816
surface of Al may prevent the formation of cohesive Al(OH) 3
passivation layer. As a consequence, the Al/H 2 O reaction can
proceed in a continuous way, even in the presence of low OH
concentration.
3.2. Study of hydrogen generation properties of the Al/
NaOH/Na 2 SnO 3 /H 2 O system
As demonstrated, the newly developed Al/NaOH/Na 2 SnO 3 /
H 2 O system possesses a series of advantages over the traditional
NaOH-promoted system in terms of HG performance.
To evaluate its potential for practical hydrogen storage
applications, we further conducted a systematic study of the
factors that influence the HG performance of the system. The
first factor we examined is the NaOH:Na 2 SnO 3 mass ratio. For
comparison purpose, the quantities of Al, H 2 O and promoting
additives were fixed at 1, 5 and 0.3 g, respectively. As seen in
Fig. 4, changing the NaOH:Na 2 SnO 3 mass ratio from 2:1 to 1:2
results in a decrease of the HG rate and yield, showing that the
system containing 0.2 g of NaOH and 0.1 g of Na 2 SnO 3 shows
the best HG performance.
Next, we examined the effect of the particle size of Al
powder on the HG performance of the system. As seen in
Table 1, both the HG rate and yield increase with reducing the
particle size of Al powder. This should be ascribed to the
variation of the surface area of Al. When using Al powder with
an average particle size smaller than 68 mm, the system
exhibited high HG rate and achieved 100% fuel conversion. In
an overall consideration of the HG performance, material cost
and availability, we selected the Al powder with an average
particle size of 68 mm for further investigation.
The newly developed HG system uses Al/NaOH/Na 2 SnO 3
mixture as solid fuel. Our study found that the mixing state
and form of the solid fuel exert important effect on the HG
performance of the system. As shown in Fig. 5, pretreatment
of the solid fuel by ball-milling instead of hand-milling results
in an increase of HG rate and a reduction of induction period.
This should be mainly attributed to the improved dispersion
Table 1 e Effect of the particle size of Al powder on the HG
performance of the Al-based system. Note: the Al-based
system is composed of 1 g of Al powder, 0.2 g of NaOH,
0.1 g of Na 2 SnO 3 and 5 g of H 2 O.
Average particle
size of Al (mm)
Max. HG rate
(ml min 1 g 1 Al)
Yield (%)
278 410 83.4
198 515 85.2
152 740 88.3
125 1025 94.3
68 1740 100
16 2100 100
of promoting additives in the solid fuel. In addition, the
extensive mechanical milling may create increased opportunity
for the close contact between the additive particles and
the fresh surface of Al, which should also contribute to the
improvements of HG performance. In spite of the clear benefits
on HG performance, the solid fuel in powder form is
undesirable for the practical hydrogen storage application due
to the poor operationality, safety concern, as well as the
complicated system design. For this reason, we pressed the
ball-milled fuel powder into the tablets. As seen in Fig. 5, the
system using the solid fuel tablets shows a similar HG rate, but
a much longer induction period than those of the systems
using solid powder fuel. Our preliminary study found that this
problem can be alleviated by adding small amount of Al
powder. For example, extra addition of 0.2 g of Al powder into
the system containing 1.3 g of solid fuel tablet leads to
a reduction of induction period from 1 to 0.5 min, as shown in
Fig. 5(d).
Finally, we examined the effect of water type on the HG
performance of the system. As seen in Fig. 6, changing the
water type from deionized water to tap water exerts no
appreciable influence on the HG performance of the system.
Among the three types of water we examined, the sea water
containing 3.5 wt% of NaCl is the best choice. In comparison
Fig. 4 e Effects of the NaOH:Na 2 SnO 3 mass ratio on the HG
rate (top) and yield (bottom) of the system. The quantities
of Al, H 2 O and promoting additives were fixed at 1, 5 and
0.3 g, respectively.
Fig. 5 e Effect of pretreatment of the solid fuel on the HG
kinetics of the system composed of 1 g of Al powder/0.2 g of
NaOH/0.1 g of Na 2 SnO 3 and 5 g of water: (a) hand-milled; (b)
ball-milled; (c) ball-milled and pressed; (d) the system (c)
with extra addition of 0.2 g of Al powder. The inset shows
a photo of the solid fuel tablet with a density of 2.45 g cm L3 .