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

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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 .
international journal of hydrogen energy 37 (2012) 5811e5816 5815 with the system using deionized water, the system using sea water exhibits an around 10% increase on HG rate and a slightly shortened induction period. Presumably, the advantage of sea water over the other two types of water originates from the enriched chlorine ions, which may accelerate the dissolution reaction of the passivation layer on the Al surface [18]. Evidently, using water as a reaction controlling agent implies the wide applicability of the newly developed HG system. 3.3. “Start/stop” hydrogen generation dynamics Rapid dynamic response to the highly varying hydrogen demands is another prerequisite for a practical HG system. In the present study, we tested the dynamic response property of the Al/NaOH/Na 2 SnO 3 /H 2 O system in a discontinuous HG process, which involves cyclic start/stop of water supply to the reactor containing solid fuel. To avoid the influence of fuel temperature on the HG performance, the reactor was cooled down to ambient temperature between each stop/start cycle. As shown in Fig. 7, feeding water into contact with the solid fuel immediately initiates vigorous H 2 release. In the first two cycles, the HG reaction reaches its maximum rate within around 1.3 min, and after switching off the water supply, the HG reaction can be terminated within around 1 min. But in the third cycle, the system exhibits a multi-step HG behavior and requires a relatively long duration for terminating the HG reaction. Most likely, this should be ascribed to the deteriorated mass transfer environment, wherein the accumulated by-products block the water from contacting the unreacted Al. But despite of the property degradation, the HG reaction in the third cycle exhibits a maximum rate of 450 ml min 1 , which is 75% of the level in the first cycle. After three start/stop cycles, the total HG yield of the system reaches up to 93%. In a general view, the newly developed system shows satisfactory dynamic response property. This is of clear significance for the design of practical hydrogen generator. Using the data obtained from “start/stop” HG dynamics measurement, we can estimate the power level of the newly developed system. If the new system is combined with Fig. 7 e “Start/stop” HG dynamics and temperature profiles of the system composed of three pieces of solid fuel tablets (as shown in the inset) and 15 g of water. The feeding rate of water was fixed at 2 g min L1 . a standard proton exchange membrane fuel cell operating at 0.7 V, a HG rate of 500 ml min 1 is equivalent to a power level of 40 W. This can meet the power demands of some portable or mobile apparatus. Using the current price of primary Al, 2.4 US$ per kg, as a calculation benchmark, the H 2 -production cost of the present system is estimated to be around 21 US$ per kg H 2 [9]. This H 2 -production cost is much lower than those of the chemical hydrogen storage systems using boroncontaining chemicals [2,5]. 4. Conclusions Our study demonstrated a new Al-based hydrogen generation system, which is composed of Al/NaOH/Na 2 SnO 3 solid mixture and water. As a consequence of the pronounced promoting effects of NaOH and Na 2 SnO 3 additives, the on-demand hydrogen generation can be readily achieved by regulating the water supply to the solid fuel. The new system possesses a favorable combination of high hydrogen generation rate, high fuel conversion, rapid dynamic response and low material cost, which makes it promising for portable hydrogen source applications. In addition, the use of solid fuel in a tablet form and the adaptability to different types of water greatly increase the applicability of the new system. Our study shows the feasibility of achieving high-performance hydrogen generation using cheap and readily available materials, thereby laying a foundation for developing practical hydrogen generators. Acknowledgments Fig. 6 e Effects of the type of water on the HG rate (top) and yield (bottom) of the system composed of a piece of solid fuel tablet (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. The financial supports for this research from the National Basic Research Program of China (973 program, Grant No. 2010CB631305), the National Outstanding Youth Science Foundation of China (Grant No. 51125003), the National Natural Science Foundation of China (Grant No. 51071155), the Natural Science Foundation of Liaoning Province (Grant No. 20102231), and the Main Direction Program of Knowledge Innovation of CAS are gratefully acknowledged.
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