Enhanced Hydrogen Generation Properties of Al-Ga-In-Sn Alloy in Reaction with Water by Trace Amount of AlTi₅B Additives

Abstract

Al-3Ga-3In-Sn (mass%) alloy with a little amount of AlTi₅B as refiner was fabricated using a simple smelting and casting method. The phase compositions and microstructure were investigated by means of XRD and SEM with EDX. The hydrogen generation property of Al-Ga-In-Sn alloy with water was investigated. The results show that the Al grains and GIS (Ga-In-Sn) particles are refined and more uniform with adding AlTi₅B to Al-Ga-In-Sn alloy. Al grains size decrease from 120 μm to 40 μm and the GIS particles are refined to 1 μm respectively. The hydrogen generation rate of Al-3Ga-3In-Snalloy with 0.1 mass%AlTi₅B reaches 44 mL/min g at 30℃, and 460 mL/min g at 60℃, which is much higher than that of Al-3Ga-3In-Sn alloy. The high hydrogen generation rate is ascribed to the Al grain refinement and increasing amounts of GIS particle.

1. Introduction

Due to the fossil fuel depletion and air pollution arising from its combustion, there is an urgent demand for renewable and clean fuel for future energy supply. Hydrogen has attracted much attention as an environmentally friendly fuel due to its high calorific value^1,2). In the past two decades, many methods have been developed to produce hydrogen, including biological water electrolysis, water splitting and steam oxidation. However, high cost, low conversion efficiency, non-clean preparation process, as well as the transportation and storage of hydrogen limit the wide-scale applications of hydrogen. Therefore, in situ production of hydrogen is extremely essential, especially in the aspect of emergency power units needed in disaster areas, portable electronic devices, on-board vehicles etc.^3–6) Recently, aluminum alloy has been identified as the most promising materials to generation hydrogen from water. Aluminum alloys have high gravimetric and volumetric energy density, low costs and non-toxic reaction by-product^7–9). However, pure aluminum does not react with water by itself due to the existence of a dense aluminum oxide film on its surface. Considerable efforts have been made to solve the problem. It was reported that Al could react with water to produce hydrogen with the assistance of alkaline or at elevated temperature^10,11). However, the harsh condition can damage the apparatus where the hydrolysis reaction occurs, which makes it not suitable for portable and household applications. Therefore, it is necessary to develop new methods to produce hydrogen. At present, aluminum alloyed with certain low melting point metals including Ga, In, Zn, Sn and Bi et al., was found to rapidly react with water to produce hydrogen, especially at room temperature. Fan et al.¹²⁾ prepared Al-30Bi (mass%) alloy using mechanical alloying method, which had high reactivity and hydrogen yield. Jeffrey T.Z. et al.¹³⁾ fabricated Al-34Ga-11In-5Sn alloy (mass%) using cast method. The hydrogen yields reached 80% and the react time was about 400 seconds at 24℃. Most investigations had indicated that the high reactivity and hydrogen yield in the hydrolysis was reached only via using high content of alloying element (such as Bi, Ga, In et al.), which increased the costs of hydrogen generation. Therefore, how to increase the hydrogen conversion efficiency and meanwhile decrease the amount of Ga and In is still a challenging task. Wang and co-workers^14,15) had indicated the microstructure and phase compositions of Al-Ga-In-Sn alloys were key factors controlling the hydrogen generation rate. In this work, we attempt to fabricate aluminum alloys with low content of Ga, In, Sn and trace amount of AlTi₅B which is regarded as a refiner to modify the microstructure using a simple smelting and casting method. The effect of AlTi₅B and reaction temperature on microstructure and hydrogen generation properties was investigated. The phase compositions and microstructure of different Al-Ga-In-Sn alloys were characterized.

2. Experimental Procedures

High purity Al (99.9 mass%), Ga (99.8 mass%), In (200 mush, 99 mass%) and Sn (200 mush, 99.9 mass%) were used as starting materials. AlTi₅B (99.9 mass%) was used as a refiner to reduce the grain size of as-cast aluminum alloy in the experiment. Ingots of 30 g in weights with a nominal composition of Al-3Ga-3In-Sn and Al-3Ga-3In-Sn-0.1AlTi₅B (mass%) were prepared by melting the mixture of Al, Ga, Sn, In and AlTi₅B under air condition in a corundum crucible. The hydrolysis reaction test was employed in a 500 mL three-necked bottle, as shown in Fig. 1. For each test, 0.5 g of aluminum alloy was added to 300 mL tap water in three-necked bottle at 30℃ and 60℃. The hydrogen volume was tested through a drainage method. The hydrogen generation rate was calculated by the hydrogen volume and the corresponding reaction time. The hydrogen yield was determined by using α = V/V_o, where V is the current volume of generation H₂ and V_o is the theoretical H₂ yield (1.244 L/(g Al)).

Fig. 1

Schematic diagram of equipment used for hydrogen generation. 1. Sample window, 2. Thermo-static water bath, 3. Thermometer, 4. Gas pipe 5. Tap water, 6. Burette.

The synthesized alloys were analyzed by X-ray diffraction (XRD) using XD-2 type diffractometer with Cu Kα (λ = 1.54 Å) radiation. The microstructure and compositions were investigated by field emission scanning electron microscopy (Quanta FEG-450) with an energy dispersed X-ray (EDX). In order to eliminate or minimize the oxidation of the fresh surface, the ingots were placed into the sample chamber immediately after carved. The temperature of tap water during the hydrolysis process was detected by a thermal couple.

3. Results and Discussion

Figure 2 shows XRD patterns of the synthesized Al-Ga-In-Sn alloy. It can be seen that all samples contain the crystalline Al phase. Ga phase of the alloys isn't found in the patterns, which may be attributed to the low melting point of Ga (29.8℃) and the formation of GIS (Ga-In-Sn) eutectic alloy (eutectic melting point is 10.7℃) when In and Sn are introduced to Ga^12,13). Furthermore, a portion of Ga may form solid solution with Al. Therefore, there are no apparent diffraction peaks of Ga and its alloys observed as the low amount of Ga in this experiment. Similar phenomenon was observed in our previous work¹⁶⁾. The diffraction peaks of In₃Sn can be observed in the pattern, which is in agreement with previous works^14,16). Si phase can be found in the XRD pattern maybe due to the import of impurity. As a refiner, the diffraction peak of AlTi₅B is hardly observed due to the fewer amounts.

Fig. 2

XRD patterns of the as-cast aluminum alloy.

Figure 3 shows the fracture surface SEM images of the as-prepared aluminum alloy. It can be seen that the grains of Al-3Ga-3In-Sn alloy are even coarser than that of Al-3Ga-3In-Sn-0.1AlTi₅B, as shown in Fig. 3 (a)–(d). With adding AlTi₅B, Al grains of the alloy are obviously refined, uniform and equiaxed. The average grain sizes decrease from about 120 μm to 40 μm. Furthermore, it is interesting that GIS phase is clearly observed at the grain boundaries and surfaces. For Al-3Ga-3In-Sn alloy, the flake-like GIS phase mostly distributed at grain boundary is about 20 μm thick, as shown in Fig. 3 (a), (b). Wang and Woodall^13,14) also observed the similar microstructure. For Al-3Ga-3In-Sn-0.1AlTi₅B alloy, most GIS particles covered on the grain surface rather than at the grain boundary. The average size is about 1 μm, as shown in Fig. 3 (c), (d), which is much smaller than that of Al-3Ga-3In-Sn alloy. This may be attributed to the addition of AlTi₅B, which extremely refines the Al grains and GIS phase with increasing nucleation site.

Fig. 3

SEM images of aluminum alloy.

Figure 4 shows EDS of the aluminum alloy. Figure 4 (a) and (b) reveals that the phase occurring at the boundary consists of In, Sn and/or Ga, which suggested that maybe most of the GIS phase is Ga-rich In-Sn intermetallic compound. Figure 4 (c), (d) reveals that the white particles covered on the surface is maybe Ga-rich In₃Sn compound due to the atom ratio of In and Sn. This is in agreement with the XRD's result of Fig. 2.

Fig. 4

EDS of as-prepared samples.

Figure 5 shows hydrogen yield versus reaction time of both aluminum alloys at different temperature. Figure 5 (a) shows the curves of hydrogen yield versus reaction time at 30℃. With increasing reaction time, the hydrogen yield increases rapidly and reaches 100% for Al-3Ga-3In-Sn-0.1AlTi₅B alloy, which indicates that the alloy has a rapid hydrolysis reaction rate. The reaction ends in 70 min. For Al-3Ga-3In-Sn alloy, the hydrogen yield only reaches 40% in 120 min. With increasing reaction temperature, hydrogen generation rate of Al-3Ga-3In-Sn-0.1AlTi₅B alloy increase rapidly and the reaction ends in only 7 min. the hydrolysis reaction rate of Al-3Ga-3In-Sn alloy is very slow, even at higher temperature. This may be attributed to the Al grain refinement and the increasing number of GIS particles covered on the grain surface. Wang and co-workers¹⁴⁾ proposed an expression (G ∝ βNR²ρ/d) to calculate the hydrogen generation rate, where β is a factor related to the GIS particles sizes, ρ is H₂ generation rate of per unit area of Ga-In-Sn. G, N, R and d are hydrogen generation rate, number and size of GIS particles, and Al grain size, respectively. It is obvious that with the decrease of Al grain size and increase of the number of GIS particles, the hydrogen generation rate will increase.

Fig. 5

Hydrogen yield versus reaction time at different temperature.

Figure 6 shows hydrogen yield of Al alloys reacting with tap water at different temperature. The dependence of the hydrogen generation rate on the reaction time of Al-3Ga-3In-Sn and Al-3Ga-3In-Sn-0.1AlTi₅B alloy is plotted in Fig. 6. The hydrogen generation rate first increases and then decreases with increasing reaction time. As shown in Fig. 6 (a), at low reaction temperature (30℃), the maximum rate for Al-3Ga-3In-Sn-0.1AlTi₅B alloy reaches 44 mL/min g, however, 30 mL/min g for Al-3Ga-3In-Sn alloy. With increasing reaction temperature, the maximum rate of Al-3Ga-3In-Sn-0.1AlTi₅B alloy increases sharply, which reaches 460 mL/min g at 60℃. The maximum rate of Al-3Ga-3In-Sn only reaches 100 mL/min g. The big difference in hydrogen generation rate is attributed to the decrease of Al grain size and the increasing numbers of the GIS particles at the same temperature.

Fig. 6

Hydrogen generation rate of Al alloys at different temperature.

4. Conclusions

Al alloys are fabricated using Al, Ga, In, Sn and AlTi₅B by a traditional smelting and casting method in this work. The hydrogen generation property of Al alloy with water is investigated at different reaction temperature. It is found that AlTi₅B can refine Al grain and increase the numbers of GIS particles. The final hydrogen production of the Al alloy depends on Al grain size, numbers of GIS particles and reaction temperature. For Al-3Ga-3In-Sn-0.1AlTi₅B alloy, the hydrogen generation rate reaches 44 mL/min g at 30℃, and 460 mL/min g at 60℃, which is much higher than that of Al-3Ga-3In-Sn alloy. The high hydrogen generation rate is ascribed to the Al grain refinement and increasing number of GIS particle.

Acknowledgments

This project is financially supported by the National Natural Science Foundation of China (Grant No. 51075129, 51375150) and Hubei Provincial Natural Science Foundation of China (Grant No. 2013CFB018).

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