**1. Introduction**

Due to the limited world proven reserves and environmental degradation caused by fossil fuel consumption, it is clear that social development based on traditional energy sources is not sustainable. Consequently, it has been an irresistible trend to find alternate green energy which appeals for a clean, co-development and e fficient energy future [1–3]. Hydrogen as an energy carrier has a high energy density about three times higher than gasoline. Furthermore, the water produced after hydrogen energy combustion is absolutely clean and will not cause environmental pollution. In the past few decades, the main approaches to produce hydrogen could be grouped into the following categories [4–7]: water electrolysis, fossil fuel gasification, chemical processing techniques and biological methods. So far, a wide range of applications of hydrogen has not been realized through these methods. Many problems need to be solved, such as expensive cost, poor conversion e fficiency, insecurity in hydrogen storage and transportation. In this case, it is extremely essential to produce hydrogen in situ, particularly in the aspects of emergency power provided in disaster-stricken areas, portable electronic equipment and on board vehicles. Hence, researchers are constantly looking for new ways of producing hydrogen gas.

In recent years, an emerging in situ method through the hydrolysis of Al alloys has been increasingly attracting the interest of researchers [8–10]. The hydrogen generated by such a method has many advantages. Firstly, the hydrogen can be produced almost anywhere, which eliminates the strict requirements for storage and transport. Secondly, the generated hydrogen has high purity and hence, can be directly supplied to internal combustion engines or fuel cells. Most of all, aluminum as a

metal is rich in the earth's crust; it also has a high hydrogen generation capacity. Through theoretical calculations, the amount of hydrogen produced per 1 g of Al can reach 1.244 L. Nevertheless, a fine and close oxide film easily forms on the pure Al surface which impedes the subsequent reaction of Al hydrolysis. Several treatment measures have been taken to activate Al as follows [11–14]: alkali aqueous activation, nano/micro-crystallization of Al, and mixing particular metallic oxide or mineral salts. Nevertheless, these processing ways have certain disadvantages, e.g., acid or alkaline solution is extremely corrosive and the pre-preparation work of milling is time consuming. Adding some low melting alloys to Al and alloying was a fascinating method of activation [15–18]. This approach can produce Al alloys in large quantities in a short time when required, and Al can be hydrolyzed at a mild temperature to generate hydrogen.

Previous studies have shown that the size and composition of GB phase play the dominant role in the process of hydrolysis reaction between Al and water [19,20]. The GB phase containing low melting point metals can prevent the oxidation of Al. Meanwhile, these activated areas also provide a transmission channel to make it easier for Al to enter the reaction site. Up to now, the Al-Ga-In-Sn alloy has been investigated in a systematic study [21,22]. The melting point of liquid Ga-In-Sn phase is 10.4 ◦C, which has a grea<sup>t</sup> correlation with the reaction temperature. In addition, In and Sn mainly exist in two kinds of interstitial phases: In3Sn (β) and InSn4 (γ). When the content of In3Sn is high, the alloy exhibits high energy transduction e fficiency and fast hydrogen generation rate. Yet, compared with Al-Ga-InSn4, Al-Ga-In3Sn alloy also su ffers from some disadvantages. Indium is expensive and hard to recycle. Furthermore, the surface of this alloy easily reacts with the moisture in the air, which requires higher storage conditions. For this reason, in consideration of cost, a fifth metal needs to be introduced to further improve the comprehensive performance of the Al-Ga-In3Sn alloy.

Combined with results of previous studies, various dopants have di fferent e ffects on the Al-Ga-In-Sn alloy. When Bi [23] or Mg [24] are added to these Al-based alloys, new GB phases (InBi or Mg2Sn) are formed, and the alloy exhibits di fferent energy conversion e fficiency. With the addition of the grain refiners (Ti, Al2O3, AlTi5B etc.) [25–28], the grain size and morphology of the alloys have changed significantly, which also a ffects hydrogen production performance. In a word, the effect of doped metals on the alloy is usually to change the GB composition or grain size.

It is acknowledged that Zn is an important metal used in industry. Studying the doping of Zn in Al-Ga-based hydrogen-producing alloys will help a wider choice of raw materials and the use of waste Al in the future. In order to achieve practical application, the process of hydrogen production needs to be controlled in an on-board hydrogen supply unit. Thus, it is worth undertaking a comprehensive investigation into the specific influence of Zn in water splitting reactions. By adjusting the mass ratio of In3Sn and Zn, alloys with di fferent hydrogen production properties can be obtained. Di fferentiated reaction temperature and start-up time help to realize the control of an on-board hydrogen supply.

For the sake of high economic benefits and excellent hydrogen production performance, Zn was introduced into Al-Ga-In3Sn alloys to reduce the contents of In and Sn. The micrographs and crystalline structure of alloys were examined by XRD, SEM with EDS. Then, the alloy melting behavior was measured by DSC and corresponding hydrogen production performance was discussed. In addition, we used an isothermal kinetic model to calculate the activation energy of alloy hydrolysis and discussed the relevant reaction mechanisms.

#### **2. Materials and Methods**

In order to explore the relationship between physicochemical properties and material properties of Al-Ga-In3Sn-Zn, a series of alloys with di fferent Zn contents were prepared and some experimental tests were performed. Zn was hypothesized to change the composition of the GB phase and thus affect the hydrogen production performance. In previous studies, the eutectic reaction in the GB phase was identified to be a key factor in the hydrolysis of Al. The continuous replacement of In and Sn, the main component of the GB phase, by Al can help verify this theory and determine other possible reaction mechanisms.

## *2.1. Preparation of Materials*

Six kinds of Alloy ingots (Table 1) were prepared using a simple melting and casting technique. The weights of the cast ingots were kept at 20 g approximately. In these samples, the contents of Al and Ga remain fixed, the mole ratio of In and Sn remained a constant of 3:1, while Zn content changed from 0 wt.% to 5 wt.%.


**Table 1.** Compositions of prepared alloys ( wt.%).

The starting mixture containing Al (purity, 99%) and dopants (Ga, In, Sn, and Zn of at least reagen<sup>t</sup> grade) was heated at a rate of 10 ◦C/min up to 800 ◦C and held at this temperature for 1 h. The smelting was protected by nitrogen in the furnace. After that, the molten metals were stirred at a constant speed for 10 min by a stirring paddle and then cast into the mold with a cylindrical recess and cooled in air. Finally, the cooled ingot was wrapped with a sealing film and stored in a vacuum sealed bag for the follow-up experiments.
