**4. Discussion**

According to the previous experimental results, it is found that the hydrogen production performance of alloys with di fferent Zn doping amounts is significantly di fferent. The composition of the material directly a ffects its physical and chemical characteristics. Therefore, the determination of the main phases of the alloy and the qualitative analysis of the distribution characteristics of Zn should be discussed first.

In addition to the Al and In3Sn phases determined by XRD testing, the existence of Ga as a solid solution of Al (Ga) has also been verified in many studies [30–32]. The EDX analysis shows that Zn exists in both the G and GB phases. As shown in Figure 6d and EDX mapping of the whole image in Figure 6b, Zn atoms are randomly dispersed throughout the entire region, which is similar to the distribution of Ga in Al alloys [12]. It is further proven that Zn is more likely to exist as a solid solution combined with the XRD analysis and Al-Zn phase diagram. In addition, a small part of Zn will appear on the grain surface in the form of Zn-rich particles.

The entry of Zn into the GB phase means that the eutectic reaction of Al and GB phases will change accordingly, which is identified to be a key factor a ffecting the hydrolysis reaction of Al. The analysis results of DSC and activation energy confirmed this point, and their change rules are highly consistent, which can be explained by the same reason: As Zn begins to entered into the GB phase, the main composition of GB phase was changed from Ga-In-Sn to Ga-In-Sn-Zn, and the eutectic temperature and Ea of the grain boundary phase decreased. On the other hand, in the process of Zn taking the place of In3Sn, the melting point of Zn is significantly above other GB elements, leading to the eutectic temperature and Ea increases gradually. In addition, high activation energy will e ffectively increase the di fficulty of the alloy to react with moisture in the air, which is conducive to long-term storage and long-distance transportation. Therefore, this can also be considered as a criterion for judging the practical value of alloys.

However, this mechanism does not perfectly explain that the sample (3 wt.% Zn) still has excellent hydrogen production performance. According to the conclusion of the previous study [37,38], two main mechanisms can be used to discuss the occurrence of these phenomena. On the one hand, the eutectic reaction of Al and GB phase is the origin of the Al hydrolysis reaction. When Zn was added, the sample became less susceptible to split when exposed to water. At the same time, the area of a single GB phase was reduced to a minimum value at 2 wt.% Zn (Figure 5), which delayed the startup time of the reaction, and further delayed the arrival of the maximum hydrogen production rate (Figure 11). On the other hand, the Zn will form di fferent micro batteries with other metals and Al corrodes rapidly as the electrode potential of Al–Zn is formed. It is clear from Figures 2a,b and 11 that the time consumed by the hydrolysis reaction of 2 wt.% Zn alloy is greater than the 3 wt.% one. Although the single GB area of the two is similar (Figure 5), the high Zn content means that more Al-Zn micro-galvanic cells will promote the reaction. Nevertheless, the content of Ga, In and Sn in the GB phase directly determines the starting temperature of the hydrolysis reaction. That is why the high-Zn content alloys (4 wt.%, 5 wt.% Zn) cannot hydrolyzed at a lower reaction temperature of 40 ◦C and low content of In3Sn makes the alloy stay intact in water.

**Figure 11.** Statistics on hydrogen production time of Al alloys at 40 ◦ C.

As shown in Figure 12, when the temperature increased from 40 to 60 ◦C, the max hydrogen production rate of Al-Ga-In3Sn alloy doubled. However, maximum release rate of hydrogen from Zn-containing alloys (1 wt.%, 2 wt.%, 3 wt.%) increased by 2.5, 8 and 6.7 times, respectively. The emergence of Al-Zn micro-galvanic cells in Zn-containing alloys has also contributed to an increase in the hydrogen production rate. Furthermore, at the same temperature, as the amount of Zn doping increases, the maximum hydrogen generation rate is constantly changing and has a maximum at 1 wt.% Zn. If the maximum hydrogen generation rate (almost with high Zn content alloy) is lower than that of the Zn-free alloys, their reaction duration is often extremely long. This is because the eutectic temperature of the GB phase in the high-Zn alloy (more than 1 wt.%) is higher, and more energy is required to promote the hydrolysis reaction.

**Figure 12.** Maximum generation rate versus Zn content.

The factors a ffecting the hydrogen production performance of di fferent alloys are summarized as follows:

(1) The occurrence of alloy hydrolysis is related to the GB phase and the external surrounding (water bath) temperature.

(2) The GB phase composition of 1 wt.% Zn sample results in its minimum activation energy, and the formation of a small portion of Al-Zn micro-galvanic cells results in the fastest hydrogen production rate.

(3) When the doping amount of Zn reaches 2 wt.%, the area of a single GB phase decreases rapidly. At the same time, when the water temperature is 40 ◦C, the Al-Zn micro-galvanic cells react slowly, and the local heat generated is small, which delays the arrival of the peak of hydrogen production rate and prolongs the reaction time.

(4) Compared with the 2 wt,% Zn sample, the single GB phase area and the number of GB in 3 wt.% alloy have not changed much, but the appearance of more Al-Zn micro-galvanic cells has increased the local temperature and accelerated the reaction.

(5) When the Zn doping amount reaches 4 wt.% or 5 wt.%, the area and number of GB phases continue to decrease rapidly. The need for higher activation energy means a higher reaction temperature.

In the present work, the mechanisms used to explain the e ffect of Zn on hydrogen production can be summarized as follows:

(1) The GB phase includes all the eutectic compounds among the grain boundary elements. This is identified to be a key factor that determines whether the Al hydrolysis reaction can proceed. While protecting Al from oxidation, the GB phase can provide a transmission channel to make it easier for Al to enter the reaction site. According to EDX data (Table 2), the GB phase contains a large amount of Al, when the sample is contacted by water, this part of Al first reacts with water. The decrease of the Al concentration will drive other Al molecules from the grains into the GB phase, and make the reaction continue. The addition of Zn will change the composition of the GB phase from Ga-In-Sn to Ga-In-Sn-Zn, which will a ffect the eutectic temperature and hydrolysis reaction. A small part of In3Sn was replaced by Zn, which will help increase the relative content of Ga in Ga-In-Sn.and make the mass ratio of the three elements in the raw material close to the eutectic point, but as more low-melting metals (In and Sn) are replaced by Zn (more than 1 wt.%), this will lead to a gradual increase in eutectic temperature.

(2) During the hydrolysis reaction, Al will form micro-galvanic cells with Zn. The release of hydrogen through this electrolytic form is relatively slow, but the heat released helps the hydrolysis reaction. In the alloys, the appearance of the Al-Zn solution and simple substance of Zn will aggravate the formation of micro-galvanic cells. Therefore, with increasing temperature, this variable of hydrogen generation rate is greater for Zn-containing alloys than Zn-free ones.
