Researchers from the Chinese Academy of Sciences and Tsinghua University have used a gallium, indium, tin and bismuth alloy to generate hydrogen, when placed in contact with an aluminum plate immersed in water. The hydrogen is then used in a PEM fuel cell. An open-access paper on their work appears in the Journal of Renewable and Sustainable Energy, from AIP Publishing.

A schematic diagram demonstrating the combination of a hydrogen extraction system and a PEMFC for power supply. Credit: Jing Liu

In their study, the Ga-In-Sn-Bi alloy-activated Al-H₂O reaction was demonstrated to be rate-stable with a hydrogen productivity of 92% and therefore superior to similar reactions activated by Ga-In and Ga-In-Sn alloys, the authors said.

Hydrolysis of active metals is a widely known hydrogen production approach. Al is a favored hydrogen generation material because of its relatively low cost, low density, and abundant geological reserves. An inevitable barrier encountered during the use of Al is that a thin oxide film forms immediately once Al is exposed to air, impeding further contact and reaction between Al and water. Only acid or alkaline solutions can dissolve this passive film, but such solutions cause corrosion.

In the 1960s, Woodall et al. found that low melting point metals such as gallium and its alloys could disrupt the oxide film on the Al surface and activate Al-H₂O reactions. Since then, considerable efforts have been made to investigate hydrogen generation performance via hydrolysis of Al-Ga based alloys. Al and liquid metal (LM) alloys are alloyed via high-temperature melting or mechanical milling in advance. However, these processes consume energy and time, and the preprepared alloys are easily passivated. In addition, the high cost of hydrogen transport is a key barrier to hydrogen utilization. Hence, it is necessary to develop real-time, on-board hydrogen production systems.

In 2015, Zhang et al. found that gallium-based LM could spontaneously encase Al foil and trigger the Al-H₂O reaction at room temperature. Before use, Al and the LM alloys were stored separately to prevent passivation. Once hydrogen was required, Al and the LM alloys were placed together in electrolyte, where they immediately began to produce hydrogen. Tan et al. compared hydrogen production from Al-H₂O reactions triggered by different binary alloys including Ga-In, Ga-Zn, and Ga-Sn. Different metal compositions performed in substantially different ways. … Adding Sn not only can enhance the hydrogen generation rate but also accelerates oxidation of the LM alloy. The Ga-In-Sn alloy-activated reaction initially proceeds quickly, but its reaction rate soon decreases.

… This paper investigates hydrogen generation using the Ga-In-Sn-Bi alloy-activated Al-H₂O reaction. This reaction meets the requirement of hydrogen supply to fuel cells, namely, stability, high purity, and high efficiency. Real-time, on-demand hydrogen generation can relieve the burdens of hydrogen storage and transport to some extent.

—Xu et al.

In the study, a 3 g Al plate activated by Ga-In-Sn-Bi alloy steadily exported an electrical power of 0.73 W via a PEM fuel cell.

Overall, hydrogen generation from the Ga-In-Sn-Bi alloy-activated Al-H₂O reaction exhibited excellent stability and durability, making it a preferred hydrogen source for fuel cells. In addition, no complex prepreparation work was required, and the reaction proceeded in a neutral solution. Thus, this is a promising method of real-time, on-demand hydrogen production with potential for use in various applications.

—Xu et al.

However, said senior author Jing Liu, a professor at the Chinese Academy of Sciences and Tsinghua University, it is important to be able to recycle the alloy in order to further reduce cost and environmental impact.

There are various problems in existing methods for post-reaction mixture separation. An acid or alkaline solution can dissolve aluminum hydroxide but also causes corrosion and pollution problems.

—Jing Liu

Other byproduct removal methods are difficult and inefficient, and the problem of heat dissipation in the hydrogen reaction process also needs to be optimized. Once these difficulties are resolved, this technology can be used for applications from transportation to portable devices, Liu said.

Resources

• Shuo Xu, Yuntao Cui, Lixiang Yang and Jing Liu (2020) “Instant hydrogen production using Ga-In-Sn-Bi alloy-activated Al-water reaction for hydrogen fuel cells” Journal of Renewable and Sustainable Energy doi: 10.1063/1.5124371