UC Santa Cruz chemists have developed a simple method to make aluminum nanoparticles that split water and generate hydrogen gas rapidly under ambient conditions. The water-splitting reaction does not require an applied potential and functions at ambient conditions and neutral pH to rapidly generate 130 mL (5.4 mmol) of hydrogen per gram of alloy. A paper on their work is published in the journal ACS Applied Nano Materials.

Aluminum is a highly reactive metal that can strip oxygen from water molecules to generate hydrogen gas. Its widespread use in products that get wet poses no danger because aluminum instantly reacts with air to acquire a coating of aluminum oxide, which blocks further reactions.

For years, researchers have tried to find efficient and cost-effective ways to use aluminum’s reactivity to generate clean hydrogen fuel. A new study by the researchers at UCSC shows that an easily produced composite of gallium and aluminum creates aluminum nanoparticles that react rapidly with water at room temperature to yield large amounts of hydrogen. The gallium was easily recovered for reuse after the reaction, which yields 90% of the hydrogen that could theoretically be produced from reaction of all the aluminum in the composite.

We don’t need any energy input, and it bubbles hydrogen like crazy. I’ve never seen anything like it.

—UCSC Chemistry Professor Scott Oliver, co-corresponding author

Bubbles of hydrogen gas are generated from the reaction of water with an aluminum-gallium composite. Movies of the reaction are available online. (Credit: Amberchan et al., Applied Nano Materials 2022)

The reaction of aluminum and gallium with water has been known since the 1970s, and videos of it are easy to find online. It works because gallium, a liquid at just above room temperature, removes the passive aluminum oxide coating, allowing direct contact of aluminum with water. The new study, however, includes several innovations and novel findings that could lead to practical applications.

A US patent application is pending on this technology. The international (PCT) filing on which it was based is here.

Previous studies had mostly used aluminum-rich mixtures of aluminum and gallium, or in some cases more complex alloys. But the lab of Bakthan Singaram, professor of chemistry and biochemistry and co-corresponding author of the study, found that hydrogen production increased with a gallium-rich composite. In fact, the rate of hydrogen production was so unexpectedly high the researchers thought there must be something fundamentally different about this gallium-rich alloy.

Oliver suggested that the formation of aluminum nanoparticles could account for the increased hydrogen production, and his lab had the equipment needed for nanoscale characterization of the alloy. Using scanning electron microscopy and x-ray diffraction, the researchers showed the formation of aluminum nanoparticles in a 3:1 gallium-aluminum composite, which they found to be the optimal ratio for hydrogen production.

Scanning electron microscopy of the composite shows aluminum nanoparticles in a matrix of gallium. (Credit: Amberchan et al., Applied Nano Materials 2022)

In this gallium-rich composite, the gallium serves both to dissolve the aluminum oxide coating and to separate the aluminum into nanoparticles.

Making the composite required nothing more than simple manual mixing. The composite can be made with readily available sources of aluminum, including used foil or cans, and the composite can be stored for long periods by covering it with cyclohexane to protect it from moisture.

Although gallium is not abundant and is relatively expensive, it can be recovered and reused multiple times without losing effectiveness, Singaram said. It remains to be seen, however, if this process can be scaled up to be practical for commercial hydrogen production.

This work was partially supported by funds from the Ima Hernandez Foundation.

Resources

• Gabriella Amberchan, Isai Lopez, Beatriz Ehlke, Jeremy Barnett, Neo Y. Bao, A’Lester Allen, Bakthan Singaram, and Scott R. J. Oliver (2022) “Aluminum Nanoparticles from a Ga–Al Composite for Water Splitting and Hydrogen Generation” ACS Applied Nano Materials doi: 10.1021/acsanm.1c04331