Aluminum-gallium gets hydrogen out of seawater

A new powder identified by UCSC researchers can be dumped into seawater to rapidly release 90% of its theoretical maximum of hydrogen.

“We don’t need any energy input, and it bubbles hydrogen fast. I’ve never seen anything like it,” said UCSC Professor Scott Oliver. He described a new aluminium-gallium nanoparticle powder that generates H₂ when placed in water – even seawater.

Aluminium rapidly oxidises in water, stripping the O out of H₂O and releasing hydrogen as a byproduct. This is a short-lived reaction. In most cases, the metal quickly attains a microscopical coating of aluminium oxide that seals it off.

Chemistry researchers at UC Santa Cruz say they’ve found a cost-effective way to keep the ball rolling. Gallium is known to remove the aluminium oxide coating and keep the aluminium in contact with water to continue the reaction. Previous research had found that aluminium-heavy combinations had a limited effect.

So when chemistry/biochemistry Professor Bakthan Singaram found out that student Isai Lopez was playing with aluminium/gallium hydrogen production in his home kitchen, there seemed to be nothing particularly special about the idea.

“He wasn’t doing it scientifically, so I set him up with a graduate student to do a systematic study,” Singaram said. “I thought it would make a good senior thesis for him to measure the hydrogen output from different ratios of gallium and aluminium.”

Moving beyond fun to scientific study of seawater

When Lopez decided to extend the experiment to test gallium-heavy mixtures, things got a little weird. Hydrogen production went through the roof, and the team started figuring out why these mixtures behaved so fundamentally differently.

After electron microscopy and X-ray diffraction studies, they realised that the most effective mix, three parts gallium to one part aluminium, was indeed doing something the lower ratios weren’t. Not only was the gallium dissolving the aluminium oxide. It was also causing the aluminium to separate into nanoparticles and keeping them separate.

“The gallium separates the nanoparticles and keeps them from aggregating into larger particles,” Singaram said. “People have struggled to make aluminium nanoparticles, and here we produce them under normal atmospheric pressure and room temperature conditions.”

With the aluminium so finely separated, its surface area is maximised. The reaction with water was spectacularly efficient. It pulled out 90% of the theoretical maximum amount of hydrogen possible for a given amount of aluminium. In a study published in ACS Nano Materials, the researchers report that a single gram of their gallium-aluminium alloy will rapidly liberate 130 ml of hydrogen when placed in water.

Seawater can work well for this experiment

Remarkably, the water source doesn’t need to be clean, either.

“Any available water source can be used,” reads the study, “including wastewater, commercial beverages, or even ocean water, with no generation of chlorine gas.”

Gallium is expensive. But the researchers say it can be fully recovered at the end of the process. It could be used with fresh aluminium to create more of this remarkable hydrogen-producing alloy. The creation of the alloy is extremely easy in and of itself. One mixes the gallium manually with aluminium, including used foil or cans, in the correct ratio.

“Our method uses a small amount of aluminium, which ensures it all dissolves into the majority gallium as discrete nanoparticles,” Oliver said. “This generates a much larger amount of hydrogen, almost complete compared to the theoretical value based on the amount of aluminium. It also makes gallium recovery easier for reuse.”

The team has slapped a patent application on the process and is beginning to examine how it’ll scale up commercially.

Solid-state storage of hydrogen from seawater

So what are we looking at here? It’s effectively a solid-state way to store and release hydrogen. Hydrogen is an important fuel that’ll be necessary for certain applications during the race to decarbonisation. Still, it’s notoriously difficult and expensive to compress into gas or cryogenically condense into a liquid for storage and transport.

On the other hand, a hydrogen-storage powder is much easier and cheaper to handle, potentially changing the cost of working with hydrogen so drastically that new applications become viable. That is why Deakin’s mechano-chemical ball-milling process and EAT’s Si+ silicon powder were such a big deal.

This stuff sounds easy to make and even easier to use for hydrogen production. It’ll store and travel well for at least three months if stored in cyclohexane gas. The fact that it works in seawater is hugely significant; access to clean water is not the sort of thing you’d want to be staking a volume business on moving forward. The gallium can be collected and recycled in the process will help keep costs down. The reaction at ambient pressures and temperatures means you can get away with less equipment.

How does this development measure up?

So how does it measure up against these other two powders? Well, the figures provided allow us at least to take a guess. The key metric is probably the mass fraction if you’re treating this stuff as a hydrogen storage medium. For a given powder mass, how much hydrogen can you get out? Well, if a gram of gallium-aluminium powder produces 130 ml or 5.4 mmol of hydrogen, that hydrogen would weigh 0.00544 grams.

That’s a mass fraction of 0.544%. Not much chop, really; EAT’s Si+ powder is probably the substance to beat at this stage, at least on this metric, claiming a mass fraction of 13.5%. Of course, there are many other considerations when talking about a commercial energy transport and release cycle – particularly one that’s not fussy about water quality – so there are still opportunities for this new powder to contribute.