Ellen Phiddian
Cosmos science journalist
Anyone who’s ever tied off a balloon knows how difficult it is to store a gas. It takes a lot more energy and effort to do at scale, which seriously vexes many people in the energy industry. The advent of hydrogen fuel and carbon capture is only going to make this a more prominent problem.
Even trickier is separating gas mixtures – like the various carbon-based gases emitted from fossil fuel mining and refining. To both separate and store, industry typically relies on cooling gases down until they become liquids, which is an energy and resource-intensive process.
But a breakthrough by Australian researchers could change this. Their method works at room temperature and pressure, and requires only a bit of boron nitride – and some ball bearings.
So far, the researchers have shown that it can separate different types of hydrocarbon gases, like methane and acetylene, which it can then cleanly and efficiently store. It could also work for hydrogen, or possibly carbon dioxide.
The technique uses a compound called boron nitride, which is made (unsurprisingly) up of boron and nitrogen. The boron nitride powder is put in a ball mill – a rotating grinder – with the gas or gases of interest.
“When we first put these hydrocarbons in the boron nitride, the gas completely disappeared,” says Dr Srikanth Mateti, a research fellow at Deakin University’s Institute for Frontier Materials.
The result was so unusual that Mateti and colleagues had to repeat it several times before they were satisfied it wasn’t a mistake.
“Then we tried it again 20, 30 times. Why was this happening? Was it the experiment, or a gas leak, was it not sealed well? We tweaked every parameter, and still it behaved the same.”
The researchers realised that their boron nitride could bind extremely well to some gases. The gases it binds to are classes of compounds called “olefins” and “alkynes” – acetylene, a common fuel, is a type of alkyne.
They were able to figure out a chemical reason for this.
“The nitrogen’s electron configuration, and certain gas molecules’ configuration – they match,” says Mateti.
The combination of boron nitride and the ball mill grinder creates a mechanochemical reaction that absorbs olefin and alkyne gases into the powder. But it ignores other gases – like methane.
This means that a mixture of methane and acetylene, which are chemically very similar, can be easily separated. It also means that acetylene could be transported safely from place to place in powder form.
When the powder is heated in a vacuum, it releases the gas again.
The boron nitride powder is reusable. The whole process is very energy efficient, using only around 80 kilojoules per second to store 1,000 litres of gas.
While the researchers have demonstrated this works for acetylene and ethylene, they’re confident it could be adapted to store other gases – like hydrogen or carbon dioxide.
“We’re working on that,” says Mateti.
He says the system would also work for separating and removing carbon impurities from hydrogen prior to using it in fuel cells. “If hydrogen is not pure, it will destroy the fuel cell. So you need to have some kind of purification system. You can use ours.”
The researchers have been able to separate a couple of litres of gas in their lab easily. Because it uses a common industrial method, it should be an easy process to scale up.
“We just need to optimise some parameters – cooking recipes,” says Mateti. “Like the content of the gases, and how much material you need, how much ball milling.”
They’ve submitted a provisional patent application for the process, and are looking to pilot it at an industrial scale.
A paper describing their discovery is published in Materials Today.