MIT boffins claim liquid sodium battery could one day power aircraft while sucking up CO2

Prototype packs triple the energy density of lithium-ion, they claim

MIT boffins have built a prototype fuel cell using liquid sodium and air that could one day power aircraft, and may help capture carbon through its byproducts.

Skeptical? You can be forgiven. Even MIT materials science professor Yet-Ming Chiang, one of the lead researchers, admits it sounds a bit out there.

We expect people to think that this is a totally crazy idea

"We expect people to think that this is a totally crazy idea," Chiang told MIT News. But the team has demonstrated a prototype fuel cell with more than three times the energy density of today's lithium-ion EV batteries, according to a research paper published on Tuesday. 

While the paper is nearly as dense as the fuel cell's output, the concept itself is surprisingly straightforward. Liquid sodium metal acts as the fuel, stored in compartments that could one day take the form of sealed cartridges. As oxygen flows in from ambient air, it reacts with the sodium across a ceramic electrolyte, producing electricity.

By carefully managing humidity, the researchers were able to form a liquid byproduct - sodium hydroxide (NaOH) - which can be more easily removed and potentially used to capture carbon dioxide.

And here's where things get really crazy. 

If that sodium hydroxide is released into the air, say by a jet airliner, it can react with carbon dioxide to form sodium carbonate. If it ends up in the ocean, it may go further and form sodium bicarbonate, better known as baking soda, potentially offsetting ocean acidification.

The researchers imagine future systems powering aircraft, ships, or trains. In some scenarios, the NaOH could be captured and sold for industrial use or deployed for point-source carbon capture. In others, it might be "discarded and allowed to naturally capture atmospheric CO₂ as well as to deacidify marine bodies," the paper says.

That's some dense energy you've got there

Lithium-ion batteries typically deliver between 200 and 300 watt-hours per kilogram. This is not even close to gasoline's energy density of over 12 kilowatt-hours per kilogram, or to green hydrogen, which packs more still by weight.

MIT's prototype liquid sodium fuel cell doesn't rival fossil fuels in raw energy terms, but it does outclass lithium-ion: the researchers report a stack-level energy density of 1,200 Wh/kg, about three to four times greater than today's EV batteries.

Those levels of energy density approach generally agreed-upon requirements for marine shipping and aviation, Chiang told The Register in an email. "But we have not tested more granular vehicle-specific drive cycles or flight cycles."

With promising lab results in hand, the team is moving toward commercialization.

Anyone who likes drones should love electric aviation batteries since that is the logical starting point

Chiang and his colleagues have spun up a startup called Propel Aero inside MIT's incubator to commercialize the tech with plans to build a 1,000-watt-hour fuel cell, roughly the size of a brick, within the next year. The prototype is expected to power large drones and serve as a proof of concept for applications like agriculture.

"Anyone who likes drones should love electric aviation batteries since that is the logical starting point," Chiang said. He noted that the team has already had interest from drone manufacturers and the electric aviation industry. 

With fuel easily procured, a working lab design in hand, and a clear development roadmap, liquid sodium metal fuel cells may be closer to takeoff than you'd expect.

"To fully develop the tech and get it into production will probably take about 3 years," Chiang told us. Keep an eye on the skies - aircraft with net negative emissions might be flitting about sooner rather than later. ®