MIT’s new fuel cell could pave way for electric airplanes

By Stephen Beech

A new fuel cell could enable electric airplanes that produce zero carbon emissions, according to a study.

Scientists say the devices could pack three times as much energy per pound as today’s best EV batteries - offering a lightweight option for powering planes, lorries or ships.

Batteries are currently nearing their limits in terms of how much power they can store for a given weight.

Researchers explained that it is a serious problem for energy innovation and the search for new ways to power airplanes, trains and ships.

Now, researchers at the Massachusetts Institute of Technology (MIT) have come up with a solution that could help electrify transportation systems.

Instead of a battery, the new concept is a kind of fuel cell - which is similar to a battery but can be quickly refuelled rather than recharged.

In the new system, the fuel is liquid sodium metal - an inexpensive and widely available commodity.

The other side of the cell is just ordinary air, which serves as a source of oxygen atoms.

In between, a layer of solid ceramic material serves as the electrolyte, allowing sodium ions to pass freely through, and a porous air-facing electrode helps the sodium to chemically react with oxygen and produce electricity.

The research team showed in a series of experiments with a prototype device that the cell could carry more than three times as much energy per unit of weight as the lithium-ion batteries used in virtually all electric vehicles today.

Professor Yet-Ming Chiang said, “We expect people to think that this is a totally crazy idea.

"If they didn’t, I’d be a bit disappointed because if people don’t think something is totally crazy at first, it probably isn’t going to be that revolutionary.”

He says the technology does appear to have the potential to be quite revolutionary, particularly for aviation, where weight is especially crucial.

The researchers believe such an improvement in energy density could be the breakthrough that finally makes electrically powered flight practical at a significant scale.

Chiang said: “The threshold that you really need for realistic electric aviation is about 1,000 watt-hours per kilogram.

"Today’s electric vehicle lithium-ion batteries top out at about 300 watt-hours per kilogram — nowhere near what’s needed."

Even at 1,000 watt-hours per kilo, he says, that wouldn’t be enough to enable transcontinental or trans-Atlantic flights.

That’s still beyond reach for any known battery chemistry, but Chiang says that getting to 1,000 watts per kilo would be an enabling technology for regional electric aviation, which accounts for about 80% of domestic flights and 30% of the emissions from aviation.

The technology could be an enabler for other sectors as well - including marine and rail transportation, according to the study published in the journal Joule.

Chiang said: “They all require very high energy density, and they all require low cost.

“And that’s what attracted us to sodium metal.

“People have been aware of the energy density you could get with metal-air batteries for a very long time, and it’s been hugely attractive, but it’s just never been realized in practice."

He said tests using an air stream with a carefully controlled humidity level produced a level of nearly 1,700 watt-hours per kilo at the level of an individual “stack” - which would translate to over 1,000 watt-hours at the full system level.

The researchers envision that to use this system in an aircraft, fuel packs containing stacks of cells, like racks of food trays in a cafeteria, would be inserted into the fuel cells.

The sodium metal inside the packs gets chemically transformed as it provides the power.

A stream of its chemical by-product is given off, and in the case of aircraft, it would be emitted out the back, not unlike the exhaust from a jet engine.

However, there would be no carbon dioxide emissions.

Instead, the emissions, consisting of sodium oxide, would actually soak up carbon dioxide from the atmosphere.

The researchers say the compound would quickly combine with moisture in the air to make sodium hydroxide - a material commonly used as a drain cleaner - which readily combines with carbon dioxide to form a solid material, sodium carbonate, which in turn forms sodium bicarbonate, otherwise known as baking soda.

Chiang added: “There’s this natural cascade of reactions that happens when you start with sodium metal.

“It’s all spontaneous. We don’t have to do anything to make it happen, we just have to fly the airplane.”

He said that, as an added benefit, if the final product, the sodium bicarbonate, ends up in the ocean, it could help to de-acidify the water, countering another of the damaging effects of greenhouse gases.

Chiang also emphasized that the new fuel cell is "inherently safer" than many other batteries.

He explained that sodium metal is extremely reactive and must be well-protected.

As with lithium batteries, sodium can spontaneously ignite if exposed to moisture.

Chiang said: “Whenever you have a very high energy density battery, safety is always a concern, because if there’s a rupture of the membrane that separates the two reactants, you can have a runaway reaction.

"But in this fuel cell, one side is just air, which is dilute and limited.

"So you don’t have two concentrated reactants right next to each other. If you’re pushing for really, really high energy density, you’d rather have a fuel cell than a battery for safety reasons.”

While the device so far exists only as a small, single-cell prototype, Chiang says the system should be "quite straightforward" to scale up to practical sizes for commercialization.

Members of the research team have already formed a company, Propel Aero, to develop the technology.

The system they envisage would use a refillable cartridge, which would be filled with liquid sodium metal and sealed.

When it’s depleted, it would be returned to a refilling station and loaded with fresh sodium.