An H-cell modified with electrodes and an ion-conducting ceramic membrane to conduct sodium-air fuel cell experiments. Credit: Gretchen Ertl.

A new fuel cell could provide a lightweight option for powering trucks, ships and aircraft, according to new research.

The devices could pack three times as much energy per pound as today’s best EV batteries.

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

That’s a serious obstacle for energy innovation and the search for new ways to power airplanes, trains, and ships.

Now, researchers at MIT and elsewhere have come up with a solution that could help electrify these 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 this case, 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.

In a series of experiments with a prototype device, the researchers demonstrated that this 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.

Yet-Ming Chiang, who is the Kyocera Professor of Ceramics at MIT, 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.”

Electrically powered flight

And this technology does appear to have the potential to be quite revolutionary. In particular, for aviation, where weight is especially crucial, such an improvement in energy density could be the breakthrough that finally makes electrically powered flight practical at 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 kilogram that wouldn’t be enough to enable transcontinental or trans-Atlantic flights.

That’s still beyond reach for any known battery chemistry, but Chiang said that getting to 1,000 watts per kilogram would be an enabling technology for regional electric aviation, which accounts for about 80% of US 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.

Chaing said: “They all require very high energy density, and they all require low cost, and that’s what attracted us to sodium metal.”

Metal-air

A great deal of research has gone into developing lithium-air or sodium-air batteries over the last three decades, but it has been hard to make them fully rechargeable.

Chiang said: “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 realised in practice.” Chiang says.

By using the same basic electrochemical concept, only making it a fuel cell instead of a battery, the researchers were able to get the advantages of the high energy density in a practical form.

Unlike a battery, whose materials are assembled once and sealed in a container, with a fuel cell the energy-carrying materials go in and out.

Prototypes

The team produced two different versions of a lab-scale prototype of the system.

In one, called an H cell, two vertical glass tubes are connected by a tube across the middle, which contains a solid ceramic electrolyte material and a porous air electrode.

Liquid sodium metal fills the tube on one side, and air flows through the other, providing the oxygen for the electrochemical reaction at the centre, which ends up gradually consuming the sodium fuel.

The other prototype uses a horizontal design, with a tray of the electrolyte material holding the liquid sodium fuel. The porous air electrode, which facilitates the reaction, is affixed to the bottom of the tray. 

According to Chiang, tests using an air stream with a carefully controlled humidity level produced a level of nearly 1,700 watt-hours per kilogram 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 these packs gets chemically transformed as it provides the power.

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

But there’s a very big difference: There would be no carbon dioxide emissions.

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

Natural cascade

This 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 explained: “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.”

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.

Using sodium hydroxide to capture carbon dioxide has been proposed as a way of mitigating carbon emissions, but on its own, it’s not an economic solution because the compound is too expensive.

“But here, it’s a byproduct,” Chiang explained, so it’s essentially free, producing environmental benefits at no cost.

The study is published in Joule.