The energy transition in the transport sector faces a monumental challenge, especially when it comes to the electrification of heavy vehicles such as aircraft, trains, and ships. Existing battery technologies are reaching their physical limits in terms of the amount of energy they can store per unit of mass, which represents a significant obstacle to innovation. However, a scientific team from the prestigious Massachusetts Institute of Technology (MIT) where you can also find accommodation and collaborating institutions recently presented a concept that could fundamentally change this paradigm and pave the way for the electrification of these key transport systems.

Revolutionary approach to power: Liquid sodium fuel cell

Instead of a conventional battery, the new solution is based on the principle of a fuel cell – a device that, similar to a battery, produces electricity through a chemical reaction, but with a key difference: it can be quickly refueled instead of undergoing lengthy charging. The core of this innovative system is liquid metallic sodium, an inexpensive and widely available raw material. On the other side of the cell is ordinary air, which serves as a source of oxygen atoms. Between them is a layer of solid ceramic material that functions as an electrolyte, allowing free passage of sodium ions, while a porous electrode facing the air facilitates the chemical reaction of sodium with oxygen, thereby generating an electric current.

In a series of experiments conducted with a prototype device, researchers demonstrated that this cell can store more than three times the amount of energy per unit of weight compared to the lithium-ion batteries that dominate the world of electric vehicles today. Detailed findings of this research were recently published in the scientific journal Joule, in a paper authored by MIT doctoral students Karen Sugano, Sunil Mair, and Saahir Ganti-Agrawal, professor of materials science and engineering Yet-Ming Chiang, and five other collaborators.

Professor Chiang, who holds the title of Kyocera Professor of Ceramics, commented on the potential perception of their work: “We expect people to think this is a completely crazy idea. If it weren’t so, I’d be a bit disappointed, because if people don’t initially think something is completely crazy, it probably won’t be that revolutionary.” It is precisely this revolutionary potential that makes this technology extremely promising.

New hope for electric aviation

Especially in aviation, where weight is a critical factor, such an improvement in energy density could represent a turning point that will finally enable the practical application of electric propulsion on a significant scale. “The threshold that is really needed for realistic electric aviation is around 1,000 watt-hours per kilogram (Wh/kg),” explains Chiang. Today’s lithium-ion batteries for electric vehicles reach a maximum of about 300 Wh/kg, which is far from what is needed. Even with 1,000 Wh/kg, he notes, it would not be enough for transcontinental or transatlantic flights.

Although that goal is still beyond the reach of any known battery chemistry, Chiang points out that achieving 1,000 Wh/kg would represent a technology that enables regional electric aviation. Such flights account for approximately 80 percent of domestic flights and are responsible for about 30 percent of aviation emissions. The electrification of this segment would therefore have a significant positive impact on the environment.

The technology could also be a driver of progress in other sectors, including maritime and rail transport. “They all require very high energy density and low costs,” says Chiang. “And that’s what drew us to metallic sodium.”

From battery to fuel cell: Overcoming limitations

Over the past three decades, much research effort has been invested in developing lithium-air or sodium-air batteries, but it has proven difficult to make them fully rechargeable. “People have long been aware of the energy density they could achieve with metal-air batteries, and that was extremely attractive, but it was simply never realized in practice,” states Chiang.

By using the same basic electrochemical concept but transforming it into a fuel cell instead of a battery, researchers managed to harness the advantages of high energy density in a practical form. Unlike a battery, whose materials are assembled once and sealed in a casing, in a fuel cell, the energy-carrying materials enter and exit the system. This dynamic approach allows for quick "refueling," eliminating the long charging times associated with batteries.

The team produced two different versions of a laboratory prototype of the system. In one, called an H-cell, two vertical glass tubes are connected by a transverse tube containing a solid ceramic electrolyte and a porous air electrode. Liquid metallic sodium fills the tube on one side, while air flows through the other, providing oxygen for the electrochemical reaction in the center, which gradually consumes the sodium fuel. The second prototype uses a horizontal design, with a crucible of electrolytic material holding the liquid sodium fuel. A porous air electrode, which facilitates the reaction, is attached to the bottom of the crucible.

Tests conducted using an air stream with a carefully controlled humidity level showed a level of over 1,500 watt-hours per kilogram at the single “stack” of cells level, which, according to Chiang, would translate to more than 1,000 Wh/kg at the complete system level.

Environmental benefits and carbon dioxide capture

Researchers envision that for use in aircraft, fuel packs containing stacks of cells would be inserted into the fuel cells, similar to food trays in a cafeteria. The metallic sodium within these packs chemically transforms as it provides energy. The byproduct of this chemical reaction is released, and in the case of an aircraft, it would be emitted from the rear, similar to jet engine exhaust gases.

But there is a very big difference: there would be no carbon dioxide emissions. Instead, the emissions, consisting of sodium oxide, would actually absorb carbon dioxide from the atmosphere. This compound would quickly combine with moisture in the air to form sodium hydroxide – a material commonly used as a drain cleaner – which readily reacts with carbon dioxide to form a solid material, sodium carbonate, which in turn creates sodium bicarbonate, better known as baking soda.

“There is this natural cascade of reactions that occurs when you start with metallic sodium,” explains Chiang. “Everything is spontaneous. We don’t have to do anything for it to happen, we just have to fly the plane.” As an added benefit, if the final product, sodium bicarbonate, ends up in the ocean, it could help de-acidify the water, counteracting another of the harmful effects of greenhouse gases.

Using sodium hydroxide to capture carbon dioxide has been proposed as a way to mitigate carbon emissions, but it is not an economical solution on its own because the compound is too expensive. “But here it’s a byproduct,” Chiang clarifies, so it is essentially free, bringing environmental benefits at no additional cost.

System safety and scalability

It is important to note that the new fuel cell is inherently safer than many other batteries, says Chiang. Metallic sodium is extremely reactive and must be well protected. As with lithium batteries, sodium can spontaneously ignite if exposed to moisture. “Whenever you have a very high energy density battery, safety is always a concern, because if there is a breach in the membrane separating the two reactants, you can have an uncontrolled reaction,” says Chiang. But in this fuel cell, one side is just air, “which is diluted and limited. So you don’t have two concentrated reactants right next to each other. If you are aiming for really, really high energy density, you would rather have a fuel cell than a battery for safety reasons.”

Although the device currently exists only as a small, single-cell prototype, Chiang says the system should be fairly simple to scale up to practical sizes for commercialization. Members of the research team have already founded a company, Propel Aero, to develop the technology. The company is currently located in MIT’s startup incubator, The Engine, in Cambridge, a popular destination for visitors.

Producing enough metallic sodium to enable widespread, full global implementation of this technology should be practical, as the material has been produced on a large scale before. When leaded gasoline was the norm, before it was phased out, metallic sodium was used to produce tetraethyllead, which was used as an additive, and was produced in the USA at a capacity of 200,000 tons per year. “This reminds us that metallic sodium was once produced on a large scale and was safely handled and distributed throughout the USA,” says Chiang.

Moreover, sodium primarily comes from sodium chloride, or salt, so it is abundant, widely distributed around the world, and easily extracted, unlike lithium and other materials used in today’s electric vehicle batteries. The team of scientists, whose work predominantly originates from research centers in Massachusetts, relies on this availability.

Charging system and future steps

The system they envision would use replaceable cartridges, which would be filled with liquid metallic sodium and sealed. When depleted, they would be returned to a charging station and refilled with fresh sodium. Sodium melts at 98 degrees Celsius, just below the boiling point of water, so it is easy to heat it to its melting point for filling the cartridges.

The initial plan is to produce a brick-sized fuel cell that can deliver about 1,000 watt-hours of energy, enough to power a large drone, to prove the concept in a practical form that could be used, for example, in agriculture. The team hopes to have such a demonstration ready within the next year, i.e., by May 2026.

Key scientific insights and teamwork

Karen Sugano, who conducted most of the experimental work as part of her doctoral dissertation and will now work at the startup Propel Aero, says that a key insight was the importance of moisture in the process. While testing the device with pure oxygen and then with air, she discovered that the amount of moisture in the air was crucial for the efficiency of the electrochemical reaction. Humid air resulted in the sodium forming its discharge products in a liquid rather than solid form, which significantly facilitated their removal by the air stream through the system. “The key was that we could form this liquid discharge product and easily remove it, unlike a solid discharge that would form in dry conditions,” she says.

Saahir Ganti-Agrawal notes that the team drew on knowledge from various engineering subdisciplines. For example, there is a lot of research on high-temperature sodium, but none with a humidity-controlled system. “We draw from fuel cell research in terms of our electrode design, we draw from older high-temperature battery research, as well as from some newer sodium-air battery research, and we kind of bring it all together,” which led to the “big leap in performance” that the team achieved, he says.

The research team also included Alden Friesen, a summer intern at MIT attending Desert Mountain High School in Scottsdale, Arizona; Kailash Ramana and William Woodford from Form Energy in Somerville, Massachusetts; Shashank Sripada from And Battery Aero in California, and Venkatasubramanian Viswanathan from the University of Michigan. The work was supported by ARPA-E, Breakthrough Energy Ventures, and the National Science Foundation, and utilized the facilities of MIT.nano.