New Fuel Cell Technology Could Power Electric Aviation

The limitations of traditional batteries are becoming increasingly clear, particularly regarding power storage relative to weight. This presents significant challenges in advancing new energy solutions for airplanes, trains, and ships. However, researchers at MIT and other institutions have developed a groundbreaking solution that could revolutionize these transportation systems.

Instead of a conventional battery, this innovative design utilizes a fuel cell, which, unlike batteries, can be swiftly refueled rather than requiring lengthy recharging. The fuel of choice is liquid sodium metal, an economical and widely accessible resource. Pairing this with regular air—which supplies oxygen—the system incorporates a solid ceramic electrolyte layer that facilitates the free flow of sodium ions. A porous electrode facing the air enables sodium to react with oxygen, generating electricity.

In a series of prototype tests, the research team demonstrated that this fuel cell can deliver more than three times the energy per weight compared to the lithium-ion batteries commonly used in electric vehicles today. Their findings are detailed in a recent publication in the journal Joule, co-authored by MIT doctoral students Karen Sugano, Sunil Mair, and Saahir Ganti-Agrawal, along with Professor Yet-Ming Chiang and additional colleagues.

“We anticipate that people might initially view this as a radical idea,” notes Chiang, the Kyocera Professor of Ceramics. “However, if everyone thinks it’s completely feasible from the start, it probably won’t impress. Revolutionary concepts often seem far-fetched at first.”

This technology has the potential to be transformative, particularly within aviation, where weight is critical. The increased energy density might finally pave the way for practical electric flight on a larger scale.

According to Chiang, “The energy density threshold necessary for viable electric aviation is approximately 1,000 watt-hours per kilogram.” Currently, lithium-ion batteries used in electric vehicles only reach about 300 watt-hours per kilogram—far from what’s required for significant advancements. Even achieving 1,000 watt-hours per kilogram would not suffice for transcontinental flights, yet Chiang emphasizes that it would enable regional electric aviation, which contributes to a sizable portion of air travel emissions.

The innovation could also impact other sectors such as marine and rail transportation, both of which demand high energy density and cost-effectiveness. “That’s why we turned to sodium metal,” he explains.

Despite decades of research into lithium-air and sodium-air batteries, creating fully rechargeable systems has proven challenging. “The potential energy density of metal-air batteries has always been appealing but has remained largely theoretical,” Chiang states.

By adapting the electrochemical principles to a fuel cell format, the researchers harnessed the advantages of high energy density in a practical solution. Unlike conventional batteries that are sealed for life, fuel cells allow the energy materials to be cycled in and out.

The team created two distinct lab-scale prototypes. In one design, termed the H cell, two vertical glass tubes are linked by a central tube containing the solid ceramic electrolyte and porous air electrode. One side is filled with liquid sodium metal, while air is channeled through the other, providing essential oxygen. The second prototype features a horizontal arrangement with the electrolyte material holding the sodium, and the air electrode secured underneath.

Testing with air streams of controlled humidity revealed energy densities exceeding 1,500 watt-hours per kilogram per individual stack, potentially translating to over 1,000 watt-hours at the complete system level.

For practical implementation in aircraft, the researchers envision using fuel packs that contain stacks of cells—similar to trays in a cafeteria. As the sodium is consumed for power, it generates a chemical byproduct that is emitted out the back, akin to jet engine exhaust.

However, a significant distinction lies in the emissions: instead of carbon dioxide, the byproducts consist of sodium oxide, which has the ability to absorb carbon dioxide from the atmosphere. This compound reacts with moisture, forming sodium hydroxide, commonly used as a drain cleaner, which then combines with CO2 to create solid sodium carbonate, or baking soda.

“This natural series of reactions begins with sodium metal,” Chiang explains. “It occurs spontaneously; we simply need to operate the aircraft.”

Interestingly, if sodium bicarbonate ends up in the ocean, it could help reduce seawater acidity, counteracting one of the adverse effects of greenhouse gas emissions.

While sodium hydroxide has been considered for carbon capture, it has not been economically viable due to cost, but in this setup, the byproduct presents a “free” opportunity to benefit the environment.

Moreover, the safety profile of this new fuel cell is inherently better than many current batteries. Sodium metal is highly reactive and requires careful management, as exposure to moisture can cause spontaneous ignition, similar to lithium batteries. “High-energy-density systems always raise safety concerns—particularly regarding chemical reactions if there’s a membrane breach,” notes Chiang. However, with one side utilizing just air, the risks are significantly reduced. “For high energy density demands, a fuel cell offers superior safety compared to a conventional battery.”

Although only a small prototype currently exists, Chiang insists that the technology is easily scalable for commercial applications. The research team has already founded a startup, Propel Aero, situated in MIT’s incubator, The Engine.

With sodium’s large-scale production being feasible, given its historical use in leaded gasoline additives—once manufactured in the U.S. at a rate of 200,000 tons annually—widespread deployment of this technology appears attainable. “It’s essential to recognize that sodium metal was once produced safely and extensively across the U.S.,” Chiang emphasizes.

Moreover, sodium is abundantly sourced from salt, making it readily obtainable compared to lithium and other materials used in today’s electric vehicle batteries.

The envisioned system would involve refillable cartridges filled with liquid sodium metal. Depleted cartridges would be returned to refilling stations for replenishment. Sodium’s melting point is just 98 degrees Celsius, slightly below that of water’s boiling point, making it straightforward to heat for cartridge refueling.

The team is initially targeting the development of a compact, brick-sized fuel cell capable of delivering about 1,000 watt-hours of energy to power large drones, aiming to prove this concept within the year for applications in agriculture.

Sugano, who conducted much of the experimental work for her doctoral thesis and will join the startup, highlights the crucial role of humidity: “Through testing with pure oxygen and then with air, we discovered that air’s humidity is vital for optimizing the electrochemical reaction.” The humidity allowed the sodium to produce discharge products in liquid form, facilitating their removal through the airflow—streamlining the process significantly.

Ganti-Agrawal adds that their approach drew from various engineering disciplines, stating, “We leveraged research from high-temperature sodium systems, combined with insights from fuel cell design and nascent sodium-air battery studies, leading to our significant performance improvements.”

The team also included Alden Friesen, a summer intern from Desert Mountain High School in Scottsdale, Arizona; Kailash Raman and William Woodford from Form Energy in Somerville, Massachusetts; Shashank Sripad 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 facilities at MIT.nano.