Electric flight has long been constrained by one key limitation: battery weight. Even the best lithium-ion packs top out at around 300 Wh/kg, sufficient for electric vehicles but well below the 1,000 Wh/kg threshold often cited as the tipping point for practical regional aviation.
In fact, weight remains the most significant limiting factor in aviation, where energy per kilogram directly determines range and payload. However, a new sodium-air fuel cell developed by researchers at MIT may have just crossed that line. They’ve developed a new type of fuel cell in collaboration with other institutions, which could shift the balance.
Rather than further developing lithium-ion chemistries, the researchers turned to a liquid sodium-air system that achieves higher energy density by functioning as a “true” fuel cell rather than a sealed battery.
The findings are being published in the journal Joule, in a paper by MIT doctoral students Karen Sugano, Sunil Mair, and Saahir Ganti-Agrawal; professor of materials science and engineering, Yet-Ming Chiang; and five others — representing a promising step toward enabling electric propulsion for aircraft, trains, and ships.
An H-cell modified with electrodes and an ion-conducting ceramic membrane to conduct sodium-air fuel cell experiments. (Image: Gretchen Ertl)
The core of the system is a high-temperature ceramic electrolyte that allows sodium ions to move between a reservoir of liquid sodium metal and a porous air cathode. The fuel cell oxidizes the sodium in the presence of humidified air, producing electricity while forming sodium oxide byproducts.
In lab-scale tests, the MIT team demonstrated an energy density of over 1,500 Wh/kg at the cell stack level. That would exceed the 1,000 Wh/kg mark even after accounting for the balance-of-system components in a full-scale configuration. Critically, sodium is abundant and inexpensive, often derived from common rock salt. It also sidesteps the extraction, cost, and supply chain challenges associated with lithium and cobalt. The system is designed for modularity.
Fuel cartridges containing sodium can be loaded and unloaded like food trays in a galley, potentially making the turnaround time in aircraft faster than conventional battery charging. The electrochemical byproducts offer an additional environmental benefit. Sodium oxide reacts spontaneously with moisture and carbon dioxide in the atmosphere to form sodium bicarbonate, also known as baking soda. This chemical pathway not only avoids greenhouse gas emissions but also passively captures carbon dioxide during operation. If the byproducts reach the ocean, they may help counteract acidification by slightly raising pH levels.
From a safety perspective, the cell design minimizes risk by keeping the reactive fuel physically separated from the oxidizer until the point of reaction. Only one side of the system is flammable at any given moment. That contrasts with lithium-ion batteries, where short circuits and thermal runaway remain serious concerns. Although still in the prototype stage, the system appears to be scalable.
The team is developing a brick-sized fuel cell module that can power a large drone, with plans to further scale it up for aviation and heavy transport applications. The researchers have formed a company, Propel Aero, to commercialize the technology and are currently incubated at MIT’s The Engine. If successful, sodium-air fuel cells could serve as a viable power source for short-haul aviation, replacing fossil fuels without the mass penalties of current battery systems.
The approach combines proven fuel cell architecture with low-cost materials, opening a new path for decarbonizing sectors that have remained out of reach for electrification. Unlike sealed batteries, this sodium-air cell works as a refillable system. The liquid sodium is gradually consumed and can be replaced in cartridge form. The exhaust is a sodium compound that reacts with air to form sodium bicarbonate, effectively removing some carbon dioxide from the atmosphere during flight.
Producing enough sodium metal to support widespread implementation appears feasible. During the era of leaded gasoline, sodium was manufactured at industrial scale in the US to produce tetraethyl lead, reaching volumes of 200,000 tons annually. What’s more, sodium primarily originates from sodium chloride, or salt, so it’s abundant, widely distributed around the world, and easily extracted (unlike lithium and other materials used in today’s EV batteries). This makes it a more sustainable and scalable alternative for hard-to-electrify sectors.
The team believes that, beyond aviation, the fuel cell’s high energy density and low-cost materials make it a viable candidate for other hard-to-electrify sectors, such as shipping and rail. Sodium is not only abundant and easily extracted from salt, but it was once produced at an industrial scale for use in tetraethyl lead production, highlighting its logistical feasibility.
While the work remains at prototype scale and scaling challenges remain, including cathode flooding and the safe handling of reactive sodium, the research has already spun off into a commercial venture, Propel Aero, housed at MIT’s incubator, The Engine.
As Prof. Yet-Ming Chiang, the Kyocera Professor of Ceramics, put it: “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.”
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The research team also included Alden Friesen, an MIT summer intern who attends Desert Mountain High School in Scottsdale, Arizona; Kailash Raman and William Woodford of Form Energy in Somerville, Massachusetts; Shashank Sripad of And Battery Aero in California, and Venkatasubramanian Viswanathan of the University of Michigan.
The work was supported by ARPA-E, Breakthrough Energy Ventures, and the National Science Foundation, and used facilities at MIT.nano.
Original paper: “Sodium-Air Fuel Cell for High Energy Density and Low-Cost Electric Power”
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