New fuel cell could enable electric aviation

These devices could pack three times as much energy per pound as today’s best EV batteries, offering a lightweight option for powering trucks, planes, or ships.

Massachusetts Institute of Technology

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

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 refueled 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. Their findings are being published today 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.

“We expect people to think that this is a totally crazy idea,” says Chiang, who is the Kyocera Professor of Ceramics. “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.”

And this technology does appear to have the potential to be quite revolutionary, he suggests. 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.

“The threshold that you really need for realistic electric aviation is about 1,000 watt-hours per kilogram,” Chiang says. 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, 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 kilogram would be an enabling technology for regional electric aviation, which accounts for about 80 percent of domestic flights and 30 percent of the emissions from aviation.

The technology could be an enabler for other sectors as well, including marine and rail transportation. “They all require very high energy density, and they all require low cost,” he says. “And that’s what attracted us to sodium metal.”

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. “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,” 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.

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 center, 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.

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, Chiang says.

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. 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.

“There’s this natural cascade of reactions that happens when you start with sodium metal,” Chiang says. “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 explains, so it’s essentially free, producing environmental benefits at no cost.

Importantly, the new fuel cell is inherently safer than many other batteries, he says. Sodium metal 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’s a rupture of the membrane that separates the two reactants, you can have a runaway reaction,” Chiang says. 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 company is currently housed in MIT’s startup incubator, The Engine.

Producing enough sodium metal to enable widespread, full-scale global implementation of this technology should be practical, since the material has been produced at large scale before. When leaded gasoline was the norm, before it was phased out, sodium metal was used to make the tetraethyl lead used as an additive, and it was being produced in the U.S. at a capacity of 200,000 tons a year. “It reminds us that sodium metal was once produced at large scale and safely handled and distributed around the U.S.,” Chiang says.

What’s more, sodium primarily originates 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 EV batteries.

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. Sodium melts at 98 degrees Celsius, just below the boiling point of water, so it is easy to heat to the melting point to refuel the cartridges.

Initially, the 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, in order to prove the concept in a practical form that could be used for agriculture, for example. The team hopes to have such a demonstration ready within the next year.

Sugano, who conducted much of the experimental work as part of her doctoral thesis and will now work at the startup, says that a key insight was the importance of moisture in the process. As she tested the device with pure oxygen, and then with air, she found that the amount of humidity in the air was crucial to making the electrochemical reaction efficient. The humid air resulted in the sodium producing its discharge products in liquid rather than solid form, making it much easier for these to be removed by the flow of air through the system. “The key was that we can form this liquid discharge product and remove it easily, as opposed to the solid discharge that would form in dry conditions,” she says.

Ganti-Agrawal notes that the team drew from a variety of different engineering subfields. For example, there has been much research on high-temperature sodium, but none with a system with controlled humidity. “We’re pulling from fuel cell research in terms of designing our electrode, we’re pulling from older high-temperature battery research as well as some nascent sodium-air battery research, and kind of mushing it together,” which led to the “the big bump in performance” the team has achieved, he says.

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.

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Written by David L. Chandler, MIT News

Journal

Joule

DOI

10.1016/j.joule.2025.101962

Article Title

Sodium-Air Fuel Cell for High Energy Density and Low-Cost Electric Power

Article Publication Date

27-May-2025

FAMU-FSU researchers design cryogenic hydrogen storage and delivery system for next-generation aircraft

Florida State University

Researchers at the FAMU-FSU College of Engineering have designed a liquid hydrogen storage and delivery system that could help make zero-emission aviation a reality. Their work outlines a scalable, integrated system that addresses several engineering challenges at once by enabling hydrogen to be used as a clean fuel and also as a built-in cooling medium for critical power systems aboard electric-powered aircraft.

The study, published in Applied Energy, introduces a design tailored for a 100-passenger hybrid-electric aircraft that draws power from both hydrogen fuel cells and hydrogen turbine-driven superconducting generators. It shows how liquid hydrogen can be efficiently stored, safely transferred and used to cool critical onboard systems — all while supporting power demands during various flight phases like takeoff, cruising, and landing.

“Our goal was to create a single system that handles multiple critical tasks: fuel storage, cooling and delivery control,” said Wei Guo, a professor in the Department of Mechanical Engineering and corresponding author of the study. “This design lays the foundation for real-world hydrogen aviation systems.”

WHAT THEY DID
Hydrogen is seen as a promising clean fuel for aviation because it packs more energy per kilogram than jet fuel and emits no carbon dioxide. But it’s also much less dense, meaning it takes up more space unless stored as a super-cold liquid at –253°C.

To address this challenge, the team conducted a comprehensive system-level optimization to design cryogenic tanks and their associated subsystems. Instead of focusing solely on the tank, they defined a new gravimetric index, which is the ratio of the fuel mass to the full fuel system. Their index includes the mass of the hydrogen fuel, tank structure, insulation, heat exchangers, circulatory devices and working fluids.

By repeatedly adjusting key design parameters, such as vent pressure and heat exchanger dimensions, they identified the configuration that yields the maximum fuel mass relative to total system mass. The resulting optimal configuration achieves a gravimetric index of 0.62, meaning 62% of the system’s total weight is usable hydrogen fuel, a significant improvement compared to conventional designs.

The system’s other key function is thermal management. Rather than installing a separate cooling system, the design routes the ultra-cold hydrogen through a series of heat exchangers that remove waste heat from onboard components like superconducting generators, motors, cables and power electronics. As hydrogen absorbs this heat, its temperature gradually rises, a necessary process since hydrogen must be preheated before entering the fuel cells and turbines.

HOW IT WORKS
Delivering liquid hydrogen throughout the aircraft presents its own challenges. Mechanical pumps add weight and complexity and can introduce unwanted heat or risk failure under cryogenic conditions. To avoid these issues, the team developed a pump-free system that uses tank pressure to control the flow of hydrogen fuel.

The pressure is regulated using two methods: injecting hydrogen gas from a standard high-pressure cylinder to increase pressure and venting hydrogen vapor to decrease it. A feedback loop links pressure sensors to the aircraft’s power demand profile, enabling real-time adjustment of tank pressure to ensure the correct hydrogen flow rate across all flight phases. Simulations show it can deliver hydrogen at rates up to 0.25 kilograms per second, sufficient to meet the 16.2-megawatt electrical demand during takeoff or an emergency go-around.

The heat exchangers are arranged in a staged sequence. As the hydrogen flows through the system, it first cools high-efficiency components operating at cryogenic temperatures, such as high-temperature superconducting generators and cables. It then absorbs heat from higher-temperature components, including electric motors, motor drives and power electronics. Finally, before reaching the fuel cells, the hydrogen is preheated to match the optimal fuel cell inlet conditions.

This staged thermal integration allows liquid hydrogen to serve as both a coolant and a fuel, maximizing system efficiency while minimizing hardware complexity.

“Previously, people were unsure about how to move liquid hydrogen effectively in an aircraft and whether you could also use it to cool down the power system component,” Guo said. “Not only did we show that it’s feasible, but we also demonstrated that you needed to do a system-level optimization for this type of design.”

FUTURE STEPS AND COLLABORATORS
While this study focused on design optimization and system simulation, the next phase will involve experimental validation. Guo and his team plan to build a prototype system and conduct tests at FSU’s Center for Advanced Power Systems.

The project is part of NASA’s Integrated Zero Emission Aviation program, which brings together institutions across the U.S. to develop a full suite of clean aviation technologies. Partner universities include Georgia Tech, Illinois Institute of Technology, University of Tennessee and University at Buffalo. FSU leads the effort in hydrogen storage, thermal management and power system design.

At FSU, key contributors include graduate student Parmit S. Virdi; professors Lance Cooley, Juan Ordóñez, Hui Li, Sastry Pamidi; and other faculty experts in cryogenics, superconductivity and power systems.

FUNDING
This project was supported by NASA as part of the organization’s University Leadership initiative, which provides an opportunity for U.S. universities to receive NASA funding and take the lead in building their own teams and setting their own research agenda with goals that support and complement the agency’s Aeronautics Research Mission Directorate and its Strategic Implementation Plan.

Guo’s research was conducted at the FSU-headquartered National High Magnetic Field Laboratory, which is supported by the National Science Foundation and the State of Florida.

An artist's rendering of a 100-passenger hybrid-electric aircraft that uses hydrogen as fuel.

A schematic showing the design of a hydrogen-powered aircraft.
Credit
Courtesy of Wei Guo