A sodium-air fuel cell could packs 3x more energy per kg than today's lithium-ion batteries - enough to electrify aviation and more.
29 Jun, 2025
4 min read
An H-cell modified with electrodes and an ion-conducting ceramic membrane to conduct sodium-air fuel cell experiments. Credit: Gretchen Ertl
Metals can be used as fuels, just like oil, gas, or hydrogen, because they release energy through burning or electrochemical reactions. Light metals like lithium and sodium have exceptionally high energy levels, making them particularly useful for transportation sectors that are challenging to make environmentally friendly.
Researchers have discovered a new method to power airplanes, trains, and ships as batteries reach their storage capacity limits. Instead of a battery, they propose using a fuel cell, which can be refueled quickly rather than recharged.
This fuel cell uses liquid sodium metal, a cheap and widely available material, as its fuel. On the other side, ordinary air provides oxygen atoms. A solid ceramic layer in between acts as the electrolyte, letting sodium ions move through.
A porous electrode then facilitates the reaction between sodium and oxygen to generate electricity. Researchers tested a prototype fuel cell and found that it stores over three times more energy per unit of weight than the lithium-ion batteries commonly used in electric vehicles today.
Scientists have spent decades developing lithium-air and sodium-air batteries, but making them fully rechargeable has proven to be a significant challenge. While metal-air batteries have long been known for their high energy density, they haven’t been successfully implemented in practice.
By adapting the same electrochemical principles to a fuel cell rather than a battery, researchers have found a way to utilize the high energy density in a practical form. Unlike batteries, which are sealed with fixed materials, fuel cells allow energy-carrying substances to flow in and out, making refueling easier and more efficient. This innovation could lead to new possibilities for electrifying transportation.
The team created two lab-scale prototypes of the system. One called an H cell, consists of two vertical glass tubes connected by a middle tube containing a ceramic electrolyte and an air electrode. Liquid sodium fills one side while air flows through the other, enabling a reaction that gradually consumes the sodium fuel.
The second prototype is a horizontal design, where a tray holds the liquid sodium fuel with the electrolyte material. The air electrode, which helps the reaction, is attached to the bottom of the tray.
Tests with controlled humidity showed that the system could generate over 1,500 watt-hours per kilogram in a single unit and more than 1,000 watt-hours at full scale.
Researchers propose using this system in aircraft by inserting fuel packs containing stacked cells, similar to food trays in a cafeteria. As the sodium metal in these packs reacts, it generates power while releasing a byproduct.
Unlike jet engine exhaust, this system wouldn’t emit carbon dioxide. Instead, it would release sodium oxide, which absorbs CO₂ from the air. This reaction forms sodium hydroxide, which further combines with CO₂ to create solid compounds, such as sodium carbonate and baking soda, making it a potentially eco-friendly alternative for aviation.
This fuel cell offers extra environmental benefits. If its byproduct, sodium bicarbonate, reaches the ocean, it could help reduce water acidity caused by greenhouse gases.
Using sodium hydroxide to absorb carbon dioxide has been suggested before, but it’s too costly on its own. Here, though, it’s a free byproduct, making carbon capture more practical.
Additionally, the fuel cell is safer than many batteries; however, sodium metal must be well-protected, as it reacts strongly and can ignite if exposed to moisture.
Chiang says, “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.” 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.”
Although the device is currently just a small prototype, researchers believe it can be easily scaled up for real-world use. To bring this technology to market, the team has founded Propel Aero, a company that will focus on its development and commercialization.
Scaling up sodium metal production for this technology should be feasible, as it has been manufactured in large quantities before. When leaded gasoline was widely used, sodium metal played a key role in making tetraethyl lead, with U.S. production reaching 200,000 tons annually.
The team plans to build a brick-sized fuel cell that can provide 1,000 watt-hours of energy, enough to power a large drone, with hopes of demonstrating it within a year.
A key discovery was the role of moisture. When tested with humid air, the sodium released its byproducts as a liquid instead of a solid, making removal easier. This improved the efficiency of the electrochemical reaction.
The research combines insights from multiple fields, including fuel cells, high-temperature batteries, and sodium-air battery studies. By integrating these ideas, the team significantly boosted performance.
Journal Reference
- Karen Sugano, Sunil Mair, Saahir Ganti-Agrawal et al. Sodium-air fuel cell for high energy density and low-cost electric power. Joule. DOI: 10.1016/j.joule.2025.101962
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