Fuel Cell Breakthrough Could Spur Electric Aviation Applications
MIT engineers have developed a fuel cell that uses liquid sodium. Photo courtesy Massachusetts Institute of Technology
CAMBRIDGE, MA—Batteries are nearing their limit 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 next-generation aircraft.
Engineers at the Massachusetts Institute of Technology (MIT) have developed a lightweight option that generates three times as much energy per pound as today’s best EV batteries. The fuel cell can be quickly refueled rather than recharged.
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. A porous air-facing electrode helps the sodium chemically react with oxygen and produce electricity.
A tremendous amount of research has gone into developing lithium-air or sodium-air batteries over the last three decades, but it’s been difficult 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,” says Yet-Ming Chiang, Ph.D., a materials science and engineering professor at MIT. “By using the same basic electrochemical concept, only making it a fuel cell instead of a battery, [we] 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.”
According to Chiang, this breakthrough could soon make 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,” he explains. “Today’s EV 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, that wouldn’t be enough to enable transcontinental or trans-Atlantic flights.”
Long flights are still beyond reach for any known battery chemistry, but Chiang believes that getting to 1,000 watts per kilogram would be ideal for regional applications, which account for about 80 percent of domestic flights and 30 percent of the emissions from aviation.
Looking for quick answers on assembly and manufacturing topics? Try Ask ASM, our new smart AI search tool. Ask ASM
Chiang and his colleagues 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 more than 1,500 watt-hours per kilogram at the level of an individual stack, which Chiang says would translate to over 1,000 watt-hours at the full system level.
To use this system in an aircraft, fuel packs containing stacks of cells would be inserted into the fuel cells. The sodium metal inside the packs would get chemically transformed as it provides the power. A stream of its chemical byproduct would be given off, similar to conventional exhaust from a jet aircraft engine. But, there would be no carbon dioxide emissions.
Looking for a reprint of this article?
From high-res PDFs to custom plaques, order your copy today!
