How sodium-air fuel cells could empower electric aviation

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. Researchers at Massachusetts Institute of Technology (MIT) have come up with a solution. Instead of a battery, the new concept is a fuel cell – like a battery but able to be quickly refueled rather than recharged. The fuel is liquid sodium metal, inexpensive and widely available. The other side of the cell is air serving as a source of oxygen atoms. 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 react chemically with oxygen to produce electricity.

In a series of experiments with a prototype device, researchers demonstrated this cell could carry more than 3x as much energy per unit of weight as lithium-ion batteries used in electric vehicles today. For aviation, where weight is especially crucial, such an improvement in energy density could be the breakthrough that makes electrically powered flight practical at significant scale.

“The threshold you need for realistic electric aviation is about 1,000 watt-hours per kilogram,” says Yet-Ming Chiang, professor of materials science and engineering and the Kyocera Professor of Ceramics at MIT. “Today’s electric vehicle lithium-ion batteries top out at about 300 watt-hours per kilogram – nowhere near what’s needed.” Chiang says getting to 1,000 watts per kilogram would be an enabling technology for regional electric aviation, which accounts for about 80% of domestic flights and 30% of the emissions from aviation.

The team produced two different versions of a lab-scale prototype. In the 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 gradually consumes the sodium fuel. The other prototype uses a horizontal design, with a tray of electrolyte holding the liquid sodium fuel. The porous air electrode facilitating 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, equivalent to 1,000 watt-hours at the full system level.

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. To develop the technology, members of the research team have formed a company, Propel Aero, in MIT’s startup incubator, The Engine.

Massachusetts Institute of Technology (MIT)
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