A Cleaner Alternative For Fueling Heavy Transport

With batteries nearing their energy-density ceiling, electrifying heavy transport—aircraft, ships, trains—has largely remained out of reach. MIT researchers have unveiled a rapid-refuel alternative: a sodium–air fuel cell that packs over three times the energy-density of today’s lithium-ion packs.

Instead of storing all reactants internally, this design feeds liquid sodium metal into one electrode and draws oxygen from ambient air at the other. A solid ceramic electrolyte in between conducts sodium ions, while a porous air electrode catalyzes the reaction. In lab prototypes, one a vertical “H-cell” and the other a horizontal tray design, controlled humidity in the incoming air yielded discharge products as liquids, streamlining their removal and sustaining stable operation.

Tests of individual cell stacks achieved nearly 1,700 watt-hours per kilogram (Wh/kg) at the cell level, projecting over 1,000 Wh/kg at system scale—surpassing the estimated threshold for regional electric aviation. At that density, short-haul electric flights become viable, cutting domestic flight emissions by up to 30 percent. Marine and rail sectors, where weight and cost constraints parallel aviation, stand to gain similarly.

Beyond energy, the sodium–air system offers an ecological bonus: its exhaust of sodium oxide absorbs CO₂ from the atmosphere, ultimately forming sodium carbonate and bicarbonate (baking soda). If vented over oceans, the byproducts could even help to buffer the increasing acidity of the oceans.

Safety advantages also emerge: unlike sealed high-energy batteries where internal short circuits can trigger runaway fires, this fuel cell’s oxidant is just ambient air—dilute and self-limiting.

The research team has already formed the company Propel Aero to scale the technology. Refuelable sodium cartridges, heated above 98 °C to melt the metal, could slide into aircraft fuel-cell racks much like trays in a galley. With sodium production once reaching 200,000 tons annually for industrial uses, the material’s abundance and low cost promise a practical path toward next-generation electric transport.