Hydrogen fuel cells have had a hard time making inroads as power sources for ground-based electric vehicles, but things are starting to look up for the zero-emission propulsion tech. A new air taxi startup, Alaka’i Technologies, this week unveiled a liquid-hydrogen-powered, five-passenger electric aircraft that it claims will be more efficient and powerful than the battery-powered aircraft its many, many competitors are developing.

The Massachusetts-based “flying car” company, led by veterans of NASA, Raytheon, Airbus, Boeing, and the Department of Defense, unveiled a mock-up of the six-rotor aircraft, called Skai, in Los Angeles at the offices of BMW Designworks, with which it partnered on the aircraft’s design. Alaka’i says the final product will be able to fly for up to four hours and cover 400 miles on a single load of fuel, which can be replenished in 10 minutes at a hydrogen fueling station. It has built a functional, full-scale prototype that will make its first flight “imminently,” a spokesperson says.

It won't be the first fuel-cell-powered plane; Boeing made that happen in 2008. But it would be the first of its kind. Skai’s boxy configuration doesn’t have the aerodynamic look of prototypes from the likes of Lilium, Bell, and, yes, Boeing. It tops out at just 118 mph, while other eVTOL (that’s electric, vertical-takeoff and -landing) concepts promise speeds of over 150 mph. The Skai is designed, rather, for efficiency, which matters more than top speed if it’s going to make dozens of short hops daily. “Our goal was to keep it simple, and we focused on accommodating a certain mission profile that’s repeatable over an entire day,” says NASA veteran engineer Bruce Holmes, who serves on Alaka’i’s board of directors.

To get the Skai off the ground, Alaka’i plans to skip air taxi service early on, focusing instead on emergency services, search and rescue missions, and hauling cargo. Alaka'i Technologies

The argument for fuel cells boils down to energy density: A pound of compressed hydrogen contains over 200 times more energy than a pound of battery, says Alaka’i founder Brian Morrison. That means the Skai can meet the speed, range, and payload requirements that Alaka’i thinks will make it competitive while saving a lot of weight—a top-line consideration for anything that flies. Though the company won’t reveal specifics surrounding the power system, it suggests that it and its fuel cell provider (also not disclosed) have made “breakthroughs” with the technology that enable this performance.

Hydrogen fuel cells are proving themselves able to significantly boost run times for vehicle systems, with certain small unmanned aircraft jumping from 30- to 45-minute run times with batteries to more than two to four hours with fuel cells, says Thomas Valdez, a chemical engineer with Teledyne Energy Systems. And they offer a safety benefit by eliminating the risk of thermal runaway. Even a punctured tank is no big deal: “Pressurized hydrogen would very quickly dissipate in the air, so it won’t pool or catch fire the way conventional fuels do,” Valdez says.