In the U.S., air travel accounts for about a third of all Co2 emissions. A startup called ZeroAvia wants to clean things up in a big way.

ZeroAvia recently emerged from stealth with a zero-emission powertrain for small aircraft. It’s electric, but there are no big, bulky batteries involved. ZeroAvia opted for compressed hydrogen instead.

Why not use batteries? Compressed hydrogen is about four times as energy-dense as today’s best batteries. Founder, physicist and pilot Valery Miftakhov told FastCompany “actually getting a sizable aircraft in the air for a reasonable amount of time will be quite difficult with batteries.” That’s something he believes isn’t likely to change in the near future.