Weight is everything in aviation, and it's even more important when you're talking about energy-intensive vertical takeoff and landing like the coming eVTOL air taxi contenders are promising. Nearly every company on that list is building its prototypes using lithium batteries, but the simple fact is, lithium batteries have terrible energy density.

Commercial air taxis will need to fly all day, not spend long hours hooked up to charging stations, so every company that's planning around lithium is praying for a magical new chemistry to emerge from a test lab that doubles or triples the capacity of the best batteries available today.

That may well happen, but hydrogen is beginning to look like an excellent alternative. It's a pain in the butt when it comes to ground transport, but in electric aviation it might find a perfect use case. It offers much higher energy density, promising excellent flight endurance, and it lets you fill up with fuel in minutes, just like topping up your car's gas tank.