Hydrogen. Hydrogen fuel cells combine Hydrogen And Oxygen to create a flow of electrons around a circuit to power a motor. And as we all know, Hydrogen Plus Oxygen equals h20. The problem is. We cant do much about the fundamental properties of hydrogen. Volumetrically, we need to compress it to get sort of in the ballpark of not quite a third of the efficiency of fuels that we use. That means that we need often very large, very heavy tanks, because they need very thick walls to deal with the incredibly high pressures we need them at. What you end up with is an aeroplane that cant have any passengers in. This is the challenge faced by zeroavia, one of several companies working to make hydrogen powered flight a reality. They retrofit existing planes with hydrogen engines. The company had several successful test flights with this plane last year