plus because hydrogen is lighter than air it means the aircraft that run on it would be even more efficient due to the lower weight
Hydrogen is lighter then air because it’s less dense. So it takes a lot of space to store very little hydrogen. So to have get any useful amount of hydrogen on a plane, you need to store it cryogenically under high pressure. That makes hydrogen planes much heavier and less efficent … even if you could figure out how to keep the hydrogen cooled on longhaul flights.
It’s also just a general saftey nightmare. And on top if that you’d still produce water vapour as an exhaust, which is a potent greenhouse gas when emitted at altitude.
Overall hydrogen planes are a terrible idea that don’t really solve anything.