If you've got H2 on a plane it's probably lighter to just feed it to the jet engine directly rather than try to replace that with a fuel cell/motor combo which will not buy you enough efficiency gain to compensate for its weight.
[0] Considering the Carnot efficiency of a jet engine at ~30%, a combination of fuel cell (~60%), motor (~95%) and propeller (~80%) will still beat it by a factor of 1.5.