slowrotor & arismount,
The hydrogen fuel cell may be a solution, but it appears that the current discharge rate is not fast enough for the needs of rotorcraft.
As you mention, aircraft can have a higher than normal power requirement during takeoff. One feature of the hydrogen fuel cell that MIGHT be advantageous is that of applying its byproduct (water) to the motor,s coils, for cooling by evaporation.
Wick Idea
This MIGHT be the the source of power for very future rotorcraft;
Nuclear Power We Might Be Able to Live With
Dave