The UK-based company Intelligent Energy already makes hydrogen fuel cells which can be integrated into UAV drones:
Intelligent Energy makes a 2.4 kW fuel cell module, which equates to about 3.2 hp:
If you used that 2.4 kW in connection with batteries or a supercapacitor, then the batteries/supercap could give you “takeoff power” while the fuel cell itself could be enough for regular cruising.
Here’s a demonstration involving the Tarot-X4, which is a known consumer-level drone:
Ballard Power Systems, one of the most well known fuel cell makers, has been touting their own fuel cell-powered drone::
