A hydrogen fuel cell is a device that converts hydrogen gas & air into electricity and water. Although these devices are much lower power density than batteries, they can pump out that power for many hours, depending on the amount of fuel you are storing.
At Loughborough University UK, we are working on a fuel cell and LiPo hybrid, to give you the oompf for take off (and go-arounds!) and the long endurance for cruise and loiter.
The base aircraft is the Skywalker X8, modified to carry considerably more weight than it is designed for (currently 4.5kg all up, and pushing towards 6kg with more testing).
This video briefly explains the platform setup, shows a video using ArduPilot's autonomous take-off, and concludes with a short 4.5kg flight.
In the coming weeks look forward to videos of the payload approaching 6kg, and the fuel cell being connected properly and providing some of the power!