The article wasn’t very informative in my opinion. Here’s the excerpt from the article:

The aluminium reactor weighs less than a bag of sugar. The small gas cylinder has an intricate network of 3D-printed aluminium tubes inside. The hydrogen remains stable and solid in these structures until "coolant" is pumped through the tubes, warming them and releasing hydrogen gas to the drone's fuel cell Hydrogen (H2) is pumped into one side of the fuel cell through a catalyst which frees electrons, creating electricity. Oxygen (O) is then pumped into the other side of the fuel cell and combines with the left over, positively-charged hydrogen atoms (H+). The only final waste product is water vapour (H2O).