 The fuel cells are built into the top of the wing, where they combine oxygen in the ambient air with hydrogen produced internally by the MAV through reaction of a hydride material with water.The fuel cell system is expected to provide three times the endurance of batteries of comparable weight, though early flights were limited by the tendency of the fuel cells to dry out.DARPA is actually more interested in the battery-powered Wasp, but other interested parties in the US defense establishment, particularly the NRL, are very intrigued by fuel cells, and so DARPA is hedging its bets.