Among other things, a device for use in electrolyzing water is described. The device comprises an electrolysis unit that includes a chamber, an ion exchange structure in the chamber, a cathode, an anode, a high pressure chamber, and a reservoir. The chamber is separated by the ion exchange structure into a first compartment and a second compartment. The cathode is in the first compartment and the anode in the second compartment. The reservoir is disposed in the high pressure chamber for storing water to be supplied to the chamber of the electrolysis unit. In some implementations, the ion exchange structure is a proton exchange membrane.