Submarines travel underwater, but they all need oxygen. Diesel-electric submarines need it for their engines, and must surface to get it, though modern AIP (Air Independent Propulsion) supplemental systems allow one to cruise at moderate power for over a week without surfacing. Nuclear-powered submarines could cruise underwater at full power for years, of course, as their engines do not need air. Their crews, however, do. Hence Electrolytic Oxygen Generators (EOGs), which break up water molecules and keep the oxygen for use aboard ship.
Treadwell Corp’s Model 6L16 EOG was first introduced in 1965. It breaks up distilled water by passing an electric current through an electrolyte solution (30% potassium hydroxide) in 16 high-pressure cells, connected in series. This equipment can produce 150 standard cubic feet per hour of oxygen, and variants remain the primary oxygen producers aboard the USA’s SSN-688 Los Angeles Class fast attack submarines and SSBN-726 Ohio Class ballistic missile submarines. Treadwell also produces Oxygen Generation Plants (OGPs) for the new SSN-21 Seawolf Class fast attack submarines, which include OGP electrolysis modules that depend on proton exchange membranes for oxygen separation.
The US Navy is moving to upgrade both of these systems.