Pressure Tolerant Electrochemical Fuel Cell and Cycle for Autonomous Underwater Vehicles

An underwater liquid supply unit includes a first bladder containing a first liquid, a second bladder containing a second liquid, and a third bladder containing a third liquid. The combined volume of the first liquid, second liquid, and third liquid is neutrally buoyant relative to a surrounding medium the liquid supply unit is disposed in (e.g., in seawater). As the first liquid, second liquid, and third liquid are dispensed from the bladders, the bladders may reduce in size in at least one dimension. As the liquids are dispensed, the liquids may be dispensed in a predetermined volumetric ratio based on the density of the liquids to maintain neutral buoyancy of the combined volume of liquid. The underwater liquid supply unit may also include an integrated generator such as a fuel cell, as well as a propeller.