Ordinarily, underwater ROVs (remotely operated vehicles) and AUVs (autonomous underwater vehicles) are designed to be neutrally buoyant. This means they neither sink to the bottom nor float to the surface when underwater.

In order to move vertically in the water column, they typically use their powerful electric thrusters. Utilizing an electric pump to draw water in and out of onboard ballast tanks is another option. In either case, a considerable amount of battery power is expended in the process.

Seeking a more efficient alternative, Rice University's BayMax team of engineering students looked to reversible water-splitting fuel cells. They integrated four of the devices into an ROV they built themselves, called the Monarch.

Water-splitting fuel cells utilize the electrolysis process to split liquid water into its two components, hydrogen and oxygen gas. Reversible cells, as their name implies, can also convert the two gases back into liquid. Because the gases have a lower density than the liquid, they have a greater volume.

This phenomenon is exploited by the Monarch.

If the ROV needs to increase its buoyancy, distilled liquid water in its fuel cells is converted into hydrogen and oxygen gas, which inflate balloons located on top of each cell. When the vehicle needs to decrease buoyancy, those gases are converted back to liquid, causing the balloons to deflate.

The technology can additionally be utilized to keep the vessel sitting horizontally level in the water, by automatically activating individual cells as needed. What's more, because the buoyancy of even "neutrally" buoyant ROVs and AUVs changes at different depths, or in fresh versus salt water, the system can be set to hold the vehicle at any given depth.