In the Clarion-Clipperton Zone, researchers found that polymetallic nodules-metal-rich rocks spread across the seafloor-may be creating oxygen in complete darkness. Acting like tiny natural batteries, these nodules split seawater into hydrogen and oxygen through electrochemical reactions, without sunlight or microbes.
This process, now called "dark oxygen," challenges the long-standing idea that oxygen can only be produced through
photosynthesis. It could offer clues about how life began on Earth-and how it might exist on other planets.
The nodules are basically batteries. There's a reaction at their surface that splits water into oxygen and hydrogen, and also reduces Mn(IV) in the nodules to Mn(III).
At some point the nodules will be "exhausted".
https://www.nature.com/articles/s41561-024-01480-8
Harvesting them off the deep seafloor en masse to make a bit of hydrogen will be vastly less efficient than just hooking up a solar panel to an electrode inside a water tank.