I ran across an article which describes the laboratory model for the extraction of dissolved oxygen from water, which has been discussed here.
One of the legitimate concerns expressed here was the massive quantity of water which would have to be processed to obtain enough breathing gas. The author says this is true for an open system, but not for a closed system...a rebreather type system with the extraction of dissolved oxygen from water as the oxygen supply. A one Kg battery can supply enough power for 1 hour (theoretical?). The separation is done by centrifuge. They're still in the "proof of concept" stage of development.
See for yourself....
Stan