You can gain energy by mixing fresh water and salt water, or you can use energy to extract salt from sea water, but you can't gain energy extracting salt from sea water.

Edit: “The desalination battery is essentially the same device, but reversed,” explains La Mantia, who worked again with Cui and other researchers, on the new study.

The desalinated water that comes from the battery still contains too much salt for drinking, La Mantia says: “We removed up to 50% of the original salt, but we need to arrive at 98%.”

PS: Reverse osmosis uses more energy the saltier the water is so depending on how much energy this process uses it could be useful as a first stage prior to reverse osmosis filtering even if it can only remove 50% of the salt.