Does it need to be sea water, or could it just be brine?
It can be just brine. I don’t know why they specify seawater. Had to relate salt water to seawater maybe?
But a full solution brine with some hydrogen peroxide to raise the oxygen level will produce the best voltage output. Of course if you add peroxide it will have to be replenished and will off gas flammable Hydrogen.
Here is something wild. If you live where the soil is higher alkali you can just jam a copper rod and a zinc rod in the ground about four inches apart and then wet the soil. Give it a few minutes and it will produce around 1.2 volts from the soil and water as an electrolyte.
Now for how long or if a dirt battery can be recharged and hold a charge I don’t know, haven’t had the time to test that far yet. But I do know that the dirt battery actually put out more initially than the salt batteries I was experimenting with. The Alkali salt is better than just a salt brine I’m sure.
So the next test will be a mixture of sodium and alkali solution.