You would spend more energy obtaining sodium ions than you would recover in the reaction. Most of the good primary chemical energy sources on the earth have already reacted with something and expended their reaction energy. The nuclear furnace 93 million miles away bombards us with electromagnetic energy which enterprising plants convert to chemical energy. We exploit the resulting plant energy as firewood, peat, coal and petroleum.
It is only because of plants that there is any free oxygen in the atmosphere, it would quickly have bonded with some other element otherwise.
You would spend more energy obtaining sodium ions than you would recover in the reaction.
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the sodium ions Na+ are already present in solution in saltwater.
the question is —thanks to the previous poster— where do you get the extra free electron from to attach to the sodium ion Na+ and how do you attach it in situ in solution so that suddenly a sodium metal Na is in the exothermic presence of water H2O