Extract electrons from a liquid?

[Magnethos]

Hi, recently I have made a Volta´s Battery. This is a simple device that consists on two different metals and a glass of water. Using this procces you can obtain a very little amount of energy. But with the time, the screw oxides because one metal gives electrons to make possible obtain energy.

 

But I have a question, recently I read that you can obtain these electrons from the water instead from the screw. But I don´t know how do it.

[Mr Skeptic]

That would be interesting, to make a battery that uses liquid only. It could in theory be infinitely rechargeable, as there would be no electrodes to corrode. However, it would need a membrane to separate the liquids, and would probably have a poor energy density.