Many of us have heard of the Potato battery, lemon battery, and maybe even the water battery. This is where you take something like a penny which is copper and place that in the potato or water based substance with another metal such as aluminum. You put you meter across the metals and you'll get voltage. The voltage is coming from the galvanic reaction, one of the metal plates is giving up electrons to the other plate and over time that plate will corrode away and the cell will stop producing voltage.
Now lets say we get rid of this galvanic reaction so that the plates last longer. This is where we start to run into problems as many textbooks and websites will state that you need two dissimilar plates to get a voltage. If you use the same plates in water, such as distilled water, then what everyone says is that this can not produce a voltage. Well I got proof that you can get voltage from the same plates in distilled water. As to where the electricity is coming I don't know, but it exist.
Its the fact that plates are exposed to a dielectric material such as water that creates the voltage. This same idea also applies to capacitors, they too have aluminum plates separated by a dielectric and also can produce a voltage when never been charged or have been shorted out. There is something mysterious about plates separated by a dielectric that I can't seem to find information about anywhere.
I'm not going to state that this type of battery cell will last forever because water is the universal solvent, so the plates will break down over time. The time frame can be anywhere from weeks to centuries depending on what plates are used. I have test Graphite plates and they work just as good, if not better, than the aluminum plates. Aluminum will dissolve before Graphite in water so Graphite might be something to look at especially since graphite is not a metal.