In reference to http://docsfreelunch.blogspot.com/ . Doc mentions a honey cotton wool and copper zinc plates as a good starter point and the low power circuit is handy.
So I built a rochelle salt ,copper and steel plated battery with cotton wool insulation.
The rochelle salts is made by mixing sodium carbonate and creme of tartar. Firstly heat gently vinegar and add slowly the sodium carbonate. Allow enough sodium carbonate to be soluble. Add the creme of tartar until no longer solubable. Cool down after the mixture has thickened.Place cotton wool on a steel plate and pour mixture on the cotton wool. Press the copper plate on top and allow to cool. Once cooled ready to measure the voltage.
Here is a picture of the battery measuring 964 millivolts with a capacitor in series. The capacitor had no charge before attached to the battery. Measuring 20 micro amps puts this battery at about 18-15 micro watts.