A microbial fuel cell uses anaerobic biological decomposition of organic matter to produce electricity.
A simple cell using coffee grounds and briny newspaper in an aluminum tray and separated by a produce bag produces 3-4 μA at 30-50 mV, but the polarity is reversed. The tray containing the decomposing organic matter is the cathode; it is positively charged. Perhaps this is because of the aluminum.
The salt solution can also draw excess water from the anaerobic chamber, and controlling evaporation from the brine will control the moisture level of the decomposing material.
Those 'breathable' produce bags may work. Natural rubber latex is a possibility. A toy balloon filled with air gradually loses its pressure as the gases diffuse out of it. (Mylar is now widely used for helium balloons because it holds helium better than natural rubber, and so is likely to hold hydrogen and hydronium longer, as well.)
It should be entirely possible to use an egg1) suspended in weak brine, so long as it (the egg) is vented.
Materials Needed