This design is made of two containers, bridged together with a salt bridge (rope dipped in saturated salted water, wrapped in insulating tape), one with pond sludge, another with clear water.

An airpump feeds clean water with oxygen for the reduction reaction: protons coming from sludge by the salt bridge, together with electrons coming from sludge by the wires, recombine with oxygen to make water. The airpump allows high current to be produced.

Performances: 6V peaks, 200+ mA with airpump, 170 mA without.

This design is made from one container, sludge at the bottom, smart gel proton exchanger on top of it, and clean water on the very top. No air pump, but lower performances. Something like charcoal is used as an electrode material.

• Is the air pump giving more than what it pumps in terms of energy ?
• Could we make a mechanical airpump coupled with a windmill ?
• Could we improve the cathode design to remove the airpump ?

• Could we use compost tea as a sludge source ? Does it also work with usual soil ?
• How can we keep feeding the sludge, to make the fuel cell operation continuous ?

• Does the efficiency drops if we use flat electrodes from dead batteries ?