A Microbial Fuel Cell (MFC) is a renewable electricity source based on soil microbial life. Anaerobic bacteria (living without oxygen) are produce electrons we can harness to feed our electrical circuits. It is a low-power energy source, working fine for lighting, or communication circuits sending data at low frequency.

MFCs are the object of advanced studies in many laboratories across the world. My objective is to design a MFC which can be build by anyone without specific prior knowledge out of abundant waste or bio- materials found in his/her close environment. The power is this quick & dirty MFC should be able to power a low-power LED.

I am currently playing with parameters: sample source, design geometry, electrode materials, nature of the proton exchange membrane (PEM), water source… To understand better the drivers of electrical power delivered by these Quick & Dirty MFCs.

So far, the peak power I measured was 0.5 mW during my experiments in Brussels, which is still not sufficient to power a LED.

(see also fuel_cell for futher info…)

These preliminary experiments layed the ground for the “Power of the mud” project created with robotic artist Paul Granjon.

I started this research by looking for existing quick & dirty designs online. I found two main categories of designs

The two-container + air pump 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+ mV with airpump, 170 mV without.

The one-container 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.

I also found a presentation made by a group of students on low-tech horse-crap-based MFCs, with actual power figures in it (about 100mW for 120g of horsecrap). Interesting aspect of their designs: they use a small sponge covered with rocks as a proton exchange membrane.

• Nantes1
• Bruxelles2
• Marseille3
• Nantes4 (in French)

* Try to integrate the MFC with a Joule Thief and supercapacitors to see if we can obtain a functional system for intermittent power generation.

• Could we use compost tea as a sludge source ?
  • See here for the quite good performances obtained with compost tea.
• Does it also work with usual soil ?
  • A brief experiment in Marseille gave bad signs. The battery was flat after two hours on measurements. However, longer-term experiments should be performed to let time to the anaerobic bacterial consortium to develop.
• How can we keep feeding the sludge, to make the fuel cell operation continuous ?
  • Reverse the design, to allow waste to be piled up on top of a nutrients pile for the MFC, and be slowly degraded.

• Does the efficiency drops if we use flat electrodes from dead batteries ?
  • Actually, the best power value measured so far was obtained with copper electrodes from dead batteries (scraps from the supercapacitors research workshop).

• Is the electric air pump giving more energy than what it pumps ?
  • Most probably not.
• Could we make a mechanical airpump coupled with a windmill ?
  • Probably. But how useful would this be ? We should maybe focus on gathering the energy of the windmill.
• Could we improve the cathode design to remove the airpump ?

• Other battery designs:
  • Molten salt batteries
    • Donald Sadaway, TED
    • Zebra battery
• Rhubarb battery