Powering any moving machine or appliance requires a portable source of electricity. The normal source used is a battery, that steadily converts the chemically reactive substances in its interior into some other products. The difference of free energy released in the reaction is taken by electrons that travel in the outer circuit so that the electrical power is obtained via the electrodes. Fuell cells make a similar function, but the operation is not limied by the amount of chemical reactives in its interiors. Instead, a chemical fuel can be continuously fed that drives the chemical reactions from which the electrical power is obtained. The fuel cell runs for so long as the fuel is available.
Microbial fuel cells (MFC) are special in that the supply fuel can be organic matter or waste watter. The MFC contains bacteria in contact with electrodes. The bacteria are very small microoganisms (size appr. 1 µm) which can convert many organic compounds into CO2, water and energy. The bacteria extract electrons (oxidize) some organic substrate such as glucose, and simultaneously produce protons. The micro-organsisms carry out this process in order to use the produced energy to grow and to maintain their metabolism, but we can intercept the process to extract a part of the energy as electricity. Electrons can be transfered from the bacteria to the negative potential electrode (the anode) as in a battery. The converse process, electron injection, occurs at the positive electrode, at which other micoorganisms can take advantage of the cathode as a source of electrons before transfer to a final oxidant. The circuit is closed by proton transfer across a selective membrane between the electrodes.
The origin of energy output of this process in terms of electricity is explained in a recent review article (Falk Harnisch and Uwe Schröder Chem. Soc. Rev., 2010, 39, 4433-4448). The chemical energy (free energy) of the fuel Efuel can be converted to electricity down to the energy of the substrate oxidation, Eox, except for some energy losses. The losses at the anode are related to biological energy dissipation such as anabolic cell processes. Losses at a microbial cathode can have a similar origin.
One exciting application of MFC is to create autonomous artificial systems that can obtain energy from organic matter in the surroundings (like predators). The Bristol Robotics Laboratory has a history of creating robots that can run off the decomposition of organic matter. Their latest is the Ecobot-III Bio-Regulation and Energy Autonomy with Digeston or simply the BREADbot. This robot has an ‘artificial gut’ that holds a host of sludge bacteria to decompose all kinds of waste. 48 MFC harness the decomposition process for small amounts of electricity. BREADbot ‘eats’ wastewater for fuel and freshwater to replace moisture lost to evaporation. The robot travels back and forth between supplies for each along a stainless steel track.
And of course it has a defecation system…