Microbial Fuel Cells (MFC)

Microbial Fuel cells are drastically different, not in their operation, but in how they produce hydrogen. MFCs are referred to as bio-electrochemical devices and they use bacteria and their enzymes to generate a current. A useful way to think about a microbial fuel cell is to think about the bacteria as the catalyst, rather than a precious metal such as platinum.

Functions and Reactions of MFCs

Just like any standard fuel cell, fuel is oxidized in the anode compartment to produce protons and electrons. Protons are free to travel through the membrane, while electrons are forced to travel for an external circuit where they can do useful work. Once the protons of electrons are recombined and reacted with oxygen in the cathode compartment, they form water. microbial fuel cells come in two different types: mediator-less and mediator.

  • Mediator MFCs Most microbial fuel cells are “electrochemically inactive.” This means that the bacteria within the fuel cell are unable to transfer electrons out of themselves without the assistance of a mediator such as humic acid. Mediators are generally undesirable because they are expensive and toxic. Nevertheless, this is currently the most common type of fuel cell.
  • Mediator-free MFCs These fuel cells do not require a mediator because the bacteria are capable of transferring electrons directly to an electrode. Bacteria that are capable of this include Aeromonas hydrophila and Shewanella putrefaciens. Besides the fact that these fuel cells avoid the toxic mediators present in other microbial fuel cells, they are also capable of running on wastewater and of deriving energy directly from specific plants.

Benefits of MFCs

Benefits of using MFCs include the ability to derive energy from otherwise useless sources, such as wastewater and organic matter.  The ecological impact of such a strategy is twofold. First, organic waste that would otherwise be mechanically or chemically treated can be directly utilized in these fuel cells, which reduces energy expenditure for treatment while achieving the same end. Second, this process does not rely on fossil fuels and therefore has a lower overall impact in terms of greenhouse gas emissions than other fuel cells.

There is current interest in the use of microbial fuel cells in municipal wastewater treatment. It is proposed that these fuel cells be coupled with wastewater treatment in order to make wastewater treatment facilities self sufficient or even possibly produce excess electricity.

A second benefit to using MFCs is that they avoid the use of expensive catalysts such as platinum. Bacteria are relatively inexpensive and easily grown, providing an abundant supply for the production of fuel cells.

Drawbacks of MFCs

Like all fuel cells, MFCs have their drawbacks. The first drawback it the limited power that they can produce. The highest power densities reported as of 2006 were 50 watts per cubic meter. Due to their low power density, microbial fuel cells have only found application in powering remote monitoring devices that have low power needs.

To combat the problem of low power density, researchers are currently working on a variation of the mediator-free MFC called the microbial electrolysis cell or MEC. These fuel cells are rather complicated, but the general idea is that applying an external voltage to the bacteria can actually increase their efficiency. While this fuel cell does consume some electricity (rather than strictly making it), it has been shown to produce 144% more usable energy than it consumes. These fuel cells are experimental and currently suffer from a reliance on platinum.