Microbial Fuel Cell - Electrical Generation Process

Electrical Generation Process

When micro-organisms consume a substance such as sugar in aerobic conditions they produce carbon dioxide and water. However when oxygen is not present they produce carbon dioxide, protons and electrons as described below:

C₁₂H₂₂O₁₁ + 13H₂O ---> 12CO₂ + 48H+ + 48e- (Eqt. 1)

Microbial fuel cells use inorganic mediators to tap into the electron transport chain of cells and channel electrons produced. The mediator crosses the outer cell lipid membranes and bacterial outer membrane; then, it begins to liberate electrons from the electron transport chain that normally would be taken up by oxygen or other intermediates.

The now-reduced mediator exits the cell laden with electrons that it shuttles to an electrode where it deposits them; this electrode becomes the electro-generic anode (negatively charged electrode). The release of the electrons means that the mediator returns to its original oxidised state ready to repeat the process. It is important to note that this can only happen under anaerobic conditions; if oxygen is present, it will collect all the electrons as it has a greater electronegativity than mediators.

In a microbial fuel cell operation, the anode is the terminal electron acceptor recognized by bacteria in the anodic chamber. Therefore, the microbial activity is strongly dependent on the redox potential of the anode. In fact, it was recently published that a Michaelis-Menten curve was obtained between the anodic potential and the power output of an acetate driven microbial fuel cell. A critical anodic potential seems to exist at which a maximum power output of a microbial fuel cell is achieved.

A number of mediators have been suggested for use in microbial fuel cells. These include natural red, methylene blue, thionine or resorufin.

This is the principle behind generating a flow of electrons from most micro-organisms (the organisms capable of producing an electric current are termed Exoelectrogens). In order to turn this into a usable supply of electricity this process has to be accommodated in a fuel cell. In order to generate a useful current it is necessary to create a complete circuit, and not just shuttle electrons to a single point.

The mediator and micro-organism, in this case yeast, are mixed together in a solution to which is added a suitable substrate such as glucose. This mixture is placed in a sealed chamber to stop oxygen entering, thus forcing the micro-organism to use anaerobic respiration. An electrode is placed in the solution that will act as the anode as described previously.

In the second chamber of the MFC is another solution and electrode. This electrode, called the cathode is positively charged and is the equivalent of the oxygen sink at the end of the electron transport chain, only now it is external to the biological cell. The solution is an oxidizing agent that picks up the electrons at the cathode. As with the electron chain in the yeast cell, this could be a number of molecules such as oxygen. However, this is not particularly practical as it would require large volumes of circulating gas. A more convenient option is to use a solution of a solid oxidizing agent.

Connecting the two electrodes is a wire (or other electrically conductive path which may include some electrically powered device such as a light bulb) and completing the circuit and connecting the two chambers is a salt bridge or ion-exchange membrane. This last feature allows the protons produced, as described in Eqt. 1 to pass from the anode chamber to the cathode chamber.

The reduced mediator carries electrons from the cell to the electrode. Here the mediator is oxidized as it deposits the electrons. These then flow across the wire to the second electrode, which acts as an electron sink. From here they pass to an oxidising material.