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3.1

Electrical Generation Process

In this video explains a brief explaination of the electrical generation process (3.2)

HOW MICROBIAL FUEL CELLS WORK

The Microbial Fuel Cell is divided into two parts: aerobic and anaerobic. The Aerobic half contains positively charged electrode and oxygen gas while in the anaerobic half oxygen is not present, allowing negatively charged electrode to act as electron receptor for bacterial processes. The two halves are seperated by a semi-permeable membrane to keep oxygen out of the anaerobic chamber while still allowing hydrogen ions (H+) pass through. 

C12H22O11 + 13H2O → 12CO2 + 48H+ + 48e−

  • The electrons from the cathode combine with dissolved oxygen and the H+ ions to form pure H2O.

  • The H+ ions flow through the semi-permeable membrane to the cathode. This process is driven by the electro-chemical gradient resulting from the high concentration of H+ ions near the anode.

  • The electrons flow up from the anode, through a wire, and onto the cathode. While flowing through the wire, an electrical current is generated that can be used to perform work.

  • The electrons flow from the bacteria to the anode, sometimes assisted by a mediator molecule. 

  • The bacteria on the anode decompose organic matter and free H+ ions and electrons. (1)

In the anaerobic chamber, a solution containing food (glucose and or acetate) for the bacteria is circulated. The bacteria metabolizes food by first breaking down the food molecules into hydrogen ions, carbon dioxide, and electrons. The bacteria then uses the electrons to produce energy by way of the electron transport chain. The microbial fuel cell disrupts the electron transport chain using a mediator molecule to shuttle electrons to the anode. 

  • The electron transport chain begins with NADH which releases a high energy electron and proton

  • The electrons then follow the red path according to the figure above, which goes through the proteins of the mitochondrial membrane

  • Electrons will pump hydrogen ions as they pass through the proteins and throughout the membrane

  • In the microbial fuel cell, the electrons will continue on the red path until it reaches mediator molecule and is picked up and taken to an anode (1)

In Simple Terms

Microbial fuel cells use inorganic mediators to go into the electron transport chain of cells where electrons are produced. The mediator crosses the outer cell lipid membranes and bacterial outer membrane, where it begins to release electrons from the electron transport chain. The reduced mediator exits the cell laden with electrons that transfers to an electrode where it deposits them. The electrode becomes an negatively charged electrode. The release of the electrons allows the mediator to return to its original oxidised state and ready to repeat the process. This process can only happen in anaerobic conditions. As a result, if oxygen is present, it will collect all the electrons, due to its greater electronegativity compared to the mediators. In a microbial fuel cell operation, the anode is the terminal electron acceptor in the anodic chamber. The microbial activity is strongly dependent on the redox potential of the anode. (2) Mediators suggested for use in microbial fuel cells include natural red, methylene blue, thionine, or resorufin. In order to turn this into a usable supply of electricity, this process has to be accommodated in a fuel cell meaning a complete circuit must be created, and not just transfer electrons to a single point.(3) The mediator and micro-organism are mixed together in a solution which is added and is placed in a sealed chamber to stop oxygen entering, forcing the micro-organism to use anaerobic respiration. An electrode is then placed in the solution that will act as the anode. In the other chamber of the MFC, there is another solution and electrode, which is called the cathode (positively charged) and is the equivalent of the oxygen sink at the end of the electron transport chain. The solution is an oxidizing agent that collects the electrons at the cathode. Connecting the two electrodes is a wire which may include some electrically powered device for example a light bulb, completing the circuit and the two chambers is a ion-exchange membrane. The reduced mediator carries electrons from the cell to the electrode. The mediator is oxidized as it deposits the electrons then flows across the wire to the second electrode, acting as an electron sink. From here they pass to an oxidising material.(4)

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