User:Anand Gupta/Notebook/Microbial Fuel Cell/2009/04/20

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Project Proposal
Implementation of a Microbial Fuel Cell for Efficient Energy Production

Objective

Cellular respiration pathways are highly efficient in producing large quantities of energy from simple organic compounds, and are even able to power the most enormous organisms on the planet. We hope to construct a system that can harness cellular respiration to produce electrical energy with in order to provide a sustainable energy source to meet rising global energy demands.

Goals

To design and build a microbial fuel cell that harnesses the redox reactions involved in cellular respiration to produce electrical energy, assessing its costs and benefits in production, as well as calculating the standard reduction potential of cellular respiration.

Proposed Methods

Our implementation of the microbial fuel cell (MFC) will intervene in the respiratory pathway by tapping H+ production, capturing those ions at the biological anode and using a proton exchange membrane to reduce O2. The movement of electrons from biological anode to ion cathode should produce DC current, which would be used to power electronic appliances.

Biological Systems

Our selection of an organism to catabolize supplied glucose at the anode must provide an easily-accessible pathway to collect both electrons and protons for the subsequent reduction of oxygen. The bacteria Escherichia Coli is a facultative anaerobe, capable of both aerobic and anaerobic respiration. Under conditions where oxygen is present, E. Coli will perform aerobic respiration, using electrons and protons collected through the oxidation of glucose to reduce oxygen to water. Aerobic respiration in E. Coli is much like that present in eukaryotes, following glycolysis with the Citric Acid (Krebs) cycle, membrane-embedded electron transport chains (ETCs) and ATP synthase to produce additional ATP by oxidative phosphorylation. The primary advantage of implementing E. Coli is that their electron transport chain is embedded in the membrane, pumping protons into the extracellular space, whereas in eukaryotes, ETCs exist in the mitochondrial inner membrane, pumping protons into an space between mitochondrial membranes. This difference puts electrons and protons at the surface of the cell in E. Coli, allowing for their capture and use in an electrochemical cell. Note that under oxygen-poor conditions, E. Coli will convert products of glycolysis to ethanol via fermentation, consuming collected protons and electrons this way.

Biological Anode Reaction

An enabling result of E. Coli’s unusual respiratory pathway is the ability to draw electrons directly off the ETC. If the oxygen that is usually reduced by E.Coli is replaced by an electron-accepting material (anode material), it becomes possible to draw a current from the bacteria. In the proposed fuel cell design, E. Coli will be placed at the anode of an electrochemical cell, acting to reduce the anode material by its metabolic half-cell reaction. The half reaction for the oxidation of acetate (produced through glycolysis and the link reaction), C2H4O2+ 2H2O => 2CO2 + 8e- + 8 H+ represents the net products of glycolysis, the link reaction, and Kreb’s cycle, with electrons separated from their electron acceptors by the ETC. Note that bacteria may use other electron acceptors beside oxygen, including sulfates and nitrates, at reduced potentials.

Proton Exchange

Hydrogen ions produced at the anode are necessary for the cathodic reduction of oxygen gas to water: O2 + 4 H+ + 4 e- => 2 H2O Thus, we will need a system by which hydrogen ions can be transported to the cathode from the anode. In non-biological electrochemical cells, a salt bridge is used, allowing for the selective movement of ions to balance charge changes, and generally transports a spectator ion such as NO3-. In the MFC, we will use a proton exchange membrane (PEM), commonly used in hydrogen fuel cells, to allow for the one-way transfer of hydrogen ions from anode to cathode. This membrane is made of the polymer Nafion, and will divide the two half reactions of the MFC. Both diffusion and the spontaneous reduction of oxygen will drive the transport of H+.

Cathode Reactions

At the cathode, oxygen will be reduced to H2O, using the electrons from the anode and the protons from the “salt bringe”, i.e. the PEM.

Overall Reaction

The overall balanced equation for the reaction is as follows: C2H4O2 + 2O2 => 2CO2 + 2 H2O


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