The world’s current way to produce, convert and consume energy comes with a price. Development of sustainable energy technologies which can continue providing the society with energy-derived benefits without further environmental destructions is highly desired. A series of green energy solutions, such as solar, wind and biomass energy, are employed in the hope of preventing the impending global energy crisis (Wang, Chem and Huang 2010). Microbial Fuel Cells (MFCs) which capable of harvesting electricity from renewable biomass and organic wastes arise as a promising yet challenging candidate to the existing sustainable energy portfolio (Picioreanu, et al. 2010).
II. Historical Overview of Microbial Fuel Cells
The idea of extracting energy from organic matters through the catalytic reactions of microorganisms emerged a century ago (Lovley 2006). Although it was popular in the 1960s, interests in MFCs diminished due to their low power density and the lack of durability. A recent revival of MFC research was observed. Significant advancements in elevated power density and improved understanding on underlying microbiology concepts provided us new and valuable insights into improved MFCs designs with potential practical applications.
III. Microbial Fuel Cell Fundamentals
MFC is a bio-electrochemical device which, with the aid of bacteria through microbial-catalyzed redox reactions, converts the energy stored within bio-convertible substrates to electricity directly. (0 net carbon emission) The fundamental physical components of a typical dual-chamber MFC are the electrolyte, an anode and a cathode partitioned by a proton exchange membrane as shown in figure1 (Du, Li and Gu 2007).
At the anode, microbial respiration oxidizes available substrates to carbon dioxide results in liberation of electrons and protons. These electrons are transported out of the cell to the electrolytes via electrochemically active carriers, also known as electron mediators, and further to the cathode through an integrated external circuit. At the same time, for each electron that is transferred, the corresponding proton must travel to the cathodic chamber to sustain charge neutrality. At the cathode, a terminal electron acceptor (e.g. oxygen) becomes reduced by accepting the electrons and protons (Logan 2007). The two compartments are connected externally through an electrical wire which typically includes a resistor and separated internally by a proton exchange membrane (PEM) in a laboratory setup. The membrane acts as a barrier which restricts the oxygen diffusion from the cathode to the anode while permeable to proton migration the other way around (Logan 2007). The following equations are the simplified reactions involved in a glucose fed MFC in the anode and oxygen as terminal electron acceptor in the cathode.
Anode: C6H12O6 + 6H2O 6CO2 + 24H+ + 24e-
Cathode: 6O2 + 24H+ + 24e- 12 H2O
Overall reaction: C6H12O6 + 6O2 6CO2 + 6H2O
IV. Performance of Microbial Fuel Cell
Among numbers of expressions exist to describe the power production of MFC, the power density per anode surface area (mW/m2) is commonly used (Rabaey and Verstraete 2005). The actual power output of a MFC is a result of the collective effects of various biological, physical and electrochemical factors.
Biologically, the substrate-induced variations in the rate of catabolic degradation by different bacterial species become the primary constraint in MFC design (Logan and Regan 2006). Screening of suitable substrate which facilitates the enrichment of electrochemically active microbes is a viable mechanism to improve the MFC performance (Reference).
Physically, optimizing power generation of MFC entails a system architecture which shortens the proton migration distance therefore the internal resistance and raises the cathode oxidation efficiency. The internal resistance of a typical MFC...