Microorganisms are main electron producer in the MFCs. They uptake and decompose organic substrates to carbon dioxides, protons, and electrons. Although the electrons are consumed by reacting with oxygen or inorganic electron acceptors in nature, the anode of the MFCs works like inorganic electron acceptor. The electrons that transferred to the anode are moved to the cathode through the outer circuit. Meanwhile, the produced protons are transferred through electrolyte or ion exchange membrane to the cathode. On the cathode surface, the transferred electrons and protons are reacted with oxygen that comes from outside air to produce water. From the process, the substrates are changed into carbon dioxide, water and electricity.

 

  What should we do for the efficient production of electricity in microbial fuel cells? Although there are several key factors for increasing MFCs’ performance, we focus mainly anode material, and oxygen reduction catalyst. The main research direction of the anode material is improvements in electron transfer efficiency from bacteria to electrode and increasing anode surface area for the microbial attachment. To improve electron transfer efficiency we utilize conductive molecular wires, and nanosized carbon particles. They allow direct electron transport to the anode electrode. Although few kinds of bacteria can transfer electrons to the anode directly, the great part of bacteria cannot. The molecular wires or carbon particles connect electrode and microbial electron transfer system, thereby, allowing direct electron transfer for every microorganism. For the direct electron transfer, bacteria have to attach on the anode electrode surface. This is the reason that the anodes have to have large surface area. Also, since the electrode cannot expand unlimitedly, three dimension structures are needed for the anode electrode. So, we are also searching for three-dimensional conductive materials with macropores.

 

  The cathode reduction reaction is another driving force in the MFCs. The oxygen reduction reaction is the well-used reaction in the practical fuel cells because the oxygen is free and omnipresent resource, also, the oxygen reaction has high redox potential. Thus, the oxygen is commonly chosen as the cathodic fuel in the practical MFCs. However, this reaction is needed the expansive catalyst, platinum. The platinum is widely used catalyst but very expensive precious metal. Due to the price of the platinum, studies on the alternative substance for platinum are vigorously progressed. We also research on the non-platinum-based oxygen reduction catalysts using polyaniline, metal phthalocyanine compound, and carbon nanomaterials.