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TYPES

2.7 , 2.8

Mediator Microbial Fuel Cell

In this process, in order to generate electricity and form water a mediator is used because microbial cells are usually electrochemically inactive. Mediators are usually thionine, methyl, viologen, methyl blue, humic acid, and neutral red. In figure 2.1, it shows a model of how it is produced in a two-chamber system with the bacteria in the anode chamber separated from the cathode chamber where the electron transfer from the microbial cell to the electrode is done through the mediator, Methylene blue. This process is usually toxic and more expensive due to the materials needed. (1)

2.1

Mediator-Free

Instead of using a mediator, and electrochemically active bacteria is used in order to transfer electrons to the electrodes in this microbial fuel cell design. Some of these electrochemically active bacteria include Shewanella putrefaciens and Aeromonas hydrophillia (2). Because Mediator-less MFCs are a more recent area of research factors that affect optimum efficiency are not understood; these factors include the strain of bacteria used in the system, type of ion-exchange membrane, and system conditions. Mediator-less microbial fuel cells can derive energy from plants known as plant microbial fuel cell (3)

2.2

Microbial Electrolysis

2.3

The microbial electrolysis cell is a variation of the mediator-free MFC. This type of microbial fuel cell produces an electrical current by bacterial decomposition of organic compounds in water. MECs reverse the process to generate hydrogen and methane by applying and outside source of volts to the bacteria in order to supplement the electrical current generated by the microbial decomposition of organics resulting in the electrolysis of water. (4) In the photo above explains the process of how a microbial electrolysis cell operates; a plant is grow and chopped up and is then fermented which will produce acetic acid, the bacteria will then consum the acetic acid which will release electrons, protons, and CO2, volts from an outside source are then added allowing the electrons to join with the protons forming hydrogen gas, a clean source of fuel.

2.4

Soil-Based

In soil-based microbial fuel cells, the soil acts as an anoidic media, inoculum, and the proton exchange membrane. The anode or the electrode is placed in the soil at a certain depth and the cathode rests on top of the soil, exposed to the oxygen in the air. Soil is composed of naturally diverse microbes and electrogenic microbes needed for MFCs, meaning there are mediator-free. Some microbes located in the soil are aerobic meaning they consume oxygen acting as a filter which can be compared to the PEM materials used in the laboratory MFC systems making it popular to use in science classrooms. (5) The main microbial fuel cell used to help droughts are Sediment Microbial Fuel Cell which have structures that can generate electrical energy while decontaminating wastewater using plants that mimic constructed wetlands (6) 

Phototrophic Biofilm

2.5

This type of microbial fuel cell uses the anode with the phototrophic biofilm that contain photosynthetic microorganisms and act as producers of organic metabolites and electron donors. Through electrochemical analysis and biofilm 16S rRNA gene profiling of biocathodes of sediment/seawater-based microbial solor cells (MSC) inoculated from the biocathode of a previously described sediment/seawater-based MSC it is indicated that second generation of MSC biocathodes, catalytic activity diminishes over time if illumination is provided during growth, whereas it remains stable if growth occurs in the dark (7)

2.6

Nanoporous Membrane Microbial Fuel Cells

The Naval Research Laboratory helped developed the nanoporous membrane microbial fuel cell which its main function is harvest energy from aerobic aqueous environments. NMMFCs are powered by passive nutrient diffusion whereas other MFCs are powered by energy draining pumps. This design isolates electrochemically active microbes in the cell rather than relying bacteria that are electrochemically active that are available in the environment, allowing it to be used in a wide range of aerobic aqueous environments. This design is much more efficient due to the fact that it uses inexpensive nanoporous membranes made from polycarbonate or other materials to confine the microbes. The resulting MFC designs are capable of generating microwatts to milliwatt (8)

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