|Figure 1: During photosynthesis and respiration, plants expel Oxygen and absorb Carbon dioxide. The plant roots release small molecular weight carbohydrates. These carbohydrates along with organic carbons are utilized by the micro-organisms yielding protons and electrons. The electrons travel through the anode. Then, the electrons flow, due to the potential difference, from the anode through an electrical circuit with a load or a resistor to the cathode, creating a current that can be harnessed for energy usage. Source: Wikimedia Commons|
Research for possible large-scale green energy initiatives is becoming more popular and vital in today's energy climate. Harnessing energy from sources such as the sun and human produced waste products are areas where much research and development is happening. One such area that can combine these two sources are organic fuel cells. In essence, these microbial fuel cells are devices that convert chemical energy to electrical energy by the catalytic reaction of microorganisms. Through oxidation and reduction of certain chemicals, bacteria and microbes transfer electrons to attached anodes, thus creating an electric current. 
Living plants such as rice and seaweed can be used in microbial fuel cells to create renewable, efficient, and sustainable bio-energy harnessed from the sun (Fig. 1). Roots of plants produce rhizodeposits, mostly in the form of proteins, sugars, plant cells, and carbon. Bacteria and other microbes present in the soil feed off these deposits and create energy through the sediment microbial fuel cell (SMFC). In particular, the organic carbon deposits can be used to generate energy. Plants deposit a large amount of unharnessed organic carbon in the soil. Such carbon is a waste of input energy (through the form of fertilizers, tilling, sunlight, etc. SMFC's contain an anode buried in a reduced matrix and a cathode floating in the overlying, oxidized water layer. The submerged matrix can also serve as a support for plant growth. At the anode, a microbial catalyzed oxidation of reduced compounds is responsible for a delivery of electrons to the anodic electrode.  The electrons pass through an electrical circuit, containing a power user. Arriving at the cathode electrode, they react with the available electron acceptor, such as oxygen. Benefits of plant microbial fuel cell energy systems include: (1) nondestructive, in-situ harvesting of bio energy; (2) potential implementation in wetlands and poor soils without competition to food or conventional bio energy production, which makes it an additional bio energy supply; and (3) carbon neutral and combustion emission-free operation. 
Research studies using Plant microbial fuel cells have achieved levels of electrical power production of around 67 mW m-2 anode surface. It is estimated that a potential electricity production of 21 GJ ha-1 year-1 (5800 kWh ha-1 year-1) in Europe.  Another feature of microbial fuel cells is their ability to produce bio-hydrogen as a byproduct. This hydrogen can be used to power hydrogen fuel cells. Microbial fuel cells can also be used in a variety of situations. Urban waste water can be collected and used in conjunction with microbial fuel cells in order to create electricity and hydrogen. There is enough biodegradable material in all of Netherlands urban waste water to generate 1.1 billion m3 of hydrogen gas per year. Assuming a fuel efficiency of 0.5-1 kg hydrogen per 100 km for a fuel cell powered vehicle, this is already enough hydrogen for driving 9.4-19% of the total car km in the Netherlands.  At the current time however microbial fuel cells have low coulombic yields and low power outputs (Ranging from 1.3 mW/m2 to 6.7 mW/m2). Many more improvements will be necessary, before biological fuel cell production and use can be commercialized. 
© Sidharth Kumar. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
 R. M. Allen and H. P. Bennetto, "Microbial Fuel Cells: Electricity Production from Carbohydrates," Appl. Biochem. Biotechnol. 39-40, 27 (1993).
 L. D. Schamphelaire et al, "Microbial Fuel Cells Generating Electricity from Rhizodeposits of Rice Plants," Environ. Sci. Technol. 42, 3053 (2008).
 D. Strik et al., "Green Electricity Production with Living Plants and Bacteria in a Fuel Cell," Int. J. Energy Res. 32, 870 (2008).
 R. A. Rozendal et al., "Principle and Perspectives of Hydrogen Production Through Biocatalyzed Electrolysis," Int. J. Hydrogen Energy 31, 1632, (2006).
 M. M. Ghangrekar and V. B. Shinde, "Microbial Fuel Cell: A New Approach of Waste Water Treatment with Power Generation," Bioresource Technol. 98, 2879 (2007).