|Fig. 1:Schematic of two-step energy generation process using microbial batteries. (After Xie et al. )|
There is substantial energy in organic matter that is currently wasted or lost in treatment processes. If not managed, the reservoirs of organic matter, such as marine sediment, wastewater, and waste biomass, can become eutrophic dead zones and sites of greenhouse gas generation. By harnessing the oxidative power of microorganisms, energy can be recovered from the these reservoirs. Microbial fuel cells (MFCs) offer an option for direct electricity generation from electron donors oxidized by microorganisms and have been used to recover electricity from domestic wastewater and marine sediment. [1-5]
MFCs are similar to chemical fuel cells. Oxidation occurs at an anode, and electrons pass through an external circuit to a cathode where O2 is reduced. At the anode, however, chemical catalysts are replaced by exoelectrogens - microorganisms that oxidize the electron donors and transfer the electrons to an electrode.
For MFCs, energy recovery is limited by a voltage loss when O2 is reduced at the cathode. This loss is exacerbated by MFC operating conditions - atmospheric pressure, ambient temperature, and an aqueous electrolyte at near-neutral PH. Diffusion of dissolved O2 into the anode compartment is also a problem, allowing formation of aerobic biomass and oxidation of organic matter without energy production. A recent work uses of a solid-state cathode to replace the oxygen gas cathode of a MFC, thus largely improved the energy recovery efficiency.  Operation of the anode is like that of a MFC anode, but operation of the cathode is like that of a rechargeable battery. They therefore refer to this device as a microbial battery (MB).
|Fig. 2: Microbial battery energy recovery. The heights of the boxes indicate the energy percentages. (After Xie et al. )|
As shown in Fig. 1, energy generation process using MBs consists of two steps. In the first step, electron donors are oxidized by microorganisms at the anode, generating free electrons that pass through an external circuit to a cathode. The cathode contains a solid-state oxidant, silver oxide, which becomes reduced to silver metal. This step is similar to the discharge process of batteries. In the second step, the same cathode is removed and then oxidized under favorable conditions. The silver electrode is then oxidized into silver oxide. This step is similar to the recharge process of batteries. The two-step process is repeated, recovering energy from the reservoirs. This device design uses single-chamber operation, which avoids the drawbacks of voltage losses at oxygen cathodes and diffusion of oxygen into anode in MFCs.
The energy flow of the MB is shown in Fig. 2. About 11% of the energy in the added glucose is retained in the electrolyte as residual electron donors; of the 89% removed, about 7% is used for biomass synthesis, leaving about 82% for microbial energy production; about 12% is used for microbial metabolism; over-potential at the solid-state electrode resulted in another 16% energy loss; and 10% energy loss resulted from losses due to internal resistance and diffusion limitations. The gross efficiency of electrical energy production was about 44%. This number increases to about 49% if the energy recovered is divided by the energy of the organic matter consumed. The energy required for electrochemical re-oxidation ranged from about 14 - 24%, depending upon the current density (0.01 - 1 mA/cm2) applied. This step involves a tradeoff: re-oxidization at higher current densities consumes more energy but is more rapid, so less electrode material is required for continuous electricity generation. The net energy conversion efficiency for energy production and electrode re-oxidation was about 20 - 30% based on the organic matter added and about 21 - 33% based on the organic matter consumed.
© Shuang Wang. 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.
 B. E. Logan and K. Rabaey, "Conversion of Wastes into Bioelectricity and Chemicals by Using Microbial Electrochemical Technologies," Science, 337, 686 (2012).
 D. R. Bond et al., "Electrode-Reducing Microorganisms That Harvest Energy From Marine Sediments," Science 295, 483, (2002).
 L. M. Tender et al., "Harnessing Microbially Generated Power on the Seafloor," Nat. Biotechnol. 20, 821 (2002).
 S. K. Chaudhuri and D. K. Lovley, "Electricity Generation by Direct Oxidation of Glucose in Mediatorless Microbial Fuel Cells," Nat. Biotechnol. 21, 1229 (2003).
 H. Liu, R. Ramnarayanan and B. E. Logan, "Production of Electricity During Wastewater Treatment Using a Single Chamber Microbial Fuel Cell," Environ. Sci. Technol. 38, 2281 (2004).
 X. Xie et al., "Microbial Battery For Efficient Energy Recovery," Proc. Nat. Acad. Sci. 40, 15925 (2013).