Intermittent Energy Harvesting Improves the Performance of Microbial Fuel Cells ALIM DEWAN, † HALUK BEYENAL, † AND ZBIGNIEW LEWANDOWSKI ‡, * The Gene and Linda Voiland School of Chemical Engineering and Bioengineering, Center for Environmental, Sediment and Aquatic Research, Washington State University, Pullman, Washington 99163-2710, and Center for Biofilm Engineering, Department of Civil Engineering, Montana State University, Bozeman, Montana 59717-3980 Received December 30, 2008. Revised manuscript received March 31, 2009. Accepted April 6, 2009. In this study, we compare the efficiencies of harvesting energy from microbial fuel cells (MFC) using two modes of operation: (1) continuous-passing the current through an electrical load-and (2) intermittent-first accumulating the energy in a capacitor and then discharging it through the load. Each of these two modes of operation has advantages and disadvantages: the first mode of operation allows the continuous powering of low- power-consuming devices, and the second mode of operation allows the intermittent powering of high-power-consuming devices. We used a two-compartment MFC: in the anodic compartment, Shewanella oneidensis MR-1 was grown using lactate as an electron donor, whereas in the cathodic compartment we used an electrode made of manganese- based catalyzed carbon bonded to a current-collecting screen made of platinum mesh and oxygen as the electron acceptor. The maximum power generated by harvesting energy intermittently was 152 μW, which is 111% higher than the 72 μW generated by harvesting the energy continuously. We conclude that in the operation of MFCs it is beneficial to harvest the energy intermittently. This not only allows the powering of external devices of high power consumption but also allows generating power with greater energy efficiency than does harvesting the energy continuously. Introduction The known attempts to increase the power generated by a microbial fuel cell (MFC) can be summarized as manipulating various internal components of the fuel cell to improve its performance, and the following studies exemplify these efforts. Different types of microorganisms have been selected: Bond and Lovley (1) used Geobacter sulfurreducens to increase the efficiency of organic substrate oxidation, Choi et al. (2) used thermophilic bacteria Bacillus licheniformis and Bacillus thermoglucosidasius to produce power at elevated temper- atures, and Prasad et al. (3) used yeast Hansenula anomala, which can transfer electrons directly to the electrode. Various substances have been tried as electron donors: Kim et al. (4) used various carbon sources: glucose, galactose, sucrose, maltose, and trehalose; Liu et al. (5) used acetate and butyrate; and Oh et al. (6) used food and animal wastewater. A variety of electrochemically active substances have been added to enhance the rate of electron transfer: Park and Zeikus (7) used neutral red (also known as toluylene red) as the electron transfer mediator to increase the rate of the anodic reaction, and Schaetzle et al. (8) used laccase to increase the rate of the cathodic reaction. Various types of electrode materials have been used: Logan et al. (9) used carbon fiber brush, Morris et al. (10) used lead oxide catalyzed platinum as the anode, Rhoads et al. (11) used biomineralized manganese deposited on stainless steel as the cathode, and Pham et al. (12) used platinum-coated graphite electrodes. The design of the reactor has also been manipulated: Liu et al. (5) used a single-chambered microbial fuel cell to decrease the internal resistance caused by the membrane, Rabaey et al. (13) used a tubular microbial fuel cell to increase the power generation, and Reimers et al. (14) used a membrane-less microbial fuel cell to generate power from marine sediment. Much attention has been given in the literature to the design of the microbial fuel cell (MFC) and to the function of its internal components. Much less attention has been given to the difficulties that need to be overcome before these devices can become reliable power sources able to power commercially available electronic devices. These difficulties can be traced to the fact that MFCs generate notoriously low levels of power, so that powering external electrical devices directly from MFCs is problematic, except for devices with very low power requirements. The sensors normally used for environmental monitoring require more power than microbial fuel cells can generate. For example, electrochemical sensors require 50 mW; laser diodes, 225 mW; metal oxides, 280 mW; light-emitting diodes, 30 mW; and temperature sensors, 11 mW (15, 16). These sensors can not be powered directly using the microbial fuel cells reported in the literature. We are interested in powering devices consuming more power than can be continuously generated by MFCs, and to achieve this we will manipulate the manner of harvesting energy from MFCs. The most popular manner of harvesting energy from a MFC is inserting an electrical loadsa device to be powereds between the anode and the cathode. In laboratory setups used to measure the power generated by MFCs, the load is frequently simulated by a resistor and the electrical energy generated by the MFC is dissipated as heat (17). Using this mode of operation simplifies the computations: the power delivered by the MFC is just the product of the current and the potential drop across the resistor, both measured by suitable devices. However, this continuous mode of harvest- ing energy has limited practical applications because the energy from the MFC is harvested at a slow rate and, consequently, only devices consuming low power can be operated. To power a device consuming high power, an alternative solution is used: the electrical energy is first collected in a capacitor and then dispensed intermittently, in bursts of high power (16, 18, 19). This intermittent mode of energy harvesting allows the powering of electrical devices consuming high power but limits the applications to devices that can be operated intermittently. Clearly, each of these two modes of operation has limitations and their use needs to be examined each time a MFC is considered as a power source. We use MFCs to power sensors and wireless telemetry systems (16), for which intermittent powering is an acceptable solution. Comparing the two modes of powering electrical devices using MFCs reveals that the mode of operation not only determines the size of the electrical devices that can be * Corresponding author e-mail: zl@erc.montana.edu. † Washington State University. ‡ Montana State University. Environ. Sci. Technol. 2009, 43, 4600–4605 4600 9 ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 43, NO. 12, 2009 10.1021/es8037092 CCC: $40.75  2009 American Chemical Society Published on Web 05/14/2009